Chapter 12 Connections to Light Summary

Choose any TWO Sections you would like to summarize from chapter 12 and report on.  You should also be able to draw some connections to everyday life or a career.

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Chapter 12
Connections to Light
Why Do Lightsticks Glow?
Few of us recognize that the fascinating glow of a lightstick at a
carnival or fair arises from a complex series of chemical reactions.
The chemist’s palette–a variety of ingredients with specific chemical
structures–produces the rainbow of colors seen in these novelties.
The Chemical Basics
Lightsticks and other glow-in-the-dark products involve a phenomenon similar to
the processes that lead to a firefly’s glow or a lightning strike. This phenomenon is
called chemiluminescence–the process whereby chemical energy produced by a
chemical reaction is transformed into light energy, a different form of energy. In
a firefly the light-producing chemical reaction is triggered by a catalyst known as
an enzyme. In a lightning storm, an electrical discharge in the atmosphere sets off
a sequence of reactions that leads to the flash of light in the sky. In a lightstick,
when you follow a manufacturer’s instructions to bend, snap, and shake the stick,
you initiate a chemical reaction by mixing the substances contained within separate compartments in the plastic tube. Energy generated during the course of the
reaction is accepted by a dye molecule contained within the lightstick and then
released in the form of colored light with no accompanying heat.
The Cyalume lightsticks marketed by Omniglow Corporation have found an
extensive range of applications beyond the initial toy and novelty market. The intense yet cool light provided by a chemical lightstick is ideal for emergency lighting, traffic control, and hazard identification. Numerous sports products, including golf balls, footballs, hockey pucks, badminton birdies, and wiffle balls, use
replaceable lightsticks to create innovative nighttime activities. In virtually every
amusement park open past sunset, we can find varieties of glow-in-the-dark colored necklaces, bracelets, and bands. Military covert, nighttime, and emergency
operations have been enhanced with the use of visible and infrared chemiluminescent products. The United States and Allied forces during Operation Desert
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Chapter 12 Connectionsto Light
Storm employed lightsticks for underwater operations, nighttime personnel and
ship-to-ship identification, hazard demarcation, and color-coding of units, vehicles, and equipment. Commercial fishing fleets have found deep sea fish such
as tuna and swordfish to be attracted to the light emitted by lightsticks when the
fish swim in surface waters at night. In particular, the longline fishing industry
utilizes lightsticks to illuminate their monofilament long lines, often extending in
length up to 80 miles with more than 3000 hooks (see color Fig. 12.1.1). With the
variations in color, intensity, and duration of light emission possible, numerous
applications in other commercial industries are likely.
T h e C h e m i c a l Details
The color, intensity, and duration of light that we observe depend upon the exact
composition of a lightstick. Let’s explore how the chemical composition affects
these properties of the lightstick. The amount of energy produced by the chemical
reaction between the original components in the lightstick dictates the color of the
glow. Certain chemical reactions generate large amounts of energy and give rise
to short-wavelength light (i.e., energy cx 1/wavelength) that is blue or violet in
color. Other reactions are somewhat less energetic and give rise to red and orange
glows at the longer wavelength end of the visible spectrum. The efficiency of
the conversion of energy from the chemical reaction into light is one factor that
affects the intensity of the light emission. The more efficient the conversion, the
brighter the lightstick’s glow. In addition to a characteristic energy output, each
chemical reaction has its own efficiency.
The intensity of a lightstick is also sensitive to temperature (see color Fig.
12.1.2). An increase in temperature accelerates the rate of the chemical reaction,
increasing the intensity or amount of light that can be generated from the dye
molecule in a given time period. By the same token, you can prolong the life of a
lightstick by storing it in the freezer. At lower temperatures, the chemical reaction
slows down, producing less energy per unit time and yielding less intense light
that persists for a longer period of time. By making light intensity measurements
at a variety of temperatures, adherence to the Arrhenius law can be demonstrated
(i.e., intensity cx rate cx e-l/T). The duration of the lightstick’s glow is dictated
by the amount of chemical reactants contained in the tubing. At any temperature,
once all of the original chemicals contained within the lightstick are consumed by
the reaction, chemiluminescence is no longer possible, and the lightstick fades to
darkness.
Let’s look at a general chemical reaction for the chemiluminescence of a
lightstick in some detail. Hydrogen peroxide, H202, is the primary ingredient
contained in an aqueous solution within an inner thin-glass ampule within the
lightstick. By bending the plastic tubing of the lightstick, the thin vial of H202
is broken, mixing H202 with a surrounding solution of a phenyl oxalate ester
((COOC6H5)2) and a fluorescent dye. The ester and peroxide react in a series of
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Question 12.1
Why Do Lightsticks Glow?
141
several steps to generate a highly energetic C204 intermediate as in Fig. 12.1.3. [1]
O
O
Figure 12.1.3 9 The reaction ofphenyl oxalate ester and hydrogen peroxide to generate
a highly energetic C204 intermediate.
Variations in the substitution on the phenyl rings of the oxalate ester primarily
affect the yield of the C204 intermediate. The energetic nature of the intermediate
presumably arises from the significant strain imposed by the four-membered ring.
Recall that the ideal bond angle around a planar spZ-hybridized carbon is 120 ~
not 90 ~ The chemielectronic step–the step in which the chemical energy of the
intermediate is converted into electronic energy in the fluorescent d y e – – m a y be
written as in Fig. 12.1.4.
O-O
O
Figure 12.1.4
-I-
FLUORESCER
=-
FLUORESCER*
+
2 CO 2
O
9 A chemielectronic step, i.e., a step in which the chemical energy of an
intermediate is converted into electronic energy in a fluorescent dye. Here the C204 intermediate releases energy as it dissociates into two carbon dioxide molecules. The energy is
transferred to the accompanying fluorescent dye to generate an excited state of the dye.
The release of energy to the dye molecule or fluorescer is driven by the conformational instability of the C204 intermediate (the flat highly strained C204
prefers to be two linear CO2 molecules). The sensitized fluorescer, denotedfluorescer*, returns to the ground state via the emission of light:
fluorescer* –~ fluorescer + hr.
Emission spanning the visible and near-infrared wavelengths has been obtained through the choice of fluorescent dye. For example, 9,10-diphenylanthracene (Fig. 12.1.5) generates blue light; 9,10-bis(phenylethynyl)anthracene
(Fig. 12.1.6) yields yellow-green emission with maximum output at 486 nm;
rubrene (5,6,11,12-tetraphenylnaphthacene; Fig. 12.1.7) emits orange-yellow light
at 550 nm; violanthrone (Fig. 12.1.8) emits orange light at 630 nm; 16,17(1,2-ethylenedioxy)violanthrone (Fig. 12.1.9, R… R = – O C H z C H 2 0 – ) gives rise
to a red glow at 680 nm; and 16,17-dihexyloxyviolanthrone (Fig. 12.1.9, R -OC6H13) provides infrared luminescence at 725 nm.
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142
Chapter 12
Figure 12.1.5
9 The molecular structure of 9,10-diphenylanthracene.
~
Figure 12.1.6
Figure 12.1.7
naphthacene).
Connections to Light
c–~—_
c
c -~-c.–~
9 The molecular structure of 9,10-bis(phenylethynyl)anthracene.
9 The molecular structure of rubrene (5,6,11,12-tetraphenyl-
o
Figure 12.1.8
9 The molecular structure ofviolanthrone.
o
R R
Figure 12.1.9 9 The molecular structure of 16,17-(1,2-ethylenedioxy)violanthrone with
R… R = – O C H 2 C H 2 0 – and 16,17-dihexyloxyviolanthrone with each R = -OC6H13.
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Question 12.2 Why Is an Astronaut’s Visor So Reflective?
143
References
[1] B. Z. Shakhashiri, Lightsticks in Chemical Demonstrations.” A Handbook for Teachers of
Chemistry, Volume 1 (Madison: Univ. of Wisconsin Press, 1983), 146-152.
R e l a t e d W e b Sites
9 “The Chemiluminescence Home Page.” Dr. Thomas G. Chasteen, Sam Houston State
University, http://www.shsu.edu/~chm_tgc/chemilumdir/chemiluminescence2.html
P. OmniglowCorporation, http://www.omniglow.com/
“Lightstick Chemistry.” Moravian College, Chemistry Department,
http:llwww’cs’m~
st scheme.html
“Lightstick Spectra.” Moravian College, Chemistry Department,
http://www.cs.moravian.edu/chemistryllightstickll st spec.html
Why Is an Astronaut’s Visor So Reflective?
The bright reflection of the sun’s rays on an astronaut’s visor is keenly
apparent in many of the photographs taken during spacewalks. The
chemistry of the visor reveals the origin of its highly reflective nature.
The Chemical Basics
We are familiar with mirrors–polished surfaces that divert light according to
the law of reflection. In ancient times, polished castings of solid tin, bronze,
copper, gold, and silver served as the first mirrors. Modem mirrors consist of a
plate of glass with a thin layer of aluminum or silver deposited on the backing.
Thus, glass serves as the substrate – – the underlying material to which a coating
is applied. The brilliant white luster of aluminum and silver metal contributes to
their selection as mirror coatings. While the decorative beauty of silver has been
known for centuries, the silvering process of making mirrors was discovered in
1835 by the German chemist Justus von Liebig.
A typical mirror reflects both visible and near-infrared light. The thickness of
the silver layer further delineates a mirror’s function. Without the transmission
of visible radiation, one cannot see through a silvered mirror. The coating on an
astronaut’s visor permits the visor to act as both a mirror and a transmitting shield.
Coated with an ultrathin film of gold, infrared light reflects off the visor’s surface
while still permitting the astronaut to see visible light through the visor shield
(see color Fig. 12.2.1). The relative amount of reflection to transmittance can be
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144
Chapter 12 Connectionsto Light
controlled by the thickness of the film. A “gold-plated” visor is absolutely necessary to protect the astronaut from the infrared rays of the sun that are essentially
unfiltered in space. Even the general public can benefit from this technology-gold-mirrored lenses are popular on some brands of sunglasses. Surfaces other
than glass have been treated with gold for infrared protection. For example, gold
tape served as a coveting on the tetherlines connecting the Gemini-Titan 4 astronauts with their spacecraft during spacewalks (“extravehicular activity”). Thin
films of gold also coat satellites in order to control the temperatures that could
result from infrared heating in space. The exterior of the canopies of F 16 aircraft
are also treated with ultrathin layers of gold metal, presumably to reduce the radar
signature of the aircraft (although the purpose of the gold treatment on the cockpit
is officially classified).
The Chemical Details
An astronaut’s visor is coated with thin films of the element gold. This coating
reflects up to 98% of the infrared radiation incident on the visor, protecting the
astronaut from the intensity of the sun’s heat. The same principle dictates why
gold films (as thin as even 20 pm!) Ill are placed on the inside face of windows
in office buildings, reducing heat losses in winter and reflecting infrared radiation
(and thus heat) in summer. Gold is more malleable and ductile than any other
metal, enabling the creation of thin films, and its high thermal conductivity (only
silver and copper have higher thermal conductivities) [2] enables efficient cooling.
However, the high degree of reflectivity of gold in the infrared region (98%) [3]
minimizes the absorption of radiation by the gold coating deposited on the visor
or glass.
References
[1] N.N. Greenwood, and A. Earnshaw, “Copper, Silver, and Gold,” in Chemistry of the Elements, 2nd ed. (New York: Pergamon Press, 1997), Chap. 28, pp. 1173-1200.
[2] “Thermal and Physical Properties of Pure Metals.” Handbook of Chemistry and Physics,
74th ed., ed. David R. Lide (Boca Raton, FL: CRC Press, 1993), 12-134 to 12-137.
[3] “Metallic Reflector Coatings, Oriel Instruments, http://www.oriel.com/netcat/Volumelll/
pdfs/v36metct.pdf
Related Web Sites
I,. NASA Photos of Astronaut Edward H. White II during Extravehicular Activity Performed
during the Gemini-Titan 4 Space Flight,
http ://science. ksc. nasa. gov/mifrors/imag es/i mag es/pao/G T4/ 10074015. htm
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Question 12.3 Why Is the Aurora Borealis So Colorful?
145
http ://www.ksc.nasa.govlmirrorslimageslimageslpaolGT4l 10074016.htm
http :llwww.ksc.nasa. govlmirrorslimageslimageslpaolG T4110074017.htm
http :llwww.ksc.nasa.govlmirrorslimageslimageslpaolG T4110074018.htm
“Gold Occurrences.” R. James Weick, Geological Survey of Newfoundland and Labrador,
http :llwww.geosurv.gov.nf.ca/education/occgold,html
Why Is the Aurora Borealis So Colorful?
And the Skies of night were alive with light, with a throbbing, thrilling
flame;
Amber and rose and violet, opal and gold it came.
It swept the sky like a giant scythe, it quivered back to a wedge;
Argently bright, it cleft the night with a wavy golden edge.
Pennants of silver waved and streamed, lazy banners unfurled;
Sudden splendors of sabres gleamed, lightning javelins were hurled.
There in our awe we crouched and saw with our wild, uplifted eyes.
Charge and retire the hosts of fire in the battlefield of the skies.
–Robert W. Service, The
Ballad of the Northern Lights
How does chemistry explain the ethereal and breathtaking Aurora Borealis?
The Chemical Basics
Have you ever been treated to the breathtaking display of the northern lights or
aurora borealis of the Northern Hemisphere (or the corresponding southern lights
aurora australis of the Southern Hemisphere)? The colorful auroral rings and
patches of light in the nighttime sky are a wonderous sight. Aurora was the Roman
goddess of dawn, and aurora borealis and aurora australis literally mean “dawn of
the north” and “dawn of the south.” In fact, auroras are the final event of a chain of
reactions that begin with the sun. Solar flares eject energetic particles that travel
throughout interplanetary space at a high velocity (400 km/s or 250 miles/s). [11
These particles are charged and can be trapped by the earth’s magnetic field. As
the particles travel to the earth’s magnetic poles, they collide with oxygen (i.e.,
dioxygen, O2) and nitrogen (i.e., dinitrogen, N2) gases in our upper atmosphere
40-600 miles above Earth. These collisions result in a transfer of energy to the
gaseous molecules that may even be sufficient to ionize the molecules (create
a charged species by removing an electron, e.g., O2 + e- —) O~- + 2 e – ) or
dissociate (separate) the molecules into individual atoms (e.g., O2 + e- –~ 2 0 +
+ e-). The collection of energized molecules, ions, and atoms retums to a less
energetic state by releasing the acquired energy in the form of light. The color of
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146
Chapter 12 Connectionsto Light
the light observed depends on the identity of the species (see color Figs. 12.3.1
and 12.3.2).
The Chemical Details
An aurora is generated by streams of energetic charged particles (mostly electrons) that emanate from explosive activity, such as solar flares, on the surface
of the sun. Interaction of the solar wind with the earth’s magnetic field restricts
these particles to higher latitudes. The characteristic wavelengths observed in
auroras depend on the chemical identity of the energetic species created and the
energy transferred from the solar particles. Highly energetic electrons can penetrate to great depths in the atmosphere, whereas low-energetic electrons only
reach to the top of the thermosphere. Violet and blue light with wavelengths of
391.4 and 470.0 nm is emitted by N~-, predominantly located in the lower atmosphere. The sensitivity of the human eye generally does not readily detect
the blue wavelengths. Crimson-red light at 630 nm is discharged by O~-. Both
a greenish-yellow light at 557.7 nm (high energy) and a deep red light at 630.0
and 636.4 nm (low energy) are characteristic of O atoms, created via dissociation
of diatomic oxygen by ultraviolet light. Yellow-green auroras arise from oxygen
atoms at lower altitudes where energetic electrons penetrate. The combination of
a large influx of low-energy electrons and high-altitude oxygen (200 miles up) is
responsible for the rare all-red auroras. [2]
References
[ l] “The Sun.” NASA Goddard Space Flight Center,
http://www-s pof. gsfc. nasa. gov/E d ucation/I sun. html
[2] “The Rare Red Aurora.” Article 918, Carla Helfferich, Alaska Science Forum, March 22,
1989, http:/lwww.gi.alaska.edu/ScienceForum/ASF9/918.html
Related Web Sites
9 “Auroras: Paintings in the Sky.” Mish Denlinger, The Exploratorium,
http://www.exploratorium.edu/learning_studio/auroras
9 “The Aurora Page.” Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI, http://www.geo.mtu.edu/weather/aurora/
9 “The Exploration of the Earth’s Magnetosphere.” NASA Goddard Space Flight Center,
http://www-spof.gsfc. nasa. gov/E d ucation/I ntro.html
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Question 12.4 What Causes the Pearlescent Appearance of Some Paints?
147
9 “Space Weather: A Research Perspective–The Elements of Near-Earth Space.” Space
Studies Board, Commission on Physical Sciences, Mathematics, and Applications, National Research Council, http://www.nas.edu/ssb/elements.html
9 Photo of a Red Aurora, Alyeska Pipeline, http://www.alyeska-pipe.com/Photolibrary/
AuroraBorealis.html
What Causes the Pearlescent Appearance of
Some Paints?
Pearlescent lusters are commonly seen in many paints, inks, and cosmetics. The pearlescent pigment technology that brings us these unusual effects relies on a common mineral to achieve these opalescent
qualities.
The Chemical Basics
Pearlescent pigments contain small flakes or platelets of the mineral mica that are
additionally coated with a very thin layer of titanium dioxide. The simultaneous
reflection of light from many layers of small platelets creates an impression of
luster and sheen. By varying the thickness of the coating on the surface of the mica
particles, pigment manufacturers can achieve a range of colors for the pearlescent
effect.
The Chemical Details
Pearlescent pigments are derived from microscopic mica platelets ranging in size
from 1 to 2 ktm in thickness and up to 180 r
in diameter. Mica consists of
an aluminum silicate with a crystalline structure that easily permits cleavage into
very thin platelets. In fact, mica is a generic term for any one of several complex hydrous aluminosilicate minerals characterized by their platy nature and pronounced basal cleavage. The general formula for mica is AB[2_3](A1, Si)Si3Olo(F,
OH)2. [j] Generally A represents the metal potassium (K) and B corresponds to aluminum (A1). However, some micas contain calcium (Ca), sodium (Na), or barium
(Ba) for A and lithium (Li), iron (Fe), or magnesium (Mg) for B. This subclass
of silicates contains rings of SiO4 tetrahedrons linked by shared oxygens to other
rings in a two-dimensional plane. This bonding arrangement produces a sheet-like
structure that gives rise to flat, platy crystals with good basal cleavage. Ill
The presence of mica in pearlescent pigments only partly accounts for the appearance of the pigment. A very thin layer of the inorganic oxide titanium dioxide
(TiO2) or iron oxide (Fe203) or both is coated on the mica platelets. The various
colors and pearlescent effects are created as light is both refracted and reflected
from the titanium dioxide layers. The very thin platelets are highly reflective and
transparent. With their plate-like shape, the platelets are easily oriented into parallel layers as the paint medium is applied. Some of the incident light is reflected
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Chapter 12 Connectionsto Light
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148
,’
/
Microns
t
“,
‘..
Reflected light:
Red
i
Green
”
Blue
Figure 12.4.1 ~ The multiple reflection of light from microscopic oxide layers of different dimensions leads to constructive and destructive interference of light waves, producing
a particular color effect. Different thicknesses reflect different colors.
from the uppermost layers, while a portion is transmitted to lower layers and
then reflected. A pearlescent luster is produced from this multiple reflection of
light from many microscopic layers. Smaller mica particle size leads to smoother
sheens, while larger particle size produces a higher luster or sparkled effect. In
addition, the thickness of the oxide layer dictates the color observed. Multiple
reflections and refractions of light lead to both constructive and destructive interference of light waves. The oxide layer thickness determines the narrow range of
wavelengths of light that interfere constructively, thus producing a particular color
effect. All other wavelengths of light interfere destructively and are not observed.
Fig. 12.4.1 summarizes these concepts.
References
[ 1] The Silicate Class, The Mineral Gallery,
http://www.galleries.com/minerals/silicate/class.htm
Related Web Sites
I~ “Interference and Iridescent Acrylics.” Golden Artist Colors, New Berlin, NY,
http://www,golden pai nts. co m/irid int. htm
P, Microscopic photo of pearlescent paint,
http://www.standox.de/standote/englisch/color/3schicht.htm
I., “Understanding Vehicle Palm Technology: What Are Metallic Finishes?”
http ://www. sta nd ox. de/lack/e ng Iisch/g u id e3. htm#Meta Ilic
“Standothek: Design and Paint.” http:llwww.standox.comlstandotelenglischldesignl
effekt.htm
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Question 12.5 Why Is It Incorrect to Call U.S. Bills “Paper Currency”?
149
Why Is It Incorrect to Call U.S. Bills “Paper
Currency”?
Most Americans recognize the portraits that appear on the front face
of U.S. currency: George Washington ($1), Thomas Jefferson ($2),
Abraham Lincoln ($5), Alexander Hamilton ($10), Andrew Jackson
($20), Ulysses Grant ($50), Benjamin Franklin ($100). As with U.S.
coinage, the Secretary of the Treasury, in consultation with the Commission on Fine Arts, selects the designs shown on U.S. currency.
However, it is the Bureau of Engraving and Printing that is responsible
for designing and printing U.S. currency at its facilities in Washington,
DC, and Ft. Worth, TX. While the printed features of U.S. currency
have undergone significant changes over the years, the chemistry of
the “paper” has a much more stable history.
The Chemical
Basics
The constant circulation of a national currency demands a durable, yet highquality material. Beginning with the first series of U.S. bank notes issued in 1861,
U.S. currency has never been printed on paper but on a cotton/linen fabric with
the linen content held at 25 4- 5%. Often this fabric is referred to as cotton and
linen rag paper. Red and blue silk fibers from scraps and cuttings of clothing
manufacturers are embedded in the cotton/linen sheet to deter counterfeiters. A
commercial company, Crane & Company Inc. of Dalton, MA, has been manufacturing the rag paper since 1879, and it is illegal for anyone to manufacture,
possess, or use this material or a fabric of a similar type. The history of Crane &
Company is a fascinating story.Ill The association of the Crane family with U.S.
currency began with the sale of paper in 1775 by Steven Crane to Paul Revere
for the Massachusetts Bay Colony’s first currency. Having been taught the art
of paper-making by his father, Zenas Crane carried on the family business and
built his first paper mill along the Housatonic River in Dalton, MA in 1801. The
following advertisement appeared in the Pittsfield Sun in 1800[2]:
Americans-Encourage your own manufactories, and they will Improve. Ladies
save your Rags. As the Subscribers have it in contemplation to erect
a Paper-Mill in Dalton, the ensuing Spring; and the business being
very beneficient to the community at large, they flatter themselves
they shall meet with due encouragement.
–[Signed] Henry Marshall,
Zenas Crane, and John Willard
As the above notice indicates, cloth rags were the basic raw material for conversion into pulp for high-quality, rag-based paper products. Although Zenas
Crane retired in 1842, his sons James Brewer Crane and Zenas Marshall Crane
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150
Chapter 12 Connectionsto Light
continued the business and begin to make paper for banknotes, bonds, and securities. Two generations later in 1879, W. Murray Crane won a competition to
manufacture the paper for U.S. currency. Crane & Company has held an exclusive contract with the U.S. Treasury Department to produce the specially threaded
cotton and linen paper for U.S. currency. The company also manufactures paper
for the foreign currencies of Canada, Mexico, Indonesia, and the Ukraine [3] in
addition to its renown high-quality stationery business.
T h e C h e m i c a l Details
In ancient Egypt the fibers and glue-like sap of the reedy papyrus plant were
the main constituents of the sheets formed for writing materials. Other woody
fibers, such as mulberry, were introduced by the Chinese in the first century AD.
Paper mills existed in Europe by the fourteenth century, and linen and cotton rags
served as the basic raw materials through the eighteenth century. Paper mills often
solicited publicly for rags because the shortage of raw materials could not keep up
with the demand for paper. The manufacturing of paper from wood pulp began in
1800 to relieve the paper industry from its demand for cotton and linen rags. Even
today grades of paper requiring strength, durability, permanence, and fine texture
employ cotton and linen fibers derived from textile and garment mill cuttings.
One of the final steps in the paper-making process is the coating of the paper
surface to achieve a variety of effects. For example, coating can enhance the
uniformity of the surface for printing inks or enhance the opacity or whiteness of
the paper. Titanium dioxide (TiO2) is used to whiten and opacify all U.S. paper
currency and many other forms of paper.H] Why is titanium dioxide an optimal
coating selected to whiten paper? For the human eye to perceive the color white,
an object must scatter all wavelengths of light. Thus, the ability of a sheet of
paper, or its coating, to scatter light will define its opacity and brightness. One
mechanism of light scattering is by refraction, and the high refractive index of
titanium dioxide makes it an effective scatterer of light. Refraction is defined as
the bending of light as it passes from one medium, such as air, to another medium,
such as a paper coating. The larger the difference in the refractive index of the
two media, the greater the extent to which the light is bent or refracted. The
extremely high refractive index values of both commonly used crystal forms of
titanium dioxide – – anatase and rutile – – make them ideal commercial pigments
for achieving brightness with low volumes of sample. [5]
References
[ 1] “The State of One Small Family Business: Crane & Co.” Mary Furash, Inc. Online,
http://www.inc.com/incmagazine/archives/27961141.html
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Question 12.5 Why Is It Incorrect to Call U.S. Bills “Paper Currency”?
151
[2] “Crane Museum, Dalton, Massachusetts 01226.”
http://www.berkshireweb.com/themap/dalton/museum.html
[3] “Opening up Money Supply Worries Dalton Paper Firm.” JeffDonn, Associated Press, The
Standard-Times, http ://www. s-t. co mid a i ly/04-97/04-13-97/f03 bu299, h tm
[4] “Basics: Why is TiO2 Used in Paper?” Millenium Chemicals,
http ://www.millenniumchem.com/Products+and+ Services/Products+by+ Type/
Titanium+Dioxide+-+Paper/rPaper+Basics/
[5] “DuPont Ti-Pure: Paper: Rutile and Anatase.”
http://www.dupont.com/tipure/paper/anarut.html
Related Web Sites
9 “Crane Bro’s Paper Manufacturers.” http://www.holyokemass.com/historic/
pichampden/p 157. html
9 “History of Paper Money.” Federal Reserve Bank of San Francisco,
http ://www.frbsf.org/federalreserve/money/funfacts. html#A3
9 “Fundamental Facts about U.S. Money.” Federal Reserve Bank of Atlanta,
http :llwww.atl.frb.org/publicalbrochurelfundfaclmoney.htm
9 “Money Hot Off the Press.” Bureau of Engraving and Printing, United States Treasury, A
30-second video capsule of how currency is printed, http://www.treas.gov/bep/money.mpg
“The Making of Money.” Bureau of Engraving and Printing, United States Treasury, A
39-second video with sound on the currency-making process, http://www.treas.gov/bep/
making.mpg
9 “Anatomy of a Bill: The Currency Paper, Secrets of Making Money.” NOVA Online,
http ://www.pbs. o rg/wg bh/novalmoolahlanatomypaper, html
9 “Money Making.” The AFU and Urban Legend Archive,
http://www.urbanlegends.com/misc/money_making.html
9 “Six Kinds of United States Paper Currency.” Kelley L. Ross, Ph.D.,
http ://www.friesia n. com/notes, htm
“A Brief History of Our Nation’s Paper Money.” 1995 Annual Report, Federal Reserve
Bank of San Francisco, http://www.frbsf.org/publications/federalreserve/annual/
1995/history.html
9 “Treasury Home Page: Existing Contracts: Massachusetts.”
http :/Iwww.treas, g ov/s balm a. html
9 “DuPont Ti-Pure Titanium Dioxide.” http://www.dupont.com/tipure/paper/index.html
9 “The Mineral Rutile.” Amethyst Galleries, Inc.,
http://mineral.galleries.com/minerals/oxides/rutile/rutile.htm
9 “The Mineral Anatase.” Amethyst Galleries, Inc.,
http ://mineral. gal le ries. co m/m i n e ra Is/oxid es/a n ata se/a n ata se. htm
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152
Other Interesting References
9 “Zenas Crane,” in Encyclopedia of Entrepreneurs, ed. Anthony Hallett and Diane Hallett
(New York: Wiley, 1997), 131-132.
What Is the Purpose of the Thread That
Runs Vertically through the Clear Field on
the Face Side of U.S. Currency?
The Bureau of Engraving and Printing in Washington, DC, is responsible for the design, engraving, and production of U.S. banknotes.
Several of the special design features evident in U.S. currency today
take advantage of technological advancements in the ink and paper
industries. The highly sophisticated chemistry of such materials is an
effective deterrent against counterfeiting.
The Chemical Basics
Have you ever examined the details on paper U.S. currency? The denomination,
portrait, and back design are commonly recognized. The Treasury seal, Federal
Reserve seal, signature of the U.S. Treasurer, motto, and serial numbers are also
clearly identified. Some design features are not as readily apparent. One such
feature introduced in 1990 series currency is a clear thread embedded in paper
currency as a security measure against counterfeiting. The security thread is a
polyester thread on which a denomination identifier is printed. Both the thread
and the printing are visible only with a light source and can be viewed from either
the face or the back of the note. For the two highest denomination bills, $100
and $50, the security thread repeats the wording “USA 100 USA 100” and “USA
50 USA 50,” respectively. For the next three lower denominations ($20, $10, and
$5), the printing consists of the abbreviation “USA” followed by the written denomination in capital letters, as in “USA TEN,” repeated along the length of the
thread. No security thread is included in the $1 note. Beginning with the 1996
series notes, the placement of the thread varies, with the position indicative of the
bill’s denomination. In addition, the thread on the new $100 note first issued on
March 25, 1996 was redesigned to glow or fluoresce a red color when exposed to
ultraviolet light in a dark setting. The thread is positioned to the immediate left
of the oval frame on Ben Franklin’s portrait. The new $50 notes first issued on
October 27, 1997 also have the added feature that the security thread, now to the
right of the portrait of President Grant, glows yellow when exposed to ultraviolet
light in a dark environment. The printing on the thread also includes a flag in
addition to “USA 50.” The redesigned $20 bill was unveiled in May 1998 and
put into circulation in the fall of that year. The vertical thread is embedded to the
far left of President Jackson’s portrait (to the left of the Federal Reserve Seal).
The words “USA TWENTY” and a flag can be seen from both sides against a
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Question 12.6 What Is the Thread Running through U.S. Currency?
153
light. The number “20” appears in the star field of the flag. The thread fluoresces
green under an ultraviolet light. The design features for the new $10 and $5 notes,
released in May 2000, include security threads that glow orange and blue, respectively, when held under an ultraviolet light. The introduction of the new security
measures has not always occurred without error. In November 1996 officials at the
Bureau of Printing and Engraving of the Treasury Department discovered that the
positions of the polymer security thread and the watermark on $4.6 million worth
of the newly designed $100 bills were swapped–the thread incorrectly appeared
on the right side of Benjamin Franklin’s portrait and the watermark on the left.J1]
These misprinted bills are still legal tender, yet they are likely to be worth much
more than their face value to collectors. You might wonder why the U.S. Treasury
is varying the position of the security thread in its currency. Counterfeiters often
attempt to “raise notes,” that is, bleach out the paper of a low denomination and
reprint a higher denomination onto the authentic paper. With the position of the
security thread constrained to the bill’s denomination, this type of fraud will be
easily detected.
The Bank of Latvia (Latvijas Banka) also incorporates invisible fluorescent
fibers in their currency as a security measure. As with the security thread on
U.S. bank notes, the Latvian thread fluoresces under ultraviolet light, emitting
three different colors. In addition, the printing ink employed for the red serial
numbers located in the upper center portion of the face of the notes fluoresces
upon exposure to ultraviolet light. Singapore currencies include specially formulated inks to fluoresce under ultraviolet light, including black serial numbers that
glow green and a red seal of the Minister of Finance that emits orange illumination. Fluorescent inks are employed for denomination figures, seals, and other objects in Hong Kong dollars, Malaysian dollars, Taiwan yuan, and Indonesian rupiahs. Special fluorescent fibers were incorporated in the paper-making process for
Deutsch marks, Italian lire, Netherlands goldens, and Belgian francs. Fluorescent
spots also appear on Swiss francs and Canadian dollars. A number of commercial
products are manufactured to detect embedded fluorescent denomination-specific
security threads. [2]
The Chemical Details
The security thread introduced in the Series 1990 banknotes is a thin metallized
polyester strip that is 1.4 to 1.8 mm in width and 10 to 1 5 / t m in thickness. [3]
Polyester threads are standard synthetic fibers consisting of large linear (chainlike) or cross-linked (network) polymers formed from a large number of smaller
molecules or monomers. The monomers are connected via ester linkages as in
Fig. 12.6.1. Polyesters most commonly are prepared from equivalent amounts
of two different monomers: glycols and dibasic acids. Glycols are organic compounds containing two hydroxyl groups, -OH, and dibasic acids are organic molecules containing two carboxyl functionalities,-COOH.
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154
Chapter 12 Connections to Light
O
R
0 1
Figure 12.6.1 9 Ester linkages that connect monomers to form polymeric molecules
known as polyesters.
References
[ 1] “Officials report $100 printing error.” National/World News, March 29, 1997, TH On-Line,
Telegraph Herald of Dubuque, IA, http:/lwww.thonline.com/thlnews10329971
National/52409.htm
[2] “Ultraviolet Applications.” UVP Products, Inc., Upland, CA,
http://www.uvp.com/html/bulletins.html
[3] “Description and Assessment of Deterrent Features,” in Counterfeit Deterrent Featuresfor
the Next-Generation Currency Design, National Materials Advisory Board (Washington,
DC: National Academy Press, 1993), Chap. 4.
Related Web Sites
9 “Know Your Money.” United States Secret Service, United States Treasury,
http://www, t rea s. g ov/u sss/kn ow_yo u r_m o ney. shtml
9 “US Treasury and Federal Reserve Introduce New $50 Bill.” Treasury News, June 12,
1997, http ://www.treas.gov/press/releases/archives/1997. html
9 “Features of the New Twenty.” Bureau of Engraving and Printing,
http://www.bep.treas.gov/section.cfrn/4130157
9 “Security Features of the Lats Banknotes.” Latvijas Banka,
http://www.bank.lv/naudas/English/index_security.html
9 “Your Online Guide to Singapore Currencies: Telling Genuine from Fake Notes: Features
Recognizable under Ultraviolet Light.” Board of Commissioners of Currency, Singapore,
http://www.bccs-sin.com/telling.html
9 “Counterfeit Money Detector.” Trade2000 (USA) Inc., http://www.trade2000.com/
intro.htm and http://www.trade2000.com/detector3.htm
9 “Money Hot offthe Press.” Bureau of Engraving and Printing, United States Treasury, a 30second video capsule of how currency is printed, http://www.treas.gov/bep/rnoney.mpg
9 “The Making of Money.” Bureau of Engraving and Printing, United States Treasury, a 39second video with sound on the currency-making process,
http://www, treas, gov/be p/m a ki ng. mpg
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Question 12.7 Why Do U.S. Bills Shift in Color at Different Angles?
155
9 Phosphors for Security, Tagging and UV/IR Detection, Phosphor Technology,
http :llwww.phosphor, demon, co. uk/iruv.htm
9 “Physical Sciences and Technology. New Greenbacks: How to Make a Buck–Literally.”
Richard Lipkin, http://www.sciencenews.org/sn_edpik/ps_4.htm
Why Do U.S. Bills Shift in Color When
Viewed from Different Angles?
After nearly four generations, the currency of the United States has
undergone a noticeable change in appearance using several technological advances. In particular, complex and innovative chemistry has
been incorporated in the design of the ink used to denote the denomination in the lower right-hand corner. Is it an optical illusion, or is the
color of the ink shifting from green to black as you tilt the bill?
The Chemical
Basics
With advancements in the technologies of color copiers, scanners, and printers,
maintaining the security of U.S. currency is increasingly difficult. Since 1990 the
U.S. Treasury has added a number of features to new currency as security measures against counterfeiting. One of the new design features for series 1996 notes
is the use of optically variable ink on the number in the lower right-hand corner
of the bill. As one tilts the bill in light, the use of a color-shifting ink becomes
apparent. When viewed straight-on, the numerals appear green, but when viewed
at an angle the numbers look black.
The technology was developed by chemist Roger Phillips, currently product
technology manager and manager of intellectual properties at Flex Products in
Santa Rosa in Sonoma County, CA. Phillips and two colleagues, Pat Higgins and
Peter Berning at Optical Coating, originally designed an optically variable foil
that would be applied to U.S. currency as an anticounterfeiting measure. The
Federal Reserve initially liked the idea and spent $17 million to develop the product, but by 1985 federal officials decided to abandon the new technology because
the process would require expenditures for new machinery. With the company
facing massive layoffs, Phillips came up with the idea to translate the technology
into a printing ink that would not require costly new equipment.
The United States is not the only government using optically variable ink as
a security feature. Over 40 countries use the pigment technology. On the 500lats note issued by the Bank of Latvia (Latvijas Banka), optically variable ink
creates the effect of changing colors for the nominal value printed in the lefthand comer of the obverse side of the bill. Security measures involving optically
variable ink have also been taken by the Central Bank of Ireland for its bank notes.
The United Kingdom also recently used optically variable ink on first-class rate
stamps reissued in gold to commemorate the 50th anniversary of the marriage of
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156
Chapter 12 Connections to Light
Her Majesty the Queen to Prince Philip. The Queen’s head changes from gold
to green with a change in viewing angle. Roger Phillips has also extended the
technology of optically variable ink to new areas, for example, revolutionizing
the automotive industry. In 1993 he developed a durable automobile paint that
changed colors when viewed from different angles.
T h e C h e m i c a l Details
The process for developing optically variable ink involves the layering of several
extremely thin metal-containing pigment coatings of precise thickness, followed
by grinding of the coating into tiny platelets or flakes. The flakes are typically
1 /~m thick and 2 to 20/zm in diameter with an average aspect ratio (i.e., ratio
of width to height) of 10 to 1.[1] These color-shifting thin-film flakes are then
suspended in a mixture of regular ink, and the ink is then applied to a surface.
The high-aspect ratio helps align the flakes parallel to the surface of the ink. As
light shines on the flakes, light is reflected. Because of the random positionings
of the metallic platelets, certain wavelengths of light are selectively reinforced
(“constructive interference”), while other wavelengths are canceled (“destructive
interference”). This phenomenon, known as color diffraction, creates the appearance of color through reflection (see Fig. 12.7.1).
Incident
Reflection
Incident
Constructive
ct’on
i]iiAt)
Transmission
Figure 12.7.1 ~ The phenomena of light reflection, absorption, and interference to
create the appearance of color. Color arises as certain wavelengths of reflected light are
selectively reinforced through constructive interference, while other wavelengths of light
are canceled through destructive interference.
The particular colors that are observed at different angles will depend critically on the thickness of the thin film coating. Precision instrumentation is required to carefully control film thickness during production. The magnitude of
the optical effect depends on the density of flakes in the ink, while the quality of
the optical effect depends on the precise orientation or alignment of these flakes
with respect to the paper surface.
What are the typical materials used to create the color-shifting flakes? A
symmetrical layering pattern of absorber/dielectric/reflector/dielectric/absorber is
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Question
12.7
Why Do U.S. Bills Shift in Color at Different Angles?
157
employed, since the flakes can be oriented either up or down on the ink surface. The role of the absorber is to absorb particular wavelengths of light to
enhance or modify the observable color change of the ink as the viewing angle is varied. The dielectric is a material that does not absorb visible light and
also possesses a low refractive index. The interference colors of such materials are highly angle-dependent [z] The reflector is the material that produces the
unique optical effect through the reflection of light. Typical materials used in the
multilayer structure are chromium as the absorber, magnesium fluoride or silicon
dioxide as the dielectric, and aluminum as the reflector: Cr/MgFz/A1/MgFz/Cr
and Cr/SiOz/A1/SiOz/Cr. Layer thicknesses of 50, 4000, 900, 4000, and 50 A,
respectively, [1] are typical. Some additional combinations of materials for optically variable flakes are illustrated in Fig. 12.7.2.
Partially reflective
AI
Cr
MoS2
Fe203
Fe2Oz
Low refraction
SiO2
MgF2
SiO2
SiO2
SiO2
Inner reflector
AI
AI
AI
AI
Low refraction
SiO2
MgF2
SiO2
SiO2
Partially reflective
AI
AI
MoS2
Fe203
Fe20.~
SiO2
Fe203
Figure 12.7.2 9 Layering patterns of absorber (partially reflected)/dielectric (low refractive)/reflector (inner reflector)/dielectric/absorber used to create the optical effect of
color-shifting ink.
References
[ 1] “Description and Assessment of Deterrent Features,” in Counterfeit Deterrent Featuresfor
the Next-Generation Currency Design, National Materials Advisory Board (Washington,
DC: National Academy Press, 1993), Chap. 4.
[2] “Luster Pigments with Optically Variable Properties.” Raimund Schmid, Norbert Mronga,
Volker Radtke, Oliver Seeger, BASF Corporation, http://www.coatings.de/articles/
schmid/schmid.htm
Related Web Sites
9 “Know Your Money.” U.S. Secret Service, U.S. Treasury,
http://www, t tea s. 9 ~ sss/kn ~176 u r–m ~ neY s htm I
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158
9 “Color-ShiftingInk.” Unites States Treasury,
http://www.bep.treas.gov/cdO42500/color.html
9 “BankNotes and Security Features.” Banque de France,
http://www, banque-france.fr/gb/billets/main.htm
9 “SecurityFeatures of the Lats Banknotes.” Latvijas Banka,
http://www.bank.lv/naudas/English/index_security.html
9 “Stories from a Few Who Teach, Innovate, Create.” Mary Fricker, The Press Democrat,
Outlook Sonoma County and Its Economy,
http://www.pressdemo.corn/outlook97/tech/stories.html
9 “MoneyHot off the Press.” Bureau of Engraving and Printing, United States Treasury, a 30second video capsule of how currency is printed, http://www.treas.gov/bep/money.mpg
9 “The Making of Money.” Bureau of Engraving and Printing, United States Treasury, a 39second video with sound on the currency-making process, http://www.treas.gov/
bep/making.mpg
9 “Secretsof Making Money.” NOVA Online, http:llwww.pbs.org/wgbhlnovalmoolah
What Is an Optical Brightener?
“Whiter whites! Brighter brights!” You’ve heard such claims made by
many manufacturers of laundry detergents. The chemical structure of
the additives called “optical brighteners” provides the essential factors
that make these superior detergents possible.
The Chemical Basics
Natural fibers such as wool and silk and cellulose-containing paper often have a
yellowish tinge. To appear yellow these materials must absorb the complementary
c o l o r – – a violet to blue hue, i.e., light of wavelength near 400 nm. (The complementary color is the color on the opposite side of a color wheel.) Natural pigments
in these fabrics and in cellulose are responsible for the absorption. Whitening of
fabrics and paper can be accomplished using bleaches, but this treatment often degrades the material. Alternatively, an optical brightener may be added to replace
the blue-violet light that is lost (i.e., not transmitted to our eyes but absorbed by
the fiber). An optical brightener is a compound that accomplishes this function by
absorbing ultraviolet light and subsequently emitting (“fluorescing”) blue visible
light. The emitted blue light from the optical brightener replaces the blue light
absorbed by the fabric, thereby creating a “complete” white light that contains all
of the frequencies of the color spectrum. The further “brightening” action of an
optical brightener arises when a slight excess of the fluorescent compound is used
to convert even more ultraviolet light into visible light. The amount of optical
brightener used should be carefully controlled, for an excessive level of optical
brightener can lead to a blue cast for the fabric due to the blue emission of light.
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Question 12.8 What Is an Optical Brightener?
159
The Chemical Details
While many commercial optical brighteners are trade secrets, most of these fluorescent compounds contain one or more ring systems and are derivatives of
stilbene (Fig. 12.8.1), coumarin (Fig. 12.8.2), imidazole (Fig. 12.8.3), triazole
Figure 12.8.1 ~
The molecular structure of stilbene.
~
Figure 12.8.2 ~
0
The molecular structure of coumarin.
H
N
,)
Figure
12.8.3 ~
The molecular structure of imidazole.
(Fig. 12.8.4), oxazole (Fig. 12.8.5), and biphenyl (Fig. 12.8.6). The uninterrupted
chain of alternating single and double bonds (“conjugated double bonds”) in these
substances contributes to the absorption of these compounds in the ultraviolet region and their fluorescence in the blue wavelengths of light. One such compound,
7-amino-4-methylcoumarin (Fig. 12.8.7), absorbs near 350 nm and emits at 430
nm. These optical parameters are evident in the normalized absorption and emission spectra Ill of this compound that appear schematically in Fig. 12.8.8. An
absorption spectrum is a record of how much light of a particular color is absorbed by a substance as a function of color (i.e., wavelength). An emission (or
fluorescence) spectrum, a similar concept, measures the intensity of light of a
given color emitted by a substance as a function of color (i.e., wavelength). The
wavelength of maximum absorption (-350 nm) is in the ultraviolet region of the
electromagnetic spectrum. When light principally of this wavelength is absorbed,
blue light with a maximum wavelength near 430 nm is emitted.
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160
H
H
NH – S – O
H.,,N-H
Figure 12.8.4
9 The molecular structure of triazole.
~ ,>
O
N
Figure 12.8.5
9 The molecular structure of oxazole.
Figure 12.8.6
9 The molecular structure ofbiphenyl.
H
H~N~ 84
I
H
Figure 12.8.7
H
9 The molecular structure of 7-amino-4-methylcoumarin.
tl)
o
t-G)
o
o0
t..
O
Lt.
Wavelength/nm
Figure 12.8.8 9 A schematic representation of the normalized absorption and emission
spectra of 7-amino-4-methylcoumarin.
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Question 12.9 What Is the Difference between a Sunscreen and a Sunblock?
161
References
[]] “spectra: 7-Amino-4-methylcoumarin.”Molecular Probes, Inc.,
http ://www.probes.com/servlets/spectra?fi leid= 191 ph7
Related Web Sites
“Structure and Colour in Dyes.” Bryan D. Llewellyn,
http://members.pgonline.com/~bryand/StainsFile/dyes/dyecolor.htm
~, “The Color Wheel.” Painter On-Line, The Maryland Institute, College of Art,
http://www.mica.edu/painter/on_line/colorwhe.htm
What Is the Difference between a Sunscreen
and a Sunblock?
The invisible ultraviolet rays of the sun can cause immediate and longterm skin damage in the form of sunburn, rashes, premature wrinkling, and skin cancer. To avoid overexposure, we are encouraged
to apply sunscreens and sunblocks to protect the health of our skin.
Chemistry clearly distinguishes between these two formulations, and
the chemical structure of these products dictates how well these materials perform.
The Chemical
Basics
Sunblocks are opaque substances such as zinc oxide, titanium dioxide, and iron
oxide that protect by forming a shield on the skin, which reflects and scatters
incident radiation. In essence, sunblocks provide physical protection against sun
exposure, including both visible and ultraviolet light. Sunscreens are substances
that chemically absorb ultraviolet light in the top layer of the epidermis, protecting
the underlying layers.
Visible light ranges in wavelength from 400 to 700 nm. The spectrum of
ultraviolet light from the sun ranges in wavelength from 200 to 400 nm and is
divided into three broad classifications. [1] UVA rays have the longest wavelength
(320-400 nm), are fairly constant year-round, and penetrate deeper into the layers of skin. Shorter UVB rays (290-320 nm) are more intense during the summer
months than the longer wavelength UVA radiation. UVB radiation is also stronger
at higher altitudes and in areas closer to the equator. UVC radiation, even shorter
in wavelength from 200 to 290 nm, is absorbed by the stratospheric ozone layer
and does not reach the Earth’s surface. (As ozone is depleted in the stratosphere,
however, the range of wavelengths of UV light that reaches the Earth’s surface
will become a greater concern for skin exposure.) The particular chemical structure of a sunscreen determines the wavelengths of ultraviolet light preferentially
absorbed by a sunscreen.
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162
Chapter 12
Connections to Light
The Chemical Details
The sunblocks zinc oxide, titanium dioxide, and iron oxide are inorganic chemicals that are not absorbed into the skin. These substances consist of opaque particles that reflect both visible and ultraviolet light. In addition, zinc oxide blocks
virtually the entire UVA and UVB spectrum []] and thus offers overall protection.
The particulate nature of these sunblocks enhances their effectiveness at reflecting
sunlight. The smaller the particle size, the greater the surface area available for
reflection, and the more effective the sun protection offered by the formulation. [2]
Sunscreens are transparent organic substances that penetrate into the skin and
absorb ultraviolet radiation. Common classes of sunscreens include benzophenones, PABA derivatives, cinnamates, salicylates, and dibenzoylmethanes. [3] Benzophenones have a primary protective range in the UVA region and include oxybenzone (Fig. 12.9.1), 270-350 nm; dioxybenzone (Fig. 12.9.2), 206-380 nm;
o
Figure 12.9.1
H”Lo
9
The molecular structure of oxybenzone.
OH
0
OH
Figure 12.9.2 9 The molecular structure ofdioxybenzone.
and sulisobenzone (Fig. 12.9.3), 250-380 nm. PABA and PABA esters have
OH
Figure 12.9.3
0
O=S=O
I
OH
9
The molecular structure of sulisobenzone.
a primary protective range in the UVB range (290-320 nm), including PABA
(Fig. 12.9.4), 260-313 nm; Padimate O (Fig. 12.9.5, also known as octyldimethyl
PABA), 290-315 nm; Padimate A, 290-315 nm; and glycerol aminobenzoate
(Fig. 12.9.6), 260-315 nm. Cinnamates are derivatives of cinnamon, with a primary protective range in the UVB range (290-320 nm). Examples include octyl
methoxycinnamate (Fig. 12.9.7), 280-310 nm; and cinoxate (Fig. 12.9.8), 270EBSCO Publishing : eBook Academic Collection (EBSCOhost) – printed on 11/12/2019 5:43 AM via BARTON
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Question 12.9
What Is the Difference between a Sunscreen and a Sunblock?
163
0
H
“N
I
H
Figure 12.9.4 ..
The molecular structure of PABA.
HH
~. H H
H ~
“r HN.’H 2H”
/~ X
IH H H H
N’H
H
H
H
H
Figure 12.9.5 ~ The molecular structure of Padimate 0 (or octyldimethyl PABA).
328 nm. Salicylates, protecting in the UVB range (290-320 nm), include homosalicylate (Fig. 12.9.9), 290-315 nm; ethylhexyl salicylate (Fig. 12.9.10), 260310 nm; and triethanolamine salicylate (Fig. 12.9.11), 269-320 nm. Dibenzoylmethanes serve best as protectors for the UVA range (320-400 nm), offering no
protection from UVB. These substances include 4-tert-butyl-4’-methoxydibenzoylmethane (Fig. 12.9.12), 310–400 nm; and 4-isopropyldibenzoylmethane
(Fig. 12.9.13), 310–400 nm. What do all of these molecules have in common
that enhances their ability to absorb ultraviolet light?
One common structural element is the presence of an aromatic or benzene
ring structure. The aromatic ring is the chromophore of sunscreens, that is, the
functional group of atoms capable of absorbing ultraviolet light. Benzene, C6H6,
absorbs in the ultraviolet at 280 nm. The presence of certain substituents on the
benzene ring alters the electron distribution in the ring and shifts the absorption
wavelength. In addition, the conjugation of double bonds (i.e., the presence of
alternating double and single bonds as in C=C-C=C or C=C-C=O) can lead
to tremendous shifts in absorption wavelength. In the class of benzophenones,
the presence of hydroxyl (-OH) groups on the aromatic rings and the additional
H
I~
H%,,, OH
O
/ ” H OH
Figure 12.9.6 ~, The molecular structure of glycerol aminobenzoate.
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164
Chapter 12 Connections to Light
HH. O –
O
H
Figure 12.9.7 9 The molecular structure of octyl methoxycinnamate.
H
O
HH
HH HH HH
O
Figure 12.9.8
O
9 The molecular structure of cinoxate.
H3C
Figure 12.9.9
f ~ O ~ c H 3
~ I~
+
isomer
v
OH
9 The molecular structure ofhomosalicylate.
H
Hf’~
HO..~
Figure 12.9.10
HH
HH HH HH
9 The molecular structure of ethylhexyl salicylate.
HO
O
OH
Figure 12.9.11
H
“-‘~- OH
HO
9 The molecular structure of triethanolamine salicylate.
O
O
H3C
Figure 12.9.12
9 The molecular structure of4-tert-butyl-4 -methoxydibenzoylmethane.
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Question 12.9 What Is the Difference between a Sunscreen and a Sunblock?
O
165
O
CH3
Figure 12.9.13 ~, The molecular structure of 4-isopropyldibenzoylmethane.
conjugation shift the absorption to longer wavelengths, particularly in the UVB
range. Amino substituents (-NH2) and carboxyl groups (-COOH) also increase
the wavelength of absorption. Sunscreens thus have a chemical structure that
matches their function–the absorption of ultraviolet light.
References
[1] “The Protective Role of Zinc Oxide: Sunscreens.” Mark Mitchnick, sunSmart Inc.,
http://www, iza. co m/zh e_o rg/Articl es/Art-09, htm
[2] “TiO2sperse? Ultra: Product Profile, Collaborative Laboratories,
http://www.collabo.com/tioultra.htm
[3] “Sunscreens.” http://www.geocities.com/HotSprings/4809/sunscr.htm# ACTIVE INGREDIENTS IN SUNSCREENS
Other Questions
U4
to Consider
Why does a kitchen gas burner glow yellow when a pot of boiling water
overflows? See p. 2.
Why is hydrogen peroxide kept in dark plastic bottles? See p. 40.
Why does chlorine in swimming pools work best at night? See p. 70.
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