In the Laboratory
The Spectrophotometric Analysis and Modeling of Sunscreens Christina Walters, Allen Keeney, Carl T. Wigal, Cynthia R. Johnston, and Richard D. Cornelius* Department of Chemistry, Lebanon Valley College, Annville, PA 17003 The absorption of light is central to many instrumental techniques in chemistry, and students are familiar with the use of sunscreens to absorb the light that causes sunburn. Thus, the quantitative measurement of the absorption of ultraviolet light using sunscreens is a logical first use of instrumentation in the introductory laboratory. Previously described experiments involving suncreeens and suntan preparations include a spectrophotometric investigation published before the current rating scheme for sunscreens was introduced (1) and a liquid chromatographic determination of sunscreen components (2). Because the wavelength range important for absorption of light by sunscreens covers only about 25 nm, small differences in molecules can cause drastic differences in their effectiveness as sunscreens. Sunscreens therefore also constitute an appropriate class of compounds to use for modeling electronic spectra. This experiment has been developed as part of a curriculum called “Chemistry Domesticated” (3), which uses everyday experiences of students to organize the chemical topics typically found in the general chemistry sequence for science majors. Chapters have titles such as “Fuels,” “Beverages,” and “Clothing,” and include the chemical topics necessary for an understanding of these subjects. The first chapter in the curriculum is “The Sun”, and this instrumentation-based experiment on sunscreens fits well with that first chapter.
Relationship between Absorbance and SPF The formal definition of SPF (6) can be expressed in terms of the “minimal erythemal dose” (MED), the length of time that one can stay in the sun before getting sunburned. Standard testing conditions define a dose of sunscreen (7) as 2 mg/cm2, comparable in household units to 1fl oz over the entire body (5). The SPF is the ratio of the MED when a person is using a sunscreen of this dosage to the MED when the same person is not using a sunscreen.
SPF =
Background Ultraviolet light is often broken into categories called UVA, UVB, and UVC as shown in Table I. UVC light is dangerous, but it is absorbed by the ozone layer and other gases in the atmosphere. As pollution depletes the ozone layer, more UVC radiation reaches the earth’s surface, and more cases of skin cancer and other diseases may result. UVB light is responsible for the characteristic sunburn that humans acquire after prolonged exposure to sunlight. Tanning occurs when UVB light activates the melanocytes found in the skin so that they produce melanin. UVA reportedly causes such adverse effects as loss of collagen, a decrease in the quantity of blood vessels, and an alteration of connective tissue of the dermis (4). Although protection from UVA light by a sunscreen has been described as “highly desirable” (5), the rating scheme for sunscreens addresses only the sunburn effect of UVB light. Manufacturers have developed sunscreens which protect humans from most of the harmful UVA and UVB radiation from the sun. The manufacturers identify levels of protection by their “sun protection factors” (SPFs). The larger the SPF value of a sunscreen is, the longer an individual using the sunscreen can stay out in the sun without burning. Sunscreen packaging identifies the active ingredients that block ultraviolet light. In this experiment, *Corresponding author.
Coppertone® brand sunscreen is used. The active ingredients ethylhexyl para-methoxycinnamate and oxybenzone appear in Coppertone sunscreens graded SPF 4, 6, 8, 15, 30, and 45. The ultraviolet spectra of these two compounds1 appear in Figure 1. The sunscreens rated SPF 30 and 45 contain additional sunscreen agents: SPF 30 contains 2-ethylhexyl salicylate and homosalate, while SPF 45 also contains 2-ethylhexyl salicylate and octocrylene. The chemical structures of these five UV-blockers are shown in Figure 2. Each of these sunscreens is an aromatic compound containing a conjugated carbonyl group. Coppertone and most other sunscreens are free of para-aminobenzoic acid (PABA) and derivatives, which were once widely used but which were reported to cause dermatitis.
MED for skin with sunscreen (2 mg/cm 2) MED for skin without sunscreen
(1)
Although the nature of this measurement depends upon a biological response related in an unknown manner to chemical behavior, we can imagine a simplified relationship between absorbance and SPF. If for example, a sunscreen has an SPF value of 2, a person could stay out in the sun two times as long as without the sunscreen. A chemist might well expect that the amount of light passing through the applied sunscreen would be half the amount that would otherwise reach the skin. If I0 represents the intensity of light reaching the skin in the absence of sunscreen and I the intensity of light in the presence of sunscreen, then the absorbance (A) would be 0.30:
A = – log 10 I = – log 10(0.5) = 0.30 I0
(2)
In general, the expected relationship between absorbance and SPF would be given by eq 3. Table 1. Ranges of UV-Light Category
Wavelength Range (nm)
Health Effects
UVC
100–290
dangerous, but absorbed by the atmosphere
UVB
290–315
causes sunburn
UVA
315–400
less dangerous
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In the Laboratory
(3)
In spite of attempts to devise instrumental procedures using dilute solutions to mimic the results of measurements on human patients, no laboratory substitute for testing with humans has emerged (8). Still, we can explore the extent to which the absorbance of dilute solutions serves as a predictor of SPF. Beer’s law provides a relationship among absorbance, molar absorptivity (ε), path length (b) and molar concentration (c): A = εbc
(4)
Eq 4 applies both to the undiluted sunscreen (designated ss) and to the solution used in the laboratory (designated soln). Dividing eq 4 written for the sunscreen by eq 4 written for the solution yields eq 5:
A soln = ε solnbsolnc soln A ss ε ssbssc ss
(5)
Considering the molar absorptivity to be constant for the sunscreen and the solution, and substituting log10(SPF) for Ass, we can rearrange eq 5 into a form suitable for analysis by a straight line plot of Asoln vs. log10 (SPF):
A soln =
bsolnc soln log 10(SPF) bssc ss
molar absorptivity (M-1 cm-1)
1 = log SPF 10 SPF
A = – log 10
wavelength (nm) Figure 1. Experimental ultraviolet spectra of p–methoxycinnamate and oxybenzone.
(6) CH3
O H
Experimental Procedure S AFETY. Normal laboratory precautions are called for, including adequate ventilation and the use of proper eye protection. Propanol is flammable and has a flash point below room temperature. For each sunscreen, weigh 0.050 ± 0.005 g (about one drop) of one of the sunscreens into a 150-mL beaker and add 50.0 mL of distilled water. Stir for at least 1 min until the sunscreen lotion is off the bottom and sides of the beaker and the mixture has a uniform white color of a colloidal suspension. Pipet 1.00 mL of the mixture into a 50-mL beaker. Add 9.00 mL of 1- or 2-propanol and stir for 10 s to produce a clear solution. Cover the 50-mL beaker with Parafilm® to prevent evaporation. Follow this procedure for sunscreens having SPF values of 4, 6, 8, 15, 30, and 45. Record the absorbance of each solution at 312 nm. Correct each absorbance to a standard mass of 0.05 g by dividing by the actual mass and multiplying by 0.05 g. Plot the corrected absorbance at 312 nm vs. the log10(SPF).
C
C
C
CH CH2
CH2
CH2
CH3
H CH3O 2-ethylhexyl p-methoxycinnamate
O
OH
C OCH3 oxybenzone
CH3
O C
CH2 O CH2
CH CH2
CH2
CH2
CH3
OH 2-ethylhexyl salicylate
Molecular Modeling
O
Because of the importance of spectral features to the success of sunscreens, students can readily understand the commercial value of modeling the spectra of sunscreen candidates. We present here our use of molecular modeling as an optional extension to the experiment. We have used the ZINDO algorithm of CAChe™ molecular modeling software to predict the ultraviolet spectra of the compounds that appear in Figure 2. All of these compounds have peaks in the UVC region, but our attention focused on the absorption peaks at the longest wavelengths that have the potential to appear in the UVB region as shown in Table 2. Examples of the predicted spectra of ethylhexyl para-methoxycinnamate and oxybenzone including the UVB region are shown in Figure 3. A comparison to the actual UV spectra shown in Figure 1 provides assurance that the software typically
C
100
CH2 O CH2
CH3 O
OH
CH3 CH3
H
homosalate
O C C C
CH3 CH2
O CH2 CH CH2 CH2 CH2 CH3 C N
octocrylene
Figure 2. Chemical structures of commercially important sunscreens.
Journal of Chemical Education • Vol. 74 No. 1 January 1997
In the Laboratory
predicts the absorption maxima within 10 nm. An exploration of the compounds shown in Figure 2 using molecular modeling permits students to draw conclusions regarding the importance of structure as it relates to UV absorption. Students are asked what structural features the sunscreens in Figure 2 have in common (an aromatic ring in conjugation with a carbonyl group). Upon identifying the chromophore, students examine the predicted UV spectra of benzene, benzoic acid, and para-aminobenzoic acid (PABA). Students are asked to predict whether these compounds would be potential sunscreens for the UVB region. Using the calculated results, students readily conclude that benzene and benzoic acid are not suitable candidates, whereas PABA would be effective. The contrast in the ultraviolet spectra among these compounds demonstrates the effect of substitution compared to a base compound.
Table 2. Spectral Data for Common Sunscreens Sunscreen 2-ethylhexyl p-methoxycinnamate oxybenzone
Experimental Maximum (nm)
Calculated Maximum (nm)
310
308
288, 326
280, 333
2-ethylhexyl salicylate
306
299
octocrylene
310
306
Results Figure 4 shows the results from a group of Lebanon Valley College freshman using a Hewlett-Packard 8452A diode-array spectrophotometer to measure the absorbance of solutions. The data are plotted for 312 nm where a maximum in absorbance occurs for the sunscreen solutions.2 The slope of the least-squares line fitted to the data is 0.86, and the intercept is 0.35. Results at 330 nm gave a similar plot having a smaller slope (0.64) and intercept (0.28). The longer wavelength makes the experiments accessible using routinely available Spectronic 21 spectrometers. Possible explanations for the nonzero intercept include the uneven nature of the surface of the skin and physical differences between the action of dissolved species and those spread on the skin. Figure 3. Ultraviolet spectra of p–methoxycinnamate and oxybenzone calculated using CAChe™ molecular modeling software.
Conclusion Although instrumental tests have been judged inadequate as a substitute for biological testing, the results show the correlation between the absorbance and log10(SPF). Molecular modeling lets students explore which compounds have the potential to function as sunscreen agents and thereby see the importance of a knowledge of chemistry to the formulation of household items. Acknowledgment This work was supported in part by National Science Foundation grants DUE-9354642 and DUE-9551199. Notes 1. Generously supplied as a sample by Haarmann & Reimer Corporation, Springfield, NJ. 2. A reviewer suggested an alternate form of analysis in which data over the UVB range would be read into a spreadsheet and summed.
Literature Cited 1. 2. 3. 4. 5. 6. 7.
Evans, G. O. J. Chem. Educ. 1976, 53, 315. Davis, M. R.; Quigley, M. N. J. Chem. Educ. 1995, 72, 279. Hixson, S. H.; Sears, C. T. J. Chem. Educ. 1994, 71, 506. Siegel, M. Safe in the Sun. Walker and Company: New York, 1990. Consumer Reports; June 1991; p 400. Fed. Regist. 1978, 43, 38259–38269. Sunscreens. Development, Evaluation, and Regulatory Aspects; Lowe, N. J.; Shaath, N. A. Eds.; Marcel Dekker: New York, 1990. 8. Sayre, R. M; Agin, P. P.; LeVee, G. J; Marlowe, E. Photochem. Photobiol. 1979, 29, 559–566.
Figure 4. Students’ results showing the experimental relationship between absorbance of dilute solutions and SPF values of sunscreens. The solid line represents a least squares line fitted to the data at 312 nm. The dotted line is a least squares line fitted to the data at 330 nm.
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