SOLVENT EFFECTS ON SPF AND BROAD-SPECTRUM PROTECTION 153 2.3 g of UV fi lters and 7.7 g of solvent and were prepared using the aforementioned method. Each mixture was allowed to sit for 10 min after mixing to determine solubility. Signs of physical separation were visually determined. IN VITRO SPF AND CRITICAL WA VELENGTH TESTING AND WAVELENGTH OF MAXIMUM ABSORBANCE The Food and Drug Administration (FDA) 2011 method (21 ) was used for testing in vitro SPF (Labsphere UV-2000S, North Sutton, NH) using polymethylmethacrylate (PMMA) plates (HelioScreen HelioPlates HD6, Labsphere). A 5 × 5-cm PMMA plate was tared on an ana- lytical balance with a readability of 0.001 g. As per the FDA method, 2 mg/cm2 of the UV fi lter–solvent mixture (total of 50 mg) was applied to the plate, and using a fi nger cot, the mixture was spread in a circular motion for 30 s, vertical motion for 15 s, and horizontal mo- tion for another 15 s to ensure complete and even coverage. Then the plate was placed in a dark drawer for 15 min. Each plate was scanned in fi ve different locations, each UV fi lter–solvent mixture was tested on three different plates. The software (UVS2000 Application, Labsphere) measured absorbance and converted it to in vitro SPF using an algorithm. In the United States, to label a sunscreen broad spectrum, the product has to have a critical wavelength of at least 370 nm. Critical wavelength is the wavelength below which 90% of the area under the absorbance curve resides (21). The same PMMA plates and same amount of UV fi lter–solvent mixture was used for broad-spectrum testing. Critical wavelength was calculated by the Labsphere software. The λmax values were obtained f rom the absorbance results. SPREADABILITY TESTING Spreadabil ity of each solvent wa s determined using a TA.XTPlus texture analyzer (Texture Technologies Corp., Hamilton, MA) and a spreadability fi xture. Test mode was set to “measure force in compression,” and “return to start” option was used. Trigger type was set to “button.” The male cone’s penetration distance was set to 1 mm less than the start- ing point distance. Test speed and post-test speed were set to 3.0 mm/s. Exponent stable micro-systems software (version 6.1.10.0, Texture Technologies Corp., Hamilton, MA) was used to generate spreadability curves. STATISTICAL ANALYSIS Differences in in vitro SPF, λmax, critical wavelength, and spreadability were evaluated using one-way analysis of variance followed by Tukey’s multiple comparison test using SPSS Statistics 21 software (IBM, Armonk, NY). A p value less than 0.05 was taken as the minimal degree of statistical signifi cance. RESULTS FFE FFE calculated the IAG for eac h UV fi lter–solvent mixture (Table I). Among the solvents tested, there were excellent solvents (arbitrary IAG range: 1–5), good solvents (arbitrary

JOURNAL OF COSMETIC SCIENCE 154 IAG range: 6–10), and poor solvents (arbitrary IAG range: 10) for each UV fi lter. IAG ranged from 0 to 39 when looking at all 167 solvents in FFE for good solvency, the num- ber should be as low as possible. In the case of homosalate, ethylhexyl salicylate, and the UV fi lter blend, most solvents (i.e., 16 of 24) were excellent based on FFE predictions. As for butyl methoxydibenzoylmethane, most solvents (i.e., 16 of 24) were considered good. Four solvents were ranked poor for all UV fi lters and the UV fi lter blend: two silicones, including dimethicone and a blend of cyclotetrasiloxane and cyclopentasiloxane ethanol, a semipolar solvent and pentylene glycol, a polar solvent. Next in line was isododecane, which was ranked good for the UVB fi lters and the UV fi lter blend and poor for butyl methoxydibenzoylmethane. Butyl methoxydibenzoylmethane did not dissolve in any of the solvents ranked poor and fi ve good solvents (i.e., 5 of 16). We did not observe a trend in the IAG number and abil- ity of a good solvent to dissolve butyl methoxydibenzoylmethane. Some good solvents had higher numbers and worked, e.g., caprylic/capric triglyceride, whereas others did not work, although they had a lower number, e.g., Helianthus annuus (sunfl ower) seed oil. Three solvents were ranked excelle nt for all UV fi lters and the UV fi lter blend, including C12-15 alkyl benzoate, butyloctyl salicylate, and ethylhexyl methoxycrylene. IN VITRO SPF AND CRITICAL WAVELENG TH TESTING AND WAVELENGTH OF MAXIMUM ABSORBANCE Most solvents tested had in vitro SPF values close to 1 (Table II). Four solvents had in vitro SPF values above 2, including butyloctyl salicylate (SPF 19.5), diethylhexyl 2,6-naph- thalate (SPF 35.2), polyester-8 (SPF 50.3), and ethylhexyl methoxycrylene (SPF 330.2). The in vitro SPF of homosalate alone was 13.7 ± 2.8, and ethylhexyl salicylate 12.7 ± 1.9. Measuring the in vitro SPF of butyl methoxydibenzoylmethane alone was not possible because of its waxy nature. Butyl methoxydibenzoylmethane has to be dissolved to pro- vide sun protection. Solvents change their absorbance spectra therefore, calculating the SPF of butyl methoxydibenzoylmethane from any of the mixtures was not possible either. Measuring the in vitro SPF of each UVB fi lter alone helped understand the extent of SPF boost. Testing was not performed for mixtures in which avobenzone was insoluble this is indicated in the tables. A theoretical in vitro SPF was calcu lated for each UVB fi lter–solvent mixture by adding 10% of the UV fi lter’s SPF to 90% of the solvent’s SPF. Any measured number higher than the theoretical number was considered a boost. In any 1:9 mixture, homosalate was assumed to have an in vitro SPF of 1.4 and ethylhexyl salicylate 1.3. The theoretical SPF of the blend could not be calculated because of butyl methoxydibenzoylmethane’s un- known theoretical SPF. All UVB fi lter–solvent mixtures—exce pt for the isododecane mixture—had a higher in vitro SPF than the sum of the SPF of the UV fi lter and solvent would yield, indicating a synergistic and not just additive relationship. The four solvents with a high inherent SPF (diethylhexyl 2,6-naphthalate, butyloctyl salicylate, ethylhexyl methoxycrylene, and polyester-8) did not boost the SPF of the individual UVB fi lters. The SPF of these mix- tures was signifi cantly higher (p 0.05) in most cases than the rest of the mixtures, but the high SPF was a result of the solvents’ inherent SPF and not a synergism between the UV fi lter and solvent.