410 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS components the spectrums become complex and diffuse, and it becomes increasingly difficult to recognize anything other than particular functional groups. Consequently, the infrared spectrum of a commercial surfactant may bear only a slight resemblance to its isolated major component. Useful information can be obtained if water is carefully removed from the sample, since the presence of even traces of moisture can mask surfactant absorptions in the 3400-3000 cm -•, 1700-1500 cm -•, and 600 cm -• region (all associated with H20). The more common assignments of functional groups associated with anionic surfactants are the hydroxyl-OH stretching frequency occurring between 3400-2400 cm-L This band is usually complex and broad. The CH- stretching frequency is usually superimposed on the low side of the broad OH- band. Absorption arising from the CH- stretching of the methylene (CH2) and methyl (CH3) groups on the carbon chain occurs between 2950-2750 cm -•. Unsaturation can be observed at 3020 cm-•, and its intensity relative to the methylene antisymmetric stretch at 2920 cm-• can give an indication as to the degree of unsaturation (9-14). More recently, gas chromatography has played an increasingly important role in the characterization of anionic surfactants, the best known being sodium dodecyl sulfate. This material can be easily hydrolyzed in acid media to yield the original fatty alcohol. It has been shown that about 98% of sodium lauryl sulfate is hydrolyzed in 1N HCI at 100 ø in seven hours (15). The hydrophobic oil is extracted from the reaction solution with suitable solvents, then gas chromatographed with or without pretreatment. Mixtures of fatty alcohols have also been separated by forming the acetates (16). Further work has been reported in the literature on the analysis of alkyl sulfates using on-line pyrolysis gas chromatography with and without reagents (17). At 650øC pyrolysis gas chromatography of commercial sodium lauryl sulfate gave peaks for the C•2 and C•4 fatty alcohols and the C•2 and C•4 ce-olefins. On-line P205 pyrolysis gas chromatography gave peaks due only to a mixture of olefins. The formation of the alkyl alcohols was not observed. A second class of anionic surfactants used widely in cosmetics are the ethoxylated ether sulfates of fatty alcohols, obtained by reacting the alcohol with ethylene oxide followed by sulfation. The general formula for these compounds is: CH3(CH2).O(CH2--CH2-O)mSO3-Na + where n = 11, 13, 15, 17 and m = 1-20. Naturally occurring sources usually contain the C•0, C14, and C16 alcohols in varying amounts. If one ethoxylates lauryl alcohol to an average of 2 ethylene oxide (EO) units, the number of reaction products obtained is considerable. Not only is the 2EO derivative present, but the homologs of the 1, 3, 4, 5, and 6EO derivatives are also formed. Impurities of the C•0, C•4, and C•6 alcohols along with their homologous EO derivatives are also present. Since cosmetic products are delicately balanced formulas, differences in the homologous ratio can affect both stability and performance of the product. This has generated research into the analysis and distribution of the homologous series contained in this class of surfactants. As with the alkyl sulfates, the alkyl ether sulfates can be hydrolyzed back to the ethoxylated alcohols. Gas chromatography of the hydrolyzed mixture on the porous polymer tenex or the liquid phase OV-17 has been used to separate the underivitized C•2EO oligomers with up to 8 ethylene oxide units, while the trimethylsilyl ether of the C•2 alcohol with up to 16EO units have been separated on Dow Corning high vacuum grease as the liquid phase (18).

ANALYTICAL CHEMISTRY OF COSMETICS 411 Researchers have found better resolution with capillary GC by converting the hydroxyl group to the methyl ether. The methylation was carried out with methyliodide and dimethylsulfonyl anion (19,29). An OV-101 glass capillary column was used for the separation, and a commercial C12-C14 alcohol ethoxylated with an average of 3 moles of ethylene oxide gave eleven peaks under these conditions. This method is quantitative and therefore suitable for fast routine quality control (21). Mass spectroscopy has been used for absolute identification of the separated peaks obtained from the hydrolysis of ethoxylated sodium alkyl ether sulfates. Traditional electron impact mass spectrometry on these compounds does not show a molecular ion. However, we have demonstrated in our laboratory that isobutane chemical ionization mass spectroscopy can be used to obtain the molecular weights of the separated components. Under these conditions, the alcohols show a strong (M - 1) ion whereas the ethoxylated alchols show a strong (M + 1) ion (22). We have also demonstrated (23) that silylation of the hydrolyzed surfactant will yield electron impact spectra that are suitable for structure elucidation. There is no doubt that mass spectroscopy yields the most complete picture of the chemical distribution of this class of surfactant. Non-ionic surfactants consist of ethylene oxide (or propylene oxide) capped with a hydroxyl group on one end and a long chain alkoxy group or alkyl phenol group at the other end. Examples of this class are alkyl ether alcohols, or the ethoxylated octyl and nonylphenols. These surfactants are very amenable to analysis by gas chromatography and other advanced analytical techniques (24). An excellent paper has been published on the applications of mass spectroscopy to the analysis of non-ionic surfactants, using electron impact solid probe mass spectroscopy. The authors characterized the most common ions obtained from ethoxylated alcohols, octyl and nonylphenols, fatty amines and ethoxylated fatty alcohols (25). The degree of ethoxylation in ethoxylated non-ionic surfactants is an important parameter which influences product performance and stability. Nuclear magnetic resonance spectroscopy has been used for the determination of this parameter (26-29). The technique is based on the observation that a proton placed in an external magnetic field can be aligned with the external field. Energy can be absorbed by the proton and flipped so that the alignment is against the external field. This is the less stable situation. Upon realignment with the external field, energy is released. These spectra are usually obtained as a plot of signal intensity vs. change in strength of applied magnetic field. The CH 3 protons of trimethylsilane (TMS) are used as an internal standard and are arbitrarily set at 0 ppm or 10% Since TMS contains 9 equivalent protons, a strong singlet is obtained. The methylene protons associated with the ethylene oxide chain are shifted downfield to 6.0-6.5% while the methylene protons from the alkyl chain occur at 8.0-8.5% By silylating the terminal alcohol with a strong silylating agent (i.e., Bis-(trimethylsilyl) trifluoroacetamide), one can ratio the integrated areas in the NMR for the 9 methyl protons of the derivitizing agent, with either the integrated areas for the protons from the ethylene oxide chain or the protons from the alkyl chain. Average alkyl chain length or average degree of ethoxylation can then be calculated. If the surfactant is octyl or nonylphenol, the integrated area corresponding to the four aromatic protons, which are shifted downfield to 2.0-3.03', can also be used for the ratioing. This technique has also been applied to propylene oxide adducts of alkyl phenols, fatty alcohols, and ethoxylated mercaptans.