154 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS specified and surfactants with different molecular weights have different properties. These include water solubility, physical appearance, viscos- ity, and emulsifying properties. Once a particular average molecular weight is chosen, the property which most influences its performance is the molecular weight distribution. In most cases, one would prefer a narrow distribution since this would insure more uniform characteristics from the material chosen. The molecular weights of polymers can be determined by chemical or physical methods of functional group analysis, by measurement of the colligative properties, light scattering, or ultracentrifugation, or by mea- surement of dilute solution viscosity. The colligative properties which are used to determine molecular weights are based on vapor pressure lowering, boiling point elevation, freezing point depression, and the os- motic pressure. Of these four methods, the most useful is osmotic pres- sure since the largest effect is observed in this measurement. End-group analysis as well as colligative methods give the number-average molecu- lar weight (Mn). The number-average is very sensitive to changes in the weight fractions of low molecular weight species. In contrast, the method of light scattering to determine molecular weight gives the weight-average molecular weight (Mw), since the amplitude of scattered light is proportional to the mass of the scattering particle. The weight- average is particularly sensitive to the presence of high molecular weight species. For example, if equal weights of molecules with M ---- 10,000 and M = 100,000 are mixed, Mw = 55,000 and Mn: 18,200 if equal numbers of each kind of molecule are mixed, Mw: 92,000 and Mn = 55,000. Mw is always greater than Mn except for a monodisperse system. The ratio Mw/Mn is a measure of the polydispersity of the system. For polymers with a very narrow molecular weight distribution, this value will approach one. It can be seen from the above that the molecular weight alone is not very useful when dealing with a p.olymeric material. The property of interest is the molecular weight distribution. Certain characteristics, such as solubility, which are important in the formulation of a product, require that one knows what type of distribution he is dealing with. This knowledge is essential for determining what molecular weight range of a particular polymer one is interested in and whether a particu- lar sample from a supplier meets specifications. The literature contains several references to work on the determina- tion of the molecular weight distribution of polyethers. Considerable

MOLECULAR WEIGHT OF POLYETHER SURFACTANTS 155 work has been done on polyethylene glyco.1 very little has been reported on polypropylene glycol. Gildenberg and Trowbridge (1) used gas-liquid chromatography to separate ethylene oxide adducts of fatty alcohols. The hydroxyl groups were converted to the acetate esters. This increased the stability of the molecule to high temperatures and permitted temperature programming, enabling them to obtain separate peaks for adducts with up to 13 ethyl- ene oxide units. Calzolari (2) studied the molecular weight fractionation of polyethyl- ene glycol by gas chromatography. Of several derivatives of the poly- ethylene glycol which were tried, the trimethylsilyl derivative was se- lected because it had the highest volatility and thermal stability and the lowest energy of absorption on the support, thus making it the most suit- able for the analysis of polyethylene glycol products with a molecular weight of less than 1000. Withers (3) has also reported on the gas chromatography of poly- ethylene glycols. Identification of the series of polyethylene glycols was made by running the first four members of the series since these were available as pure materials. The assumption was then made that suc- cessive peaks following these four each represent an increment of one ethylene oxide residue since the standards correspond to the first four peaks. Ludwig (4) used a variety of techniques for the separation of oligo- mers of ethylene oxide and propylene oxide adducts of alkylphenols. The procedures applied were gas chromatography on derivatives, thin- layer chromatography, and gel permeation chromatography. In addi- tion, nuclear magnetic resonance spectroscopy was used to determine the ethylene oxide/propylene oxide ratio in a copolymer. This work gave the average molecular composition, but did not concern itself with the exact molecular weight distribution. Another technique which has been widely used successfully to deter- mine molecular weight distributions is gel permeation chromatography. One advantage of this method is that it permits one to obtain separations in molecular weight ranges far exceeding 1000. The advent of high- speed liquid chromatography has done much to eliminate one of the drawbacks of column chromatography, namely, the time involved for an analysis. The area of gel permeation chromatography is a very broad field in itself. An excellent monograph by Determann (5) describes its varied applications.