]. Soc. Cosmet. Chem., 23, 153-163 (March 3, 1972) Determination of the Molecular Weight Distribution of Polyether Surfactants NORMAN KALISH, M.S., • LEONARD WEINTRAUB, Ph.D., • and MARIE KOWBLANSKY, M.S.* Presented May 24, 1971, Seminar, Washington, D.C. Synopsis--Many of the characteristics which are important in formulating with POLY- ETHER SURFACTANTS are dependent on MOLECULAR WEIGHT DISTRIBUTION. The determination of the molecular weight distribution of a commercial quaternary am- monium polyether surfactant is reported. The intermediate tertiary aminopolyether alco- hol which contains polypropylene glycol as a by-product has a distribution identical to that of the quaternary products and was used in the analyses. The amine and glycol fractions were separated by esterification of the mixture, which permits partitioning of the esterified amine and glycol fractions into different solvents. The makeup of each fraction was determined by integrated programmed GAS CHROMATOGRA- PHY. In addition, the aminopolyether fraction was analyzed by a comb[nation o,f MASS SPECTROSCOPY and NUCLEAR MAGNETIC RESONANCE. The amine fraction was found to have a symmetrical distribution of molecular weights, while the glycol fraction had more high molecular weight than low molecular weight com- ponents. This method for determination of the molecular weight distribution is readily adapted to other polyether surfactants. INTRODUCTION Polyether surt•actants are employed in a variety of cosmetic prod- ucts. In most cases, the average molecular weight of the polyether is * Research and Development Laboratories, Bristol-Myers Products, 1350 Liberty Ave., Hillside, N. J. 07207. ? Present address, Warner-Lambert Research Institute, Morris Plains, N.J. 07950. 153

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