Determination of the average molecular weight of nonionic emulsifiers by vapor-phase osmometry

Standard nonionic emulsifiers are heterogeneous by nature. Their reported molecular weight is unreliable, especially when several lots of the product are used in a study. The number-average molecular weights of two nonionic emulsifiers, poloxamer 188 and polyoxyethylene (23) lauryl ether were determined by vapor-phase osmometry. This determination is essential when the concentration should be given in molarity rather than in weight per volume. A discrepancy was noted between the number-average molecular weights of two lots of poloxamer 188. That difference is taken into account prior to the establishment of any comparison of the behavior of the emulsifiers.