It has been shown that the weight average molecular weight determined from light scattering measurements for freshly prepared solutions of an ionic surface active agent in toluene is much greater than the number average molecular weight determined from vapour pressure measurements. This discrepancy is caused by small quantities of water solubilized by the surface active agent. The water initially forms a relatively small number of very large aggregates which disintegrate as the solution ages until at equilibrium the weight average and number average molecular weights are in agreement. It is suggested that similar discrepancies observed for non-ionic surface active materials in non-aqueous media are also caused by solubilized impurities.
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