Partial molal volumes and adiabatic compressibilities of dodecylpolyoxyethylene glycol monoethers in water at 5, 25, and 45°C

Density and ultrasonic velocity measurements were made on a series of dilute aqueous solutions ofn-C₁₂H₂₅O (CH₂CH₂O)_nH (n=6,7 and 8) at 5, 25, and 45°C. The partial molal volumes (V) and adiabatic compressibilities (K)of the solutes were independent of the concentration in the micellar solutions. This finding suggests that the mode of aggregation does not change over the concentration range studied. Larger V and K were found for the CH₂CH₂groups in micelles than in water, and the micellar core was shown to be liquidlike. The V and K of the monomeric surfactants could not directly be measured because of the very low c.m.c.'s. The infinite dilution values V^° and (K^°) were, however, estimated from the V^° and K^° for the constituent groups on the basis of additivity. Thus, sharp increases were found in both V and K on micellization, which was interpreted in terms of dehydration effects.