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Apparent molar volumes of sodium decyl (NaDS), dodecyl (NaDDS), and tetradecyl sulfate (NaTDS) are calculated from densities measured by a vibrating capillary densitometer in normal and 99.85% heavy water at 25 °C. On the basis of a pseudophase approach, the apparent molar volume of monomers is separated from that of aggregated surfactants. With the concept of group additivity being utilized, the apparent molar volume of the headgroup is separated from that of monomer and aggregated alkyl chains. The apparent molar volumes of a particular surfactant in normal and in heavy water solutions are slightly, but significantly, different. Both the magnitude and the direction of this isotope effect were found to depend on alkyl chain length. From the apparent molar volume of aggregated alkyl chains (determined in normal water), the (apparent) density inside the core of sodium alkyl sulfate micelles is calculated, and its comparison with the bulk densities of liquid n-alkanes indicates that the packing of the alkyl chains in the two media should be different. When the infinite dilution, aggregated apparent, and bulk molar volumes of the CH3 and CH2 groups are compared, the number of solvent molecules that penetrated into the micellar core per alkyl chain is estimated and a good agreement is found with an upper limit arising from a small-angle neutral scattering study. 7 This work is supported by the OTKA Foundation of the Hungarian Academy of Sciences.

The study was extended to analysis of mass, size and conformation of micelles formed in aqueous solutions of ethoxylated nonyl phenols. The results obtained by ultracentrifugal technique between 293 and 323 K have proved that the slightly ethoxylated nonyl phenols form micelles with high molecular mass and larger size at constant temperature, while the increasing length of the ethylene oxide chain favours formation of micelles of smaller molecular mass and size. The transformation of conformation from oblate to spherical shapes ensues with increasing temperature at constant ethoxy number or with ethoxylation at constant temperature. The second virial coefficient decreases with increasing temperature and decreasing ethoxy number. In accordance with the earlier conclucions, the change of the second virial coefficient relates to enhanced variation of monomer solubility, stabilization of micelle structure and increased deviation from ideal behaviour of a given micellar system.

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