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Small-angle neutron- (SANS) and X-ray-scattering (SAXS) patterns obtained at 40 °C from 0.0729 m D2O solution of cesium dodecyl sulfate (CsDDS) have been simultaneously evaluated in terms of the conventional two-shell model, a three-shell model created for demonstration purposes, and a newly developedand partly testedfour-component model. The simultaneous fitting is based on the fact that the two types of coherent scattering patterns differ only in the neutron- and X-ray scattering lengths. For comparison, the SANS and SAXS patterns were evaluated separately, too. In contrast to the two- and three-shell models, the four-component model is able to represent the continuously varying spatial distribution of the scattering contrast. From the results, it seems that the most reliable data are obtained from fitting the four-component model simultaneously to both patterns. Along with (approximate) core- and counterion profiles, application of the latter model can result in the spatial distribution of the solvent molecules around the micellar center. If an atom or a molecular group has well-distinguishable scattering contrast relative to both types of scattering (such as Cs+ counter-, and -SO4 -- headgroup-ions), utilization of the four-component model enables their molecular volumes to be treated as variable model parameters, thus providing a unique method for determining their hydration properties in a structured nonsimple liquid.

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.