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The present work deals with an interpretation of the differences in micellar structure of two surfactants, sodium decyl sulfonate and sodium decyl sulfate, whose only difference in molecular structure is the presence, for sulfate, of an oxygen atom bridging alkyl chains from a common SO 3 - charged head group. Using a simple geometrical approach, a different exposure of the surfactant head groups on the micellar surface is proposed. It is also shown that the different interphase microstructure could be the reason that decyl sulfate forms ellipsoidal aggregates, in contrast to decyl sulfonate, which forms nearly spherical micelles.

Differential scanning calorimetry (DSC), dynamic light scattering (DLS), and turbidity measurements have been used to study the effect of sodium decylsulfate, C10OS, on 1,2-dipalmitoylphosphatidylcholine, DPPC, vesicles. DSC measurements, performed soon after addition of C10OS solutions to DPPC vesicular dispersion, at several C10OS/DPPC molar ratios, showed the contemporary presence of maxima ascribable both to transitions of an unmodified DPPC membrane and to transitions related to membrane domains in which C10OS is also present. In all the considered samples only unilamellar vesicles are present as shown by DLS measurements carried out on aqueous DPPC and C10OS-DPPC systems. Also the aging of the samples confirmed the presence of SUV in the time scale of DSC measurements. On the basis of DSC results the pseudo-phase diagram of the system was drawn. This diagram does not represent a system in equilibrium, from a thermodynamic point of view, but only kinetically probable states. In fact the prolonged aging of the solutions used both for DSC and DLS scans highlight the disappearance of the unmodified DPPC membrane and its conversion into a mixed membrane. Turbidity measurements at 25 degrees C, performed as a function of surfactant composition, showed that C10OS initially contributes to increase the vesicles dimensions and allowed to calculate the saturation ratio C10OS/DPPC in the membrane. In contrast, at 45 degrees C the presence of surfactant gives origin immediately to smaller structures in comparison to vesicles, probably gel-state micelles.