Aggregate structures in water-in-CO2 microemulsions were studied by high-pressure small-angle neutron scattering (SANS). With liquid CO2 at 15 °C, the partially fluorinated, di-chain surfactant bis(1H,1H,5H-octafluoro-n-pentyl) sodium sulfosuccinate (di-HCF4) stabilized single-phase microemulsions at pressures above ∼400 bar. The maximum water loading (w) investigated was 30 ([water]/[di-HCF4]), representing formation of relatively large water droplets in the microemulsion. Between w = 5 and 30, the SANS data were consistent with a model for attractive polydisperse spherical droplets. A linear relationship between the water droplet radius (R c) and w was found, which gave an apparent head group area for the surfactant of 87 Å2 at the water−CO2 interface. In Winsor II type microemulsions the value of R c, measured in the presence of excess water, increased with pressure from 36 Å at 400 bar to 56 Å at 550 bar.

Tensiometry and neutron reflection (NR) have been used to investigate the adsorption at the air/water interface of four fluorinated surfactants. These compounds were two single-chain carboxylatessodium perfluorononanoate (NaPFN) and sodium 9H-perfluorononanoate (HNaPFN)and two double-chain sulfosuccinatessodium bis(1H,1H-perfluoropentyl)-2-sulfosuccinate (DCF4) and sodium bis(1H,1H,5H-octafluoropentyl)-2-sulfosuccinate (DHCF4). The replacement of a terminal fluorine for hydrogen creates a permanent dipole in the hydrophobic chain tip which significantly affects the surfactant behavior. This change in structure leads to an increase in the critical micelle concentration (cmc) which was 10.0, 40.0, 1.6, and 16.0 mmol dm-3, respectively. At their cmc's the interfacial area per molecule was 41 and 44 Å2 for NaPFN and HNaPFN and 63 and 66 Å2 for DCF4 and DHCF4 as determined from NR. From tensiometry the values obtained were 43, 51, 56, and 65 Å2, respectively. For all four compounds the surface excess measured by tensiometry, using a prefactor of 2 in the Gibbs equation, was in reasonable agreement with that obtained from NR. Partial structure factors of the adsorbed layers were determined for all four surfactants. The widths of the surfactant distributions were found to be significantly broadened by roughness at the interface. The widths of the solvent distribution were about 5 Å for the carboxylates and essentially 6 Å for the sulfosuccinates. In all cases the measured separations of the water and surfactant distributions were 5 Å, indicating a greater penetration of water into the sulfosuccinate layers. The contribution of capillary waves to the surface roughness was examined in order to assess the static disorder of molecules in the adsorption layer. The presence of the chain dipole does not appear to cause any significant structural differences normal to the interface.

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The spin-probe detected polarity index H(25 °C) of SDS micelles decreases linearly with the number of inserted sugar-based nonionic surfactant molecules. This decrease is interpreted as being due to the expulsion of water molecules by the sugar groups from the polar shell surrounding the hydrocarbon core of the dodecyl micelle. Employing the geometrical model described in the companion paper immediately preceding this work, the effective volume of water expulsion is found to be similar to the volume of the sugar groups after taking into account that the OH bonds of the sugar groups also contribute to the polarity index H(25 °C). The estimate of the hydration of pure SDS micelles as a function of their aggregation number from these studies with the spin probe 16 doxylstearic acid methyl ester is similar to that with 5 doxylstearic acid methyl ester. This confirms that both spin probes are located similarly in the polar shell.