SMALL-ANGLE NEUTRON-SCATTERING STUDY OF W/O N-HEXADECANE, POTASSIUM OLEATE, WATER, AND CNH2N+1OH (N = 5-8) MICROEMULSIONS - EFFECT OF WATER CONCENTRATION

In an extension of previous work, microemulsions comprised of n-hexadecane, potassium oleate, water, and cosurfactant have been studied by small-angle neutron scattering (SANS) at constant ratios (moles/ mole of surfactant) of hydrocarbon and alcohol and a varying ratio of water (as D2O). The alcohols used were 1-pentanol, 1-hexanol, 1-heptanol, and 1-octanol. The range of volume fractions of the dispersed phase (0.3–0.5) covers most of the region of phase stability. The ability to solubilize water decreases with increasing chain length of the cosurfactant; this may indicate that the driving force in the phase separation is the spontaneous curvature of the interface. Independent of the alcohol used, SANS data in the whole range examined are consistent with a droplet structure. With allowance for “permeability”, a monodis-perse oblate ellipsoid model consisting of a central aqueous core and an essentially hydrocarbon shell gave satisfactory fits. As the alcohol chain length decreases the interface rigidty decreases, and the permeability of the shell to the solvent or to the shell of the other particles increases. As expected, the droplet sizes were found to increase primarily with the water to surfactant molar ratio; the deviation of the ratio between the water phase volume and total interfacial area from a linear trend is interpreted as due mainly to an increased amount of alcohol at the interface, but a variation of the interfacial area per mole of alcohol and surfactant cannot be definitely excluded. At a fixed water-to-surfactant molar ratio, the particle dimensions increase from 1-pentanol- to 1-octanol-containing systems. The trend is interpreted as due to a higher fraction of larger alcohols in the hydrocarbon phase, in agreement with increase of hydrophobicity. © 1990, American Chemical Society. All rights reserved.