Influence of alkyl chain length compatibility on microemulsion structure and solubilization

Abstract

The solubilization capacity of water/oil microemulsions formed with fatty acid soaps and alcohols was studied as a function of alkyl chain length of oil (C 8 to C 16), soap (C 14 and C 18), and alcohol (C 4 to C 7). For n-butanol with both soaps, the maximum amount of water solubilized in the microemulsion continuously decreased with increasing oil chain length whereas for n-heptanol the solubilization capacity continuously increased. For pentanol with both soaps and for hexanol with sodium stearate, the solubilization reached a maximum when the oil chain length, l o, plus that of the alcohol, l a, was equal to that of the surfactant, l s, (i.e., l a + l o = l s). When excess water was added to these systems, a birefringent aqueous phase in equilibrium with an isotropic upper phase formed if l a + l o < l s whereas a different phase behavior, namely, the formation of two isotropic phases, was observed if l a + l o ⩾ l s. Both of these observations indicate a chain length compatibility effect similar to that observed in monolayers, foams, emulsions, and lubricants. The solubilization behavior is interpreted in terms of partitioning of alcohol among oil, water, and the interface depending upon the chain length of oil and alcohol, as well as in terms of molecular packing at the interface in relation to the disorder produced by the chain length compatibility effect. It was concluded from the oil-alcohol titration study that the molar ratio of alcohol to soap at the interface increases with oil chain length. We propose that the increase in the alcohol/soap molar ratio leads to increased spacing between carboxyl groups resulting in a greater degree of ionization. Electrical resistance and dielectric constant measurements yielded results consistent with this interpretation. At a given water/oil ratio, the electrical resistance of microemulsions decreases, the dielectric constant increases, and the thickness of ionic atmosphere within the water droplets increases as the oil chain length is increased. These phenomena are consistent with the concept that microemulsions form, and vary their structure, under the influence of surfactant and alcohol partitioning between aqueous and oleic phases and the consequent interfacial activity of both species. Here, the partitioning of alcohol and soap is subtly varied by changing the oil chain length.