The HLD-NAC Model for Extended Surfactant Microemulsions

It has been confirmed that the structure of the alkyl group of an extended surfactant plays an important role in defining its interfacial properties. Alkyl groups containing a higher degree of beta-branching (C2-branching) produce microemulsions with a larger characteristic length (xi, the extent of solubilization in middle phases). This effect is explained on the basis that beta-branching increases the hydrophobicity of the surfactant and decreases the optimal salinity of the microemulsion. Higher salinities produce a dehydration of the surfactant groups that lead to shorter extent of the interactions with the oil and the water. Larger characteristic lengths are desirable if the objective of the formulation is obtaining greater solubilization of oil and water, and lower interfacial tensions. Large characteristic lengths are, in most cases, associated with high interfacial rigidities, which are undesirable if rapid coalescence is required. However, mixtures of branched and linear extended surfactants produce large characteristic lengths and lower interfacial rigidities. The HLD-NAC model is able to reflect the experimental trends in solubilization of oil and water. The differences between the predictions of the model for the solubilization of oil and water in Type I and II formulations, respectively, highlight the complexities in the conformation of extended surfactants, particularly their PO groups, at oil-water interfaces and the need for advanced scattering techniques to evaluate these conformations.