Dualistic Role of Alcohol in Micelle Formation and Structure from iSAFT Based Density Functional Theory and COSMOplex

Alcohols are effective and commonly used additives in surfactant self-assembling systems. Their effect on the behavior of these systems is complex and depends on a balance of the interacting forces. It is known that alcohols can act as cosolvents by altering solvent properties, and can also act as cosurfactants by inserting themselves into the palisade layer of the micelle and coaggregate with surfactants. The most recent models require user input to select the fraction of alcohols partitioning to each role. Two molecular theories iSAFT and COSMOplex are able to capture both roles and successfully predict the effect on micellar structure and critical micelle concentration of different alcohols ranging from short-chain to long-chain in qualitative agreement with experimental data. In this paper, the effect of adding linear 1-alcohols to aqueous solutions of nonionic poly(ethylene oxide) alkyl ether (CxEy) surfactant is investigated. To achieve this, the interfacial Statistical Associating Fluid Theory (iSAFT), a classical density functional theory, is applied to these systems and compared to the newly developed COSMOplex model and to experimental data. iSAFT and COSMOplex can provide detailed density profiles of each component in the system to illustrate the locus of an alcohol within a micellar structure. We observe from DFT that all alcohols studied are more present in the palisade layer than the micelle core, while COSMOplex shows greater accumulation of alcohol in the center of the micelle. Partition coefficients, aggregation numbers, micelle size, and total number of surfactant and alcohol molecules are also predicted and discussed. Furthermore, results from different surfactant architectures are compared and analyzed.

Automated summary

Alcohols are commonly used additives in surfactant self-assembling systems and their effect on these systems is complex. They can act as cosolvents by altering solvent properties and as cosurfactants by coaggregating with surfactants. Two molecular theories, iSAFT and COSMOplex, can accurately predict the effect of different alcohols on micellar structure and critical micelle concentration.