Thermodynamic Modeling of the Duality of Linear 1-Alcohols as Cosurfactants and Cosolvents in Self-Assembly of Surfactant Molecules

Abstract

The effect of adding an alcohol to surfactant systems depends much on the alcohol chain length. Investigations on the effect of alcohols in micellar systems point out that medium-chain alcohols are appreciably incorporated in the micellar phase whereas short-chain alcohols are localized mainly in the aqueous phase. Nonetheless, penetration of the hydrocarbon chain of alcohols in the micellar shell has been experimentally observed for the entire homologous series of linear 1-alcohols. We present a thermodynamic model in which the alcohol molecules play two roles: cosurfactant and cosolvent. The cosurfactant effect of the alcohols is included by assuming that the alcohol molecules are nonionic surfactants. The cosolvent effect is modeled by accounting for the changes in the free energy to relocate the surfactant tail from the solvent to the aggregate core. The effects of short-chain alcohols in the macroscopic interfacial tension and dielectric constant of the solvent medium are also taken into account. For short-chain alcohols the partition coefficient of the alcohols between water and liquid hydrocarbons provides knowledge of the fraction of the molecules that participate in each function. Our proposed thermodynamic model improves the modeling of the effect of short- and medium-chain alcohols in self-assembly of molecules that are of increasing importance in modern scientific research and technological processes.

Molecular Thermodynamics of Micellization: Micelle Size Distributions and Geometry Transitions

“…The model used to calculate the total Gibbs free energy of the system is similar to the one proposed by Nagarajan and Ruckenstein (1991) and Moreira and Firoozabadi (2009). Given a global specification of T (temperature), P (pressure), SA N (total number of surfactant molecules in the solution) and w N (number of water molecules), the Gibbs free energy is calculated as a sum of two contributions: free energy of formation and free energy of mixture.…”

“…(1991) and Moreira and Firoozabadi (2009). When the micelles size distribution is narrow enough, it is common to use the Maximum Term approximation.…”

“…However, in those works, only the necessary condition for equilibrium is considered. The Nagarajan model was improved by Moreira and Firoozabadi (2009) when they started considering the minimum of Gibbs free energy as a condition for the thermodynamic equilibrium of the solution. However, Moreira and Firoozabadi (2009) used the maximum term approach, which assumes that the micelle size distribution can be represented by one characteristic micelle size and a single number of micelles formed.…”

“…The Nagarajan model was improved by Moreira and Firoozabadi (2009) when they started considering the minimum of Gibbs free energy as a condition for the thermodynamic equilibrium of the solution. However, Moreira and Firoozabadi (2009) used the maximum term approach, which assumes that the micelle size distribution can be represented by one characteristic micelle size and a single number of micelles formed. Therefore, Moreira and Firoozabadi (2009) did not take into account the fact that micelles can be formed with a large distribution of sizes.…”

“…However, Moreira and Firoozabadi (2009) used the maximum term approach, which assumes that the micelle size distribution can be represented by one characteristic micelle size and a single number of micelles formed. Therefore, Moreira and Firoozabadi (2009) did not take into account the fact that micelles can be formed with a large distribution of sizes. They also assumed that only small micelles could be formed, with either a spherical or globular shape.…”