Van der Waals free energy model for solubilization of oil in micelles

Abstract

This work introduces the first of a two part thermodynamic framework to estimate the solubilization of nonpolar oils in micelles conformed by nonionic surfactants with linear alkyl tails, considering their configuration and the molecular properties of the constituents. This first part introduces a formal approach to account for the lipophilic (van der Waals) contributions to the free energy of solubilization in spherical micelles. To this end, this work uses two recently developed integration methods for sphere-shell and cone-shell VDW interactions that allow the calculation of surfactant-oil and surfactant-surfactant interactions that take place within the micelles of the solubilization process studied here. The method consists in calculating the free energy of transferring a normal alkane from its continuum, and surfactants monomers from empty micelles to produce an oil swollen micelle. The lipophilic interactions are estimated using the microscopic approach of Hamaker with Lifshitz-based Hamaker constants. The influence of n-alkane and surfactant tail length on the solubilization capacity predicted by the van der Walls free energy model (VDW-FEM) are consistent with experimental trends and it is also consistent with the lipophilic terms included in the semi-empirical Hydrophilic-Lipophilic-Difference + Net-Average-Curvature's (HLD-NAC) equation that predicts the phase behavior of microemulsions. As a result, these lipophilic terms can now be defined in terms of molecular interactions and molecular properties.

Robust Flash Calculation Algorithm for Microemulsion Phase Behavior

“…Winsor [9] defined the R-ratio to account for the overall effect of molecular forces at the interface on surfactant affinity. The R-ratio, however, is difficult to measure and calculate [13]. Salager et al [6,[14][15][16] made a significant and practical advance by defining the hydrophilic-lipophilic difference (HLD) based on the change in molar excess Gibbs free energy involved in the transfer of a surfactant molecule from the excess water phase to the excess oil phase.…”

“…Their published correlation is valid as long as only two formulation parameters change simultaneously, one of them being the salinity. With these new correlations, the HLD-NAC model has been successfully used to model many properties of real microemulsion phase behavior [13,36]. The model has become one of the standard surfactant models for benchmarking new thermodynamic developments [37,38].…”

Rhamnolipid Biosurfactants for Oil Recovery: Salt Effects on the Structural Properties Investigated by Mesoscale Simulations

“…By adding the nonane molecules, it is easier for RLs to form aggregates as the overall hydrophobicity of the system is increased. The resided oil molecules either aggregate as a hydrophobic core or pack with surfactants within the tail region based on the counter-balanced oil–oil and oil–surfactant interactions discussed in ref . For the zero-salinity cases, oil in the C10-RL micelles is visually closer to each other, especially in the spherical micelles.…”

Simulation of Surfactant-Polymer Floods with a Novel Microemulsion Equation of State

“…Acosta (2003) developed the HLD-NAC equation to fit phase behavior data from a salinity scan. The HLD-NAC model has been successfully used to predict many properties of real microemulsion phase behavior (Troncoso and Acosta, 2015), such that the model has become one of the standard surfactant models to benchmark new thermodynamic developments (Fraaije et al 2013, Browarzik et al 2010). Johns (2014, 2016) made a significant advance in the prediction capability of HLD-NAC by developing a new correlation for optimum solubilization as a function of optimum salinity.…”