Micellization and Partition Equilibria in Mixed Nonionic/Ionic Micellar Systems: Predictions with Molecular Models

Abstract

In practical applications, surfactant solutions are mostly used in mixtures of nonionic and ionic surfactants because they have improved characteristics compared to those of single surfactant solutions. By adjusting the composition of the micelles and the pH value, the solubilization of solutes can be enhanced. Nevertheless, the partitioning of solutes between nonionic/ionic mixed micelles and the aqueous phase is studied to a much lesser extent than for single surfactant solutions. Theoretical methods to predict partition equilibria in mixed micelles are of interest for screening studies. For those, the composition of the mixed micelle has to be known. Here we investigate mixtures of TX-114 (Triton X-114), Brij35 (C12E23), SDS (sodium dodecyl sulfate), and CTAB (cetyltrimethylammonium bromide). First, to investigate the surfactant compositions in the micelles, molecular dynamics (MD) self-assembly simulations were applied. Thereafter, the predictive COSMO-RS model, which applies the pseudophase approach, and its extension to anisotropic systems termed COSMOmic were compared for the prediction of partition equilibria in mixed micelles, where various molar ratios of the surfactants were considered. It could be demonstrated that both methods are applicable and lead to reasonable predictions for neutral molecules. However, taking into account the three-dimensional structure of the micelle is beneficial because the calculations with COSMOmic are in better agreement with experimental results. Because the partitioning behavior of ionizable molecules in mixed micelles is of particular interest, the partitioning of ionized isovanillin in mixed Brij35/CTAB micelles at different micelle compositions was calculated with COSMOmic. Using a thermodynamic cycle, the position-dependent pK of isovanillin within the micelle is calculated on the basis of COSMOmic free energy profiles. As a result, the protolytic equilibrium of isovanillin within the micelles can be taken into account, which is crucial for the reliable prediction of partition coefficients.

Preparation and Characterization of an Anionic Gemini Surfactant for Enhanced Oil Recovery in a Hypersaline Reservoir

“…It could be seen that the oil/aqueous IFT values of 6501 or AGS alone were all larger than 0.01 mN m −1 at 50 and 70 °C, which indicated that using 6501 or AGS alone could not achieve high EOR. This result was consistent with the existing conclusion that the EOR effect of an independent surfactant was not good enough (Vilasau et al, ; Yordanova et al, ). The oil/aqueous IFT values first decreased and then increased with the decrease of mass ratio of 6501 to AGS, and all were lower than 0.01 mN m −1 when the mass ratio of 6501:AGS was in the range from 2.86:1 to 2.19:1 regardless of whether the temperature was at 50 or 70 °C, indicating that 6501 and AGS have a very good synergistic effect (Jin et al, , , b, ).…”

Effect of Emulsification on Surfactant Partitioning in Surfactant‐Polymer Flooding

“…After emulsification, most of oil and water separate and the surfactants at the oil–water interface maintain the stability of the emulsion. Due to the combination of oil and KPS hydrophobic groups (Fischer et al, ; Yordanova et al, ), surfactants may remain in the oil phase or form a reverse micelle while demulsifying, which will solubilize the water in oil, resulting in the decrease of surfactants in the water phase.…”

In Silico Prediction of the Thermodynamic Equilibrium of Solute Partition in Multiphase Complex Fluids: A Case Study of Oil–Water Microemulsion

“…As shown in Figure 11, most of the predicted values agreed well with the experimental data, having an accuracy within 0.5 in the logarithmic scale. The RMSE values are in line with previous studies on octanol−water, 67 micellar systems, 16,21,68 and lipid systems. 17,69,70 Only the partition coefficient for p-nitroaniline was severely underpredicted, with an error considerably higher than 0.5 in the logarithmic scale.…”

“…The atomistic level molecular assembly of liquid structure can be achieved by performing molecular dynamics (MD) simulations. The method that combines MD with COSMOmic has been proven to accurately predict partition coefficients of not only homogeneous fluids such as octanol–water but also structured fluids of micelles containing anionic, cationic, and zwitterionic surfactants and surfactants mixtures. − …”