Self – assembly of model surfactants as reverse micelles in nonpolar solvents and their role as interfacial tension modifiers

Abstract

The self-assembly of linear surfactants into reverse micelles (RMs) in nonpolar solvents, and their efficiency in reducing the interfacial tension is studied using dissipative particle dynamic simulations. Given the importance of RMs as thickeners, among many other applications, their properties are studied here when formed in oil and in supercritical carbon dioxide (scCO2). Our simulations are found to be in agreement with experimental results of surfactant self-assembly in scCO2 that found viscosity increments of up to 90% with 10 wt % of surfactants. The role played by a small number of water molecules in RM formation is studied as well in both solvents, corroborating experiments reporting the enhancement of RM formation with the addition of a small quantity of water. The dynamics of water surfactant aggregation in nonpolar solvents is also discussed. Lastly, the performance of the model surfactant as an interfacial tension modifier is studied in detail. The results show that RMs are more easily formed in oil than in scCO2 and that the effectiveness of the surfactant in reducing the interfacial tension lies in its preference to adsorb at interfaces rather than selfassociation.

The persistence length of linear surfactants modulates the self-assembly of reverse micelles and their diffusion in nonpolar solvents

“…It has been proved that the number of RMs formed depends on the water-surfactant ratio. 12 Additionally, here we find that the number of aggregates can be changed for a fixed water-surfactant ratio through the variation of the persistence length of the surfactants. In general, we find that the number of water-surfactant RMs formed tends to be higher for surfactants with low persistence length (L p = 0.728, Fig.…”

“…This setup has been implemented in a previous work that successfully achieved the micellization of water/HT4 surfactant models into reverse micelles in non-polar solvents. 12 The formation of RMs of the HT5 and HT9 surfactants is studied as a function of the persistence length (L p = −r 0 /ln[coth(k A ) − 1/k A ]), 25 1 and were taken from previous work with a slight variation in the interaction between surfactants' headand tail-groups to promote faster aggregation. 12 The parameters corresponding to the bonding forces (F S and F A ) are listed in Table 2.…”

“…12 The formation of RMs of the HT5 and HT9 surfactants is studied as a function of the persistence length (L p = −r 0 /ln[coth(k A ) − 1/k A ]), 25 1 and were taken from previous work with a slight variation in the interaction between surfactants' headand tail-groups to promote faster aggregation. 12 The parameters corresponding to the bonding forces (F S and F A ) are listed in Table 2. All simulations performed in this work were run in 50 blocks of 10 4 time steps.…”

“…There are several simulations that have studied the formation of RMs, as well as their morphology and structural properties, using molecular dynamics 8,9 and mesoscopic-scale simulations. [10][11][12] S. Salaniwal and co-workers 13 were the first group to study the formation of RMs in a supercritical nonpolar solvent. They found that reverse micellization in supercritical carbon dioxide occurs faster than that for micelles in liquid solvents.…”

“…The self-assembly of HT4 model surfactants (one particle head and a four-particle tail) into RMs in non-polar solvents has been studied recently by Mayoral and co-workers. 12 They found that the number of aggregates (RMs) depends on the water/surfactant ratio, and the formation of RMs occurs at a lower surfactant concentration in strong non-polar solvents. However, the influence of the surfactants' stiffness on micelle formation has not yet been studied thoroughly.…”