Aggregation Behavior of Nonionic Surfactants Synperonic A7 and A50 in Aqueous Solution

Confinement effects on micellar systems with a hydrogen-bonding solvent

Space confinement greatly influences the properties of liquids, such as their viscosity and capillary critical point. For aqueous solutions of amphiphiles, this effect is extended to the mobility and micellization properties of these molecules, changing important characteristics of micellar solutions such as the critical micelle concentration (CMC). In the present work, we use a lattice Monte Carlo model, which allows for orientational freedom and hydrogen-bond formation for the water molecules, to investigate confinement effects on a solution of surfactants limited by two parallel walls perpendicular to one of the Cartesian axes. This configuration aims to reproduce a small pore, and walls with a hydrophilic or hydrophobic character are studied. We find that, for hydrophilic walls, there is an increase in the value of the CMC for small pores, caused by space confinement effects and also by the interactions of the amphiphile polar heads with the walls. Micelles are able to adhere to the walls as a whole, and their shape shows little change compared to micelles in the bulk solution. Hydrophobic walls show a more dramatic effect on the properties of the solution, arising mainly from the strong adsorption of the amphiphile tails on the walls, driven by the hydrophobic effect. The process of adsorption of amphiphiles with increasing concentration shows a behavior very similar to the one observed in experiments and simulations of such systems. Micelles adsorbed to the hydrophobic walls also show significant changes in their moments of inertia compared to the bulk ones, which is attributed to the formation of half-micelles that have their tails attached to the walls and the polar heads facing the solution. We extend our analysis to the change in the hydrogen-bonding properties of the solvent caused by the confinement, and how that is directly related to the number of free amphiphiles in our system for different pore sizes. Finally, we test different surfactant sizes and how they affect the micellar shape for different concentrations.

Dynamics of micelle formation from temperature-jump Monte Carlo simulations

In the present work we perform temperature jumps in a surfactant solution by means of Monte Carlo simulations, investigating the dynamics of micelle formation. We use a lattice model that allows orientational freedom and hydrogen bonding for solvent molecules, which can make a connection between the different time scales of hydrogen bond formation and amphiphilic aggregation. When we perform a large jump between a high-temperature nonmicellized state and a micellized state, there is strong hysteresis between the heating and cooling processes, the latter showing the formation of premicelles that act as nucleation centers for the assembly of larger aggregates and the former is a drive for dissociation of the existing aggregates. Hysteresis is not seen when we perform a small jump between two states that can be both micellized or nonmicellized. Looking for a more detailed analysis of the hydrophobic effect that drives aggregation, we compare the time evolution of the solvent hydrogen bonds in our system close and far from micelles and how that is affected by the formation of large clusters at low temperatures. We find a strong connection between them, with the total number of hydrogen bonds in the system always increasing when micelles are formed. To gain insights into the mechanism of premicellar formation and growth, we measure the lifetime of micellized amphiphiles as a function of the aggregate size and the stage of the aggregation process. Our results indicate that the premicelles are always unstable, quickly exchanging amphiphiles with the solution due to their low probabilty in equilibrium. Furthermore, we find that the stability of individual surfactants in micelles increases with the aggregate size, with the lifetime of amphiphiles in large micelles being as much as 35 times longer than in the case of the unstable premicellar region.