Micelle stability in water under a range of pressures and temperatures; do both have a common mechanism?

Abstract

The formation of sodium dodecylsulfate (SDS) micelles in water and heavy water at different pressures and temperatures using molecular dynamics simulations was used to analyze their stability and structure under different conditions and to evaluate the agreement with existing experiments. The results show the assembling of micelles at 1 bar and the presence of larger aggregates under high pressure (over 3 kbar). These large aggregates are not micelles but small finite pieces of bilayers in rod-like shapes. The results obtained using systems at different temperatures showed that both high and low temperatures produce lamellar structures. It is well-known that micelles expose polar residues to water and leave non-polar residues inside where they interact via hydrophobic interactions. High pressure as well as low and high temperatures inhibit the hydrophobic interactions, and under these conditions other structures are produced instead of micelles. This process seems to be similar to protein denaturation under certain temperatures and pressures.

Pressure effect on micellization of non-ionic surfactant Triton X-100

“…By contrast, when the systems are coupled to gradual increases in pressure, they do not lose mobility immediately and give rise to new configurations. 7 The 20 ns of trajectory in each pressure increase was selected following criteria determined in previous studies performed by our group. 7,38 Basically, the simulation time was determined on the ground of the stability of the potential energy for each pressure increase.…”

“…7 The 20 ns of trajectory in each pressure increase was selected following criteria determined in previous studies performed by our group. 7,38 Basically, the simulation time was determined on the ground of the stability of the potential energy for each pressure increase. Also, we ensured that the pressure coupling manages to represent the liquidliquid transition from a low-density to high-density state in water.…”

“…Furthermore, new types of conformational assemblies can be arranged in some cases. 7,8 A particular issue that remains under discussion is the volume change under high pressures. In agreement with Le Chatelier's principle, pressure augmentation is accompanied by a negative volume change in the system.…”

“…Thus, qualitative descriptors, such as variations in solvation, critical micelle concentration, and aggregation number, have a) Author to whom correspondence should be addressed: ernesto.caffarena@ fiocruz.br been taken into account aiming at the understanding of the hydrophobic stability, leading to the self-assembly of the surfactant and its pressure dependence. 7,8,15 At the beginning of the 1960s, studies pioneered by Hamann, 16 about the influence of pressure on the micelles in aqueous solutions, started to be accomplished, wherein the critical micelle concentration (CMC) was determined for the first time for each increase in hydrostatic pressure. By measuring the specific conductivity of the solutions as a function of the sodium dodecyl sulfate (SDS) molality, it was observed that the CMC initially increases with increasing pressure.…”

“…This transition was also perceived by our group using Molecular Dynamics (MD) simulations in an aqueous system with SDS micelles. 7 Unfortunately, there is not yet a detailed description that pressure can exert on nonionic surfactants as there is for the ionic ones. In this line, we can mention Hara's group work where they found that the aggregation number of the Triton X-100 (TRX100) micelle reached its minimum around 150 MPa and then it increased to almost 230 when the pressure varied from 200 MPa to 500 MPa.…”