Aggregation Thermodynamics of Sodium Octanoate Micelles Studied by Means of Molecular Dynamics Simulations

Abstract

The present work is aimed at studying the computation of the thermodynamic potentials that describe the stability of anionic surfactant molecules in micellar aggregates. We report a set of molecular dynamics simulations of a sodium octanoate micelle in aqueous solution using the umbrella sampling method along with the Jarzynski equality in order to compute the potential of mean force for the dissociation process of one surfactant molecule from a previously assembled micellar aggregate. The Jarzynski average was computed at several different temperatures in order to estimate the Gibbs energy of association for the octanoate anion, which was split into its enthalpic and entropic contributions. We also estimated the contributions arising from the polar head and the apolar tail for each thermodynamic potential, and a detailed picture emerged from these simulations. The aggregation is driven mostly by the Gibbs energy contribution arising from the hydrophobic tail, which was large enough to cancel out the unfavorable contribution from the polar head. Although the association process may be ascribed mostly to the transfer of the apolar tail to the micellar core, it should be noted that the polar head also contributed with a favorable entropic term to the overall Gibbs energy. These findings were rationalized by comparing the energetic and structural patterns of the hydration process of a free monomer in solution to an aggregated molecule. The interaction energy distributions presented at least two discernible populations and each population was related to a different structural pattern, as characterized by the radial distribution functions. Altogether, the changes in both the energy and structure of the hydration layer are consistent with the entropy-driven association of the surfactant into the micellar aggregate.

Shape of Long Chain Alkyl Sulfonate Micelle upon Salt Addition: a Molecular Dynamics Study

“…Surfactant molecules are able to self-assemble above the critical micelle concentration (CMC) into various structures and these kinds of structures are usually used to replicate biological systems [1]. As such, surfactant systems have been the subject of numerous experimental, theoretical and computational studies [2], with various aspects considered, including fission/fusion mechanisms [3][4][5][6], influence of salt [7,8], effect of variations in concentration and temperature [9], etc. In this study, we report the results of atomic scale molecular dynamics (MD) simulation of long chain alkyl sodium pentadecyl sulfonate (SPDS) micelles in the presence of 0.5 M NaCl.…”

Free energy of micellization of dodecyl phosphocholine (DPC) from molecular simulation: Hybrid PEACH‐BAR method

“…For our purpose, selected micelle sizes (including 1,3,8,9,14,15,19,20,24,25,29,30,34,35,39,40,44,45,49,50,54,55,59, 60, 64, 65, 69, 70, 74, 75, 79, 80) were chosen to perform BAR analysis.…”

“…The addition or removal of a single monomer to or from a micelle can in principle give information on the difference between ΔG i and ΔG i + 1 , or equivalently the equilibrium constant association or dissociation of a single monomer. A few reports have been made of the PMF associated with removing a monomer from a micelle, [35][36][37][38] tions to the affinity of a monomer to a cluster is, however, being counted in the US estimate for that quantity entered in Table 2. The original PEACH-BAR calculations therefore were missing a factor that contributes to the stability of aggregates formed.…”

Effect of alkyl-group flexibility on the melting point of imidazolium-based ionic liquids

“…For instance, introduction of urea to an aqueous solution 26 or the transference of ions from a hydrophobic to a hydrophilic environment affects the population of different conformers. 27 This brief review shows that the flexibility of both cations and anions is important for the comprehension of the melting point and possible crystalline structures of ILs. However, several questions remain.…”