Molecular Dynamics Study of Spherical Aggregates of Chain Molecules at Different Degrees of Hydrophilicity in Water Solution

Abstract

We present molecular dynamics simulations of three spherical aggregates constituted of the same number of monomeric chains of increasing hydrophilic character in water solution. The three different chains are dodecane, CH3(CH2)10CH3; dodecan-1-ol, CH3(CH2)10CH2OH; and oligoethylenoxide, CH3(CH2)11(OCH2-CH2)3OH, and the systems are denoted C12, C12E0, and C12E3, respectively. The three solutions are simulated at the same temperature and pressure and at about the same concentration in weight of the solute. We investigate the structural changes of the aggregate and the conformational changes of its chains after increasing the hydrophilicity of the monomers. In the C12 system, the density of the aggregate is higher than the density of the pure hydrocarbon liquid in the same thermodynamic conditions. Dodecane chains are quite rigid and mostly in the all-trans conformation. This is an effect of the strong hydrophobic repulsion exerted on the aggregate by the water. A density depletion develops at the interface between the oil core and the solvent because of the mismatch between the two components. This extra pressure is gradually released after increasing the hydrophilicity of the monomers. In the C12E3 system, the density depletion at the interface is completely canceled, and the system is homogeneous through the interface. In this system, the interfacial regions appear to be divided into an inner part, where methylene groups, oxyethylene groups, and water molecules are present, and into an outer region formed by a mixture of oxyethylene groups and water only. The local density of oxyethylene groups in the interfacial region is strongly fluctuating, and its histogram shows an effective attraction among those groups. Thus, a considerable portion of the interfacial volume is filled by the solvent only. We observe a strong tendency for the OCCO dihedrals in the hydrophilic tails to be in the gauche state, and at the same time, we observe the presence of H-bond bridges that stabilize this structure.

Molecular Simulations of Dodecyl-β-maltoside Micelles in Water: Influence of the Headgroup Conformation and Force Field Parameters

“…A single conformation for 1-dodecanol was considered in charge derivation, and the selected geometry was optimized in its extended conformation ( i. e. “all trans dihedrals”). Indeed, it has been previously shown that the surfactant alkyl chain adopts mainly this conformation in the micelle core (see for instance refs.44,59,60). Frequencies were calculated for all the molecules, and transition state structures were excluded from the charge derivation procedure.…”

Modelling the interfacial behaviour of dilute light-switching surfactant solutions

“…However, even with commonly used CG approaches, pre-cmc calculations are strongly dependent on (i) the capability of the model to preserve the molecular character of surfactant-solvent interactions and, (ii) the adopted methodology. [30][31][32][33][34][35][36][37][38] However, in these simulations, the key limitation to performing a full-fledged simulation including the surfactant and the full amount of solvent molecules associated with these dilute conditions remains, even with the reduced resolution of the coarse grained models.…”

Computer simulations of micellar systems

“…Another series of works [35][36][37][38][39][40] was devoted to long term study of micelles formed from nonionic surfactants. The authors of [35,36] studied the hydra tion of a spherical C 12 E 6 micelle in order to ascertain both the structure of the hydration layer and the mech anism of dehydration upon heating from 10 to 45°C.…”

“…The authors of [35,36] studied the hydra tion of a spherical C 12 E 6 micelle in order to ascertain both the structure of the hydration layer and the mech anism of dehydration upon heating from 10 to 45°C. The micelle was composed of 45 surfactant molecules at a concentration in water of 0.01 M. The former of the mentioned works described the preliminary stage of the study.…”