Structural properties and dynamics of C12E5 molecules adsorbed at water/air interfaces: A molecular dynamic study

A many-body dissipative particle dynamics parametrisation scheme to study behaviour at air-water interfaces

In this article, we present a general parametrisation scheme for many-body dissipative particle dynamics (MDPD). The scheme is based on matching model components to experimental surface tensions and chemical potentials. This allows us to obtain the correct surface and mixing behaviours of complex, multicomponent systems. The methodology is tested by modelling the behaviour of nonionic polyoxyethylene alkyl ether surfactants at an air/water interface. In particular, the influence of the number of ethylene oxide units in the surfactant head group is investigated. We find good agreement with many experimentally obtained parameters, such as minimum surface area per molecule; and a decrease in the surface tension with increasing surfactant surface density. Moreover, we observe an orientational transition, from surfactants lying directly on the water surface at low surface coverage, to surfactants lying parallel or tilted with respect to the surface normal at high surface coverage. The parametrisation scheme is also extended to cover the zwitterionic surfactant lauryldimethylamine oxide (LDAO), where we provide good predictions for the surface tension at maximum surface coverage. Here, if we exceed this coverage, we are able to demonstrate the spontaneous production of micelles from the surface surfactant layer.

Surfactant and Plasticizer Segregation in Thin Poly(vinyl alcohol) Films

The vertical depth distributions of individual additive components [cetyltrimethylammonium bromide (CTAB), deuterated pentaethylene glycol monododecyl ether (d25-C12E5), and deuterated glycerol (d-glycerol)] in PVA films have been isolated and explored by ion beam analysis techniques and neutron reflectometry. The additives display an unexpectedly rich variety of surface and interfacial behaviors in spin-cast films. In separate binary films with PVA, both d-glycerol and CTAB were evenly distributed, whereas d25-C12E5 showed clear evidence for surface and interfacial segregation. The behavior of each surfactant in PVA was reversed when the plasticizer (glycerol) was also incorporated into the films. With increasing plasticizer content, the surface activity of d25-C12E5 systematically decreased, but remarkably, when glycerol and CTAB were present in PVA, the surface and interfacial activities of CTAB increased dramatically in the presence of glycerol. Quantification of the surface excess by ion beam analysis revealed that, in many cases, the adsorbed quantity far exceeded what could reasonably be explained by a single layer, thus indicating a wetting transition of the small molecules at the surface or interface of the film. It appears that the surface and interfacial behaviors are partly driven by the relative surface energies of the components, but are also significantly augmented by the incompatibility of the components.

A numerical investigation on the drainage of a surfactant-modified water droplet in paraffin oil

A volume of fluid approach is used in numerical simulations of the settling motion of a surfactant modified water droplet in a continuous paraffin oil phase. The droplet is millimeter-sized and confined in a square two dimensional domain. The surfactant interfacial and bulk concentration-equations are solved together with the incompressible Navier-Stokes equation. The role of boundary walls in the overall settling dynamics is described. As the droplet moves downwards the interfacial shear creates non-homogeneous interfacial surfactant concentrations and Marangoni driven phenomena come into play. A decrease of the drainage velocity is then evidenced indicating that buoyancy forces are counter balanced by Marangoni induced lift-forces. The lateral migration of the droplet due to boundary wall proximity is discussed. It is shown to increase with wall proximity and to decrease when increasing the interfacial concentration. Finally, a simplified model is used to investigate the evolution of the bulk concentration assuming the surfactant is insoluble in paraffin oil and poorly soluble in water.

Structure and dynamics of nonionic surfactants adsorbed at vacuum/ionic liquid interfaces

Structural and dynamical properties related to the adsorption of nonionic surfactants at vacuum/ionic liquid interfaces were studied using molecular dynamics simulations. Specifically, the surface activity of pentaethylene glycol monododecyl ether (C12E5) was investigated at the free interface of an imidazolium-based room temperature ionic liquid (RTIL), at different surface densities. At low surface coverages, the incorporation of C12E5 does not produce meaningful changes in the vacuum/RTIL interface: the C12E5 hydrophobic tails remain entangled with those of the RTIL cation groups in the outer shell, whereas the C12E5 hydrophilic heads reside at an inner layer. At high surface coverages, the structure in the substrate-in terms of the features exhibited by the local density profiles-practically vanishes; the interface becomes wider and the surfactant molecules shift toward more external positions. Information about the local structure of the interface at high surface densities can be recovered by performing a tessellation procedure. For the sake of comparison, the surface behavior of two commonly used ionic surfactants, sodium dodecyl sulfate and dodecyl trimethyl ammonium chloride, were also studied. The modifications in the width and structure of the bare vacuum/RTIL interface due to the presence of the ionic surfactants are markedly milder than those observed for the nonionic surfactant. Moreover, the RTIL seemed to behave as a better solvent for the chloride counterions than for sodium ones; which were found to remain bound to the surfactant head groups. An analysis of the dynamics at the surface was also performed. Our results indicate that the presence of increasing amounts of nonionic surfactants leads to a gradual reduction of the mobility of the RTIL species. When ionic surfactants are adsorbed, these retardations are even more severe for the surfactant head groups, where the corresponding diffusion coefficients show reductions of practically 1 order of magnitude.

Atomistic simulation study of linear alkylbenzene sulfonates at the water/air interface

Molecular dynamics simulations with the CHARMM atomistic force field have been used to study monolayers of a series of linear alkylbenzene sulfonates (LAS) at the water/air interface. Both the numbers of carbon atoms in the LAS alkyl tail (1 to 11) and the position of attachment of the benzene ring on the alkyl chain have been varied. Totally 36 LAS homologues and isomers have been investigated. The surface tensions of the systems and the average tilt angles of the LAS molecules are found to be related to both the length and the degree of branching of the alkyl tails, whereas the solubility and mobility are mostly determined by the tail length.

C12E6 and SDS surfactants simulated at the vacuum-water interface

The effect of surface coverage on the aggregate structure for the nonionic hexaethylene glycol monododecyl ether (C(12)E(6)) and anionic sodium dodecyl sulfate (SDS) surfactants at vacuum-water interface has been studied using molecular dynamics simulations. We report the aggregate morphologies and various structural details of both surfactants as a function of surface coverage. Our results indicate that C(12)E(6) tail groups orient less perpendicularly to the vacuum-water interface compared to SDS ones. Interfacial C(12)E(6) shows a transition from gaslike to liquidlike phases as the surface density increases. However, even at the largest coverage considered, interfacial C(12)E(6) aggregates show more disordered structures compared to SDS ones. Both surfactants exhibit a non-monotonic change in planar mobility as the available surface area per molecule varies. The results are interpreted on the basis of the molecular features of both surfactants, with particular emphasis on the properties of the surfactant heads, which are nonionic, long, and flexible for C(12)E(6), as opposed to ionic, compact, and rigid for SDS.