Computer simulation of adsorption kinetics of surfactants on solid surfaces

Exploration of the Adsorption Kinetics of Surfactants at the Water-Oil Interface via Grand-Canonical Molecular Dynamics Simulations

It is well-known that surfactants tend to aggregate into clusters or micelles in aqueous solutions due to their special structures, and it is difficult for the surfactant molecules involved in the aggregation to move spontaneously to the oil-water interface. In this article, we developed a new grand-canonical molecular dynamics (GCMD) model to predict the saturated adsorption amount of surfactant with constant concentration of surfactant molecules in the bulk phase, which can prevent surfactants aggregating in the bulk phase and get the atomic details of the interfacial structural change with increase of the adsorption amount through a single GCMD run. The adsorption of anionic surfactant sodium dodecyl sulfate (SDS) at the heptane-water interface was studied to validate the model. The saturated adsorption amount obtained from the GCMD simulation is consistent with the experimental results. The adsorption kinetics of SDS molecules during the simulation can be divided into three stages: linear adsorption stage, transition adsorption stage, and dynamic equilibrium stage. We also carried out equilibrium molecular dynamics (EMD) simulations to compare with GCMD simulation. This GCMD model can effectively reduce the simulation time with correct prediction of the interfacial saturation adsorption. We believe the GCMD method could be especially helpful for the computational study of surfactant adsorption under complex environments or emulsion systems with the adsorption of multiple types of surfactants.

13C NMR chemical shift calculations of charged surfactants in water — A combined density functional theory (DFT) and molecular dynamics (MD) methodological study

Structural and magnetic properties of one anionic and one cationic amphiphile molecule (sodium octanoate and hexadecyltrimethylammonium chloride, respectively) in water are studied comparing different methods to account for the presence of the solvent. Calculated 13C NMR chemical shifts are used as the probe for accuracy of the theoretical electronic structures obtained with different descriptions of the surfactants in water solution. The best agreement with the experimental data are obtained by averaging 13C NMR isotropic chemical shifts over a large number of conformational structures of sodium octanoate while considering the electronic structure of the solvent molecules. The 13C chemical shift values of the hexadecyltrimethylammonium alkane chain are systematically overestimated by 10–15 ppm even if an extensive conformational sampling and water as the polarized continuum medium have been taken into consideration. The role of the basis set quality has been studied and discussed as well.

Stability and rupture of archaebacterial cell membrane: a model study

It is known that the thermoacidophilic archaebacterium Sulfolobus acidocaldarius can grow in hot springs at 65-80 degrees C and live in acidic environments (pH 2-3); however, the origin of its unusual thermal stability remains unclear. In this work, using a vesicle as a model, we study the thermal stability and rupture of archaebacterial cell membrane. We perform a simulation investigation of the structure-property relationship of monolayer membrane formed by bolaform lipids and compare it with that of bilayer membrane formed by monopolar lipids. The origin of the unusually thermal stability of archaebacterial cell and the mechanism for its rupture are presented in molecular details.

Computer simulation of the epitaxy of surfactant-templated inorganic nanomaterials on patterned surfaces

For a surfactant-inorganic-water system, we study systemically the epitaxy process of the surfactant-templated inorganic nanomaterial on a patterned surface with a lattice Monte Carlo method. It is found that by using ring-patterned substrate as a template, vertically oriented inorganic material may be formed through the nanometer-scale epitaxy. For all the cases studied in this work, a hemicylinder initially formed on the ring pattern behaves as nucleation sites for the following growth process. Different parameters, such as surfactant architecture, bulk surfactant concentration, fraction of inorganic component, and pattern size, are found to affect the epitaxial growth process of the inorganic nanomaterial. The change of surfactant architecture alters the structure of formed aggregates significantly, thus affecting the epitaxial growth. For the effects of surfactant concentration, it is found that there exists a critical value. If and only if the bulk surfactant concentration is higher than the critical value is the epitaxial growth of inorganic material nucleated from the patterned substrate possible. For the effects of the fraction of the inorganic component, simulation results indicate that there also exists a volume fraction above which the nanomaterial growth is dominated by macrophase separation but not templated by the substrate pattern. The geometry of the chemical modification of the surface also shows significant effects on the nanometer-scale epitaxy, depending on whether its sizes are commensurate with the morphology of the formed inorganic nanomaterials or not.

Computer simulations of solute exchange using micelles by a collision-driven fusion process

In this work, the kinetic process of collision-driven solute exchange in an aqueous phase in which micelles are used as solute carriers is investigated by dissipative particle dynamics simulations. Here, we try to answer two questions about the exchange process of hydrophobic solute molecules: How the solute molecules are exchanged and what factors affect the process. For the first question, the simulation results indicate that, after a stage of intermittent collision between two neighboring aggregates, there are roughly three sequential events in a coalescence stage: (1) molecular contact, (2) neck formation, and (3) neck growth. The coalescence stage is followed by a stage of solute transfer and diffusion. It is found that there are two rate-limiting steps in the whole process of solute exchange, i.e., the break of the water film between two neighboring aggregates and the nucleation of a pore between two surfactant films. For the second question, the effects of the collision velocity, the surface tension, the repulsive interaction between the surfactant films of the colliding aggregates, as well as the steric repulsion are examined. For example, the simulation results show that the depletion force plays an important role during the coalescence stage, while the initial collision velocity basically does not change the fusion ratio. The results also demonstrate that the surface tension and interaction show different effects on the different stages of a solute exchange process.

Macrophase and microphase separations for surfactants adsorbed on solid surfaces: a gauge cell monte carlo study in the lattice model

By combining the gauge cell method and lattice model, we study the surface phase transition and adsorption behaviors of surfactants on a solid surface. Two different cases are considered in this work: macrophase transition and adsorption in a single-phase region. For the case of macrophase transition, where two phases coexist, we investigate the shape and size of the critical nuclei and determine the height of the nucleation barrier. It is found that the nucleation depends on the bulk surfactant concentration. Our simulations show that there exist a critical temperature and critical adsorption energy, below which the transition from low-affinity adsorption to the bilayer structure shows the characteristic of a typical first-order phase transition. Such a surface phase transition in the adsorption isotherm is featured by a hysteresis loop. The hysteresis loop becomes narrower at higher temperature and weaker adsorption energy and finally disappears at the critical value. For the case where no macrophase transition occurs, we study the adsorption isotherm and microphase separation in a single-phase region. The simulation results indicate that the adsorption isotherm in adsorption processes is divided into four regions in a log-log plot, being in agreement with experimental observations. In this work, the four regions are called the low-affinity adsorption region, the hemimicelle region, the morphological transition region, and the plateau region. Simulation results reveal that in the second region the adsorbed monomers aggregate and nucleate hemimicelles, while adsorption in the third region is accompanied by morphological transitions.