Interfacial Ordering of Tristearin Induced by Glycerol Monooleate and PGPR: A Coarse-Grained Molecular Dynamics Study (original) (raw)
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Mass transport across surfactant-covered oil-water interfaces of microemulsions plays an important role in numerous applications. In the current work, we use coarse-grained molecular dynamics simulations to investigate model systems containing flat hexadecane-water interfaces covered by monolayers of nonionic surfactants of various lengths. Several properties of the surfactant monolayers relevant to the mass transport are considered, including the monolayer microstructure, dynamics, and a free energy barrier to the solute transport. It is observed that the dominant contribution of a surfactant monolayer to the free energy barrier is a steric repulsion caused by a local density increase inside the monolayer. The local densities, and hence the free energy barriers, are larger for monolayers composed of longer surfactants. Since it is likely that the solute transport mechanism involves a sequence of jumps between short-lived pores within a monolayer, we perform a detailed analysis of s...
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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994
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The localization of nanoparticles (NPs) at fluid/fluid interfaces has emerged as an effective self-assembly method. To understand the fundamentals of this localization mechanism, it is necessary to quantify the physical behavior of NPs in the vicinity of a fluid interface. Conventional theories treat the NP as a rigid object whose equilibrium position is dictated by the balance of its surface tensions with the two fluids. However, most NPs are functionalized with “soft ” organic surface layers which play a large role in determining the shape of the NP. Through molecular dynamics simulations, we show that the functionalizing layer also greatly alters the interfacial behavior of the NP beyond the scope of common theory. Furthermore, we characterize the effect of the surface density of functionalizing molecules on the NP deformability. Our results have implications on the experimental interpretation of NP contact angles and may be useful for future theory development.
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Interactions between surfactants, and the resultant ordering of surfactant assemblies, can be tuned by the appropriate choice of head-and tailgroups. Detailed studies of the ordering of monolayers of long-chain n-alkanoic and n-alkanol monolayers at the water-vapor interface have demonstrated that rigid-rod all-trans ordering of the tailgroups is maintained upon replacing the alcohol with a carboxylic acid headgroup. In contrast, at the water-hexane liquid-liquid interface, we demonstrate that substitution of the -CH 2 OH with the -COOH headgroup produces a major conformational change of the tailgroup from disordered to ordered. This is demonstrated by the electron density profiles of triacontanol (CH 3 (CH 2 ) 29 OH) and triacontanoic acid (CH 3 (CH 2 ) 28 COOH) monolayers at the water-hexane interface, as determined by X-ray reflectivity measurements. Molecular dynamics simulations illustrate the presence of hydrogen bonding between the triacontanoic acid headgroups that is likely responsible for the tail ordering. A simple free energy illustrates the interplay between the attractive hydrogen bonding and the ordering of the tailgroup.
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Hydrotropes are substances containing small amphiphilic molecules, which increase solubility of nonpolar (hydrophobic) substances in water. Hydrotropes may form dynamic clusters (less or about 1 ns lifetime) with water molecules; such clusters can be viewed as "pre-micelles" or as "micellar-like" structural fluctuations. We present the results of experimental and molecular dynamics (MD) simulation studies of interfacial phenomena and liquid-liquid equilibrium in the mixtures of water and cyclohexane with the addition of a typical nonionic hydrotrope, tertiary butanol. The interfacial tension between the aqueous and oil phases was measured by Wilhelmy plate and spinning drop methods with overlapping conditions in excellent agreement between techniques. The correlation length of the concentration fluctuations, which is proportional to the thickness of the interface near the liquid-liquid critical point, was measured by dynamic light scattering. In addition, we studied the interfacial tension and water-oil interfacial profiles by MD simulations of a model representing this ternary system. Both experimental and simulation studies consistently demonstrate a spectacular crossover between two limits in the behavior of the water-oil interfacial properties upon addition of the hydrotrope: at low concentrations the hydrotrope acts as a surfactant, decreasing the interfacial tension by adsorption of hydrotrope molecules on the interface, while at higher concentrations it acts as a co-solvent with the interfacial tension vanishing in accordance to a scaling power-law upon approach to the liquid-liquid critical point. It is found that the relation between the thickness of the interface and the interfacial tension follows a scaling law in the entire range of interfacial tensions, from a "sharp" interface in the absence of the hydrotrope to a "smooth" interface near the critical point. We also demonstrate the generic nature of the dual behavior of hydrotropes by comparing the studied ternary system with systems containing different hydrocarbons and hydrotropes.
Journal of Dispersion Science and Technology, 2018
The molecular dynamics (MD) simulation method was used to simulate the aggregation of four types of surfactants at the oil/water interface. The effect of different cations such as Na þ , Mg 2þ and Ca 2þ was compared. The results show that the interaction between different types and concentrations of cationic and head group of surfactants causes the surfactant adsorption layer to bend and changes the aggregation morphology of the micelles. The radial distribution function (RDF) of the surfactant head groups with water molecules and cations is calculated. The calculated results show that it exists a chemical hydration layer and a physical hydration layer between the surfactant head groups and the water molecules. Cation has different degrees of impact, thus changes the original hydration structure. Nonionic and zwitterion surfactants have good salt resistance. Potential of Mean Force (PMF) was used to analyze the energy change when the cation interacted with the polar head. The nonionic and zwitterion surfactants are determined to have a good oil displacement effect even under the conditions of high calcium-magnesium mineralization. The present results could help in choosing surfactants used in oil/water with inorganic salts.
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Scientific Contributions Oil and Gas, 2020
Surfactants have been intensively used for Enhanced Oil Recovery (EOR). Nevertheless, environmental issues cause some surfactants to become unfavored in EOR application. Biodegradable surfactants are the suitable choice to make the environment safer. However, screening surfactants that have a good performance for EOR are time-consuming and costly. Molecular Dynamics (MD) simulation is an alternative solution to reduce cost and time. In the present study, oleic acid-based surfactants that combined with the various length of polyethylene glycol were studied. The potential surfactants were screened by MD simulation to evaluate their ability to reduce the Interfacial Tension (IFT) between oil and water layers, which is the by GROMACS software with Gromos force field and SPC water model. Carboxyl-terminal of the oleic acid was substituted by a different length of polyethylene glycol. All MD simulations were prepared in octadecanewater mixture with temperature ranges of 303-363 K. Our sim...
We present a simple model of single chain, non-ionic surfactant adsorption at gas/liquid interface. Our model takes explicitly into account the effect of conformations the surfactant hydrocarbon chains can assume at the interface. We applied our model for the description of a dependence of surface tension on solution concentration of homologous series of n-alkanols. Aliphatic alcohols with the chain length from four to ten carbon atoms were studied. We found that our model correctly describes experimental data and predicts a distribution of chain conformation at the interface for all n-alkanols studied. We detected small but distinct even-odd effect in the distribution of conformations.
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In the present report molecular dynamics (MD) simulations are used to study the dependence of the superficial energy and entropy of a model heptane/water system as a function of surfactant concentration. For that purpose the total energy of three model cells representing the heptane and water surfactant solutions, and the heptane/water interface, had been followed as a function of temperature for different nonylphenol triethoxylated concentrations. It was found that the surface free energy changes linearly with temperature but presents a minimum with respect to surfactant concentration. That minimum has been studied under the scope of a simple theoretical model which was previously developed to relate molecular structure to interfacial properties. The minimum value of the interfacial energy is caused by optimum surfactant-solvent interaction energies. These energies account for a decrease of the interfacial tension with respect to surfactant concentration at constant temperature and influence its reduction with respect to temperature at constant surfactant concentration. As expected however, detailed variation of the interfacial tension for temperatures close to the solvent boiling points cannot be reproduced using constant density models, neither for the clean heptane/water system nor for the ternary heptane/water/ surfactant system. For this last case, the appropriate consideration of the surfactant excluded volume was found to be very important. The effects of excluded volume corrections for the adequate MD simulation of surfactant molecules at interfaces within the present framework are separately discussed in the second part of this series.