Molecular Simulation of Surfactant Self-assembly: From Mesoscale to Multi-scale Modeling
Molecular Simulation of Surfactant Self-assembly: From Mesoscale to Multi-scale Modeling

Author:

Publisher:

Published: 2001

Total Pages:

ISBN-13:

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Fully atomistic computer simulations of surfactant self-assembly are extremely challenging because of the different length scales and the associated different times scales, implying large system sizes and tediously long simulations. To overcome this, the uninteresting degrees of freedom at the atomistic level can be integrated out leading to a meso-scale model, which can span the required length and time scales with less computational burden. We use such a meso-scale model to study surfactant self-assembly and how alcohols affect this self-assembly behavior in supercritical carbon dioxide. Here the surfactants and alcohols are represented as a chain of beads where each bead represents a set of atoms. This model is implemented into lattice Monte Carlo simulations. We show that short chain alcohols act as cosurfactants by concentrating in the surfactant layer of the aggregates, strongly decreasing micellar size and increasing the number of aggregates. In contrast long chain alcohols act as cosolvents by concentrating more in the solvent and increasing the micellar size. We then focus on systematically constructing a meso-scale model that preserves the important aspects of the atomistic model, while spanning these different length and time scales. The process of constructing this meso-scale model from the corresponding atomistic model is called coarse-graining. We first explore the rigorous coarse-graining technique in which we match the partition function of the atomistic model with that of the meso-scale model. Such a rigorous procedure has the advantage that it leads to the reproduction of all the structural and thermodynamic properties of the atomistic model in the meso-scale model. We develop a procedure to calculate the rigorous 1, 2 ... N-body effective interactions using Widom's particle insertion method. We implement this rigorous procedure for a binary ArD r system, where the degrees of freedom of Ar are integrated out. We observed that the structure at.

Molecular Simulation in Interface and Surfactant
Molecular Simulation in Interface and Surfactant

Author: Shiling Yuan

Publisher: Mdpi AG

Published: 2023-04-28

Total Pages: 0

ISBN-13: 9783036574714

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The application fields of colloid and interface chemistry are very wide, covering many aspects such as industrial and agricultural production, daily chemistry, enhanced oil recovery, and so on. The traditional experimental analysis of a colloid and interface chemistry system includes various instrumental analysis methods such as spectroscopy, rheometer, microscopes, etc. In recent decades, molecular simulation has become an important research method in this field. It can investigate at the molecular level and provide mechanisms or insights that are hard or even impossible to obtain from an experiment. Many applications of molecular simulation have been reported in the literature, such as the behavior of surfactant, self-assembly, enhanced oil recovery, adsorption around interface, etc. This reprint has collected ten research articles which relate to the application of molecular dyamics simulation in colloide and interface chemistry.

Self-assembly of Gemini Surfactant Molecules and the Properties of Nano-confined Water
Self-assembly of Gemini Surfactant Molecules and the Properties of Nano-confined Water

Author: Sriteja Mantha

Publisher:

Published: 2016

Total Pages: 110

ISBN-13:

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Gemini surfactants are dimeric surfactants, formed by connecting two single tail surfactants with a linker at or near their headgroups. These surfactants in water have tendency to self-assemble into lyotropic liquid crystalline (LLC) morphologies with percolating water channels, which have potential applications as membranes for selective ion transport, controlled release formulations, media for chemical separation and catalysis. Using molecular dynamics simulations, this thesis investigates self-assembly behavior of gemini surfactants and properties of water confined in their LLCs. Length of the linker is known to significantly effect the self-assembly behavior of gemini surfactants in water. Dicarboxylate gemini surfactants with odd number of carbon atoms in the linker are shown to favor bicontinuous cubic (Gyroid) LLC morphologies over hexagonally(H) packed cylinders. On the other hand, surfactants with even number of carbon atoms favor H morphologies at same water content. This thesis suggests that, differences in LLC phase behavior of odd and even gemini surfactants is due to differences in the headgroup/headgroup repulsion caused by inherent conformational preferences of the surfactants. In the LLCs formed by dicarboxylate gemini surfactants, water molecules are under charged confinement. Structure and dynamic properties of water in these confinements depend on width of the water channel and nature of the counterion. Simulation results reported in this thesis suggest that water dynamics is at least an order of magnitude slower than that in bulk water, and decrease in the size of water channel results in further decrease in the dynamic properties. Counterion effect investigated in this thesis suggests that excluded volume effects play a significant role in the dynamics of confined water. Water molecules in surfactant systems with TMA+ as counterion have access to less volume for mobility than the systems with Na+ and K+ as counterion. At high water content, these excluded volume effects dictate the water dynamics. Whereas at low water content, strength of counterion-water interaction coupled with excluded volume effects dictate the water dynamics. Confinement provides an ideal environment to explore the unusual properties of supercooled water. At a temperature close to 225K, water molecules are known to undergo change in their dynamic behavior. This thesis attributes change in the dynamic behavior of supercooled water to changes in the dynamical regimes of potential energy landscape. At temperatures above dynamic crossover, water dynamics is dictated by landscape influenced regime and below crossover it is governed by landscape dominated regime.

Molecular Dynamics Study of Solvation Phenomena to Guide Surfactant Design
Molecular Dynamics Study of Solvation Phenomena to Guide Surfactant Design

Author: Vishwanath Haily Dalvi

Publisher:

Published: 2009

Total Pages: 282

ISBN-13:

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Supercritical carbon-dioxide has long been considered an inexpensive, safe and environmentally benign alternative to organic solvents for use in industrial processing. However, at readily accessible conditions of temperature and pressure, it is by itself too poor a solvent for a large number of industrially important solutes and its use as solvent necessitates concomitant use of surfactants. Especially desirable are surfactants that stabilize dispersions of water droplets in carbon-dioxide. So far only molecules containing substantially fluorinated moieties e.g. fluoroalkanes and perfluorinated polyethers, as the CO2-philes have proved effective in stabilizing dispersions in supercritical carbon-dioxide. These fluorocarbons are expensive, non-biodegradable and can degrade to form toxic and persistent environmental pollutants. Hence there is great interest in developing non-fluorous alternatives. Given the development of powerful computers, excellent molecular models and standardized molecular simulation packages we are in a position to augment the experiment-driven search for effective surfactants using the nanoscopic insights gleaned from analysis of the results of molecular simulations. We have developed protocols by which to use standard and freely available molecular simulation infrastructure to evaluate the effectiveness of surfactants that stabilize solid metal nanoparticles in supercritical fluids. From the results, which we validated against experimental observations, we were able to determine that the alkane-based surfactants, that are so effective in organic fluids, are ineffective or only partially effective in CO2 because the weak C-H dipoles cannot make up for the energetic penalty incurred at the surfactant-fluid interface by CO2 molecules due to loss of quadrupolar interactions with other CO2 molecules. Though the effectiveness of purely alkane-based surfactants in carbon-dioxide can be improved by branching, they cannot approach the effectiveness of the fluoroalkanes. This is because the stronger C-F dipole can supply the required quadrupolar interactions and a unique geometry renders repulsive the fluorocarbons' electrostatic interactions with each other. We have also determined the source of the fluoroalkanes' hydrophobicity to be their size which offsets the effect of favourable electrostatic interactions with water. Hence we can provide guidelines for CO2-philic yet hydrophobic surfactants.