Insights into Associating Fluid Properties and Microstructure from Classical Density Functional Theory

2013

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Aqueous solutions of alcohols are interesting because of their anomalous behavior that is believed to be due to the molecular structuring of water and alcohol around each other in solution. The interfacial structuring and properties are significant for application in alcohol purification processes and biomolecular structure. Here we study aqueous mixtures of short alcohols (methanol, ethanol, 1-propanol, and 2-propanol) at a hydrophobic surface using interfacial statistical associating fluid theory which is a perturbation density functional theory. The addition of a small amount of alcohol decreases the interfacial tension of water drastically. This trend in interfacial tension can be explained by the structure of water and alcohol next to the surface. The hydrophobic group of an added alcohol preferentially goes to the surface preserving the structure of water in the bulk. For a given bulk alcohol concentration, water mixed with the different alcohols has different interfacial tensions with propanol having a lower interfacial tension than methanol and ethanol. 2-propanol is not as effective in decreasing the interfacial tension as 1-propanol because it partitions poorly to the surface due to its larger excluded volume. But for a given surface alcohol mole fraction, all the alcohol mixtures give similar values for interfacial tension. For separation of alcohol from water, methods that take advantage of the high surface mole fraction of alcohol have advantages compared to separation using the vapor in equilibrium with a water-alcohol liquid. © 2013 AIP Publishing LLC.

Section: Introductionmentioning

confidence: 99%

Hydrophobic and hydrophilic interactions in aqueous mixtures of alcohols at a hydrophobic surface

Ballal

2013

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“…(29) and (37) is independent of the external potential and any density dependant terms, meaning that a generated conformation is used to evaluate the densities at each point in the domain. The chain conformations to evaluate Eq.…”

Section: Single Chain and Single Ring Simulationsmentioning

confidence: 99%

“…10,[24][25][26][27] In DFT, a grand potential functional is constructed and minimized with respect to segment densities to obtain equations for the spatially varying densities. DFT has proven to be a powerful tool in the study of interfacial systems; [28][29][30] a small set of applications of DFT includes adsorption of chain molecules in slit pores, 31 effect of association on the polymer phase diagram, 11 phase behavior of polymer-colloid mixtures, 32 and orientations of rod coil molecules adsorbed at liquid interfaces. 33 Recently, Marshall et al 34 extended the approach of SJ to interfacial systems in the context of DFT by treating Wertheim's theory in inhomogeneous form.…”

Section: Introductionmentioning

confidence: 99%

A Monte Carlo density functional theory for the competition between inter and intramolecular association in inhomogeneous fluids

Marshall

García-Cuéllar

2013

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A Monte Carlo density functional theory is developed for chain molecules which both intra and intermolecularly associate. The approach can be applied over a range of chain lengths. The theory is validated for the case of an associating 4-mer fluid in a planar hard slit pore. Once validated, the new theory is used to study the effect of chain length and temperature on the competition between intra and intermolecular association near a hard wall. We show that this competition enhances intramolecular association near wall contact and inverts the chain length dependence of the fraction bonded intermolecularly in the inhomogeneous region. © 2013 AIP Publishing LLC.

“…Rooted in statistical mechanics, density functional theory (DFT) has become a popular tool to model polymeric systems. [40][41][42][43] In comparison with simulation, DFT provides a more computationally efficient approach, especially when the solvent is explicitly included. McCoy et al 44,45 and later Jain et al 46 have extended DFT to polymer brush system.…”

Section: Introductionmentioning

confidence: 99%

Modeling lower critical solution temperature behavior of associating polymer brushes with classical density functional theory

Gong

Marshall

2013

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We study the lower critical solution temperature (LCST) behavior of associating polymer brushes (i.e., poly(N-isopropylacrylamide)) using classical density functional theory. Without using any empirical or temperature-dependent parameters, we find the phase transition of polymer brushes from extended to collapsed structure with increasing temperature, indicating the LCST behavior of polymer brushes. The LCST behavior of associating polymer brushes is attributed to the interplay of hydrogen bonding interactions and Lennard-Jones attractions in the system. The effect of grafting density and molecular weight on the phase behavior of associating polymer brushes has been also investigated. We find no LCST behavior at low grafting density or molecular weight. Moreover, increasing grafting density decreases the LCST and swelling ratio of polymer brushes. Similarly, increasing molecular weight decreases the LCST but increases the swelling ratio. At very high grafting density, a partial collapsed structure appears near the LCST. Qualitatively consistent with experiments, our results provide insight into the molecular mechanism of LCST behavior of associating polymer brushes.