(452e) Hydrophobic and Hydrophilic Interactions in Aqueous Mixtures of Alcohols At a Hydrophobic Surface

Aqueous solutions of alcohols are important because of applications in separation processes and biological systems. They are also  interesting because of the strong concentration dependence of interfacial properties as well as their anomalous properties such as maxima in heat capacity[1] and viscosity [2]. It is believed that the molecular structuring of water and alcohol around each other in solution is responsible for the anomalous behavior[3, 4]. The structure and thermodynamics of these mixtures have been extensively studied in the bulk. But less work has been done towards studying water-alcohol mixtures at interfaces. 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- and 2-propanol) at a hydrophobic surface using interfacial Statistical Associating Fluid Theory (iSAFT) [5, 6] which is a perturbation density functional theory. We find that the addition of a small amount of alcohol to water decreases its interfacial tension drastically. This trend in interfacial tension can be explained by the structure of water and alcohol. The presence of a hydrophobic surface disrupts the hydrogen bonding network of water. The hydrophobic group of an added alcohol preferentially goes to the surface preserving the structure of water in the bulk. The different alcohols structure in different ways with 1-propanol being the most effective in packing the surface since the hydrophilic segment in 1-propanol is an end segment as compared to 2-propanol where the hydrophilic segment is in the middle. 1-propanol also has a longer hydrophobic tail than ethanol and methanol making it more effective. We also consider the structuring of aqueous solutions of a mixture of alcohols.

References 

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