Surface Behaviour of Water-Miscible Non-Electrolytes

Contfary to electrolytes consisting of small ions that generally increase the surface tension of water, solutions of non-electrolytes tend to have surface tensions lower than that of water. Liquid non-electrolytes at ambient conditions, i.e., solvents that are immiscible with water but have a limited solubility in it, were already dealt with in Sect. 4.2. The surface tensions of aqueous organic solutes were reported in Adamson s book (Adamson 1990). The molecules of organic molecules tend to concentrate at the solution surface, they are surface active. 

Molecules of substances that are miscible with water are rather small and of them alkanols have received the most attention. Bandyopadhyay et al. (2010) attempted to interpret the data in terms of the balance between hydrophobic and hydrophilic 

S-shaped curves rs(cs) result for all the alkanols. Methanol, as well as 1,2-ethanediol, continue to have Ts 0 values as the concentration increases. The other alkanols studied revert to negative values of Ts after the maximum but then increase again to positive values at higher concentrations. Water is desorbed from the surface, the Fw values are negative for all the alkanols over the entire concentration range, except for aqueous 2-methyl-2-propanol, where Fw 0 was noted at 0.22 xs 0.43. 

The molecular dynamics simulation by Paul and Chandra (2005) of aqueous acetonitrile estimated the thickness of the interfacial layer tg/i (Sect. 4.1) that increases nearly linearly with xs from 0.34 nm in water to 0.45 in CH3CN. Contrary to the previous work, they found the orientation of the CH3CN molecules to be parallel to the surface rather than perpendicular, but the models used for the simulations yielded surface tension values to be nearly twice the experimental ones at xs 0.25, so that some of the conclusions need revision. A subsequent molecular dynamics study by Partay et al. (2009) using the ITIM (Sect. 4.1) concept was applied at four concentrations up to xs = 0.15. The strong interfacial enrichment of CH3CN observed experimentally by VSFG was confirmed with compositions (total, surface) of (0.03, 0.23), (0.05, 0.42), (0.10, 0.70), and (0.15, 0.88). It was concluded that also 

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