Temperature and Length Scale Dependence of Tetraalkylammonium Ion Solvation in Water, Formamide, and Ethylene Glycol

• Published in: The journal of physical chemistry. B Vol. 115; no. 49; pp. 14551 - 14555
• Main Authors: Kustov, Andrey V, Smirnova, Nataliya L
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15-12-2011
• Subjects: B: Statistical Mechanics, Thermodynamics, Medium Effects; Bromides; Enthalpy; Ethylene glycol; Heat capacity; Hydration; Solvation; Solvents; Specific heat
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp205331y

This paper focuses on the temperature and length scale dependence of solvation of familiar hydrophobic solutes: tetraalkylammonium ions in highly associated solvents with H-bond networks. Standard enthalpies and heat capacities of solution of symmetrical tetraalkylammonium bromides in formamide (FA) and ethylene glycol (C2H4(OH)2) have been computed using experimental ΔH m(sol) values and compared with previously reported enthalpies and heat capacities in pure water. Heat capacities of hydration are found to be large, positive, and increase almost linearly up to tetrapentylammonium bromide. Hydration of tetrahexyl- and tetraheptylammonium bromides is accompanied by much smaller heat capacity changes than would be expected from the length scale dependence observed. The same behavior reveals enthalpies and heat capacities of solute transfer from FA and C2H4(OH)2 to water. The results obtained rather indicate that solvation of large tetraalkylammonium ions containing more than twenty carbon atoms is accompanied by much less water orientational ordering than for solutes of a moderate size. It results in smaller fluctuations in the water–water pair energy in a hydration shell and consequently reduces the heat capacity of hydration. The comparison of tetraalkylammonium ion behavior in water and the nonaqueous solvents studied shows that the length scale dependence of solvophobic solvation is strongly pronounced in water but not in nonaqueous media.