Enthalpies of dissolution of long chain-length alkyltrimethylammonium bromide salts in water at temperatures from 278.15 to 308.15Â K

Molar enthalpies of dissolution, ΔslnHm, of decyl-, dodecyl-, tetradecyl- and hexadecyl-trimethylammonium bromide salts (RMe3NBr) in water have been measured calorimetrically as a function of concentration and temperature. Standard state values, ΔslnHmo, were obtained by extrapolation to infinite dilution and showed a smooth variation with the number of carbon atoms, nC, in the alkyl chain R, unlike their shorter congeners (nC ⩽ 6). The corresponding molar isobaric heat capacity changes, ΔslnCp,mo, were calculated from the temperature dependence of ΔslnHmo and also varied smoothly with nC but were more than an order of magnitude greater than those of their shorter congeners, probably reflecting a marked increase in hydrophobicity with increasing nC. The behaviour of the RMe3NBr salts was found to parallel broadly that of the symmetrical tetraalkylammonium salts, R4NBr.