Enthalpies and Excess Enthalpies of Transfer of n-Alkylamine Hydrobromides from H2O to D2O at 25 °C

Abstract

Differences in heats of solution and in heats of dilution of electrolytes in H2O and D2O were measured directly with a previously described cell-type differential microcalorimeter. In addition to NaCl and NaBr, most of the homologous salts RNH3Br were studied from R = methyl to R = n-octyl. The results of the two simple salts are in excellent agreement with the best literature values.The enthalpies of transfer at infinite dilution, ΔHtr0 (H2O → D2O), are mainly a measure of the structural ion–solvent interactions and of the exchange reaction[Formula: see text]Up to R = n-propyl, the relative apparent molal enthalpies of transfer, [Formula: see text], are negative, approximately zero for n-butyl and positive for the higher members. This gradual change in the excess enthalpies of transfer is interpreted in terms of the increasing importance of hydrophobic interactions between the cations. When association occurs through micelle formation (R = n-octyl), there is a rapid increase in [Formula: see text] which results from hydrophobic bonding.