Apparent Molal Heat Capacities of Transfer from H2O to D2O of Tetraalkylammonium Bromides

The volumetric specific heats (in J-K-1-cm(-3)) of tetraalkylammonium bromides (R4NBr) have been measured at 25 degrees C in the concentration range 0.02 to 0.4 aquamolal in H2O and D2O with a differential flow microcalorimeter. The apparent molal heat capacities phi(c) calculated from the specific heats and known densities, were fitted with the equation phi(c) = phi(c)degrees + A(c)m(1/2) + B(c)m where A(c) is the Debye-Huckel limiting slope and B-c is an adjustable parameter. The standard heat capacity of transfer, Delta C-ptr degrees = phi(c)degrees(D2O) - phi(c)degrees(H2O) of R4NBr is positive for R equal to n-propyl and n-butyl and negative for methyl and ethyl. Except for Me4NBr in H2O, all B-c are negative and become more so as the size of the cation increases; B-c is usually more negative in D2O. These results can be interpreted with a two-state model for water and show that a positive Delta C-ptr degrees is evidence that the solute is an overall structure maker, while a negative value indicates a net structure breaker. The negative B-c is consistent with the existence of strong solute-solute structural (mostly hydrophobic-hydrophobic and hydrophobic-hydrophilic) interactions in the solution.