Hydration sphere of tetrabutylammonium cation. FTIR studies of HDO spectra

The hydration of tetabutylammonium cation(Bu4N+) has been studied in aqueous solutions of Bu4NBr, Bu4NCI, and Bu4NF salts by means of the FTIR method. Isotopically diluted HDO in H2O has been applied by using the stretching vibration OD band as a probe of the solutes' hydration. The spectra have been analyzed in a way that led to removal of the contribution of the bulk water and thus to the: separation of the spectra For salt-affected HDO. Ln several steps of a curve-fitting procedure the Bu4N+- and the respective anion-affected OD bands have been refined from the salt-affected HDO spectra. The Bu4N+-affected OD band profile and the bulk spectrum have been converted to a probability distribution function of interatomic O ... O distances for influenced water, by the use of a correlation curve. Comparison of the band for HDO affected by cation with that of the bulk water, as well as the derived respective distributions of O ... O distances, leads to the conclusion that "iceberg formation" is a wrong expression for the effect of hydrophobic groups in wafer. The results have been discussed with respect to enthalpy and entropy effects.