Raman spectroscopic study on the hydration structures of tetraethylammonium cation in water

Conformational changes of tetraethylammonium ion (Et4N+) in aqueous solution have been studied by Raman spectroscopy as functions of pressure and concentration. The difference in the partial molar volume (Delta V-tg.tg -> tt.tt) between the trans-gauche(.)trans-gauche (tg(.)tg) and trans-trans(.)trans-trans (tt(.)tt) conformers of Et4N+ ion has been calculated from the pressure dependence of the relative Raman intensity ratio between the conformers. We discuss about the Delta V-tg.tg -> tt.tt with two contributions from the molecular and hydration volumes. Delta V-tg.tg -> tt.tt is found to be negative, and this is mainly due to the large molecular volume contribution. The value of Delta V-tg.tg -> tt.tt becomes smaller with increasing R probably due to the hydration volume contribution. In view of the pressure and concentration dependences, the water molecules around Et4N+ ions at R = 19 (R = moles of water/moles of salt) mainly form the hydrophobic hydration. The hydrophobic hydration may prefer the tt.tt conformer to the tg.tg conformer. On the other hand, the hydration structure at R = 4 no longer forms the hydrophobic hydration and changes to another type of hydration, where the medium H-bond component is the dominant species. The increase of the ion-ion interactions with decreasing R induces large fractions of the tg.tg conformer.