Evidence for strengthening of hydrogen bonds in hydrophobic hydration-Hydrogen isotope effects in dehydration of tetraalkylammonium forms of Dowex -50 W resins

Abstract

Hydrogen isotope effects in dehydration of tetraalkylammonium [tetramethylammonium (TMA⁺), tetraethylammonium (TEA⁺) and tetrabutylammonium (TBA⁺)] forms of Dowex-50W resins at 353 K by a Rayleigh distillation type technique has been studied. The single stage separation factor, ɑ<em>, </em>is less than ɑ_w (the ɑ value for bulk water at 353 K) at the lowest n_w, (number of moles of water per equivalent of resin) for all the cationic forms. It increases gradually and crosses the ɑ_w at <img src='http://www.niscair.res.in/jinfo/equation105.gif' border=0> (n_w, at which ɑ = ɑ_w) value of 12, 17 and 22 for TMA⁺, TEA⁺ and TBA⁺ resins, respectively. For n_w > n<img src='http://www.niscair.res.in/jinfo/equation105.gif' border=0>, ɑ <em>> </em>ɑ_w for all resins. The value of ɑ reaches a maximum value, ɑ_max, of 1.049 at n<img src='http://www.niscair.res.in/jinfo/equation106.gif' border=0> ≈ 28 for TMA⁺ and then decreases slowly at still larger n_w values. For TEA⁺, the increase in ɑ in the region n_w> n<img src='http://www.niscair.res.in/jinfo/equation105.gif' border=0> is much greater, reaching a value of 1.067 at the highest value of n_w (=41) studied, whereas for TBA⁺, the ɑ at the highest studied n_w (= 39) is 1.045, indicating that ɑ_max for TEA⁺ and TBA⁺ would be observed at even higher n_w, values than for TMA⁺. These results show that the nearest neighbour water molecules around these ions form a clathrate-type structure, as predicted by the theory of hydrophobic effects, with n<img src='http://www.niscair.res.in/jinfo/equation105.gif' border=0> as the number of water molecules required to form a monomolecular layer on the cation surface, n<img src='http://www.niscair.res.in/jinfo/equation106.gif' border=0> as the number of water molecules in the hydrophobic hydration shell of the cation and ɑ_max as the average H-bond strength of water molecules in this shell. These data are also consistent with the general observation that the hydrophobic character of an ion increases with its size. The study provides the first experimental evidence for the strengthening of H-bonds (ɑ > ɑ_w for n_w > n<img src='http://www.niscair.res.in/jinfo/equation105.gif' border=0>) in the hydrophobic hydration shell.