Deuterium longitudinal nuclear magnetic relaxation of heavy water in alkylammonium chloride solutions. Effect of deuterium exchange between water and polar heads

Abstract

The water deuterium longitudinal relaxation rate is shortened in systems comprising alkylammonium chlorides (with 8, 10, 12 and 14 carbon atoms in the aliphatic chain) and D₂O upon addition of a small quantity of DCl, whose effect is to slow down exchange between water and the polar headgroups. On the other hand, experiments carried out at different isotopic ratios, [D₂O]/([H₂O]+[D₂O]), exclude any contribution from polar heads to the observed water relaxation rate. This apparent paradox is removed by a separate determination of the deuterium relaxation rates of water (R₁)_w and the polar heads (R₁)_s in acidified solutions of octylammonium chloride: two distinct signals are observed. The signal corresponding to the polar heads exhibits a marked non-exponential recovery, thus allowing the determination of τ_w, the deuterium residence time on the water molecule, in addition to (R₁)_w and (R₁)_s. It is also shown that, for all the systems investigated here and under any pH conditions, the apparent water deuterium longitudinal relaxation rate is equal to (R₁)_w+ 1/τ_w. In a more general way, the conditions of validity of this result are discussed.