The Effects of Tetraalkylammonium Salts on the Structure and the Temperature of the Maximum Density of Water

Abstract

Abstract The effects of the tetraalkylammonium halides on the temperature of the maximum density of water were measured by means of dilatometry. Contrary to the expectation that the hydrophobic nature of the alkyl radicals would elevate the temperature of the maximum density of water, these salts lowered it, as other, more usual salts do, obeying the Despretz rule. Among the salts represented as R4NBr, where R is either H or an n-alkyl radical, the Despretz constant, kD, increases in the sequence: H<CH3<C2H5<n-C3H7<<n-C4H9, while the deviation of kD from k, which is a hypothetical constant of the ideal mixing where there is no interaction between the solute and the solvent, varies in the following sequence: H\simeqCH3\simeqC2H5\simeqn-C3H7 <<n-C4H9. The latter fact is attributable to the specific occurrence of a clathrate-like structure formed in the aqueous solution, only when R is n-C4H9. This is in accordance with another peculiarity of (n-C4H9)4NBr which shows a minimum in its partial molar volume at a certain concentration in water.