Apparent molal volumes of ammonium chloride and some symmetrical tetraalkylammonium chlorides at various temperatures

The apparent molal volumes, φ_v's, of (0.05 m) NH₄Cl, Me₄NCl, Et₄NCl, n-Pr₄NCl, and n-Bu₄NCl have been determined at one-degree intervals from 20 to 40° from precision density measurements. The apparent molal expansibilities, φ_E's, have been calculated from the φ_v values at various temperatures and have been equated to the infinite dilution molal expansibility, Ē⁰. The Ē⁰ values of the R₄NCl's are not a linear function of molecular weight of the cation, R₄N⁺. The Ē⁰ of NH₄Cl is low, apparently due to the ability of the NH₄⁺ ion to form an "icelike" species in solution. The Ē⁰'s of Pr₄NCl and Bu₄NCl appear to be high compared to the lower molecular weight R₄NCPs. This nonlinear behavior is interpreted as being due to expansibility changes in the structure of water caused by the R₄N⁺ cations, Ē⁰(struct). This structural effect decreases as the temperature increases and increases as the size of R₄N⁺ increases. The similarity of V̄⁰ properties of R₄N⁺ cations and the aliphatic alcohols is discussed, and it is postulated that the abnormal properties of R₄N⁺ salts may be normal for solutes able to cause hydrophobic bonding.