Compressibility changes in protonation of some organic aliphatic nitrogen compounds in aqueous solution

Density and sound velocity of aq. solns. contg. monofunctional primary, secondary and tertiary amines, as well as bifunctional primary and secondary amines (B), and their corresponding hydrochlorides, (BHCl), were measured; the apparent molar compressibilities were calculated.  From the limiting apparent molar compressibilities of B, BHCl and HCl in aq. soln. the changes in compressibilities, Δϕ°K,S, for protonation of B was obtained.  The pattern shown by the Δϕ°K,S values was similar to that presented by the changes, ΔX°r, of other thermodn. properties, such as Cp and V, for the formation of BHCl species from B and HCl.  The different values of Δϕ°K,S obsd. for amines having the same no. of hydrogens bonded to N but differing in the presence of a second hydrophilic group was attributed to the removal of the hydrophobic hydration of the alkyl groups brought about by this center.