Transfer energetics of some tetraalkylammonium ions from water to aqueous alkanols: The solvent effect on hydrophobic hydration and the 3D-structuredness of solvents

Abstract

Transfer Gibbs energies (∆<em>G⁰</em>) and entropies (∆<em>S⁰</em>) of tetrapropyl-ammonium ion (Pr₄N⁺) in aqueous mixtures of methanol (MeOH), ethanol (EtOH) and isopropanol (2-PrOH) and also those of tetramethylammonium (Me<span style="text-decoration: line-through;">₄</span>N⁺), tetraethylammonium (Er₄N⁺) and tetrabutylammonium (Bu₄N⁺) ions in aqueous 2-PrOH have been determined by measuring the solubility of tetraphenylborate (Ph₄B^-) salts of Me₄N⁺, Et₄N⁺, Pr4N⁺ and Bu₄N⁺ at five equidistant temperatures by spectrophotometric method. Chemical contribution of solvent effect, i.e., ∆<em>G0_t</em>_ch (Pr₄N⁺) values obtained by subtracting the 'cavity, Born-type electrostatic and ion-dipole interaction effects, are found to increase with co solvent composition in the order: MeOH&lt;EtOH&lt;2-PrOH&lt;t-BuOH. This suggests that destabilization of Pr4N+ ions is dictated by decreased hydrophobic hydration (H_bH) effect overcoming the stabilizing effect of increased dispersion and basicity of alkanols. The observed order of destabilization of R4N+ ions in aqueous 2-PrOH, viz., Me₄N⁺&lt;Et₄N⁺&lt;Pr₄N⁺&lt;Bu₄N⁺ also substantiates the same. The observed T∆<em>S⁰</em>_tch (Pr₄N₊)-composition profiles are, however, found to pass through 'characteristic maxima' at the respective 'magic compositions' of aqueous alkanols and in the same composition in the case of R₄N⁺ ions with their relative order: Bu₄N₊&gt;Pr₄N₊&gt;Et₄N⁺&gt;Me₄N⁺ in aqueous 2-PrOH. These are found to be dictated by H_bH effect resulting from decreased availability of monomeric H₂O molecules as induced by graded 3D-structure promoting propensity of the alkanols and by the hydrophobicity of respective alkylammonium ions. Values of ∆<em>G⁰</em>_tch(-CH₂-) and ∆<em>S⁰_t</em>_ch(-CH2-),as obtained by subtracting those for (Cn,H2D'~1)4N+ from (CnH_2n+1)₄N⁺ where n'=n-I and n=2,3 or 4, are however found to be somewhat non-additive,as in the case of t-BuOH, ACN and NaN03 &nbsp;reported earlier. But when those for R₄N+ ions computed with n=4 and n'=3, are compared with the corresponding values in the latter solvent systems, it is revealed that the solvent effect of hydrophobic hydration (H_bH) that solely dictates the behaviour of -CH2-group, serves as a better indicator of not only the solvent effect of hydrophobic hydration but also the 3D-structuredness of aquo-ionic and non- ionic cosolvents.