Transfer energetics of a series of homologous mono-alkylammonium ions and hence of (-CH₂-) group - A probe for the solvent effect on hydrophobic hydration and the 3D-structuredness of aquo-cosolvent systems

Abstract

Transfer Gibbs energies, &Delta;<em>G^o</em>,. and entropies, &Delta;<em>S^o</em> of -CH₂- group have been evaluated in aqueous mixtures of 3D-structure promoting (SP) 2-propanol (2-PrOH) and 3D-structure breaking (SB) acetonitrile (ACN) by determining the solubilities of tetraphenylborate (Ph₄B-) salts of a series of homologous mono-alkylammonium (RNH₃⁺) ions at five equidistant temperatures ranging from 15^o to 35&deg;C. The transfer energetics of Ph₄B: based on Ph₄AsPh₄B reference electrolyte assumption for the said solvents reported earlier, helped determine the same for the (RNH₃⁺)ions. The chemical contribution of transfer energetics &Delta;P^o_Lch(RNH₃⁺) [P=<em>G</em> and <em>S</em>]as determined by subtracting the cavity and the relevant electrostatic effects reflect that solvation of (RNH₃⁺)ions is' chiefly guided by the effect of decreased hydrophobic hydration (H_bH) of 2-PrOH and the increased H_bH of ACN molecules as compared to that of water molecules. But the observed 'characteristic maxima' in <em>T</em>&Delta;<em>S^o</em>_Lch (RNH₃⁺)-composition profiles in 2-PrOH and absence of the same in ACN-water mixtures though likely to be guided by the 'buffer bonds' are seemingly guided by the predominant effect of 3D-SP propensity of 2-PrOH and 3D-SB propensity of ACN molecules respectively. The &Delta;<em>P^o</em>_Lch values of (-CH₂-) group have been obtained by subtracting the &Delta;<em>P^o</em>_Lch (RNH₃⁺) values of lower homologues from those of the next higher homologues and are found to be more or less additive. Also, while the observed increased destabilization of (-CH₂-) group in both the cases is shown to be dictated by decreased H_bH, the observed 'characteristic maxima' of <em>T</em>&Delta;<em>S^o</em>_Lch (-CH²-)-composition profiles in aqueous 2-PrOH and absence of the same in aqueous ACN are adduced to result from the effect of 3D-SP propensity of 2- PrOH and 3D-SB propensity of ACN molecules. A detailed analysis in the light of Kundu's 4-step transfer process also helped establish that <em>T</em>&Delta;<em>S^o</em>_Lch of the smallest (-CH₂-) group is seemingly a better probe for the 3D structuredness of aquo-cosolvents as compared to those of HBz, pNA, HX and their individual ions with the involved structural and electronic complexities.