1.16.4: Ion-Water Interactions
The seminal paper in this subject was published in 1933 by Bernal and Fowler [1]. These authors drew attention to the possible impact of ions on water - water interactions in aqueous solutions beyond nearest-neighbour water molecules. The paper is also notable for the fact that the authors in their examination of the properties of ice, water and salt solutions did not use the term ‘hydrogen bond’ [2]. At that time there was much debate concerning the nature of this interaction bearing in mind that the ‘valency’ of hydrogen is unity. Rather Bernal and Fowler concluded that ‘the unique properties of water are due to a structure of an extended complex characterized by tetrahedral co-ordination’.
Verwey drew attention to the importance of the interactions between an ion and near-neighbor water molecules in aqueous solution [3,4], these water molecules often being described as ‘electrostricted ‘ by strong ion- solvent dipole interactions. Solvent water plays an important role in the control of partial molar entropies of ions in aqueous solutions [5] and transfer entropies of ions from aqueous to non-aqueous solvents [6].
A major landmark was a paper published by Frank and Evans who developed the concept that solutes, polar and apolar, have an important impact on water - water interactions in aqueous solutions [7]. In the development of models for ionic hydration, a distinction is drawn between hydrophilic and hydrophobic ions. Hydrophilic ions (alkali metal cations and halide anions) have strong attractive interactions with neighbouring dipolar water molecules. Neutron scattering data reveal important information concerning the arrangement of water molecules contiguous to ions [8-10].
For example in the case of chloride ions, the \(\mathrm{Cl} – \mathrm{~H} - 0\) configuration is essentially linear. Nevertheless, there is clear evidence, albeit often secondary, that strong water - ion interactions have an impact on water - water interactions beyond the immediate hydration sheath. Viscosity data indicate that ions, such as iodide and potassium, have a structure breaking effect. The cospheres, for these ions, are drawn, showing two parts [11,12]; an inner zone A and an outer zone B [11].
In zone A, ion - water dipole interactions are strong, leading to the general description ionichydration [12]. An indication of the structure of hydrated ions in solution emerges from X-ray crystallographic studies [13]. In the case of \(\mathrm{KF}.4\mathrm{H}_{2}\mathrm{O}\), the structure comprises \(\mathrm{K}^{+} \left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\) and \(\mathrm{F}^{-} \left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\) octahedra; the \(\mathrm{K}^{+} -\mathrm{O}\) distance is \(0.279 \mathrm{~nm}\). Kebarle showed that mass spectrometry could be used to study ion - water interactions and, interestingly, step-wise hydration in the gas phase [14].
If a given ion in aqueous solution is indeed surrounded by two zones identified as zones A band B, the expectation is that ion - ion interactions in solution will reflect the impact of these structural features [15].
In the context of the impact of zone B, the suggestion was that with increase in size of ions so zone B should increase. Hence the expectation was that, for example, the partial molar isobaric heat capacity of tetra-n-butylammonium bromide in aqueous solution would be large in magnitude and negative in sign. Such not the case; the sign is positive [16,17]. A link was therefore established between the hydration characteristic of tetra-alkylammoniun ions and the structures of the corresponding salt hydrates [18]; e.g. tetra-iso-amylammonium fluoride hydrate, \((\text{iso}-\mathrm{Am})_{4}\mathrm{N}^{+} \mathrm{~F}^{-} 38 \mathrm{~H}_{2}\mathrm{O}\) [19]. Generally, therefore, tetra-alkylammonium ions of \(\mathrm{C}_{4} - \mathrm{~C}_{9}\) carboxylates and tri-alkylsulphonium ions are often identified as hydrophobic where the interaction between these ions and neighboring water molecules is weak. [20-32]. Interestingly, constricting the alkyl chains to form azoniaspiroalkane cations diminishes the hydrophobic character [33,34]. The impact of replacing a hydrophobic terminal group in \(\mathrm{R}_{4}\mathrm{N}^{+}\) ions by a hydrophilic group on the properties of aqueous solutions is dramatic and offers an interesting insight into the role of ion – water interactions [35]. In contrast Finney and co-workers report that neutron diffraction data for aqueous solutions, containing \(\mathrm{Me}_{4}\mathrm{N}^{+} \mathrm{~Cl}^{-}\), show no evidence for increased ice-like structure compared to pure water [36]. Nevertheless thermodynamic and transport properties generally point to the conclusion that the ion \(\mathrm{Me}_{4} \mathrm{~N}^{+}\) does not promote near-neighbor water-water hydrogen bonding [37,38].
Footnotes
[1] J. D. Bernal and R. H. Fowler, J.Chem.Phys.,1933, 1 ,515.
[2] See for example, P. A. Kollman and L. C. Allen, Chem. Rev.,1972, 72 ,283.
[3] E. J. Verwey, Rec. Trav. Chim.,1942, 61 ,127.
[4] See also F. Vaslow, J. Phys.Chem.,1973, 67 ,2773.
[5] C. M. Criss, J. Phys. Chem.,1974, 78 ,1000.
[6] K. K. Kundu, Pure Appl. Chem.,1994, 66 ,411.
[7] H. S. Frank and M. W. Evans, J. Chem. Phys.,1945, 13 ,507.
[8] J. E. Enderby, Chem.Soc.Rev.,1995, 24 ,159.
[9] G. W. Neilson and J. E. Enderby, Adv.Inorg.Chem.,1989, 34 ,195; and references therein.
[10] J. E. Enderby and G.W. Neilson in Water A Comprehensive Treatise; ed.F. Franks, Plenum Press, New York, 1973,volume 6, chapter 1.
[11] H. S. Frank and W.-Y. Wen, Discuss Faraday Soc.,1957,24,756.
[12] W.-Y.Wen,in Ions and Molecules in Solution; ed. N. Tanaka, H. Otaki and R. Tamamushi, Elsevier, Studies in Physical and Theoretical Chemistry, Amsterdam, 1983, p.45.
[13] G. Beurskens and G. A. Jeffrey, J.Chem.Phys.,1964, 41 ,917 and 924.
[14] P. Kebarle in Modern Aspects of Electrochemistry, ed.B.E.Conway and J. O’M. Bockris, 1974, 9 ,1; and references therein
[15] H. S. Frank, J. Phys Chem.,1963, 67 ,1554.
[16] E. M. Arnett and J. J. Campion, J.Am.Chem.Soc.,1970, 92 ,7097.
[17] K. Tamaki, S. Yoshikawa and M. Kushida, Bull. Chem. Soc. Jpn., 1975, 48 ,3018.
[18] G. A. Jeffrey, Prog. Inorg. Chem., 1967, 8 ,43; and references therein
[19] D. Feil and G. A. Jeffrey, J.Chem.Phys.,1961,35,1863.
[20] C. Shin, I. Worsley and C.M. Criss, J. Solution Chem.,1976, 5 ,867.
[21] S. Lindebaum, J. Phys. Chem.,1971, 75 ,3733; and references therein
[22] P.-A. Leduc and J. E. Desnoyers, Can. J. Chem.,1973, 51 ,2993.
[23] S. Lindenbaum, J.Chem.Thermodyn.,1971, 3 ,625.
[24] A. H. Narten and S. Lindenbaum., J. Chem. Phys.,1969, 51 ,1108.
[25] R. H. Boyd, J. Chem.Phys.,1969, 51 ,1470.
[26] O. D. Bonner and C. F. Jumper, Infrared Physics, 1973, 13 ,233.
[27] R. L. Kay, Adv. Chem. 1968, 73 ,1
[28] T. S. Sarma and J. C. Ahluwalia, J. Phys. Chem.,1970, 74 ,3547.
[29] T. S. Sarma, R. K. Mohanty and J. C. Ahluwalia, Trans. Faraday Soc.,1969, 65 ,2333.
[30] D. A. Johnson and J. F. Martin, J. Chem. Soc. Dalton, Trans.,1973,1585.
[31] T. S. Sunder, B. Chawla and J. C. Ahluwalia, J. Phys. Chem.,1974, 78 , 738.
[32] E. M. Arnett, M. Ho and L. L. Schaleyer, J.Am. Chem. Soc.,1970, 93 ,77039.
[33] A. LoSurdo, W.-Y. Wen, C. Jolicoeur and J.-L.Fortier,J.Phys.Chem.,1977, 81 , 1813;and references therein.
[34] W.-Y. Wen and S. Saito, J.Phys.Chem.,1965, 69 ,3569.
[35] G.P. Cunningham, D.F. Evans and R.L. Kay, J.Phys.Chem.,1966,70,3998.
[36] J. Turner, A.K. Soper and J.L. Finney, Mol.Phvs.,1992,77,411.
[37] R. L. Kay, D. F. Evans and M.A. Matesich, in Solute-Solute Interactions, ed. J. F. Coetzee and C. D. Ritchie, Marcel Dekker, New York, 1976,voume.2, chapter 10.
[38] R. L. Kay, Water A Comprehensive Treatise, ed. F. Franks, Plenum Press, New York, 1973, volume 3, chapter 4.