The
properties of all electrolyte solutions, whether the solvent
is aqueous or nonaqueous, are strongly dependent on the nature of
the ions in solution. The consequences of these specific-ion effects
are significant and manifest from biochemistry to battery technology.
The “law of matching water affinities” (LMWA) has proven
to be a powerful concept for understanding and predicting specific-ion
effects in a wide range of systems, including the stability of proteins
and colloids, solubility, the behavior of lipids, surfactants, and
polyelectrolytes, and catalysis in water and ionic liquids. It provides
a framework for considering how the ions of an electrolyte interact
in manifestations of ion specificity and therefore represents a considerable
conceptual advance on the Hofmeister or lyotropic series in understanding
specific-ion effects. Underpinning the development of the law of matching
water affinities were efforts to interpret the so-called “volcano
plots”. Volcano plots exhibit a stark inverted “V”
shape trend for a range of electrolyte dependent solution properties
when plotted against the difference in solvation energies of the ions
that constitute the electrolyte. Here we test the hypothesis that
volcano plots are also manifest in nonaqueous solvents in order to
investigate whether the LMWA can be extended to nonaqueous solvents.
First we examine the standard solvation energies of electrolytes in
nonaqueous solvents for evidence of volcano trends and then extend
this to include the solubility and the activity/osmotic coefficients
of electrolytes, in order to explore real electrolyte concentrations.
We find that with respect to the solvent volcano trends are universal,
which brings into question the role of solvent affinity in the manifestation
of specific-ion effects. We also show that the volcano trends are
maintained when the ionic radii are used in place of the absolute
solvation energies as the abscissa, thus showing that ion sizes, rather
than the solvent affinities, fundamentally determine the manifestation
of ion specificity. This leads us to propose that specific-ion effects
across all solvents including water can be understood by considering
the relative sizes of the anion and cation, provided the ions are
spherical or tetrahedral. This is an extension of the LMWA to all
solvents in which the “water affinity” is replaced with
the relative size of the anion and cation.
We present an experimental investigation of specific-ion effects in non-aqueous solvents, with the aim of elucidating the role of the solvent in perturbing the fundamental ion-specific trend. The focus is on the anions: CHCOO>F>Cl>Br>I>ClO>SCN in the solvents water, methanol, formamide, dimethyl sulfoxide (DMSO), and propylene carbonate (PC). Two types of experiments are presented. The first experiment employs the technique of size exclusion chromatography to evaluate the elution times of electrolytes in the different solvents. We observe that the fundamental (Hofmeister) series is observed in water and methanol, whilst the series is reversed in DMSO and PC. No clear series is observed for formamide. The second experiment uses the quartz crystal microbalance technique to follow the ion-induced swelling and collapse of a polyelectrolyte brush. Here the fundamental series is observed in the protic solvents water, methanol, and formamide, and the series is once again reversed in DMSO and PC. These behaviours are not attributed to the protic/aprotic nature of the solvents, but rather to the polarisability of the solvents and are due to the competition between the interaction of ions with the solvent and the surface. A rule of thumb is proposed for ion specificity in non-aqueous solvents. In weakly polarisable solvents, the trends in specific-ion effects will follow those in water, whereas in strongly polarisable solvents the reverse trend will be observed. Solvents of intermediate polarisability will give weak specific-ion effects.
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