Zusammenfassung
This contribution aims to elucidate the connection between ion-ion-solvent interactions in the bulk of aq. electrolyte solns. and the properties of their liq.-air interface. In particular, we were interested in the conditions under which ion pairs form at the surface and whether this is linked to ion pairing in the bulk. For this reason different combinations of hard (Cl-, Li+) and soft ...
Zusammenfassung
This contribution aims to elucidate the connection between ion-ion-solvent interactions in the bulk of aq. electrolyte solns. and the properties of their liq.-air interface. In particular, we were interested in the conditions under which ion pairs form at the surface and whether this is linked to ion pairing in the bulk. For this reason different combinations of hard (Cl-, Li+) and soft ions (I-, Cs+) were investigated. Ion hydration and possible ion assocn. in the bulk was probed with dielec. relaxation spectroscopy. This technique monitors the cooperative reorientation of the dipolar solvent mols. and detects all ion-pair species possibly present in the soln. At the interface, the formation of contact ion pairs was investigated by IR-visible-sum frequency spectroscopy (SFG). This nonlinear optical technique possesses an inherent surface specificity and can be used for the characterization of interfacial water. The intensity of the SFG-active vibrational stretching modes depends on the no. of oriented water mols. The elec. field at the surface of a charged aq. interface aligns the water dipoles, which in turn increases the SFG response. Hence, the enhancement of the oscillator strengths of the water vibrational modes can be used to draw some conclusions on the strengths and geometrical extension of the elec. field. The formation of ion pairs at the interface reduces the intensity of the band assocd. with hydrogen-bonded water. The underlying theory is presented. The combined data show that there are no contact ion pairs in the bulk of the fluid and-at best-only small amts. of solvent shared ion pairs. On the other hand, the combination of hard/hard or soft/soft ions leads to the formation of ion pairs at the liq.-air interface. (c) 2014 American Institute of Physics.
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