Specific Ion Effects at the Air–Water Interface: Experimental Studies

Abstract

Ion specificity is important in all soft matter systems at high electrolyte concentrations. Specific ion effects that influence many biological processes fall within theHofmeister paradigm, whereby the strength of action of the anions and cations follow a well-defined order, independent of the co-ion. In contrast, the ion specificity seen when salts inhibit bubble coalescence depends on the combination of ions present. Single electrolytes at 0.1M may inhibit bubble coalescence or have no effect, as determined by simple ion combining rules and empirical assignments (α or β) of the cation and the anion. The coalescence of mixed electrolyte systems can also be predicted by an extension of these rules. Bubble coalescence in some non-aqueous solvents can also show ion-specificity that depends upon ion combination in a way analogous to the combining rules observed in water — which suggests that this is a general property of the air–solution interface. Here, we summarise evidence that the contrasting specific ion effects observed in bubble coalescence and in other soft matter systems have the same origin: Ion specificity arises from differences in the affinity of ions for interfaces. This leads to the conclusion that combining rules seen in bubble coalescence inhibition arise from the dynamic nature of the air–solution interface.