Bubble coalescence inhibition

Christenson HK, Bowen RE, Carlton JA, Denne JRM, Lu Y (2008) Electrolytes that show a transition to bubble coalescence inhibition at high concentrations. J Phys Chem C 112 794-796... 

The specificity of ion effects on bubble coalescence is apparent on two levels. Most significantly some electrolytes have little or no effect on bubble coalescence whereas others are seen to significantly inhibit coalescence.Moreover, those electrolytes that inhibit bubble coalescence do so with different levels of effectiveness. If the conditions of the experiment are held constant, this effectiveness can be quantified by the concentration of electrolyte that gives rise to a given level of coalescence inhibition. This effectiveness is usefully characterised by the transition concentration , the concentration at which 50% of the full effectiveness of bubble coalescence inhibition is observed. ... 

Fig. 4. Ion-specificity and combining rules in electrolyte bubble coalescence inhibition. Coalescence is plotted as a function of electrolyte concentration on a log scale. Electrolytes NaCl (aa) (A) and HCIO4 (/S/3) ( ) inhibit at 0.1 M, while their cross-products NaC104 (a/S) ( ) and HCl (/Sa) (A) have no effect up to 0.5 M concentration. 100% coalescence is defined in pure water 0% is a stable low value in inhibiting electrolytes.

Table 1. Illustration of the empirical assignment of the property a and which are used with the combining rules to codify bubble coalescence inhibition behaviour.

So two interesting questions immediately arise (i) What is the property of an ion that determines if it is an a ion or a 8 ion and (ii) What property of a salt determines if it inhibits bubble coalescence or not These questions are currently unresolved. Up until now there has been no known tabulated properties of the ions (i.e. thermodynamic properties, polarity, shape, size, etc.) that correlates with the a and f assignments. However, as we shall see below, there is now strong evidence that the assignments a and f describe how the ions arrange themselves within the air-water interface and it is this arrangement of ions within the interface that controls bubble coalescence inhibition. 

Table 3. Summary of bubble coalescence inhibition by mixtures of electrolytes categorised using the empirically assigned properties a and /S. The cation is listed first.

Table 4. Codification of bubble coalescence inhibition by electrolytes in formamide.

The influence of electrolytes on bubble coalescence in both aqueous and non-aqueous systems is ion specific. The available evidence suggests that ion specific effects at the air-water interface are strongly correlated with the partitioning of ions in the interfacial region. Ion partition coefficients determined from surface tension data correlate well with the empirically assigned a and parameters used to predict bubble coalescence inhibition. Bubble coalescence is inhibited when there is an accumulation of both ions at the interface or a removal of both ions from the interface, though the precise mechanism of inhibition remains elusive. 

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