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J. Phys. Chem. C 2008, 112, 794-796
Electrolytes that Show a Transition to Bubble Coalescence Inhibition at High Concentrations H. K. Christenson,* R. E. Bowen, J. A. Carlton, J. R. M. Denne, and Y. Lu School of Physics and Astronomy, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom ReceiVed: July 12, 2007; In Final Form: October 18, 2007
We have studied the coalescence of bubbles in electrolyte solutions by measuring the fraction of contacting bubble pairs that coalesce as a function of electrolyte concentration. At low concentrations, we have reproduced earlier results in the literature, but by extending our measurements to higher electrolyte concentrations, we have found that some electrolytes previously thought not to inhibit bubble coalescence do show a transition to coalescence inhibition at higher (>1 M) concentrations. These results suggest that coalescence inhibition should be studied over wider concentration ranges if full insight into the factors governing coalescence inhibition is to be obtained.
Introduction The last 15 years have seen considerable interest in the factors governing bubble coalescence in the physical chemistry literature, whereas previously this topic was largely the province of chemical engineering journals. This change may be traced to a series of experiments by Craig et al.1 using the turbidity of a gas-sparged column as a semiquantitative measure of bubble coalescence. Their results were in broad agreement with earlier studies of bubble-size distributions in gas-sparged columns2 as well as experiments with contacting bubble pairs.3 All these investigations showed that coalescence occurs easily in pure water and that with increasing concentrations of added electrolyte there is a transition to coalescence inhibition. This transition concentration is sharp and occurs over a narrow concentration range characteristic of a particular electrolyte, usually at concentrations of the order of 0.1 M. Craig et al.1 extended the investigations to cover a much larger range of electrolytes and, in particular, found a number of electrolytes that did not appear to show a transition concentration, although concentrations above 0.5 M were not investigated. These electrolytes that apparently lack a transition concentration had scarcely been mentioned in the many earlier studies of bubble coalescence.2-7 A possible exception is (CH3)4NBr, which had been found to have unusually short coalescence times for concentrations up to 0.2 M,5 suggesting minimal coalescence inhibition. A number of models, beginning with that of Marrucci,8 assume that coalescence inhibition in electrolyte solutions has the same origin as the transient stability of foams (although the time scales involved are shorter and the magnitude of the effect much smaller). The difference is that the surfactants or proteins that stabilize the more long-lived foams are positively adsorbed, whereas the ions in electrolyte solutions are desorbed from the air-solution interface. These models5-9 also invoke intermolecular forces, in various descriptions, across the thinning water film between the two bubbles, but the common denominator is that the transition concentration is found to be proportional to (dγ/dc)-2, the so-called Marangoni factor. The existence of these * To whom correspondence should be addressed. E-mail:
[email protected].
h.k.
models was pointed out by one of us in a short note10 that showed that there was a weak but definite correlation between the transition concentrations measured by Craig et al. and the values of (dγ/dc)-2 for the respective electrolytes. The electrolytes that did not show a transition concentration below 0.5 M had large values of (dγ/dc)-2, i.e., a small change in surface tension with concentration. At the time one of us showed11 that the increased turbidity of the gas-sparged columns is indeed related to the absence of coalescence but that the coalescence that takes place in water and dilute electrolytes occurs mainly at the bubble-generating orifices rather than in the bulk of the solution. We also demonstrated that a 5 M solution of one electrolyte (NaClO4) gave a turbidity and density of small bubbles similar to what was found with other electrolytes at low concentrations. A 1 M solution showed a turbidity similar to pure water, suggesting that there was indeed a transition concentration between 1 and 5 M. Subsequent to the experiments of Craig et al., considerable controversy has centered on the correlation between the transition concentration and the Marangoni factor (dγ/dc)-2. Experiments12,13 have shown that there are systems of mixed electrolytes where coalescence inhibition occurs at low concentrations even though the measured magnitude of (dγ/dc) is near zero. The situation has been summarized in two recent reviews14,15 and in the study of mixed electrolytes.13 The possible involvement of van der Waals forces, electric double-layer forces, and hydrophobic forces has been discussed in these papers. Briefly, the situation appears to be as follows: The experiments on mixed electrolytes show that the Marangoni factor cannot account for the coalescence inhibition in mixed electrolytes, but alternative suggestions based on ion binding and double-layer forces16 are faced with the difficulty of explaining how shortrange (of the order of 1 nm) interactions can affect coalescence events when the film thickness at rupture is believed to be 2050 nm.17 The reader is referred to refs 13-16 for a full discussion of this and other factors. In view of the observation presented in ref 11, we decided to carry out some simple experiments at higher concentrations for some of the electrolytes that have previously been assumed not to show a transition concentration. We studied the coalescence of pairs of bubbles formed at adjacent capillary orifices, in
10.1021/jp075440s CCC: $40.75 © 2008 American Chemical Society Published on Web 12/21/2007
Bubble Coalescence Inhibition
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Figure 2. Percentage coalescence vs solution concentration for LaCl3 (open squares), MgSO4 (open diamonds), CaCl2 (open triangles), and NaCl (open circles).
TABLE 1 transition concentrations electrolyte LaCl3 CaCl2 MgSO4 NaCl NH4OOCCH3 NaClO4 KOOCCH3
Figure 1. Bubble coalescence as observed in pure water (top) and coalescence inhibition in solutions at higher electrolyte concentrations. The inside diameter of the capillaries is 2 mm.
electrolytes known to show a transition concentration, as well as in several that have previously been deemed not to exhibit one. Materials and Methods Air bubbles were formed at the orifices of glass tubes of 2 mm inside diameter immersed in electrolyte solution. A pair of glass syringes was mounted on a support so that the plungers could be moved smoothly and evenly with a translation stage. The bubbles were slowly expanded using the translation stage until they made contact, as ascertained by visual inspection. The bubbles would either coalesce almost immediately or remain stable for a period of several seconds or longer, in which case coalescence inhibition was deemed to occur (see Figure 1). Measurements were made on 75-100 bubble pairs for each electrolyte concentration. The electrolytes (sodium chloride, magnesium sulfate, calcium chloride, ammonium nitrate, lanthanum chloride, ammonium acetate, potassium acetate, sodium perchlorate) were of analytical grade and used without further purification, and the water was from a Millipore unit. Results Figure 2 shows the percentage bubble coalescence vs solution molarity for electrolytes with small values of (dγ/dc)-2. Three of these salts have been studied previously, using contacting bubble pairs3 and/or the turbidity of a gas-sparged column.1 Our results show the characteristic, sharp transition to coalescence inhibition found in previous work. The transition concentration for each electrolyte was taken as the concentration at which the percentage of coalescing
ref 3
ref 1
this work
(dγ/dc)-2
0.055 0.032 0.175
0.037 0.020 0.078 none none none
0.025 0.060 0.036 0.208 1.1 1.7 2.8
0.029a 0.06a; 0.08b 0.17a; 0.18b 0.23a; 0.32b 21a; 2.6b 1.7a; 1.8b
a Values from measurements of γ at 1.5-s bubble intervals using the maximum bubble pressure method.18 b Values from measurements of γ at 5-s bubble intervals using the maximum bubble pressure method.13
bubbles was halfway between 100% (the invariable result in pure water) and the baseline measurement, which varied from 0 to 15%. No significant difference in the values would result if the transition concentration were instead deemed to be that at which 50% coalescence occurs. Table 1 gives our determined transition concentrations, together with literature results where available, and values of (dγ/dc)-2 from surface tension measurements by Weissenborn and Pugh18 and Henry et al.13 Our results are close to those of Lessard and Zieminski, who also used the contacting bubble-pair method, and differences could easily be accounted for by differences in bubble size19,20 or rate of approach of the interfaces.21 Both theirs and our transition concentrations are slightly larger than those found by Craig et al. from the turbidity of a gas-sparged column. This discrepancy is not surprising in view of the difference in the methods, although the order is the same in all three sets of measurements. Figure 3 shows the results of measurements with electrolytes (sodium perchlorate, potassium acetate, and ammonium acetate) with large (dγ/dc)-2 values. As can be seen, all of these also show a transition to coalescence inhibition, although the transition may be more gradual, particularly for sodium perchlorate. We also found a transition concentrations for oxalic acid (reported to have no transition concentration in ref 1), at a concentration close to 1 M, but the incomplete dissociation of this acid places it in a different category from the other electrolytes and we have hence excluded these results. Discussion These results show that a transition to bubble coalescence inhibition may occur at high electrolyte concentrations, and it follows that investigations should be carried out over much larger concentration ranges before a complete picture of the
796 J. Phys. Chem. C, Vol. 112, No. 3, 2008
Christenson et al. water.1,23 We cannot completely discount any influence at the higher concentrations studied here, although we note that the increase relative to pure water is still small. The viscosity of 2 M NaClO4 is only 15% higher than that of pure water24 (data for potassium and ammonium acetate are not available). In summary, while our results do not give any new insight into the mechanism behind coalescence inhibition, they do suggest that it may be unwise to claim that some electrolytes do not show a transition to coalescence inhibition unless experiments are carried out with solutions at higher concentrations, in each case up to the solubility limit.
Figure 3. Percentage coalescence vs solution concentration for KOOCCH3 (open squares), NaClO4 (open circles), and NH4OOCCH3 (open triangles).
factors determining bubble coalescence is likely to emerge. Indeed, it has already been shown that some degree of coalescence inhibition occurs with some electrolytes at concentrations much lower than the transition concentration.22 Unfortunately, the equipment available to us in a physics department did not allow us to undertake any purification of the salts employed or to measure accurately the surface tension as a function of concentration. We accept that these two facts may cast some doubt on the results, but we nevertheless considered that these results should be made available to the scientific community so that those better equipped might be able to carry the investigations further. Any attempt to correlate the transition concentrations at high concentrations with literature values of (dγ/dc)-2 for these salts is rendered difficult by the sometimes large difference in the literature values of dγ/dc. In the extreme case of NaClO4, the resultant values of (dγ/dc)-2 differ by almost a factor of 10, although others are much closer. Note, however, that the higher transition concentrations are broadly consistent with a smaller change in surface tension with concentration, although any quantitative correlation would clearly be very poor. Bubble coalescence, or inhibition thereof, is a dynamic event, and it is not clear that values of equilibrium surface tensions should be used when attempting to make a correlation with the timedependent elasticity of vapor-liquid interfaces. Any effect of solution viscosity on bubble coalescence inhibition at lower concentrations (
