10192
J. Phys. Chem. 1993,97, 10192-10197
The Effect of Electrolytes on Bubble Coalescence in Water Vincent S. J. Craig,' Barry W. Ninham,* and Richard M. Pashley**+ Departments of Chemistry and Applied Mathematics, The Australian National University, P.O. Box 4, Canberra ACT 2601, Australia Received: December 26, 1992; In Final Form: June 30, 1993'
A series of common electrolytes were found either to reduce the rate of bubble coalescence in water or to have no effect. For a range of cations and anions, a system of combining rules emerges that characterizes the behavior of all the salts studied. The specific effects of electrolytes on bubble coalescence may be related to their effect on water structure and hence the hydrophobic interaction. The phenomenon may have applications in various fields, including decompression sickness, hydrophobic chromatography, and possibly evolution.
otherwise uncontaminated coastal waters. The transient nature of the effect of salt on bubble persistence is easily observed. On That there have been advances in our understanding of surface shaking pure water, induced air bubbles break up within less forces over the past two decades is generally agreed. Mainly than 1 s, whereas addition of pure, sodium chloride produces a experimental, but theoretical too, those advances have thrown up marked increase in bubble lifetime (up to 10 s). However, it is a whole hierarchy of new forces both long and short range. A well-known that surfactants can produce bubbles (or foam) stable consequence is that the limitations of the older DLVO theory of for hours. In both cases the bubbles are metastable, differing colloid stability can be seen more clearly, and its regime of only in lifetimes. Surfactant adsorption at the air-water interface applicability circumscribed and quantified. Specifically, deproduces both a repulsive force in the water film between pending on the situation, we now have to take into account: (1) approachingbubbles and a surface elasticity. Both of these factors the awareness that the Debye length in unsymmetrical and mixed are accepted as stabilizing soap films or foam. electrolytes can be very different to that usually assumed;' (2) The reasons for the effects of salt are not understood. Salts the very long range attraction between hydrophobic surfaces, generally increase the surface tension of water and are desorbed 10-100 times larger than any conceivablevander Waals f o r ~ e ; ~ , 3 , ~ from the air-water interface. These factors might be expected (3) the depletion force in micellar system^;^.^.^ (4) the related to destabilize bubbles. Salts would also be expected to reduce oscillatory forces due to liquid structure;* ( 5 ) forces due to any electrostatic repulsion produced by charge buildup on bubble fluctuations in correlated surface dipo1es;g (6) ion correlation surfaces. The mechanisms involved in the enhanced bubble effects beyond the Poisson-Boltzmann theory of the double layer stability produced by salts have yet to be elucidated. that can give rise to attractive forces in z:l electro1ytes;"J (7) We report here on a phenomenon about as simple in concept short-range hydration forces between lipids;11J2(8) a secondary as it is dramatic, universal. and ignored. If we imagine bubbles hydration force between mica surfaces due to competitive formed by passing a gas through a glass frit at the base of a counterion adsorption;I3 (9) fluctuation forces between soft column of water, the bubbles fuse on collision and grow in size surfaces like bilayers (the Helfrich mechanism).I4 as they ascend the column. However, on addition of an alkali If short-range surface-inducedstructure (hydration) is ignored, halide beyond a certain critical salt concentration, the bubbles the Lifshitz theory of attraction (van der Waals) forces and will not fuse and remain the same size. That phenomena has repulsive (doublelayer) forces are now part of a unified description been explored by several authors.17J8 On the other hand, of the long-range part of the scheme of things and no clear increasing concentrations of HCI do not have any effect on bubble distinction can bemade between them. But whilegoodagreement coalescence.~9 between theory and experiment can be achieved from more Prompted by that observation, we have explored a whole range sophisticated theories of primitive model electrolytes for inferred of cations and anions in different combinations. The results, as parameters like surface charge, or ion binding constants,ls that we shall see, are surprising. We have found a 'combining rule' agreement is still necessarily dependent on the assumption of which gives rise to the hope that a simple explanation exists, but particular values assigned to assumed hydration ion sizes. Those none is forthcoming. Water structure has to be implicated. values are not universal and depend on the context, a fact that throws doubt and confusion on a vast body of work on specific 2. Materials and Methods ion binding to biological surfaces and proteins adduced from simpler theories. That is probably inevitable until we have a The water used in these experiments was purified by passing proper theory of water itself. Despite such nagging limitations, it through an activated charcoal column and a reverse-osmosis there has been, and perhaps deservedly, a degree of content and membrane, prior to a single-stage distillation. The water was satisfaction at the march of progress. This paper raises again the stored in a laminar flow filtered air cabinet. The salts were of specter that much of our progress is illusory and that the problem analytical grade and, where possible, were roasted before use. of water structure cannot be so easily swept away. The acids were of analytical grade and were used as purchased. What concerns us is the effect of salts in reducing bubble Rather than focus on individual bubble-bubble interactions as coalescence in aqueous solutions. This was first noted in froth have other authors,17we have developed a simple setup that allows flotation systems,where increased efficiency is related to reduced rapid and accurate characterization of the phenomenon. The bubble size.16 The observation of a transient foam produced in system is depicted in Figure 1. An expanded beam of light sea surf has also been attributed to the high level of salt, even in produced by a stable diode laser was passed through a glass cylindrical column, and was then condensed and detected by a f Department of Chemistry. light-sensitive diode. A glass sinter (type 2) was housed in the * Department of Applied Mathematics. base of the column (of inside diameter 25 mm), and high-purity Abstract published in Advance ACS Absfracrs,September 1, 1993.
1. Introduction
0022-3654/93/2097- 10192%04.00/0
0 1993 American Chemical Society
Effect of Electrolytes on Bubble Coalescence
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 10193 110
Expanding Lens
Converging Lens
Figure 1. Light beam emitted by the laser was expanded prior to passing through the bubbling tube and subsequently converged for detection. '-
1
I
I
C n n c c " /M
Figure 3. Results of two experiments with CaC12. The curve is indicative of the reproducibility achieved. .- I
404
.
,001
.
. . ...., 3 1
. .
. .
. ..., 1
. .
. . ..A 1
COnunQnirmM of NeCI
Figure 2. Effect of gas flow rate on the percentage of coalescence.
N2 gas was bubbled through the sinter into the column containing water or aqueous electrolyte solution. The gas flow rate was typically 30 mL/s. The column was set up inside a large perspex container into which N2 was continuously flushed to reduce contamination. The bubble column acts as a flotation cleaning cell which will transfer any surfactant contamination to the top. That the detector signal did not vary over more than 1 h for the case of pure water suggests that no significant contamination was introduced from the nitrogen gas stream. When the column contained pure water, the bubbles produced at the sinter readily coalesced to produce a low density of large bubbles giving the lowest level of scattering of the beam and hence maximum detector signal. The width of the expanded beam had to be sufficiently large to produce an averaged, stable diode detector signal, which was further damped using a graphical storage device. As will be demonstrated later, this system gave stable, reproducible results, quite independent of gas flow rate. To study the effect of each electrolyte, known volumes of concentrated solution were added to the column from the top. Typically, less than 1 min was required for the column to return to a steady state. Addition of an electrolyte produced either no change in the signal (i.e. bubble size/density) or a large reduction and visible opacity in the column. The concentration at which the latter occurred was found to be quite critical and could be accurately reproduced as discussed below. 3. Results
An initial study was carried out on the effect of gas flow rate on the transition observed for a typical electrolyte (NaCl). The results are shown in Figure 2. For this electrolyte the transition occurs in the range 0.02-0.2 M and was found to be quite independent of gas flow rate. The 'percentage coalescence' axis was determined on the assumption that the pure water signal corresponds to 100% and the low, constant signal obtained at
ConccntratidM
Figure 4. Percentage of coalescenceversus concentration for a range of
common electrolytes.
high concentration 0%. For convenience, we define the transition concentration to be that value corresponding to 50% coalescence, in this case 0.078 M. Repeat experiments carried out at the same gas flow rate for CaC12 solutions gave an indication of the reproducibility of the method as shown in Figure 3. The results obtained using this technique for a range of typical electrolytes are shown in Figure 4. It is clear from this that there is a correlation between the valency of the salt and the transition concentration, with a more highly charged salt effective at a lower concentration. In all previous work it had appeared that all electrolytesproduce this bubble coalescence prevention. However, as is clearly illustrated by the results obtained with mineral acids, shown in Figure 5 , this is not the case. Bases such as KOH behave in a manner similar to that of common salts, but the mineral acids HCl and H2SO4 have little or no effect on bubble coalescence, even at concentrations up to 0.5 M. Further study produced some other electrolytes which also behaved like the mineral acids. This complicated situation is summarized in Table I. Earlier researcher^'^ attempted to explain some rather restricted data using a viscosity model, whereby an increase in solution viscosity was believed to produce an increased hydrodynamic barrier, so reducing coalescence. However, the results presented in Figure 6 show the absence of any correlation, with some salts decreasing the solution viscosity but reducing coalescence. The sugars fructose and glucose also significantlyincreasetheviscosity of aqueous solutions, but the viscosity changes have little effect on bubble coalescence when compared with those of salts. We have also studied the effect of temperature on coalescence caused by NaCl and MgS04. The results are shown in Figure 7 and indicate only a slight decrease in coalescence with
Craig et al.
10194 The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
I
I
101
,
. . .. _ _ . ,
.lo ,001
.01
. .
i..
. _.,
I
~
1
0.02
Figure 5. Effect of mmmon acids and base on bubble coalescence.
0.00
0.04
TABLE I
o.oe
0.10
0.12
I
0.14
Tmsilialcmceaartim/M
salt
transition mncn (M) 0.002“ 0.020 0.025 0.03 1 0.036 0.037 0.040 0.046 0.053
HClO4 NaCl KBr
CsBr LiNO, NH4Cl NaNO3 KNO3 KCl CH,COO(CH,)dN NH4NO3 NaC104 NaClO3 Mg(CW2 NH4C104 CHiCOOK CH3COONa CH3COONH4 (CH3C0012Mg HCl HBr
Figure 6. Viscosity of salt solutions relative to water at their transition
concentrations. 100
@OI BO
0.070 0.078 0.083 0.090 0.095 0.100 0.101 0.115 0.120 0.125 0.140
no transition no transition no transition no transition
no transition no transition no transition no transition no transition no transition no transition HNO3 no transition (COOHI2 no transition (CH3)4NCl no transition a Significantly lowers the surface tension of water. temperature. The viscosity of water falls with increase in temperature, and this would suggest a correlation in the opposite direction, that is, an increase in coalescence with temperature. In an attempt to rationalize our results, we have produced a tableof anions and cations that demonstratestheir effect on bubble coalescence (see Table 11). There is clearly a remarkable correlation between the ions present in a salt and their effect on coalescence. We can find a self-consistent system by assigning a ‘property’(aor 8)to each anion or cation, which when combined in the salt ‘predicts’ the coalescence behavior. So far, we have not found any exceptions to this rule! We have also used a range of gases with quite different water solubilities in a further attempt to elucidate the mechanism involved. The results are summarized in Table 111. The sugars sucrose, fructose, and glucose have also been found to affect bubble coalescence. These sugars on addition to water
L . r o . l . , 280
290
.
, 300
.
I
310
.
,
320
.
I 330
Tempawe (K)
Figure 7. Effect of temperature on bubble coalescence. The solutions of 0.09 M NaCland 0.018 M MgS04 are close to their respective transition concentrations.
raise the surface tension and are desorbed from the air-water interface. Thus their effect on bubble coalescenceequally cannot be described in terms of surfactant-like behavior and certainly no charge effectsare involved. Hence, even if an ‘explanation’could be found within the confinesof the primitivemodel of electrolytes, that explanation could not accommodate this observation. The reduction in bubble coalescence achieved with increasing concentration is shown in Figure 8. 4. Discussion
Although previous work had focused on electrolytes which exhibited coalescence inhibition, we have now shown that some other salts and mineral acids have no effect whatsoever. We have developed a phenomenological combining rule which covers a wide range of acids and salts but which has no obvious explanation. For those electrolytes inhibiting coalescence, there does appear to be a correlation with the ionic strength, which brings the results into a relatively narrow band (see Figure 9). Salts containing trivalent ions behave anomalously (more like 2:2 electrolytes), probably due to hydrolysis or incomplete dissociation. However, and to repeat, as yet there is no obvious explanation why some electrolytes produce no effect on coalescence. In seeking an explanation we can definitely rule out any correlation with changes in surface tension at the air-water interface that are responsible for the foaming of surfactant
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 10195
Effect of Electrolytes on Bubble Coalescence
-
- w hC12 NaCI --C KBr -C NMCI KCI
-C
- w
1
CaBr
ConccnuatiowRatioof ionic stnngth (IM)
Figure 9. For electrolytes that inhibit coalescence, all results scale if expressed in units of inverse Debye length.
Combining Rules: aa or pp gives
4
I. Sol-Insufficiently soluble
ap or Pa gives
)(
Unavail=Salt unavailable
Addition of salt: Prevents c o a l e s c e n c e 4 Ha5 no effect on coalescence
x
TABLE IIk The Effect of Gas on Transition Concentration ~~~~
transition wncn (M)
SFs salt NaNO3
KBr CaC12
MgS04
(1.0 mL/L)O 0.085 0.085 0.078 0.032 0.017 0.017
NaCl
0.043 0.027 0.095
a Solubilities
'-
helium (9.2 mL/L)
nitrogen
argon
(23.3 mL/L)
(34.3 mL/L)
0.103 0.108 0.098 0.085 0.036 0.037 0.020 0.020 0.078
0.106 0.087 0.022
given in par en these^.^^.^
I
I
CoMlauionlM
Figure 8. Effect of a range of sugars on bubble coalescence. Note that the effect of sucrme equals the effect of glucose plus fructose. The effect of an equimolar mixture of fructose and glucose lies between the fructose and glucose curves.
solutions. The electrolytes studied increase the surface tension of water.20 Changesin the hydrodynamic forcecausedby viscosity can also be excluded because some electrolytes decrease the viscosity, yet inhibit coalescence.20 The temperature dependence if any is weak. Predictions of the conventional DLVO theory
that underlies the theory of colloid stability go in the wrong direction-added salt reduces repulsive double-layer forces to suggest enhanced coalescence. There is no evidence of significant bubble charging in salt solutions. A different kind of doublelayer force that increaseswith salt concentration can beconsidered. This can come about through the differing cation and anion hydrated size that can give rise to an electrostatic potential at the water surface. Detailed and sophisticatedstatistical mechanical calculationsof this repulsion force have been carried out assuming a range of ion sizes and surface charges.21 However, the predicted short range (C2 nm) and weak magnitude of this force have ruled it out as a possible explanation. Fluctuations in the thin aqueous membrane between two approaching bubbles cannot play a role because the theory predicts that fluctuations increase in magnitude with added salt and favor rather than oppose coalescence.*2 As two bubbles approach at any reasonable rate, there must arise a hydrodynamic repulsive force due to the need to expel water molecules from the film between bubbles. In water, coalescence is observed. Thus, there must exist an attractive force which overcomes the hydrodynamic repulsion. The calculated attractive van der Waals force between two bubbles in water is found to be orders of magnitude smaller than the hydrodynamic repulsion present at reasonable approach rates. As bubbles are highly hydrophobic ( ~ ~ = b72 mJ - m-2), ~ ~it is~ reasonable to assume that the hydrophobic force is present and acts to produce coalescence. Force measurements between solid, hydrophobic surfaces are found to give an attraction of sufficient magnitude to overcome the hydrodynamic r e p u ; ~ i o n ~(see ~.~ Figure 10). This implies that, for salts and sugars to reduce bubble coalescence, the hydrophobic forceof attraction is reduced in their presence. Indeed the limited investigationsof the effect of salt on the hydrophobic attraction between solid ~ u r f a c e s ~ * ~ ~ and between bubbles26 show this. The measured long-range hydrophobic attraction between hydrocarbon or fluorocarbon surfaces in water is 10-100 times larger than any conceivable van der Waals force. The range of this (roughly exponential) force is 1CL-100nm, depending on the surface. However, the mechanism of the hydrophobic force is not understood. It has been suggested that the long-range (>lo nm) interaction involves some type of microbubble cavitation between the approaching hydrophobic surfaces. The formation of such cavities is energetically favorable between hydrophobic surfaces and has been discussed by Yamin~ky.~'Further, spontaneous cavitation has been observed between hydrophobic surfaces at close separation~.~,~* However, the formation of a small vapor cavity is a high-energy process, a circumstance that poses a problem for the cavitation mechanism. A modification of this approach might eliminate this difficulty. In all cases of long-range (>10nm) hydrophobicforce measurements,reported so far, the aqueous solutions were apparently in equilibrium with
101%
Craig et al.
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
Cmccntration/M
m o a "
Figure 10. Comparison of the calculated hydrodynamic repulsion23 (approach rate 1 mm/s) and a typical hydrophobic attraction between highly hydrophobic surfaces.24
the atmosphere. Hence, they contain about 25 mL per liter of dissolved nitrogen and oxygen gas, close to saturation. These dissolved gases might accumulate in the vicinity of hydrophobic surfaces so releasing high-energy water surrounding the gas molecules back into the bulk state. Fluctuations in density near hydrophobic surfaces due to this 'adsorption' of gas could give rise to the long-range attraction. For hydrophobic surfaces in close proximity, this accumulation appears to develop into bubble nucleation which has been observed directly.28 Further support for this model comes from the simple observation of the effect of dissolved gas on emulsion stability. We have found that degassing a mixture of dodecane and water increases the stability of an oil in water emulsion produced by shaking. This simple observation suggeststhat the hydrophobic interaction responsible for oil droplet fusion is reduced by the removal of dissolved gases. It is relevant here to examine what is known about bubble nucleation. The induced nucleation of dissolved gas has been well studied with respect to decompression sicknessor the 'bends'. Nitrogen gas in water does not produce bubble nucleation at supersaturations of less than 180 atm.29 However, the bends occurs with decompressions of less than 2 atm. It now seems clear that the presence of hydrophobic surfaces in the human body enables bubble formation at these much lower pressures. The hydrophobicattraction has also been measured in the presence of surfactants and found to be ~ n a f f e c t e d . ~Hence . ~ ~ phospholipids present within tissue offer no protection. Physiological manifestations of the bends support this hypothesis. Physical exertion before decompression is known to increase the incidence and severity of the bends. Such exertion is likely to mildly damage tissue and increase the number of hydrophobic sites where cavitation is favored. Measurements of the hydrophobic attraction, between methylated silica surfaces, using an atomic force microscope have been performed in solutions of NaCl and NaClO3.3' The attraction was found to be equivalent to that in water in 0.2 M NaC103, while in 0.2 M NaCl, the attractive force was much reduced, supporting our expectation. It has also been observed previously that the range of the attractive force acting between bubbles is substantially reduced from about 100 to 40 nm on addition of KCl above the transition concentration.26 The effect of NaCl on bubble nucleation in the presence of hydrophobic surfaces has also been examined. Excess nitrogen gas was dissolved in solution by equilibration under 25 atm of pressure. Immediately following decompression,the solution was supersaturated with nitrogen gas. In water and 0.02 M NaCl, it was found that bubbles quickly (
