Bubble Coalescence and Specific-ion Effects

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Review

Bubble coalescence and specific-ion effects

Vincent S.J. Craig*

Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University,

Canberra ACT 0200, Australia

Abstract

Major recent advances: Recent advances that contribute to our understanding of specific-ion effects in bubble coalescence include new

theoretical and simulation efforts to determine the arrangement of ions at interfaces and a clearer recognition that specific-ion effects in

bubble coalescence are related to many other phenomena that exhibit ion specificity.D 2004 Elsevier Ltd. All rights reserved.

Keywords: Bubble coalescence; Specific-ion effects; Electrolytes; Thin film stability; Hydrophobic; Hofmeister; Nanobubbles

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

2. The importance of ion specificity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

3. Bubble–bubble coalescence in the presence of surfactant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

4. Films between two bubbles in the absence of surfactant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

5. Films formed between bubbles and solid surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

6. Specific-ion effects and bubble stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

1. Introduction

Perhaps, one of the most surprising and easily ob-

served examples of a macroscopic effect of ion specificity

is the influence that dissolved electrolytes have on bubble

coalescence in aqueous solutions. Above a certain con-

centration, which is dependent on the particular salt

chosen (typically f0.1 M), many common electrolytes

inhibit the coalescence of bubbles, whereas others have no

influence on bubble coalescence [1,2..]. This simple

observation is not understood. Here, we will attempt to

determine if the ions influence coalescence by action at

the interface or in bulk and investigate their possible

influences on film rupture.

2. The importance of ion specificity

The Derjaguin, Landau, Verwey and Overbeek (DLVO)

theory of colloidal interactions has been the dominant

paradigm for 50 years. This theory generally works well

under the circumstances for which it was intended, low salt,

inert surfaces and interactions at separations greater than a

few nanometers. Under these conditions, the charge on an

ion, rather than the particular type of ion, is important.

Therefore, the theory explicitly excludes specific-ion effects

and can largely be seen as a ‘‘chemistry-free’’ regime.

However, since the work of Hofmeister in the 19th century,

it has been known that the nature of an ion can have a very

great influence on the stability of a colloidal dispersion,

particularly at high salt concentrations. At concentrations

exceeding 0.1 M, the range of the electrostatic interactions is

greatly reduced, and specific-ion effects dominate. There-

fore, interactions in biological systems [3.], as well as

complex fluids [4] and slurries [5], often cannot be under-

1359-0294/$ - see front matter D 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cocis.2004.06.002

* Tel.: +61-2-6125-3359; fax: +61-2-6125-0732.

E-mail address: [email protected] (V.S.J. Craig).

www.elsevier.com/locate/cocisCurrent Opinion in Colloid & Interface Science 9 (2004) 178–184



stood within the DLVO framework, but rather, the specific

nature of the ions is important. Our understanding of ion

specificity has barely progressed from the empirical work of

Hofmeister more than 100 years ago, with the terms ‘‘ion

size’’ and ‘‘polarisability’’ giving names to our ignorance, in

the absence of an ion-specific theory. Without theoretical

predictions, molecular recognition, drug design, formula-tions science and protein crystallization will remain largely

empirical arts.

Whilst there are a great number of systems that

exhibit ion specificity, attempts to investigate these

effects are challenging. Currently, short-range interactions

cannot be adequately measured for most systems, par-

ticularly when soft surfaces are involved. Therefore,

efforts necessarily concentrate on indirect methods. Stud-

ies in protein solubility, complex fluids, enzyme action,

surface tension, pH and bubble coalescence have pro-

vided a great deal of evidence against which theories

can be tested. The challenges for theoreticians are

extensive, requiring recognition of the granularity and

chemical nature of the solvent, the geometry and nature

of the surfaces, inherent nonlinearity and an appropriate

description of the ion, which may include ion size,

polarisability, solvent interactions and ion geometry,

amongst other descriptors. Simulation studies must con-

tend with all this and find means to deal effectively

with a range of length scales that extend from the

classical down to the quantum regime. Underlying all

this is a belief that a theory of ion specificity should be

able to describe all of the very diverse specific-ion

effects, such as Hofmeister, anti-Hofmeister behavior,

strong co-ion influences and competition between ions.Therefore, a theoretical description that is adequate in one

system may lead to an increased understanding of other

systems, even if they do not exhibit the same ion-specific

effects. If this is true, the apparently simple system of

two bubbles colliding in an aqueous electrolyte solution

may be useful in elucidating the nature of short-range

interactions.

Over a decade ago, we described simple combining rules

that predict the bubble coalescence behavior of electrolytes

based on assigned properties of the ions that make up the

electrolytes [1,2..]. Ions were empirically assigned a type, a

or h and the combination of ions present in the added

electrolyte determined, if the solution would inhibit bubble

coalescence. Bubble coalescence was inhibited relative to

pure water with an aa or hh combination and unchanged

when an ah or ha combination was employed (see Fig. 1).

As yet, no exceptions have been found to these rules,

although it has been claimed that NaClO4 inhibits bubble

coalescence at very high concentrations [6]. However, at

such high concentrations, the level of contaminant ions is

likely to be sufficient to produce coalescence inhibition

independently.

What does the existence of these combining rules indi-

cate? Primarily, it enables us to separate the ions into four

classes; a anions, h anions, a cations and h cations. We

should expect a fundamental difference between a and h

anions and, similarly, between the a and h cations. How-

ever, the labeling does not necessarily mean that a cations

and a anions or h anions and h cations share a common-

ality, although this is possible. The combining rules tell us

that it is not the absolute behavior of a single ion that matters but the combination of ions. In my opinion, this is

an important and significant point that has often been

overlooked. The behavior is inherently nonlinear and spe-

cific to both ions present. Therefore, any suitable description

must identify a property or properties that separates the a

from the h ions and describe a means by which this property

or properties, in combination, can switch the coalescence

inhibition on or off. Ideally, in time, our ignorance will pass,

and the terms a and h will be replaced by terms that

describe the appropriate property. Clearly, the combining

rules provide a very strong test of any theory.

3. Bubble–bubble coalescence in the presence of

surfactant

At this stage, we still do not have a clear understanding

of the influence of electrolytes on bubble coalescence.

Indeed, our ignorance is deeper than it first appears, as we

do not fully understand the coalescence process that takes

place when two air bubbles collide in pure water. In the

following, I will examine what is known of this process.

During the collision of two bubbles, a film of liquid is

formed between the two gaseous phases. The propensity of

the film to drain and rupture ultimately determines theoutcome of the collision process, and whilst it is obvious

that these two processes are not independent, they are often

treated separately for convenience, with the acknowledg-

ment that once the film has drained below a certain

thickness, then film rupture is possible, and at another

thinner film dimension, film rupture is often assured.

Drainage is often treated within the Reynolds model of

two circular parallel plates separated by a fluid, although

under some conditions, it is known that dimpling of the

interfaces occurs [7. –9..]. Additionally, the influence of

interfacial forces must be taken into account.

The case of a thin film stabilized by a surface active

agent is relatively well understood. For films that are

electrostatically stabilized, the double-layer repulsive sur-

face forces act to provide a disjoining pressure that slows or

prevents film drainage. The film thickness increases with

the range and magnitude of the repulsive force, as does its

stability to rupture. Addition of electrolyte screens the

repulsive force and allows a thinner film to form. As

drainage proceeds, the film becomes sufficiently thin, such

that the rupture of the film becomes likely. Thermal dis-

turbances cause surface waves that can grow under the

action of attractive Van der Waals forces. If these waves

grow sufficiently, the liquid film will be pinched off, and

V.S.J. Craig / Current Opinion in Colloid & Interface Science 9 (2004) 178–184 179





the hydrophilic system [19..] does and therefore cannot be

explained by a reduction in electrical double layer repulsion.

The degree of surface hydrophobicity need not be great;

generally, a contact angle in excess of 45j is sufficient to

lead to an unstable film [8..]. Clearly the hydrophobicity of

the surface plays an important role in the film stability,

although a direct correlation with the contact angle has not been shown [8..].

Films formed between a bubble and a hydrophilic solid

surface act similarly to films formed between two bubbles

formed from surfactant solution. Their behavior is well

understood in both cases. Films formed between a bubble

and a hydrophobic solid surface are unstable, unless

electrolyte is added, as is the case for films formed

between two bubbles in the absence of surface-active

material. In this case, our understanding is poor, but

given the similarity of behavior, it is reasonable to suggest

that the same destabilisation mechanism is operating in

both cases. The evidence suggests that the hydrophobicity

of the substrate is important to film rupture. Previously

[1,2..], we have proposed that an uncontaminated air–

water interface is a strongly hydrophobic surface, and this

has been supported in later studies [20]. Therefore, it is

reasonable to assume that a film separating two bubbles

will act similarly to a film confined between a bubble and

a hydrophobic solid surface.

Direct force measurements between hydrophobic surfa-

ces are often characterized by a strong, long-range attractive

interaction. The range of the measured force varies consid-

erably and appears to be strongly dependent upon the

surface preparation method employed rather than the contact

angle at the surface. This suggests that the hydrophobicity of the interface is not the determining factor in the magnitude

of the interaction. Many theories have been proposed to

explain the interaction, but none are completely satisfactory

in terms of explaining its range and magnitude. An extensive

review by Christenson [21.] is recommended to the inter-

ested reader.

In many systems where a hydrophobic attraction is

measured, it can be argued that ‘‘nanobubbles’’ are present

on the surface and that these nanobubbles play an important

role in the attraction and often give a false impression of the

range of the attraction. The separation is determined exper-

imentally as the solid–solid separation distance, when, in

fact, the solid– nanobubble or nanobubble –nanobubble sep-

aration may be considerably less, depending on the size of

the nanobubbles. There is clear evidence of the existence of

nanobubbles on some hydrophobic surfaces [22– 24]. In the

opinion of the author, many published measurements can be

attributed to the presence of nanobubbles, but there exist

several measurements between hydrophobic surfaces that

exhibit a hydrophobic attraction in circumstances where

nanobubbles are absent. For example, Ishida et al. [23.]

measured an attraction larger than is expected based on Van

der Waals forces, between hydrophobic surfaces that had

never been exposed to air and where the absence of nano-

bubbles was assured by AFM examination. In the absence

of nanobubbles, the long-range attraction measured can be

considered the true long-range hydrophobic attraction.

Mahnke et al. [8..,9..] have elegantly demonstrated that

the film separating a bubble and a hydrophobic surface

ruptures at separations exceeding 40 nm in the absence of

nanobubbles. They attribute this to the action of hydrophobic forces. There is also evidence that the drainage rate is

faster on a hydrophobic surface [7.], suggesting that an

additional attraction may be present, providing further

evidence for a hydrophobic attraction between a bubble

and a hydrophobic surface.

Some time ago, we suggested that the hydrophobic

attraction may be operating between colliding bubbles to

produce coalescence [1,2..]. Let us examine this in more

detail. If a hydrophobic attraction (of unclear origin) that is

considerably larger than the Van der Waals interaction is

acting between bubble surfaces, this may play a role in the

destabilization of the aqueous film. First, we will consider

the capillary wave mechanism. Calculations reveal [11.] that

the range at which films rupture is consistent with capillary

waves, but the measured time of rupture is not sufficient if

the interfaces are considered immobile. However, the pres-

ence of an attractive force much larger in magnitude and

range than the Van der Waals interaction will lead to more

rapid growth of capillary waves and a large decrease in

rupture time. Therefore, the capillary wave mechanism of

film rupture driven by the hydrophobic attraction may be

valid in the pure water system.

Let us consider nucleation as a rupture mechanism.

The nucleation of a vapor phase between two nonwetting

surfaces at separations below 500 nm is energeticallyfavourable, and in the presence of dissolved gas, the

range at which nucleation is possible is extended consid-

erably [25.]. However, there is a large activation barrier

to nucleation of a gas phase between hydrophobic surfa-

ces, unless the surfaces are at a very small separation.

When the hydrophobic surface is itself a bubble, the gas

within the bubble is able to enter the film, and this

should reduce the barrier to nucleation. The amount of

gas entering the liquid film could be increased by a

strongly attractive force; therefore, it is feasible that the

hydrophobic attraction could increase the range and

probability of film rupture through nucleation. Further-

more, the origin of the hydrophobic attraction may be

related to the metastability of the intervening film, but

until the mechanism of the attraction is understood, any

relationship is speculative.

If one accepts a role for the long-range hydrophobic

attraction in bubble coalescence in aqueous systems, a

similar force must be proposed in other pure liquids, as

stable films cannot be produced in any pure liquid. Meas-

urements have revealed a similar, long-range interaction in

other liquids [26]; however, these forces were attributed to

the presence of nanobubbles, and other liquids studied did

not exhibit a long-range attraction.




In the absence of solute, Gibbs Elasticity will be absent

and Marangoni effects minimised. One could therefore

argue that capillary waves could grow undisturbed or

mechanical disturbances could have catastrophic effects on

film stability; however, the considerable film stability ob-

served between bubbles and hydrophilic substrates in pure

water are strong evidence that this is not the case. We havealready indicated that if the interface is mobile, film thinning

progresses more rapidly and this can lead to short rupture

times. A highly mobile bubble–aqueous interface could be

invoked to explain the short rupture times of films between

a bubble and a hydrophobic surface. However, films formed

between a bubble and a hydrophilic surface are stable for

long periods. In both systems, the bubble–aqueous interface

is identical and equally mobile, hence, this is clearly not the

dominant influence. Therefore, the rupture of films between

a bubble and solid surface or between two bubbles in pure

water remains unresolved.

6. Specific-ion effects and bubble stability

Any discussion of the role of electrolytes in bubble

stability is going to be speculative given our lack of

understanding of the coalescence process in pure water.

Electrolytes have only a little [27] influence on bubble

coalescence up to the transition concentration (typically

f0.1 M for a 1:1 salt). Therefore, the electrostatic

double layer is highly compressed at the concentrations

that concern us. The challenge then is to resolve how

the short-range effect of electrolyte can influence the

bubble coalescence process that occurs at separations of approximately 100 nm. We must determine if the in-

fluence of electrolyte is at the interface or if it is a bulk

phenomenon.

In the past, we have proposed that electrolyte may reduce

the range of the hydrophobic attraction between two bubbles

sufficiently to prevent film instability [1,2..]. However,

subsequent measurements of the hydrophobic attraction in

concentrated salt solutions showed that the attraction is not

reduced by the addition of salt [28]. Therefore, we can

abandon this hypothesis. Two other possibilities will be

investigated here: the possible damping of the growth of

capillary waves by electrolyte, thereby preventing film

rupture, and film rupture by nucleation being reduced in

the presence of electrolyte.

Both surface elasticity and surface diffusion [29.] will

oppose the growth of surface waves. Gibbs [30] has defined

the surface elasticity as

E ¼ 4C22ð1Þ

dl2

d M 2

M 1

ð1Þ

where E is the Gibbs Elasticity, the suffixes 1 and 2 indicate

components of the film, the latter being in excess, C is the

surface excess, l is the chemical potential and M is the total

quantity of material per unit area of the film. The Gibbs

Elasticity is also written in the form [31],

E ¼

4c

dcdc

2

k BTD ð2Þ

where c is the concentration, c is the surface tension, k B is

Boltzmanns constant, T is temperature and D is a poorly

defined measure of thickness. Note that surface elasticity is

often interpreted as arising from an increase in surface tension

when an expansion of the interface leads to a reduction in the

amount of adsorbed material at the interface in the presence of

a surface-active component. However, it equally applies

when the solute is depleted from the interface, as is the case

with many electrolytes (surface tension increases with con-

centration); thus, the mechanism giving rise to elasticity is not

a local increase in surface tension, as is often described.

Inspection of Eq. (2) reveals that the magnitude of the surface

tension change with concentration of solute is important, but

not the sign. Indeed, a correlation between the transition

concentration at which bubble coalescence is prevented and

the magnitude of the surface tension gradient has long been

recognized [31–34..], suggesting that the elasticity of the

film during rapid stretching associated with film thinning, or

capillary waves, is crucial in the prevention of bubble

coalescence. However Stoyanov and Benkov [29.] argues

that the Gibbs Elasticity disappears from the equations that

describe the drainage and hydrodynamic stability of thin

films and that surface diffusion dominates the behavior. Like

the Gibbs Elasticity, the drainage velocity determined using

the surface diffusion approach [29.

] is also approximately proportional to (dc/dc)2. Thus, the observed experimental

correlation with (dc/dc)2 may arise from the diffusion of

solute in the thin film rather than from Gibbs Elasticity. The

correlation with (dc/dc)2 also includes electrolytes that have

no influence on bubble coalescence, as they generally have

lower values of (dc/dc)2 [31]. Values of (dc/dc)2 below f1.0

(mN2 m 2 M 2) indicate no bubble coalescence inhibition.

Large values of (dc/dc)2 may dampen capillary waves and

reduce film thinning and rupture. However, the coalescence

inhibition of some salts is not described by their influence on

surface tension. The tetramethylammonium acetate electro-

lyte is a strong test of the validity of the combing rules, as it is

a hh salt and bubble coalescence is prevented. However, the

(dc/dc)2 value of 0.25 predicts that it will have no effect on

bubble coalescence. Sodium acetate has a (dc/dc)2 value of

2.1, yet, has no significant influence on bubble coalescence;

thus, the value of this term as a true indicator of the role of

electrolyte must questioned.

Perhaps then, electrolytes inhibit rupture by preventing

nucleation in the film separating bubbles. It is known that

most electrolytes have an electrorestrictive influence on

water, causing a contraction in the volume occupied by

water upon the addition of ions. It is possible that electro-

restriction is accompanied by an increase in the cohesiveness

V.S.J. Craig / Current Opinion in Colloid & Interface Science 9 (2004) 178–184182



of water. However, no reasonable correlation can be found

with electrorestriction. A reduction of gas concentration in

the film could reduce the probability of rupture. Indeed, a

possible influence of dissolved gas on bubble coalescence

has been raised [1,2..,33.,34..]. Ions are known to ‘‘salt

out’’ dissolved gas, and the electrolytes that more strongly

salt out gas are known to effect bubble coalescence inhibitionat lower concentrations [20,33.,34..]. The diffusivity of the

sparging gas has also been related to t he amount of electro-

lyte required to prevent coalescence [35]. However, suffi-

cient data on a complete series of electrolytes are lacking to

fully test these correlations. These data would be useful, as it

is possible that a reduction in the amount and ease with

which gas enters the aqueous film separating bubbles could

inhibit nucleation and film rupture. A possible mechanism

for rupture in the absence of electrolyte is the migration of

gas molecules into the liquid film under action of an

attractive force. If electrolyte prevents or reduces this mi-

gration, film rupture could be arrested.

7. Conclusions

The bubble coalescence process in aqueous electrolyte

solutions remains unresolved. The mechanism of film rup-

ture remains unclear, and the means by which ions influence

coalescence behavior remains elusive. Additionally, the

influence of both ions in combination, as described by the

combining rules, remains a major challenge. However,

recent efforts at understanding the specific-ion interfacial

behavior at high salt concentrations may provide important

clues. A recent suggestion by Marcelja (personal communi-cation and in this issue) focuses on the behavior of ions at

the interface. The recent work by Jungwirth et al. [36.. –38.]

and Ninham et al. [3.,39.,40..], which indicates that some

ions are attracted to the interface despite the image charge

repulsion, has inspired his interpretation. Marcelja proposes

categorizing ions based on their preference for the surface or

the bulk. The beauty of this proposal is that it naturally

incorporates a combining law where the influence of an ion

is dependent upon the nature of the other ions present. Thus,

ions that are both located in the bulk or both at the interface

will have little effect, but ion combinations that are located

in the bulk and at the interface will prevent bubble coales-

cence. Whilst in its infancy, this is an exciting proposal that

can be tested using mixtures of electrolytes. This permits a

tentative model to be proposed. Ion combinations that are

separated at the interface result in a reduction in the mobility

of the interface. The reduced mobility ensures that thermal

capillary waves grow less quickly, leading to an increase in

the stability of the film. At a sufficient concentration of ions

in bulk, the surface mobility is suppressed to an extent that

bubble coalescence does not take place within the lifetime of

a collision. Another related challenge awaiting experimental

attention is an investigation of electrolyte effects on bubble

coalescence in nonaqueous solvents. I propose that specific-

ion effects will again be important, but due to the different

properties of the interface, the influence of ion combinations

may be very different from one system to the next. Such a

study may see ion-specific behavior relevant to biological

systems.

Acknowledgements

Discussions with Barry Ninham, Stjepan Marcelja,

Hakan Wennerstrom and Pavel Jungwirth have been

illuminating. The assistance of Chiara Neto and Mika

Kohonen in the preparation of this manuscript is appreci-

ated. I would like to acknowledge the support of the

Australian Research Council, through the provision of a

research fellowship.

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Bubble Coalescence and Specific-ion Effects