Adhesion versus cohesion in liquid-liquid and solid-liquid dispersions

Adhesion Versus Cohesion in Liquid-Liquid and Solid-Liquid Dispersions S Y D N E Y ROSS Department o~ Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12181

(Received April 22, 1971 ; accepted March 7, 1972) It is found experimentally that, in the absence of specific interactions or electrical effects at the interface, the more stable liquid-in-llquid films are those formed by the liquid of smaller surface tension, which surrounds and separates droplets of the liquid of larger surface tension. A thermodynamic theory postulates that the condition for stability of the film is that the work of adhesion per unit area of interface between the two liquids shall be larger than the specific surface free energy of the liquid of which the film is composed. The deflocculation or flocculation of a dispersed solid in a liquid medium is also shown to depend on the same relation between the work of adhesion and the specific surface free energy of the medium, or, in equivalent terms, on whether the solid particle is "wetted" or not "wetted" by the medium. OBSERVATION S

familiar form: it does not consist of spherical drops of one phase (A) sinking or rising in the medium of the other phase (B), as is found when even low concentrations of an interfacially active solute is present. This pattern is perhaps subconsciously expected. Instead, these unstable mixtures consist of thin films of B carried into A, where they temporarily surround small volumes of A. The clue to recognizing the type of the mixture is to observe the volumes of the liquid phases. Only a minute quantity of liquid B is transferred into phase A, so that its phase volume after agitation is scarcely diminished; the large drops of encapsulated liquid that are visible in phase A cannot, therefore, consist of liquid B, as practically all of B is still to be seen in its original place. Obvious as this reasoning m a y sound, no uncoached person to whom I have shown the effect has interpreted it correctly, although all the evidence required to do so is there to be seen. This would seem to be a useful test by means of which to stimulate the powers of observation of some future Faraday. A different kind of difficulty occurs when an intelligent and interested p a r t y is asked to predict the result of these experiments. An

T h a t two immiscible liquids do not, when pure, form a stable emulsion is well known. With this generalization the subject is usually considered to be exhausted; at least no further inquiry into the behavior of such unstable emulsions appears to have been made. Yet it is of intellectual interest and possibly of some future use to know which of the two possible types of the mixture, that is, whether water-inoil (W/O) or oil-in-water (O/W), is the more stable when two immiscible pure liquids are vigorously mixed together. If such trials are made, for example b y shaking the two liquids together in a sealed glass container, one observes, soon after the agitation is stopped, the instantaneous and often turbulent coalescence of one of the two types of dispersion, followed b y the sedate and leisurely coalescence of the remaining type. The observed behavior is reproducible time after time, and so cannot be attributed to a random process such as tossing a coin. The unpracticed observer finds it difficult to recognize the type of the less evanescent of these unstable dispersions because of its un-

Journal o] Colloid and Interface Science, Vol. 42, No. I, January 1973

52

Copyright ~ 1973 by Academic Press, Inc. All rights of reproduction in any form reserved

A D t t E S I O N VS COHESION

answer sometimes given is that in the more stable type of dispersion, the more coherent liquid (i.e., the one with the larger surface tension) would be expected to be the continuous phase, because of the greater difficulty in subdividing it. This answer is, in some cases, supported by the consideration that the continuous phase is also named, or could logically be named, the coherent phase. But the observed result is the contrary one: In all trials that I have made the less coherent liquid is the continuous phase of the more stable type of dispersion. Thus, for example, for mixtures of water and an immiscible organic liquid that has, as have most of them, a smaller surface tension than water, the residual more stable dispersion is found to be W/O. The results of this research, despite the apparent triviality of its subject, are therefore not immediately obvious. The fallacy of the argument tendered above, since a fallacy there must be, lies in taking as critical the relative ease of subdivision of the two liquids rather than their respective rates of coalescence, which alone determines which of the two types of dispersion is the more persistent. When two drops of dispersed A are near to each other, molecules of the intervening liquid B are simultaneously subject to being pulled away by the homogeneous B-B attraction forces emanating from the medium, and to being retained in place by the heterogeneous A--B attraction forces at the two interfaces of the adjoining drops. If the integrated heterogeneous A-B attraction is weaker than the integrated homogeneous B B attraction, the intervening liquid (B) will retract and the two droplets of A will come together; if the heterogeneous attraction is stronger than the homogeneous attraction, the intervening film will be stabilized and will act effectively as a short-range barrier or repelling force between the drops. Of the two possible types of dispersion obtained when two immiscible liquids are mixed together, the adhesion at the A-B interface is the same for both, but the opposing force due to cohesion of the medium is greater for the liquid of larger surface tension. The

53

type of dispersion that has this liquid as the continuous phase would therefore, on these considerations, be expected to coalesce more rapidly than the opposite type, which is just the result observed in practice. The rule for dispersion type can then be stated as: The more stable of the two possible types of dispersion is the one that has the liquid of smaller surface tension (i.e., the smaller homogeneous attraction) for the continuous phase. The rule is capable of direct experimental test, by shaking together two pure immiscible liquids and observing the type of the more stable of the two kinds of dispersion for comparison with the prediction of the rule. The most important criterion of purity is provided by the liquid-liquid interfacial tension, which is sensitive to the presence of surface-active solutes in either liquid. A reduction of the interfacial tension significantly below that of the accepted value is prima facie evidence of contamination of a kind and to a degree likely to interfere with the prediction given by the rule. Three general classes of binary liquid systems have been used for this test. The f~rst of these consists of water and organic liquids immiscible with water. A list of 88 such liquids, with reported values for each of surface tension and of interfacial tension against water at 20 °, is given by Girifalco and Good (1). The surface tensions of these organic liquids are invariably smaller than that of water, so that the rule predicts that the more stable type of dispersion will be water-in-oil. Eight organic liquids (octanol, benzaldehyde, n-hexane, cyclo-hexane, benzene, toluene, xylene, and carbon tetrachloride), when agitated with pure water, were found to confirm the prediction of the rule. For each of these liquids the measured interfacial tension against water was close to the values reported by Girifalco and Good. A second class of systems consists of two immiscible liquids neither of which is water, such as fluorocarbon derivatives E(C4Fg)20 or (C4Fg)aN~ plus hydrocarbons, halogenated hydrocarbons, etc., or polar liquids like gylcerol or formamide plus hydrocarbons, halo-

Journal oJ Colloid and Inter/ace Science, Vol, 42, No, 1, J~nuary 1973

54

ROSS

genated hydrocarbons, etc. Tests made with formamide plus chlorobenzene and formarnide plus methylene chloride confirmed the predictions of the rule. A third class of systems consists of mercury plus nonmetallic liquids. In these systems the surface tension of mercury is overwhelmingly larger than that of the other liquid. The rule predicts that in every such combination, mercury would be the discontinuous phase. In a test using purified mercury and water, encapsulated water can be seen to coalesce rapidly; also visible are encapsulated mercury drops, which coalesce many times more slowly. Further corroborative evidence for the rule can be drawn from some observations reported about eighty years ago by Gossart (2), who investigated the production of liquid boules in liquid-liquid systems. A flat solid substrate, such as a polished metal or glass, was provided at the bottom of a trough, which was filled with the liquid medium. Drops of an immiscible second liquid were then carefully added, in a manner determined by much previous experience as requisite for the successful formation of a boule. The existence of a boule is evident when the liquid drop is observed to be able to move freely and rapidly back and forth across the solid substrate. Mter a time, which may be too brief to measure at one extreme, to several minutes, even hours in some cases, at the other, the boule flattens out and becomes anchored on the substrate in the form of a sessile drop. The existence of the boule depends on a thin film of the liquid medium that separates it from the substrate. Ultimately the film withdraws, and the boule wets and adheres to the substrate. Gossart found that when the medium is a liquid of larger surface tension than that of the boule, tile duration of the boule was very brief; but when the medium is a liquid of smaller surface tension than that of the boule, the boule would persist for several minutes. The nature of the solid substrate did not appear to affect the result. These findings confirm the conclusion that in the absence of specific interactions or electrical effects at the interface, the more stable liquid-in-liquid films

are those formed by the liquid of smaller surface tension, which surrounds and separates droplets of the liquid of larger surface tension. THEORY

1. Disjoining Pressure

A thin film of liquid that covers a solid or liquid substrate does not throughout its whole depth necessarily retain the same structure as the bulk liquid. Derjaguin and his coworkers maintain that thin films of liquid adhering to substrates are either totally oriented throughout, or are oriented over at least a contiguous fraction of their total thickness, the remainder of the film consisting of unoriented or bulk liquid. If this is true the surface tension of a thin film of adhering liquid would vary with its thickness. Films of dual thickness, such as sessile drops in equilibrium with thinner films of liquid, have been observed and can be explained only as the result of a lower surface tension in the thinner parts of the film. To take this variation into account Derjaguin and subsequent authors (3) have defined a disjoining pressure II as the change of surface free energy with thickness : H

=

B1

T"

Equation [-1] can be applied to a liquid-vapor surface or to a liquid-liquld or solid-liquid interface. Thus, for a liquid-vapor surface, let IILL be the disjoining pressure due to forces of cohesion in the liquid; let ~Ll~be the specific surface free energy of a thin liquid film of thickness h, and let ~L be the (usual) specific surface free energy of an infinitely thick film. Then --

/( // do" =

l~ L Ld h

h

hence ~rLh = crL +

IILLdh.

Again, for a solid-liquid interface, let

Journal o] Colloid and Interlace Science, Vol. 42, No. 1, January 1973

[-23 IIsL

be

ADHESION VS COHESION the disjoining pressure due to forces of adhesion between the solid and the liquid; let GSh be the specific interfacial free energy of the solid when covered to a depth h with only a very thin film of liquid or of adsorbed vapor; let asc be the (usual) specific interfacial free energy when the solid is covered with an infinitely thick layer of liquid; and let c~so be a boundary value, which takes into account the limiting effect of the liquid on the solid substrate. This constant is not to be taken as designating the specific surface free energy of the solid substrate in vacuo but rather the limiting case of ~sh at infinite dilution (h --~ 0) ; the analogy is with the specific conductivity of a solution at infinite dilution, which is not the same as the conductivity of the unperturbed solvent. The total disjoining pressure in the liquid film is taken as the sum of IILL and IIsz. This assumption disregards possible interrelations between them. Then: --

£ f[ dG

( l l z z + IIsL)dh;

E3]

hence

55

quantities, each one representing the specific change of surface free energy for some process significant in the phenomenology of surfaces. These quantities are expressed as follows: S = - A O (spreading)

AG (emersion) = ash -- ~SL =

f: /0 d~ =

O'SL,

(Ilsz + IILL)dh

E83

where S is the final spreading coefficient (4), A0(emersion) is the final change of specific free energy for the process of emersion of the solid from the liquid medium, and I~(adhesion) is the final specific work of adhesion of the liquid to the solid substrate. The barred letters signify that the quantities are for unit area of interfacial surface. If the liquid makes a finite angle of contact with the solid substrate, the Young-Dupr4 equation holds : ~L

=

~L cos0.

s = ~L(cos0 -

hence

[-9]

1),

AG(emersion) = O'L COS0, *~c = ~ s o -

f0 °

(IIsc+

IILDdh.

[-5]

From Eqs. [-4] and E5] crzt~ -- crss =

/;

E7-]

Where finite contact angles exist, the three quantities defined in Eqs. E6-]-[-8] can be expressed in terms of measurable properties of the system, namely, ~L, the surface tension of the liquid, and 0, the angle of contact:

Again, integrating between wider limits, --

(IIsL + II~L)dh,

1~ (adhesion) = c~sh + O'L - -

~sJ~ -

~0 h

/;

(IIsc + IILL)dh.

2. Degrees of Interaction at a Substrale

The degree of interaction of a liquid with a solid substrate is expressed by any one of a number of different but not independent

W(adhesion) = ~z(cos0 + 1).

Do3

[-11-] [12-]

The complete range of behavior is shown schematically in Fig. 1. The field is subdivided into three parts: I. Nonfinite contact angle; II. Finite angles of contact less than 90°; HI. Finite angles of contact larger than 90 °. The behavior of the first two regions is associated with high-energy solid substrates, and that of the third region is associated with lowenergy substrates. In the first region the liquid spreads on the substrate, which can hold a uniform film of liquid that is stable at any

Journal o] Colloid and Interlace Science, VoI,

42, No. i, January 1973

56

Ross

8 cos8

S

r

~G

adhesion

emersion

I

]"

SPREADING

0%--I+1-]~

O---]-

-

+CrL~ i ....

NON-SPREADING WETTING

a~ -(VrSL+7r~0

I

flocculoEon

+20-LJ--

- +0-1. LW'LL

DEFLOCCULATION

I

9 0 %- 0 . . . . .

cr.-- i. . . . . 0

-t . . . .

where the primed symbols are intended to indicate that the expressions refer to the two liquid phases under equilibrium conditions (i.e., when mutually saturated). The relations between the three quantities are:

+O-L. . . . . .

S = ~xG(emersion) -- o-A, = ~-(adhesion)

3

--20.A,. E16~

/

]E

NON-WETTING FLOCGULATION I

FIc. i. Chart of the relative degrees of wetting of solid substrates by a liquid of surface tension at, in terms of various related quantities. The scale of the disjoining pressures has different numerical values and dimensions than those of the other quantities.

thickness. In the second region the liquid "wets" but does not spread; it disproportionates to make an acute angle of contact between a thin film in equilibrium with a thicker lens or sessile drop. In the third region, the liquid neither spreads nor wets the substrate; it forms a sessile drop with an obtuse angle of contact in equilibrium with the unwetted substrate, which is dry save for adsorbed multilayers. The thickness h of the thin film is certainly not the same in the second and third regions of behavior, nor even necessarily constant throughout each region, and therefore o-sh is not constant for all values of 0; hut whatever its value the term cancels out at any 0 in deriving Eqs. [-10]-[-12~. The chart of Fig. 1 is expressible therefore in uniform terms throughout all three regions of behavior. If, instead of considering a solid substrate, we consider the spreading, wetting, or nonwetting of one liquid (A) with respect to another (B) with which it is immiscible, Eq. [-91 is inapplicable But the final (or equilibrium) spreading coefficient, the free-energy change on emersion, and the work of adhesion, can all be evaluated by means of determinable properties of the liquid-liquid system. Thus: S = o-s, -- o-A, -- o-.v~, ; AG(emersion) = ~B' -- ~a,s, ; W(adhesion) = o-s, + 0.~, -- ~a,s,,

[-131 [-14-~ [-15-]

The three quantities can, therefore, still be represented as in Fig. 1, merely by substituting the value of o.A, for that of o-L in the chart. Thus the same chart applies to both solidliquid and liquid-liquid systems, although only the last five scales to the right apply to the latter.

3. Flocculation of Dispersed Particles A fourth process is worth considering, namely, the flocculation of dispersed particles. Flocculation is not restricted to solid particles, though for convenience we may consider a solid-liquid system first. The initial (deflocculated) state would be represented by two dispersed particles of solid separated by an infinite thickness of liquid medium; the final (flocculated) state would be represented by the two particles each with one-half unit area of its interfacial surface in contiguity. Contiguity, for a flocculated system, means that the solid surfaces are separated from each other by a very thin film (adsorbed vapor only) of total thickness 2h. The specific change of surface free energy for flocculation is, therefore,

Jo' 2 /:

(nsL + n~L)dh

zXO(flocc) = -

+

=

(Ilsc + IILs)dh

(nsL + ~LL)d~.

[17]

Equation [17] describes the difference in specific surface energy free between a substrate covered with a liquid film of infinite thickness and the same substrate covered with a very thin film of thickness h. But this is precisely the same as that described by Eq. [7] for the

Journal of Colloid and Interlace Science, Vol. 42, No. I, January 1973

ADHESION VS COHESION

free energy change on emersion of unit area of interface from the liquid medium. Therefore : AG(flocc) = AG(emersion).

[-181

On the chart of Fig. 1, the fourth ordinate axis could be read as the AG of flocculation. This is placed on the chart as the sixth ordinate axis; it indicates that flocculation occurs spontaneously for dispersed particles on whose surface the liquid medium would make a contact angle greater than 90 ° . The third region of behavior, that of nonwetting, where 180 ° > 0 > 90 °, can therefore also be described as the region of flocculation. The turning point between flocculation and deflocculation occurs where l~(adhesion) = ¢~; if the work of adhesion is less than eL the particles will flocculate; if the work of adhesion is greater than eL the particles will be deflocculated. Liquid will spontaneously enter a capillary space only when the angle of contact is nonfinite or, if finite, less than 90 ° . It is evident then that the very thin film of thickness 2h that separates flocculated solid particles is not equivalent to liquid held in a capillary space, as flocculation occurs spontaneously only in the region of surface behavior where the angle of contact is greater than 90 ° , and consequently where capillary liquid could not be held between the solid substrates. Furthermore, in this region the film of the medium held by the solid substrate consists of adsorbed polymolecular layers rather than nucleated liquid. With liquid-liquid systems the same relations hold as shown in Fig. 1, save for the first two ordinates that include contact angles. The criterion for flocculation of liquid drops pivots about a value of the work of adhesion equal to cx, where A' represents the saturated liquid medium, rather than about a value of 90 ° for the contact angle. But flocculation of liquid drops is an immediate precursor of coalescence. The change of surface free energy for the process of coalescence is AG(coal) ~ [-o-h-- aG(flocc)-]AA

=~ABAA, [-191 where ~h is the A B interracial tension when the

57

surface of A is covered with only a thin layer of B, of thickness h. AG(coal) is always negative whatever the sign of A0 (flocc). But if A0 (flocc) is positive the rate of coalescence is greatly reduced. The foregoing considerations provide an explanation of the observations pertaining to stability of boules and of emulsion drops in liquid-liquid systems. For the ordinary case of water and an immiscible organic liquid (oil), the work of adhesion can have a value in either of two areas: it may be such that ~a~o, > l~(adhesion) > ¢olv.

[-20-]

For the condition described by Eq. [201, films of water should not be stable and drops of dispersed oil in water should immediately coalesce; but films of oil should be stable, and drops of dispersed water in oil should not immediately coalesce. The second possible range of values for the specific work of adhesion is such that l/V(adhesion) > ~2o' > O-oi1,.

[-21~

For the condition described by Eq. (21), films of water and also films of oil should be stable; neither oil drops dispersed in water nor water drops dispersed in oil should coalesce immediately. Gossart (2) does not seem to have investigated boules of oil in water corresponding to the conditions of Eq. [-21-]; in all his experiments with oil-in-water boules, the oil boules were immediately destroyed; nevertheless, some organic oils of sufficiently high surface tension could meet the conditions described by Eq. [-21-], leading to relatively stable water films, and such oil boules should not be immediately destroyed. Another way to express these relations is to say that Eq. ['20-] means that the water does not "wet" the oil drop, but the oil "wets" the water drop. If the drop is "wetted" by the medium it is relatively stabilized compared to a drop that is not "wetted" by the medium. Equation 1-21~ means that the water "wets" the oil, and the oil "wets" the water: drops or boules corresponding to W/O or to O/W should

Journal o] Colloid and Interface Science, Vol. 42, No. 1, January 1973

58

ROSS

both be relatively stabilized. For watermercury systems, the work of adhesion between water and mercury is greater than the specific surface free energy of water but less than that of mercury. The water "wets" the mercury but the mercury does not "wet" the water; consequently, mercury boules in water are more stable than water boules in mercury; and concomitantly, water films in mercury are more stable than mercury films in water. All these conclusions are perfectly in accord with the observations of Gossart on liquid-in-liquid boules and of the present author on liquid-inliquid films. Molecules of the medium are subjected to the competition between the forces of cohesion and adhesion only when drops of the dispersed phase are separated from each other by a thin film of intervening liquid. In typical W/O dispersions, as we have seen, the surface of the water drops is wetted by the medium (oil). The resulting retention of a thin layer of intervening medium between adjacent water drops is then effectively equivalent to a repulsive force, extending to distances of a few molecular diameters and opposed to the London dispersion force of attraction between the drops. In typical O/W dispersions, the surface of the oil drops is not wetted by the medium (water) ; the tendency of the medium to retract from the intervening space between adjacent drops at small distances of separation, then effectively acts as an attractive force, which is added to the van der Waals attractive forces between the drops. In both types of liquidliquid dispersion, droplets, whether of water or of oil, are subject to homogeneous A-A or B-B attractive forces, which in both cases lead to their ultimate coalescence; but the mutual repulsion of the water drops, caused by the presence of a metastable film of oil between them, creates an energy barrier sufficient to retard appreciably their rate of coalescence; whereas the mutual attraction of the oil drops, caused by the instability and instantaneous retraction of the water film as soon as the drops approach one another, increases their rate of coalescence. These forces of repulsion

and attraction are not taken into account by the LDVO theory of emulsion stability (5) although they are as significant at the same distances, of separation as the homogeneous van der Waals forces between particles of the dispersed phase. In Padday's (6) recent analysis of the components of the disjoining pressure, the contributions due to cohesion in the liquid film (llrz) and to adhesion between the liquid and the solid (IIsL) are indeed made explicit. Padday commented: "In the absence of electrical charges and of specific interactions riLL must always be negative and HSL always positive." That is to say, the cohesion of the medium tends to an unstable film, and the adhesion of the two disparate phases tends to a stable film. The present report provides the experiential referents that these concepts have hitherto lacked. We can also add to Padday's characterization of these two components of the disjoining pressure, that, in the absence of electrical charges and of specific interactions, then : IIZL = 0 I~SL :

- - 1-~LL

HSL = -- 2IILL

when 0 = 180 °, when 0 = 90 °, when0= 0 °.

The sum IIsL -b rILL is negative

when 0 lies between 90 and 180 °, IIsL q- IILL = 0 when 0 = 90 °, IIsL q- IILL is positive when 0 is less than 90 ° or is nonfinite. The relation between HSL and IInL is included in Fig. 1. The reader should be aware that these quantities are only two of the factors that constitute the whole disjoining pressure, which may also include contributions from the electrical repulsion between particles and the van der Waals or London dispersion forces of attraction between particles, Ilss. This contribution does not affect the value of the contact angle, but it promotes flocculation, especially in the second region of wetting behavior where the offsetting sum of H~n + Ylzn is still small. Finally, in the third region, the

Journal of Colloid and Interface Science, Vol. 42, No. 1, January 1973

ADHESION VS COHESION liquid films are sufficiently thick to ensure deflocculation; but the negative disjoining pressure would not necessarily be sufficient by itself to confer stability even on a deflocculated suspension. A stable dispersion or emulsion usually requires the presence of another chemical component, capable of being strongly adsorbed at the interface. BANCROFT'S RULE

How do these considerations apply when an interfacially active solute is introduced to confer stability on the dispersion of one liquid in another? The surface-active solute reduces the interfacial tension ~A'B' to t , ~ , , thereby indicating an increased molecular interaction between the two liquid phases, hence increased adhesion. Dissolved in water such a solute causes a marked reduction of the surface tension, but it does not affect the colligative properties of the solution, properties such as the vapor pressure, to anything like the same extent. The ability to lower surface tension is a peculiarity of this kind of solute, and does not indicate that there has been a corresponding reduction of the molecular cohesion of the solvent. Hence, neither the cohesion of the water nor of the oil is much affected by the presence of low concentrations of a surfaceactive solute. The work of adhesion of such a solution is given by Eq. [-15~, where o-A, and ~B, retain their meaning as the surface tensions of the mutually saturated liquid phases of the two-component system excluding solute ; #A,B,, however, is the actual interfacial tension as reduced by the solute. The molecular cohesion, likewise, is measured by ~A, and ~B, of the two-component system excluding solute. The net effect of the solute then is to increase l~(adhesion) so that it is larger than either ~A, or ~ , , i.e., the oil can wet the aqueous phase, and the aqueous phase can wet the oil phase; or, phrased in the terminology of disjoining pressures, IILL + IIsc is positive for both kinds of film, O/W or W/O. The superior cohesion o5 water compared to that of oil means that oil films in an aqueous medium have a larger negative free energy of formation than water

59

films in an oil medium : this would slightly favor a W/O type of emulsion rather than the inverse type. But, as we have seen, this by itself is not sufficient to create a stable emulsion for more than several minutes; coalescence of droplets, the result of the homogeneous van der Waals attraction, is merely delayed for a short time. The calculation of the relative magnitudes of the London dispersion forces of attraction between water drops in an oil medium compared to oil drops in an aqeueous medium shows them to be about the same (7). For both types also the probability of like charges being developed at the surface of the emulsion droplets when a surface active agent is dissolved in the medium, is about the same, although the mechanisms of charge separation are different; while the electrostatic repulsion is exerted to greater distance through an oil medium than through an aqueous medium. This factor therefore also tends to act in favor of the stability of W/O types of emulsion. It might therefore seem that the superior cohesion of water to that of most organic liquids, and the nature of the charging mechanisms and electrostatic repulsions in the two types of media, would result in W/O types of emulsions as a general rule. We know this is not so. Bancroft pointed out many years ago (8) that, when an interfacially active solute is present along with two immiscible liquids, then, after agitation, the liquid that is the better solvent appears as the continuous phase. The interfacially active solute must, therefore, function otherwise than merely by its enhancement of the adhesion. It operates to promote stability of an emulsion or a dispersion by introducing a means or mechanism whereby the dispersed droplets or particles are prevented from coming close to one another. Attractive forces, whether they are due to the cohesion of the medium (II~;) or to the mutual van der Waats attraction of droplets, are not significant at sufficiently wide distances of separation. Certain features of the molecular structure of interfacially active solutes automatically provide the requisite mechanism to keep the droplets apart. Such solutes would not have

Journal o] Colloid and InterJace Science, Vol. 42, :No. I, January 1973

60

ROSS

demonstrated their acknowledged usefulness as emulsifying or dispersing agents unless this were so. In an aqueous medium, a preponderantly hydrophilic solute will have either an ionic charge or a polyethylene-oxide moiety, which because of its ability to attach water by hydrogen bonding creates a thick gelated layer on the aqueous side of the interface. The electric charge entails an electrostatic field, by which a force of repulsion for like charges extends outwards from the interface into the aqueous medium. Therefore these hydrophilic characteristics confer on the solute molecule either an electrostatic or a steric field of repulsion that extends farther into the polar than into the nonpolar medium. The converse holds for molecules of a preponderantly lipophilic solute. Here again both steric and electrostatic fields of mutual repulsion are possible. The mechanism of charge separation in nonpolar solvents is now better understood and has recently been reviewed by Fowkes (9). I t depends on the solubilization of counterions by oil-soluble micelles, which are created by the preponderantly lipophilic solute. In nonpolar media, because of the low dielectric constant, the repulsive field of force can extend for a considerable distance from the interface. Steric repulsion in a nonpolar medium is created by the same mechanism as in a polar medium, namely, by adsorption of solute molecules in WATER

01L

!

m m

d

OIL

WATER

m

OIL

C

d

FIG. 3. An hydrophilic solute adsorbed at the oil/ water interface is more effective in keeping apart oil drops in a water medium, (c), than in keeping apart water drops in an oil medium, (d).

an orientation of minimum potential energy at the interface. If solute molecules are represented diagrammatically in terms of their respective fields of force rather than by a literal interpretation of the actual volumes of their atomic constituents, we should expect to see for the preponderantly hydrophilic molecule a larger space occupancy in the polar medium, and for the preponderantly lipophilic molecule a larger space occupancy in the nonpolar medium. The former molecules are therefore more effective in separating oil drops in an aqueous medium than in separating water drops in an oil medium; the latter molecules are more effective in separating water drops in an oil medium than in separating oil drops in an aqueous medium. These comparisons are shown diagramatically in Figs. 2 and 3. They make Bancroft's rule intelligible. The destabilization of an emulsion or a dispersion can be achieved by destroying or diminishing the repulsive field of force created by the interfacially active solute.

c~

DISPERSIONS OF SOLIDS IN LIQUIDS \

FIG. 2. A lipophilic solute adsorbed at the oil/water interface is more effective in keeping apart water drops in an oil medium, (b), than in keeping apart oil drops in a water medium, (a).

The preceding considerations apply also to solid particles dispersed in a liquid medium. These particles are likewise subject to a repulsive force when the adhesion of the medium to the solid substrate stabilizes the liquid film between the particles, or to an attractive force

Journal o] Colloid and Interlace Science,

1973

WATER

h

Vol.

42,

No.

1, J a n u a r y

ADHESION VS COHESION

when the adhesion is not great enough to overcome the retracting force of the bulk medium on the film. The pivotal point, where the net forces just balance, is where the angle of contact of the medium on the solid substrate is 90 °. A solid-in-liquid cannot respond by forming different types of dispersion, as can a system of two immiscible liquids. Instead, the solid-in-liquid dispersion responds by changes in its degree of flocculation. If the angle of contact is less than 90 ° the particles are deflocculated, whether or not an interfacially active solute has been used to raise the adhesion to the necessary level. If on the other hand the adhesion is not great enough to reach this level, or, what is tantamount to the same thing, the angle of contact of the medium on the surface of the solid is greater than 90 ° , or, to say it again in other terms, if the medium does not "wet" the solid, then the dispersed particles respond by a flocculation, which is the more pronounced the higher the contact angle, or the poorer the wetting. As an example of this behavior cite the results of Kopelman and Gregg (10) who measured the floc size of tungsten powder in media of varying polarity, and found that the particles are deflocculated only in polar media whose dielectric constants are greater than a certain limiting value (e > 25). In terms of the present discussion, these findings are interpreted as follows: The oxidized surface of the tungsten particles is wetted (contact angle less than 90 °) only by polar liquids. In such a liquid the adhesion of the medium is enhanced by polar interactions such as hydrogen bonding. In terms of disjoining pressure, IIsL more than overcomes the negative value of IILL: the liquid films between particles are stabilized, the force between particles at close distances of approach is one of repulsion, so that the particles remain deflocculated. At the critical value, where the medium has just sufficient polarity to deflocculate the suspension, HSL + IILL = 0 ; and this condition corresponds to a contact angle of 90 ° . Media of lower polarity

61

interact less strongly with the substrate, thus increasing the contact angle, decreasing the adhesion, and promoting the formation of flocs. The measurement in different media of the volume of the sediment, which is at its minimum when the dispersion is completely deflocculated and begins to increase progressively with increasing degree of flocculation (11) would enable the investigator to determine the polarity of the medium corresponding to the critical point at which the angle of contact is 90 ° . These experimental results are well enough known, although the present attempt is the first to give a unified account of the different behavior of solid-liquid and liquidliquid systems in response to similar conditions of adhesion vs cohesion of the medium. REFERENCES 1. G1RIFALCO,L. A., AND GOOD, R. J., J. Phys. Chem. 61,904 (1957). 2. GOSSART,E., Ann. Chim. Phys. 4, 391 (1895). 3. DERJAGUIN, B. V. AND OBUCHOV, E., J. Colloid Chem. 1, 385 (1935); Acta Physicochim. 1, 5 (1936); DERJAGU1N, B. V., and KUSSAKOV~M., Bull. Acad. Sci. URSS, 471 (1936); ActaPhysicochim. USSR, 10, 25 (1939); DERJAGUIN, B. V. AND SCHERBAKOV, •. L., Colloid J. USSR 23, 33 (1961). 4. HAI~KINS, W. D., "The Physical Chemistry of Surface Films," Reinhold, Ch. 2, New York, 1952. 5. KITCHENER,J. A., AND MUSSELLWHITE,P. R., in "Emulsion Science" (P. Sherman, Ed.), pp. 77-130. Academic Press, New York, 1968. 6. PADDAY, J. F., Spec. Discuss. Faraday Soc. 1, 64 (1970). 7. Fow•xs, F. 1V[., in "Surfaces and Interfaces, I - Chemical and Physical Characteristics" (Burke, Reed and Weiss, Eds.), p. 217. Syracuse University Press, Syracuse, 1967. 8. BANCROFT,W. D., J. Phys. Chem. 16, 179 (1912) ; "Applied Colloid Chemistry," p. 262. McGrawHill, New York, 1921. 9. FowK~s, F. M., in "Solvent Properties of Surfactant Solutions" (K. Shinoda, Ed.), pp. 65-115. NIarcel Dekker, New York, 1967. 10. KOPELMAN,B., AND GREGG, C. C., J. Phys. Chem. 55, 557 (1951). 11. Ross, SYDNEY, AND SCI:IAEF~'ER,H. F., J. Phys. Chem. 58, 865 (1954).

Journal oJ Colloid and Inter~ace Science, Vol. 42, No. 1, January i973