604
Langmuir 1993,9, 604613
Foam Breakdown by Hydrophobic Particles and Nonpolar Oil Robert Aveyard,’??Philip Cooper,$ Paul D. I. Fletcher,? and Christine E. Rutherford? School of Chemistry, University of Hull, Hull HU6 7RX, U.K., and RhSne-Poulenc Chemicals, P.O.Box 3, Beverley HU17 ONW,U.K. Received July 20, 1992. In Final Form: October 28, 1992 Foam breaking and the inhibition of foam formationare important procaeses industrially. Foam reduction is often achieved by the addition of a dispersion of hydrophobic particles in nonpolar oil to potentially foamingsystems. Both particles and oil alone may be capable of foam breaking, but they act synergistically when present in combination. A number of possible mechanisms for foam breaking have been proposed in the literature involving, variously, surfactant adsorption by particles, particles acting aa supporta for oil dropleta, oil acting aa a medium to distribute particles along lamellae surfaces, and effecta involving contact angles between surfactant solutions and solid particles. The overall aim of the present work hae been to assess in a methodical way, and for a range of systems,the likely applicability of a number of the possible mechanisms for foam breaking in the systems investigated. We have studied foaming behavior of aqueoussolutionsof three surfactants (SDS,CTAB, and AOT) in the presence and absenceof hydrophobic particles (paraffin wax, PTFE, and ethylenebis(8tearamide)) and dodecane. The effectivenessof oil alone on reducing foaming and single film lifetimes is discussed in terms of entry and bridging coefficients for dodecane with the surfactant solutions. The role of contact angles of surfactantwith the solids, with and without dodecane present in the system, is assessed, and the effect of surfactantadsorption by the particles on their effectiveness in foam reduction is also probed.
lamellae surfaces can be of great importance. Particle Introduction shape is known to be crucial in foam breaking,’t6J as is the Foaming of aqueous surfactant solutionscan be reduced surface roughness of the particles.8 Some polar hydroby the presence of water-insolubleoils, hydrophobic solid phobic solids, such as ethylene bis(8tearamide) (EBS), particles (of dimensions comparable to foam lamellar which is used as a component in commercial antifoams thickness, say between 1 and 10pm)and (more effectively), and has been studied in the present work, can also spread by a combination of oil and particles together as in many on aqueous surfactant solutions,causing tension gradients commercial antifoam formulations.’ which could contribute to the antifoam properties. KulkarOil drops present in a foaming solution may, if they are ni et have suggested that hydrophobic particles can capable of entering lamellae surfaces, spread macroscopadsorb surfactant rapidly from the foam films, resulting ically over the surfaces causing film thinningand rupture.’P2 in surface stresses which rupture the films. Presumably, Nonetheless,it is known that nonspreading oils can often for this mechanism to operate, the rate of adsorption by be effective antifoam agents. In this case, if the various the solid must exceed the rate of replenishment of interfacial tensions have appropriate values, an oil drop surfactant by adsorption from “bulk” solution and also can act by forming an unstable bridge between the two cause film rupture before Marangoni flow into the region surfaces of a lamella, causing rupture in this way.3 of increased film tension can cause film thickening. However, even if entry of a drop into a film surface is Of great importance commercially is the synergistic feasiblethermodynamically, the entry process may be slow, antifoam action of hydrophobic particles and oil together. depending on the rate of thinning of the thin aqueous film Kulkarni et al.*I1 suppose that the role of the oil (in which between the drop and lamella ~urface.~ the particles are dispersed) is to spread along the films, Hydrophobicparticles can also bridge lamellae surfaces, thus transporting particles to the film surfaces; as already and if the contact angle which the foaming solution makes mentioned, these authors supposed the particles then with the solid is sufficientlyhigh (dependingon the particle caused film rupture by surfactant adsorption. Another geometry), this can lead to film rupture as the particle possible cause of synergism has been proposed by Frye becomes dewetted by the aqueous ~ u r f a c t a n t .Frye ~ ~ ~ ~ ~ and Berg.I2 They suppose that the particles, which are andBerg7point out that effective antifoam particles must assumed to be completely wetted by oil in the presence be able to produce rapid film rupture, and they carried of aqueous surfactant, simply ensure that the “oil drops” out an analysis of film thinning and particle dewetting (i.e. oil-coated particles) are in the correct size range to rates. It was further emphasized by these authors that effect film rupture by a bridging mechanism. Thisrequires during foam formation, conditions can be far from that the contact angle of aqueous surfactant with solid equilibrium and that the rate of surfactant adsorption to immersed in the oil be M O O . If the angle is less, particles will be expected to rest in the oiVwater interface rather + University of Hull. than being fully immersed in the oil. Dippenaar6 and Rhhe-Poulenc Chemicals. Garrett’ have proposed explanations of the synergistic (1) Garrett, P. R. Book chapter entitled Mode of Action of Antifoams, inpreee. TheauthorsaregratefultoDr.Garrettforsightofthemanwript prior to publication. (2) Rose, S. J . Phya. Colloid Chem. 1950, 54, 429. (3) Garrett, P. R. J. Colloid Interface Sci. 1980, 76, 587. (4) K m o , K.; Lobo, L. A.; Waean, D. T. J. ColloidInterface Sci. 1992, 150, 492. (5) Garrett, P. R. J. Colloid Interface Sci. 1979,69, 107. (6) Dippenaar, A. Int. J . Miner. Procesa. 1982,9,1. (7) Frye, G. C.; Berg, J. C. J. Colloid Interface Sci. 1989, 127, 222.
(8)Aronson, M. P. Langmuir 1986,2,653. (9)Kulkani, R. D.; Goddard, E. D.; Kanner, B. Ind. Eng. Chem. Fundam. 1977,16,472. (10) Kulkani, R. D.; Goddard, E. D.; Kanner, B. J. Colloid Interface Sci. 1977, 59, 468. (11) Kulkani, R. D.; Goddard, E. D.; Kanner, B. Croat. Chem. Acta 1977, 50, 163. (12) Frye,G. C.; Berg, J. C. J . Colloid Interface Sci. 1989, 130, 54.
0743-7463/93/2409-0604$04.00/0Q 1993 American Chemical Society
Foam Breaking
Langmuir, Vol. 9, No. 2, 1993 605
effect of oil and particles in terms of contact angles. A useful review of variouspossible mechanisms for antifoam action has been given by Pe1t0n.l~ The work described in the present paper is concerned with assessing the likely origins of the effectsof a nonpolar oil (dodecane)and hydrophobic particles (wax, EBS, and PTFE), both alone and in combination, on foaming and on single film lifetimes of aqueous solutions of three surfactants (cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Aerosol OT (AOT)). We have considered the spreading behavior of dodecane and of EBS on aqueous surfactant solutions. From measured values of appropriate interfacial tensions, the possibility of foam reduction caused by lamellar bridging by oil droplets is assessed. From these tensions, together with values of contact angles of aqueous surfactant solutionswith the solids, both in air and under dodecane, it is possible to estimate the extents of surfactant adsorption by the solids. The contact angles also allow an assessment of the bridging-dewetting mechanism for foam reduction in the systems studied.
Experimental Section Materials. Three surfactants were used sodium bis(2ethylhexyl) sulfosuccinate (AOT) supplied by Sigma; cetyltrimethylammoniumbromide (CTAB)from F l u b , sodium dodecyl sulfate (SDS) from Fisons Scientific. All had a claimed purity of >99 % . The CTAB, on which most of the measurements were made, was extracted with diethyl ether and recrystallized twice from 5050 acetone/methanol mixtures and finally from ethanol. The purified sample showed no minimum in the tension-ln concentration plot (consistent with surface chemical purity) and had a critical micelle concentration (cmc) in water of 0.9 mM. The AOT and SDS were used as received. AOT showed no minimum in tension around the cmc, which was found to be 2.3 mM from the tension data. The SDS showed a shallow minimum in tension and the cmc was estimated to be approximately 8 mM. Dodecane was supplied by Aldrich with a claimed purity >99.9 % . It was passed through chromatographicaluminashortly before use. Water was distilled in a laboratory still, passed through an Elgastat ion exchange column, and finally through a Milli-Q reagent water system. All samples used had a surface tension of 71.9 f 0.1 mN/m at 298 K. The solid particles used in the study were ethylene bis(stearamide) (EBS, of the grade used in commercial antifoam formulations),paraffin wax (supplied by BDH), and PTFE (DLX 6000, Du-Pont). The wax (previously cooled) and EBS were subdivided in a micropulversier for 5 min. The particle sizes of the wax and EBS treated in this way were generally below 200 pm, although size distributions were not obtained. The primary particle size of the PTFE was around 1pm, but agglomerates up to 150 pm in size can form. The axial ratios of all the particles used were small, as estimated microscopically. In the context of the experiments performed, the particle sizes and size distributions do not appear to be crucial. Foam reductions given by addition of 2 mg of particles to 10 mL of surfactant solution are similar for all three solids with all the surfactants (with the exception of the effect of PTFE on AOT solutions in the absence of dodecane) as can be seen in Figure 9. The EBS particles used in measurement of spreading rates (see below) were separated into size ranges using Endecott sieves. For contact angle measurementsTeflon was obtained as smooth sheets and cleaned in chromic acid before use. EBS pellets were melted onto glass slides and allowed to cool, and paraffin wax was compressed into smooth disks under 10 tons pressure. Contact angles of water with PTFE and paraffin wax were 112O and llOo, respectively, in excellent agreement with values reported by Dann.14 Methods. Contact Angles. Contact angles of surfactant solutions with smooth solid surfaces were determined using a (13)Pelton, P. Pulp Pap. Can. 1989,90 (2), T61. (14) Dann, J. R. J. Colloid InterfaceSci. 1970,32, 302.
Rcnl to penetrate film
-4
, 1
I
Surfactant solution
Sinter to retain dmplets from direct contact with film Padding to reduce vibrations
Solution surface
Closed faces
I Frame immersed below solution surface
Figure 1. (a) Apparatus for studying film lifetimes. (b) Glass frame for studying effects of additives incorporated into only one side of a soap film. contact angle microscope (Kruss Model Gl). For measurements a t the solid-air interface a drop of liquid was added to the solid surface using a microsyringe. The volume of the drop was subsequently increased and the drop then left for 10 min before a measurement was made. New drops (five or more for each solution) were formed at different locations on the surface. The reproducibility of contact angle for a given system was generally f 2 or 3O by this method. All experiments were carried out at ambient room temperature. For contact angles of aqueous solutions with solids immersed in dodecane, the solid plate was placed in the oil in a glass cuvette. A drop of aqueous solution was then formed under the oil and allowed to fall under gravity onto the solid surface. Spreading Rates. The rates at which EBS spreads from particles added to the surface of a surfactant solution were determined using a Langmuir trough (Joyce-LoeblMinitrough). The film barrier was a constant perimeter Teflon tape and surface pressures were measured using a filter paper Wilhelmy plate. A weighed amount of solid particles was added to the surface of the surfactant solution in the area enclosed by the barrier and away from the vicinity of the Wilhelmy plate. Then the change in surface pressure was recorded as a function of time for a fixed surface area. Single Film Lifetimes. The apparatus used for the determination of the lifetimesof singlesoap filmsunder various conditions is shown in Figure la. Films were formed on a glass frame constructed of 1mm diameter glass rod and having a height of
606 Langmuir, Vol. 9, No.2, 1993
30 mm and width 20 mm. The frame was attached to the movement of a hand-operated traveling microscope via a glass rod. Films were formed in a reproducible way over a period of about 5 s. The base of the frame remained just below the surface of the surfactant solution, which was contained in a 50-mL cylindrical glass vessel. A cylindrical-shaped glass sinter of pore size 15-40 pm (number 3 sinter) was placed in the glass vessel as shown. Drops of dodecane could be placed on the surface of the surfactant solution on the outside of the sinter allowing dodecane to spread over the whole surface as a solubilized molecular layer without liquid lenses coming into contact with, and ascending, the soap film. Before measurement of film lifetimes, the atmosphere inside the apparatus was allowed to reach saturation with water vapor so as to avoid film rupture as a result of evaporation. To aid vapor saturation, six 10-mL beakers containingwater were placed around the vessel containing the surfactant solution. The apparatus had a provision for passing a thin rod (suitably coated, and with a diameter of approximately 0.3 a m ) horizontally through a film shortly after formation. A modificationof the standard glass frame is depicted in Figure lb. This allowed dodecane added to the surface to be confined to one side only of the soap film,thus producing different tensions at the two surfaces of the film. Surface and Interfacial Tension Measurement. All surface tensions were measured using a Kruss K10 tensiometer with a du Nouy ring attachment. The instrument detects the maximum pull exerted on the ring, which does not become detached during the measurement. Most of the oil-water interfacial tensions were also determined in the same way. The lowest of the oil-water tensions however were measured using a spinning drop tensiometer (Kruss Model SITE 04). The various tensions were used to calculate the entry, spreading, and bridging coefficients presented in Figure 4. Foaming Experiments. In most of the experiments on foams, the simple 'shake test" method was employed. A sample of 10 mL of aqueous surfactant solution (containing 2 mg of solid and/ or 0.5 mL of dodecane when appropriate) was placed in a 100-mL graduated cylinder fitted with a ground glass stopper. (Oil and solid were added separately rather than as a dispersionof particles in oil.) The cylinder was then shaken vigorously with an up and down motion for 5 s and the volume of foam formed noted. This volume is referred to as the initial foam volume. In some experiments the foam volume was monitored over a period of time to obtain foam breakdown rates. Although obviously crude, the shake test gives remarkably reproducible results when performed by the same operator. In experiments relating to possible effects of contact angle on foam breakdown, the contact angle has been varied by changing the surfactant concentration. However, the inherent foam stability can also change with surfactant concentration. This problem is in part surmounted by considering the relationship between contact angle and percent foam reduction (see later). Nonetheless it is obviously desirable to know in what concentration regime the foam stability is changing. One possible measure of stability is the initial foam volume. In addition to this however we have determined (for SDSsolutions)the 'foaming efficiency" using a simple foaming column of the type described by Bikerman.15 The glass column containing the surfactant solution had a glass sinter at the base through which nitrogen was passed at a known rate to generate a foam. The foaming efficiencyis defined as the volume of foam produced in unit time divided by the gas flow rate. Foaming efficiencies of SDS solutions are shown in Figure 2 as a function of surfactant concentration. Also included in the figure are initial foam volumes. Both initial volumes and the foaming efficiency fall at concentrations somewhat below the cmc. At concentrations in excess of about a third of the cmc, however, the foam stability is effectively constant.
Results and Discussion Effects of Dodecane on Single Film Stability and on Foam Volumes. Potentially an oil can act in various ways to reduce film stability and foaming. A lamella can (15) Biker",
J. J. Trans. Faraday SOC.1938,34,634.
Aveyard et al. cmc
- In((SDS]/M) Figure 2. Foaming efficiency ( 0 , O separate runs) and initial foam volumes ( 0 )of SDSsolutions as a function of concentration. air (a)
aqueous (wl Figure 3. Representation of a lens of oil resting a t an aqueous solution/air interface. The angle B is given in terms of the various interfacial tensions by eq 5.
be bridged by an oil drop ( 0 ) if the drop is capable of entering the lamella (i.e. air (a)/water (w)) surface (Figure 3) and if having done so it does not spread macroscopically along the surface, but remains as a lens. Even if a lens remains, however, "microscopic" spreading can occur to give very thin films and in the limit a mixed surfactant + oil monolayer. Such microscopic spreading causes a reduction in the surface tension of the surfactant solution. Both microscopic and macroscopic spreading may give rise to Marangoni thinning and film rupture. For an oil drop in an aqueous phase to enter an aw interface, the entry coefficient E defined as must be positive. The value of E in systems of present interest can depend very significantly on whether the system is at equilibrium or not because the tension of the air-aqueous solution interface, Yaw, can be substantially lowered by dissolution of the oil in the surfactant monolayer at equilibrium,16 as mentioned above. Since, at equilibrium (assuming Yaw to be the largest of the three tensions)17 ~ o+ a
Yaw
ow
(2)
it follows that at equilibrium E (=Ee) 1 0. If entry does occur, and if the drop does not spread macroscopically to give a duplex film, it can ultimately bridge the lamella and if the bridging coefficient B > 0, where B is defined as 2
2
2
(3) B = Yaw + Yow - Yoa this will lead to rupture of the film.3 As for the entry (16) Aveyard, R.; Cooper, P.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1990,86,3623. (17) Rowlinson, J. S.; Widom, B. Molecular Theory of Capillarity; Oxford University Press: Oxford, 1989; Chapter 8.
hngmuir, Vol. 9, No.2, 1993 607
Foam Breaking 100
100
E
'
-20 -11
-10
-8
-8
-7
I
-6
In([CTAB]/M)
Figure 4. Entry (E) (eq l),spreading (S)(eq 4), and bridging (B)(eq 3) coefficients for dodecane in CTAB solutions as a function of the surfactant concentration at 25 OC. Filled and open symbols are for final and initial coefficients,respectively. Units of E and S are mN/m and of B are (mN/m)2.
coefficient, B can take on different values for equilibrium and nonequilibrium systems. It should be noted that a positive value of E does not necessarily mean that an oil drop can enter the interface sufficiently rapidly to inhibit foam formation. As mentioned, Kono et aL4have recently pointed out that the stability of the 'pseudoemulsion" film present between the drop and the air/water interface prior to entry can be a very important factor in foam reduction. As mentioned, spreading in one form or another can lead to film rupture. Spreading to give a duplex film is discussed in terms of a spreading coefficient S (for equilibrium or non-equilibrium systems), defined by (4) S = r a w - r o w - 7, In equilibrium systems (S= Se)the coefficient is zero if
spreading occurs, otherwise it is negative. For nonequilibrium systems S can be positive, when spreading will occur. Negative values again correspond to nonspreading as a duplexfilm. In the present work, the nonequilibrium values of E, B , and S used are the so-called initial values, obtained using y a w for the surfactant solution in the is equal to that for pure absence of oil. The value of ror. dodecaneboth for equilibrium and nonequilibrium systems because surfactant does not enter the oil phase (seelater), and in any case surfactants are not expected to be surface active at the oil-air surface. Values of E, B, and Sfor the CTAB + dodecane system are shown in Figure 4 as a function of surfactant concentration. BothE and E,, as well as B and Be are positive, indicating that dodecane drops, if sufficiently large, are capable of causing film rupture by bridging. T h e initial spreadingcoefficienta are positive whereas the equilibrium values are negative and progeasively closer to zero as the surfactant concentration is increased. This reflecte the observation that a dodecane lens placed on the surface of a CTAB solution will initially spread to give a duplex film (whichexhibits interference colours) and then later retract to give small, flat lenses. It is possible therefore that dodecane could reduce initial foam volumes of CTAB solutions by a macroscopic spreading mechanism, but if the alkane is active in foam reduction over a longer period of time, the effect is likely to be due to bridging of the foam lamellae. Even though at equilibrium dodecane does not spread macroscopically on CTAB solutions, it does become
surface tension w%hdodecane presentlmNlm
Figure 5. Surface tension lowering of solutions of AOT and CTAB over a range of surfactant concentrations at 25 OC caused by lenses of dodecane resting at the surface. Table I. Mean Lifetimes of Single Films Drawn from 4 mM CTAB Solutions no. of additive films lifetime18 system without Rods none 107 17 14 5 solubilized C12 (both sides) 92 14 82 f 18 drop C12 (both sided 5 5 44 36 solubilized C12 (one side) 5 drop C12 (one side) 15 15 Systems with Rods PTFE rod 91 17 EBS rod 87 13 wax rod 95 15 solubilized Cl2 + PTFE rod 90 18 102 8 solubilized CIS+ EBS rod 106 15 solubilized Cl2 + wax rod drop Cl2 + PTF'E rod all films ruptured drop C12 + EBS rod immediately on contact of drop and rod drop Clz + wax rod
*
* *
*
incorporated in the surfactant monolayers, lowering ym. Values of the lowering (AT) are shown in Figure 6 as a function of the tension of the surfaces containing mixed films. Also shown for comparison are similar results for AOT. It is clear that the penetration of dodecane into films of AOT (a twin-tailsurfactant) is much less than for CTAB (single chain surfactant) monolayers, the reasons for which are discussed elsewhere.16 In the light of the above findings we have investigated the effects of the presence of dodecane in aqueous CTAB systems on (a) the mean lifetimes of single surfactant films (to represent lamellae within a foam) and on (b) foam volumes, both initial and over a period of time. The reeulta of the single film experiments are summarized in Table I. The presence of solubilized dodecane on both sides of singlefilms has only little effecton film lifetimes. However the lifetimes are reduced by around 26-30 % when small lenses of dodecane are placed on the surface of the surfactant solution in contact with the f h ; the lenses can be seen to amend the films. T h e angle B in Figure 1, which is given by1'
B = cos-'
[yaw
2
- 7- 2 - r o w2I/2~or.~ow
(6)
is however very close to zero for the system studied so that the oil lenses, although visible by eye, are very flat, and presumably of thickness comparable to the fiilm t h i c k . It is interesting that if the solubilized alkane ie constrained to be present on one side only of the film (me
608 Langmuir, Vol. 9, No.2, 1993
Aveyard et al.
3
1
go .10
-9
-7
-8
Figure 6. Reduction of initial foam volumes of CTAB solutions of varying concentration in the presence and absence of dodecane (see text). 140
-
120
-
100
-
80
-
E 3
9 5
6
0
4 0
2000
4000
6000
8000
10000
vs
70
60 1
nc
2
3
4
5
6
In (Vmin)
7
n
without dodecane
u-
40t
2otY with dodecane
01
1
0
3.0
log(diametedmicron)
Figure 8. Surface pressure generated as a function of time for the addition of 50 mg of EBS to the surface of 10 m M SDS solution in the Langmuir trough (see Experimental Section). The inset shows the initial rate of spreading as a function of the particle diameter.
4 - 2 60-
2 0
I
In ([CTAB]/M)
2
3
4
5
8
I
7
In(t/min)
Figure 7. Foam volumesof 1mM solutionsof CTABasa function of time, with and without dodecane present. The inset shows the percent foam reduction (FR) (eq 6) caused by dodecane over the period of the experiment.
Experimental Section), so that the tensions of the two sides of a film are different (by about 8 mN/m in this case), the film lifetime is substantially reduced. Similar experiments with AOT films did not show this effect, presumably because the differencein tension between the two sides is much lower (see Figure 3). The effect of dodecane (0.6 mL in 10mL CTABsolution) on initial foam volumes over a range of surfactant concentration is shown in Figure 6. In line with the single film experiments and the sign of E and B, the reduction in initial foam volume is substantial, about 50% at all concentrations. For a fiied concentration of CTAB (1 mM,just abovethe cmc) the fall in foam volume with time (over a period of 400 min) is significantly greater with dodecane present (Figure 7). This long term reduction caused by dodecane must presumably be a result of bridging of lamellae rather than spreading which, as discussed, could only be expected to reduce initial foam volumes. It thus appears reasonable to suppose that (a) film rupture by dodecane droplets in the CTAB systems ie occurringthrough a bridging mechanism and (b)tension differences of about 7-8 mN m-l on the two sides of the
filma, caused by dodecane solubilizationon only one side, can significantly reduce film stability. Poesible Role of Solid Spreading on Surfactant Solution. PTFE and paraffin wax do not dissolve in surfactant monolayers, but commercial EBS, mainly because of the presence of impurities (seelater), is capable of spreading on the surface of a surfactant solution. The surface pressure of commercial EBS placed on aqueous SDS solutions in a Langmuir trough has been monitored with time for various conditions. We show in Figure 8 surface pressures generated when 50 mg of EBS is placed on 500 cm2 of 10 mM SDS solution. As seen, surface pressures of up to about 12 mN m-l are given, the initial rise in pressure being quite rapid. The pressures achieved and the initial rate of spreading depend on surfactant concentration, amount of EBS added (for a constant particle size range), and particle size range (for constant mass). The latter effect is seen in the inset to Figure 8. The possibility exists then that commercial EBS could inhibit foam formation through a Marangoni spreading effect. We find that EBS particlesdo indeed reduce initial foam volumes (see later), but PTFE and paraffin wax particles, which do not spread,ale0 reduce the foam volume by a similar amount. Further, it is found that when the commercial EBS is purified, the initial spreading rate is reduced by a factor of about 10and yet the purified sample is as active as the commercial sample in reducing initial foam volume. It is probable then that spreading is not a major cause of foam inhibition in this case. To strengthen this conclusion we have also carried out single film experiments in which narrow cylindrical rods (diameter about 0.3 mm) of (or coated with) PTFE, wax, and EBS are pushed into single f i e . The results, obtained in this case using CTAB as surfactant (where EBS is again expected to spread on the surface), are recorded in Table I. It is found that none of the rode causes much reduction in f i lifetime. (We discuss later why this should be so when the same solids as particles are active in reducing foaming.) The presence of solubilized dodecane in the monolayers does not alter the situation. Summarizing the findings thus far, the entry and bridging coefficients (both initial and equilibrium) of
Langmuir, Vol. 9, No. 2, 1993 609
Foam Breaking
dodecane with CTAB solutions are positive. Correspondingly, liquid dodecanedropletsreduce both the initialfoam volumesand (toa less striking extent) single film lifetimes in CTAB systems. Dodecane is solubilized in close-packed CTAB monolayers giving rise to surface pressures of around 8 mN m-l. Film stability is lowered if alkane solubilization occurs on only one side of the film, whereas solubilization on both sides has little effect. Hydrophobic rods (PTFE, wax, and EBS) do not significantlyreduce single film lifetimes even though, in the case of EBS, substantial spreading pressures could be generated. We now consider the possible role of contact angles in film and foam breaking in the systems studied. Effects of Hydrophobic Solid in the Presence and Absence of Nonpolar Oil. Singlefilm experimentswere described above where dodecane was added to a system or where solid rods were incorporated into the films. If liquid alkane lenses and a rod are present together in a film, the film has a very short lifetime (Table I). In fact, visual observation shows that when a small lens on the surfaceof the film encountersthe rod passing through the film, the film ruptures immediately. In addition to the single film experiments, we have determined initial foam volumes of aqueous solutions of AOT, CTAB, and SDS alone (foam volumes VO)and in the presence of either solid particles (wax,EBS, or PTFE) alone or in combination with dodecane (foam volumes V), all over a range of surfactant concentration, c. Particles alone reduce foam volumes to an extent which depends on c; over a certain range of concentration, VOalso depends on c (see earlier). It is therefore convenientto expressthe foam knockdown in terms of the percent foam reduction (FR) defined as FR = loO[(Vo- v>/Vol
(6)
We show the FR results in Figure 9. In all cases FR decreaseswith increasing c and is much greater in systems containing solid and oil in combination. For SDS and CTAB all the solids give similar results whereas for AOT there are somedifferences, but the generaltrends described remain the same. The single film experiments described suggest that in foam breaking (a) the role of the oil need not be one of transporting particles to lamella surfaces and (b) rapid surfactant adsorption at the solid/aqueous solution interfacemay not be an important factor in foam breakdown by solid particles, at leastfor the present systems. Possible effects due to longer term surfactant adsorption are discussed later. The potentially important role of contact angles (0) in foam and film breaking has been discussed in detail by Garrett? Frye and Berg:J2 and Dippenaar? among others. Consider a solid spherical particle resting at an aw or ow interface as depicted in Figure 10a (left). The interface is assumed to be planar and to have its equilibriumcontact angle with the particle. The assumption of planaritymeans that the particles are sufficientlysmall for gravitational effects to be unimportant. As the film thins, the upper (aw) surface ultimately meets the particle and forms a curved meniscus as shown in Figure 10a on the right-hand side. The Laplace pressure18causes a flow of liquid away from the particle, aiding thinning of the film. When the two three-phase contact lines meet, the film will rupture. For the situation shown (spherical particle), if the phase (18) With reference to Figure loa, the meniscus shown also has a curvature,of oppositesign,at rightanglee around the girth of the particle. The argumentinvolvingthe Laplace pressure implicitly msumesthe latter curvature is the smaller of the two curvatures.
8
loo
n
U
40
,o
I
*O
-7
-5
-6
-3
-4
-In([SDS]/M) 100 80 C
.-
2 60 3
5 40
P
20
0
10
-9
-8
-7
-6
-5
-4
In([CTAB]/M)
* O t 0 -7.0
e*
I
-6.5
-6.0
-5.5
I -5.0
In([AOTj/M)
Figure 9. Percentagefoam reduction(eq6) of aqueoussurfactant solutions over a range of concentrationscause by solid particles: (a) for SDS; (b) for CTAB; (c) for AOT solutions. The circles refer to paraffin wax, squares to EBS, and triangles to PTFE particles. Open symbols are for systems without dodecane and filled symbols refer to systems with dodecane. (a) spherical solid particle in lamella surface a
-kA
w
)jo
(b) corresponding contact angles for plane solid surface
u Figure 10. Contact angles 13r sa ids (shaded) in contact wil h aqueous solutions (w)iii air (a) or oil io). P& a represents a spherical particle in a lamella surface; Part b shows equivalent angles for drops of aqueous phase resting on plane solid surfaces, which are the experimentally determined angles.
on the lower side is air, 8 (=el) must be greater than 90° for film rupture to occur. If the particle is not spherical (as is the case for the systems investigated here), it has been arguedl9697 that O1 < 90° can give rise to film rupture.
Aveyard et al.
610 Langmuir, Vol. 9, No.2, 1993
Frye and Berg' have considered dynamic aspectsof particle dewetting using a fluid mechanical model. They show that rapid film rupture (necessary for antifoam action) can occur for contact angles only a few degrees above the critical angle. Ifthe lower phase is oil (Figure lo), then it can be shown that for the two three-phase lines of contact to meet and cause rupture' ""
e1>1800-e2
.12
(7)
where 82 is the contact angle of the oil-water interface with the solid particle. This means that if 82 > 90°, 81 need not be as high as 90° for film rupture to occur for a spherical particle. This provides a possible mechanism for the synergistic action of particle and oil in foam breaking.3 As mentioned earlier however, Frye and Berg12 also suggest the possibility that the solid particles are completely wetted by oil in the presence of the surfactant solution and that they simply act as support for the oil which causes rupture by a bridging mechanism already alluded to. For this to be the case however, the contact angle of the surfactant solution with the solid immersed in oil would need to be 180'. It should be borne in mind that during foam formation the operative contact angles may be considerablydifferent from the equilibrium ones, even if the solid is preequilibrated with the surfactant solution. Thisis because surfactant adsorption at a newly formed lamella surface will be less than the equilibrium ads~rption.~ In order to probe, for the system studied here, the possible involvementof the mechanismsdescribed, we have determinedcontact angles of the surfactant solutions with the three solids (in the form of smooth flat plates) in air (el) and under dodecane (82). Apart from the direct relevance of contact anglesto foam breaking, contact angles can also be used in conjunction with appropriate tensions to estimatethe extent of surfactant adsorptionat the solid/ solution interfaces. Contact angles of aqueous CTAB, SDS, and AOT solutions with smooth flat surfacesof wax, EBS, and PTFE, both in air and under dodecane (see Figure lob), have been determinedover a range of surfactant concentration, c. T h e results are depicted in Figure 11 where contact angles are plotted against In c. For aqueous surfactant solutions in air, the angles for all the solids overlap and fall with increasing c. In the presence of dodecane the angles are much higher and independent of c, with PTFE giving an angle of around 160° and the other solids both giving angles around 140'. Foam reductions are plotted against 81 in Figure 12 for SDS and CTAB solutions. It is clear that, in the absence of dodecane, large values of FR are achieved for 81 < 90' and that FR increases with increasing 81. We indicate in Figure 12 the regime where VO is constant, and the results strongly suggest that at least in the region of constant Vo(where the inherent foam stability is independent of c), foam reduction is directly associated with the value of the contact angle. If this relationship does in fact exist, then foam reduction for 81 less than90' can be ascribed to the nonsphericalgeometry of the particles used. This would explain why cylindrical rode of the same solids do not rupture single f h , since in this case angles in excess of 90° are required for film rupture. It is expected from eq 7 that in the presence of dodecane the value of 81required for foam breaking should be much reduced. Indeed on this basis, for spherical particles, a value of 81 > 40' (or 20° in the case of PTFE) should be sufficientfor film rupture. It appsarsthat, since 82 < 180' for all the solids and surfactants studied,
.10
.8
-6
-4
-2
In([SDSYM)
In([CTAB]/M)
lcll
180,
In ([AOTJ/M)
Figure 11. Contact angles of surfactant solutions in contact with plane solidsurfaces, as a functionof Surfactantconcentration. Key is the same as for Figure 9.
dodecane is unlikely to completely wet the particles and the mechanism in which particles simply act as "formers" for oil drops, as proposed by Frye and Berg,12 does not operate in the present system. Surfactant Adsorption by Solid Particles and Possible Effects on Antifoam Action. The reduction of contact angles of water with the solids caused by addition of surfactant can come about by the reduction of either or both of raw and r,resulting from surfactant adsorption. As discussed by Blake in a review of wetting of solids by surfactant s~lutions,~g estimates of surfactant adsorption at the sw interface can be made using an approach of Lucassen-Reynders.20 For a drop of liquid (w) resting on a plane solid substrate (e) in air (a) or an immiscible oil (01, the equilibrium contact angle of the liquid drop with the solid, 8, is related to the various tensions by Young's equation21 YW
= Yaw + Yow
(9
It follows that = dr&how - dr,&, (9) The Gibbs equation, giving the adsorption r of surfactant (activity a in solution) at the various interfaces i in the system is
&row cos Wdr,
(19)Blake, T.D.In Surfactante;Tadrrs, Th.F.,Ed.;Academic F ' r e ~ London, 1 9 w p 221. (20)Lucauen-Reynders, E.H.J. Phy8. Chem. 1968,67, 969, (21)In what follom immediately, the WUI~~OM u e thm for ryabmn with oil. For ryatema without oil, qmtitibfor the M)and ow int6rfacea may be replaced by corresponding quantitiea for the ea and aw surfaces.
Foam Breaking (a)
Langmuir, Vol. 9, No. 2, 1999 611
,
20
fall
yhN
I S5
70
75
SO
85
90
contact anglddegrees (b) 100
-20
so
.30 20
t
d
K 0
80
4
m1
30
50
40
y/mN
60
70
80
m-1
40
20 n 85
-10
-
.20
-
.40
-
CTAB and AOT w i t h wax 2nd EBS
r
75
85
95
E
105
contact anglddegrees
Figure 12. Percentage foam reduction (eq6) of (a)SDS solutions and (b) CTAB solutions caused by solid particles, as a function of contactangle. The key tothe symbolsis the same as in Figures 9 and 11.
0
dyld In a = -RTTi Combination of eqs 9 and 10 then gives
(10)
d(yowCOS 8)ldrow= (r, - rsw)/row (11) Thus the slopes of plots of row cos 8 against row give ratios of surface excesses of surfactant at the various interfaces. For the cases of interest we may reasonably suppose that the adsorption of surfactant at the planar solid-oil or solid-air interface is zero. In air + water systems there is no means of transport to the so surface. In the oil + water systems investigated it is known that all the surfactant (both as monomer and micelles) resides in the aqueous and so again there would appear to be no meana of transport of surfactant to the so interface. This being the case, eq 11becomes d(r, COS 8)ldyow -rS,Jrw= -t (12) and thus the ratio, t, of adsorptions at the sw and either the aw or ow interfaces can (in the case of flat solid plates at least) be obtained. T h e necessary surface and interfacial tensions have been determined, and the plots of yowcos 8 against yowand of yawcos 8 against yawshown in Figure 13 are reasonably linear and very different for systems with and without oil present, in line with previous findings in the literature (see ref 19). For systems without oil the slopes are close to unity (Table II). For wax and EBS in the presence of dodecane 4 is about 0.8. Thus the extents of surfactant adsorption at the sw, ow, and aw interfaces are all very Similar. Although the adsorptions (slopes) are all very similar, the intercepts of the plots for the air + water systems are
(a) Aveyard, R.;B&, TTOM. 1 1986,82,125.
B.P.;Clark,S.;Mead, J.J.Chem.Soc.,Forodoy
CTAB and AOT wth PTFE
10
20
30
10
50
60
y/mN m i
Figure 13. Plots of y cos 0 against y: (a) for CTAB solutions; (b) for AOT solutions, both in the absence of dodecane. Here y is the surface tension of the surfactant solution. The symbols are the same aa in the earlier Figurea. Part c is for both CTAB and AOT solutions in the presence of dodecane, and y is now the interfacial tension between surfactant solution and dodecane. Table 11. Slopes 6 (Equation 12). wax EBS FTFE CTAB &/water 0.97 1.03 0.99 dodecane/water 0.80 0.80 0.96 AOT &/water 0.96 1.05 1.03 dodecane/water 0.83 0.82 0.96 0 Values of -08 0 O for solid/dodecane/pure water system, which according to eq 17 should equal 4, are for wax 0.80, for EBS 0.81, and for PTFE 0.96.
large, whereas the lines for the oil + water systems pass, within experimental error, through the origin. For the systemswithout dodecane, where = 1and I'r is aasumed to be zero, it can readily be shown thatIB yaw COS 8
= " ~ a w+ Wa
(13)
where Wa is the work of adhesion between solid and aqueous solution in air, defined as
Wa
y a w + ~ r y i-w
(14)
Inspection of eq 13 indicates that the intercept on the abscissa of a plot of yawcos 8 against yawis the value of Wafor the case where 8 = 90° (cos 8 = 0). It transpires that, on the assumptions made, Wa is independent of surfactant concentrati~n,~~ and hence the same as WEo, the work of adhesion of pure water with solid, which can be calculated also using the Young-Dupre equation
Aveyard et al.
612 Langmuir, Vol. 9,No.2,1993
~
Table 111. Intercepts on 7.- Axis (Equation 13) and ' . W (Eauation 15) for Systems in Air at 25 "C (See Text)' WBl[ EBS FTFE (a) CTAB intercept/mJ m-2 50 49 49 Pldeg 110 111 112 Wao/mJm+ 47 46 45 (b)AOT intercept/mJ m-2 50 45 47 0'ldeg 110 111 112 Wa0lmJm-2 47 46 45 a
Surface tension of pure water (rawo) taken aa 71.9 mN m-l,
q
.c 10
0
5
0
0
,
,
,
5
10
15
1
20
5001o0o15002m25003ooo3500 equilibration time, to I minutes
Figure 15. Initial foam volumes of 1.6mM CTAB solutions in the presence of EBS particles which have been pre-equilibrated with the surfactant solutions for various times, to, before the formation of the foams. Inset has results for t , up to 20 min. 35,
000
002
004
006
008
-
0 10
1OO([CTAE]/M)
Figure 14. Work of adhesion between aqueous CTAB solutions and solids immersed in dodecane.
w; = yawo(i+ COS eo)
(15) where superscript O refers to systems without surfactant present. The agreement between values of W,and W," for CTAB and AOT solutions in contact with the three solids is seen (Table 111) to be good. For systemscontainingdodecane,extrapolations of plots cos 0 against yowpass through the origin, and it can of row be shown that in this casela
woo= yowO(l- 0
1
(16)
and
E = -COS eo (17) where Woois the work of adhesion of water with the solid in the presence of oil. Values of and cos Bo, shown in Table 11,are in good agreement. It is seen from eq 17that 4 cannot exceed unity in these systems. We also note that the work of adhesion between aqueous solution and solid in dodecane now depends on surfactant concentration, as shown in Figure 14. From Table I11 and Figure 14 it is seen that the work of adhesion of surfactant solutions with the solids is very much smaller when dodecane is present, as could be anticipated. This means that the (positive) free energy change associated with separating a solid particle from a lamella in the presence of the oil is much less than in the absence of the oil. From the analysis above it is clear that adsorption of surfactant occurs at the solid-aqueous solution interface to roughly the same extent as at the aqueous solutionvapor and aqueous solution-oil interfaces. In order to explore the possible effects of surfactant adsorption by the particles on their effectiveness in foam breaking, we have performed the followingexperiments. Sampleswere set up containing 10 mL of surfactant solution (1.6 mM
I 0
50
100150200250300350400450500 tlme I minutes
Figure 16. Foam volumes for 1.6 mM CTAB solutions as a function of time for foams formed in the presence of EBS particles pre-equilibrated with the surfactant solutions for time t,. Key: points 0, A, m, 0 , A and 0 are for t. = 0 min, 10 min, 20 min, 60 min 48 h, and 51 h, respectively.
CTAB, about double the cmc) and 4 mg of EBS particles. The systemswere gently swirled for various times at room temperature, after which a foam was formed (sea Experimental Section). Thereafter foam volumes were monitored over a period of time. We show in Figure 15 initial foam volumes as a function of equilibration times, tea For to less than about 10-20, min initial foam volumesare little affectad(insetto f i i ) . For larger t e , however, initial foam volumes rise substantially with equilibrium time indicating the decreased effectiveness of the particles in reducing foaming. If thia is indeed due to adsorption of surfactant by the particles, the adsorption occurs more slowly than at the plane solidsolution interface. For about the fmt 200 min after foam formation, the rates of foam breakdown (Figure 16)are similar for all te (except for to = 0 ) and similar to the breakdown rate in the absence of particles. For te= 0 the foam breakdown rate is somewhat larger. For the systems containingparticlespretreated for 10and 44h before foam formation, there is a rapid fall in foam volume between 150and 250 min. This effect is reproducible,but at pregent we have no reasonable explanation for it. Nonethelw we can say that for the most part (relatively slow) surfactant adsorption affects the foam formation procesa (i.e. initial foam volumes) rather than subsequent breakdown. Adsorption on an initially hydrophobicsolid will presumably give rise to a reduction in contact angle of the surfactant solution with the particles, which could be the cause of the fall in antifoam effectiveness.
Foam Breaking
Summary of Conclusions
An attempt hae been made to systematically assess
poeeible origins of the antifoam effecte of nonpolar oil and hydrophobic solid particlee, alone and in combination, in a range of systems. We have drawn the following conclusions for the syatems studied: 1. Dodecane alone is quite an effective foam breaker for CTAB solutions; lenses of this oil also substantially reduce singlefilm lifetimea. These effecta can be explained in terma of the signa of the entry and bridging coefficients for dodecane with aqueous solutions of CTAB. 2. Single film experiments show quite strikingly how oil lenses at a film surface act together with hydrophobic solid to rupture lamellae of surfactant solution. 3. The hydrophobic particles studied are all reasonably effective in reducing initial foam volumes, the percent foam reduction (FR) achieved depending on surfactant concentration. FR is much greater in the presence of dodecane but still dependent on surfactant concentration,
Langmuir, Vol. 9, No. 2, 1993 613 4. Foam reduction by particles alone or in the presence of dodecane varies smoothly with the contact angle (61) of the surfactant solution with solid in air. T h e presence of dodecane reduces the value of 61 required for effective foam breaking. 5. Contact angles have been used in conjunction with interfacial tensions to estimate the extent of adsorption of surfactant on the particles. Adsorption at the solid/ aqueous solution interface is very similar to that at the air/solution interface and a little less than that at the oil/ solution interface. It is found that leaving solid particles in contact with surfactant solutionsprior to foaminglowers the effectiveness of the particles in reducing initial foam volumes, although subsequent foam breakdown ratee appear to be little affected.
Acknowledgment. We are grateful to RhBne-Poulenc Chemicals (Beverley, England) for the provision of fuUy funded studentships for P.C. and C.E.R.
