liquid systems

Measurement of Polymer Bridging Forces in Liquid/Liquid Systems F. VAN VOORST VADER AND H. D E K K E R Unilever Research Laboratorium, P.O. Box 114, 3130 A C Vlaardingen, The Netherlands Received October 22, 1980; accepted February 2, 1981 When a paraffin oil droplet, submerged in water, is contacted with a paraffin oil/water interface at which polyvinyl alcohol has been spread, the interface is found to adhere to the drop by polymer bridging: during withdrawal of the droplet, a neck of oil stretches from the interface to the drop. However, the oil contained in the drop and that in the neck remain separated by a polymer-stabilized aqueous film. The free energy of adhesion between drop and interface was found to decrease with the spread amount of polymer, and is independent of the time of contact at low adsorption values. Its value is smaller by an order of magnitude than the free energy of adhesion found for polymer bridging between solids. 1. INTRODUCTION

tended our investigation to bridging between liquid interfaces. This phenomenon occurs in milk and cream (5), and also in paraffin oil emulsions (6).

Polymer bridging has been accepted as one of the mechanisms causing adhesion between dispersed particles (1). When a polymer is adsorbed at the interface between a particle and a solvent, only a part of its segments is adsorbed as trains at this interface, while the residual segments stick as loops or tails into the solution. When the latter segments encounter the clean surface of a second particle, they may adsorb on it, so that both particles become connected by a series of polymer chains. While later research has supported this mechanism (2), very few data exist on the free energy change caused by polymer bridging (3). Earlier, we reported a series of measurements on the free energy of polymer bridging caused by polyvinyl alcohol (PVA) present between two carbon filaments that were submerged in an aqueous solution of KNO3 (4). This free energy of bridging was found to depend markedly on both the time of contact and the pressure applied during contact, decreasing in both cases to a final value of - 2 5 mJ m -2. Because the conformation of adsorbed polymer molecules at a solid interface is difficult to characterize, we have now ex-

2. EXPERIMENTAL

To obtain polymer bridging between two liquid phases, a droplet of oil with a clean surface was submerged in an aqueous solution and placed below a water/oil interface at which a polymer had been spread (Fig. 1A). First, the droplet moves up to this interface. Part of the polymer segments of the loops and tails of the spread polymer now adsorbs on the droplet surface. The remainder serves as spacers between the two surfaces and prevents coalescence, so that a thin aqueous film remains between the oil phases (Fig. 1B). When the droplet is retracted, the polymer bridges formed entrain the upper interface, so that a neck of the upper oil phase stretches down to the droplet (Fig. 1C). A photograph of the latter situation is shown in Fig. 2.

Apparatus The apparatus (Fig. 3) consists of a thermostated vessel with optical windows 377 002 i -9797/81/00377-07502.00/0

Journal of Colloid and Interface Science, Vol. 83, No. 2, October 1981

Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.

378

VAN VOORST VADER AND DEKKER

®

@

.

A

FIG. 1. Polymer bridging between liquids. (1) Oil phase, (2) spread polymer layer, (3) aqueous phase, (4) oil droplet, (5) support.

through which photographs of the interface profiles are made at 20x magnification. The vessel contains the aqueous phase, and paraffin oil on top. A 4 x 4-cm glass frame is placed at the interface, which forms a Langmuir trough and is equipped with a glass barrier attached to the frame with Teflon strips. Using a motor-driven logarithmic spiral (7), this barrier can be m o v e d at a constant rate of compression. P V A was spread into this trough by the modified Trurnit technique developed by Lankveld (8); an aqueous solution containing 60 mg polymer per liter and a constant flow rate of 4.2 x 10-11 m s sec -1 were used. A capillary was fixed perpendicularly on a float, and filled with paraffin oil. After its submersion in the aqueous phase, a small droplet of oil was placed on its top. The float was moved slowly and smoothly up and down by changing the mercury level below the float. To avoid motion of the upper oil/ water interface, mercury was withdrawn simultaneously by a compensating reservoir.

FIG. 2. Droplet of paraffin oil after contact with interface paraffin oil/aqueous sodium sulfate solution (5 mole m-Z), pH 7.5. PVA was adsorbed previously at the interface. Journal of Colloid and Interface Science, Vol. 83, No. 2, October 1981

379

MEASUREMENT OF POLYMER BRIDGING FORCES

The mercury levels in both reservoirs were controlled by leveling vessels attached to a rack and pinion mechanism, driven by a synchronous motor (9). A final rising/lowering speed of 10-8 m sec-' of the droplet is used to avoid deformation of the interface. For the photographs, a monochromatic light source is placed so that its light beam just skims the interface from the aqueous side. Unless indicated otherwise, the droplet motion was stopped for 15 min before photographing.

Properties of Spread PVA Monolayers The interfacial tension of PVA layers spread between paraffin oil and the aqueous phase was measured by the Wilhelmy plate technique. Spreading conditions were identical to those used in the adhesion measurements. The interfacial tensions found immediately after spreading and after 16 hr standing are shown in Fig. 4. Just as found by Lankveld (8), the spread layers showed considerable aging. All adhesion measurements were performed on layers spread initially up to 2.78 x 10-7 kg PVA/m~; these were first aged overnight, and then compressed to various degrees. In contrast to fresh layers, such aged layers show no irreversible change of their interfacial ten-

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,,o

,,,

__

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FIG. 3. Measurement of free energy of adhesion between liquids. (1) Thermostated vessel, (2) paraffin oil phase, (3) Langmuir trough, (4) optical window, (5) aqueous phase, (6) compensating mercury level, (7) oil droplet, (8) float, (9) mercury level, (10) mechanical leveling vessels, (11) flexible tubing, (12) light source, (13) microscope and camera.

c~/mNm-1

~7

\ %z -v i

zx

20

"% . . . . .

10

o

i

;.5

i

FIIO -6 kg m-2

FIG. 4. Interfacial tension tr against adsorption F of PVA 8-88 as found on spreading at the paraffin oil/ Na2SO4 solution (5 mole m -3) interface. (~7) spread over interface of constant area, direct measurement; (A) spread at 2.78 × 10-T kg m -2, aged overnight and then measured after compression.

sion when they are repeatedly expanded and compressed. Buckling occurs when aged layers are compressed over 70%. These observations suggest that during aging a change in the conformation of the adsorbed PVA occurs which after 16 hr reaches an apparent equilibrium. The bridging phenomena described above were only observed when aged layers were used. Approach of a droplet to a freshly spread PVA layer always leads to coalescence of the oil phases.

Materials Polyvinyl alcohol 8-88 from Hoechst (molecular mass 41,000, 88% hydrolyzed, i.e., 12% acetate) was used as bridging polymer. As oil .phase, paraffin oil from Baker Chemicals, analytical grade, was used. A solution of Na2SO4 at pH 7.5 in distilled water (5 mole m -3) constituted the aqueous phase. The interfacial tension between the pure phases was 52.8 mN m -~ at 20°C (Wilhelmy plate technique) and their density difference 140 kg m -3 (pycnometer). Journal of ColloM and Interface Science,

Vol. 83, No. 2, October1981

380

V A N VOORST V A D E R A N D D E K K E R

3' =/3

-

( -20-1AFi )1/2 \ 0-2(0-1 + 0"2)

= ( -2o2AFi )1/~ - or. \ o"2(o"1 + 0-D

FIG. 5. Profile of adhering drop.

3. E V A L U A T I O N OF E X P E R I M E N T S

Theory In the absence of liquid motion, the liquid configuration as shown in Fig. 1C is determined completely by gravity, the value of the film tension O-F, and the interfacial tensions of the single interfaces, 0-1 and 0-2The value of the free energy of bridging can be determined as follows. The force balances at the line of contact between the interfaces read (Fig. 5) O"F COS T = O'1 COS O( dr 0"2 COS /3,

0-7 sin 3' = -0"1 sin a + 0"2 sin/3,

[1]

where a, /3, and 3' are the angles of the extrapolated macroscopic interfaces with the horizontal at the contact line. When interaction between the film interfaces occurs, the film tension will be lower than the sum of the interracial tensions of the free interfaces by an amount equal to the free energy of interaction AFi,

[4]

When AFi < 0, it follows that a + /3 > 0, so that a stable liquid neck is formed between interface and droplet. Inversely, the existence of such a neck constitutes sufficient proof for the existence of attraction between the interfaces (Eq. [3]). If AFi = 0, and/3 become zero, so that on slow retraction the droplet will separate again from the interface.

Calculations The direct measurement of small values of angles a and/3 from photographs is very inaccurate. Therefore, their values were obtained indirectly. The value of a was calculated from an approximate equation describing the shape of the liquid meniscus around a cylinder with its axis perpendicular to the interface (10) (Fig. 5) ZmaXrN-- sin o~[ 0.809 + In rN(1 +acos c0 ] , [5]

[3]

where rN is the radius of the contact line, Zmax is the height of the neck, and a = (0-J Ao'g) lj2, where A0 is the density difference between oil and aqueous phase, and g, the acceleration due to gravity. Using data relevant to the present investigation, the values of c~calculated from Eq. [5] were compared with those derived from the tables of Huh and Scriven (11). Up to a = 75 °, the differences were less than 1%. In our experiments, no deviation from the spherical shape of the droplet due to the presence of the neck was observed. Consequently, the value of/3 was calculated from sin/3 = rN/R, [6]

The value of 3' conforms to the same degree of approximation

where R is the radius of the droplet. When considering the cohesion of emulsion droplets by bridging, their small dimen-

0-V = O"1 + 0-2 + AFt,

[2]

where AFj has a negative value when the film surfaces attract each other. On combining [1] and [2] and neglecting higher-order terms, we obtain -AF~-~

[1 - c o s (a + / 3 ) 1 -

0-10-2

-

0-i + 0-2

(a + 13)2

o-lo-2

2(o-1 + 0-2)

Journal of Colloid and Interface Science, Vol. 83, No. 2, October 1981

MEASUREMENT OF POLYMER BRIDGING FORCES TABLE I

sions will generally cause the force exerted by pressure differences across the film to be small in comparison to the adhesional force at the contact line, Ka, Ka = 2~-o-rN sin a = 27ro-R × sin a sin/3.

[7]

If a fixed value of the free energy of interaction is assumed, the sum of o~ + /3 is constant (Eq. [3]); then Eq. [7] attains its maximum for a =/3. Consequently, the present experiments, where usually o~ ¢ / 3 , can be used to estimate the force of adhesion of emulsion droplets joined by polymer bridging, Ka = 27ro'R sin 2 (1/2)(a + /3).

[8]

4. R E S U L T S

First, we determined whether the free energy of adhesion, as calculated from the sum o f the observed contact angles, a +/3, constitutes a static property o f a given system. To this end an aged PVA layer was compressed to 4.63 × 10 -7 kg/m 2. A droplet of paraffin oil, R = 4.75 × 10-4 m, was brought into contact with this interface, left standing for 30 rain, and then withdrawn at a speed o f 10-8 m s e c - k After stopping the motion, photographs were taken after 5 and 180 rain. The shape of the interfaces remained unchanged: for both photographs, rN = 4.10 -5 m, Z m a x = 10-5 m. The value of o-1 is interpolated from Fig. 4 and equals 19.7 mN m -I. The values of the contact angles can be calculated from Eqs. [5] and [6]: a = 3.1°;/9 = 4.8 ° . If the droplet interface remains uncontaminated by the PVA, 0-2 = 52.8 m N m -1 and Eq. [3] leads to AFi = - 0 . 1 4 m N m -1. It might also be possible that on contact, PVA spreads from the upper interface to the droplet surface. Then, o-2 = 19.7 m N m -1, and AFt = - 0 . 0 9 mN m -1. It should be noted that the determination of the contact angles is independent of the value of the interfacial tension of the droplet, and that

381

Contact Angle o f Paraffin Oil Droplet Against O/W Interface ~F~, Position

rs (10 -~ m)

Zm.~ (10 -~ In)

a (")

B (°)

a + 13 (°)

o-~ ¢ o'~, (raN in-')

I II III IV

4.37 3.50 3.26 4.37

2.5 7.5 8.75 2.5

0.72 2.56 3.10 0.72

6.97 5.58 5.20 6.97

7.69 8.14 8.30 7.69

-0.13 -0.14 -0.15 -0.13

Note, Droplet: R = 3.6 × 10 -4 m. O/W interface: 5.21 x 10-7 kg P V A / m ~. oq = 19.3 m N m - l ; ~re = 52.8 m N m - l ; Ap = 0.14 × 10~ kg m -~.

also the value of AFi is fairly insensitive to the value of o-z. If contamination o f the droplet occurred after formation of the neck a decrease of o~ + /3 by 23% should be observed. In fact, no change is found. Thus, contamination must either be completed during the time that the droplet is pressed against the monolayer, or it will not occur. To check whether the values of the contact angles are independent of the method of forming the configuration (Table I), an aged PVC layer was compressed to 5.21 × 10-7 kg m -2 at the O/W interface. A paraffin droplet was pressed against this interface for 1 hr, and was then r e m o v e d at a speed of 10-8 m sec -1. It was photographed 30 min after stopping the withdrawal (I). The motion was then continued and stopped again (II, III). Finally, the droplet was restored to position I = IV. Table I shows that the contact angles of the receding drop are the same as those of the advancing drop. The value of AFi appears to decrease slightly when the droplet is lowered, i.e., when the contact area is decreased. The influence of the history of the spread polymer layer was investigated in a series of experiments, in which the time during which the droplet was pressed against the interface and the compression of the polymer layer were varied (Table II). In most experiments the aged PVA layer was compressed once before establishing contact Journal of Colloid and Interface Science, Vol. 83, No. 2, October 1981

382

VAN VOORST VADER AND DEKKER T A B L E II

Free Energy of Interaction AFi as a Function of Contact Time (t) and P o l y m e r Adsorption (F) AF, Expefimerit

F (10 -7 k g . m -2)

t (hr)

trt (raN m-')

a +/3 (°)

o-~ + o'~ (raN rn -~)

1 2 3 4 5 6a 7a

2.78 2.78 4.64 9.3 9.3 9.3 9.3

1 3 1.5 2 2.5 1 2

23.3 23.3 19.7 18.2 18.2 18.2 18.2

7.33 6.90 10.27 12.92 13.92 8.49 18.69

-0.14 -0.12 -0.23 -0.34 -0.39 -0.14 -0.71

Note. AO = 0.14 x l0 s kg m-3; c% = 52.8 m N m -1. Interface c o m p r e s s e d and e x p a n d e d s e v e n times.

between droplet and interface. However, in Experiments 6 and 7 the interface was expanded and recompressed seven times before contact. When the interface is compressed only once, variation of the contact time does not influence the value of AFi appreciably. The value of AFi decreases nearly proportionally with the PVA adsorption. However, pronounced time effects are observed when the interface has been repeatedly compressed and reexpanded. In combination with long contact times, a large value ~of AF~ is obtained. Under comparable conditions the values of AF~ in Table II are somewhat lower than those found earlier (Table I). This may be due to slow configurational changes in our PVA stock solution. 5. D I S C U S S I O N

The present investigation has established that polymer-stabilized aqueous films can be formed between immiscible liquid phases, even if only one of the interfaces was covered with polymer before contact. Such films form contact angles with the adjoining single interfaces whose order of magnitude is comparable to that found for Newton detergent films (12). The force of adhesion between emulsion droplets due to bridging can now be estimated using Eq. [8]. For paraffin oil dropJournal of Colloid and Interface Science, Vol. 83, No. 2, October 1981

lets with R = 10-6 m, cr = 10 mN m -1, a + fl = 8° , the bridging force equals 3 x 10-l° N. If we assume the distance between the droplets to be 15 nm, which is the average thickness of an adsorbed PVA layer (13), and estimate the Hamaker constant for the system at 1.9 × 10-22 J, then the van der Waals attraction between the droplets will be at most 7.10 -14 N. This large difference illustrates the effectiveness of polymers as flocculation agents, e.g., in wastewater purification. The formation of stable films requires a rather high polymer coverage of the interface of the order of 10-4 kg m -2, and is promoted by aging of the adsorbed polymer layer. If aging or coverage is insufficient, coalescence of the oil phases is observed. However, deformation of the aged layer prior to contact enhances the buildup of the free energy of interaction. Over a fairly extensive range of polymer adsorptions and contact times, the contact angles are nearly independent of contact time. The free energy of interaction, AFi, is of the order of - 0 . 1 to - 0 . 2 mJ m -2, and its absolute value is far larger than can be ascribed to van der Waals forces (at most - 2 . 3 × 10-~ mJ m -2 under the above conditions). This and the fact that the free energy of interaction is about proportional to the polymer adsorption support our assumption that the interaction is due to polymer bridging. The value of AFi observed in the present experiments is smaller by an order of magnitude than the interfacial pressure of the polymer layer. This suggests that only a few segments of the loops and trains are actually adsorbed on the clean surface. Moreover, AFi is also smaller by an order of magnitude than the interaction energy found for polymer bridging between solids (4). We ascribe this to differences in the polymer conformation at a solid and at a fluid interface. When a polymer loop adsorbed on a solid is pressed against a second surface, relaxation of the tension in the loop can only occur by its partial adsorption on the second

MEASUREMENT OF POLYMER BRIDGING FORCES interface. H o w e v e r , for a loop o f a p o l y m e r a d s o r b e d at a fluid interface, the tension i n d u c e d b y c o m p r e s s i o n can also relax b y tangential sliding o f the trains at b o t h sides o f the loop. T h u s , bridging will d e c r e a s e w h e n the p o l y m e r molecules can m o v e freely o v e r the interface, while e n t a n g l e m e n t o f the a d s o r b e d p o l y m e r m o l e c u l e s will prom o t e bridging. This is in c o n f o r m a n c e to o u r o b s e r v a t i o n that aging and c o m p r e s sion o f the surface p r o m o t e the f o r m a t i o n o f p o l y m e r - s t a b i l i z e d films. 6. REFERENCES I. Ruehrwein, R. A., and Ward, D. W., Soil Sci. 73, 485 (1952). 2. Fleer, G. J., Thesis, University of Wageningen, 1971. 3. Brooks, D. E,, J. Colloid Interface Sci. 43, 714 (1973).

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4. Van Voorst Vader, F., and Dekker, H., in "Tr. Mezhdunar. Kongress Poverkhn. Akt. Veshchestvam, 7th Moscow 1976," Vol. 2(II), p. 691 (1978). 5. Mulder, H., and Walstra, P., "The Milk Fat Globule," p. 179. Wageningen, 1974. 6. Sylvester, N. D., Byeseda, J. J., and Jadeta, B., Ind. Eng. Prod. Res. Develop. 18, 57 (1979). 7. Van Voorst Vader, F., Erkens, Th. F., and Van den Tempel, M., Trans. Faraday Soc. 60, 1170 (1964). 8. Lankveld, J. M. G., Thesis, University of Wageningen, 1970. 9. Van Voorst Vader, F., and Dekker, H., in "Proceedings, Int. Congress Surf. Act., Vlth, Ztirich, 1972," Vol. 2(II), p. 735. 10. Deryagin, B. V., Acad. Sci. USSR 51,519 (1946). 11. Huh, C., and Scriven, L. E., J. Colloid Interface Sci. 30, 323 (1969). 12. De Feyter, J. A., and Vrij, A.,J. Colloidlnterface Sci. 64, 269 (1978). 13. De Feyter, J. A., Benjamins, J., and Veer, F. A., Biopolymers 17, 1759 (1978).

Journal of Colloidand Interface Science, Vol.83, No. 2, October1981