Effects of Adsorption of Low-Molecular-Weight TriblockCopolymers on Interactions between Hydrophobic

Surfaces in Water

K. Eskilsson,*,† B. W. Ninham,‡ F. Tiberg,§ and V. V. Yaminsky‡,|

Physical Chemistry 1, Centre for Chemistry and Chemical Engineering, Lund University,P.O. Box 124, S-221 00 Lund, Sweden, Department of Applied Mathematics, Research Schoolof Physical Engineering, Institute of Advanced Studies, The Australian National University,

Canberra, ACT 0200, Australia, and Institute for Surface Chemistry, P.O. Box 124,S-114 86 Stockholm, Sweden

Received October 20, 1998. In Final Form: January 20, 1999

In this work, we report on the interaction forces between hydrophobed silica surfaces immersed inpolymer solutions. The polymers studied were a series of poly(ethylene oxide)-polytetrahydrofuran-poly(ethylene oxide) (PEO-PTHF-PEO) triblock copolymers and a poly(ethylene oxide) homopolymer.The interaction forces were measured by the interfacial gauge technique. We show how the interactionsare changed by the adsorbed state of the copolymers. This depends on both the copolymer concentrationand the adsorption time. Above a critical surface coverage, the interaction between approaching surfacesat first shows a steric repulsion due to overlap of the adsorbed polymer layers. This repulsion increasesas the distance between the surfaces decreases. In this regime the energy-distance curve could be accountedfor by the theory of grafted polymer brushes of de Gennes. However, for small surface-to-surface distancesthe interaction curves do not follow this prediction. Instead, the repulsion stabilized at a more or lessconstant level with decreasing intersurface separation. Finally, however, hard wall contact was establishedbetween the two surfaces. We infer that adsorbed copolymers to a large extent are expelled from the gapbetween the surfaces in this small surface-to-surface distance range. The force needed to expel copolymersfrom the intersurface gap was shown to be equal to the surface pressure at the solid-liquid interface. Wealso studied the influence of the rate of approach and the separation of the surfaces on the energy-distancecurves. The process of expelling polymers from the surface-to-surface gap was shown to depend on thevelocity of the approaching surfaces and the surface coverage. For high approach rates and/or large surfacecoverages, the lateral mobility of the polymers was such that it inhibited the expulsion process of polymersfrom the gap. However, rapidly repeated force curves, measured at a constant rate, and successively, werefound to be perfectly reproducible.

Introduction

Low-molecular-weight amphiphilic block copolymersexhibit phase behavior and adsorption properties remi-niscent of both surfactants and polymers. The industrialimportance of these molecules is motivation enough toinvestigate their properties in solution and adsorption atinterfaces. Nonionic surfactants and copolymers are todayfrequently used to modify wetting properties, createprotein resistant surfaces, and stabilize colloidal particles,foams, and emulsions. Success in such applications is oftendependent on the interaction forces between surfaces thatcarryadsorbedcopolymers.Thetraditional interferometricsurface force apparatus (SFA) developed a few decadesago opened up the possibility of measuring forces betweensurfaces in solutions containing polymers or surfactants.Interactions between different kinds of surfaces immersedin polymer solutions have since then been studied quiteintensively with the SFA technique. Most studies havebeen performed with adsorbed homopolymers or termi-nally attached polymers. Terminally attached polymersare nonadsorbing polymers that are chemical grafted tothe surface or more commonly block copolymers anchored

preferentially at the surface by one of the blocks. It hasbeen shown in many of these studies that unchargedpolymers of high molecular weight adsorb irreversibly tomany solid interfaces, that is, that these polymers cannotbe expelled from the gap between the two surfaces. Theinteractions between surfaces covered by high-molecular-weight polymers in both good and poor solvents are todayrelatively well understood; see reviews.1-3 Some inves-tigations of interactions between surfaces covered by low-molecular-weight polymers have also been performed.4-6

In these studies it has been inferred that the interactionforces have the same overall appearance as terminallyattached high molecular weight polymers. It has, never-theless, been shown that polymers are expelled from thegap between the surfaces under some conditions.5 Theforce-distance curves obtained during successive com-pressions, however, exhibited a large degree of hysteresis.

Several studies of force interactions in surfactantsystems have been published recently.7-17 These articles

* To whom all correspondence should be addressed. E-mail,Krister.Eskilsson@fkem1.lu.se. Fax. 46-46-2224413.

† Lund University.‡ The Australian National University.§ Institute for Surface Chemistry.| On leave from the Institute of Physical Chemistry, The Russian

Academy of Science, Moscow.

(1) Luckham, P. F. Adv. Colloid Interface Sci. 1991, 34, 191-215.(2) Klein, J. Pure Appl. Chem. 1992, 64, 1577-1584.(3) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove,

T.; Vincent, B. Polymers at Interfaces, 1st ed.; Chapham & Hall, London,ed. 1, 1993.

(4) Ansarifar, M. A.; Luckham, P. F. Polymer 1988, 29, 329-335.(5) Schille’n, K.; Claesson. P. M.; Malmsten, M.; Linse, P.; Booth, C.

J. Phys. Chem. B 1997, 101, 4238-4252.(6) Claesson, P. M.; Golander, C. G. J. Colloid Interface Sci. 1987,

117, 366.(7) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984,

98, 500.

3242 Langmuir 1999, 15, 3242-3249

10.1021/la981469z CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 04/07/1999

display some incompatible views on how to interpret theforce data. The nature of the long-range attraction betweenhydrophilic surfaces covered by small amounts of op-positely charged surfactants is one topic that was greatlydebated. Yaminsky and co-workers,13,15,18,19 analyzedforce-distance curves between surfactant-covered mac-roscopic surfaces within the general framework of theGibbs adsorption equation. They showed, for instance,that the hydrophobic attraction was due to an increasingadsorption of ionic surfactants at the oppositely chargessurfaces with decreasing surface-to-surface distance. Theunderstanding of the repulsive force interactions observedbetween surfactant-covered hydrophobic surfaces has alsobeen improved during recent years. It is clear thatadsorbed surfactant layers can be expelled from theintersurface gap when the surfaces are brought intocontact. The height of the repulsive force barriers appearsto be proportional to the surface pressures exerted by theadsorbed surfactant films.15

In the present work we have studied the effects ofadsorbed poly(ethylene oxide)-polytetrahydrofuran-poly(ethylene oxide) (PEO-PTHF-PEO) triblock copoly-mers on interaction forces between hydrophobic surfaces.A corresponding study of forces between hydrophilicsurfaces was published earlier.20

Experimental SectionSurface Preparation. Melting one end of a 2 mm thick glass

rod (Pyrex) provides a sphere with a diameter of severalmillimeters. Such spheres provided the substrates for the surfaceforce experiments. Unlike the case for SFA interferometry, thescaling radii of such spheres coincide with the macroscopic radii,measured with high accuracy by a micrometer. A freshly moltenglass sample was rinsed in water, dried in a stream of nitrogen,and placed in a desiccator in a saturated vapor of dimethyldi-chlorosilane (DMDCS) for 20 h. After the silane treatment thesamples were rinsed repeatedly with chloroform, ethanol andwater. The way the samples are prepared is very important forthe final result.21 There is a continued discussion about thechemical status of silane layers.22 However, the surfaces preparedas above have been shown to be stable in pure water as well asorganic solvents for many days.23

Surface Forces. Surface forces were measured with a solid-state sensor in the basic setup described earlier.15,24-27 The

apparatus is referred to as the interfacial gauge. The hydrophobicsamples installed in the gauge were immersed in a liquid in aquartz beaker. The polymer concentration was varied by smallamounts of stock solution added to the sample beaker, and stirringwas achieved by rotating the beaker. Forces were determinedfrom measurements of the electric response of a piezoelectricsensor, which results from an external load induced by a magnet.This gives interaction forces between the two surfaces as afunction of their mutual displacement as well as rate ofdisplacement. The acquisition system enables collection of datapoints at a desired frequency (up to 50 kHz). Changing the periodand the amplitude of the loading ramps alters the speed and theload of the samples. The surfaces were moved at a constant speed(≈60 nm/s) in all measurements unless specified otherwise. Anadvantage of using interferometric measurements of surfaceforces is that the absolute zero position can be directly observed.Instead, separation distances quoted here are relative to hardwall contact. This means that there may be polymers sandwichedbetween the surfaces; the actual zero position should then beshifted outward by a factor that is unknown. This, however, doesnot alter the form of the interaction before the contact. Theobtained force-displacement curves are then transferred intoenergy-distance curves by the Derjaguin approximation.

Polymers. Five different (ethylene oxide-tetrahydrofurane-ethylene oxide) triblock copolymers, EOn/2THFmEOn/2, have beenstudied in this work. Depending on the chemical composition,these are referred to as P224-28, P172-14, P146-28, P128-14,and P50-14, respectively (see Table 1). The first number, n, refersto the number of EO groups, and the second, m, refers to thenumber of THF groups of the polymer. The triblock copolymerswere produced by Akzo Nobel Surface Chemistry, by ethoxylatingpolytetrahydrofurane (PTHF) polymers of molecular weights of1000 and 2000 g/mol, respectively. The latter were purchasedfrom BASF. Important properties (i.e. molecular weight andcritical micelle concentration (cmc)) of the copolymers have beencharacterized by NMR diffusion and fluorescence spectroscopy.A description of these characterization procedures is given in ref28. A summary of the properties of these copolymers is presentedin Table 1. The polydispersity index, Mn/Mw, of the copolymersamples is between 1.1 and 1.2 according to the manufacturer.The triblock copolymer samples are free of homopolymers, butthere may be a small contamination by diblock copolymers. ThePEO homopolymer with a molecular weight of 4000 g/mol(SERVA, Feinbiochemica) was used with no further purification.All aqueous solutions were prepared from Millipore water. Theethanol used was freshly distilled.

Results and DiscussionAs a basis for our surface force discussion, we need first

to go through some details of the adsorption of copolymersat a single surface. The adsorptions of the PEO-PTHF-PEO (Pn-m) copolymers and the PEO homopolymer arediscussed in detail in refs 28-29. All copolymers wereobserved to form monolayers on hydrophobized silica. Themiddle tetrahydrofuran block was always preferentiallyanchoredat thesurface, resulting in the formationofPTHFtrains. PEO segments are either anchored at the surfaceor protrude into the aqueous phase, depending criticallyon the surface coverage. The copolymer adsorption iso-therms were well described by the conventional Langmuirexpression, with a plateau adsorption value of about 2.4

(8) Pashly, R. M.; McGuggian, P. M.; Ninham, B. W.; Evans, D. F.Science 1985, 229, 1088.

(9) Luckham, P. F.; Klein, J. J. Colloid Interface Sci. 1986, 117, 149-158.

(10) Kekicheff, P.; Christensson, H. K.; Ninham, B. W. Colloids Surf.1989, 40, 31.

(11) Herder, P. J. J. ColloidInterface Sci. 1990, 134, 346.(12) Rutland, M. W.; Waltermo, A° .; Claesson, P. M. Langmuir 1992,

8, 176.(13) Yaminsky, V. V.; Ninham, B. W.; Christenson, H. K.; Pashly, R.

M. Langmuir 1996, 12, 1936-1943.(14) Yaminsky, V. V.; Jones, C.; Yaminsky, F.; Ninham, B. W.

Langmuir 1996, 12, 3531-3535.(15) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996,

12, 836-850.(16) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir

1997, 13, 4349-4356.(17) Lachlan, M. G.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998,

102, 4288-4294.(18) Yaminsky, V. V. Langmuir 1994, 10, 2710-2717.(19) Yaminsky, V. V.; Christenson, H. K. J. Phys. Chem. 1995, 99,

5176-5179.(20) Eskilsson, K.; Ninham, B. W.; Tiberg, F.; Yaminsky, V. V.

Langmuir 1998, 14, 7287.(21) Yaminsky, V. V. Colloids Surf. A 1997, 129-130, 415-424.(22) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149.(23) Yaminsky, V. V.; Claesson, P. M.; Eriksson, J. C. J. Colloid

Interface Sci. 1993, 161, 91-100.(24) Parker, J. L. Langmuir 1992, 8, 176.(25) Parker, J. L.; Stewart, A. M. Prog. Colloid Polym. Sci. 1992, 88,

162.(26) Stewart, A. M.; Parker, J. L. Rev. Sci. Instrum. 1992, 63, 5626.(27) Stewart, A. M. Meas. Sci. Instrum. 1995, 6, 114.

(28) Eskilsson, K.; Tiberg, F. Macromolecules 1997, 30, 6323-6332.(29) Eskilsson, K.; Tiberg, F. Macromolecules 1998, 31, 5075-5083.

Table 1. Molecular Weight of the Polymer, Number ofEthylene Oxide Groups, n, Number of TetrahyrdrofuranGroups, m, Value of the Critical Micellar Concentration,cmc and the Adsorbed Amount, Γp. Corresponding to the

Ellipsometric Values at the Adsorption Plateau

polymer MW (g mol-1) n m cmc (wt %) Γp (mg m-2)

P224-28 11900 224 28 0.03 2.4P146-28 8400 146 28 0.04 2.3P172-14 8600 172 14 0.2 2.3P128-14 6700 128 14 0.15 2.3P50-14 3200 50 14 0.02 2.4

Interactions between Hydrophobic Surfaces in Water Langmuir, Vol. 15, No. 9, 1999 3243

mg/m2.Sinceall copolymers haveroughly thesameplateausurface coverage (in mass per unit area), the surface areaper molecule must increase more or less linearly with themolecular weight. The isotherm measured for the PEOhomopolymer was a typical high-affinity type polymerisotherm.3 The plateau adsorption was about five timeslarger for the triblock copolymers than for the homopoly-mer. This is explained by accounting for the contributionof the short hydrophobic PTHF blocks in the copolymer.Representative adsorption isotherms for the copolymersand homopolymer are shown in Figure 1. The curves showthe isotherms measured for the P224-28 copolymer (MW) 11 900 g/mol) and the P90 homopolymer (MW ) 4000g/mol), respectively.

Interactions in Air and in Pure Water.Hydrophobicsurfaces in air were observed to jump into adhesive contactat a surface-to-surface separation of about 13-14 nm. Anadhesion energy, F/(2πR), of about 20-22 mN/m2 wascalculated from the pull-off force needed to separate thetwo surfaces. The range of the attractive force was slightlysmaller in water as compared to that in air. The jump intocontact in water was observed at an intersurface distanceof 10 nm (see Figure 2), that is, a few nanometer less thanthat in air. By contrast the adhesion energy in water wasclose to 40 mN/m. This is almost twice the measured valuein air. In water, we also observed a small repulsiveinteraction for intersurface distances larger than the jump-in distance. The repulsion is due to the existence of residualelectrical charges on the methylated glass substrates,which give rise to a weak double-layer interaction. It iswell-known that residual charges often are present onsilica after the surface has been methylated.30 However,the charge density of the hydrophobized surface isgenerally small as compared to that at the bare silicasurface. This was also the case in our study. We remarkthat the measured jump-in distances and adhesion ener-gies agree well with previously published data for thesame type of surfaces.15,31 The measured jump-in distances

in both air and water are larger than expected for thegiven spring stiffness if van der Waals forces alone areresponsible for the attraction between the solid surfaces.32

The origin of the extra attraction has been variouslyattributed to “water structure”, small air bubbles at thesurface, cavitation and/or entanglement, and bridging byprotrudingpolysiloxanechains.Bridging is consistentwiththe fact that the attraction is increased in both air andwater.15,21 However, there is no firm proof that this is thelikely adhesion mechanism. Capillary condensation andbubble formation (in air and water, respectively), forinstance, may very well prove to be the dominant adhesionmechanism. More work is still needed in order to reacha good understanding of the interactions between hydro-phobic surfaces in air and water. However elucidation ofadhesion mechanism is not the aim of the present work.Therefore, we finish this discussion of interactions betweenbare hydrophobic surfaces by noting that there was a cleartendency for the surfaces to slow, about 1 nm before hardwall contact. This soft contact is due to the compressibilityof the hydrophobic coating.

Interaction in Polymer Solutions. We now turn ourattention to some general observations on interactionsbetween surfaces immersed in aqueous polymer solutions.We note first that force curves measured successivelyduring rapidly repeated compressions and separationcycles were essentially identical. All the polymers studied,at all concentrations, shared this feature. Our results showthat the adsorbed polymer layers are able to relax to theiroriginal nonperturbed state during the time the surfacesare separated. A complete approach-separation cycle lastsfor about 1 min. However, interactions were not measuredunder perfect equilibrium conditions. We did indeedobserve a reproducible dependence on the speed of surface-to-surface approach. This will be further discussed below.

The adsorption-desorption kinetics of the copolymersystems was slow compared to the time needed for acomplete force curve measurement. The adsorption-desorption processes due to molecular exchange of poly-mers between the adsorbed layers and the bulk solutioncould in fact be neglected during the measurements. Wecould therefore study situations at which the adsorbed(30) Blake, T. D.; Kitchener, J. A. Trans. Faraday Soc. 1972, 68,

1435.(31) Yaminsky, V. V.; Yusupov, R. K.; Amelina, E. A.; Pchelin, V. A.;

Shchukin, E. D. Kolloidn. Zh. 1975, 37, 918. (32) Yaminsky, V. V.; Ninham, B. W. Langmuir 1993, 9, 3618-3624.

Figure 1. Adsorption isotherms for the triblock copolymerP224-28 and the PEO homopolymer P90. The dashed line isthe best fit to the Langmuir isotherm; the solid line is drawnto guide the eye. The data are taken from ref 20.

Figure 2. Energy-distance curves for four different concen-trations of the triblock copolymer P224-28; the curve for thebare surfaces is displayed as a reference.

3244 Langmuir, Vol. 15, No. 9, 1999 Eskilsson et al.

layer was not at equilibrium with the bulk. To check thatadsorption equilibrium was reached, force-distance curveswere always compared with corresponding curves mea-sured after another hour of equilibration time. Systemswere considered to be in equilibrium with the bulk solutionwhen the force curves before and after this period wereshown to be identical. Now, we proceed to discuss theinteraction forces obtained in the presence of adsorbedhomo- and copolymers, respectively.

PEO Homopolymer Solutions. Measurements of theinteractions in PEO homopolymer solutions were per-formed mainly to provide a bench mark for the copolymersystems. A comparison of the energy-distance curvesmeasured in homo- and copolymer solutions is essentialfor understanding the importance of the middle hydro-phobic PTHF block. The energy-distance curves duringapproach with adsorbed PEO turned out to be similar tothe curves obtained in the absence of adsorbed polymers;that is, only a small electrostatic repulsion was observedprior to the adhesive jump into contact. The jump-indistance was observed to be about 8 nm, which is 2-3 nmshorter than the corresponding distance between baresurfaces. The polymer concentration was varied between10-4 and 10-2 wt %, but only very small concentrationeffects were noticed. However, the depth of the adhesionminimum was decreased. This together with the changeof the jump-in distance shows that some polymers wereindeed adsorbed at the surface. The adhesion energy inwater decreased from a value of F/(2πR) ) 40 mN/m inwater to 28 mN/m at 10-4 wt % and 24 mN/m at 10-2 wt% of PEO homopolymer. The relatively small change inadhesion with concentration change was expected. Theadsorption isotherm measured by ellipsometry shows thatthe surface has a saturated polymer coverage of roughly0.5 mg/m2 already at the polymer concentration 10-5 wt% (see Figure 1).

The absence of nonelectrostatic repulsion at distanceslarger than 8 nm indicates that these polymer layers arethin. The number density of polymer chains stretchingfurther out from the isolated surface than 4 nm musttherefore be small. This is a reasonable result, since anadsorbed layer thickness of 4 nm is almost twice the radiusof gyration of a P90 homopolymer.33 The ellipsometricallydetermined adsorbed layer thickness for the PEO ho-mopolymer at hydrophobized silica was only around 3 nm.Other studies34,35 also show that PEO polymers formrelatively thin adsorbed layers at hydrophobic latexsurfaces. Hence, we can conclude that steric repulsiveinteractions are not observed during the mutual approachof the two hydrophobic surfaces covered by the P90polymer. The adsorbed polymer remaining in the gap afterhard wall contact does, however, decrease the adhesiveinteraction between surfaces.

Triblock Copolymer Solution. Figure 2 shows theenergy-distance curves measured in aqueous solutionscontaining different amounts of the copolymer P224-28.As a reference, we also show the force curve measured inpure water. All measurements were performed after bulkequilibration. The energy-distance curves have the samecharacteristic features at all polymer concentrations. Theforce curves begin to diverge from the water referencecurve as the distance between the surfaces becomessmaller. The deviation results from a larger repulsive forcecontribution. This begins at a distance that from here onis referred to as the steric force onset distance. This

distance should be roughly speaking equal to twice theadsorbed layer thickness. We define the onset distance asthe distance where the repulsive energy measured for thepolymer system is 0.05 mN/m larger than that at thecorresponding distance on the reference curve. Therepulsion continues to increase as the surface-to-surfacedistance decreases to values smaller than the onsetdistance. But close to the hard wall contact, we find thatthis increase in the repulsive interactions levels off andfinallybecomesalmost constantwithdecreasing thickness.We will refer to the energy where the repulsive interactionslevel off as the barrier height.

The steric force onset distance increased from 14.5 to21 nm when the copolymer concentration was increasedfrom 5 × 10-5 to 10-3 wt %. Above 10-3 wt %, no furtherchange was observed in the force onset distance withincreasing concentration. The high sensitivity of theinterfacial gauge makes the measurements rather sensi-tive for small volume fractions of polymer tails protrudinginto the solution. We believe then that the onset distanceshould be a representative measure of the hydrodynamicthickness of the adsorbed layer at the isolated surface.The increase in layer thickness from 7.2 to 10.5 nm occurswhen the adsorbed amount increases from 1 to 2.4 mg/m2.In our previous ellipsometric study, we found that at asurface coverage above 1 mg/m2 the adsorbed layerthickness increases only slowly with increased surfacecoverage due to interactions between the PEO chains.28

In Figure 3, we have plotted the adsorbed amount(obtained from the adsorption isotherm) versus the barrierheight at different polymer concentrations. The barrierheight in this concentration interval increases as theadsorbed amount increases. There also exists a criticaladsorbed amount, above which the steric repulsiveinteractions dominate over attractive interactions. Thisobservation agrees with the fact that we did not observeany steric repulsion between surfaces with adsorbed PEOhomopolymers for which the plateau adsorption value wasabout 0.5 mg/m2. Hence, we note that, at these rather lowcoverages, surface force measurements provide littleinformation about the adsorbed layer characteristics.

(33) Bhat, R.; Timasheff, S. N. Protein Sci. 1992, 1, 1133-1143.(34) Baker, J. A.; Berg, C. J. Langmuir 1988, 4, 1055-1061.(35) Killmann, E.; Maier, H;, Baker, J. A. Colloids Surf. 1988, 31,

51-71.

Figure 3. Height of the repulsive barrier for four differentconcentrations of the triblock copolymer P224-28, as a functionof adsorbed amount. The adsorbed amount is taken from Figure1 and put into correspondence with the force data according tothe concentration used.

Interactions between Hydrophobic Surfaces in Water Langmuir, Vol. 15, No. 9, 1999 3245

At higher surface excess values, in the presence of therepulsive barrier, the height of the barrier is inferred tobe a measure of the surface pressure exerted by theadsorbed copolymer layers. The very reason for thedistance independent force, observed at short surface-to-surface separations, is related to molecules beinglaterally displaced from their original positions in the gap.Copolymers become expelled from the intersurface gap,as thedistancebetweenthesurfacesbecomessubstantiallysmaller than twice the adsorbed layer thickness of thecopolymer. The force needed to expel the copolymersdepends on the equilibrium surface pressure and also tosome extent on the rate of surface-to-surface approach.The copolymers are forced out from the gap when thepressure between the surfaces exceeds the surface pres-sure outside the contact zone. The fact that the energyover this range is net repulsive rather than net attractiveis due to the slow kinetics of the depletion process and thefact that not all copolymers are depleted between thesurfaces. It was furthermore observed that the size of thedepletion range decreased slightly as the adsorbed amountincreased. It was also noticed that, for surfaces moved athigher speeds over the depletion range at lower surfacecoverages, the resistance is lower and the interactiontherefore more attractive. All measurements in Figure 2were performed with a measurement time-resolution of100 points per second.

On separation, the surfaces could be separated by almost3 nm, before they jumped out of contact. This distance didnot change much as the polymer concentration wasincreased, but the magnitude of the adhesion againdecreased substantially. The decrease may be partiallyrelated to the increased surface pressure with increasingsurface excess. But still the decrease is larger thanexpected from the changed surface pressure alone. Thisfact again indicates that some copolymers are trappedbetween the surfaces after the compression into hard wallcontact.

Influence of Surface Compression and Separation Rates.The rate at which the surfaces approach each other wasfound to be an important parameter that to some extentalso influences the energy-distance curves. In all mea-surements presented so far, the rate of the moving surfacewas about 60 nm s-1. By changing the period andamplitude of the magnetic loading ramps, we were ableto vary the rate of approach and separation. Energy-distance curves obtained at different rates are shown inFigure 4. The measurements were carried out at twodifferent P224-28 polymer concentrations, 5 × 10-5 and1 × 10-3 wt %, respectively. It is clear from the graph thatthe repulsive barrier increases with an increasing ap-proach rate. At the largest speeds and copolymer con-centrations, the force curves do not stop increasing closerto the hard wall contact. Rather, the force continues toincrease all the way into contact, but with a much smallerslope. These kinetic effects are related to the rate of lateralmovement of the copolymers at the surface. When thesurfaces are compressed rapidly, the copolymers do nothave enough time to move out of the intersurface gapduring the compression. This results in a continuedincrease of the barrier height. Ultimately this is aconsequence of the fact that more copolymers remain inthe gap under a certain force load. Finally, however, mostof copolymer molecules are expelled, but higher loads areneeded to do so. It should be emphasized that the increaseof barrier height due to increased approach speed is smallrelative to the total barrier.

Adsorption Kinetics and Surface Forces. In the above,we have discussed situations in which the adsorbed layer

is inequilibriumwith thebulksolution.Weproceed furtherto the relation between adsorption kinetics and the forceof interaction, that is, interaction forces during situationsin which the adsorbed layer was clearly not in equilibriumwith the bulk. It was possible to study such situationsbecause the adsorption rates of the copolymers at low bulkconcentrations were slow compared to the time neededfor a force measurement. In Figure 5, we show the energy-distance curves that were obtained when force measure-ment runs that were performed first at a bulk P224-28concentration of 10-4 wt % were then subsequentlyincreased to 10-3 wt %. The curve at time zero correspondsto the equilibrium energy-distance curve at 10-4 wt % ofcopolymer. The other curves represent measurements atdifferent times after the addition of copolymer. The energy-distance curves obtained with increasing time are rep-resentative of the evolution of the forces with increasing

Figure 4. Energy-distance curves for different approachspeeds. The curves are shown for concentrations 5 × 10-5 and10-3 wt % of the triblock copolymer, P224-28.

Figure 5. The energy-distance curve at different times afteran increase of the triblock copolymer P224-28 concentrationfrom 10-4 to 10-3 wt %. The curve at time zero corresponds tothe equilibrium energy-distance curve at 10-4 wt %.

3246 Langmuir, Vol. 15, No. 9, 1999 Eskilsson et al.

surface coverage. We can therefore follow the adsorbedlayer during its continued buildup with time. As theadsorbed amount increases with time at constant bulkconcentration, the height of therepulsivebarrier increases.We also notice that the increase of the repulsive interactionwith decreasing distance becomes steeper. Both theseobservations are in agreement with an increased polymerdensity in the surface region. We further observe that thegeneral characteristics of the force-distance curves aresimilar to those obtained under equilibrium conditions.It is important to note that during this kinetic study wedid not observe any hysteresis in consecutive compressionand separation cycles. This shows that the dynamic surfacepressure (acting at a given instance of time) can bemeasured by the interfacial gauge technique. The mea-surement is the surface analogue of a dynamic surfacetension measurement at the liquid-vapor interface.

Effects of Polymer Chemical Composition. It is of interestto study how force interactions are affected by changingthe chemical composition and size of copolymer segments.For this purpose, a number of different Pn-m copolymerswere used. All experiments were performed at a polymerconcentration of 10-2 wt %. All systems were, furthermore,allowed to equilibrate before the measurements wereperformed. In Figure 6, we show the different energy-distance curves obtained on approach. The shapes of allcopolymer force curves are similar to that of the curveobtained for the largest copolymer P224-28, which hasbeen discussed above. For the copolymers P146-28 andP50-14, we did not notice the distance indifferent repulsiveforce region obtained close to hard wall contact. Therepulsion in this region continued to increase slowly withdecreasing intersurface distance all the way into contact.This resembles the situation obtained during measure-ments with adsorbed P224-28 at higher approach rates.Our earlier ellipsometric measurements showed that theP146-28 and P50-14 copolymer systems form the mostdense adsorbed layers. Hence, we can interpret the kineticeffect as being due to changing lateral mobility. The higherinner densities of P146-28 and P50-14 compared to thoseof other polymers result in a decreased lateral mobility.This result may prove useful for the optimization ofsterically stabilized systems. Another observation thatdeserves to be mentioned is that the copolymers can be

divided into two groups depending on the THF blocklength. For each group, the repulsive barrier increasessteadily with the number of EO segments, both with regardto onset distance and the barrier height.

Barrier Height in Relation to Surface Pressure. Whatis measured in a surface force experiment is essentiallythe compressibility of the adsorbed layer. Resistance tocompression results in a repulsive force, while attractiveinteractions can arise if the amount of material is increasedin the intersurface gap during a surface-to-surface sepa-ration decrease. Hydrophobic and van der Waals interac-tions also result in attractive interactions between thetwo surfaces. In our case (for adsorbed amounts and layerthicknesses significantly larger than about 0.5 mg/m-2

and 5 nm, respectively), repulsive interactions dominatethe total force interaction during surface-to-surface ap-proach. This repulsion is due to the overlap of the adsorbedcopolymer layers. Attractive forces dominate the interac-tion between bare surfaces and surfaces with low copoly-mer coverage. We have also shown that the adsorbedpolymers to a large extent are expelled from the inter-surface gap at distances close to hard wall contact.However, the layers quickly re-form when the surfacesare separated, so that no hysteresis is observed betweentwo consecutive force measurement cycles. This was alsoshown to be the case when the adsorbed layers were notin equilibrium with the surrounding bulk solution. Thetransport of polymer from the bulk is very slow comparedto the time needed for two consecutive force runs.29 Wecan conclude from this that the adsorbed layers aredisrupted on approach but retain the capacity to re-formin the contact region even without an efficient exchangewith the bulk solution. The reservoir for this self-regulating system is simply the surface area outside thecontact zone. In a Langmuir trough, a monolayer readjustsitself in response to changed surface area. The equilibriumdensity profile in the lateral direction is maintainedwithout exchange with the bulk phase. The Gibbs equationcan be used to understand this phenomenon. We will hereuse the same approach to interpret our force curves.

The repulsive barrier height is assumed to representthe pressure needed to expel copolymers from the inter-surface gap. This should correspond to roughly twice thesurface pressure at the undisturbed solid-liquid interface.A simplified way to calculate the surface pressure is tocombine the Langmuir isotherm with the Gibbs equation.The resulting Szyszkowski equation is strictly only validfor highly ideal systems.36,37 Nevertheless, it can often besuccessfully applied to describe more complex systems.The surface pressure is given by the following equation.

The constants Γm and a are obtained by fitting theexperimental copolymer isotherms to the Langmuir equa-tion. The calculated surface pressure at the nondisturbedinterface (using eq 1) and half the value of the repulsivebarrier for P224-28 are plotted in Figure 7 against thebulk concentration. We can see from the graph that themeasured values of the barrier height agree relativelywell with the surface pressure values calculated (by theSzyszkowski equation) from the independently measuredadsorbed amounts. This may appear a little surprisingdue to the extreme simplifications used and the fact thatwe have already shown that there exists a force depen-dence of the approach speed of the surfaces (i.e. a

(36) Von Szyszkowski, B. Z. Phys. Chem. 1908, 64, 385.(37) Jones, P.; Hockey, J. A. Trans Faraday Soc. 1971, 67, 2679.

Figure6. Energy-distance curves for the different copolymers.All curves are obtained at equilibrium adsorption of polymersat 10-2 wt % concentration.

πsl ) γ° - γ ) RTΓm ln(1 + aC) (1)

Interactions between Hydrophobic Surfaces in Water Langmuir, Vol. 15, No. 9, 1999 3247

nonequilibrium effect). However, the effect of approachrate is relatively small compared to the absolute value ofthe barrier height. We conclude by noting that adsorptiondata can indeed be used to estimate the repulsive forcebarrier height between copolymer-covered hydrophobicsurfaces. This is probably also true for other systems, suchas surfactants.

Barrier Shape. In an attempt to further understandthe force curves and the process of lateral displacementof adsorbed polymers from the gap between two ap-proaching surfaces, the repulsive part of the force-distance curves was fitted to the theory of de Gennes.This was developed to interpret interactions betweenterminally grafted polymer brushes.1,38,39 The relationbetween the energy of interaction of tethered polymersand the distance between surfaces is given by the followingequation.

In this theoretical formula, 2L0 is the onset distance forthe steric repulsion, s0 is the spacing between two polymerchains, k is the Boltzmann constant, and T is the absolutetemperature. The spacing between two polymer chains s0can be approximated from the adsorbed amount deter-mined by ellipsometry. We calculated the area permolecule and divided this by a factor of 2 to account forthe fact that the copolymers have two PEO chains thatare anchored preferentially to the surface by the THFblock. The value so obtained was then used in eq 2 to fitthe experimental data. The prefactor, which is missing ineq 2, was used as the fitting parameter (between 0.2 and0.3 in our case). The experimental and fitted energy-distance curves obtained for two different polymer con-centrations (and surface coverages) are shown in Figure

8. The theoretical curve fits the experiment very well atlarge separation. At smaller separations, however, the fitbreaks down. We infer that the fit begins to deviate whenthe polymers begin to be laterally forced out from the gap.Hence, as discussed earlier, as they move out of the gapbetween approaching surfaces, this results in an increaseof s0, in the interaction zone, and thereby also in a decreaseof the interaction energy.

It is interesting to note that the theoretical energy versusdistance curve obtained at the lower copolymer concen-tration (i.e. 10-4 wt %) can be fitted almost to the maximumheight of the repulsive barrier, while the fit starts todeviate relatively early for the more concentrated sample.The reason for this is probably the fact that changes of s0are relatively speaking larger at high surface coveragesand large forces. Note the very strong dependence of theinteraction energy on s0. Finally, we want to emphasizethe fact that most of the force-distance curve charac-teristics can be accounted for by using eqs 1 and 2 for longand short surface-to-surface distance regimes, respec-tively.

Conclusions

We have come to a number of general conclusionsregarding the interaction forces between hydrophobicsurfaces covered by adsorbed low-molecular-weight tri-block copolymers. We confirm the earlier describeddevelopment of the adsorbed layer with coverage. Thisincluded a transition from a mixed pancake layer at lowconcentrations to a brush-type structure at higher con-centrations. We have also shown that the polymers studiedin this work do not act as stabilizers for hydrophobiccolloids below a critical adsorbed amount and layerthickness. These are roughly 0.5 mg/m-2 and 5 nm,respectively. The copolymers become more and moreefficient as the adsorbed amount increases. The force atlarger intersurface distances at higher coverages was welldescribed by the theory of de Gennes for terminally graftedpolymer chains. We further show the maximum value ofthe steric repulsive barrier (obtained closer to hard wallcontact) is equal to twice the surface pressure of theadsorbed polymer layer. This pressure can be estimatedand calculated independently from the measured adsorp-

(38) de Gennes, P. G. Polymer adsorption. In Liquids at interfaces;Charvolin, J., Joanny, J. F., Zinn-Justin, J., Eds.; North-Holland:Amsterdam, 1990.

(39) de Gennes, P. G. Adv. Colloid Interface Sci. 1987, 27, 189.

Figure 7. Surface pressure of the triblock copolymer P224-28calculated from the Szyszkowski equation (open circles) and ashalf of the barrier height from Figure 2 (filled squares).

E(D) ∝ kTs0

3([ (2L0)9/4

1.25(D)5/4+ D7/4

1.75(2L0)3/4] -

[ 2L0

1.25-

2L0

1.75]) (2)

Figure 8. Experimental energy-distance curves and the fitby the theory of de Gennes (eq 2), for concentrations 10-4 and10-2 wt % of the triblock copolymer P224-28.

3248 Langmuir, Vol. 15, No. 9, 1999 Eskilsson et al.

tion isotherm by the use of the Szyszkowski equation.Close to hard wall contact, polymers are expelled fromthe gap between the surfaces. This process begins whenthe pressure between the surfaces exceeds the surfacepressure at the solid-liquid interface. We have also shownthat the surface forces exhibit a dependence on both therate of the surface-to-surface approach and the volumefraction of polymers in the adsorbed layer. Higher speedand increased volume fraction result in an increase of therepulsive barrier. This rate dependence occurs becausepolymers become kinetically trapped in the intersurfacegap. The adsorbed layers were also observed to re-form

rapidly when the surfaces were separated. Rapidlyrepeated force curves were shown to be perfectly repro-ducible. This is an important observation in the contextof colloidal stability, where the stabilizing polymer layersare subjected to repeated collisions.

Acknowledgment. We thank the Swedish ResearchCouncil for Engineering Sciences (TFR) and the SwedishNational Board for Technical Development (NUTEK) forfinancial support.

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