Polymer-Surfactant Interaction As Revealed by the TimeDependence of Surface Tension. The EHEC/SDS/Water

System

Inger Nahringbauer

Department of Physical and Inorganic Pharmaceutical Chemistry, Uppsala University,Uppsala Biomedical Center, P.O. Box 574, S-751 23 Uppsala, Sweden

Received October 7, 1996. In Final Form: February 10, 1997X

Adsorption properties at an air/water interface and interaction between a nonionic polymer ethyl-(hydroxyethyl) cellulose (EHEC)andsodiumdodecyl sulfate (SDS)werestudiedbypendantdrop tensiometry,enhanced by video-image digitization and computerized data handling. Three studies were performedover the concentration range 0-19 mM SDS with fixed concentrations of EHEC: 0.2% w/w, 12 ppm, and2 ppm. Particular emphasis was placed on the investigation of the dynamics of surface tension in thepresence and absence of SDS. This study demonstrates that the onset of complex (cluster) formation canbe identified by surface-tension measurements even when a very hydrophobic polymer such as EHEC ispart of the complex. Considerable synergism in surface activity is observed in the EHEC-SDSmixtures.Below the critical aggregation concentration (cac), the degree of surface-tension reduction was found todepend on the bulk concentration of surfactant molecules, as well as the number of polymer segments inactual contactwith the surface at the time ofmeasurement. At andabove cac an extraordinary accelerationof polymer adsorption was observed following the addition of SDS. Considerable evidence regarding thepresence of a polymer/surfactant complexat the air/water interface is discussed in thepaper. Thedynamicsof surface tension, at and above cac, are explained in terms of the observations that charged clusters areboundalong thepolymer chainand causeanappreciable change of thepolymer conformation. Inagreementwith the results, polyelectrolytic properties of the EHEC/DS- complex cause the complex to become firmlyadsorbed to the air/water interface. The adsorption kinetics strongly suggested a progressive ordering ofthe polymer/surfactant aggregates within the monolayer, based on observations of the surface film afteraging (17 h). The findings can be of practical use in applications where quick wetting and absorption arerequired, e.g., in injected, spread, or inhaled drug formulations.

Introduction

Interaction between polymers and surfactants in aque-ous solutions has attracted much interest, due to theapplication of mixed polymer/surfactant systems in vari-ous fields, and a number of reviews have been publishedin this area.1-5 As described by Goddard,6 the previousstudies have primarily addressed bulk properties ofaqueous polymer/surfactant solutions. In contrast, fewstudies have attempted to examine the polymer-surfac-tant interaction at interfaces (air/liquid and liquid/liquid)or thedynamic adsorptionbehavior of polymer/surfactantmixtures. Synergisminsurfaceactivity, andthedynamicsof the adsorption of the particles involved, are of greatimportance in many processes where colloid stability,surface modifications, and wetting are fundamentalphenomena. The time scale of adsorption can varyappreciably. Surfactants and surface-active polymers, aswell asmixturesof thesespecies, reachequilibriumsurfacetension or equilibrium adsorption concentration at timesranging from milliseconds to several hours.The present work is concerned with the interaction

phenomena at air/liquid interfaces. It applies to a well-defined system, viz., aqueous solutions of the nonionicpolymer ethyl(hydroxyethyl) cellulose (EHEC) and theanionic surfactant sodium dodecyl sulfate (SDS). Theprimary objectives were (a) to study and demonstratesynergism in surface activity of polymer/surfactant mix-tures, especially in very dilute surfactant solutions, (b) to

investigate thedynamicsof surface tension, in thepresenceand absence of an anionic surfactant, and the kinetics ofpolymer adsorption, and (c) to elucidate the effects ofaggregation on adsorption properties and the relationbetween bulk structures and structures formed at inter-faces.

Previous Works on EHEC

Aqueous solutions containing surfactants and cellulosederivatives are important to several industrial applica-tions.7 During the last 10 years, especially, studies onhydrophobicpolymers suchasEHEChavebeenexpanded.Awide range of techniqueshave beenused to characterizevarious grades of EHEC8-11 and to monitor the array ofcomplex interactions involved in bulk aqueous solutionscontaining EHEC and cosolutes.12-32 From changes in

X Abstract published inAdvanceACSAbstracts,March15, 1997.(1) Robb, I. D. Surfactant Sci. Ser. 1981, 11, 109.(2) Goddard, E. D. Colloids Surf. 1986, 19, 255.(3) Goddard, E. D. Colloids Surf. 1986, 19, 301.(4) Hayakawa, K.; Kwak, J. C. T. Surfactant Sci. Ser. 1991, 37, 189.(5) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22,

85.(6) Goddard, E. D. J. Am. Oil Chem. Soc. 1994, 71, 1.

(7) Greminger,G.K., Jr.;Krumel,K.L. InHandbookofWater-SolubleGums andResins; Davidson, R. L., Ed.;McGraw-Hill: NewYork, 1980.

(8) Carlsson, A.; Lindman, B.; Nilsson, P. G.; Karlsson, G. Polymer1986, 27, 431.

(9) Malmsten, M.; Lindman, B. Langmuir 1990, 6, 357.(10) Nahringbauer, I. J. Colloid Interface Sci. 1995, 176, 318.(11) Nilsson, S.; Sundeloef, L.O.; Porsch,B.Carbohydr.Polym.1996,

28, 265.(12) Carlsson, A.; Karlstroem, G.; Lindman, B. Langmuir 1986, 2,

536.(13) Carlsson, A.; Karlstroem, G.; Lindman, B.; Stenberg, O.Colloid

Polym. Sci. 1988, 266, 1031.(14) Carlsson, A.; Karlstroem, G.; Lindman, B. J. Phys. Chem. 1989,

93, 3673.(15) Carlsson, A.; Lindman, B.; Watanabe, T.; Shirahama, K.

Langmuir 1989, 5, 1250.(16) Karlstroem, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990,

94, 5005.(17) Lindman,B.; Carlsson,A.;Karlstroem,G.Adv.Colloid Interface

Sci. 1990, 32, 183.(18) Karlstroem, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990,

94, 5005.(19) Lindman, B.; Carlsson, A.; Thalberg, K.; Bogentoft, C. Actual.

Chim. 1991, 181.

2242 Langmuir 1997, 13, 2242-2249

S0743-7463(96)00976-6 CCC: $14.00 © 1997 American Chemical Society

thebulkproperties of solutions, e.g., rheological propertiesandphase behavior, it has beenwell established that bothanionic and cationic surfactants interact strongly withtheEHECpolymer. Anionic surfactants generally exhibitsignificantly stronger interactions than cationic surfac-tants of similar chain length. The dramatic change inbulk properties is explained by the onset of a stronginteraction between polymer and surfactant. Mixedhydrophobic clusters of polymer and surfactant alkylchains are formed. For EHEC, the process is favored,since the macromolecule contains a low concentration ofethyl groups. Also, these groups tend not only to self-associate in solution but also to attract surfactants bysupplying “nuclei” for theiraggregation. Asaconsequenceof the chain structure of the polymer, complex 3-dimen-sional networks are formed at high polymer concentra-tions. The onset of cluster formation in the bulk phaseof solution is assumed to take place when the surfactantconcentration reaches a value often referred to as thecritical aggregation concentration (cac). Apparently, thepresence of the polymer facilitates the surfactant micel-lization, since cac is always less than the normal criticalmicelle concentration (cmc) of the surfactant.In spite of the huge number of studies on cellulose

derivatives and surfactants, the mechanism of the inter-actions between the particles is still inadequately un-derstood. For instance, it is unknown whether clustersalso form at the surface of polymer solutions. As pointedoutbyGoddard,3 insomeaqueouspolycation/SDSsystems,thepolymer itself ispart ofahighlysurface-active complex.On the basis of this finding, he proposed a model of acomplex that was continuously growing at the air/waterinterface. Furthermore, he reasoned that themechanismof aggregation at the interface might be similar to thatone in bulk solution.33,34 One aimof the presentworkwasto further develop his model. In consequence, someimportant questions were raised. For instance, can anonionic polymer such as EHEC also form surface-activecomplexes with surfactants, and if so, can the onset ofcomplex (cluster) formation be identified by surface-tension measurements?An exhaustive analysis of the interaction between

EHEC and surfactants at liquid interfaces necessitatesspecific knowledge regarding the adsorption to liquidinterfaces of the polymer itself. When a flexible surface-active polymer such asEHECadsorbs at an interface, theconformation of its chain changes. Some portions of thebackbone of the polymer will be in direct contact with theinterfaceas trains, and the remainingparts of thepolymer

will extend into the solution as loops or tails. Recentstudies by the author regarding the concentration andtime dependence of surface tension10 and interfacialtension20 have provided information as to the surfaceactivity of EHEC and the kinetics of polymer adsorptionto air/liquid interfaces. A very high surface activity ofEHEC was observed. Obviously, the heterogeneoussubstitution of hydrophilic ethylene oxide groups andhydrophobic ethyl groups along the cellulose backboneand the flexibility of the polymer chain of EHEC are veryfavorable in respect to its surface activity. Furthermore,based on the time dependence of the surface tension ofextremely dilute EHEC solutions, three consecutivekinetic regions of polymeradsorptionwere observed.Theyappeared as an induction region (t < tcr1), a surface-coverage region (tcr1 < t < tcr2), and a mesophase region(tcr2 < t). A generalized curve, dynamic surface tensionversus logarithmic time, is shown in Figure 1.The induction period of EHEC adsorption, t < tcr1, was

found to be concentration-dependent, and the onset of thesurface-tensionchangewassuggested tobeduetoasuddenincrease in the number of adsorbed polymer segments,most likely caused by the uncoiling and spreading ofpolymer chains. In the case of a polymer molecule, itshould be noted that the process affecting the surfacetension is adsorption/desorption of the segments of thepolymer.35 The transport ofmacromolecules fromthebulkto the subsurface is certainly governed by diffusion; i.e.,the rate of transport of EHECmolecules depends on bulkconcentration andmolecular weight, but the dynamics ofsurface-tension reduction are related to the change in thenumber of polymer segments in actual contact with thesurface. Consequently, the number of active polymersegments isgenerallyhigher thanthenumberofmoleculestransported to the subsurface, and less than the totalnumber of segments in the monolayer.In the surface-coverageperiod, tcr1 < t< tcr2, thekinetics

of EHEC adsorption were proven to be governed by anonlinear Langmuir mechanism. According to the hy-pothetical model suggested, all surface-active polymersegments are adsorbedand theactual adsorptiondependson the fraction of surface coverage. Furthermore, since

(20) Nahringbauer, I. Prog. Colloid Polym. Sci. 1991, 84, 200.(21) Holmberg, C.; Nilsson, S.; Singh, S. K.; Sundeloef, L. O. J. Phys.

Chem. 1992, 96, 871.(22) Zhang, K.; Karlstroem, G.; Lindman, B. Prog. Colloid Polym.

Sci. 1992, 88, 1.(23) Zana, R.; Binana-Limbele, W.; Kamenka, N.; Lindman, B. J.

Phys. Chem. 1992, 96, 5461.(24) Nilsson, S.; Holmberg, C.; Sundeloef, L. O. Colloid Polym. Sci.

1994, 272, 338.(25) Kamenka,N.; Burgaud, I.; Zana,R.; Lindman,B.J.Phys.Chem.

1994, 98, 6785.(26) Nilsson, S.; Holmberg, C.; Sundeloef, L. O. Colloid Polym. Sci.

1995, 273, 83.(27) Nystroem, B.; Walderhaug, H.; Hansen, F. K.; Lindman, B.

Langmuir 1995, 11, 750.(28) Nystroem, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11,

1994.(29) Nystroem, B.; Lindman, B. Macromolecules 1995, 28, 967.(30) Walderhaug,H.;Nystroem,B.;Hansen,F.K.; Lindman,B.Prog.

Colloid Polym. Sci. 1995, 98, 51.(31) Walderhaug, H.; Nystroem, B.; Hansen, F. K.; Lindman, B. J.

Phys. Chem. 1995, 99, 4672.(32) Holmberg, C.; Sundeloef, L.-O. Langmuir 1996, 12, 883.(33) Shirahama, K.; Tsujii, K.; Takagi, T. J. Biochem. (Tokyo) 1974,

75, 309.(34) Cabane, C. J. Phys. Chem. 1977, 81, 1639.

(35) Lankveld, J. M. G.; Lyklema, J. J. Colloid Interface Sci. 1972,41, 454.

Figure1. Curve showinggeneralizeddynamic surface tensionversus logarithmic time. The curve illustrates three stages: t< tcr1, the induction period, tcr1 < t < tcr2p, the period of quicklydecreasing surface tension, and tcr2 < t, the period of mesoequi-librium and the transition to the steady state.

Polymer-Surfactant Interaction Langmuir, Vol. 13, No. 8, 1997 2243

a large number of adsorbed segments belong to the samemacromolecule, a cooperative attachment of segmentsenhances the process. The following equations, derivedby the author,10 quantitatively describe the kinetics ofpolymer adsorption from bulk solution to air/liquidinterfaces. In eq 1,n(t) represents thenumber of polymersegments per unit area of monolayer transported to thesubsurface at time t.

NA is Avogadro’s number, D (m2/s) is the diffusioncoefficient, Mseg (g/mol) is the average molecular weightof one polymer segment, and co (g/m3) is the bulkconcentration. Equation 2 provides the relationshipbetween surface tension and n(t):

γo (N/m) represents the surface tension of solvent, γ(t)(N/m) is the dynamic surface tension at time t, γm is thesurface tension at surface saturation (see Figure 1),Kn isthe overall equilibrium constant for the cooperativebinding of n* molecules, and ν is the average fraction ofn(t) in actual contact with the surface, i.e., polymersegments contributing to the surface coverage.In the mesophase region, t > tcr2, following complete

surface coverage, the equilibration of surface tensionwasfound to continue for several days. Loopingwas suggestedto start at tcr2 since, according to statistical predictions,36,37macromolecules adopt a more extended conformationperpendicular to thesurface,athighsurface concentration.Each adsorbed polymer chain will have an increasingnumber of its segments below the surface, i.e., in the formof loopsand tails, andadecreasingnumber of its segmentsattached to the surface. Several observations workingwith aqueous solutions of pure EHEC indicated a pro-gressive transition of the surface film to amore condensedphase.A comparison with various nonionic alkyl celluloses

shows that the adsorption kinetics of EHEC to the air/liquid interfaces are not unique. Probably, similar pat-terns occur for many nonionic alkyl celluloses. Observedvariations in surface tension (surface pressure) at surfacecoverage can generally be explained in terms of theflexibility of themacromolecule and/or thehydrophobicityof the various polymer segments. For instance, the samepatterns of adsorption regionsdescribedabove (seeFigure1) were found by Chang and Gray38 for (hydroxypropyl)-methyl cellulose (HPMC). Furthermore, referring to thestudies by Zografi and collaborators39-42 of spread andadsorbed monolayers of hydroxypropyl cellulose (HPC)and hydroxyethyl cellulose (HEC), it is clear that theadsorption kinetics of HPC,HEC, and EHEC are similar.The formation of irreversible monolayers at air/waterinterfaces seems to be characteristic of alkyl celluloses,as shown in the studies of HPMC, HPC, HEC, andEHEC.10,38-42 In the works on HPC and HEC, someproperties of adsorbed monolayers (e.g. compressibility)

were found to be the same as those formed by spreadingfrom a solvent. Furthermore, as in the case of EHEC, anappreciable looping was thought to occur after theadsorbed amount of the polymer (HPCandHEC) reachedits limiting value.42

Experimental SectionMaterials. EHEC powder (Bermocoll cst-103) was kindly

supplied by Berol Nobel AB, Stenungsund, Sweden (from thesamebatchpreviouslyutilized10,20). This grade ofEHEC is fairlyhydrophobic and highly polydisperse. The average degree ofsubstitution is 1.5 with respect to ethyl groups and 0.7 withrespect to hydroxyethyl groups. Values reported for the averagemolecular weight are Mw ) 480 00024 (light scattering), Mv)150 000-200 00023 (viscosity), andMn ) 80 0000.22 The valueof ηred is 510 mL/g for a 0.05% w/w solution at 20 °C,32 and thecloud point varies from 28 to 37 °C, depending on the polymerconcentration.26 Sodium dodecyl sulfate (>99%) was suppliedbyMerck (Germany), recrystallized four times fromethanol, andvacuum-dried.Preparation of Solutions. A stock solution of EHEC (1%

w/w) was prepared according to a standard technique43 anddialyzed for 5 days in tube membranes (molecular weight cutoffof approximately 10 000) against membrane-filtered water(exchanged three times a day) to remove salt (NaCl) present inthe sample. The stock solution was further diluted to 0.2% w/wto obtain a convenient concentration. This “new” stock solutionwas allowed 1 month to attain equilibrium properties. Subse-quently, the SDS solutions, with and without EHEC, werepreparedbyweighingequalamounts ofEHECsolutionandwaterintoanappropriately concentratedSDSsolution. All of thewaterused was deionized (specific conductance 3.3 10-6 Ω-1 cm-1) anddouble-distilled (Millipore).Surface Tension Measurements. Pendant drop tensiom-

etry, enhanced by video-image digitization, has emerged as anextremely accurate method for measuring the surface andinterfacial tension.10,20,44-47 This technique enables preciseobservations of the time-dependence of surface tension over anextended time scale (several hours), without disturbing the air/liquid or liquid/liquid interfaces. The suitability of the pendantdropmethod and the limitations of other less popular techniquesused to measure the surface tension of protein and polymersolutions have been discussed by Wu.48 Descriptions of thependantdrop tensiometer andmethodsused in thepresent studyand of the numerical analysis of the data are previously givenbyNahringbauer10,49 (i.e., the constructor of theequipment).Eachvalue of surface tension is based on theoretical curves that wereobtained by the numerical integration of eq 3 fitted to the dataat approximately 400 points by the least-squares refinement ofthe parameters b and â.44

In eq 3, â is equal to (g∆Fb2)/γ, where g is the acceleration dueto gravity, ∆F is the difference in the densities of solution to air,b is the radius at the apex of the drop, and γ is surface tension.All measurements were made in a climate room at 20 ( 0.5

°C.ADMA02Cdigitaldensitometer fromAntonPaarK.G.,A-8054

Gratz, Austria, was used to determine the densities of solutionsat 20.00 °C.

Results and DiscussionFigures 2-4 show the results obtained from studies of

aqueous solutions of EHECwith andwithout SDS,where(36) Scheutjens, J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1979, 83,1619.

(37) Scheutjens, J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1980, 84,178.

(38) Chang, S.; Gray, D. G. J. Colloid Interface Sci. 1978, 67, 255.(39) Johnson, B. A.; Kreuter, J.; Zografi, G. Colloids Surf. 1986, 17,

325.(40) McNally, E. J.; Zografi, G. J. Colloid Interface Sci. 1990, 138,

61.(41) McNally, E. J.; Gau, C. S.; Zografi, G. J. Adhes. Sci. Technol.

1992, 6, 445.(42) Gau, C.-S.; Yu, H.; Zografi, G.Macromolecules 1993, 26, 2524.

(43) Manley, R. S. J. Arkiv for kemi 1956, 44, 519.(44) Rotenberg, Y.; Boruvka, L.; Neuman, A. W. J. Colloid Interface

Sci. 1983, 93, 169.(45) Hansen, F. K.; Rodsrud, G. J. Colloid Interface Sci. 1991, 141,

1.(46) Lin, S.-Y.; Hwang, H.-F. Langmuir 1994, 10, 4703.(47) Thiessen, D. B.; Chione, D. J.; McCreary, C. B.; Krantz, W. B.

J. Colloid Interface Sci. 1996, 177, 658.(48) Wu, S. J. Macromol. Sci., Rev. Macromol. Chem. 1974, 10, 1.(49) Nahringbauer, I. Acta Pharm. Suec. 1987, 24, 247.

n(t) ) [2NA(D/π)1/2/Mseg]cot

1/2 (1)

[γo - γ(t)]/[γ(t) - γm] ) Kn[νn(t)]n* (2)

b[ d2y/dx2

(1 + (dy/dx)2)3/2+ dy/dxx(1 + (dy/dx)2)1/2] ) 2 - â (γ/b) (3)

2244 Langmuir, Vol. 13, No. 8, 1997 Nahringbauer

the polymer concentration is kept fixed at 0.2% w/w, 12ppm (10-4 % w/w), and 2 ppm and the SDS concentrationis varied. The polymer concentration, 0.2% w/w, waschosen as equal in magnitude as the concentration usedinmanystudiesof thebulkproperties of thesystemEHEC/SDS. The choice of the extremely low concentrations, 12and 2 ppm, was based on the observation that 3.6 ppm isequal to a critical concentration. To be more precise, inthe case of EHEC solution without cosolute, the authorfound recently10 that a polymer concentration of 3.6 ppmor more is necessary to cover the surface within the timeof observation (≈17h). Isotherms illustrate the variationof surface tension, at different concentrations of SDS,versus surface age (the time period after the formation ofthe drop analyzed).For the sake of clarity, not all of the mixtures inves-

tigated are included in Figures 2-4. The surface tensionof SDS solutions without polymer and above cmc,γ(t)(SDS)∞, is included in Figure 2 for comparison. Here,γ(t)(SDS)∞ is assumed equal to γ(t) for the solution 8.5mM SDS.

SynergisticEffects. Todate, hydrophobic attractionsbetweenpolymerandalkyl chains of surfactantmoleculesare generally believed to be the main driving force forpolymer/surfactant associations. According toGoddard,6the simplest indicator of “reactivity” (hydrophobicity)should be the surface tension of an aqueous solution ofthe polymer itself. On the basis of the fact that EHEC isahighly surface-activemacromolecule, a strongpolymer-surfactant interaction is expected. As discussed above, anumber of investigations have established that this isindeed true in the aqueous bulk phase. In the followingpresentation, these ideas are further developed by dem-onstrating that the EHEC molecule is also very reactivetoward surfactants at the surface of an aqueous solution.Most convincing is the apparent enhancement in surfaceactivity found for the mixed solutions. Careful compari-sons of the surface-tension isotherms of the EHEC-SDSmixtures, with those of pure EHEC and pure SDSsolutions, clearly show a reduction in surface tension forall of the mixed solutions. The pure SDS isotherms areincluded in the figures only when they are considerednecessary for the discussion. A significant synergism in

b

a

Figure 2. (a) Effects of SDS addition on the surface tensionversus time for an aqueous solution of 0.2% w/w EHEC. Therange of SDS concentrations inmM is shownbeside the curves.(+) refers to the isotherm 8.5 mM SDS without polymer. (b)Effects ofSDSadditiononthesurface tensionversus logarithmictime for an aqueous solution of 0.2% w/w EHEC. The range ofSDSconcentrations inmMis shownbeside the curves. (+) refersto the isotherm 8.5 mM SDS without polymer.

b

a

Figure 3. (a) Effects of SDS addition on the surface tensionversus time for an aqueous solution of 12 ppm EHEC. Therange of SDS concentrations in mM is shown beside theisotherms. (b) Effects of SDS addition on the surface tensionversus logarithmic time for an aqueous solution of 12 ppmEHEC.The range of SDSconcentrations inmMis shownbesidethe isotherms.

Polymer-Surfactant Interaction Langmuir, Vol. 13, No. 8, 1997 2245

surface activity was also found in extremely dilutemixtures of SDS and EHEC, far below cac (cf. Figures 4and 5). This is very interesting, since no polymer-surfactant interactions having a meaning are thought tobepresent in thebulkphasebelowthecacof thesurfactant.Reported cac values for the EHEC/SDS system, asmeasured by conventional methods such as sodiumactivity13 and equilibrium dialysis,21 are 5 and 3.5 mM,respectively. Significantly lower cac values have beenobtained by conductivity (1.8 mM) and DS- self-diffusionmeasurements (1.4 mM),25 and recently by fluorescencequenching (1.5 mM).50

Surface-Tension Effects Due to the Surface Ac-tivities of Polymer and of Surfactant. According toFigures2a-4a,most of theaqueousEHEC-SDSmixturesseem to attain a state close to equilibriumduring the timeof observation. The curves in Figures 3a and 4a indicatethat at fixed EHEC concentration, the equilibrium valueof surface tension decreases with increasing addition of

SDS. Then, for surfactant concentrations above 5 mM ofSDS, all equilibrium values are 32 mN/m. This value issignificantly lower than the equilibrium value, 37mN/m,above the cmc of pure SDS. However, an examination ofFigures 2b-4b (showing surface tension versus logarith-mic time) reveals that few of the aqueous EHEC-SDSmixtures attain a “real” equilibriumstate during the timeof observation. The extremely slow change of surfacetension indicates that the adsorbed monolayer is progres-sively transferred to an irreversible semicrystallinephase(vide supra).From a comparison of Figures 2-4, another important

effect is obvious. Besides being dependent on the surfaceactivity of surfactant, the surface tension of the EHEC-SDSmixtures seems tobehighlydependent on the surfaceconcentration of EHEC. This is further illustrated inFigures 5 and 6, which show isotherms at fixed SDSconcentrations of 1 and 3 mM, respectively. However,the largedifferencesbetween thesamples canbeexplainedat least partly as a kinetic effect, considering that thesurface tensiondependson the fractionof surface coverageat the time ofmeasurement. On the otherhand, although

(50) Evertsson, H.; Nilsson, S.; Holmberg, C.; Sundelof, L.-O.Langmuir 1996, 12, 5781.

Figure 4. (a) Effects of SDS addition on the surface tensionversus time for an aqueous solution of 2 ppmEHEC. The rangeof SDS concentrations in mM is shown beside the isotherms.(b) Effects of SDS addition on the surface tension versuslogarithmic time for an aqueous solution of 2 ppm EHEC. Therange of SDS concentrations in mM is shown beside theisotherms.

Figure 5. Effects of 1 mM SDS on the surface tension versuslogarithmic time for aqueous solutions of EHEC: 2 ppm (0);12 ppm (O); 0.2% w/w (4). Closed and open symbols representpolymer solutions with and without 1 mM SDS, respectively.(×) refers to the isotherm 1 mM SDS without polymer.

Figure 6. Effects of 3 mM SDS on the surface tension versuslogarithmic time for aqueous solutions of EHEC: 2 ppm (0);12 ppm (O); 0.2% w/w (4). Closed and open symbols refer topolymer solutions with and without 3 mM SDS, respectively.(×) refers to the isotherm 3 mM SDS without polymer.

2246 Langmuir, Vol. 13, No. 8, 1997 Nahringbauer

the plateau values might be independent of polymerconcentration, the appreciable dependence during theequilibration of the surfacemust be strongly emphasized.This is particularly true for diluteEHECsolutions,wherethe transient times are extraordinarily extended.On the basis of the observations of the EHEC-SDS

solutions, it seems as though the overall reduction ofsurface tension is due toa joint action of polymer segmentsand surfactant molecules, where the polymer concentra-tion and the hydrophobicity of polymer segments are ofutmost importance for the effect. Up tonow,many reportson thesurfaceactivityofpolymer/SDSmixtures containinganuncharged polymer have been based on the hypothesisthat the surface tension is solely reflective of the con-centration of the surfactant. One reason for this mightbe that the studies have dealt with systems containingless hydrophobic polymers such as PEO51 and PVP.52Lately, however,Chari andHossain53 foundunexpectedlylow surface-tension values in the systemPVP/SDS/water,which they explained in terms of the surface activity ofpolymer.Dynamics of Surface Tension. Parts a and b of

Figure 2 show that the surface-tension values ofmixturesof the highest polymer concentration, 0.2% w/w withvarious concentrations of SDS, 0.5 mM and higher, areapproximately constantand independent of the surfactantconcentration. Another important finding is that thepresence of SDS induces a significant change in thedynamics of surface-tension reduction. Prudhomme andLong54 have reported similar results regarding the agingof polyacrylamide solutions. When SDS was added, theagingof thepolyacrylamidesolutionswasmuchmorerapidthan without the surfactant.Clearly, a discussion of the polymer-surfactant inter-

action, as it appears in the surface film, cannot be basedon the results presented in Figure 2a,b. Instead, thephenomenon is better studied by using polymer solutionsthat aremuch less concentrated than the 0.2%w/wEHECsolution. Indilutepolymersolutions, entanglementeffectsare reduced, thus favoring studies of the adsorptionkinetics specific for polymer chains. Furthermore, thedynamics of monomer adsorption and macromoleculeadsorption differ considerably. Adsorption of DS- mol-ecules is fast, whereas the adsorption of polymer chainsis extraordinarily slow, especially at low concentrations.Moreover, for polymers, the time dependence of surfacetension follows a sigmoidal curve with a well-definedinduction period.For very dilute mixtures of SDS and EHEC, the time

dependence of surface tension is mainly governed by theadsorption kinetics of the polymer. However, the induc-tionregionofpolymeradsorptionseemstobeverysensitiveto the presence of surfactant molecules (cf. Figure 4b),and the time scale of the subsequent surface-tensionchange (surface coverage) is dramatically reduced byaddition of SDS (cf. Figure 3b). In previous work by theauthor,10 the dramatic change in surface tension after thelag time period was interpreted as being due to a suddenincrease in the number of adsorbed polymer segments.Consequently, the onset of surface-tension reductionshould reflect the uncoiling and spreading of polymerchains. If this interpretation is true, then the extraor-dinary acceleration of polymer adsorption in the presenceof SDS can be explained in terms of changes in theconformation of polymer chains inducedby the surfactant.

LinkbetweenSurfaceTensionandtheFormationof Polymer/Surfactant Complexes. Owing to a wide-spread interest in the mechanism of complex formation,much effort was focused on identifying the onset ofpolymer/surfactant aggregation. In the following discus-sion, the EHEC-SDS mixtures containing less than 2mMofSDSare first considered. Next, theEHECsolutionswith more surfactant added are discussed.A careful analysis of Figure 3b shows that some of the

isotherms have the same critical time point of phasetransfer, tcr2 (≈25 min). More precisely, these isothermsdescribe the solution12ppmofEHECwithout surfactant,and the mixtures containing EHEC and less than 2 mMof SDS. In contrast, the remaining isotherms in Figure3b seem to have values of tcr2 too small to be observed bythe pendant dropmethod. Furthermore, in Figure 4b theisotherms of the mixtures containing less than 2 mM ofSDS and 2 ppm of EHEC show a time period of constantsurface tension before the onset of the sharp decrease ofsurface tension. All lag times are equal in length to theinduction period t < tcr1 observed for 2 ppm of EHECwithout surfactant. Another interesting fact is thatwhilethe curve shape is equal, the level of surface tensiondecreases with increasing concentration of SDS. More-over, the magnitude of the surface-tension values of thelast points on each curve indicates an incomplete surfacecoverage. This observation implies that the total surfaceconcentration of surfactant molecules and adsorbedpolymer segments is very low. Probably, the surfaceconcentration of DS- molecules is too low to facilitate anyassociationwithpolymer segments. However, the resultsshow clearly that polymer molecules as well as DS-

molecules contribute to the surface-tension change. Asurface-tension reduction due to the DS- concentrationsolely is expected to be fast and independent of the surfaceage.When 2mMandmore of SDSwas added to the polymer

solution, a drastic change of the adsorption kineticsoccurred. Asdisplayed inFigures3band4b, the isothermsof the actual EHEC-SDS mixtures and the isotherm ofthe pure polymer differ significantly. The initial surface-tension values of the mixtures are unexpectedly low, andany critical timepoints, tcr1 and tcr2, cannot be determined.Absence of lag time and the drastic reduction of the timebefore complete surface coverage point to a dramaticchange of the polymer conformation. It should be notedthat the onset of the extraordinary adsorption kineticsappears at the same surfactant concentration, indepen-dent of the polymer concentration. The same findingsare generally established for the onset of polymer-surfactant association.2,51 Moreover, from recent studiesof bulk properties for the EHEC/SDS/water system, thevalue of cac is near 2 mM SDS at 20 °C.25,26,50 Veryplausibly, the change in adsorption kinetics is due to thepresence of a complex betweenEHECandDS-molecules,as will be shown in the following discussion.In order to illustrate the discussion, hypothetical

polymer conformations are drawn in Figure 7. Thedrawings illustrate the case in which there are no stronginteractions between surfactant and polymer molecules.Figure 7a illustrates “uncoiling” of a polymer chain at t) tcr1. At the formation of a surface layer of polymermolecules, a competition exists between the surfaceactivity of the polymer segments, which tends to spreadthe segments on the interface, and the entropy of thepolymer chain, which tends to preserve the coil. The lagtime observed in dilute solutions is mainly due to thiseffect. Figure 7b shows the onset of polymer “looping” att ) tcr2, which occurs when the surface is completelycovered. Since the adsorption of polymer segments to the

(51) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36.(52) Arai, H.; Murata,M.; Shinoda, K. J. Colloid Interface Sci. 1971,

37, 223.(53) Chari, K.; Hossain, T. Z. J. Phys. Chem. 1991, 95, 3302.(54) Prud’homme, R. K.; Long, R. E. J. Colloid Interface Sci. 1983,

93, 274.

Polymer-Surfactant Interaction Langmuir, Vol. 13, No. 8, 1997 2247

surface and the association of particles are highly coop-erative phenomena,10 the processes at tcr1 and tcr2 arethought to be irreversible (although theadsorption of eachpolymer segment can be reversible). The aging of surfacebeyond t) tcr2 is clarified in Figure 7c, where the polymerchain isextendingveryslowly,perpendicular to thesurfacelayer. As an increasing number (amount) of polymermolecules are adsorbed, each individual macromoleculehas an increasing number of segments below the surface.In bulk solution it is well established that a complex

formed between a nonionic polymer and charged surfac-tant molecules has polyelectrolytic properties. The phe-nomenon is generally explained in terms of the linearshapeandcharges of the complex. Whenstrongpolymer-surfactant interactions appear at cac, a dramatic changeof polymer conformation is assumed to occur. The onsetof this process is induced by electric repulsion forcesoccurring between the charged surfactant clusters whenthey form along the uncharged polymer molecule. Ac-cordingly,when theEHECmolecule is a part of a complexwith DS- molecules, most of the polymer segments areexposed for adsorption to the interface. Thismeans that,at and above cac, there is no basis for a lag time ofadsorption, since the uncoiling (cf. Figure 7a) happens inbulk solution. Furthermore, the clusters spread over thepolymer chain supply the macromolecule with chargesthat make it more polar. The findings for the EHEC-SDS system, elimination of lag time and enhanced rateof surface coverage, are clearly in accordance with thepresence of a complex betweenEHECandDS-molecules.Another observation of special interest is the very slow

change in surface tension (surface pressure) found formixtures containing2mMandgreater ofSDS. Evidently,the rearrangement of polymer segments (laterally andperpendicularly directed to the surface film cf. Figure 7c)occurs for extraordinarily long times. As previouslydiscussed, these adsorption kinetics are characteristic ofmonolayers that are progressively transferred to a morecondensed phase. Thus,with regard to the very extendedtime scale of adsorption, a semisolid phase or a sort of2-dimensional gel phase, seems to form. As illustrated inFigure 8a, it is easy to imagine a polymer/surfactantcomplexwhere loops of polymer segmentsprovide “reactorrooms” for a cooperative association of the surfactantmolecules that leads to hemimicelle formation at the air/water interface. Further observations support the as-sumption of an interfacial layer consisting of complexessimilar to those found to form in bulk solution. Forinstance, the slopes of the isotherms in Figure 4b are ofspecial interest. Studies of the EHEC/SDS/water systemby steady-state fluorescence quenching and dialysisequilibriummeasurements26,50 showthat indilutepolymer

solutions (0.03% w/w) the distance between clusters isalmost constant, whereas the cluster size (charge) in-creases for increasingsurfactant concentration. The latterchange leads to a gradual restriction of the flexibility ofpolymer chain. In addition, the surface pressure issupposed to increase. Accordingly, the compressibility ofa monolayer consisting of EHEC/DS- complexes shouldgradually decrease when the surfactant concentrationincreases. This statement agrees with the results inFigure4b,where theslopeof the isotherms is lessnegative,and the surface tension smaller, with increasing SDSconcentration. The imagined process upon surface agingis illustrated in Figure 8b.Contrary to ionic surfactants, nonionic surfactants are

generally not found to form complexes with nonionicpolymers suchasalkyl celluloses. For instance, in a studyusing surface-tension measurements of the adsorption ofHPCandHEC in thepresence of penta(oxyethylene)decylmonoether,C10E5, nopolymer-surfactant interactionwasobserved.41 However, a work by Fourier transformNMRself-diffusion showed that octaethylene glycol mono-n-dodecyl ether, C12E8, forms complexeswithEHEC.55 Thepresence of a complex between EHEC and C12E8 was alsoproposed from phase diagram findings.56 In the case ofthe systems HPC/C10E5/water and HEC/C10E5/water theauthors reported that the extremely surface active HPCand the less surface active HEC molecules desorb fromthe air-water interface above a particular surfactantconcentration. The critical concentrationof the surfactantwas related to the surface tension of the pure polymerand was about 1 order of magnitude smaller for the lesshydrophobic polymer, i.e., HEC. Due to the strongcooperative effect, the polymer adsorption is generallyassumed to be irreversible (as pointed out above). Thus,a complete desorption of the uncoiled polymer moleculesseems to be an incorrect explanation of the findings.Another interpretation of the observed critical concentra-tions is in terms of critical aggregation concentrations,i.e., cac for C10E5 in association with HPC and HEC,respectively. Since the complexes will have no charges,no ordered change of polymer conformation (uncoiling) isexpected to follow the complex formation. Furthermore,the interaction between polymer and surfactant is prob-ably due to a hydrophobic/lipophilic balance similar tothat present inmixedmicelles of nonionic surfactants. Inconsequence, complexes such as HPC/C10E5 and HEC/C10E5 are not expected to be surface active, in contrast to

(55) Zhang, K.; Jonstroemer, M.; Lindman, B. J. Phys. Chem. 1994,98, 2459.

(56) Zhang, K.W.; Karlstroem, G.; Lindman, B.Colloids Surf. 1992,67, 147.

Figure7. Hypothetical polymer conformationsat theair/liquidinterfacewhentherearenostrong interactionsbetweenpolymerand surfactant molecules.

Figure8. Schematicdiagramof (a)polymer/surfactant clustersat the air/water interface and (b) the imagined process uponsurface aging.

2248 Langmuir, Vol. 13, No. 8, 1997 Nahringbauer

the complex formed between the nonionic EHEC and thecharged DS- molecules.Applicationsof theResults. Fromthe foregoing, one

can confidently state that the potential of polymericsurfactants such as EHEC is considerable, not only asthickeners but also as specialty surfactants. One keyadvantage is the strong surfaceactivity observed inhighlydilute solutions. The presented results are technically ofextraordinary importance, since they prove that the rateof surface-tension reduction can be dramatically acceler-ated by the addition of a minute amount of surfactant tothe polymer solution. For instance, a solution containinga mixture of 12 ppm EHEC and about 2 mM SDS attainsnearly constant surface tension in a few minutes (seeFigure 3). The technical importance of this findings isapparent when comparing with the isotherm, 12 ppmEHEC without SDS, where the equilibration appears togo on for several hours. In addition, note the very lowresultantsurface-tensionvalue: 34mN/m. It is even lowerthan γ(t)(SDS)∞ (≈37 mN/m).The findings concerning extremely dilute EHEC solu-

tions should be of practical use, for instance, when fastwetting is required, as in injected, spread, or inhaledapplications. In addition to being “water-like”, suchformulations should wet rapidly to mucous in order topromote a quick absorption of the active substancesolubilized in the surfactant clusters. Up tonow, researchon these kinds of applications has dealt with rather highEHEC concentrations, 0.1% w/w and over.19

ConclusionsThis study demonstrates that the onset of complex

(cluster) formation can be identified by surface-tensionmeasurements, even when a very hydrophobic polymersuch as EHEC is part of the complex. In this case, thegenerally employed “surface tension method”, accordingto Jones,51 is less informative and more difficult to

interpret. In the present work the value of cac isdetermined from the change in the dynamics of polymeradsorption. For the EHEC/SDS/water system cac wasfound to be close to 2 mM.In the case of very dilute mixtures of EHEC and SDS

(bellowcac), thekinetics ofadsorptionaremainlygovernedby the adsorption of the polymer. The same behavior hasbeenreported for other systems, e.g.,HPC/C10E5andHEC/C10E5.41 As is evident from the present study, thealteration of the surface tension is a joint action of polymersegments and surfactant molecules. When the additionof SDS to the aqueous EHEC solution is less than 2 mM,the adsorption kinetics of pureEHECand of themixturesare indeed similar. Furthermore, the surface-tensiondecrease is undoubtedly caused by DS- molecules andpolymermolecules thought of as “separate particles”. Thedegree of reduction depends on the bulk concentration ofsurfactantmoleculesand thenumberof polymer segmentsin actual contact with the surface. The surface equilibra-tion is extremely slow.Addition of 2mMandmore ofSDSdramatically reduces

the time scale of the sharp surface-tension change, i.e.,the surface coverage. The results providemanyevidencesfor the presence of a complex between EHEC and DS-

molecules. Thus, the observed “new” dynamics of adsorp-tionare explained in termsof a change of the conformationand flexibility of the polymer chain induced by surfactantclusters bound over the polymer chain. The complex isstrongly adsorbed to the air/water interface due to itspolyelectrolytic properties. Furthermore, while the sur-face film ages, a progressive transfer of the monolayer toa 2-dimensional “gel phase” seems to occur.

Acknowledgment. Financial support from the Swed-ishResearchCouncil forEngineeringScience is gratefullyacknowledged.

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