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Acid-Base Equilibria of Weak Polyelectrolytes inMultilayer Thin Films

S usan E. B ur k e an d C hr is to phe r J . B a r r e t t*

Departm ent of Chemi stry, M cGill U ni versit y, 801 Sherbr ooke Street West,

M on t r ea l , Q C , H 3 A 2 K 6 , C a n a d a

Received September 2, 2002. I n F in al F orm: Ja nuar y 17, 2003

In this paper, we report on the local apparent dissociation constants of poly(acrylic acid) and poly-(al lylamine hyd rochlorid e) incorporate d in polyelectrolyte mult i laye r thin f i lms. We asse mble d 10polyelectrolyte layers on colloidal silica by the sequential electrostatic adsorption of the polyacid andpolybase from aq ueous solutions at different pH va lues and then mea sured the zeta potentia l as a functionof the solution pH t o determ ine the pKa(app)of each surfa ce lay er. The results suggest th at the dissociat ionconstant decreases upon adsorption for poly(acrylic acid) and increases in the case of poly(allylaminehydrochloride). These deviations from ideal behavior can be substantial, changing by as much as 4 pHunits, and the shifts become more pronounced as the number of adsorbed layers increases. In addition,we found tha t these pKa(app)shifts a re influenced by the pH of the solution used to assemble the th in filmsbut show little dependence on the salt concentration used in the assembly baths.

Introduction

Over the past decade, the layer-by-layer electrosta tica dsorption technique intr oduced by Decher and Hong hasreceived much at tention as a route to prepa re thin polymerfilms.1,2 This self-a ssembly method involves the sequent iala dsorption of polyelectrolytes onto a n oppositely cha rgedsubstrate from dilute aqueous solution leading to chargereversal on th e surfa ce. This simple requirement ma keslay er-by-lay er self-a ssembly a pplicable to a w ide variet yofpolyelectrolyt es ran ging from complex biopolymers, sucha s p r ot ein s a n d D N A, t o p ol y el ec t r ol y t es c on t a i n i n gnonlinear optical functional groups.3,4 The stra ti f iedstru cture of polyelectrolyte multila yer films ha s a lso beencombined w ith sma ll molecules t o prepa re more complexsyst ems such a s ca psules for enzymes an d nan opa rticles,templates for nanoparticle growth, l iquid crysta l a l ign-

ment, a nd na nowire assemblies.5-9 In a ddition, polyelec-trolyte multi layers have been assembled on substratesdif fering in size, composition, and geometry.10 Conse-quently, the versatility oft he layer-by-layer method ma kest h is t e ch n iq u e a t t r a ct i ve f or a n u m be r of p ot e n t ia lapplications such as electro-optic devices, microcapsules,and sensors.11-13

The th in films prepar ed using the lay er-by-la yer method

a re most fr equently composed of str ong polyelectrolytesbeca use they remain fully charged over a wide pH range.14

Although ma nipulating the ionicst rength of the assemblysol u t ion ca n b e u sed t o c on t r o l t h e m or p h ol ogy a n dthickness of such polyelectrolyte mult ilayer films t o someextent, the effectiveness of using this par a meter is limitedto a sma ll ra nge of sa lt concentra tions because increa singthe ionicst rength of the system ca n lead to either solubilityproblems or decomposition of t he multi layer f i lms.15

However, more recent st udies have shown tha t preparingmulti layer t hin f i lms from w eak polyelectrolytes canproduce systems with a rich suite of properties becausethe behavior of this class of polyelectrolytes is sensitivenot only to the ionic strength of the solution but a lso toi t s p H .16-18 In f a c t , i t wa s r ec en t l y sh o wn t h a t ev en a

single wea k polyelectrolyte la yer embedded at the bottomof a 10-l a y er f il m i s gr ea t l y i n fl u en ced b y t h e l oca lenvironment a t the surface layer .19

One of the most studied polyelectrolyte combinationstha t yields multilayer thin films whose physica l propertiesare strongly dependent on solution pH is that of poly-(a crylic acid) (P AA) a nd poly(a llylamin e hy drochloride)(P AH). Surfa ce wet ta bility, sur face roughness, film mor-phology, dielectr ic properties, and layer thickness areexamples of such properties that experience significantv a r i a t i o n s wi t h c h a n ges i n p H o f t h e sy st em .16-18,20-23

Perha ps most nota bly, bi layers with thicknesses rangingfrom less than 10 to more tha n 120 ha ve been preparedfrom P AH/P AA at dif ferent pH values, w hile certa in

* To w ho m cor r espond ence s houl d be a d d r es sed . E -m a i l :[email protected].

(1) Decher,G . ;H ong,J .D . M a k r o m o l . C h e m . , M a c r o m o l . Sy m p .1991,46, 321.

(2) Decher, G . Science1997, 277, 1232.(3) Caruso, F.; Schuler, C. L a n g m u i r 2000, 16, 9595.(4) Lee, S .-H.; Ba lasubrama nian, S . ; K im, D. Y. ; Viswa nat han, N.

K.; Bia n, S.; Kuma r, J . ; Tripat hy, S. K. M acromolecul es2000,3 3, 6534.

(5) Ca ruso, F.; Tra u, D.; Moh wa ld, H.; Renneber g, R.L a n g m u i r 2000,16, 1485.(6) Ca ruso,F.; Kichtenfeld,H .; Giersig, M.; Mohw ald, H. J . Am . C h e m .

Soc. 1998, 12 0, 8523.(7) Wang, T. C.; Rubner, M. F.; Cohen, R. E. L a n g m u i r 2002, 18,

3370.(8) Pa rk, M.-K.; Advincula, R. C. L a n g m u i r 2002, 18, 4532.(9) Guld i, D. M.; Luo, C.; Kokt ysh , D.; Kotov, N. A.; Da Ros, T.; B osi,

S . ; P r a t o, M . N a n o L et t . 2002, 2, 775.(10) Caruso, F. Chem.sE u r . J . 2000, 6, 413.(11) Cheun g, J . H.; Fou, A. F.; Rubner , M. F. T h i n S ol i d F i l m s 1994,

244, 985.(12) Donath, E. ; Sukhorukov,G . B. ; Caruso,F. ; Davis,S . A.;Mohwa ld,

H . Angew. Chem., Int. Ed. 1998, 37, 2202.(13) Dai, J . ; J ensen, A. W.; Mohant y, D. K .; Ern dt, J . ; Bruen ing, M.

J . L a n g m u i r 2001, 17, 931.

(14) Decher, G .; Eckle, M.; Schmitt, J . ; S trut h, B . C u r r . O p i n . C o l l o i d In terface Sci. 1998, 3, 32.

(15) Duba s, S . T.; Schlenoff, J . B . M acromolecul es2001, 34, 3736.(16) Yoo, D.; Shiratori, S. S.; Rubner, M. F. M acromolecul es1998,

31, 4309.(17) Shira tori, S. S.; Rubn er, M. F. M acromolecul es2000, 3 3, 4213.(18) Mendelsohn, J . D.; Ba rrett , C. J . ; Cha n, V. V.; Pal, A. J . ; Mayes,

A. M.; Rubner, M. F. L a n g m u i r 2000, 16, 5017.(19) Xie, A. F .; G ra nick, S. M acromolecul es2002, 35, 1805.(20) Ha rris, J . J . ; B ruening, M. L. L a n g m u i r 2000, 16, 2006.(21) Fery , A.; Scholer, B .; Ca ssa gnea u, T.; Ca ruso, T.L a n g m u i r 2001,

17, 3780.(22) Sta ir, J . L.; Har ris, J . J . ; Br uening, M. L. Chem. Mater. 2001,

13, 2641.(23) Durs tock, M. F.; Ru bner, M. F. L a n g m u i r 2001, 17, 7865.

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assembly pH combinations completely prevent multilayerassembly of PAH and PAA.17 This w ide diversity of filmcharacteristics,duetotheeffect ofpH on thechargedensityof the polyelectrolyte chains in solution, is not easilyra tiona lized, a nd no existing a dsorption models of singlelayers of wea k polyelectrolytes are suita blet o explain suchphenomena , but it ha s been speculat ed tha t incorporat inga polyelectrolyte into a multi layer thin f i lm shif ts theapparent dissociat ion consta nt (pKa(app)) from its di lutesolution value due to chan ges in t he local electrosta tic

environment upon adsorption of the polymer chains,lead ing to unusua l behavior .17,18 Thus, it is particularlyrelevant to gain insight into these acid-base equilibriaa nomalies of P AH/P AA multila yer films beca use ma ny ofthe potentia l applica tions for these systems , such as dru gcapsules a nd templates for na nopa rticle growth, dependentirely on thechargedensity of thepolyelectrolytechainswithin the f i lm.7

Acid-base equil ibria deviations from ideal behaviorhave been observed previously in a number of differentsingle-lay er self-a ssembled organ icfilms.24-29 For example,th e dissociat ion const a nt of poly((2-dimet hyla mino)-eth ylmethacrylate-b-metha crylat e)a ssembled in a monolayerf ilm at the air-wa ter interfa cew a s observed to experiencea negat iveshif t of1 unit f rom the dilute solution value.24

A similar shift in pKaw a s observed for the sma ll moleculespecies docosylamine incorporated into a free-standingf il m a t t h e a i r-water interface.25 In both cases, the pKashifts w ere at tributed t o cha nges in the dielectric permit-t i v i t y o f t h e l o c a l en v i r o n m en t a n d a dec r ea se i n t h edegrees of f reedom of the species immobilized a t theinterface. La rger shif ts in the a cid-base equil ibria werereported for s elf-a ssembled m onolayers (SAMs) of -sub-sti tuted a lka nethiol m olecules chemisorbed t o a goldsurface.26-29 The dissociat ion consta nts of -carboxylalkanethiol molecules incorporated into self-assembledmonolayers increase from the dilute solution value by 0.5-4.5 units, w hile in the case of -am ino a lkan ethiols t hepKava lues decreas e by3-4 units. I t wa s concluded tha tthe trend in the pK

a values resulted from a combination

of effects including contribut ions from electr osta tic repul-si o n b et ween t h e c h a r ged m o i et i es , a dec r ea se i n t h ep er m i t t i vi t y a t t h e i n t er fa c i a l l a y er , a n d s t r on g l a t er a lhydrogen bonding among the surface functional groups.Nota bly, a cid-base equilibria shifts observed for func-tiona lized alka neth iol molecules incorporat ed into S AMsare not surprising considering the extreme local envi-ronmental changes experienced by each individual mol-ecule upon chemisorption from dilute solution into adensely packed surfa ce film. In contra st, it is possible form a n y of t h e r ep ea t u n i t s i n a p ol y el ect r o ly t e c h a i n t omaintain a similar local environment and mobility uponphysisorption on a surface,pa rticularly if the chains adsorbin a coiled st a te or w ith loopy conforma tions. Addit iona lly,

it is still un clea r h ow t he presence of multiple polyelec-trolyte underlayers and the conditions used for polymeradsorption affect the dissociation constants of polyelec-t r o ly t es i n m u l t il a y er t h i n f i l ms. H en ce, i t i s o f gr ea tinterest to measure th e acid-base equilibria of polyelec-trolyte multi layer assemblies because of the many dif-

ferences that exist between polymers and small moleculeson surfaces.

One common t echniqu e used t o determine t he effect ofvar ious para meters (e.g., sa lt, polymer, pH) on the surfa cecharge properties of colloidal particles is microelectro-phoresis,from which zeta potential can be calculat ed usingthe electrophoretic mobili ty dat a . 30 The plots of zetapotentia l versus pH for colloida l par ticles cont a ining acid-base functional groups represent titrat ion curves,a nd suchcurves can be used to determine th e dissociation const a nt

of a cid funct iona l groups on colloidal pa rt icles by a pplyingthe basic principles of acid-base equilibria in solution,wh i ch i n di ca t e t h a t pKa ) pH at the inflection point onthe curves.31

The acid-base equilibria of weak polyelectrolytes insol u t ion h a v e b een a t o pi c o f i n v est i ga t i on f or m a n ydecades.32-35 The dissociation behavior of weak polyelec-trolytes in solution is commonly described as a n a pparentdissociat ion const a nt (pKa(app)), wh ich reflects t he overalla cid dissociat ion equilibrium of the polyelectrolyte. Dueto the electrosta tic intera ctions of the individual char gedfunctional groups a long the polyelectrolyte chain, thepKa(app) is strongly influenced by the degree of dissociat ionan d the electrostat icscreening of added sa lt molecules.32,33

In the case of P AAfor example, the pKa(app) ha s been shownto vary from 6.79 in the absence of added salt to 4.68 inthe presence of 1.0 M Na Cl.33 In t he present study, i t isou r go a l t o det er m in e h o w su ch f a c t or s a s t h e c h a r gedensity on the surface (silica or polyelectrolyte), the degreeof dissociation of the adsorbing polyelectrolyte, and thedissociat ion beha vior of the polyelectrolyt e underla yer a sa function of pH influencet he pKa(app) of the polyelectrolytespecies termina ting t he surfa ce of t he multi layer f i lms.

In this paper, we report measurements of the acid-base equilibria ofP AAa nd PAH as pa rt of a polyelectrolytem u lt i la y e r a s s em b ly . We l a y er ed P A H a n d P A A o ncolloidal si l ica a t dif ferent pH values, and the resultingzeta potential of the particles was followed as a functionof the solution pH to obtain an acid-base equilibrium

p r of il e f o r t h e su r fa c e a f t er t h e deposi t ion of ea c hpolyelectrolyte layer in the assembly, f rom which thepKa(app) wa s evaluat ed from the inflection point oft he curve.Because the chains within each layer of the assembly havethe abil i ty to assume a variety of conformations and topenetrate intot he neighboring layers,16-18 weinvestigatedthe ef fect of layer a ccumulat ion, t he a ssembly solutionpH, and the salt concentration in the assembly bath onthe dissociation behavior of th e film surfa ce. In a ddition,the dependence of the dissociation const a nt s on the pH ofthe a ssembly solutions and t he salt concentra tion in thea ssem bl y b a t h s w a s eva l u a t ed.

Experimental Section

Multilayer Assembly.The multila yers were prepared by first

depositing a P AH la yer (MW) 60 000 g/mol, P olys ciences ) onSnowtex silica part icles (70-100 nm) obtained from NissanChemica l Indus tr ies . T his inv o lv ed a dding a n a queo us s a l tsolution of P AH to a colloidal suspension. The salt wa s add ed tocreate sufficient electrostatic screening to promote polymeradsorption onto the particles. Both solutions were adjusted tothe same pH with HCl or NaOH prior tomixing.The final solutionconcentr at ions were 0.01 M PAH, 0.1 or 0.2 M NaCl a s required,

(24) An, S. W.; Thomas, R. K. L a n g m u i r 1997, 13, 6881.(25) Zhao, X.; Ong, S.; Wan g, H.; Eisent ha l, K. B. Chem. Phys. Lett.

1993, 214, 203.(26) Vezenov, D. V.; Loy, A.; Rozsnyai, L. F .; Lieber, C. M. J . A m .

Chem. Soc. 1997, 119, 2006.(27) Hu, K.; B ar d, A. J .L a n g m u i r 1997, 13, 5114.(28) Sa by, C.; Ortiz, B .; Cha mpagn e, G. Y.; Bela nger, D. L a n g m u i r

1997, 13, 6805.(29) Kakiuchi, T.; Iida, M.; Im aba ya shi, S.; Niki, K.L a n g m u i r 2000,

16, 5397.

(30) Lyklema, J . Fundam ental s of Int erface and Colloid Science;Academic P ress: London, 1991; Vol. 2.

(31) Schulz, S. F.; Gisler, T.; Borkovec, M.; Sticher, H. J . C o l l o i d In terface Sci. 1994, 16 4, 88.

(32) Katchalsky, A.; Spitnik, P. J . Po l y m . Sci . 1947, 2, 432.(33) Gr egor, H. P .; Frederick, M. J. Polym. Sci. 1957, 13, 451.(34) Strauss, U . P . ; Vesnaver, G . J. Phys. Chem. 1975, 79, 1558.(35) Vink, H. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2353.

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and 0.5 g/L of silica part icles. After 30 min, t he solution wa scentrifuged, followed by removal of th e superna ta nt . The sam ple

wa s wa shed wit h wa ter (Millipore, Milli-Q 18.2 M

cm)adjustedto the sa me pH a s the prepara tion solutions. The part icles wereredispersed by sonication, and the suspension was allowed tosta nd for 30 min followed by centrifuga tion. This wa sh procedurewa s completed th ree times to remove una bsorbed polyelectrolyteto prevent the format ion of polyelectrolyte complexes in solution.A s ma ll s a mple o f pa rt ic les (30 mg) was then removed forcha ra cteriz a t io n a n d dried a t 6 5 C f or 1 2 h. The rema iningcolloidal particles were added to a solution of 0.02 M PAA ( Mw90 000g/mol, P olysciences)/0.2or 0.4 M Na Cl for th e depositionof PAA on top of the PAH layer. The adsorption and washingprocedures for the PAA layer were similar to those used for thedeposition oft he P AH layer. Alterna te polyelectrolyte layers wereassembled on the colloidal silica to a ma ximum of 10 layers.

Titration with Strong Polyelectrolyte. Fo r the t i tra t ionexperimen ts , an a qu eous stock solut ion of 2.0 g/L of PAH /P AAlayered particles (assembled at pH ) 9.0)a nd 1.0 mM NaC l wa s

prepared a t pH ) 9.0. The st ock w as divided int o several 15 mLs a mples . A v a ria ble a mo unt o f s to ck poly(dia l ly ldimethyl-ammonium chloride) (PDADMAC, Aldrich, MW ) 200 000-350 000 g/mol) solution conta ining 1.0 mM Na Cl an d a djustedto a pH va lue of pH ) 9.0 wa s a dded to each colloid sa mple tocrea te a s eries o f t i t ra nt s olut ions di ff ering in PD ADMACconcentration ranging across the point of half neutralization forth e P AH/P AA lay ered pa rt icles.

Zeta Potential Measurements. Solutions of 30 mg of drycolloidal particles in 15 mL of 1.0 mM NaCl were prepared forthe electrophoretic mobility measurements, w hich were deter-mined using a Microelectrophoresis Apparatus Mk II (RankBr others, Bottisha m, U .K.). We calculated t he zeta potentia l ofour assemblies from the Smoluchowski equat ion using an avera gemobility valu e based on 20 mea surement s. The concentr at ion ofNaCl in the colloidal solutions wa s mainta ined at 1.0mM because

higher sa lt concentrat ions lead t o f locculation of t he colloidalsuspension.

Results and Discussion

Multilayer Buildup. In the f ield of polyelectrolytemulti layer thin f i lms, zeta potentia l measurements arecommonly employed to determine whet her th e electricalcharge on the f i lm is posit ive or nega tive as a means ofmonitoring the stepwise growth of sequentia l polymerlayers.36 An exam ple of this is sh own in F igure 1 for th ea ssembly of 10 la yers of P AH a nd P AA on colloidal silicaa t n eu t r a l pH ) 7.0. Bare silica displays a zeta potentialof -42 mV, reversing to +30 mV with the a ddition of 1

la yer of th e polycat ion PAH. The sign of th e zeta potent ia lthen shif ts ba ck to negat ive with the a ddition of the f irst

P AAla yer. This alt ernat ion in the sign oft he zeta potentia loccurs with ea ch additiona l la yer. Nota bly, the cha nge inthe sign of the zeta potentia l does not indicate w hetheradsorption or desorption has occurred with each poly-electrolyte cycle. H owever, 13C cross-pola riza tion/ma gic-an gle spinning (CP-MAS) NMR a nd elemental an alysisstudies were used t o confirm t he sequentia l a ddition ofeach polyelectrolyte layer .37,38 I t h a s p r ev iou sly b eendetermined tha t the thickness of the P AH/P AAm ultilayerf il ms g r ow s i n a l in ea r f a s h ion u n d er con s t a n t p Hconditions, but t ha t t he a verage thickness of the layersvaries depending on the pH of the assembly solutions. 17

Changing the solution pH af fects the charge density ofthese w eak polyelectrolytes, an d th is influences the filmchara cteristics such a s t he surface roughness, the la yer

thickness, a nd the electr ical properties. Since the zetapotential is a reflection of the electrical character of thecolloidal pa rticles, i t is used in t his case t o monitor t hecha nges in th e electr ical properties of wea k polyelectrolytefilms wit h varia tions in the pH of the colloidal suspension.The result ing plots represent a cid-base t i tra tion curvesfrom which the local apparent dissociation constant ofthe surface a fter the deposition of new polyelectrolyte layeris obtained from the inflection point of the curve.31 Anexample of th is is i l lustrat ed in Figure 2 for a 10-layerP AH/P AAfilm ass embled at pH )7.0for wh ich the pKa(app)wa s found to be 2.76 ( 0.06. Reversibility w a s confirmedby increasing t he pH ba ck up to pH ) 7.0 and recoveringthe origina l strongly cha rged film. The tit ra tion curve forbare silica is shown for comparison.

Dependence of pKa(app) on Layer Number. Toinvestigate the effect of the number of underlying poly-electrolyte layers on the pKa(app) of the surfa ce lay er , weassembled f i lms a t pH ) 7.0 containing 1-10 layers ofpolyelectrolyte on colloidal silica. B oth PAH and PAAw erefound to be fully charged a t neutra l pH w hen ad sorbedontoa surfa ce.17 Thus, by comparing th e pKa(app) of simila rlay ers (i.e., P AH la yers: 1, 3, 5, 7, a nd 9), we a re able toi so l a t e t h e ef f ec t s o f l a y er n u m b er o n t h e a c i d-b a seequilibria of P AH a nd P AA. Figures 3 a nd 4 illustra te thet r en ds i n t h e pKa(app) as a function of layer number for

(36) Caruso, F.; Mohw ald, H. J. Am. Chem. Soc. 1999, 12 1, 6039.

(37) Rodriguez, L. N.; DeP aul, S. M.; Ba rrett , C. J . ; Reven, L.; Spiess,H. W. Ad v . M a t er . 2000, 12, 1934.

(38) Smith , R. N.; Reven, L.; Ba rrett , C. J .M acromolecul es, in press.

Figure1. Alterna tion in zeta potentia l with la yer number forP AH/P AA mult ilay er thin films a ssembled at pH ) 7.0. Er rorin electrophoretic mobility, (0.08-1.9%.

Figure 2. Acid-base equilibrium profile for a 10-layer PAH/P AA f i lm asse mb le d a t p H ) 7.0 (b) a n d b a r e s i li ca (O).Un certa inty in electrophoretic mobility , (1.9%.

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P AH/P AA multila yer films a ssembled at pH ) 7.0. Thecomplete set of titra tion curves is provided as S upportingInformation. Both PAH and PAA experience a shif t intheir dissociation constants upon adsorption onto thecolloid surfa ce from th e dilute solution va lues 8.56( 0.08and 6.68 ( 0.05, respectively, in the presence of 1.0 mM

NaC l (Figures Aa nd B , Supporting Informat ion);

40

thesevalues ar e in good a greement wit h the reported litera tur evalues of 8.7 and 6.71 (Figure C, Supporting Informa-tion).33,39

It is clear tha t the a cid-bas e equilibria shifts are layerdependent and t ha t w ith increasing the tota l number oflay ers the dissocia tion const a nt s for both polyelectrolytesa ppea r to level off to a const a nt devia tion from the dilutesolution values. The dissociation constant of PAH as-

sembled in a multi layer f i lm increases from the dilutesolution va lue, but this cha nge levels off aft er layer 3. Inthe ca se of PAA, th e pKa(app) shifts to sma ller values uponadsorption and continues todo so up unti l the fourth layeris reached. The va riable shifts in pKa(app) observed for thef irst 3-4 l a y er s a r e a t t r i b u t ed t o i n f l u en c es f r o m t h echarged silica substrate or to inhomogeneous polyelec-trolytecoverageon thesurfacein agreement with previous

suggestion.41

Notably, in the t i tra t ion curve for t he f irstPAA layer (assembled a t pH ) 7.0), t he zeta potentia lremains negat ive over the entire pH ran ge investigat ed(Figure 5A). It is speculat ed tha t the zeta potent ial neitherreaches a value of zero nor switches sign because of acombination of effects. First, it is believed that there issparse coverage of the substra te by the f irst P AH la yer ,wh i c h i s c o n si s t en t wi t h t h e f i n di n g t h a t t h e a v er a geincremental thickness contribution from PAH is fa ir lylow when the multilayer assembly is carried out at pH )7.0.17 Therefore, one might expect t here t o be little t o nocontribution to the overall surface charge from the firstPAH layer , otherwise the zeta potentia l would become

(39) Fang, M.; Kim, C. H.; Sa upe, G. B.; Kim, H.-N.; War aksa , C. C.;Miwa , T.; Fujishima, A.; Ma llouk, T. E . Chem. M ater. 1999, 1 1, 1526.

(40) The polyelectrolyte solutions used for the titration experimentsconta ined 0.010 M polymer an d 1.00 mM NaC l. NaOH (0.398 M) wa sused as the titr an t. The solutions were allowed to equilibrate for 10 minbefore ea ch mea sur em en t . (41) S a h a i, N. Environ. Sci. Technol. 2002, 36, 445.

Figure 3. Dependence of the a pparent dissociat ion constantof PAH on the tota l number of layers in P AH/P AA mult ilay erfilms assembled at pH ) 7.0. The dashed line indicates thedilute solution pKa(app). The dotted and solid lines are visualguides.

Figure 4. Dependence of the a pparent dissociat ion constantof PAA on t he tota l num ber of layers in P AH/P AA multilayerfilms assembled at pH ) 7.0. The dashed line indicates thedilute solution pKa(app). The dotted and solid lines are visualguides.

Figure5. Acid-base equilibria curves for P AA surface layers

in P AH /P AA mu lt i lay e r f i lms asse mb le d a t d i f fe r ent p Hvalues: (A) 2-layer film a ssembled a t pH ) 3.0 (3), pH ) 5.0(0), pH ) 7.0 (4), an d pH ) 9.0 (O); (B ) 8-lay er film a ssembledat p H ) 3.0 (1), pH ) 5.0 (9), pH ) 7.0 (2), and pH ) 9.0 (b).The dott ed lines indicat e the isoelectric point of P AA in dilut esolution. Uncertainty in electrophoretic mobility, (1.0-2.1%.



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positive once a l l of the COO- groups from PAA werep r o t o n a t ed a t l o w p H . F u r t h er m o r e, a ssu m i n g sp a r secoverage of the f irst few layers, i t is possible that thenega tive zeta potentia l at low pH from the 2-lay er coat ingma y result from cont ributions from th e low residua l char geof P AA a n d t h e s i li ca su b st r a t e . Alt h o ugh t h e s i la n o lgroups of the particle substrate are weakly acidic, theisoelectric point does not occur unt il pH )2.4. It is possiblet h a t t h i s v a l u e m a y ev en b e l o wer i n t h e p r esen c e o f

polyelectrolyte on the surface, which has been shown tobe the case for colloidal silica in the presenceof a copolymerof acryla mide a nd ((3-metha crylam ido)propyl)trimet hyl-ammonium chloride.42 This w ould a lso explain why thezeta potentia l is slightly more negative tha n tha t observedfor subsequent layers.

The overa ll increas e in the ba se strengt h of PAH fromdilute solution to the a dsorbed sta te ma y be a t tr ibuted tot h e f a ct t h a t t h e i n fl u en ce o f t h e n ega t i vely ch a r gedfunctionality of the P AA or silica und erlay er in promotingthe protonation of the amino groups of PAH is greatert h a n t h e r esi s t a n c e t o p r ot o n a t i on f r om elect r o st a t i crepulsion of neighboring groups. In the case of PAA, it

was previously speculated that the presence of the fullyp r o t o n a t ed P AH u n der l a y er m a y i n c r ea se t h e ea se o fdissociat ion of the ad sorbing P AA layer an d thus fa cilita tethe increase in the acid strength of the adsorbing PAAchains.17 Our results do indeed indicat e tha t t he surfacecharge strongly influences the acid-bas e equilibrium ofthe adsorbing polyelectrolyte layers.

Dependenceof pKa(app) on the pH of the AssemblySolutions. One means of investigating t he nat ure of theinfluenceofthesurfacechargeofthepolyelectrolyte-coatedpart icles on the pKa(app) of the adsorbing polyelectrolyte isto va ry t he degree of dissocia tion of the surfa ce (i.e., silicaor polyelectr olyte la yer(s))on which it a ssembles. We choset o a l t er t h e su r f a c e c h a r ge b y c h a n gi n g t h e p H o f t h esolutions used to as semble the layers since pH affects thedegree of dissociation of both polyelectrolytes and thecharge density on t he si l ica part icles. The d issociat ion

consta nts were determined for P AH/P AA multi layersassembled a t pH ) 3.0, 5.0, 7.0, 8.0, and 9.0. Covering al a r ge p H r a n ge i s p a r t i c u l a r l y i m p o r t a n t b ec a u se t h epolyelectrolytes in d ilute solution a re fully cha rged a t onep H e xt r em e b u t on ly p a r t ia l ly ch a r g ed a t t h e ot h e rextreme, wit h the polya cid and polybase showing oppositetrends in dissociat ion with respect to th e solution pH.

Figures 5 and 6 contain plots of zeta potentia l versuspH for four different total layer numbers (2, 3, 7, and 8)within multi layer f i lms assembled using a range of pHconditions. The resulting a pparent dissociation consta ntsa re summa rized in Ta ble 1. These findings indicate th a tfor all assembly pH conditions stud ied, the pKa(app) valuesof t h e f il m s t er m i n a t ed w i t h P AA (l a y er s 2 a n d 8) a r esubsta ntia lly lower tha n those observed in dilute solution,

while the opposite is true for th e surface of fi lms composedof a tota l of 3 a nd 7 lay ers whose composition is domina tedby PAH.

F or b ot h of t h e f il m s i n v est i ga t ed t h a t h a v e su r f a c elayers terminated with PAA, the apparent dissociationconsta nt decreases with d ecreasing a ssembly solution pH.To rationalize these findings, one needs to consider theinf luence of t he cha rge density of the surface (si l ica orpolyelectrolyte) on which t he polyelectrolyte a dsorbs, thedegree of dissociat ion of polyelectrolyte in solution, a ndthe a verage incremental thickness contributions from bothP AH and P AA. For exam ple, a t low a ssembly solution pHva lues (i.e., 2.5 < pH < 5.5), Shira tori an d Rubner foundtha t th e avera ge incremental thickness contribution fromPAH is larger than that f rom PAA.17 Therefore, there is(42) Shubin, V.; Linse, P . M acromolecul es1997, 30, 5944.

Figure6. Acid-ba se equilibria curves for PAH surfa ce lay ersin P AH /P AA mu lt i lay e r f i lms asse mb le d a t d i ffer e nt p Hvalues: (A) 3-layer film a ssembled a t pH ) 3.0 (3), pH ) 5.0(0), pH ) 7.0 (4), a nd pH ) 9.0 (O); (B ) 7-lay er film a ssembledat p H ) 3.0 (1), pH ) 5.0 (9), pH ) 7.0 (2), and pH ) 9.0 (b).The dotted lin es indicat e the isoelectric point of P AA in dilut esolution. Uncertainty in electrophoretic mobility, (0.9-2.0%.

Table 1. Dependence of the pKa(app)Values of PAH andPAA on the pH of the Assembly Solutions

polymer la yer no. a ssembly pH pKa(app)

P AA 2 3.0 2.75P AA 2 5.0 2.83P AA 2 7.0 4.14P AA 2 8.0 4.26P AA 2 9.0 4.48P AA 8 3.0 2.46P AA 8 5.0 2.59

P AA 8 7.0 2.85P AA 8 8.0 3.01P AA 8 9.0 3.16P AH 3 3.0 10.10P AH 3 5.0 10.27P AH 3 7.0 10.47P AH 3 8.0 10.62P AH 3 9.0 9.91P AH 7 3.0 10.32P AH 7 5.0 10.46P AH 7 7.0 10.64P AH 7 8.0 10.80P AH 7 9.0 10.94



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a h igher concentra tion of a mino functiona l groups in theunderla yer tha n carboxylic a cid groups on the surfa ce forfilms a ssembled in t his pH regime. P AH/P AA films a realsoknown toexperienceinterpenetration between layersa nd to undergo rea rra ngement of their count erions wh enexp osed t o sol u t ion s o f v a r i a b le p H .16-18 D u r in g t h eti tra tion of f i lms termina ted in P AA, the charge densityof the P AA chains decreas es with d ecrea sing pH, but th ePAH chains from the previously adsorbed layer becomemoreprotona ted. Sincet he concentra tion of am inogroups

i n t h e u n der l a y er i s h i gh er t h a n t h a t of t h e ca r b o xy l ica cid groups on the surfa ce, the underlying P AH st ronglypromotes the dissociation of the COOH groups to neutral-i ze t h e h i gh r esi dua l p osi t iv e c h a r ge ob t a i n ed w i t hdecreasing pH, leading t o a decrease in the pKa(app)of thePAA-terminated surface. However, when the f i lms areassembled a t higher pH va lues (pH > 5.5), t he a verageincrementa l thickness contributions from PAA a nd P AHare essentia l ly the same at matched assembly solutionpH values.17 Therefore, since the ratio of amino groups toca r b ox yl ic a c id g r ou ps i s l ow e r w h e n t h e f il ms a r ea ssembled a t high pH va lues, the underlying PAH chainsha ve less of an influence on the dissociat ion of the surfa ceCOOH gr oups going from high t o low pH on the titr a tioncurves.

Similar ra tionale can be used to explain t he trends int h e pKa(app) va lues for the f i lms termina ted in P AH a s afunction of assembly solution pH. In this case, t he basest r en gt h o f P AH i n cr ea ses w i t h i n cr ea si n g a ssem b lysolution pH. When th e films a re ass embled using the low-pH regime, th e ra tio of the carboxylic a cid groups of theunderla yer to the amino groups in the surface PAH layeri s l o w i n c o m p a r i so n t o i t s v a l u e wh en t h e f i l m s a r ea ssem bl ed u sin g t h e h i gh -p H r egi m e b eca u se of t h edifferences in the average incremental thickness contri-butions from PAH an d P AA.17 Hence, in the high solutionpH region of the titr a tion curve wh ere the COOH groupsof the und erlayer become dissociat ed an d the P AH cha inson the surface become weakly charged, the ability of thePAAchains to promote the protonation of the aminogroupson t h e P AH -t er m in a t ed su r f a c e i s gr ea t er i n f i lm s a s-sembled at higher pH values because of the higher ratioo f CO O H t o N H 2 functionali ty making up the surfacecomposition of th e film.

B ecause of the prevalence of the substr at e ef fects an dinhomogeneous polyelectrolyte covera ge in t he first 3-4lay ers with in the multilay er film, we exam ined the effectof the assembly pH on t he a cid-base equilibria of PAHa n d P A A a t b ot h l ow a n d h ig h l a y er n u m be rs . Wedetermined the a pparent dissociat ion const an ts for lay ers2 and 8 as well as layers 3 a nd 7, wh ich a re composed ofPAA a nd PAH, respectively. For both polyelectrolytespecies, the trends in the pKa(app) wit h increasing a ssemblypH ar e the same for th e two compara ble films exam ined.

However, the apparent dissociation constants depend onthe layer number for all assembly pH values studied.Theseresults a re in agr eement with t hose found in the study ofpKa(app) a s a fun ction of la yer number for films assembleda t n eu t r a l pH ) 7.0.

In t he case of the 2-lay er film termina ted w ith P AA, theti tra tion curves do not pass t hrough zero zeta potentia lfor films ass embled at pH )7.0, 8.0, an d 9.0. As mentionedea r l i er , t h i s a n o m a l y i s a t t r i b u t ed t o i n h o m o gen eo u scoverag e of the first P AH layer on silica a nd th e residualcharge on the silica particles since the charge density ofthe silica is known to increase in the presence of positivelycharged polyelectrolyte.42 In a ddition, due to the decreas-ing surface cha rge of the PAH layers a t higher pH valuesand PAA layers a t lower pH values, the stabil i ty of the

colloidal suspension is reduced under these conditions,resulting in flocculat ion of th e suspension in some cases.This wa s observed a t high solution pH va lues for a 3-la yerfilm a ssembled at pH )9.0. This beha vior wa s not observedfor a 7-lay er film a ssembled at pH) 9.0, suggesting thatthe f locculat ion of th e 3-layer f i lm ma y be a ttr ibuted toinhomogeneous coverage of the polymer on the particlesu r f a c e, wh i c h i s c o m m o n f o r t h e f i r s t f ew l a y er s a s-sembled. This explains the discrepancy in the trend ofpKa(app) for 3-lay er films a ssembled at d ifferent pH va lues(Table 1).

SaltEffectontheAcid-BaseEquilibria.In a dditionto examining t he dependence of the a cid-base equilibriaof PAH/P AA mult ila yer film s on th e num ber of polyelec-t r o l y t e l a y er s i n t h e f i l m a n d t h e p H o f t h e a ssem b l ysolutions, we a lso investigated the ef fect of varying thesalt concentration in the assembly solutions. This pa-rameter is of interest because the presence of salt in theassembly step screens the electr ical charge in the sys-t em , w h i ch i n fl u en ces su c h f a c t o rs a s t h e a m o u n t ofpolyelectrolyte a dsorbed, the conforma tion of the cha inson the surface, and the stabil i ty of both the multi layerfilm and theparticlesuspension.15,21,43 In a ddition, studieshave shown that the salt concentration directly a f fectst h e a c id-b a s e e q u il ib r ia of b ot h P A H a n d P A A insolution.44,45 To determine t he effect of the Na Cl concen-tration in the assembly solutions on the pKa(app) of t h e

multi layer f i lms, we assembled fi lms a t t wo combinationsof salt concent ra tions. The results discussed so far a re forfilms assembled using a sa lt concentra tion of 0.2 M NaC lin the P AH solution an d 0.4 M NaC l in the P AA solution.Four other f i lms were assembled using a reduced saltconcentra tion of 0.1 M NaCl in t he P AH solution a nd 0.2M NaC l in the PAA solution. The resulting pKa(app) valuesare summarized in Table2. Nota bly, the salt concentra tiond oe s n ot h a v e a s ig n if ica n t e ff ect on t h e a c id-b a seeq u il ib r ia of t h e f ou r f il m s i n v est i ga t ed. O n e w a y t oaccount for this observation is to note that most of theNa + a n d C l- ions present from t he adsorption ba th ar erinsed a wa y in t he thr ee consecutive wa sh cycles. There-fore, it is reasona ble to believe tha t a ny of the changes inthe film caused by the presence of salt in the adsorption

s t ep m a y b e a l t er ed b y t h e s a l t -f re e w a s h s ol ut i on spreventing us from observing an y salt effects on the a cid-base equilibria of the surfa ce lay er. However, assemblingthe m ultilay ers on colloidal pa rticles from salt -free poly-electrolyte solutions led to insta ble f i lms tha t desorbedfrom t he part icles a f ter 3 la yers w ere deposited. In thisstudy, t he concentra tions of Na Cl used in t he a ssemblysolutions werechosen becausethey yielded themost robustf i lms for which growth wa s possible for many layers. In

(43) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. L a n g m u i r 2001, 17, 6655.

(44) Kodama, H. ; Miyajima, T.; Tabuchi , H . ; I shiguro, S . ColloidPolym. Sci. 2000, 27 8, 1.

(45) Kodama , H.;Miyajima, T.;Mori, M.;Takasha sha, M.;Mishimura,H.; Ishiguro, S . Colloid Polym. Sci. 1997, 275, 938.

Table 2. Summary of the Effect of the SaltConcentration in the Assembly Solutions on the

Dissociation Constants of PAH and PAA in MultilayerFilms

pKa(app)

polymerlayer

no.assembly

pHP AH/P AA 0.2 M/

0.4 M NaC lP AH/P AA 0.1 M/

0.2 M NaC l

P AH 3 7.0 10.47 10.44P AH 7 3.0 10.32 10.34P AA 2 3.0 2.75 2.69

P AA 8 7.0 2.85 2.81



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addition, the salt concentration in the solutions used forthe microelectrophoresis studies wa s ma inta ined a t 1.0

mM beca use the colloida l suspensions a re unsta ble in th epresence of higher salt concentrations.Titration with aStrongPolyelectrolyte.As a second

and independent test to confirm the suitabil i ty of ourmethod for determining theapparent dissociation constantof polyelectrolyte multi layer f i lms, we carried out anadditionalexperiment that allowed us toevaluatethepointof h a l f n eu t r a l iz a t i on of ou r m u l t il a y er f il m s. Th i sexperiment involved titrating a solution of the particleswrapped with a polyelectrolyte multi layer f i lm with astrong polyelectrolyte. We chose to use PDADMAC fort h i s s t u dy b ec a u se o f i t s h i gh b i n di n g a f f i n i t y .46 Thetitra tion experiment was carried out a t pH ) 9.0 for a2-layer f i lm assembled a t pH ) 9.0 (Figure 7). Fromknowledge of t he concentra tion of functional groups a t

the point of half n eutral izat ion (inf lection point on thecu r v e), we wer e a b l e t o det er m i n e a cor r esp on di n gconcentra tion of protons required to rea ch ha lf neutral-ization of the surface. The charge density of functiona lgroups on the surface wa s calculat ed based on ma nufac-t u r er -su pp li ed da t a of t h e su r fa c e a r ea of t h e coa t edcolloidal particles, and the relative sizeof the repeat unitsbased on usua l bond lengths. The resulting inflection point

wa s found to be 4.4 ( 0.4 for the sur fa ce of a 2-la yer film,termina ted in P AA. This result is in close agr eement w itht h e pKa(app) of the f i lm obtained from t he zeta potentia lversus pH study (pKa(app) )4.48 ( 0.08). Nota bly, t hereis a higher uncerta inty associat ed with t he strong poly-electrolyte t itra tion method for determining the dissocia-tion constant than with the variable-pH zeta potentia lcurves used for the study reported here.

Conclusion

We reported here direct measurements of the localacid-base equilibria of polyelectrolytes incorporated intomultilayer assemblies on solid supports. We found sig-nif icant deviat ions in the dissociat ion consta nts of PAHa n d P AA i n cor p or a t ed i n t o m u l t i la y er t h i n f il m s i ncompa rison to th e dilute solution values. We also inves-tigat ed the inf luence of the layer number, the a ssemblypH, and the a ssembly sa lt concentra tion on the a pparentdissociat ion consta nts for P AH and P AA surfaces. In a l lcases, PAA was found to be a stronger acid and PAH as t r on g er b a s e b y 1-4 u n i t s wh en i n cor p or a t ed i n t omultila yer ass emblies. The acid strengt h of P AA a nd th ebase strength of PAH both increase w ith a n increase inth e tota l number of la yers in th e films. The pKa(app) of PAAi n c r ea ses wi t h a n i n c r ea se i n t h e p H o f t h e a ssem b l ysolutions but a lwa ys remains lower tha n the dilute solutionv a l u e, wh i le t h e pKa(app) of P AH increases wit h an increasein the pH of the assembly solution. In a ddition, the sa ltconcentra tion in the a ssembly solutions ha s no detecta bleinfluence on the acid-base equil ibria of the multi layerfilms. However, salt is required in the a ssembly solutionsfor successful as sembly of P AH/P AA mult ila yer films oncolloidal silica. Knowledge ofa cid-base anomalies in weakpolyelectrolyte systems may be important for polyelec-trolyte multilayer thin films employed as pH sensors, drugdelivery vehicles, and selective membranes.

Acknowledgment. Th e a u t hor s t h a nk N S E RCCanada and McGill University for f inancial support in

the form of grants and scholarships. Professors D. Ronisand T. van de Ven are acknowledged for helpful discus-sions, a nd Pr ofessors van de Ven, Reven, and Ka zlaus kasare a lso a ppreciat ed for use of equipment.

Supporting Information Available: Plots showingthe layer-dependent acid-ba s e t i tra t io n curv es f o r 1 0 f i lmsassembled at pH ) 7.0 and a plot showing t he titra tion curvesfor P AH an d P AA in dilute solution. This ma terial is a vailablefree of cha rge via th e Int ernet a t h tt p://pubs.acs.org.

LA026500I(46) Schwarz, S.; Buchhammer, H. M.; Lunkwitz, K.; J acobasch, H.

J . Colloids Surf., A 1998, 14 0, 377.

Figure 7. Acid-base equilibrium profile for a 2-layer P AH/PAA multilay er film assembled at pH ) 9.0 and t i t r a t e d wit hPD ADMAC. U ncertainty in electrophoretic mobility , (9.6%.


19 burke barrett

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