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ceuticals,been estab-

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changes inption was

process.ence of the

EC on talcformation,g of then

iving forcemodifiedM imag-ng out

Journal of Colloid and Interface Science 293 (2006) 322–332www.elsevier.com/locate/jcis

Mechanisms of ethyl(hydroxyethyl) cellulose–solid interaction:Influence of hydrophobic modification

Jing Wang, P. Somasundaran∗

NSF Industry/University Cooperative Research Center for Advanced Studies in Novel Surfactants, Columbia University, New York, NY 10027

Received 5 May 2005; accepted 20 June 2005

Available online 2 August 2005

Abstract

Hydroxyethyl cellulose and its hydrophobically modified derivatives are widely used in many industrial areas such as pharmacosmetics, textiles, paint and mineral industries. However, the interaction mechanisms of these biopolymers and solids have notlished. In this work, the interaction mechanism and conformation of hydrophobically modified ethyl(hydroxyethyl) cellulose (C14-EHEC)have been investigated using spectroscopic, AFM and allied techniques. Comparison was made with corresponding unmodified aorder to investigate the effects of the hydrophobic modification. Electrokinetic studies showed that polysaccharides adsorption decnegative zeta potential of talc but did not reverse the charge. EHEC adsorption on talc was not found to be affected significantly bysolution conditions such as pH and ionic strength, ruling out electrostatic force as the controlling factor. However, HM-EHEC adsorfound to increase markedly with increase in ionic strength from 0.1 to 1 suggesting a role for the hydrophobic force in this adsorptionFluorescence spectroscopic studies conducted to investigate the role of hydrophobic bonding using pyrene probe showed no evidformation of hydrophobic domains at talc–aqueous interface. Urea, a hydrogen bond breaker, reduced the adsorption of HM-EHmarkedly. In FTIR study, the changes in the infrared bands, associated with the C–O stretch coupled to the C–C stretch and O–H dewere significant and therefore support strong hydrogen bonding of HM-EHEC on the solid surface. Moreover, Langmuir modelinadsorption isotherms suggests hydrogen bonding to be a major force for the adsorption of EHEC and C14-EHEC on solid since the adsorptiofree energies of these polymers were close to that for hydrogen bond formation. All of the above results suggest that the main drfor EHEC adsorption on talc is hydrogen bonding rather than electrostatic interaction or hydrophobic force. For hydrophobicallyC14-EHEC, hydrophobic force plays a synergetic role in adsorption along with hydrogen bonding. From computer modeling and AFing, it is proposed that C0-EHEC and C14-EHEC adsorb flat on talc with ethylene oxide side chains and hydrophobic groups protrudifrom the surface into bulk water phase. 2005 Elsevier Inc. All rights reserved.

Keywords:Talc; EHEC; Hydrophobic modification; Polymer adsorption; Fluorescence; FTIR; AFM; Model; Zeta potential

ith

oxylgentain

city.

oxyl,s to

ion,lly-EC

einyh is

1. Introduction

The mineral talc is a hydrous magnesium silicate wthe chemical formula Mg3Si4O10(OH)2 [1]. Its elementarysheet is composed of a layer of magnesium-oxygen/hydroctahedra sandwiched between two layers of silicon-oxytetrahedra. The main surfaces of this sheet do not conhydroxyl groups or active ions and hence the hydrophobi

* Corresponding author.E-mail address:ps24@columbia.edu(P. Somasundaran).

0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2005.06.072

The edges of the sheets are, however, composed of hydrsilicon, magnesium and oxygen ions and this contributeits hydrophilicity.

The nonionic polymers used in the present investigatethyl(hydroxyethyl) cellulose (EHEC) and hydrophobicamodified ethyl(hydroxyethyl) cellulose (HM-EHEC) are important associative thickeners. The repeating units of EHand HM-EHEC are shown inFig. 1. Previous studies havdealt with the rheology and phase behavior of HM-EHECan aqueous solution[2–4], with less emphasis on the studof interactions between these polymers and solids, whic

J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332 323

(a)

(b)

Fig. 1. The monomeric structure of (a) EHEC, (b) HM-EHEC.

lica-chahed.

Liutesork-ysi-

o-d-e,

ing.di-tionad-ntlythegumicobi-ied

tic

lec-

i-)x-of

is-slyle

attud-

singso-HEso-di-s-em-ndx-

essential for coating, dispersion, flotation and other apptions. Specific interaction mechanisms between polysacrides and solid surfaces are generally not well establisWhile Balajee[5], Afenya[6] and Jucker[7] have attributedthe adsorption of polysaccharides to hydrogen bonding,and Laskowski[8] observed that the surface metallic siwere responsible for polysaccharide adsorption. Other wers suggested polysaccharide adsorption to involve phcal interaction via hydrophobic bonding[9–11]. Steenbergand Harris[12] have proposed a combination of hydrophbic and hydrophilic interaction, with polysaccharides asorbing firstly on the faces of talc via hydrophobic forcand then on the hydrophilic edges via hydrogen bondIf their conclusion is correct, then the hydrophobic mofication of polysaccharide would strengthen the interacbetween the polymer and the solid and improve polymersorption significantly. However, we have observed recethat hydrogen bonding rather than hydrophobic force isgoverning mechanism for polysaccharide such as guaron talc [13]. Therefore, to study the role of hydrophobforce on polysaccharide adsorption, EHEC was hydrophcally modified and two polymers were comparatively studin this work.

The objective of this work was to clarify the mechanisaspects of the interactions between EHEC/C14-EHEC andtalc using a combination of spectroscopic, microscopic, etrostatic adsorption techniques and molecular modeling.

-2. Experimental

2.1. Materials

Ethyl(hydroxyethyl) cellulose (EHEC) and hydrophobcally modified ethyl(hydroxyethyl) cellulose (HM-EHECwere obtained from Akzo-Nobel Inc. They have approimately the same weight average molecular weight100,000. The hydrophobic moiety in the HM-EHECtetradecyl (C14). All EHEC and HM-EHEC stock solutions were prepared by adding powder into vigoroustirred water and furthur stirring for 30 min. Talc sampof 40 µm size and BET surface area of 19.75 m2/g wasobtained from Cytec Industries. Mica in the form of flplate purchased from Ted Pella Inc. was used in AFM sies.

pH of the solutions and suspensions was adjusted uFisher standard hydrochloric acid or sodium hydroxidelutions. Reagent grade potassium chloride from APACChemicals Inc. was used to adjust the ionic strength oflutions. 1-pyrene butyric acid and 1,3-dicyclohexyl carboimide (DCC) bought from Aldrich were used in the fluorecence probe labeling of the polymers. Urea (Fisher Chical), phenol (EM Science) and 98% sulfuric acid (AmeDrug & Chemical Co. Inc.) were used for colorimetric eperiments. The water used was triply distilled.

324 J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332

ure-luded-hy-re

ted%enasingpertionsityial

5 mlob-op-d at–on

forl orleftea-

mererieser-

ithllets

oni-er.

dedionshto-

epa-oton

s antsg a

orP-

gesrce

orts.

eseingrides

fill-dnits.hedwneatthefe

hen0 to

io

talct

Cethe.

2.2. Experiments

Experiments done along with the adsorption measments to investigate the adsorption mechanisms incFTIR for exploring the role of hydrogen bonding in asorption, fluorescence spectroscopy to study the role ofdrophobic force, AFM and molecular modeling to explothe conformation of the adsorbed polysaccharides.

2.3. Adsorption measurements

The suspensions of solids, with ionic strength adjususing KCl and pH adjusted using HCl or KOH at 10solid loading, were ultrasonicated for 30 min and thstirred magnetically for 2 h. Polymer stock solution wthen added to the solid suspension and left for conditionovernight. The suspensions were centrifuged and the sunatants pipetted out for determination of guar concentraby a Total Organic Carbon Analyzer. The adsorption denof polymer on solid was calculated from the data for initand residual polymer concentrations.

2.4. Colorimetric method

A colorimetric method described by Dubois et al.[14]was used in the experiments. 0.05 ml 80% phenol and98% sulfuric acid were added to 2 ml of supernatanttained after centrifugation, and after 4 h of color develment under warm conditions, absorbance was measurewavelength of 487.5 nm using SHIMADZU UV-1201 UVvis spectrophotometer. Adsorption density of polymertalc was calculated from the difference in absorbance.

2.5. Electrokinetic measurements

Small amounts of talc were added to 10−3 M KNO3 solu-tion and ultrasonicated for 30 min, magnetically stirred2 h and the pH adjusted to the desired value using HCKOH. Finally, the polymer stock solution was added andfor conditioning for 16 h. The zeta potential was then msured using a Zeta meter.

2.6. FTIR study

The infrared spectra of talc, polysaccharides and polyadsorbed talc were recorded using a Model FTS7000 sFourier transform infrared spectrometer from Digilab opating in the range of 4000–350 cm−1. Approximately 2 mgof the desired powder sample was thoroughly mixed w200 mg of spectroscopic grade KBr and pressed into pefor recording the spectra.

2.7. Fluorescence spectroscopy

Suspensions of talc at 10% solid loading were ultrascated for 30 min and mixed for 2 h with a magnetic stirr

-

a

Stock solution of pyrene-labeled polymer was then adto the talc suspension. The vials containing the suspenswere quickly wrapped with aluminum foil to keep the ligaway and left to condition overnight. Finally, the hydrophbicity of the supernatant and the solid were analyzed srately using a LS-1 fluorescence spectrophotometer (PhTechnology International).

2.8. AFM analysis

The atomic force microscopy (AFM) system used waNanoscope III from Digital instruments. The measuremewere performed in air or underwater in tapping mode usinV-shaped Si3N4 cantilever covered with gold on the back flaser beam reflection. Two types of tips (MESP-10 and N20) were used in air and underwater respectively. All imawere collected in the height mode, which keeps the foconstant.

2.9. Molecular modeling

Molecular modeling (Macromodel) is a powerful tool fexploring the conformation of polysaccharide in solvenMacromodel was applied to study the conformation of thpolymers. Accelrys Material Studio was also used in helpto understand the conformation of adsorbed polysacchaon solid.

3. Results and discussion

3.1. Molecular modeling

Fig. 2shows the computer simulated molecular spaceing structure of EHEC and C14-EHEC in both vacuum anaqueous environments. Each of them contains 5 repeat uIt is seen that polymer chains tend to be in a more stretchelical form in water. The complex helical structure shoin Fig. 3 is a piece of polymer chain containing 50 repunits. It is clear that the OH groups tend to stick out ofhelix. For HM(C14)-EHEC, from the longitudinal view ostructure, it appears that the C14 hydrophobic tails stay in thcenter of the helical structure (Fig. 3). According to the sim-ulation, there is no change in the simulated structure wthe number of repeat units is increased further from 5100.

From computer simulation by Accelrys Material Stud(Fig. 4), it was found that both EHEC and C14-EHEC mole-cules chose relatively flat conformation to adsorb onsurface. Compared toFig. 3, it is clear to see the significanchange of the conformation of C14-EHEC after adsorptionon solid surface. Therefore, it can be proposed that0-EHEC and C14-EHEC adsorb on talc with ethylene oxidside chains and hydrophobic groups protruding out fromsurface into bulk water to make the surface hydrophobic

J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332 325

five

Fig. 2. Computer simulated molecular CPK (space filling) structure of: (a) EHEC, (b) C14-EHEC with and without aqueous environment (each containsrepeat units).

lec-.5,n pHorp

etautorp-talc

ayermer

ndtheH 3in

r onsur-m 3

tha

ad-

onen-tot

s in-thed-

tionfur-dly.tion

stsond

nd itn theoncha-

3.2. Electrokinetic studies

It is evident from the electrokinetic data shown inFig. 5that the surface of talc is negatively charged and the isoetric point (i.e.p.) is located around pH 2.5. Above pH 2the electronegative character increases with increase iup to 4 and thereafter remains almost constant. The adstion of EHEC and C14-EHEC decreased the negative zpotential of talc (without any significant shift of i.e.p.) bdid not reverse the charge, which indicates that the adstion of these polymers only masks the surface charge onsince the shift of shear plane due to adsorbed polymer lis the reason for the change in zeta potential upon polyadsorption.

3.3. Adsorption studies

3.3.1. Influence of pHFigs. 6 and 7show adsorption isotherms of EHEC a

C14-EHEC on talc at pH 3 and pH 9. It can be seen thatadsorption density does not vary measurably between pand 9. If electrostatic interaction plays an important rolethe adsorption process, the adsorption density of polymetalc should have been affected significantly since theface charge of talc does change with change in pH froto 9. Such is not the case here, and it can be concluded

-

t

electrostatic interaction is not the dominant force for thesorption of EHEC and C14-EHEC on talc.

3.3.2. Influence of ionic strengthAdsorption isotherms of EHEC and C14-EHEC on talc

at pH 9 at different ionic strengths are shown inFigs. 8and 9. If hydrophobic force makes a significant contributito adsorption of a polymer on a mineral, the adsorption dsity would have increased with the addition of salt due“salting-out” effect[15]. For EHEC, there is no significanchange in the adsorption density as salt concentration icreased from 0 to 0.1 and 1 M. Thus it would appear thathydrophobic force is not the main driving force for the asorption of EHEC on talc. In the case of C14-EHEC, almostno effect is observed with the increase of salt concentrafrom 0 to 0.1. However, as salt concentration increasedther from 0.1 to 1, adsorption density increased markeHydrophobic force may play a role in the case of adsorpof HM-EHEC on talc.

3.3.3. Adsorption on talc with and without ureaTo explore the role of hydrogen bonding, adsorption te

were carried out in the presence of urea, a hydrogen bbreaker. Urea is a strong hydrogen bonding acceptor acan be expected to affect the hydrogen bonding betweesolid and the polymer in solution by preferential formatiof hydrogen bonds between themselves and the polysac

326 J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332

Fig. 3. Computer simulated (a) latitudinal view and (b) longitudinal view of molecular CPK (space filling) structure of EHEC and C14-EHEC in aqueousenvironment (each contains 50 repeat units).

Fig. 4. Computer simulated structure of (a) EHEC and (b) C14-EHEC molecule on talc surface.

J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332 327

Fig. 5. Zeta potential of talc as a function of pH in the presence and absence of EHEC and HM-EHEC, 0.001 M KCl.

.

lt).

n in, re-

gth

gth

-tant

Fig. 6. Adsorption isotherm of EHEC on talc at different pH (no salt)

Fig. 7. Adsorption isotherm of HM-EHEC on talc at different pH (no sa

rides and water. Adsorption isotherms for C14-EHEC on talcin the presence and absence of urea at pH 9 are showFig. 10. It is evident that urea, the hydrogen bond breaker

Fig. 8. Adsorption isotherm of EHEC on talc at different ionic stren(pH 9).

Fig. 9. Adsorption isotherm of HM-EHEC on talc at different ionic stren(pH 9).

duces the adsorption of C14-EHEC on talc markedly. Therefore, it is proposed that hydrogen bonding plays a imporrole in the adsorption of C14-EHEC on talc.

328 J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332

ch-gs a

olidmer

ea.

onav-to

y-

-f

e of

3.3.4. Modeling of polysaccharide adsorption on solidsurface

To obtain a better understanding of the adsorption meanism of C14-EHEC and EHEC on solids, the followinparameters are extracted from the adsorption isothermshown inTable 1: standard free energy of adsorption�G0

ads,maximum amount of polymer adsorbed per mass of s(x/m)max and effective substrate area occupied per polychainσ 0 at the talc/solution interface.

Fig. 10. Adsorption isotherms of HM-EHEC on talc with and without ur

s

The adsorption isotherms of EHEC and HM-EHECsolids in this study exhibited a pseudo-Langmuirian behior. According to the Langmuir adsorption model appliedthe polymer systems[16], surface coverage

(1)θ = KC/(1+ KC) = (x/m)/(x/m)max.

Equation(1) can be expressed as

(2)m/x = 1/[K(x/m)maxCeq

] + 1/(x/m)max,

whereCeq is the equilibrium solution concentration of polmer, x is the amount of polymer adsorbed,m is the massof solid substrate,K is the Langmuir adsorption equilibrium constant and(x/m)max is the maximum amount opolymer adsorbed per mass of solid. A plot ofm/x against1/Ceq should yield a linear relationship. The values ofK and(x/m)max can be determined from the intercept and slopsuch a plot as shown inFigs. 11 and 12.

Table 1The calculated values of(x/m)max, �G0

ads andσ0 for the adsorption ofEHEC and HMC on talc atT = 25◦C

Polymer (x/m)max(mg/m2)

�G0ads

(kJ/mol)σ0

(nm2)

EHEC 1.31 −23.0262 126.3HM-EHEC 0.93 −28.3803 178.65

Fig. 11. Langmuir plot for adsorption of EHEC on talc at pH 9,T = 25◦C.

Fig. 12. Langmuir plot for adsorption of HM-EHEC on talc at pH 9,T = 25◦C.

J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332 329

Fig. 13. FTIR spectrum of C14-EHEC.

tic-y o

rea

-e

rea

e,ule

se

re-d-

edof

rol-

the

obic

ponrheanndeoly-

o

n inof

det-as

dsmThe-the

gle-

onof

The Langmuir adsorption equilibrium constant,K , can beconsidered to represent the affinity of a polymer for a parular surface. It can be related to the standard free energadsorption,�G0

ads, from the expression given in Eq.(3):

(3)�G0ads= −RT lnK,

whereR is the general gas constant (R = 8.314 J/(mol K))andT is the absolute temperature.

From the maximum adsorption density, the effective aoccupied on the substrate surface per polymer chain,σ 0, canbe calculated using the following equation:

(4)σ 0 = θA

ns2NA

,

whereNA is Avogadro’s number.θ is the fraction of the surface which is covered by polymer,A is the substrate surfacarea,ns

2 is the number of moles of polymer adsorbed.The number of moles of polymer adsorbed per unit a

of solid,ns2/A, is given by Eq.(5).

(5)ns

2

A= (x/m)max

Mw,

whereMw is the molecular weight of the polymer. Hencthe effective substrate area occupied per polymer molecσ 0, can be calculated using Eq.(6).

(6)σ 0 = θMw

(x/m)maxNA.

From Table 1, it can be seen that�G0ads, calculated for

EHEC and HM-EHEC indicates that adsorption of thepolymers on talc is highly favored, i.e.�G0

ads< 0. It is ap-parent that the values of�G0

ads are around−25 kJ/mol,close to the free energy of hydrogen bond formation. Moover, if hydrophobic interaction is the major force for asorption, then a value of about−50 kJ/mol will be expectedfor �G0

ads [17], which is much greater than the calculatvalue. These results thus further confirm the major rolehydrogen bonding rather than hydrophobic force in contling CMC adsorption. In addition,�G0

adswas determined tobe more negative for the adsorption of HM-EHEC than

f

,

other ones because of the synergetic effect of hydrophinteraction in adsorption.

For a particular polymer, the value ofσ 0 will be deter-mined by the conformation that the polymer adopts uadsorption. Clearly,σ 0 will be much greater if the polymeis adsorbed mainly in “trains” (a conformation in which tmajority of the polymer is in contact with the surface) thif it adsorbs in the form of a high degree of “loops” a“tails”. As shown inTable 1, theσ 0 of polysaccharides arvery high, which means that most segments of these pmers adsorb as trains on talc.

3.4. FTIR spectroscopy

For pure C14-EHEC (Fig. 13), the band at 2924 cm−1 isdue to C–H stretching of the –CH2 groups. The band due tring stretching of glucose appears at around 1611 cm−1. Inaddition, the bands in the region 1350–1450 cm−1 are dueto symmetrical deformations of CH2 and COH groups. Thebands due to primary alcoholic –CH2OH stretching modeand CH2 twisting vibrations appear at 1078 and 1021 cm−1,respectively. The weak bands around 770 cm−1 are due toring stretching and ring deformation ofα-D-(1–4) andα-D-(1–6) linkages.

Fig. 14 shows the spectra of talc, C14-EHEC and C14-EHEC adsorbed talc. The FTIR spectrum of talc is showFig. 14a. According to the characteristic IR frequenciestalc reported by other researchers[18], the band at aroun1039 cm−1 can be assigned to the out-of-plane symmric Si–O–Si mode. The Si–O bending vibration for talc hbeen observed at 432 cm−1 in the spectrum. The other banin the spectrum around 654, 604, 550, 481 and 450 c−1

are probably associated with various Mg–OH modes.band at around 384 cm−1 appears to involve mixed vibrations of the Si–O network, the octahedral cations andhydroxy groups. The band at 374 cm−1 is associated withthe symmetric Mg–OH vibration. Talc also shows a sinMg–OH stretching band around 3677 cm−1 due to the centrosymmetric relationship between the hydroxy groupsboth sides of the octahedral layers. The FTIR spectrum

330 J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332

Fig. 14. FTIR spectra of talc, C14-EHEC and C14-EHEC adsorbed talc.

Fig. 15. FTIR spectra of talc, C14-EHEC and C14-EHEC adsorbed talc (850–1200 cm−1).

m in

p-ionledant

d to

asidatee is

mesres-ad-

n ofausecon-ma-nd

a-sedlly

fmer

talc after interaction with C14-EHEC is depicted inFig. 14b.It is apparent that several bands appear on the spectruthe region 1000–1050 cm−1 (Fig. 15b) which is attributedto the hydrogen bond formation between the O of –CH2–CH2–O–CH2–CH2– and Si–O–H of talc edge after adsortion [19]. The changes in the infrared bands in the reg1000–1080 cm−1, associated with the C–O stretch coupto the C–C stretch and O–H deformation, were significand support strong hydrogen bonding of C14-EHEC to thesolid surface, since hydrogen bonding would be expecteaffect the C–O stretching.

3.5. Fluorescence spectroscopy

The microstructure of the polymer adsorbed layer wprobed using fluorescence spectroscopy in order to elucthe nature of HM-EHEC adsorption. Pyrene fluorescencsensitive to the medium in which it resides[20]. The ratio ofthe intensities of the first to the third peak (I3/I1) on a pyrene

emission spectrum will increase as the environment becomore hydrophobic and is termed polarity parameter. Fluocence data for the pyrene labeled EHEC and HM-EHECsorbed talc-solution interfaces are shown inFigs. 16 and 17.Pyrene fluorescence gave no indication for the formatiohydrophobic domains at the talc-aqueous interface becthe polarity parameter decreased with the increase incentration of both polymers, suggesting instead the fortion of hydrophilic domains for the adsorption of EHEC aHM-EHEC on talc.

3.6. AFM analysis

From AFM imaging[21–24] of polymer adsorbed micsurface (Fig. 18a), HM-EHEC is found to form isolated clusters evenly on the solid. As the concentration was increafrom 200 to 1000 ppm, the solid surface was almost fucovered by these clusters (Fig. 18b), however the size oeach cluster remained the same with increase in poly

J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332 331

, no

on00

asedfthesur-it isy-

alt).

rgeular

hy-

M-hat

Fig. 16. Pyrene fluorescence for HM-EHEC and talc interface (pH 8.5salt).

concentration with the topography of C0-EHEC adsorbedmica very different (Fig. 18c). C0-EHEC forms a smoothlayer with small uncovered spots uniformly distributedmica. With the increase of polymer concentration from 2to 1000 ppm, the number of the uncovered sites decrewith an increase in their size (Fig. 18d). The thickness oadsorbed layer is around 3 nm, which further provedflat adsorbed conformation of these polymers on solidface as is suggested in molecular modeling. Therefore,proposed that C0-EHEC adsorbs flat on solid surfaces by h

Fig. 17. Pyrene fluorescence for EHEC and talc interface (pH 8.5, no s

drogen bonding with ethylene oxide side chains with laaggregates forming due to intramolecular and intermolechydrogen-bonding interactions[25,26]. But for C14-EHEC,only small clusters form because of the presence ofdrophobic chains on the polymer.

4. Conclusions

(1) Electrokinetic studies of adsorption of EHEC and HEHEC on talc in the pH range of 2–11 showed t

Fig. 18. AFM images of C14-EHEC and C0-EHEC adsorbed on mica at (a) C14-EHEC, 200 ppm, (b) C14-EHEC, 1000 ppm, (c) C0-EHEC, 200 ppm,(d) C0-EHEC, 1000 ppm.

332 J. Wang, P. Somasundaran / Journal of Colloid and Interface Science 293 (2006) 322–332

but

n ofes

orce

asy-C

00–to

antese

bic-

thep-her

toef-ace.ob-olidith

g-foren-

ond

maintoho-in

nalEClid

nalag-

at

in-

12–

le-

B 9

.

.),o, IL,

, in:alia

) 77–

al.

ng.

ut-

ep.

ys.

95)

em.

.5)

is,

rf.

dr.

to-.

polymer decreased the negative zeta potential of talcdid not reverse it.

(2) Adsorption studies also showed that the adsorptioEHEC and HM-EHEC on talc is not affected by changin pH. These results suggest that the electrostatic fis not the dominant force for the adsorption process.

(3) The adsorption of HM-EHEC but not EHEC on talc waffected significantly by the ionic strength level. Hdrophobic force plays a role in adsorption of HM-EHEon talc.

(4) The changes of the infrared bands in the region 101080 cm−1, associated with the C–O stretch coupledthe C–C stretch and O–H deformation, were significand therefore support strong hydrogen bonding of thpolysaccharides to the solid surfaces.

(5) Pyrene fluorescence gave no indication for hydrophodomain formation for the adsorption of EHEC and HMEHEC at talc–aqueous interfaces.

(6) Urea, a hydrogen bond breaker, reduces significantlyadsorption of HM-EHEC on talc. This result also suports a mechanism involving hydrogen bonding ratthan a hydrophobic one.

(7) Polysaccharides were also found from AFM imagingadsorb in a very flat conformation according to thefective area per polymer chain on the substrate surfFrom molecular modeling, helical structures wereserved. The polymers were found to adsorb flat on ssurface to let more of their OH groups in contact wthe surface.

(8) Langmuir modeling of adsorption isotherm further sugested hydrogen bonding to be the dominant forcepolysaccharide adsorption since the adsorption freeergy of these polymers close to that of hydrogen bformation.

Generally speaking, all the above results suggest thedriving force for EHEC and HM-EHEC adsorption on talcbe hydrogen bonding rather than electrostatic or hydropbic force. Hydrophobic force plays a synergistic roleHM-EHEC adsorption process. In addition, conformatiostudies suggest a helical structure of EHEC and HM-EHin solution while they are found to adsorb flat on the sosurface.

Acknowledgments

The authors acknowledge the support of the NatioScience Foundation (Grant #CTS-00-89530 dynamic

gregation behavior of surfactant mixture in solutionssolid/liquid interface) and Akzo-Nobel Industries.

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