s

rface

samplet of surfacee extent ofeutral andnt at the

is manifested

Journal of Colloid and Interface Science 283 (2005) 294–299www.elsevier.com/locate/jci

Studies on the adsorption of Brij-35 and CTAB at the coal–water inte

S.K. Mishra∗, D. Panda

Regional Research Laboratory, Bhubaneswar 751 013, Orissa, India

Received 20 February 2004; accepted 9 September 2004

Abstract

The adsorption behavior of polyoxyethylene (23) lauryl ether (Brij-35) and cetyl trimethyl ammonium bromide (CTAB) on coalhas been studied. The adsorption process is found to be sensitive to pH, temperature, electrolyte concentration, and the amounactive agent. An attempt has been made to explain the adsorption behavior of the surfactants using the Langmuir equation. Thadsorption of Brij-35 on coal is found to be the highest at pH 2, which decreases with increase in pH and remains constant in the nalkaline pH regions. But, the adsorption of CTAB exhibits the opposite behavior of that of Brij-35. Adsorption of any of the surfactacoal/water interface sharply decreases the apparent viscosity of 55 wt% coal–water slurry (CWS) at a shear rate of 100 s−1. Electrostaticadsorption of the surfactants on the coal surface decreases the surface charge and renders the coal surface hydrophobic whichin the form of high apparent viscosity of the coal–water slurry under the test conditions. 2004 Elsevier Inc. All rights reserved.

Keywords: Adsorption; Brij-35; CTAB; Coal; Rheology; Apparent viscosity

al–esseseeend bsurl–theroupthatorpedthentedles

n of

ghlyvis-ni-ite

m-icaliledthetion

au-ylto

n the

amed

1. Introduction

The stability and rheological characteristics of the cowater slurry (CWS) are of great significance in the procof CWS transportation, atomization, and combustion. Thcharacteristics of CWS are related to the interaction betwcoal particles in aqueous suspension and are influencethe properties of adsorbed layers of surfactants on theface of coal[1–6]. Adsorption of surfactants at the coawater interface is controlled by many factors such asnature of the coal surface and the nature of the head gand tail part of the surfactant molecule. Coal particleshave been rendered mutually repulsive through the adstion of ionic or nonionic surfactant form a well-disperssuspension characterized by low viscosity. The thickeradsorption layer, the more the coal particles are prevefrom interaction which favors the dispersion of coal particresulting in the reduction of the viscosity of CWS[7]. There-fore, an understanding of the mechanism of adsorptio

* Corresponding author.E-mail address: sk_mis@rrlbhu.res.in(S.K. Mishra).

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.09.017

y-

-

surfactants on coal is essential for the preparation of hiconcentrated coal–water slurry (HCCWS) of the desiredcosity with good sedimentation stability. However, a uversal correlation of the work already carried out is qudifficult due to the diversity of coal in its physical and cheical contents which largely depends upon the geographsetting of the place where the coal is mined. Hence, detastudy of individual coal samples is necessary prior topreparation of HCCWS. This paper presents the adsorpbehavior of a nonionic surfactant polyoxyethylene (23) lryl ether (Brij-35) and a cationic surfactant cetyl trimethammonium bromide (CTAB) on an Indian coal sampleassess the effect of adsorption of these surfactants oviscosity of CWS.

2. Experimental

2.1. Materials

The coal sample used in the present work is the spowdered ROM coal having the d80 of 40 µm as reporte

S.K. Mishra, D. Panda / Journal of Colloid and Interface Science 283 (2005) 294–299 295

ted

s

lysisthe

thell-iate

olu-pH5 g,edinttedur-rds

loly-Ad-ter.ithliq-t ofthe

r-sing

riedor

sys-was

ring

andon--itecal-agent)duedilyes ance

s arethe

ionre-faceoal.rfacem-ater.s ob-

of

ssel

l inhities

acer pe-thentra-at

e

f

in our previous communication[8]. The nonionic surfactanpolyoxyethylene (23) lauryl ether or Brij-35 was procurfrom s.d. fine chem. India Ltd. and the cationic surfactantN -cetyl-N,N,N -trimethyl ammonium bromide (CTAB) waprocured from CDH India.

All other reagents used in the experiments and anawere of AR grade. Doubly distilled water was used in allexperiments.

2.2. Potentiometric titration

The surface charge of coal in KNO3 solutions of dif-ferent ionic strengths was determined as a function ofsolution pH by potentiometric titration. To a series of westoppered 125-ml PTFE bottles containing the approprelectrolyte, different volumes of 0.1 N HNO3 or 0.1 N NaOHwere added in such a way that the total volume of the stion was the same in each bottle. After 3 h the initialvalues were noted. Accurately weighed coal sample, 0.2was then added to each bottle and each bottle was immately stoppered. After 72 h of equilibration with intermittestirring, the pH values of the supernatant liquid were nowith the help of an Edt Research ion/pH meter. For this ppose the pH meter was calibrated with three buffer standanamely, pH values 4.0, 7.0, and 9.2.

2.3. Adsorption of surfactant

Fifty milliliters of surfactant solution of different initiaconcentrations was transferred to a series of 125-ml pethylene bottles each containing 0.25 g of coal sample.sorption studies were carried out in doubly distilled waas well as in KNO3 solutions of different ionic strengthsThe suspensions were allowed to equilibrate for 24 h wintermittent manual shaking after which the supernatantuid was carefully separated by centrifugation. The extenadsorption was determined from the difference betweeninitial and the final concentration of the surfactant.

2.4. Analysis of the surfactants

The equilibrium concentration of Brij-35 in the supenatant solution was estimated spectrophotometrically ucobalt thiocyanate method at 620 nm[9,10]and CTAB usingmethyl orange at 418 nm[11].

2.5. Rheological measurements

Rheological studies of the coal–water slurry were carout in a HAAKE rotational viscometer RV30 using sensMV1 and measuring system M5. The temperature of thetem and the coal–water slurry sample inside the cupmaintained constant (±0.1 ◦C) by circulating water from athermostatted water bath through a temperature tempevessel attached to the stator.

-

,

3. Results and discussion

3.1. Interfacial properties

Coal surface contains polar groups such as –COOHphenolic –OH attached to the hydrocarbon skeleton cnected by cross links[12,13]. Coal also contains clay minerals, kaolinite being the main component. Montmorillonand illite occur as interlayer minerals besides quartz andcite [14]. When coal is powdered there occurs the breakof both van der Waals and interatomic (ionic and covalebonds. The powdered coal surfaces are highly reactiveto the creation of unpaired electron centers, which reaadsorb oxygen from air. The coal surface thus assumheteropolar character due to the electronegative differebetween oxygen and carbon atoms. When coal particleexposed to water, surface hydroxyl groups are formed bydissociative sorption of water molecules by their interactwith clay and other mineral matters present in it. As asult of the proton transfer reactions coal acquires a surcharge that strongly affects the slurry properties of the cHence, it is necessary to know the surface charge and supotential of coal[15] to assess the influence of physicocheical interactions between dispersed coal particles and wThe proton charge characteristics of the coal sample watained by comparing the titration curve in the presencecoal with that of the medium alone[16,17]. This method wasused to ensure that the effects of impurities and the vewalls were taken into account.

The acid–base potentiometric titration results of coa0.1, 0.01, and 0.001 M KNO3 obtained using the batctitration method and expressed in excess proton dens(ΓH+–ΓOH− ) are given as a function of pH inFig. 1. Thebatch titration method yields better simulation of the surfcharge condition of the substrate because of the longeriod of equilibration used. It appears from the figure thatsurface charge increases with an increase in the concetion of electrolyte. The intersection of the curves occurspH 5.5 which is the pHpzc of coal, since at this point thadsorption density is zero. This value of pHpzc was also de-

Fig. 1. Adsorption isotherm of H+ and OH− during the batch titration ocoal (temperature, 30◦C).

296 S.K. Mishra, D. Panda / Journal of Colloid and Interface Science 283 (2005) 294–299

seswithf po-

on-

oalisithoaltheter

l surayerorplkylichne

esin-

omi-ant.qui-ith

onal

low

lues,f. Inin-

nd is,

l-in-

ties,

l atB–logde-

en-

e forthe

Ad-in the

termined by use of the solid addition method[18]. It maybe noted fromFig. 1that the surface charge on coal reverits sign at pH 5.5 and increases in absolute magnitudeincreasing ionic strength and increasing concentration otential determining ions. It can also be observed fromFig. 1that the pHpzc value does not depend on the electrolyte ccentration, indicating clearly the absence of K+ and NO3

−specific adsorption.

3.2. Adsorption of Brij-35 and CTAB on coal

Fig. 2 shows the adsorption isotherms of Brij-35 on cat 30, 50, and 70◦C. The initial increase in adsorptionattributed to the adsorption of the nonionic surfactant wthe polyoxyethylene chain partly directed toward the csurface leaving the hydrocarbon chain pointing towardbulk solution, thus displacing the weakly adsorbed wamolecules from the surface. In the second stage, the coaface that is free from water molecules achieves a monolcoverage. Upon further increase of the concentration adstion occurs through hydrophobic bonding between the agroup and the hydrophobic portion of the coal surface, whresults in a hydrophilic surface in which the polyoxyethylechain provides steric stabilization[19].

FromFig. 2it is seen that adsorption of Brij-35 increaswith temperature. This may be due to the fact that ancrease in temperature reduces the solubility with a conctant increase in the surface activity of nonionic surfactThe slope of the log–log plot of amount adsorbed vs elibrium concentration increases from about 0.3 to 0.4 wtemperature, suggesting that adsorption is not proportito concentration (slope< 1). Adsorption of Brij-35 on coais described using a Langmuir equation (Eq.(1)) in its lin-earized form to estimate the monolayer adsorption atconcentration.

(1)Ceq

M= Ceq

M ′ − 1

bM ′ ,

whereCeq is the equilibrium concentration (mol/L) of thesurfactant,M is the amount adsorbed (mol/g), M ′ is the

Fig. 2. Adsorption isotherm of Brij-35 on coal (pH 5.5 and KNO3concn= 0.001 M).

-

-

l

monolayer capacity (mol/g) andb is a constant.M ′ andb

can be evaluated from the slope and the intercept varespectively. The constantb in Eq. (1) is the measure opartitioning between the surface and the bulk solutionother words, it represents the equilibrium constant of theteraction between the surfactant and the coal surface atherefore, related to the free energy of adsorption (�G) as

(2)�Gads= −RT lnb

or

(3)�Hads− T �Sads= −RT lnb.

Dividing both sides byRT ,

(4)lnb = −�Hads

RT+ �Sads

R,

where �H , �S, R, and T have their usual and welknown thermodynamic significance. From the slope andtercept values of the linear plot of lnb vs 1/T , �Hads, and�Sadsvalues can be obtained. Knowing these two quanti�Gadscan be easily calculated (Table 1).

Fig. 3depicts the adsorption isotherm of CTAB on coa30, 50, and 70◦C, which indicates that adsorption of CTAalso increases with temperature. The slope of the logplot of amount adsorbed vs equilibrium concentrationcreases from about 0.63 at 30◦C to 0.55 at 70◦C, suggestingthe lack of proportionality between adsorption and conctration of the surfactant (slope,< 1). FromTable 1it can beseen that enthalpy and entropy of adsorption are positivboth the surfactants. But, the change in entropy in both

Table 1Thermodynamic data for coal/surfactant (Brij-35 and CTAB) system (sorption experiments of surfactants on coal were carried out at pH 5.5presence of 0.001 M KNO3 as supporting electrolyte.)

Surfactant �H0ads

(kJ mol−1)

�S0ads

(J mol−1 K−1)

�G0ads

(kJ mol−1)

Brij-35 8.78 123.36 −27.98CTAB 14.47 82.49 −10.11

Fig. 3. Adsorption isotherm of CTAB on coal (pH 5.5 and KNO3concn= 0.001 M).

S.K. Mishra, D. Panda / Journal of Colloid and Interface Science 283 (2005) 294–299 297

e

ngofpH

ial

asious-ease

re-c)the

ofglylues

l-hispH

hichbe

dsur-ad-

tesughr

sur-tha

ial

n ofnon

non-the

i-ar-ble

rp-he

e-150hear

ofsur-obicfacewardead

rease

w)B

theichcon-ABameaseinsrentereum

es-

cases, as measured by�S0adsand weighed by multiplication

with temperatureT , is just sufficient to compensate for thendothermic character of adsorption.

3.3. Effect of pH

To check if the electrostatic interaction is the driviforce for adsorption, the effect of pH on the adsorptionBrij-35 and CTAB was determined. The dependence onof adsorption of Brij-35 and CTAB on coal at constant initsurfactant concentration is illustrated inFig. 4. The variationof adsorption of Brij-35 with pH exhibits the same trendthat of NaDDBS on the same coal as reported in our prevcommunication[8]. Adsorption of Brij-35 attains the highest value at pH 2, which decreases sharply with the incrin pH and remains constant in the neutral and alkaline pHgion. The similarity in behavior between Brij-35 (nonioniand NaDDBS (anionic surfactant) may be attributed topresence of a lone pair of electrons on the oxygen atomthe ether linkage of the nonionic surfactant which is stronattracted toward the positively charged surface at pH vabelow pHpzc (5.5) of coal. Above pHpzc adsorption occursthrough hydrophobic interactions only.

However, adsorption of CTAB on coal at different pH vaues exhibits a trend just the opposite to that of Brij-35. In tcase adsorption of CTAB on coal increases slowly in therange 2–4 followed by a sharp increase above pH 4 wattains a constant value above pH 9.5. The coal surfacecomes more and more negative above pHpzc (pH 5.5) with anincrease in pH as seen fromFig. 1. The negatively chargesurface attracts the positive head group of the cationicfactant more strongly resulting in the higher extent ofsorption of CTAB above pHpzc of coal. The variation in theadsorption behavior of CTAB on coal with pH demonstrathat the predominant mechanism of adsorption is throelectrostatic interaction[20]. It may also be noted that foboth the surfactants adsorption also occurs at pHpzc of coal.Adsorption persists in the region where coal surface andfactant head group have a similar charge, which suggests

Fig. 4. Effect of pH on the adsorption of Brij-35 and CTAB and coal (initconcentrations of Brij-35 and CTAB are 1.33×10−4 and 3.5×10−3 mol/l,respectively, temperature, 30◦C).

-

t

the surface charge on coal does not affect the adsorptiothe surfactants under the test conditions. This phenomedemonstrates that for both the surfactants a specific orcoulombic interaction occurs between the surfactant andcoal surface.

3.4. Effect of adsorption of Brij-35 and CTAB on therheology of coal–water slurry

A coal–water slurry with low viscosity and good sedmentation stability is obtained by altering the surface chacteristics of coal particles through the adsorption of suitasurfactants.Fig. 5clearly demonstrates the effect of adsotion of surfactant on the apparent viscosity of CWS. Taddition of a very small amount of Brij-35 or CTAB (0.1%w/w of coal) to 55% (w/w) coal–water slurry at pH 5.5 rsults in a sharp decrease in the apparent viscosity frommilli Pascal second (mPa s) to less than 20 mPa s at a srate of 100 s−1 at 30◦C. At pH 5.5 the surface chargecoal particles is zero. Hence at pH 5.5, adsorption offactants on coal particles takes place through hydrophinteractions of the surfactant tail group and the coal surwhen the polar/charged head groups remain directed tothe solution. The repulsion among the polar/charged hgroups of the surfactant molecules results in a sharp decin the viscosity of the coal–water slurry.

The effect of pH on the apparent viscosity of 55% (w/CWS in the presence of 0.5 wt% Brij-35 or 0.25 wt% CTAat a shear rate of 100 s−1 is presented inFig. 6. It can be seenfrom the figure that the apparent viscosity of the CWS inpresence of 0.5 wt% Brij-35 is the highest at pH 2, whsharply decreases on increasing the pH and attains astant value at pH 7. But, in the presence of 0.25 wt% CTthe apparent viscosity of the CWS exhibits almost the svalue in acidic and neutral pH regions, but begins to increabove pHpzc, attains the highest value at pH 7, and remaconstant above pH 7. It may be noted here that the appaviscosity of the CWS is the highest at those pH values whthe adsorption of the surface active agent is the maxim

Fig. 5. Variation in the apparent viscosity of 55% (w/w) CWS in the prence of different amounts of surfactant at a shear rate of 100 s−1 at 30◦C(pH of the slurry= 5.5).

298 S.K. Mishra, D. Panda / Journal of Colloid and Interface Science 283 (2005) 294–299

ear

Ss-as

isbe-rgedpar

icallypor-rbedlec-rfacase

thees

ur-ead

ren-the

par-aren

-35lu-

totiones.n ofon

h-and

drp-st

thesim-ions.r-ositytedt%

p onsur-

obic,is-ns.

ra,751he.D.

indOneorcien-

nalFL,

s ofand

g-oal

nalDE

(1)

ucts,

s forrican

Fig. 6. Apparent viscosity of CWS (55 wt%) as a function of pH at a shrate of 100 s−1 at 30◦C.

(Fig. 6). In other words, the apparent viscosity of the CWis the highest below pHpzc (i.e., when the coal surface is poitively charged) in the presence of 0.5 wt% Brij-35, but hthe highest value above pHpzc (i.e., when the coal surfacenegative) in the presence of 0.25 wt% CTAB. Attractiontween the coal particle surface and the oppositely chasurfactant head group results in an increase in the apent viscosity of the CWS at the pH values shown inFig. 6.When the surfactant head group is adsorbed electrostaton the oppositely charged coal surface, the hydrophobiction of the surfactant remains on the exterior of the adsolayer thereby rendering the coal surface hydrophobic. Etrostatic adsorption of the surfactants decreases the sucharge of the coal particles thereby resulting in an increof particle to particle interactions which is manifested inform of high apparent viscosity of CWS. Adsorption takplace below pHpzc for CTAB and above pHpzc for Brij-35,respectively, through hydrophobic interactions of the sfactant tail group and the coal surface when the polar hgroup remains on the exterior. Hydrophobic adsorptionders the coal particles less hydrophobic and modifiessurface charge, which improves the dispersion of coalticles in aqueous suspension thereby lowering the appviscosity of the coal–water slurry.

4. Conclusions

From the above investigation on the adsorption of Brijand CTAB at the coal/water interface the following concsions can be drawn.

(i) The adsorption of Brij-35 and CTAB on coal is foundincrease with temperature. But, the extent of adsorpis not proportional to concentration in both the casThe free energy change shows that the adsorptioCTAB on coal is more favorable than that of Brij-35the same sample.

-

e

t

(ii) The extent of adsorption of Brij-35 on coal is the higest at pH 2 which decreases with an increase in pHremains constant above pHpzc, i.e., in the neutral analkaline pH regions. But, the variation of the adsotion of CTAB on coal with pH exhibits behavior juthe opposite of that of Brij-35.

(iii) As adsorption persists at those pH ranges wherecoal surface and the surfactant head group haveilar charges, it is clear that noncoulombic interactoccurs between the coal surface and the surfactant

(iv) Adsorption of Brij-35 or CTAB at the coal–water inteface results in a sharp decrease in the apparent viscof the coal–water slurry which is clearly demonstraby the addition of 0.1 wt% of the surfactant to 55 wCWS at a shear rate of 100 s−1.

(v) Electrostatic adsorption of the surfactant head grouthe oppositely charged coal surface decreases theface charge and renders the coal surface hydrophwhich is manifested in the form of high apparent vcosity of the coal water slurry under the test conditio

Acknowledgments

The authors express their thanks to Dr. Vibhuti N. MisDirector, Regional Research Laboratory, Bhubaneswar013, India, for his kind permission to publish this paper. Tauthors also express their gratitude to Dr. Rajeev, H.OC.P.A.F. Department, R.R.L., Bhubaneswar, for his khelp and encouragement during the course of this work.of the authors (S.K.M.) is grateful to CSIR, New Delhi, fthe award of Senior Research Associateship (under stists’ pool scheme).

References

[1] D.H. Everett, Pure Appl. Chem. 48 (1976) 419.[2] M.J. Vold, J. Colloid Interface Sci. 16 (1961) 1.[3] E.L. Mackor, J. Colloid Interface Sci. 6 (1951) 492.[4] T. Cheng, M. Cai, L. Jiang, in: Proceedings of the Sixth Internatio

Symposium on Coal Slurry Combustion and Technology, Orlando,1984, p. 313.

[5] E.Z. Casassa, G.D. Parfitt, A.S. Rao, E.W. Toor, in: Proceedingthe Sixth International Symposium on Coal Slurry CombustionTechnology, Orlando, FL, 1984, p. 251.

[6] Z. Yuhua, Q. Liuqing, D. Wenying, Z. Hongon, Y. Yuping, H. Chanming, in: Proceedings of the Sixth International Symposium on CSlurry Combustion and Technology, Orlando, FL, 1984, p. 437.

[7] T. Chen, M. Cai, J. Long, in: Proceedings of the Sixth InternatioSymposium on Coal Slurry Combustion, 1984 (CONF-840637,84 015343), p. 313, NTIS; Springfield, VA.

[8] S.K. Mishra, S.B. Kanungo, Rajeev, J. Colloid Interface Sci. 267(2003) 42.

[9] G.F. Longmann, The Analysis of Detergents and Detergent ProdWiley, New York, 1977, p. 398.

[10] A.D. Eaton, L.S. Clesari, A.E. Greenberg (Eds.), Standard Methodthe Examination of Water and Waste Water, nineteenth ed., AmePublic Health Association, 1995, pp. 5–42.

S.K. Mishra, D. Panda / Journal of Colloid and Interface Science 283 (2005) 294–299 299

3)

a-

oal

Fu-

s,

on,es,

978)

91)

uel

[11] M.M.B. Simon, A.D.E. Cozar, L.M.P. Diez, Analyst (London) 115 ((1990) 337.

[12] D.D. Whitehurst, T.O. Mitchell, M. Farcasiu, Coal Liquefaction, Acdemic Press, New York, 1980.

[13] R.M. Davidson, in: M.L. Gorbaty, J.W. Larsen, I. Wender (Eds.), CScience, Academic Press, New York, 1982, vol. I, p. 84.

[14] M.M. Konar, P. Bandopadhyay, S.K. Mazumdar, J. Mines Met.els 44 (1996) 231.

[15] A.L. Smith, in: G.D. Parfit (Ed.), Dispersions of Powders in LiquidAcademic Press, London, 1973, p. 126.

[16] C.P. Huang, The surface acidity of hydrous solids, in: M.A. AndersJ.A. Rubin (Eds.), Adsorption of Inorganics at Solid–Liquid InterfacAnn Arbor Science, Ann Arbor, MI, 1981, p. 183.

[17] R.O. James, J.A. Davis, J.O. Leckie, J. Colloid Interface Sci. 65 (1331.

[18] L.S. Balistrieri, J.W. Murray, Am. J. Sci. 281 (1981) 788.[19] P. Taylor, W. Liang, G. Bognolo, T.F. Tadros, Colloids Surf. 61 (19

147.[20] A. Guerses, S. Bayrakceken, K. Doymus, M.S. Guelaboglu, F

Process. Technol. 45 (2) (1995) 75.