Ghoshal, D. N.

Pal, Bidyut K.

English

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IDdfan JoUJ'lllll of ChemistryVol. 15A, Au~ust 1977, Plio 680-683Adsorption of Cetylpyridinium Chloride & CetyltrimethylammoniumBromide on BentoniteBlDYUT K. PAL & D. N. GHOSHALChemistry Unit, Indian Statistical Institute, Calcutta 700035Received 9 November 1976; accepted 6 January 1977A monolayer of cetylpyridinium (chloride (CPC) is formed on bentonite surface at saturationadsorption. Total surface area of bentonite calculated from the monolayer capacity accountsfor only 45% of the theoretical value. External surface area determined from the adsorptionof CPC comes close to the anticipated value. About 56% of the quantity of surfactants adsorbedat monolayer coverage is bound to the charge sites, while the rest on the hydrophobic part ofthe surface. Adsorption before and after the saturation of the cation exchange capacity (CEC)is brought about by forces that are almost entirely electrical or entirely hydrophobic respectively.Surfactants are adsorbed in the associated form above the CEC of clay. A method for determin-ing the CEC of bentonite by studying the conductance behaviour of the equilibrium solutions inadsorption experiments has been developed.SPECIFIC surface areas and cation exchangecapacities (CEC) of soils and soil clays aretwo most important factors in determiningmany of the soil colloidal behaviour. Greenlandand Quirck1,2 studied the adsorption of cationicsurfactants on clays and soils and used the datafor calculating the specific surface areas", Theycompared the specific surface areas thus calculatedwith those obtained by other methods. However,the calculation of the above quantities from theadsorption data demands a clear understandingof the nature of adsorption of the organic sorbatesas well as their orientation' in the sorbed state.Failure to take account of these factors led to di-vergent and rather confusing values of the CECand surface areas calculated from sorption datawith organic materials, particularly the dyestuffs=".Quaternary nitrogen compounds are often pre-ferred because of their strong attraction" for thenegative charge sites of clays. Basic dyestuffsare popular due to their ease of estimation colori-metrically.In the present work adsorption of cetylpyridiniumchloride (CPC) and cetyltrimethylammonium bro-mide (CTAB) on a sodium-bentonite has been studiedprimarily to understand the nature of sorption.It was suggestedt? that these surfactants utilizeboth their polar head as well as their hydrophobicpart in the adsorption procedure. The flocculationbehaviour of an originally peptized clay suspensionwith increasing addition of surfactants was explainedby suggesting first an exchange reaction leadingto complete neutralization of the surface chargeand a subsequent hydrophobic bonding of excessmolecules with their polar groups projected awayfrom the surface.In our experiment we have directly measuredthe adsorption of CPC and CTAB at different sur-factant concentrations and tested if the data fitthe Langmuir's monolayer model. No attempt680has been made to compare the surface area computedfrom the data with those obtained by other methods.So far there has been no dependable procedureto calculate the CEC of clays by organic cationexchange method. Although recently the efficacyof the methylene blue adsorption method for thecalculation of CEC has been acknowledged-", therange of its application is rather limited-''. Bymeasuring the specific conductance of the solutionsafter adsorption we have located the point of totalreplacement of the exchangeable cation by organiccations. A very good idea of the exchange capacityis thus obtained and compares well with the valuegiven by standard inorganic ion exchange method.Evidence of any molecular adsorption, apartfrom the cationic exchange, has also been elicitedby measuring the chloride concentration in thesupernatant.Materials and MethodsThe sample of bentonite (Code No. Pd/500),obtained from the Calcutta Mineral Supply Corpo-ration, Calcutta, was dispersed in water and particlesof size less than two microns were isolated by usualmethod. The separated colloidal particles wereconverted into sodium form by repeated treatmentwith 2N NaCl solution, washed with deionized waterand finally rendered chloride-free by dialysis. A2·5% well-dispersed clay suspension was utilizedfor subsequent work.The sample of cetylpyridinium chloride mono-hydrate (CPC, H20; E. Merck) was free from anypyridine contaminant. The purity of the samplewas confirmed by conductometric titration of thechloride content against a standard AgN03 solution.Cetyltrimethylammonium bromide (CTAB; BDH)was similarly tested. No further purification wasconsidered necessary. Both the surfactants weredissolved in warm water causing as little foam aspossible. Clay percentage was determined byPAL & GHOSHAL: ADSORPTION OF CPC & CTAB ON BENTONITEevaporating a known volume of clay suspensionin petridishes and. weighing the residue.The CEC was determined first by converting theclay into Ca-form and then determining, by EDTAtitration, the total amount of Ca2+ ionIa-Is liberatedfrom a known amount of it by 2N ammoniumacetate solution.Portions (5 ml each) of clay suspension weremixed with increasing volume of CTAB or CPCsolutions separately. The concentrations of thesurfactants were kept constant at 1·04 X 10-4Mby dilution. The mixtures were allowed to standovernight at 30°± JO with intermittent shaking.The specific resistances of the above solutions werethen measured at the same temperature with aPhilips conductance hridge.A parallel set of experiments was carried outwith identical surfactant concentrations to measurethe adsorption. The contents of the reaction vesselswere separately centrifuged and the supernatanttested colorimetrically for CP+ ion by van Steveninckmethod-s in a Zeiss colorimeter. The adsorbedCTAB was determined by estimating the nitrogencontent of well washed and vacuum dried clay-CTAB complexes by Kjeldahl's method. The chlo-ride content of the supernatants were determinedby conductometric titration with a standard AgNOasolution.ResultsThe adsorption data are presented in Fig. 1 wherethe amount of CPC sorbed is plotted against theamount initially applied for a given amount ofclay. The strict linearity of the Langmuir plot(Fig. 2) indicates a monolayer adsorption. Sorptionfar in excess of normal CEC is observed. TheLangmuir plot gives a value of 148 meq.jl00 g forthe monolayer capacity (X",). Without discrimi-nating between inner and external surfaces, thesurface area of the clay can be calculated from thex; value using Eq. (1),5 = Xm'Am X 6·02 X 10-Zm2g-1 •.. (1)where Am is the cross-sectional area of the CPCmolecule. We have used for Am the value 27Azafter Greenland et al.". The surface area is thusfound to be 240·4 m2g-l. However, it has beensuggested earlier-? that for a similar three layerclay with expandable lattice, 70% of the exchangesites are located on the inner lattice planes and30% on the external surfaces. The intercalatedmolecules will account for a surface area of 54 Aaeach",The amount of CP+ held is 83 meq.jl00 g (Table 1).Assuming that 70% of these, i.e. 58 meq./l00 g,are intercalated h the lattice planes and 30%,... 160...~z...140'"~'" 1200QtJc...II.a 100wCDIll:0<I> 800/)......•0~60..•..•~SO 70 90 100 130 ISO 170 190MILliMOLE5 OFCPCL ADDED PER 100g OF ~ENTONITEFig. 1 - Sorption isotherm of CPC on bentonite70..--- --,6010O~~ __~ ___L___L__~ __~ __~ __~~~o m G ~ ~ ~ ~ ro 00 ~i~ILIBRIUM CONCENTR"TlON(G) MOl"RIT't' )( 10~Fig. 2 - Langmuir plot of CPC sorption datai.e. 25 meq.JI00 g lie on the external surface, theproblem is how the rest i.e. 65 meq.jl00 g in themonolayer is distributed between the inner and theexternal surfaces.It is likely that on completion of the ionic sorption,the hydrophobic part of the intercalated moleculespulls the opposite planes causing a shrinkage inthe lattice and thus preventing further entry ofmolecules. The adsorption in excess of CEC shouldthus be attributed primarily to the external surface.The modified surface area computed on this basis is5=(58 x54+90x27) x6'02x 10-2 m2 g-1=334 m2 g-lIt is not possible from the adsorption isothermto get an idea of the CEC of the clay, for the isothermsSurfactantTABLA1 - CATIONEXCHANGECAPACITY(CEC) OF THE CLAYSurfactant Surfactant Chloride Surfactant Av. CECmeq./l00 g sorbed sorbed sorbed in meq./l00 gclay meq./l00 g meq./l00 g ionic form(a) (b) (c) (b-c)87·33 86·59 4·05 82·54 83·187·42 85·46 1·72 83·74681CPCCTABINDIAN J. CHEM., VOL. 15A, AUGUST 1977are continuous curves, and the phase of ionic ex-change reaction cannot be distinguished from thatinvolving hydrophobic bonding. However, the pointof complete exchange of inorganic ions by surfactantions could be determined following the flocculationpattern of the clay suspension. The sedimentationvolume measurement technique-", although interest-ing, is quantitatively inadequate. The total replace-ment of the inorganic counter ions by the organicones marks the maximum shrinkage of the electri-cal double layer and methods such as potentiometricmeasurement, specific conductance determination,etc. would therefore be more meaningful.The plots of specific conductance of the equi-librium solution against the initial surfactant-clayratio in the mixture for CPC and CTAB show ash':lrp change il!-the gradie~t (Fig. 3) exactly at thepoint of maximum specific conductance, whichincidentally agrees with the point of optimum floccu-lation of the clay suspension. The amount ofsurfactant actually bound to the clay surface atthis point is obtained from Fig. 1 and is taken tobe the CEC of the clay (Table 1). The value 83meq.jl00 g b~ing the average of four replicati~:ms,agrees well with the CEC value (83 ± 1) determinedby the Ca2+ exchange method.Analysis of chloride content of the supernatantshows that some amount of chloride ion is alsosorbed. The more the sorption, the more is thedepletion in chloride in the solution. But at theflocculation point the amount of chloride removedis negligible and is virtually nil below this point.It is interesting to note that the difference betweenthe amount of total CPC or CTAB sorbed andthe chloride removed at higher adsorption regioncomes close to the CEC value of the clay (Table 2).DiscussionThe total surface area (external-l-internal) of theclay computed from its CEC and lattice dimensionswas found to be 750 m2 g-l. The surface area~.-------------------------------~(QICPCL280 (bl CTAS~'«:~~ 260IIIu~u 240~az0uu 220u:UUoI0-'" 200160 '":-_--10__ ......L..__ .....L- __ 1..-_--L__ -U40 £>0 60 100 120 140 160MILLIEQUIVALENT OF eTAS 011CPO../IOO?OF SOOIUIoI BENTONITEFig. 3 - Specific conductance versus initial surfactant-clayratio in the mixture682TABLE 2 - AMOUNT or CPC AND CTAB SORBED ANDCHLORIDE REMOVEDSurfactant Totalsurfactantsorbedmeq./l00 g(a)86·5898·01115·0085·4291·31109·96Totalchloridesorbedmeq./l00 g(b)4·0511·9624·971·724·3011·78Difference(a-b)CPC 82·5386·0590·0383·7087·0198·18CTABcalculated from CPC adsorption data falls for shortof it and accounts for only 45% of the theoreticalvalue.This however is not unexpected. While theaverage area per ionic adsorption site is about 150 A2,the cross-sectional area of the CPC molecule is only27 A2 for the externally held and 54 Aa for the inter-calated ones.The external and internal surface areas calculatedfrom CPC adsorption are 146 m~ g-l and 188 m2g-1respectively. This external surface area valuecompares well with that reported by Greenlandand Quirck3 for a Cs-saturated bentonite (146 m! g-l).In calculating the surface area all the adsorptionbeyond the CEC of the clay has been attributedto the external surface, and that too not withoutreason. Doubts have been expressed-rs earlier inregard to a total penetration of the interlamellarspaces by CPC and other surfactant molecules.Moreover, Cs-saturated bentonite suffers latticecontraction and offers only the external surfacefor adsorption. CP+ ions held in the interlamellarcharge-sites might cause a similar linking of oppositelamellae by polar head and hydrophobic chainbarring further entry of CPC molecules.The CPC adsorption procedure is thus inadequatefor calculating total surface area of an expandinglattice clay. A fair measure of the external surfacearea could be obtained however by this procedure.The earlier works with surfactant adsorptionon clays did ~ot give any satisfactory procedurefor the determination of CEC. Mc Attee and co-worker18,20 however observed one to one replacementof Na" ion by dimethyllaurylbenzyl ammonium ionfrom a Na-bentonite. This type of exchange wasnoted up to 90 meq.jl00 g for a clay with a CECof 100 meq.jl00 g.In the present work a satisfactory value of theCEC is obtained by combining the adsorption andspecific conductance data (Figs 1 and 3). WhenFigs 1 and 3 are studied in conjunction, the surfac-tant adsorbed at the point of discontinuitv in thespecific conductance diagram is readily obtained.And this gives the CEC of the clay. Even thespecific conductance curve alone is adequate toindicate the CEC value, for the error involved isvery small in as much as the point of conductance-maximum is characterized by almost total removalof surfactant as cation.AcknowledgementPAL & GHOSHAL: ADSORPTION OF CPC & CTAB ON BENTONITEWe are indebted to the authorities of IndianStatistical Institute for supporting the work witha grant of a scholarship to one of us (B.K.P.).References1. GREENLAND, D. J. & QUIRCK, J. P., Clays 0- ClayMinerals, 9 (1960), 484.2. GREENLAND, D. J. & QUIRCK, J. P., Soil Sci., NewZealand, (1962), 79.3. GREENLAND, D. J. & QUIRCK, J. P., J. soil su.. 15(1964), 178.4. BERGMAN, K & O'KONSKII, P. T., J. phys. Chem., 67(1963), 2169.5. HOFFMANN, U., KOTTENHAN, H. & MARCOS, H., Angewcu«. (int. Edn), 5 (1966), 242.6. DE, D. K, Sorption and desorption of basic dyes onaluminosilicates, Ph.D. thesis, University of Kalyani,West Bengal, 1968.7. GHOSHAL, D. N. & MUKHERJEE, S. K, J. Indian chem,Soc., 49 (1972), 569.8. RAMCHANDRAN,K. S., KACKER, K. P. & PATWARDHAN,N. P., Am. Mineralogist, 47 (1962), 165.9. BARRER. R. M. & McLEOD, D. M.• Trans. Faraday Soc.•51 (1955). 1290.10. VANOLPHEN. H .• An introduction to clay colloid chemistry(Interscience, New York), 1963. 155.11. BRINDLEY, G. W. & THOMSON, T. D., Israel J. Chem.,8 (1970). 409.12. HANG. P. T. & BRINDLEY, G. W., Clay 0- Clay Minerals,18 (1970). 203.13. BERNARD, (Jr), A. J. BROAD, W. C. & FLASCHKA. W.,Chemist-Analyst, 45 (1956), 86, 111.14. BERNARD. (Jr), A. J. BROAD, W. C. & FLASCHKA, W.,Chemist-Analyst, 46 (1957). 18.15. CARLSON, R. M. & JOHNSON, C. M., J. agric. Fd cie«,46 (1961), 180.16. ROSEN, M. J. & GOLDSMITH, H. A., Systematic analysisof surface active agents (Wiley-Interscience, New York),1972, 459.17. TENAKOON. D. T. B., THOMAS, J. N., TRICKER, M. J. &WILLIAMS, JOHN 0., J. chem. Soc. (Dalton), (1974),2133.18. CHAKRAVORTY.S. K. Sci. 0- cuu., 22 (1956). 170.19. Mc ATTEE, J. L.. Clays 0- Clay Minerals. 9 (1962).444.20. Mc ATTEE. J. L. & HACKMAN.J. R.. Am. Mineralogist,49 (1964), 1569.683

Adsorption of Cetylpyridinium Chloride & Cetyltrimethylammonium Bromide on Bentonite

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Adsorption of Cetylpyridinium Chloride & Cetyltrimethylammonium Bromide on Bentonite

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