Reprinted from. AN IN1EflNAT1CIW. ~ A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS

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1 Reprinted from COLWIDS AND SUREACES AN IN1EflNAT1CIW. A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS Colloids and Surfaces A: Physicochemical and Engineering Aspects 117 (1996) Adsorption of Aerosol-aT on graphite from aqueous and non-aqueous media S. Krishnakumar, P. Somasundaran * Langmuir Center for Colwids & Interfaces, Henry Krumb School, Columbia University, New York, NYl0027, USA ELSEVIER

2 COllOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS Editors-in-Chief P. Somasundaran, 911 S.W. Mudd Bldg, School of Engineering and Applied Science, Columbia University, New York, NY 10027, USA. (Tel: (212) ; Fax: (212) ) Th.F. Tadros, c/o Elsevier Editorial Services, Mayfield House, 256 Banbury Road, Oxford OX2 7DH, UK Co-Editors D.N. Furlong, CSIRO, Division of Chemicals and Polymers, Private Bag 10, Clayton, Vic. 3169, Australia (Tel: (3) ; Fax: (3) ) T.A. Hatton, Massachusetts Institute of Technology, Department of Chemical Engineering, 25 Ames Street, , Cambridge MA 02139, USA (Tel: (617) ; Fax: (617) ) H. Mohwald, Max-Planck-lnstitut for Kolloid- und Grenzflchenforschung, Instituteil Berlin- Adlershof, Rudower Chaussee 5, Geb 9-9, D12489 Berlin, Germany (Tel: ; Fax: ) Founding Editor E.D. Goddard, 349 Pleasant Lane, Haworth, NJ 07641, USA Editorial Board S. Ardizzone (Milan, Italy) P. Luckham (London, UK) M. Aronson (Edgewater. NJ, USA) R.A. Mackay (Potsdam. NY. USA) J.C. Berg (Seattle. WA, USA) C.A. Miller (Houston, TX, USA) A.M. Cazabat (Paris. France) R. Miller (Berlin, Germany) N.V. Churaev (Moscow, Russia) I.D. Morrison (Webster. NY, USA) J. Clint (Hull. UK) B. Moudgil (Gainesville. FL. USA) K. De Kruif (Ede, The Netherlands) K. Papadopoulos (New Orleans. LA, USA) S.S. Dukhin (Kiev, Ukraine) R.M. Pashley (Canberra, Australia) G.H. Findenegg (Berlin, Germany) B.A. Pethica (Parsippany, NJ. USA) T.W. Healy (Parkville, Australia) D.C. Prieve (Pittsburgh, PA, USA) K. Higashitani (Kyoto. Japan) C.J. Radke (Berteley. CA. USA) R. Hilfiker (Basel. Switzerland) R. Rajagopalan (Houston, TX. USA) K. Holmberg (Stockholm, Sweden) J. Sjoblom (Bergen, Norway) J. Israelachvili (Santa Barbara. CA, USA) C. Solans (Barcelona. Spain) E.W. Kaler (Newart, DE, USA) J.K. Thomas (Notre Dame, IN. USA) T. Kunitake (Fukuoka, Japan) B.V. Toshev (Sofia. Bulgaria) K. Kurihara (Nagoya. Japan) F.M. Winnik (Hamilton, Ont.. Canada) R. Y. Lochhead (Szeged, Hungary) Scope of d1e Journal Colloids and Surfaces A: Physicochemical and Engineering Aspects is an international journal devoted to the science of the fundamentals, engineering fundamentals, and applications of colloidal and interfacial phenomena and processes. The journal aims at publishing research papers of high quality and lasting value. In addition. the journal contains critical review papers by acclaimed experts, brief notes. letters. book reviews. and announcements. Basic areas of interest include the followin9: theory and experiments on fluid interfaces; adsorption; surface aspects of catalysis; dispersion preparation, characterization and stability; aerosols, foams and emulsions; surface forces; micelles and microemulsions; light scattering and spectroscopy; detergency and wetting; thin films, liquid membranes and bilayers; surfactant science; polymer colloids; rheology of colloidal and disperse systems; electrical phenomena in interfacial and disperse systems. These and related areas are rich and broadly applicable to many industrial, biological and agricultural systems. Of interest are applications of colloidal and interfacial phenomena in the following areas: separation processes; materials processinq; biological systems (see also companion publication Colloids and Surfaces B: Biointerfaces); environmental and aquatic systems; minerals extraction and metallurgy; paper and pulp production; coal cleaning and processing; oil recovery; household products and cosmetics; pharmaceutical preparations; agricultural, soil and food engineering; chemical and mechanical engineering. Audience Surface and Colloid Chemists, Separation Chemists. Powder Technologists. Mineral Processors, Petroleum Engineers, Environmental, Soap, Cosmetic and Textile Scientists, Biological and Bioengineers, Tribologists. Publication Schedule and Subscription Information Colloids and Surfaces A: Physicochemical and Engineering Aspects (ISSN ). For 1996 volumes are scheduled for publication. Subscription prices are available upon request from the publisher. Subscriptions are accepted on a prepaid basis only and are entered on a calendar year basis. Issues are sent by surface mail except to the following countries where air delivery via SAL mail is ensured: Arqentina, Australia. Brazil. Canada. Hong Kong. India, Israel. Japan. Malaysia. Mexico. New Zealand. Pakistan. PR China. Singapore, South Africa. South Korea, Taiwan, Thailand. USA. For all other countries airmail rates are available upon request. Claims for missing issues must be made within six months of our publication (mailing) date. Please address all your requests regarding orders and subscription queries to: Elsevier Science, Journal Department, P.O. Box AE Amsterdam, The Netherlands. Tel: Fax: ;p -' ( ",."S.'.."

3 ELSEVIER Colloids and Surfaces A: Physicochemical and Engineering Aspects 117 (1996) COLLOIDS AND SURFACES A Adsorption of Aerosol-aT on graphite from aqueous and non-aqueous media S. Krishnakumar, P. Somasundaran * Langmuir Center for Colloids & lnterfaces, Henry Krumb School, Columbia University, New York, NYl0017, USA Received 13 October 1995; accepted 14 March 1996 Abstract Efficient dispersion of carbonaceous particles in polar and non-polar liquids is important in the processing and use of paints, pigments, printing inks and tertiary oil. In this study we examined the adsorption of an anionic surfactant, Aerosol-OT (AOT), on graphite particles in cyclohexane and water and the ensuing changes in their dispersion properties. The adsorption of AOT on hydrophobic graphite particles in cyclohexane results from the tendency of the polar head group of the surfactant molecules to partition into hydrophilic domains away from the solvent which is hydrophobic. Fluorescence studies using I-methyl-8-oxyquinolium betaine as probe reveals the presence of "reversemicelle"-like surfactant aggregates at the interface at all surfactant concentrations. The observed changes in dispersion stability suggest the fonnation of interparticle surfactant aggregates at low surface coverages while at higher coverages the aggregates are fonned on individual particles. AOT adsorbs on graphite from aqueous solutions as well, despite the unfavorable electrostatic repulsion, evidently through hydrophobic interactions. Such adsorption stabilizes the aqueous dispersions by providing increased electrostatic repulsion among the particles. Desorption of preadsorbed AOT from graphite when contacted with different solvents showed a marked solvent effect with maximum desorption occurring in solvents of intennediate polarity where the hydrophilic and hydrophobic driving forces are balanced. Keywords: Adsorption; Aerosol-OT; Graphite; Non-aqueous. Introduction While electrostatic attraction and chemical bonding are the most frequently encountered Efficient dispersion of colloidal particles in nonaqueous liquids is important in many technologies polar surfaces from aqueous solutions, it has been mechanisms in the adsorption of surfactants on such as reprography and high performance ceramics [1-4J. Adsorption of surf act ants and polymers forces do not playa pronounced role in adsorption suggested that in organic solvents electrostatic has been employed in the past to modify particle [7,8]. Acid-base interactions between the solute surfaces and to change their dispersion and rheological properties'[5,6]. The effect of these surface dominant adsorption mechanism in these systems. and the substrate have been suggested to be the modifiers depends on their electrostatic and structural properties which also determine their adsorp- interactions involving an electron or proton This mechanism essentially consists of dipolar tion behavior and the molecular orientation at the transfer between the solute and substrate molecules interfaces. resulting in the formation of an acid-base adduct at the interface. This can be understood easily for * Corresponding author. polar adsorbents but fails to explain adsorption /96/$ Elsevier Science B. V. All rights reserved P/I (96)

4 228 S. KrishnakJImor, P. SomasundaranjCo/loidf Surfaces A: Physicochem. Eng. Aspects 117 (1996) on hydrophobic surfaces such as graphite. In aqueous systems ionic surfactants are known to aggregate at low surface coverages due to lateral interactions among the adsorbed molecules and even form bilayers at higher concentrations [9]. In non-aqueous solvents adsorption has been postulated to proceed only up to a monolayer as the lateral hydrophobic interactions are very weak in this case. In this study we have investigated the adsorption of anionic Aerosol-OT on a hydrophobic adsorbent, graphite, in cyclohexane as well as water. The changes in dispersion stability and zeta potentia] (in the aqueous case) were monitored at different surface coverages. In addition, we have used steady state fluorescence spectroscopy to monitor the formation of molecular aggregates at the solid-liquid interface. Fluorescence probing utilizes the photophysical response of a probe molecule which is sensitive to changes in the structure and composition of its local environment [10]. The selection of a suitable probe depends on the following two criteria: (a) it should partition selectively and provide information on a specific microdomain; (b) its presence, even in traces, should not significantly alter the properties of the original system. For this study we selected I-methyl-8-oxyquinolium betaine as the fluorescent probe. This probe has been previously used to study the characteristics of Aerosol-OT reverse micelles in benzene and is known to partition into the hydrophilic core of the reverse micelles [11]. 2. Fxperimental 2.1. Materials Graphite particles of size 1 JUD, with a BET surface area of 15 m2 g-l, were purchased from Sigma Chemicals. Aerosol-OT (Fig. 1 (a), AOT) used in this study was purchased from Fisher Scientific and purified by dissolving in methanol and recrystallizing by solvent evaporation. Cyclohexane and other organic solvents of spectroscopic grade were obtained from Fisher Scientific and used without further purification. (a) Fig. 1. Molecular structure of: (a) Aerosol-OT; (b) I-methyl. 8-oxyquinolium betaine. The fluorescent probe, I-methyl-8-oxyquinolium betaine (Fig. l(b» was obtained from Molecular Probes Inc. and used as received. Initially the UV absorption of the probe was measured in different solvents and found to be similar to that reported in the literature [12]. The A.uax of absorption was observed to shift from 375 nm in non-polar solvents to about 364 om in polar solvents. The probe is insoluble in pure cyclohexane but dissolves in the presence of AOT micelles. The absorption spectra of this probe in AOT solutions in cyclohexane are characterized by an absorption at 364 nm signifying a highly polar environment for the probe in the interior of the reverse micelles (Fig. 2). The probe also displayed a characteristic fluorescence emission at 550 nm at an excitation wavelength of 364 nm in these solutions Methods Adsorption-desorption was estimated by measuring changes in concentration of the surfactant in the bulk solution upon contact with the solid for a period of 12 h. AOT was analyzed in solution by the two phase colorimetric titration technique [12]. The stability of dispersions was estimated by monitoring the rate of descent of the upper interface in a homogeneously dispersed suspension. The sediment volumes were also measured in all cases after allowing the dispersions to settle for 24 h. Zeta potentials of the aqueous dispersions were measured by electrophoresis using the Zetameter 3.0 setup. Fluorescence spectra were recorded using a Photon Technology International PTI-LS spectrophotometer at an excitation wavelength of 364 nm. (b)

5 S. Krishnakumar, P. Soma.fundaranICo//oids Surfaces A: Physicochem. ng. Aspects 117 ( 1996) n. QJ = J,. Q V) < {, $2.1 Wavelength, nm Fig. 2. UV adsorption spectra of 1-methyl-oxyquinolium betaine in cyclohexane at different concentrations of AOT: (a) 4 x 10-2 M; (b) 1 x 10-2 M; (c) 4 X 10-3 M; (d) 4 x 10-4 M; (e) 4 x 10-5 M Sample preparation For the adsorption studies, the graphite was dried first by heating at 200 C for 6 h followed by cooling to room temperature under vacuum. 1 g of the dried solid was then mixed with 10 ml of the surfactant solution and conditioned for 12 h. The particles were then removed by centrifugation and the supernatant analyzed for residual surfactant. For the fluorescence studies, surfactant solutions containing 10-4 M of the probe are prepared and conditioned with the solid as before. Fluorescence spectra were acquired from the solidliquid interface as well as from the supernatant. For the desorption studies the surfactant was first adsorbed on graphite from cyclohexane and then the solids separated and redispersed in solvents of different polarity. Residual concentrations of AOT in the different solvents were then measured after 12 h of conditioning. ting pattern as shown in Fig. 3. The adsorption increases sharply in the initial part suggesting high affinity of the surfactant for the solid at low concentrations. The adsorption then appears to reach a plateau. Calculations based on apparent... E "'" } a x :t:- "Vi c., -0 C.2 "'Q.. '- 0 III c"'r;:';i', I..t5 0, ;g i 3. Results and discussion 3.1. AOT adsorption in cyclohexane The adsorption isotherm of Aerosol-OT on graphite from cyclohexane follows a very interes- Residual Concentration x 10' mol/l Fig. 3. Adsorption isotherm of Aerosol-OT on graphite from cyclohexane.

6 230 S. Krishnakumar, P. SomasundaranjCo//ow Surfaces A: Physicochem. Eng. Aspects 117 ( 1996) monolayer coverage at this level yield a parking area of approximately 1.03 nm2 per molecule of AOT. This molecular parking area corresponds to an AOT molecule adsorbing flatly at the solidliquid interface. At a higher concentration, above 10-2 M, there is a further sharp increase in the adsorbed amount and this reaches about five times the initial plateau value at about 3 x 10-2 M. It should be noted at this point that the CMC of AOT in cyclohexane is about (0.8-1) x 10-3 M. Thus this sharp increase in adsorption occurs above the CMC though it does not coincide with the onset of micellization. The settling behavior of these dispersions as a function of the adsorbed AOT amount is also very interesting (Fig. 4). The settling rate rises sharply at first and then decreases as sharply, suggesting restabilization. At low surface coverages the polar head group of the flatly adsorbing AOT is exposed to the solvent with which it is not compatible. In such a case the AOT molecules, as shown in Fig. 5(a), can form interparticle aggregates that would effectively create a polar microdomain to shield the head groups from the solvent. Such an interparticle aggregation can account for the increased settling rate. As the AOT concentration is increased the adsorbed molecules are proposed. 0."Ē x.. "0 0: 0\ C :! 9. :: 70C "", iccf"fc;:riv:::"'!':' L Adsorption density x 1Q. mol/m Fig. 4. Effect of AOT adsorption on the settling behavior of graphite dispersions in cyclobexane. A Fig. 5. Schematic diagram showing the formation of: (a) interparticle surfactant aggregates which cause ftocculation; (b) aggregates with monomers in solution leading to redispersion. to form aggregates at the interface as shown in Fig. 5(b). This leads to the sharp increase in the adsorption density as well as the decrease in settling rate due to the disappearance of the interparticle aggregation observed at low surface coverages. Formation of such reverse-hemimicelle-like aggregates at the interface have been reported earlier for the adsorption of l-decanol on graphitized carbon black from non-polar solvents [13] Fluorescence studies Fluorescence studies were conducted by coadsorbing the probe, l-methyl-8-oxyquinolium betaine, with the surfactant and measuring the fluorescence response of the probe from the interface as well as in the supernatant after adsorption. The fluorescence spectra from the graphityclohexane interface were of extremely low intensities at all concentrations probably due to absorption of the emitted light by the graphite surface. From these spectra it was therefore not possible to ascertain whether the probe is incorporated into the adsorbed surfactant layer at the interface. However when the supernatant was analyzed it was found that there was no residual probe in any of the cases including those where the residual AOT concentration is above the bulk CMC value in cyclohexane. This observation clearly shows complete transfer of the probe to the interface. From the earlier studies it was shown that the probe can be solubilized only in polar domains and thus B

7 Krishnaklmlar. P. SomasundoranjColJoiJs Surfacu A: Plrysicochml. &g. Asprct.f 117 ( 1996) cannot be expected to adsorb at the graphite surface by itself. Thus the probe must be solubilized in the polar microdomains created by the adsorbed AOT molecules at the interface - in the interparticle aggregate at low concentrations and in the adsorbed micelle at higher concentration Adsorption of AOTfrom aqueous solution Fig. 6(a) shows the adsorption isotherm or AOT on graphite rrom aqueou solutions at ph 6.6. The -6 "'-..;.. '!;. ċ:.. '0 c:.!!.. 0. u u... E , -.L.. -. Residual concenlralion x ic?1 N,"G Ḍ. isotherm has a- Langmuirian shape with the onset of the plateau roughly corresponding to the CMC of AOT in aqueous solutions (8 x 10-4 M). A calculation of parking area based on monolayer coverage at this adsorption plateau gives a value of 1.03 nm2 per molecule, which is the same value as that obtained for the first plateau in the adsorption from cyclohexane. As in the previous case this corresponds to a flatly adsorbing molecule at the interface. Fig. 6(a) also shows the change in zeta potential of graphite with AOT adsorption and it can be seen that the graphite becomes increasingly negative with the adsorption of anionic AOT. This increased negative charge also stabilizes the dispersions as indicated by the decrease in the sediment volumes with surfactant adsorption (Fig. 6(b». It is interesting that the negatively charged surfactant is able to adsorb on graphite which is negatively charged to start with. This, along with the flat orientation of the AOT molecule, suggests that the adsorption in this case is driven by the interactions of the hydrophobic chains of the surfactant with the predominantly hydrophobic graphite surface. The adsorption of ionic surfactants on polar solids in aqueous solutions has been widely studied and is characterized by a sharp increase in adsorption density at a critical low concentration (hemimicelle concentration) which corresponds to the formation of molecular aggregates or "solloids" at the interface due to lateral interaction among the adsorbed molecules. However in this case we do not observe any such increase in adsorption density in the concentration range studied. This suggests that the hydrocarbon chains are firmly anchored on the graphite surface and are unavailable for solloid formation Desorption into different solvents (b) Residual concentration x 10". "/1 Fig. 6. (a) Adsorption isotherm of AOT on graphite in aqueous solutions [0] and the corresponding change in zeta potential [8]. (b) Effect of AOT adsorption on the sediment volumes of aqueous graphite dispersions. -"'" Graphite particles with preadsorbed AOT were redispersed into solvents of different polarity and the surfactant desorption was monitored. The results, as shown in Fig. 7, indicate that the desorption increases with solvent polarity, attains a maximum and then decreases again. A similar behavior has been observed for the desorption of AOT from oxide minerals [14]. This clearly demonstrates the changing mechanism of adsorption with change in

8 232 S. Krishnakumar, P. Somasundaran/Colloids Surfaces A: Physicochem. &g. AspeCU 117 (1996) solvent polarity. In low dielectric constant solvents the polar interactions of the surfactant molecule with the polar sites that may be present on the substrate cause adsorption. With increase in solvent polarity the surfactant becomes more compatible with the solvent and partitions more into the solvent. In the more polar liquids the hydrocarbon part of the surfactant becomes incompatible with the solvent and is driven to the hydrophobic graphite surface resulting in adsorption. The solvents in which the desorption is maximum correspond to those with which the surfactant is compatible on the whole and which exist in the monomolecular form [15]. 4. Conclusions Aerosol-OT adsorbs on graphite from cyclohexane as well as water, but the mechanisms of adsorption differ in the two solvents. In cyclohexane the adsorption is driven initially by the differential polarity between the graphite surface and the solvent with respect to the surfactant molecule. At higher AOT concentrations the adsorption density increases sharply and this is hypothesized to be due to the fomlation of reversemicelle-like aggregates at the solid-liquid interface. Fluorescence studies using I-methyl- 8-oxyquinolium betaine showed the probe to parti- tion to the interfacial aggregates in preference to the aggregates in solution. This suggests that the micropolarity of the interfacial aggregates is higher than that of the ones in solution. However this could not be quantified as the graphite surface appeared to quench any fluorescence signal originating from the probe on the surface. The settling rates of the dispersions in cyclohexane initially increase with AOT adsorption but later decrease. The initial increase is attributed to the formation of interparticle surfactant aggregates. At higher concentrations the adsorbed molecule aggregates with the excess surfactant in solution rather than with molecules on other particles, so the flocculation ceases to occur and the dispersion is restabilized. Aerosol-OT adsorbs on graphite from aqueous solutions through hydrophobic interactions despite the electrostatic repulsive force. The adsorbed AOT stabilizes the dispersions by providing increased repulsive force between the particles as evidenced by the change in zeta potential. The adsorbed AOT does desorb into different solvents, but this desorption is dependent on the solvent polarity. The desorption passes through a maximum with increasing solvent polarity. An analysis of this result reveals that at low solvent polarity the adsorption is driven by polar interactions while at higher solvent polarities hydrophobic interactions playa dominant role. This observation is in line with earlier findings on surfactant adsorption in aqueous media in which the adsorption was found to increase with increase in the surfactant chain length. Thus the mechanism of surfactant adsorption on hydrophobic surfaces depends on the nature of the solvent and the effect of such adsorption on the dispersibility of the suspensions depends on the orientation of the adsorbed surfactant. Ackno,,'iedgments The authors wish to acknowledge the financial support of this work by National Science Foundation (NSF-CTS ) and MMMRI, New York.

9 S. Krishnakumar, P. Somasundaran/Colloidr Surfaces A: Physicochem. Eng. Aspect.f 117 ( 1996) References [IJ V. Novotony, Colloids Surfaces, 24 (1987) 361. [2J A. Blier, Stability of ceramic suspensions, in L.L. Hench and D.R. Ulrich (Eds.), Ultrastructure Processing of Ceramics, Glasses and Composites, J. Wiley & Sons, New York, 1984, p.391. [3J R.B. McKay, Pigment dispersions in apolar media, in H.F. Eicke and G.D. Parfitt (Eds.), Interfacial Phenomena in Apolar Media, Marcel Dekker, New York:, [4J F.M. Fowkes and RJ. Pugh, Polymer Adsorption and Dispersion Stability, ACS Symp. Ser., 240, American Chemical Society, Washington, DC, 1984, p [5J P. Somasundaran in P. Somasundaran and R.B. Greves (Eds.), AIChE Symp. Series, 1975, p. 1. [6J P. Somasundaran, T.W. Healy and D.W. Fuerstenau, J. Phys. Chem 68 (1964) 3652 [7J F.M. Fowkes in S. Ross (Ed.), Physics and Chemistry of Interfaces II, American Chemical Society Special Publication, 1971, p [8] R.J. Pugh, in L. Messing, S. Hirano and H. Hauser (Eds.), Ceramic Powder Science III. American Chemical Society, Westerville, OH, 1990, p [9] P. Somasundaran and D.W. Fuerstenau, J. Phys. Chem., 70 (1966) 90. [10] P. Chandar, P. Somasundaran and N.J. Turro, J. Colloid Interface Sci., 117 (1987) 31. [11] M. Ueda and ZA. Schelly, Langmuir, 5 (1989) [12] V.W. Reid, G.H. Longman and E. Heinhirth, Tenside, 5 (1968) 90. [13] T. Gu and B-Y. Zhu, Colloids Surfaces, 46 (1990) 81. [14] S. Krishnakumar and P. Somasundaran, Langmuir, 10(8) (1994) [15] S. Krishnakumar and P. Somasundaran, J. Colloid Interface Sci., 162 (1994) 425.

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