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  • 2786 Reprinted from LANGMUIR, 1994, 10. Copyright @ 1994 by the American Chemical Society and reprinted by pennission of the copyright owner.

    Role of Surfactant-Adsorbent Acidity and Solvent Polarity in Adsorption-Desorption of Surfactants from Nonaqueous

    Media

    S. Krishnakumar and P. Somasundaran'

    Langmuir Center for Colloids & Interfaces, Henry Krumb School of Mines, Columbia University, New York, New York 10027

    Received March 8, 1993. In Final Form: February 22, [email protected]

    Adsorption of surfactants at the solid -liquid interface is governed in general by the properties of the surfactant, solid, and solvent. However, very little is known about the role of these properties in the case of adsorption from organic media. In this study the effect of adsorbent and surfactant acidities and solvent polarity on the adsorption of surfactants on oxide minerals in nonaqueous solvents has been investigated. The studies conducted using anionic (Aerosol OT) and cationic (dimethyldodecylamine) surfactants on the basic and acidic oxide minerals (alumina and silica) reveal that for a given surfactant the polarity differential between the adsorbent and the solvent is responsible for the partitioning of the solute to the solid -liquid interface. Following this, acid-base-type interactions between the solute and the adsorbent can take place to enhance the attachment of the former to the particles. The acid-base interactions depend on the nature of the surfactant and the substrate, with an acidic surfactant interacting strongly with a basic adsorbent and vice versa. By using the solvent dielectric constant as an indicator of solvent polarity, it was found that while polar interactions control the adsorption from solvents of low polarity, hydrocarbon chain interactions with the surface playa major role in determining adsorption from solvents of higher polarity.

    that the surface coverage varied as a function of their chain length.5 The bulk of the studies have been directed toward detennining the orientation of surfactants at the interface. Recent efforts have aimed toward identifying the major adsorption mechanisms in nonpolar media. Pugh6 showed in a recent paper that acid-base interac- tions are chiefly responsible for adsorption of solutes from nonaqueous solvents. In the absence of significant ionization, electrostatic mechanisms do not playa major role in adsorption processes in solvents of low dielectric constants. Labib and Williams discuss acid-base inter- actions in tenDS of the donicity of surfaces which is related to the electron donor properties of the surface species.7 They found that a solid in contact with a liquid of lower donicity developed positive surface charges by electron transfer to the liquid and vice versa.

    In the present work the effect of adsorbent-solute- solvent interactions on the adsorption process and the dominant adsorption mechanisms were systematically investigated using surfactants and solids of different acidities and solvents of different polarities.

    Materials and Methods

    The alumina used for this study was Linde Alumina Polishing Powder Type A purchased from Union Carbide Corp. Morpho- logically, the powder was constituted of micrometer-size ag- gregates composed of smaller particles (between 200 and 500 nm) with a nitrogen surface area of 14 m2/g. The absence of hysteresis on the adsorption -desorption isotherm suggested this material to be essentially nonporous. The silica used was Spherosil-B obtained from Rhone Poulanc, with a nominal size of 40-100 JIm and a Brunauer-Emmett-Teller (BET) surface area (SA) of 25 m2/g. The pore size was measured to be about 400 nm. The other oxide minerals rutile (SA = 2 m2/g) and hematite (SA = 9.2 m2/g) were obtained from Aldrich Chemicals.

    Introduction

    Nonaqueous colloidal dispersions are being widely used currently in a number of important technological pro- cesses.1 High-performance ceramic processing, for ex- ample, requires a well-dispersed system prior to firing in order to minimize the flaw population.2 Other technologies using dispersions in nonpolar media include magnetic tape manufacturing,2 electrophoretic image processing,2 re- pographic inking,2 and monodispersed colloid production.3 In all these applications it is necessary to have dispersions that have long-term stability under adverse conditions of high electric fields, high solid concentration, etc. Typically the dispersions are stabilized by the addition of surfaCtants or polymers that adsorb on the particle surface and provide electrostatic and/or steric stabilization. To understand the stabilization mechanisms, it is necessary to have a full knowledge of the nature of the adsorption process since this does control the amount and orientation of the stabilizing agent at the interface.

    There are several factors that control the adsorption process which are reviewed in detail by Parfitt and Rochester.. The nature of the adsorbent and the solute and their mutual interaction plays an important role in adsorption. The structure and orientation of the adsorbed layer depend on the relative strength of the interaction between the adsorbent and solute. The solvent can also affect the adsorption process either by competing with the solute for adsorption or by weakening the adsorbent- solute interactions.

    Adsorption of surfactants from nonaqueous media has been a subject of interest for a long time. Earlier studies on adsorption ofn-fatty acids on silica from hexane showed

    . Abstract published in Advance ACS Abstracts, July 15, 1994. (1) Novotny, V. Colloids Surf. 1987,24,361. (2) Blier, A. In Ultrastructure Processing of Ceromics. Glasses and

    Composites; Hench, L. L., ffirich, D. R., Eds.; J. Wiley & Sons: New York, 1984; p 391.

    (3) Esumi, K.; Suzuki, M.; Tano, T.; Torigoe, K; Meguro, K Colloids Surf. 1891, 55, 9.

    (4) Parfitt, G.; Rochester, C. H. In Adsorption {rom solution at the solid/liquid interface; Parfitt, G., Rochester, C. H., Eds.; Academic Press: New York, 1983; p 3.

    (5) Annistead, C. G.; Tyler,A.J.; Hockey, T.A. TraM. Faraday Soc. 1971,67,493.

    (6) Pugh, R. J. Ceram. Trans. 1990, 12,375. m Labib. M.; Williams, R. J. Colloid interface Sci. 1984,97,356.

    0743-7463/94/2410-2786$04.50/0 @ 1994 American Chemical Society

  • Langmuir, Vol. 10, No.8, 1994 2787Surfactant -Adsorbent Acidity and Solvent Polarity

    Table 1. Dielectric Constant of Relevant Solventsa

    53.5 58.5 68.63 80

    cyclohexane 2.02 methanol chloroform 4.8 glycerol I-hexanol 13.3 methanoVwater (wt %) butanol 15.8 50:50 2-isopropanol 18.3 40:60 acetone 20.3 20:80 ethanol 24.3 water

    4 From the CRC Handbook of Chemistry and Physics.

    The concentration of surface hydroxyl groups on several oxides has been determined previously by several researchers.8

    Aerosol OT (sodium bis{2-ethylhexyl) sulfosuccinate) (AOT) is the most commonly used anionic surfactant in nonpolar media. It was purchased from Fisher Scientific Co. Before use, the surfactant was purified by dissolving it in methanol and filtering off the undissolved impurities. The excess solvent is driven off by rotary evaporation followed by freeze drying. The waxy solid left. behind is stored in a dry atmosphere. Cationic dimethyl- dodecylamine (DDA) was obtained from ICN pharmaceutical and used without further purification.

    Cyclohexane, of spectroscopic grade, was obtained from Fisher Scientific Co. It was selected for this study because of its weak interactions with oxide surfaces, which allows any residual adsorption of the solvent in the prsence of the surfactant to be ignored.9 When required, the solvent was stored on molecular sieves 4A to avoid contamination by water. All the other organic solvents used were purchased from Fisher Scientific and used after drying without further purification. A list of solvents used along with some of their relevant properties is shown in Table 1.

    Samples for adsorption studies were prepared by desiccating the mineral at 200 DC for 6 h followed by cooling it for 2 h at 25 DC in a vacuum desiccator. Dehydroxylation of alumina was done by heating it at 900 DC for 72 h and confirmed by the disappearance of the OH adsorption bands (3800 cm-l) on the IR spectrum. For adsorption tests a 1-g mineral sample was added to 15 mL of surfactant solution in the desired solvent and conditioned for 12 h in a glovebox. Samples for the desorption experiment were prepared by first adsorbing the surfactant on the mineral from cyclohexane. Following this, the solids with the adsorbed AOT were separated by centrifugation, vacuum dried for 12 h, and then conditioned with different solvents for 12 h, and the resultant supernatant was analyzed for the surfactant. Analysis of the anionic and cationic surfactants was conducted by the two-phase titration technique described in the literature.lO

    Results and Discussion (8) Effect of Solid. All oxide minerals have surface

    hydroxyl groups formed by the reaction of the oxygen atoms on the surface with the atmospheric moisture. The density of these surface hydroxyls determines the relative basicities of minerals which can also be inferred from the isoelectric points (iep) of the minerals in aqueous solutions (the lower the iep, the more acidic is the mineral). Alumina has a high density of hydroxyls on the surface (10-15 OH/nm2) and a high iep (8.5) and is a basic oxide. On the other hand silica has a lower density of hydroxyls (3-4 OH/nm2) and a low iep (2.5) and is an acidic oxide. The density of the hydroxyl groups and hence the basicity of the surface can also be controlled by dehydroxylation of the surface by extended heating. Figure 1 shows the adsorption isotherms of Aerosol OT on alumina and silica, and it is observed that the anionic surfac