2786
Langmuir 1994,10, 2786-2789
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, 1994@ Adsorptjon 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 adsorptionfrom 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 (AerosolOT)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 play a major role in determining adsorption from solvents of higher polarity.
Introduction Nonaqueous colloidal dispersions are being widely used currently in a number of important technological processes.l High-performance ceramic processing, for example, 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 repographic inking,2and monodispersed colloid p r o d ~ c t i o n . ~ 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 adsorbentsolute 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 A C S Abstracts, July 15,1994. (1)Novotny, V. Colloids Sufi. 1987,24, 361. (2) Blier, A. In Ultrastructure Processing of Ceramics, Glasses and Composites; Hench, L. L., Ulrich, D. R., Eds.; J. Wiley & Sons: New York, 1984;p 391. (3) Esumi, IC;Suzuki, M.; Tano, T.; Torigoe, IC;Meguro, K. Colloids Surf. 1991, 55, 9. (4)Parfitt, G.;Rochester, C. H. In Adsorption from solution at the solid/liquid interface; Parfltt, G., Rochester, C. H., Eds.; Academic Press: New York, 1983;p 3. @
that the surface coverage varied as a function of their chain length.5 The bulk of the studies have been directed toward determining the orientation of surfactants a t the interface. Recent efforts have aimed toward identifylng the major adsorption mechanisms in nonpolar media. Pugh6 showed in a recent paper that acid-base interactions are chiefly responsible for adsorption of solutes from nonaqueous solvents. In the absence of significant ionization, electrostatic mechanisms do not play a major role in adsorption processes in solvents of low dielectric constants. Labib and Williams discuss acid-base interactions in terms of the donicity of surfaces which is related to the electron donor properties of the surface species.' 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-solutesolvent 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. Morphologically, 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-Bobtained from Rhone Poulanc, with a nominal size of 40-100 pm 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 mz/g)were obtained from Aldrich Chemicals. (5) M s t e a d , C. G.; Tyler,A. J,; Hockey, T. A. Trans. Faraday Soc. 1971, 67, 493.
(6) h g h , R. J. Ceram. Trans. 1990, 12, 375. (7) Labib, M.; Williams, R. J. Colloid Interface Sci. 1984, 97,356.
0743-7463/94/2410-2786$04.50/00 1994 American Chemical Society
Surfactant-Adsorbent Acidity and Solvent Polarity
Langmuir, Vol. 10,No. 8,1994 2787
Table 1. Dielectric Constant of Relevant Solvent@ solvent cyclohexane chloroform 1-hexanol butanol 2-isopropanol acetone ethanol a
dielectric constant 2.02 4.8 13.3 15.8 18.3 20.3 24.3
solvent methanol glycerol methanovwater (wt %)
5l
dielectric constant 32.63 42.5
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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.* Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate) (AOT) is the most commonlyused 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 dimethyldodecylamine (DDA)was obtained from ICN pharmaceutical and used without further purification. Cyclohexane,of spectroscopicgrade, 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 i g n ~ r e d .When ~ required, the solvent was stored on molecular sieves 4Ato 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 "C for 6 h followed by cooling it for 2 h a t 25 "C in a vacuum desiccator. Dehydroxylation of alumina was done by heating it at 900 "C 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.1°
Results and Discussion (a) 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 (thelower the iep, the more acidic is the mineral). Alumina has a high density of hydroxyls on the surface (10-15 OWnm2)and a high iep (8.5)and is a basic oxide. On the other hand silica has a lower density of hydroxyls (3-4 OWnm2) and a low iep (2.5)and is a n acidic oxide. The density of the hydroxyl groups and hence the basicity of t h e surface can also be controlled by dehydroxylation of the surface by extended heating. Figure 1 shows the adsorption isotherms ofAerosol OT on alumina and silica, and it is observed that the anionic surfactant has a greater affinity for the basic oxide than for the acidic oxide. The situation is reversed in the case of adsorption ofthe cationic (8)Zettlemoyer, A. C.; McCafferty, E. Croat. Chem. Acta 1973,45, 173. (9)Suda, Y.;Morimoto, T. Langmuir 1985,I , 544. (10)Reid, V. W.; Longman, G. H.; Heinhirth,E. Tenside 1988,5,90.
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dimethyldodecylamine. Dimethyldodecylamine adsorbs more on acidic silica than on alumina (Figure 2). Calculation based on a plateau adsorption of about 3 x 10+ Wm2 on alumina gives a parking area of about 0.55 nm2 per AOT molecule. This is in good agreement with the published values for AOT molecules adsorbed at the water-xylene1' and water-isooctane12 interfaces, suggesting that the AOT adsorbs as a monolayer in a n orientation perpendicular to the adsorbent with the hydrocarbon chain extending into the solution. A similar calculation based on plateau adsorption for the cationic surfactant gives a parking area of 1.6 nm2 per molecule which is much less than the area occupied by a flatly adsorbing DDA molecule (approximately 4 nm2), suggesting that this also adsorbs perpendicular to the adsorbent. Figure 1also shows the adsorption ofAerosol OT on dehydroxylated alumina. Dehydroxylation increases the acidity of the surface and hence reduces the (11)McGown, D. N. L.; Parfitt, G. D.; Willis, E. J.Colloidlnterface Sci. 1967,20,650. (12)Maitra, A. N.; Eicke, H.F.J. Phys. Chem. 1981,85,2687.
2788 Langmuir, VoE. 10, No. 8, 1994
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adsorption of anionic AOT. Thus, it is clear that Aerosol OT adsorbs by interaction with the hydroxyl groups on the oxide surface. These results also suggest that for a given surfactant the relative acidity of the oxide surface is one of the major factors that determine the extent of interaction. (b)Effect of Surfactant. Comparison of Aerosol OT and dimethyldodecylamine adsorption data from Figures 1 and 2 also shows the role of the surfactant properties on its adsorption on alumina from cyclohexane. For surfactants, the anionic surfactant being a better electron acceptor is more acidic than the cationic surfactant which can accept protons and can be considered as basic. As can be seen the affinity of the acidic Aerosol OT for alumina is greater and the plateau adsorption larger compared to those of basic dimethyldodecylamine. However, the adsorption trend is reversed on silica where the basic amine adsorbs to a much larger extent than the acidic Aerosol OT. The infrared spectrum of AOT adsorbed on alumina (Figure 3) does not show any new absorption bands or any shift in the alumina or AOT bands due to adsorption. This suggests that the interaction is weak and probably of a n acid-base type rather than of a chemical type. From these results it can be inferred that the hydrophilic groups on the surfactants interact with the hydroxyl groups on the oxide surfaces and for a given oxide surface the interaction depends mostly on the acid-base character of the surfactant. In general we observe that the acidic surface has a greater affinity for the basic solute and vice versa. ( c ) Effect of Solvent. The adsorption isotherms of Aerosol OT on alumina from three different solvents, cyclohexane, chloroform, and methanol, are shown in Figure 4. As the polarity of the solvent is increased from cyclohexane to methanol the adsorption becomes weaker (initial slope of the adsorption isotherm) and lesser (adsorption density). Figure 5 depicts the desorption of
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Aerosol OT, preadsorbed on alumina from cyclohexane, into solvents of different polarity. It can be seen that as the solvent polarity is increased the amount of surfactant desorbed rises sharply above a certain critical value of the dielectric constant of the solvent. As the solvent polarity increases the surfactant interacts more with the solvent with a concomitant reduction in the surfactantsolid interaction. This increase in solvent-surfactant interaction with increasing polarity is also evident from the data for aggregation ofAOT in different s01vents.l~In solvents of low polarities, aggregation was observed and (13)Pen, J. B.J. Colloid Interface Sci. 1969,29, 6.
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Dielectric Constant Figure 6. Desorption vs solvent dielectricconstant for different minerals showing the effect of solid surface acidity on desorption: (0)alumina, (O), silica, ( 0 )hematite, (m) rutile.
the extent of aggregation decreased as the polarity of the solvent was increased due to increased interaction of the surfactant with the solvent. In addition interaction of the solvent with the oxide surface can also be expected to increase with solvent polarity, and this in turn would lead to a decrease in the oxide-surfactant interaction. Most interestingly, beyond a certain solvent polarity the affinity of the surfactant for the oxide surface starts to increase again as indicated by the maximum in the desorption curve. This occurs in solvents where Aerosol OT is known to exist as regular micelles in contrast to the reverse micelles observed in solvents of low dielectric constant. At higher solvent polarities the hydrocarbon chain of the surfactant is less compatible with the solvent and tends to form aggregate structures to remove the hydrocarbon parts from the bulk solvent. This can be accomplished via micelle formation in solution or adsorption a t a relatively less polar interface. Evidently the latter mechanism is favored in this case. Studies with different oxides (Figure 6)showed similar results, with the curves shifting to the right in the low dielectric constant regime and moving lower in the high dielectric constant regime as the acidity of the oxide increased. The acidity of the oxides14 increases in the order alumina < hematite < rutile < silica, and the observed shifts also follow the same trend. This brings out the effect of the nature of the solid on the solvent influence on adsorption. This can be better understood if we consider the solvent dielectric constant at which the desorption rises steeply to be equivalent to the dielectric constant ofthe oxide surface. Thus, the apparent dielectric constant of the oxide surfaces increases in the order silica < rutile < hematite < alumina. It is proposed that the surfactant partitions favorably to the higher dielectric constant environment in the solvents where it forms reverse micelles. Thus, in solvents of dielectric constant less than the apparent dielectric constant of the solid, the surfactant partitions favorably to the interface, and with increasing dielectric constant it partitions more into the bulk. A similar explanation based on the relative hydrophobicities of the surfaces is used for the high dielectric constant regular micellar regime (beyond the maximum).
The relative hydrophobicities of these minerals will be in increasing order, alumina < hematite < rutile silica (based on the surface density of OH groups). In the regime of regular micelles, the more the hydrophobicity of the mineral the greater is the tendency of the surfactant to partition to the interface, and hence the desorption curve moves down.
(14)Koksal, E.; Ramachandran, R.; Somasundaran, P.; Maltesh, C. Powder Technol. 1990, 62,253.
Conclusions (1)It is shown that the adsorption of surfactants on oxide minerals in nonpolar media occurs primarily through interactions between the hydroxyl groups on the oxide surface and the polar moiety of the surfactant molecule. (2)Adsorption can be considered to be controlled by two factors: The first one is the surfactant partitioning between the bulk and the interface, the extent of which is controlled by the polarity difference between the solvent and the adsorbent. The second one is the degree of interaction via the acid-base mechanism between the surfactant and the adsorbent, the extent of which depends on the acid-base character of the adsorbent and the surfactant. In general, it is observed that a n acidic surfactant interacts more with a basic adsorbent and vice versa. However, the desorption of the surfactant indicates that these interactions are weak and easily reversed. (3) The solvent can affect the adsorption process by influencing the solute aggregation behavior and by competing for adsorption sites on the adsorbent. It was observed that the affinity of Aerosol OT for a given oxide surface initially decreased as the solvent polarity was increased and then increased again. (4) In solvents of low dielectric constant the surfactant partitions to the more hydrophilic solid surface, while in solvents of high polarity it partitions to the relatively more hydrophobic solid surface. While polar interactions control the adsorption in low-polarity solvents, hydrophobic interactions appear to play a significant role in adsorption from high-polarity solvents onto oxide minerals. (5) In the solvents of medium polarity the surfactant partitions favorably into the bulk solvent phase as both parts of the amphipathic molecule are fairly compatible with the solvent phase. Acknowledgment. Financial support from the Nais actional Science Foundation (Grant CTS-90-11991) knowledged.
