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A Study on Adsorption of Surfactant Molecules on Magnesium Oxide Nanocrystals Prepared by an Aerogel Route P. Jeevanandam and K. J. Klabunde* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received January 28, 2002. In Final Form: April 15, 2002 Adsorption of surfactant molecules, cationic as well as anionic, has been carried out on nanocrystals of magnesium oxide, prepared by an aerogel route (AP-MgO). Aerosil OT (AOT) and didodecyl dimethylammonium bromide (DDAB) have been chosen as examples of anionic surfactant and cationic surfactant, respectively. The adsorption studies were mainly carried out in nonaqueous media. For comparison, adsorption experiments were also carried out for two other magnesium oxide samples, namely, conventionally prepared MgO (CP-MgO) and commercial MgO (CM-MgO), which consist of microcrystals. The adsorption properties of the surfactants have been studied by determining the adsorption isotherms. The results have been interpreted on the basis of the fact that nanocrystals possess increased surface reactivity compared to microcrystals and particle shapes do have a role to play.
1. Introduction Adsorption of surfactants on metal oxides is a topic of great academic and industrial interest because of their importance in many processes such as detergency, mineral flotation, and oil recovery.1 Extensive work has been done to understand the process of adsorption of surfactants on oxides.2 Nonaqueous colloidal dispersions are widely being used in a number of important technological processes.3 Surfactants and polymers have been used to modify particle surface properties for many applications.4 In all these applications it is necessary to have dispersions that have long-term stability. Typically, the dispersions are stabilized by the addition of surfactants or polymers that adsorb on the particle surface.5 Polymers usually provide steric stabilization and surfactants provide electrostatic stabilization. There are many factors that control the adsorption process at the interface.6 The behavior of surfactants and polymers at interfaces is determined by a number of forces such as electrostatic, covalent, hydrogen bonding, and hydrophobic bonding. The extent and the nature of the forces involved vary depending on the adsorbate and the adsorbent and also the composition and other characteristics of the solvent. The information on the properties of adsorbed surfactant and polymer layers has been obtained from adsorption isotherms. Most of the reported studies that involve adsorption of surfactants on oxides have been carried out in aqueous medium,7 and there are relatively few reports on adsorption studies in nonaqueous media.8 MgO is a unique solid because of its highly ionic character, simple stoichiometry, and crystal structure, (1) Xiao, L.; Liao, P.; Hu, W. Colloids Surf. 1987, 26, 273. (2) (a) Koopal, L. K.; Goloub, T. ACS Symp. Ser. 1995, No. 615, 78. (b) Bohmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2649. (c) Koopal, L. K.; Lee, E. M.; Boehmer, M. R. J. Colloid Interface Sci. 1995, 170, 85. (3) (a) Novotny, V. Colloids Surf. 1987, 24, 361. (b) Fuerstenau, D. W.; Herrera-Urbina, R.; Hanson, J. S. Ceram. Trans. 1988, 1, 333. (4) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990. (5) Napper, D. H. Polymeric Stabilization of Colloidal Suspensions; Cornell University Press: Ithaca, NY, 1979. (6) (a) Parfitt, G.; Rochester, C. H. In Adsorption from solution at the solid/liquid interface; Parfitt, G., Rochester, C. H., Eds.; Academic Press: New York, 1983. (b) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964, 68, 3562.
and it can be prepared in widely variable particle sizes and shapes.9 MgO prepared by the aerogel route (APMgO) possesses surface area in the range of 300-500 m2/g and consists of very small crystallites (about 4 nm in size) that appear more irregular with polyhedral shapes rather than as cubes or hexagonal platelets (Figure 1).10 Conventionally prepared MgO (CP-MgO) consists of ordered microcrystals with large exposed areas of the (100) crystal face and possesses a surface area in the range of 130-250 m2, while commercially prepared MgO (CM-MgO) is polycrystalline in nature and possesses a surface area in the range of 10-30 m2/g. AP-MgO powder has a pore structure that has been attributed to void spaces formed upon consolidation of nanosized polyhedral particles while CP-MgO and CM-MgO show no pore structure at all. Moreover, AP-MgO possesses numerous surface sites such as crystal corners, edges, or ion vacancies, which lead to inherently high surface reactivity per unit area compared to CP-MgO and CM-MgO.11 The percentage of corner and edge sites on the surface of AP-MgO could approach as high as 20%, while on CP-MgO less than 0.5% and on CM-MgO, essentially 0%. The shape and size of crystals of MgO have considerable effects on their adsorption properties in addition to affecting the type of bonding to the surface that takes place.12 For example, monodentate adsorption of SO2 is preferred on AP-MgO nanocrystals owing to the prevalence of edge/corner sites while bidentate (7) (a) Wang, W.; Kwak, J. C. T. Colloids Surf., A 1999, 156, 95. (b) Nagashima, K.; Blum, F. D. J. Colloid Interface Sci. 1999, 214, 8. (c) Solomon, M. J.; Saeki, T.; Wan, M.; Scales, P. J.; Boger, D. V.; Usui, H. Langmuir 1999, 15, 20. (d) Dao, K.; Bee, A.; Treiner, C. J. Colloid Interface Sci. 1998, 204, 61. (e) Lee, E. M.; Koopal, L. K. J. Colloid Interface Sci. 1996, 177, 478. (8) (a) Krishnakumar, S.; Somasundaran, P. Langmuir 1994, 10, 2786. (b) Lai, C.; Harwell, J. H.; O’Rear, E. A.; Komatsuzaki, S.; Arai, J.; Nakakawaji, T.; Ito, Y. Colloids Surf., A 1995, 104, 231. (9) Morris, R. M.; Klabunde, K. J. Inorg. Chem. 1983, 22, 682. (10) Utamapanya, S.; Klabunde, K. J.; Schlup, J. Chem. Mater. 1991, 3, 175. (11) (a) Klabunde, K. J.; Stark, J. V.; Koper, O.; Mohs, C.; Park, D. G.; Decker, S.; Jiang, Y.; Lagadic, I.; Zhang, D. J. Phys. Chem. 1996, 100, 12142. (b) Richards, R.; Li, W.; Decker, S.; Davidson, C.; Koper, O.; Zaikovski, V.; Volodin, A.; Rieker, T. J. Am. Chem. Soc. 2000, 122, 4921. (12) Lucas, E.; Decker, S.; Khaleel, A.; Seitz, A.; Fultz, S.; Ponce, A.; Li, W.; Carnes, C.; Klabunde, K. J. Chem. Eur. J. 2001, 7, 2505.
10.1021/la0200921 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/23/2002
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Figure 2. Adsorption isotherms for AOT on AP-MgO and CPMgO in pentane.
Figure 1. (a) Model of MgO nanocrystal of polyhedral shape (AP-MgO). (b) Model of hexagonal microcrystal (CP-MgO). (c) Model of cubic-shaped microcrystal (CM-MgO).
adsorption is preferred on the larger microcrystals (CPMgO and CM-MgO) where flat planes are prevalent.13 The present work involves studies on the adsorption properties of anionic and cationic surfactants on MgO prepared by an aerogel method, hereafter referred as APMgO. Adsorption studies have been mainly carried out in nonaqueous solvents. For comparison, adsorption studies were also carried out for conventionally prepared MgO (CP-MgO) and commercial MgO (CM-MgO). Aerosil OT (sodium bis (2-ethylhexyl) sulfosuccinate (AOT)), which is a commonly used anionic surfactant, and didodecyldimethylammonium bromide (DDAB) were chosen as examples of anionic and cationic surfactants, respectively. 2. Experimental Section The procedures for the preparation of AP-MgO and CP-MgO have been described earlier.10 CM-MgO was from Comprehensive Research Chemical Corporation, AOT was purchased from Aldrich, and DDAB was obtained from Fluka. All the chemicals were used as received. All the organic solvents used were purchased from Fisher Scientific and used after drying. The adsorption isotherm experiments were carried out for AP-MgO, CP-MgO, and CM-MgO samples. Typically, about 0.25 g of APMgO or CP-MgO samples was suspended in about 20 mL of surfactant solutions of various known concentrations in the required solvent for about 20 h, which was found to be a sufficient time for the maximum adsorption of the surfactant at any given (13) (a) Stark, J. V.; Park, D. G.; Lagadic, I.; Klabunde, K. J. Chem. Mater. 1996, 8, 1904. (b) Itoh, H.; Utampanya, S.; Stark, J. V.; Klabunde, K. J.; Schulp, J. R. Chem. Mater. 1993, 5, 71.
concentration. In the case of CM-MgO, about 1 g of sample was used for the adsorption experiments since the surface area of CM-MgO was fairly low (30 m2/g). After the adsorption time interval, the surfactant concentration in the supernatant solutions was determined by a two-phase mixed indicator titration method. This method is widely used in the literature to determine the concentration of anionic as well as cationic surfactants in aqueous as well as nonaqueous solutions.14 From the change in the concentration of the surfactant solutions before and after the adsorption, the amount of surfactant adsorbed can be calculated in terms of adsorption density (mol/m2). The adsorption isotherm measurements for AOT were carried out in solvents such as pentane, ethanol, chloroform, and tetrahydrofuran and also in other solvents. The adsorption isotherm experiments for DDAB were carried out only in toluene in order to compare the results with those of the anionic surfactant adsorption. After the adsorption experiments, the MgO samples, which have adsorbed surfactants on them, were repeatedly washed with the respective solvents in order to remove the excess surfactant and dried in a drying cabinet at 100 °C for about an hour. Surface areas of the samples were measured from N2 adsorption at liquid nitrogen temperature and by applying the BET multipoint method. FTIR spectra were recorded on a Nicolet infrared spectrophotometer with KBr pellets in the range 400-4000 cm-1.
3. Results and Discussion Figure 2 shows the adsorption isotherms of AOT on AP-MgO (starting specific surface area ) 456 m2/g) and on CP-MgO (specific surface area ) 270 m2/g) in pentane. They show an increase in the adsorption density with the concentration of AOT and reach a saturation value at a concentration of about 0.03 M. It can also be noticed that AP-MgO adsorbs more AOT molecules compared to CPMgO at any given concentration of AOT. For a CM-MgO sample, a complete adsorption isotherm could not be determined since irreproducible adsorption density values were observed during the adsorption experiments. From the plateau in the adsorption density versus concentration curves, one can calculate the number of AOT molecules adsorbed per nm2 area of AP-MgO and CPMgO. It was found to be 1.72 for the case of AP-MgO suggesting that the AOT molecules adsorb in a slight excess of a monolayer on the surface of AP-MgO, as compared with the reported values for AOT molecules at (14) (a) Li, Z.; Rosen, M. J. Anal. Chem. 1981, 53, 1516. (b) Reid, V. W.; Longman, G. H.; Heinhirth, E. Tenside 1968, 5, 90.
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Figure 3. (a) Infrared spectra of pure AOT and AOT adsorbed on AP-MgO and (b) in the range corresponding to SdO stretching mode.
the interfaces, e.g., water-isooctane.15,16 On the other hand, the number of AOT molecules per nm2 area of CPMgO was found to be 0.93, which is much smaller compared to that for the AP-MgO sample indicating less than a monolayer coverage. The AOT molecules adsorbed on the AP-MgO sample show sharp bands in the IR spectrum compared to pristine AOT, and this could be attributed to a good alignment of AOT molecules on the surface of AP-MgO (Figure 3) and also suggests that the adsorbed AOT is in a dense, ordered monolayer rather than a partial double layer. The IR spectrum of AOTadsorbed AP-MgO does not show any new absorption bands or any shift in the oxide or AOT bands. However, a small change observed is the disappearance of the splitting attributed to asymmetric SdO stretching mode in the spectrum of AOT adsorbed AP-MgO. The asymmetric SdO stretching mode appears as a doublet at ca. 1249 and 1216 cm-1 in pristine AOT, and the doublet indicates a noncentrosymmetric interaction between the sodium ion and the sulfonate group.17 The decrease in the (15) Maitra, A. N.; Eicke, H. F. J. Phys. Chem. 1981, 85, 2687. (16) Assuming an area of 78 Å2 per AOT molecule (Li, Z. X.; Lu, J. R.; Thomas, R. K. Langmuir 1997, 13, 3681), the number of AOT molecules required to form a monolayer will be 1.3/nm2. This value is very close to the number of AOT molecules per nm2 area of AP-MgO suggesting monolayer coverage of AOT. (17) Moran, P. D.; Bowmaker, G. A.; Cooney, R. P. Langmuir 1995, 11, 738.
Figure 4. Effect of solvent on the adsorption of AOT on APMgO.
magnitude of the band splitting reflects the weakening of the interaction between the cation and the sulfonate group. In the AOT-adsorbed AP-MgO, this doublet now appears as a single band at ca. 1227 cm-1 indicating the weakening of the cation-sulfonate interaction. The disappearance of the asymmetric SdO stretch splitting was observed for AOT adsorbed on CP-MgO, too. This suggests that the interaction between the adsorbed surfactant and the MgO samples is relatively weak since one would expect changes in the band positions of AOT if the interactions were strong. On the other hand, the AOT interaction with AP-MgO is clearly somewhat stronger than that with CP-MgO. To study the effect of polarity of solvents on the adsorption properties of AOT on AP-MgO, the determination of adsorption isotherms was carried out in different solvents. Figure 4 show the typical adsorption isotherms of AOT on AP-MgO in different solvents. It can be noticed that in a more polar solvent such as ethanol, the adsorbed amount of AOT is less when compared to that in a less polar solvent such as pentane. Moreover, it is to be noted that the adsorption density maximum is observed at a much higher concentration of AOT in ethanol in comparison with that in pentane. As the polarity of the solvent increases, the interaction between the surfactant and the solvent begins to be prominent, which leads to a reduction
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Table 1. Effect of Water on the Adsorption of AOT on AP-MgO in Pentane amount of H2O (µL) 0 10 50
adsorption density (mol/m2) 10-6
1.9 × 1.8 × 10-6 1.9 × 10-6
in the surfactant-solid interaction (initial slope of the adsorption isotherm changes, see Figure 4). The increase in solvent-surfactant interaction with increasing polarity is also evident from the reported data for aggregation of AOT in different solvents.18 The fact that the saturation coverage is not the same in different solvents may be explained by assuming that there is competition between solvent molecules and AOT for the MgO surface and that some solvents interact with the MgO surface more strongly than others. When the AP-MgO sample, which has AOT adsorbed on it from pentane, was suspended in a more polar solvent such as water, it was found from IR spectral studies that desorption of the AOT molecules into water takes place but the desorption of AOT is not complete even after 48 h. This again suggests that the interaction between the surfactant and the oxide surface is relatively weak compared with that of water and the oxide surface. In general, the surface area of the surfactant adsorbed APMgO samples is less by about 25-30% compared to their original surface area. Also, the pore volume decreases after the surfactant adsorption, e.g., the AOT adsorbed APMgO possesses a pore volume of about 0.35 g/cm3 while the bare AP-MgO nanoparticles possess a pore volume of about 0.65 g/cm3. These results indicate that the AOT is obviously entering and filling much of the pore volume. To identify the role of the isolated hydroxyl groups on the surface of AP-MgO on the adsorption properties of AOT, the effect of addition of water on the adsorption of AOT in pentane on AP-MgO was studied. A 0.03 M solution of AOT in pentane was used for the adsorption experiments, and a known amount of water was introduced into the pentane solution. The adsorption density values found are tabulated in Table 1. It can be seen that the addition of water into pentaneAOT solution does not affect the adsorption properties of AOT molecules on AP-MgO. Addition of water molecules into pentane during the adsorption experiments was found to introduce more isolated hydroxyl groups on AP-MgO, as evidenced by IR spectroscopy. A sharp band at about 3700 cm-1, which can be attributed to isolated hydroxyl groups, grew in its intensity as more and more water was introduced into the solution. The adsorption of AOT in pentane solutions was also carried out for silylated samples of AP-MgO. Silylation of AP-MgO leads to protection of the hydroxyl groups of AP-MgO by (CH3)3Si groups and the silylation was carried out by allowing AP-MgO to react with (CH3)3SiOCH3. The Si/Mg ratio in the silylated APMgO was found to be about 0.5% as determined by energydispersive X-ray analysis. This suggests that the surface coverage of (CH3)3Si groups is small as expected since silylation affects the surface hydroxyls only. The adsorption density values for AOT in pentane (0.03 M) for the silylated and unsilylated AP-MgO samples were found to be 3.0 × 10-6 and 2.9 × 10-6 mol/m2, respectively. It can be concluded that the protection of the hydroxyl groups and the introduction of additional isolated hydroxyl groups do not affect the adsorption properties of AOT on APMgO. To block the Lewis as well as Bronsted acid sites on the surface of the AP-MgO sample, pyridine molecules (18) Peri, J. B. J. Colloid. Interface Sci. 1969, 29, 6.
Table 2. Number of DDAB Molecules Adsorbed per nm2 on Various MgO Samples sample
no. of DDAB molecules/nm2
AP-MgO CP-MgO CM-MgO
0.53 0.50 1.10
Chart 1. Binding Scheme for MgO Surface with AOT and DDAB
were adsorbed on the surface by allowing AP-MgO powder to react with neat pyridine at room temperature. The pyridine-adsorbed AP-MgO samples were used for further adsorption experiments: adsorption of AOT from pentane and adsorption of DDAB from toluene on AP-MgO. From the IR spectral studies it was inferred that neither the AOT adsorption nor the DDAB adsorption was affected to a noticeable extent. The adsorption isotherms of the cationic surfactant, DDAB, in toluene, on AP-MgO, CP-MgO, and CM-MgO samples were also measured. From the plateau in the adsorption isotherm curves, the number of adsorbed DDAB molecules per nm2 area of AP-MgO, CP-MgO, and CMMgO were calculated and are given in Table 2. Due to the low surface area of the CM sample, the data are less reliable, but it is interesting to note the large value for the DDAB adsorption. The reactive sites on the surface of MgO are as follows: 12 (i) Mg2+ site which is of Lewis acid type, (ii) O2- site which is of Lewis base type, (iii) lattice bound hydroxyls, (iv) isolated hydroxyls, and (v) anionic and cationic vacancies. Since the hydroxyl groups were found to have no effect on the adsorption of the surfactants, it can be proposed (Chart 1) that the anionic surfactant interacts with the Mg2+ sites/cationic vacancies through the sulfonate group, and the cationic surfactant interacts with O2- sites/anionic vacancies through the quaternary ammonium ion. The increased amount of adsorption of AOT on AP-MgO compared to CP-MgO is clearly due to its enhanced surface reactivity. The increased reactivity of AP-MgO nanocrystals is likely due to the higher surface concentration of highly reactive and sterically available edge and corner defect sites. Moreover, unusual lattice planes, also possessing greater reactivity, are also available on the polyhedral-shaped nanocrystals.
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Chart 2. Scale Model of Binding Scheme for AP-MgO-AOT Taking into Account the Amount of AOT Adsorbed/nm2, Spectroscopic Evidence for an Ordered Film, and Spectroscopic Evidence for SO3-‚‚‚Na+ Separation
Figure 5. Rate of pH change for surfactant-coated AP-MgO samples.
To summarize our findings with AOT and DDAB: (1) anionic surfactant is adsorbed in higher amounts than cationic surfactant; (2) more surfactant is adsorbed from the least polar solvents; (3) surface hydroxyl groups do not seem to play a role, nor does adsorbed pyridine play a role in affecting the surfactant adsorption; (4) nanocrystals adsorb the most anionic surfactant/nm2; and (5)
upon adsorption of the anionic surfactant, the SO3-Na+ interaction is perturbed greatly. These results suggest that the interaction is due to weak Lewis acid sites (surface Mg2+) interacting with the weak Lewis base SO3- site of the surfactant and the Na+ of the surfactant binds to a nearby surface O2- site (see Chart 1). Since the IR results suggest an alignment of AOT molecules on the surface of AP-MgO, and the adsorption isotherm studies suggest a very closely packed monolayer coverage of AOT molecules, it can be proposed that the AOT molecules adsorb on APMgO in a fashion similar to that shown in Chart 2. Since the number of Lewis acid sites and Lewis base sites on the surface of AP-MgO would be roughly the same, the difference in the adsorption capacities of AOT and DDAB toward AP-MgO can be possibly explained by considering steric arguments. DDAB will exhibit more steric hindrance due to its four alkyl groups compared to two for AOT. An approach to investigate the differences in surface reactivity of the surfactant coated AP-MgO nanoparticles and bare AP-MgO nanoparticles is to look at the pH changes (measured using a pH meter) in water as a function of time.12 Figure 5 shows the pH changes versus time observed when the surfactant-coated AP-MgO samples were suspended in water. Overall, the surfactantcoated AP-MgO nanoparticles interacted more slowly with water compared to that of pure AP-MgO.12 Further studies on the reactivity of the surfactant-coated nanoparticles are in progress. Acknowledgment. The support of this research by the Army Research Office is acknowledged with gratitude. Partial support was also provided by the National Science Foundation. LA0200921
