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Langmuir 1996, 12, 1958-1966
Solid-State 1H and 13C NMR Investigations of Dodecyl Sulfate-Alumina Interfacial Interactions Using High Surface Area Pseudo-Boehmite Solids Containing Adsorbed Surfactants Gilberto Piedra and John J. Fitzgerald* Department of Chemistry, South Dakota State University, Brookings, South Dakota 57007
Cynthia F. Ridenour† and Gary E. Maciel* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received August 28, 1995. In Final Form: December 8, 1995X Solid-state 1H NMR using CRAMPS (combined rotation and multiple pulse spectroscopy) technique and cross-polarization magic angle spinning (CP/MAS) NMR studies of the interactions between a high surface area alumina material and various surfactants adsorbed at the solid/liquid interface in aqueous media systems are reported. These studies have focused on “wet” and evacuated alumina-surfactant solids obtained by adsorption of surfactant ions (dodecyl sulfate (DDS), oleate, and dodecylammonium) from aqueous media (pH 3 and 6.5), over surfactant loading concentrations (C0) from 1 × 10-3 to 1 × 10-2 M in 2% w/v suspensions of a high surface area pseudo-boehmite material. 1H CRAMPS and 1H singlepulse (SP)/MAS NMR were used to identify the relative proton populations associated with surface Al-OH groups, “physisorbed” water, and the various protons of adsorbed surfactant ions of “wet” and evacuated solids. 1H CRAMPS data and adsorption densities show that the adsorption of the dodecyl sulfate ion on the alumina surface is increased as C0 is increased from 1 × 10-2 to 1 × 10-3 M, at both pH 6.5 and 3.0. Evacuation was found to eliminate the “physisorbed” water (4-6 ppm), permitting the observation of both “clustered” surface and/or internal Al-OH sites (7.0-7.5 ppm) and surfactant protons (1-3 ppm). Increases in the relative peak intensity of the surfactant protons as C0 was increased from 1 × 10-3 to 1 × 10-2 M, together with a decrease in the “physisorbed” water peak intensity, suggest that a competition occurs between the “physisorbed” water and the surfactant ions for the surface hydroxyl sites on the alumina material. Dipolar dephasing experiments show that the surfactant ions are quite mobile in “wet” DDSAl2O3 solids, while the removal of water leads to increased surfactant rigidity. Several of the DDS-Al2O3 solids, prepared by equilibration in 2 × 10-3 to 1 × 10-2 M DDS loading concentration, were also examined by solid-state 13C CP/MAS NMR. While the 13C CP/MAS NMR spectra revealed numerous resonances that were assigned to various carbons of the adsorbed surfactant ion DDS, and that the intensity of these peaks are dependent on the surfactant loading levels, no significant changes were observed in the chemical shifts or line widths of the 13C NMR peaks to provide additional information about the mode of attachment and mobility of these adsorbed species on the alumina surface. 13C
Introduction Studies of alumina-surfactant interactions are important for materials processing technologies such as froth flotation,1-3 for thin film deposition of inorganic binders or adhesives4-7 adsorbed from aqueous solutions onto alumina ceramics and catalysts,4-8 for dispersion and * Authors to whom correspondence should be addressed. † Present address: Chemagnetics, Inc., 2555 Midpoint Drive, Fort Collins, CO 80525. X Abstract published in Advance ACS Abstracts, March 1, 1996. (1) Leja, J. Surface Chemistry of Froth Flotation; Plenum Press: New York, 1982; pp 205-214, 265-268, 288-306, 517-532. (2) Smith, R. W.; Akhtar, S. In Flotation: A. M. Gaudin Memorial Volume; Fuerstenau, M. C., Ed.; American Institute of Mining, Metallurgical and Petroleum Engineers, Inc.: New York, 1976. (3) De Bruyn, P. In The Scientific Basis of Flotation; Ives, K. J., Ed.; Martinus Hyhoff Publishers: The Hague, 1984; pp 111-193. (4) Rothon, R. N. Thin Solid Films 1981, 11, 149-152. (5) Rothon, R. N. Chem. Ind. 1974, 11, 457-459. (6) Morris, J. H.; Perkins, P. G.; Rose, A. E. A.; Smith, W. E. Chem. Soc. Rev. 1977, 6, 173. (7) Morris, J. H.; Perkins, P. G.; Rose, A. E. A.; Smith, W. E. J. Appl. Chem. Biotechnol. 1978, 28, 756-760. (8) Marcelin, G.; Vogel, R. F.; Swift, H. E. J. Catal. 1983, 83, 42-49. (9) Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley and Sons: New York, 1976. (10) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization Colloid and Surface Properties, and Biochemistry; John Wiley and Sons: New York, 1979. (11) Fuerstenau, M. C.; Gutierrez, G.; Elgillana, D. A. Trans. Am. Inst. Min. Eng. 1968, 214, 319-323.
0743-7463/96/2412-1958$12.00/0
flocculation treatment of aluminum oxides and hydrous oxides from aqueous solution,9-11 and for understanding the “interfacial” interactions and bonding important to the preparation of alumina-containing composite materials.12,13 The phenomenon of surfactant adsorption at the solidwater interface of metal oxides such as silica and alumina has been previously studied.14-19 When a metal oxide is suspended in water, an electrical double layer has been postulated to form. This double layer is envisioned to consist of a charged region on the solid oxide surface and a layer of counterions adjacent to the surface (termed the Stern layer), which is required for electroneutrality.15 The Stern layer probably associates to the solid surface by Coulombic or van der Waals forces, whereas the residual (12) Chawla, K. K. Composite Materials; Springer-Verlag: New York, 1987. (13) Surface and Interfaces in Ceramics, Materials Science Research; Pask, J., Evans, A., Eds.; Plenum Press: New York, 1980. (14) De Bruyn, P. L. Trans. Am. Inst. Min. Eng. 1955, 202, 291-296. (15) Modi, J. H.; Fuerstenau, D. W. Trans. Am. Inst. Min. Eng. 1960, 217, 381-387. (16) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90-96. (17) Fuerstenau, M. C.; Cummins, W. F. Trans. Am. Inst. Min. Eng. 1967, 238, 196-203. (18) Wakamatsu, T.; Fuerstenau, D. W. Trans. Am. Inst. Min. Eng. 1973, 254, 123-126. (19) Chandar, P.; Somasundaran, P.; Turro, N. J. Colloid Interface Sci. 1987, 117, 31-46.
© 1996 American Chemical Society
Dodecyl Sulfate-Alumina Interfacial Interactions
counterions are weakly held to the surface only by Coulombic forces, thereby constituting a diffuse layer that extends out into the liquid.15 At low concentration, adsorption of long-chained surfactant ions to metal oxide surfaces occurs by electrostatic attraction between the counterions and the charged solid oxide surface. It has been proposed that, when the surfactant surface concentration reaches monolayer coverage, two-dimensional aggregates (hemimicelles) begin to form at the solid-liquid interface.16 This process is facilitated by ion association of the surfactant molecular ions with each other, thus causing an additional rapid increase in adsorption. The electrostatic contribution to adsorption has been proposed to disappear when the number of adsorbed surfactant ions is equal to the number of metal oxide surface sites. Any further increase in adsorption is then due primarily to association between the hydrocarbon chains of the surfactant ions and those surfactant ions that are already adsorbed on the metal oxide surface.16 Dodecyl Sulfate-Alumina Adsorption Studies. Modi and Fuerstenau15 carried out flotation studies with corundum (R-alumina) using sodium dodecyl sulfate as the surfactant. Dodecyl sulfate (DDS) ions associate with each other at the corundum surface at a solution pH of 6.5 and at an initial solution surfactant concentration of 1 × 10-5 M; by contrast, in more acidic media (pH 4), surfactant association is initiated at a solution equilibrium surfactant concentration of 1 × 10-6 M. The adsorption of DDS on alumina in basic media (pH 11) is essentially zero, since the surface charge of alumina exceeds the isoelectric point of alumina.15 [The isoelectric point, or point-of-zero charge (pzc), is normally used to refer to both the pH at which an immersed solid oxide surface has a net charge of zero and the pH at which the concentration of positive and negative species on the surface are equivalent.] The point-of-zero charge (pzc) for alumina occurs at pH 9.1-9.4, and therefore electrostatic repulsion would prevent the adsorption of negatively charged DDS ions at pH values higher than 9.4.15 Somasundaran and Fuerstenau16 examined the mechanisms of adsorption of the dodecyl sulfonate ion at the alumina solid-water interface. The adsorption isotherm for the alumina-dodecyl sulfonate system at pH 7.2 and 25 °C, given as a plot of adsorption density (Sads, mol/cm2) versus equilibrium solution concentration of dodecyl sulfonate ion (Ceq, mol/L), consists of three regions. Region 1 is characterized by an increase in adsorption density that is probably due to both the low solution concentration of dodecyl sulfonate ion and the low alumina surface potentials. Region 2 is characterized by an abrupt change to a positive slope of the isotherm as significant surfactant ion adsorption occurs, due to both the electrostatic attraction between the ions and the charged alumina surface, and the hemimicelle association of hydrocarbon chains of the adsorbed surfactant ions. In region 3, where a lower dependence of adsorption on surfactant solution concentration occurs, the electrostatic interaction disappears and the observed lower increase in adsorption levels is due to hydrocarbon chain association.16 The pH dependence of the adsorption isotherm of the DDS-Al2O3 system has also been reported to consist of three regions: In the higher pH region 1, the adsorption density increases slightly as the pH is lowered to 6.8; in region 2, extending from pH 6.8 to 5.5, the adsorption density shows a significant increase as the pH is lowered, and in region 3, the rate of increase in adsorption density decreases as the pH is lowered to 2.0.16 Chandar et al.19 utilized pyrene and dinaphthylpropane as fluorescence probes to study the organization of the adsorbed layer of dodecyl sulfate ions at the alumina-
Langmuir, Vol. 12, No. 8, 1996 1959
water interface. The adsorption isotherm for dodecyl sulfate ion adsorbed onto alumina equilibrated at pH 6.5 in 0.1 M NaCl shows four distinguishable regions which were interpreted as follows: (1) in region I, the isotherm is linear at low surfactant ion concentration, and the surfactant adsorbs as individual ions through electrostatic interactions with the positively charged alumina surface; (2) in region II, an abrupt increase in the slope of the adsorption curve is observed as the solution equilibrium surfactant concentration (Ceq) reaches a value of 7.5 × 10-5 mol/L due to increased surfactant association with the alumina surface by both Coulombic and hydrophobic tail interactions of the hydrocarbon chains; (3) in region III, where the slope of the adsorption isotherm begins to decrease as the Ceq values reaches 5 × 10-3 mol/L, adsorption begins to tail off due to increased electrostatic hindrance in the surfactant association process following interfacial charge reversal; and (4) in region IV, adsorption is observed to plateau at complete surface coverage or possibly where bulk micellization (formation of micelles within the surfactant solution) occurs. Surface Chemistry of Alumina. De Bruyn3 has proposed a general model for the interfacial interaction of metal oxides at the solid-liquid interface in aqueous media. In this model, both Lewis acid and Lewis base sites are involved as depicted in the equations below.
Ms + :OH-(aq) ) (M:OH-)s
(1)
:Os + H+(aq) ) (O:H+)s;
(2)
Ms refers to the surface Lewis acid metal ion sites, and :Os refers to the Lewis base oxide sites. The surface hydroxyl groups manifest themselves as positive or negative surface charges that are dependent upon the hydration and structural nature of the metal oxide surface. It has been proposed that the nature of the overall surface charge and thus the interfacial activity of metal oxide materials is governed by solution pH for most metal oxides.3 The pH at which the pzc occurs varies for different metal oxides, e.g. pH ∼9.1 for Al2O3, pH ∼5-8 for TiO2, and pH ∼1-2 for SiO2.20 When alumina particles become hydrated, the particles exhibit negative or positive surface charges that depend on the number of positive and negative moieties on the surface, which are dependent upon the pH of the system.16 For example, the alumina surface has been determined to be positively charged at pHs below 8.0, neutral in the pH 9.0-9.4 range, and negatively charged at pH values higher than 10.0.16 The population of the hydroxyl sites of hydrated alumina surfaces are possibly responsible for the net (+ or -) sign and magnitude of the surface charge and depends on the surface structure, the state of hydration, and/or the degree of protonation of the metal oxide surface. Another model of the surface of alumina materials is the surface model for γ-alumina envisioned for the solidgas interface, as postulated by Peri.21 In this model, five different types of surface hydroxyl sites have been proposed as shown in Figure 1. Kno¨zinger and Ratnasamy22 refined Peri’s model to include the (111) and (110) planes. The relative contribution of specific crystal faces ultimately determines the occurrence and number of each hydroxyl group.21 Type III hydroxyls are presumed to exhibit the highest acidity, while types IA and IB are probably the most basic. (20) Parks, G. A. Chem. Rev. 1965, 65, 177-198. (21) Peri, J. B. J. Phys. Chem. 1965, 69, 220-230. (22) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev.sSci. Eng. 1978, 17, 31-70.
1960 Langmuir, Vol. 12, No. 8, 1996
Figure 1. Type of surface hydroxyl groups, as proposed by Peri,8 where Oh ) octahedral and Td ) tetrahedral.
In the model proposed by De Bruyn,3 the surface Lewis acid Ms sites of alumina materials probably correspond to coordinatively unsaturated aluminum sites formed upon dehydration of the alumina material. Dehydration treatment of high surface area hydrated aluminas has recently been shown by Fitzgerald et al.23 to lead to the condensation of adjacent and “clustered” as well as more distant “isolated” Al-OH surface groups, analogous to the surface hydroxyl groups proposed by Peri21 for the surface of γ-alumina, with concomitant production of molecular water. This condensation process results in the formation of strained Al-O-Al linkages on the alumina surface, possibly due to the formation of both four-coordinate AlO4 and five-coordinate AlO5 structural moieties.23 Studies of Surfactant-Alumina Interactions at the Solid-Liquid Interface. The interactions between inorganic solid metal oxide surfaces and molecular or ionic species adsorbed from aqueous solutions have been previously studied by radioassay methods,14 flotation techniques,15,17 and contact angle determinations,18 and more recently, spectrophotometric techniques such as colorimetry,16 fluorescence,19 and infrared spectroscopy.24 In addition, in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR)25-27 and surface-enhanced Raman spectroscopy (SERS)28 have been used to study the adsorption of surfactants at the solution/ solid interface. Recently, several nuclear magnetic resonance (NMR) studies of solution interactions in surfactants systems have also been explored.29-33 Bahadur et al.,30 for example, used solution 1H NMR techniques to examine the interactions of dodecyl sulfate and other ionic surfactants with styrene-ethylene oxide block copolymers in aqueous solutions. These studies showed that the addition of anionic surfactants to copolymer solutions increases the solubility of the block copolymer, because the anionic surfactant causes the dissociation of the copolymer micelles to form more dispersed solutions. It was proposed that the surfactant ions adsorb onto both the poly(ethylene oxide) chains and the hydrophobic polystyrene chains. Inoue et al.31 studied the interactions of the homopolypeptide, poly(L-lysine), with the dodecyl (23) Fitzgerald, J. J.; Piedra, G.; Hawkins, B. L.; Davis, M. F.; Maciel, G. E. J. Am. Chem. Soc., submitted for publication. (24) Cross, W. M.; Kellar, J. J.; Miller, J. D. Proceedings of the XVII International Mineral Processing Congress; Polygraphischer: Freiberg, FRG, 1991; Vol. II, pp 319-338. (25) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1994, 10 (1), 37-44. (26) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1992, 8 (9), 21832191. (27) Sperline, R. P.; Muralidharan, S. Y.; Freiser, H. Langmuir 1987, 3 (2), 198-202. (28) Couzis, A.; Gulari, E. Langmuir 1993, 9 (12), 3414-3421. (29) Fuhrhop, J. H.; Svenson, S.; Boettcher, C.; Roesler, E.; Vieth, H. M. J. Am. Chem. Soc. 1990, 112, 4307-4312. (30) Bahadur, P.; Sastry, N. V.; Rao, Y. K. Colloids Surf. 1988, 29, 343-358. (31) Inoue, Y.; Teraoka, N.; Suzuki, Y.; Chujo, R. Makromol. Chem. 1981, 182, 1819-1827. (32) Zheng, Y.; Lu, D. Mikrochim. Acta 1992, 106, 3-9. (33) Badiger, M. V.; Rajamohanan, P. R.; Kulkarni, M. G.; Ganapathy, S.; Mashelkar, R. A. Macromolecules 1991, 24, 106-111. (34) Ziegler, R. C.; Maciel, G. E.; In Chemically Modified Surfaces in Science and Industry; Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach Science Publishers: New York, 1988; Vol. 2, pp 319-336.
Piedra et al.
sulfate ion utilizing solution-state 13C NMR spectroscopy in D2O solutions. These studies showed that at neutral pH the negatively charged end of the surfactant ion and the protonated lysine -group interact through electrostatic attractive forces, whereas in more alkaline solutions, hydrogen bonding was proposed to occur between the anionic head of the surfactant ion and the uncharged lysyl side chains. These results suggested that the surfactant molecular ions form micelle-like clusters around the poly(L-lysine) helices, exposing the surfactant polar head group to the solution medium, thereby increasing the solubility of DDS/(Lys)n complexes.31 Solid-state NMR approaches based on various nuclides, e.g. 1H, 13C, 17O, and 27Al, are currently available to study the interactions of surfactant molecular ions with alumina materials in aqueous media. 1H, 17O, and 27Al NMR approaches may be useful to study the substrate of a particular alumina-surfactant system, since proton, oxygen, and aluminum atoms constitute both the bulk and the surface structural units of an alumina material. 17O MAS (magic-angle spinning) NMR37 and 27Al MAS NMR35,36 have previously provided relevant information about the local chemical environments of aluminum and oxygen atoms constituting the bulk and surface structural units of alumina materials, including the local coordination number of aluminum and oxygen sites, as well as the quadrupole coupling constants of these various aluminum and oxygen sites. Cross-polarization (CP) techniques have also been successfully employed by Morris and Ellis (27Al CP/MAS)35 and Walter and Oldfield (17O CP/MAS)37 to study surface sites on alumina materials by selective enhancement of low-intensity resonances associated with surface moieties such as Al2OH sites. 1H NMR techniques such as SP/MAS (single-pulse with MAS) and CRAMPS (combined rotation and multiple pulse spectroscopy)38-40 offer additional, attractive means to examine directly the protons comprising the surface of the alumina material, as well as the various protons of the surface-adsorbed surfactant molecular ions, including those adjacent to the ionic head, methylene groups along the hydrocarbon chain, and terminal methyl groups. Bronnimann et al.38 utilized the 1H CRAMPS approach to examine surface moieties such as Si-OH and Si(OH)2 groups and “physisorbed” water on silica gel and silica-alumina materials. Fitzgerald et al.23 have carried out 1H NMR using the CRAMPS technique to study the proton population of a high surface area (230 m2/g) alumina (pseudo-boehmite) material (Al2O3‚2.05H2O), including measurements of solids obtained following thermal dehydration. Solid-state 1H CRAMPS approaches were used to identify two distinct 1 H NMR resonances (at about 3.0 and 7.8 ppm) that were assigned to two different types of surface and/or internal Al-OH moieties: six-coordinate, “isolated” AlOH sites and six-coordinate, “clustered” Al2OH sites, respectively. It was proposed that these two types of octahedral AlOH sites of the pseudo-boehmite material are analogous to the six-coordinate, “octahedral” IB and IIB sites of the surface model for γ-alumina (Figure 1) proposed by Peri.21 In addition to solid-state 1H NMR, 13C CP/MAS NMR approaches that have been widely utilized to examine (35) Morris, H. D.; Ellis, P. D. J. Am. Chem. Soc. 1989, 111, 60456049. (36) Huggins, B. A.; Ellis, P. D. J. Am. Chem. Soc. 1992, 114, 20982108. (37) Walter, T. H.; Oldfield, E. J. Phys. Chem. 1989, 93, 6744-6751. (38) Bronnimann, C. E.; Chuang, I.; Hawkins, B. L.; Maciel, G. E. J. Am. Chem. Soc. 1987, 109, 1562-1564. (39) Ryan, L. M.; Taylor, R. E.; Paff, A. J.; Gerstein, B. C. J. Phys. Chem. 1980, 72, 508-515. (40) Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1987, 110, 2023-2026.
Dodecyl Sulfate-Alumina Interfacial Interactions
organic compounds and polymers can also be used to study the adsorbed molecular surfactant ion of various aluminasurfactant systems. 13C SP/MAS and CP/MAS NMR approaches have been utilized by Ziegler and Maciel34 to examine the chain motion and configuration of the hydrocarbon chains in dimethyloctadecylsilane-modified silica (C18 silica) surfaces. SP/MAS experiments, in particular, have been useful when rapid molecular motions are present, whereas CP NMR techniques are most effective to examine more rigid solids in which a static component of the 1H-13C dipolar interaction is present, due to the lack of motion or the anisotropic character of the motion.34 The use of both 13C CP/MAS and SP/MAS NMR facilitates studies of the dynamics of the hydrocarbon interactions over a wide range of motional regimes in both dry and “wet” or solvated alkyl chains that are covalently bonded to silica surfaces. While 1H CRAMPS and 17O and 27Al CP/MAS NMR techniques have proved useful in examining proton, oxygen, and aluminum atoms of the structural Al-OH surface groups, 1H NMR using CRAMPS techniques offer the additional capability to detect protons of surface and/ or internal Al-OH groups, “physisorbed” water on the alumina surface, and protons of the surfactant hydrocarbon chains simultaneously. The work reported here describes 1H CRAMPS NMR38-40 studies of alumina materials containing adsorbed surfactant moieties following surfactant adsorption from aqueous solutions, using both “wet” and evacuated solid alumina/surfactant samples. The alumina material utilized in this study consists of a high surface area, partially hydrated alumina (pseudo-boehmite, R-AlOOH) material that contains a very large number of structural hydroxyl groups. The range of surfactant loading concentrations investigated spans regions II and III of the DDS-Al2O3 adsorption isotherm reported by Chandler et al.19 These 1H NMR studies have enabled us to simultaneously observe and quantify the proton signals of both Al-OH structural groups and adsorbed surfactant molecular ions, thus providing information about the surfactant loading levels and the hydrocarbon chain mobility, as well as the competition between water molecules and surfactant ions for adsorption onto the surface Al-OH structural groups.
Langmuir, Vol. 12, No. 8, 1996 1961 Table 1. Summary of Surfactant-Al2O3 Loading Concentration Based on Carbon Analysis
Chemical system
% carbon
(
mol of surfactant 1 g of Al2O3
mol of surfactant cm2 of Al2O3
Al-DDS-weta Al-DDS-drya Al-DDA-weta Al-DDA-drya Al-SO-weta Al-SO-drya
0.94 1.02 0.70 1.09 0.86 0.96
× 10 pH 3.0 6.53 7.08 4.86 7.57 3.98 4.44
Al-DDS (1 × 10-2)b Al-DDS (5 × 10-3)b Al-DDS (3 × 10-3)c Al-DDS (2 × 10-3)c Al-DDS (1 × 10-3)b
1.77 1.56 0.90 0.68 0.35
12.30 10.84 6.24 4.71 2.43
5.35 4.71 3.12 2.36 1.06
Al-DDS (1 × 10-2)c Al-DDS (5 × 10-3)c Al-DDS (3 × 10-3)c Al-DDS (2 × 10-3)c Al-DDS (1 × 10-3)c
6.25 3.56 2.38 1.87 1.19
pH 6.5 43.40 24.70 16.50 13.00 8.26
18.90 10.70 7.19 5.65 3.59
c
× 10
-5
)
-11
2.84 3.08 2.11 3.29 1.73 1.93
a C ) 1 × 10-3 M; pH 3.0. b Set no. 1 (see Experimental Section). 0 Set no. 2 (see Experimental Section).
alumina combustion boats and heated at 1100 °C for 24 h in a Lindberg single-zone tube furnace (54000 series). The samples were then transferred to a desiccator while warm (110-120 °C), allowed to cool to room temperature, and reweighed, and their water contents were calculated. The total water content calculated refers to the weight loss due to both surface “physisorbed” water and “trapped” internal water.38 Calculations of Surfactant Loading Level Based on Percent Carbon Analysis. The percent carbon analysis for alumina samples containing adsorbed surfactant were obtained from Huffman Laboratories in Golden, CO. A typical calculation of the surfactant loading levels (given in Table 1) in units of mol of surfactant/g of Al2O3 and mol of surfactant/cm2 (see Results and Discussion) is summarized below. For an Al2O3-DDS sample, wherein the %C was 0.94%, the moles of surfactant per gram of alumina were calculated using the expression:
mol of DDS/g Al2O3 )
(
)(
)
0.94 g of C 1 g of DDS × 100 g of Al2O3 0.5425 g of C 1 mol of DDS (265.83 g of DDS)
Experimental Section Adsorption of Surfactants. Two series of 2% wt/V suspensions of a high surface area alumina (HSA-Al2O3) material (Norton Lot. 08061, surface area 230 m2/g, average particle size 46 µm, pore volume > 0.5 mL/g) containing surfactants in the 10-2-10-3 M concentration range were prepared. The first series of samples (set no. 1) was prepared by stirring a suspension of alumina powder in a 0.02 M NaCl aqueous solution for 1 h, followed by the addition of one of the following three surfactant aqueous solutions: sodium dodecyl sulfate (SDS, Aldrich), sodium dodecylamine (DDA, Aldrich), sodium oleate (SO, J. T. Baker). The resulting slurries were then equilibrated at pH 3.0 or 6.5 for 23 h (adjustment of the solution pH was made by addition of 1.0 M HCl and/or 1.0 M NaOH solutions) and then centrifuged at 4000 rpm for 1 h. The supernatant was then decanted, and the “wet” solid was desiccated at ambient pressure (725.0 Torr) in air. In the second series (set no. 2), 2% wt/V suspensions were prepared with surfactants in the 10-2-10-3 M concentration range. The alumina powder was suspended in 0.02 M NaCl with stirring for 1 h, followed by addition of various DDS solutions of different concentrations. The slurries were equilibrated at pH 3.0 or 6.5 for 23 h and pressure filtered at 95 psi N2 using a 47 mm stainless-steel pressure filtration funnel (Gelman 4280), with a 45 µm Nylaflo nylon membrane filter. The wet solids were then dried in a desiccator at room temperature and ambient pressure (725.0 Torr) in air. Total Water Content of the HSA-Al2O3 Material. Duplicate samples of the HSA-Al2O3 material were weighed in tared
)(
) 6.53 × 10-5 mol of DDS/g Al2O3 For calculations of the mol of surfactant/cm2 of alumina, the mol of surfactant/g of alumina are divided by the surface area (2.3 × 106 cm2/g) of the alumina powder, e.g.,
mol of DDS/cm2 of Al2O3 )
(
)
6.53 × 10-5 mol of DDS × 1 g of Al2O3
(
1 g of Al2O3
)
2.3 × 10-6 cm2 of Al2O3
) 2.84 × 10
-11
mol of DDS/cm2 of Al2O3
Percent Surface Protons Based on the Particle Size and Surface Area. Calculations of the percent surface protons were carried out on the basis of the particle size and surface area of this material (mean diameter of 46.25 µm, 230 m2/g).23 Calculations based on the particle size, assuming spherical geometry, indicate that about 4.1% of the total protons are within the first five AlO6 layers from the surface. Calculations based on the surface area of this material indicate that ca. 27.5% of the total octahedral Al2O10 structural units are located within the first five layers from the surface. This difference is probably due to the porous nature of the pseudo-boehmite material employed
1962 Langmuir, Vol. 12, No. 8, 1996
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here, which contains channels or pores within the particles that are ill-defined at present.23 NMR Experiments. For 1H CRAMPS experiments, samples were placed into thick-walled 5 mm (o.d.) glass tubes (Wilmad PS241), evacuated at 6.5 mTorr for 24 h, and sealed off under vacuum. 1H CRAMPS spectra were obtained at 187 and 360 MHz on modified Nicolet NT-200 and NT-360 spectrometers, using the BR-24 pulse sequence41 and “home-built” probes. The 187 MHz CRAMPS spectra were obtained with (τ) spacings of 3.0 and 1.2 µs and π/2 pulses. The 360 MHz CRAMPS spectra were obtained with τ ≈ 1.4 and 3.1 µs π/2 pulses. MAS speeds were 1.2-1.5 kHz. All spectra were referred to the chemical shift of tetrakis(trimethylsilyl)silane (TTMSS) at 0.38 ppm (with liquid tetramethylsilane at 0.00 ppm). Samples were placed into thick-walled 5 mm o.d. glass tubes (Wilmad PS241), evacuated at 6.5 mTorr for 24 h, and sealed off under vacuum.
Results and Discussion Alumina-Surfactant Loading Concentrations. The surfactant loading levels of the various aluminasurfactant systems calculated on the basis of the percent carbon analysis of the solids are summarized in Table 1. The surfactant loading levels for the DDS-Al2O3 system span the ranges observed for regions II (Sads ∼ 3.5 × 10-13 to 8.0 × 10-11 mol/cm2, Ceq ∼ 5.5 × 10-5 to 2.5 × 10-4 mol/L) and III (Sads ∼ 8.0 × 10-11 to 7.5 × 10-10 mol/cm2, Ceq ∼ 2.5 × 10-4 to 1.8 × 10-3 mol/L) of the DDS-Al2O3 adsorption isotherm reported by Chandar et al.19 The first two series of samples represented in Table 1 compare the amount of dodecylsulfate ion (CH3(CH2)10CH2OSO3-, DDS), oleate ion (CH3(CH2)7CHCH(CH2)7COO-, SO), and dodecylammonium ion (CH3(CH2)10NH3+, DDA) adsorbed onto the alumina material, for both dry (evacuated) and “wet” (nonevacuated) solid systems. Calculation of the adsorption density data of Table 1 has been made on the assumption that the total amounts of surfactant ions determined from %C analysis are adsorbed only on the surface of the alumina material. The loading levels of adsorbed surfactant on the alumina material in the dry solid systems is generally higher than in the “wet” solid systems. This behavior may be due to the removal of “physisorbed” and/or extraneous water upon drying, in addition to the possible competition between the surfactant ions and water molecules for the hydroxyl groups on the alumina surface. Such competition could lead to a reduction in the surfactant adsorption levels for the “wet” samples. Comparison of the surfactant adsorption data for the various surfactant-solid systems indicates that in the “wet” system values the dodecyl sulfate ion is adsorbed onto the alumina surface at higher loading levels than dodecylammonium ion or oleate ion at similar Ceq, whereas in the dry systems, the dodecylammonium-Al2O3 system shows the highest levels of adsorbed surfactant. This observation may be due to differences in the degree of hydration of alumina in the presence of different surfactants. The DDA ion, because of the electrostatic repulsion experienced between the positively charged alumina surface and the positive NH3+ head of the surfactant ion, is not expected to adsorb appreciably onto the alumina surface at low pH. The fact that the number of surfactant molecular ions in the double layer that have a charge of the same sign as that of the surface is limited also accounts for the low adsorption of DDA at lower pHs.15 However, the fairly high loading levels observed for the DDA ion adsorbed on the alumina material seen here suggest that interactions with the alumina surface may occur by weak van der Waals forces with the aliphatic hydrocarbon chain of the surfactant, or by coadsorption with chloride ions. (41) Burum, D. P.; Rhim, W. K. J. Phys. Chem. 1979, 71, 944-956.
Figure 2. 187 and 360 MHz 1H CRAMPS spectra of pseudoboehmite (Al2O3‚2.05H2O) equilibrated at pH 6.5 and heated at 110 °C for 5 h.
The next two series of adsorption results given in Table 1 summarize the variations in surfactant adsorption as a function of the dodecyl sulfate ion loading concentration at two different suspension pHs (3.0 and 6.5) over the 10-2-10-3 M range of surfactant loading concentrations (C0). An increase in C0 increases the number of moles of surfactant adsorbed onto the alumina surface for both the pH 3.0 and 6.5 series of samples. The amount of surfactant adsorbed on the samples equilibrated at pH 6.5 is appreciably higher than for those samples equilibrated at pH 3.0. These results are not in agreement with the work of Modi and Fuerstenau,15 who showed that negatively charged surfactants are preferentially adsorbed at low pHs onto a low surface area R-alumina (corundum). A possible reason for the decreased adsorption onto the high surface area pseudo-boehmite material equilibrated at pH 3.0 may be the fact that this alumina material undergoes partial dissolution by acid neutralization at pH 3.0, with the concomitant formation of a gel-like material. Partial gelation of the alumina material by acid neutralization might be expected to decrease the surfactant adsorption because of both a reduction in the hydroxyl group population and/or a reduction in the surface area of the alumina material following acid neutralization. 1 H CRAMPS NMR Results. High Surface Area Alumina. The 187 and 360 MHz 1H CRAMPS spectra for the high surface area alumina pseudo-boehmite material equilibrated at pH 6.5, following heating at 110 °C for 5 h, are shown in Figure 2.23 The 1H CRAMPS spectra show resonances centered at about 7.8 and 3.0 ppm associated with two distinguishable types of internal and/or surface Al-OH moieties. These two types of “aluminol” sites have been previously assigned to two different six-coordinate aluminum Al2OH and AlOH sites,23 respectively, similar to those proposed in the γ-alumina surface model by Peri.24 Previous studies have shown that increases in the magnetic field of the 1H CRAMPS measurements from 4.39 to 8.45 T leads to increases in the resolution of the proton peaks for this alumina material, probably due to narrowing of the observed resonances as a result of decreased quadrupolar interference with MAS averaging of the 27Al-1H dipolar interactions.23 General Features of 1H CRAMPS NMR Spectra for Various of Alumina-Surfactant Systems. Figure 3 shows the 187 and 360 MHz 1H CRAMPS spectra of a DDSAl2O3 solid (C0 ) 5 × 10-3 M, pH 6.5) obtained following desiccation and evacuation. The increased magnetic field
Dodecyl Sulfate-Alumina Interfacial Interactions
Figure 3. 187 and 360 MHz 1H CRAMPS spectra of Al2O3SDS (C0 ) 5 × 10-3 M) system.
Figure 4. 360 MHz 1H CRAMPS spectra of solid sodium oleate, sodium dodecyl sulfate, and Al2O3-SO (C0 ) 1 × 10-3 M) and Al2O3-SDS (C0 ) 1 × 10-3 M) systems.
of the NMR measurements results in a reduction in the effect of the 27Al quadrupolar interactions, causing narrower peaks and increased peak resolution for the AlOH proton peaks. The 187 and 360 MHz 1H CRAMPS spectra show two peaks at 3.0 and 1.4 ppm attributed to the C1 and C12-C2-11 protons of the dodecyl sulfate ion, respectively, and a resonance at 7.8 ppm associated with Al2OH surface protons moieties (vide infra). The 1H CRAMPS spectra of the “wet” solid show two resonances at 7.8 and 1.4 ppm associated with the Al2OH and C2-11 protons, respectively, in addition to a broad resonance at 4 ppm assigned to the protons of “physisorbed” water (vide infra). Evacuation or heating in the 110-120 °C temperature range has been reported to eliminate the presence of physically adsorbed water from alumina surfaces.23,42 The 360 MHz 1H CRAMPS spectra of the solid surfactant reagents, sodium oleate and sodium dodecyl sulfate, and the spectra of DDS-Al2O3 and SO-Al2O3 solids, are shown in Figure 4. The peak centered at 6.0 ppm in the 1H CRAMPS spectrum of sodium oleate is assigned to the olefinic C7-8 protons and the signal at about 2.5 ppm to the C1 protons adjacent to the carboxyl group (CH2COO-); the small shoulder at 2.0 ppm is assigned to the C6 and C9 protons, while the resonance at 1.0 is attributed to the (42) DeBoer, J. H.; Fortuin, J. M. H.; Lippens, B. C.; Meijs, W. H. J. Catal. 1963, 2, 1-7.
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C18 (methyl group) proton and the 1.5 ppm resonance is assigned to the C2-5 and C10-17 protons. In the 1H CRAMPS spectrum of sodium dodecyl sulfate, the peak centered at 3.5 ppm is assigned to the C1 protons adjacent to the sulfate group (CH2OSO3-), while the intense resonance at 1.5 ppm is due to C2-11 protons, and the small shoulder at 1.0 ppm due to the methyl group protons (C12). These peak assignments are in good agreement with the reported solution 1H NMR chemical shifts for the corresponding proton resonances of pure sodium dodecyl sulfate43 and sodium oleate.44 The 1H CRAMPS spectra of the “wet” sample of the DDS-Al2O3 solid shown in Figure 4 reveal a single broad peak centered at about 5.0 attributed to physically adsorbed water23,38 and another peak at 7.0 ppm assigned to the protons of the Al2OH surface moieties of pseudo-boehmite.23 The C1 signals are presumably masked by the broad proton peak due to “physisorbed” water.34 The 1H CRAMPS spectra of the “wet” SO-Al2O3 solid features four distinguishable peaks: a sharp peak at about 1.5 that probably includes the protons of the C2-5, C6, C9, C10-17, and C18 carbons, a small shoulder at 2.5 ppm assigned to the C1 protons, a broad resonance centered at about 5 ppm attributed to the “physisorbed” water that probably masks the olefinic C7-8 proton signal (6.0 ppm), and a shoulder at about 7.5 ppm assigned to the Al2OH surface groups. Figure 5 shows the 1H CRAMPS and 1H SP/MAS spectra of various “wet” alumina-surfactant solid systems. The solution 1H NMR spectrum43 (not shown) of sodium dodecylamine exhibits three resonances centered at 2.69, 1.28, and 0.9 ppm assigned to the C1 (CH2NH2), C2-11 (CH2) and C12 (CH3) protons, respectively; the 1H CRAMPS NMR spectrum of DDA-Al2O3 solid is shown in Figure 5. This spectrum consists of a broad peak that includes two distinguishable contributions at 7.5 and at 4.0 ppm due to surface and internal Al2OH moieties and “physisorbed” water,23 respectively, in addition to a peak centered at 1.5 ppm probably due to the C2-11 and C12 protons. The C1 signal expected at ca. 2.7 ppm is likely to be masked by the broad band of the “physisorbed” water protons centered at 4.0 ppm. The 1H SP/MAS NMR spectrum of the DDAAl2O3 system depicts a single resonance at 6 ppm probably due to a combination of “physisorbed” water and Al2OH proton-containing moieties and a peak at 1.5 ppm assigned to the aliphatic hydrocarbon chain protons of DDA. The 1 H CRAMPS and 1H SP/MAS NMR spectra of the SOAl2O3 system show distinctly different results. The 1H CRAMPS spectrum consists of a shoulder at 7.0 ppm, a broad resonance at 5.0 ppm, and a peak at 1.5 ppm previously assigned to the Al2OH group protons, “physisorbed” water protons, and C18-C2-5-C10-17 protons, respectively, whereas the 1H SP/MAS spectrum shows only the resonances associated with the “physisorbed” water (which probably masks the 2.7 ppm peak) and C18C2-5-C10-17 protons. The 1H CRAMPS spectrum of the DDS-Al2O3 also shows three resonances centered at 7.0, 5.0, and 1.5 ppm attributed to the Al2OH group protons, “physisorbed” water protons, and the C12-C2-11 protons, respectively. The 1H SP/MAS also exhibits peaks due only to the “physisorbed” water and C12-C2-11 protons. The 1H CRAMPS and 1H SP/MAS NMR spectral results shown in Figure 5 indicate that MAS line-narrowing capabilities of the SP/MAS technique executed with the MAS speeds employed in this study (∼3 kHz) are sufficient (43) Pouchert, C. J. The Aldrich Library of NMR Spectra, 2nd ed.; Aldrich Chemical Co., Inc.: Milwaukee, 1983; pp I 241B, II 805A. (44) The Sadtler Standard Spectra; Sadtler Research Laboratory, Sadtler, Inc., New York, 1972; p 7451. (45) Dec, S. F.; Bronnimann, C. E.; Wind, R. A.; Maciel, G. E. J. Magn. Reson. 1989, 82, 454-466.
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Figure 5.
1H
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CRAMPS and 1H SP/MAS NMR spectra of Al2O3-DDA, Al2O3-SO, and Al2O3-SDS wet systems.
to decouple the 1H-1H dipolar interactions in the surfactant protons (1-3 ppm), but are insufficient to decouple the strong 1H-1H dipolar interactions associated with the Al2OH groups (7.0-7.5 ppm).23 This would explain the absence of a peak for the Al2OH protons in the 1H SP/ MAS spectra of the various surfactant-Al2O3 systems studied. The SP/MAS experiment with modest MAS speeds has been found to successfully average out weak 27 Al-1H, 27Al-27Al, and 1H-1H dipolar interactions and to average 1H chemical shift anisotropies,46 but does not average the strong 1H-1H dipolar interactions. In the 1H CRAMPS experiment, a multiple pulse sequence (in the present case, the BR-24 sequence) averages out strong homonuclear 1H-1H dipolar interactions, thereby increasing the resolution of the 1H NMR spectra.38 Solidstate 1H SP/MAS NMR thus provides complete results only if the 1H dipolar interactions in the sample are weak, e.g., because of low 1H concentration, fast motional averaging, or large 1H-1H distances,38,45 or if extremely high MAS speeds become available. 1 H CRAMPS NMR of Solids from the Adsorption Isotherm for the SDS-Al2O3 Systems. A series of 1H CRAMPS spectra of the DDS-Al2O3 solids are given in Figures 6 and 7 for a range of different surfactant loading concentrations (listed in Table 1). The 1H CRAMPS spectra presented correspond to alumina samples equilibrated in the DDS loading concentration range of 1 × 10-3 to 1 × 10-2 M at pH 6.5. A series of 187 MHz 1H CRAMPS spectra of the DDS-Al2O3 systems evacuated at 6.5 mTorr for 24 h, presented in Figure 6, show a peak at about 7.8 ppm associated with the protons of the Al2OH surface moieties and two peaks at about 3.0 and 1.4 ppm attributed to the sodium dodecyl sulfate C1 and C12C2-11 protons, respectively. The increased intensity of the C1 and C12-C2-11 peaks as C0 is increased from 1 × 10-3 to 1 × 10-2 M shows that 1H CRAMPS spectra may be used to monitor the adsorption density of surfactant ions on the alumina material over the loading levels studied. The C12-C2-11/Al2OH peak area ratio, obtained by deconvolution/integration of these two peaks, was 0.48/ 1.00, 0.78/1.00, 0.87/1.00, 1.25/1.00, and 2.94/1.00 for the surfactant loading concentrations studied, indicating that the surfactant adsorption density may be related to (46) Cheetan, A. K.; Day, P. Solid State Chemistry Techniques; Clarendon Press: Oxford, 1987; Chapter 6.
Figure 6. 187 MHz 1H CRAMPS spectra of the Al2O3-SDS systems evacuated at 6.5 mTorr for 24 h: (a) 1 × 10-3 M, (b) 2 × 10-3 M, (c) 3 × 10-3 M, (d) 5 × 10-3 M, and (e) 1 × 10-2 M.
increases in solution C0 values in the adsorption isotherm range studied. A series of 1H CRAMPS NMR of the nonevacuated DDS-Al2O3 solids is shown in Figure 7. The spectra show peaks at 7.8 and 1.4 ppm associated with the Al2OH and C2-11 protons, respectively, plus an additional broad feature centered at about 4 ppm assigned to “physisorbed” water on the alumina surface.23 The increase in the relative peak intensity of the surfactant protons at higher surfactant loading levels, together with a decrease in the
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Figure 8. 187 MHz 1H CRAMPS dipolar dephasing spectra of the Al2O3-SDS (C0 ) 2 × 10-3 M) system following evacuation at 6.5 mTorr for 24 h.
Figure 7. 187 MHz 1H CRAMPS spectra of the Al2O3-SDS systems desiccated at 25 °C: (a) 1 × 10-3 M, (b) 2 × 10-3 M, (c) 3 × 10-3 M, (d) 5 × 10-3 M, and (e) 1 × 10-2 M.
intensity of the “physisorbed” water proton resonance, suggests that competition exists between the surfactant ions and water for hydroxyl adsorption sites on the alumina surface. 1H-1H dipolar dephasing experiments, as described elsewhere,38 have been previously employed to qualitatively determine the relative strength of the 1H-1H dipolar coupling between protons of “physisorbed” water,23,38 “silanol”,34 and structural Al-OH groups.23 The results of a dipolar dephasing experiment for the DDS-Al2O3 system (1:4 ratio) are shown in Figure 8. This series of 1H NMR spectra shows that a dephasing period (2τ value) of 20 µs is required to eliminate the Al2OH resonance, whereas the C1 and C12-C2-11 resonances disappear at 2τ values of 40 and 160 µs, respectively. The disappearance of the resonance due to Al2OH moieties at smaller 2τ values indicates that this species is involved in stronger 1H-1H dipolar interactions (smaller or more rigid internuclear vectors), such as in hydrogen bonding situations; by contrast, the C1 protons are somewhat more “rigid” than the C12-C2-11 protons due to their proximity to the sulfate group directly associated with the alumina surface. The 2τ value of 160 µs required to eliminate the C12-C2-11 signal suggests that the 1H-1H dipolar interactions of these protons are weaker, possibly due to the increased mobility of the protons of the aliphatic chains. This NMR result is consistent with a model whereby the aliphatic chains of the adsorbed surfactant would not be lying parallel to the alumina surface in the isotherm concentration region studied here. Such previously proposed models by Chandar et al.19 are consistent with the hydrocarbon chains of the surfactant being perpendicular to the surface of the alumina particle, possibly in a micellar-type association. Such micellar aggregates are probably not large enough to keep the chains rigid, but
would facilitate their presence in a rather semimobile state as observed for the aliphatic chains on C18-modified silica gels.47,48 However, a more detailed understanding of the orientation of these aliphatic chains of the adsorbed DDS surfactant ions on the alumina surfaces studied here would require line width analysis of solid-state 2D NMR or relaxation studies by 13C CP/MAS NMR approaches such as those used to study the alkyl chains of C18-modified silica gels as reported by Maciel et al.47,48 13C CP/MAS NMR Results. Solids from Adsorption Isotherm for the SDS-Al2O3 System. Several of the identical samples from the DDS-Al2O3 series, prepared by equilibration in 2 × 10-3 to 1 × 10-2 M DDS loading concentration range at pH 6.5, were also examined by solid-state 13C CP/MAS NMR. The 13C NMR spectra for the three samples examined are shown in Figure 9 and correspond to the following loading levels in 10-5 mol of DDS/1 g of Al2O3 (10-11 mol of DDS/cm2 of Al2O3): 43.40 (18.90), 16.50 (7.19), and 13.00 (5.65), identical to the values shown in Table 1. The 13C CP/MAS NMR spectra were obtained at 64.743 MHz at 2.1 kHz sample spinning, using 7.5 µs pulses, 2.00 s delay times, with a contact time of 1.00 ms and 40 000 transients. The 13C CP/MAS NMR spectra show the presence of five peaks at 16, 24, 27, 30, and 74 ppm. These resonances are assigned to the C12 terminal methyl carbon, the C11 methylene carbon, the C3 methylene carbon, the eight C2, C4-C10 methylene carbons, and the C1 methylene carbon adjacent to the sulfate group on the chemically adsorbed DDS. The relative signal intensities of the 13C CP/MAS NMR peaks increase with loading levels as expected from the reported loading levels; however, no significant changes were observed in the chemical shifts or the peak line widths of the 13C NMR resonances as a function of surfactant loading levels to warrant continued study by this approach. However, more detailed 13C CP/MAS NMR studies of the SDS-Al2O3 system or other alumina/ surfactant systems at higher loading levels48 may be necessary to obtain additional information on both the mobility and possibly mode of association between these adsorbed surfactants on alumina surfaces. (47) Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1991, 113, 63496358. (48) Zeigler, R. C.; Maciel, G. E. J. Phys. Chem. 1991, 95, 73457353.
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Figure 9. 13C CP/MAS NMR spectra at 64.74 MHz for various Al2O3-SDS solids equilibrated at pH 6.5 and C0 values of (a) 2 × 10-3 M SDS, (b) 3 × 10-3 M SDS, and (c) 1 × 10-3 M SDS, followed by evacuation at 6.5 mTorr for 24 h.
Mechanisms of Alumina-Surfactant Adsorption Interactions. Previous studies of the adsorption of surfactants by alumina materials have demonstrated that negatively charged, long-chain surfactants are readily adsorbed at pHs below the alumina isoelectric point.15-19 The model of surfactant adsorption on metal oxides assumed here19 proposes that at low surfactant concentration the surfactant is associated with the alumina surface as individual ions by electrostatic interactions between the negatively charged surfactant ion and the alumina surface. Once the surfactant loading concentrations are elevated, the adsorbed surfactant ions adsorb to monolayer coverage. As the surfactant concentration is increased these aggregates increase in size, until nearly complete surface coverage occurs. Above monolayer coverage, the formation of two-dimensional “hemimicellar” aggregates occurs by lateral interactions of alkyl chains through weak van der Waals forces. The adsorption density results for the DDS-Al2O3 system and the 1H CRAMPS spectra presented in this work suggests that, in the surfactant loading concentration range studied here, the adsorption of dodecyl sulfate ion by alumina at low and neutral pH is proportional to the surfactant loading concentration. The adsorption loading levels of dodecyl sulfate ion reported herein are larger at neutral pH than at low pH. This result may be contrasted with the work of Modi and Fuerstenau,15 who showed that decreasing the pH of the DDS-Al2O3 suspension at constant DDS loading concentration decreases the amount of adsorption of DDS by the alumina material. A possible reason for this disagreement is that Modi and Fuerstenau15 utilized a low surface area R-alumina (corundum), whereas this work is based on a high surface area pseudo-boehmite (hydrous alumina) material (R-AlOOH) that contains structural surface and internal Al-OH groups, as shown in Figure 2. Besides this difference, the pseudo-boehmite material used here experiences partial dissolution by acid neutralization at pH 3.0, forming gel-like materials. These materials exhibit decreased surfactant adsorption probably due to a reduction in the hydroxyl group proton population due to protonation and in the surface area. Conclusion The present work has examined the applicability of 1H NMR using SP/MAS and the CRAMPS technique to study
a high surface area alumina material that contains adsorbed surfactant molecular ions. The 1H CRAMPS results have demonstrated that this technique can be used to simultaneously examine the nature and population of the hydrogen environments of surface/internal Al-OH groups, “physisorbed” water, and various sites of the hydrocarbon chains for a range of surfactants such as sodium dodecyl sulfate, sodium oleate, and sodium dodecylamine. 1H CRAMPS experiments have shown that the adsorption of the dodecyl sulfate ion on the alumina surface, at both pH 6.5 and pH 3.0, increases as C0 is increased from 1 × 10-2 to 1 × 10-3 M. In addition, 1H CRAMPS experimental results may be used to monitor the surface Al-OH proton population following surfactant adsorption from aqueous solutions. These studies suggest that a competition exists between water molecules and surfactant ions for the alumina surface Al-OH groups. Dipolar dephasing experiments have also provided qualitative information about the 1H-1H dipolar interactions in the DDS-Al2O3 system. The mobility of the surfactant hydrocarbon chain is larger than the mobility of the protons of the Al2OH groups or protons of the “physisorbed” water. The dipolar dephasing results also indicate that the protons adjacent to the DDS anionic head are more rigid than the protons of the methylene groups along the hydrocarbon aliphatic chain and the terminal methyl groups. In addition, 13C CP/MAS NMR results have also shown that the NMR signal intensities can be related to the surfactant loading levels. However, limited changes in the 13C CP/MAS NMR line widths at the loading levels studies have not been fruitful to obtain information on the mobility, mode of association, and possibly orientation of adsorbed surfactants. Such questions, however, may be addressed using line width analysis of 2D MAS NMR results or relaxation studies using 13C CP/MAS NMR at different loading levels of various surfactant-alumina solid systems. Acknowledgment. The authors gratefully acknowledge partial support of this research by the National Science Foundation, Grants CHE-9021003 and RII8902066. In addition, the authors acknowledge the technical assistance of Dr. Mark Davis. LA950722H
