Langmuir 1996, 12, 2263-2270
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Forces between Crystalline Alumina (Sapphire) Surfaces in Aqueous Sodium Dodecyl Sulfate Surfactant Solutions Zhenghe Xu,† William Ducker,‡ and Jacob Israelachvili* Department of Mining and Metallurgical Engineering, McGill University, Montreal, Canada H3A 2A7, Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand, and Materials Research Laboratory, College of Engineering, University of California, Santa Barbara, California 93106 Received October 26, 1995X Using the surface forces apparatus, we have studied the adsorption of negatively charged sodium dodecyl sulfate (SDS) surfactant onto positively charged surfaces of Al2O3 and the resulting interactions between these surfaces in aqueous solutions. The adsorbed layer thicknesses, adhesion forces, and long range colloidal interactions were measured at SDS concentrations from 0.01 to 5 mM (below the critical micelle concentration, cmc ) 8 mM). Our results show that an SDS bilayer of thickness ∼3.2 nm forms at bulk concentrations above 1 mM (g1/10(cmc)) and that beyond bilayer-bilayer contact the measured forces are well described by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (with a constant surface potential at low surfactant coverage and constant surface charge density at high coverage). The lack of any hydrophobic monolayer formation well below the cmc was noticed in the force measurements, confirmed indirectly by wettability studies, and attributed to the low surface charge density on the sapphire basal plane (52.7 nm2 per charge). With one exception, the results are in good agreement with previous data on similar systems obtained using other techniques. The results clarify how electrostatic binding interactions determine the stepped adsorption of, and transitions between, hydrophobic and hydrophilic surfactant layers. The results also provide a basis for understanding why certain surfactants (under appropriate solution conditions) can be used as effective additives in the colloidal processing of ceramic materials.
Introduction Surfactant adsorption at solid-liquid interfaces has long played an important role in many practical applications, including lubrication, detergency, mineral flotation and extraction of petroleum resources, pharmaceuticals and cosmetics, surface coatings, food science, and agriculture.1 More recently, colloidal processing of composite materials has become one of the most promising technologies for developing new special performance materials in which the modulation of colloidal interactions is crucial.2 Understanding the adsorption mechanisms and other properties of surfactant species is therefore of both practical and theoretical importance, and considerable efforts have recently been devoted to this area. The systems investigated typically include the adsorption and colloidal interactions of ionic and nonionic surfactants with organic and inorganic solid surfaces in aqueous or nonaqueous solutions.3-7 Concerning adsorption, both in-situ and ex-situ techniques have been useful in determining and characterizing surfactant adsorption. Complementary information is often provided by different techniques, such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), ellipsometry, photoacoustic spectroscopy, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, electron spin resonance, fluorescence, neu* Author to whom correspondence should be addressed at the University of California, Santa Barbara. † McGill University. ‡ University of Otago. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Myers, D. Surfactant Science and Technology; VCH Publishers, Inc.: New York, 1988. (2) Velamakanni, B. H.; Chang, J. C.; Lange, F. F.; Pearson, D. S. Langmuir 1990, 6, 1323. (3) Rutland, M. W.; Christenson, H. K. Langmuir 1990, 6, 1083. (4) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412. (5) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (6) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31. (7) Chen, Y. L.; Xu, Z.; Israelachvili, J. N. Langmuir 1992, 8, 2966.
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tron reflection and scattering, cyclic voltammetry, and microcalorimetry.8 More recently, the surface forces apparatus (SFA)3 and atomic force microscope (AFM)4 have been used to study surfactant adsorption and aggregation at solid-liquid interfaces. From the measured forces, the thickness, structure, and elastic properties of adsorbed surfactant layers can often be inferred.5-6 Earlier studies showed that, in the absence of chemical binding interactions, the driving force for the adsorption (self-assembly from solution) of a surfactant monolayer on a solid surface well below the cmc is mainly due to the electrostatic attraction between oppositely charged ionic or dipolar (zwitterionic) groups on the surfactant headgroups and the solid surface. The cooperative hydrophobic interaction between the hydrocarbon chains of the surfactant molecules leads to further adsorption close to the cmc, forming a bilayer of surfactant molecules restricted by steric packing constraints. The formation of full monolayers or bilayers often occurs at surfactant concentrations in the range 0.05-1.0(cmc). The adsorption of negatively charged SDS on aluminum oxide (Al2O3) at a solution pH much below its point of zero charge (PZC ) 8.0), where Al2O3 is positively charged, follows this adsorption pattern.9 The adsorption mechanisms of nonionic surfactants are less well understood. A limited number of studies3-4 suggest that hydrogen bonding and/or dipole interactions could be responsible for the adsorption. Similarly, little has been reported on the adsorption of ionic surfactants on neutral surfaces, which is complementary to the system of nonionic surfactant and charged surfaces. The present communication is aimed at exploring the adsorption and interactions of an ionic surfactant (SDS) at a polar but almost electrically neutral solid surfacesthe sapphire basal plane at neutral pH. (8) Sivakumar, A.; Somasundaran, P. Langmuir 1994, 10, 131 and references cited therein. (9) Wakamatsu, T.; Fuerstenau, D. W. Adv. Chem. Ser. 1968, 79, 161.
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The adsorption of SDS at alumina-aqueous interfaces is considered as a model system of metal oxide flotation and of interparticle interactions during ceramic processing and has been previously studied by the “adsorption from solution” method9,10 and by in-situ spectroscopic techniques.11 These early investigations, conducted at neutral pH, presented some discrepancies regarding the states of the adsorbed SDS surfactant molecules. To explain three distinct adsorption regimes, it was inferred9 that initially, at low SDS concentration, amphiphilic surfactant species adsorb as individual molecules (submonolayer adsorption) until about one tenth of the cmc, above which they adsorb as two-dimensional hemimicelles. However, a spectroscopic investigation11 showed that adsorbed SDS on a sapphire crystal is not in a pseudomicellar state but in one of the more condensed SDS phases such as the hexagonal liquid crystal, lamellar, or coagel state. A recent report10 further demonstrated that the type of counterions present (e.g., Li+, Na+, K+, or Cs+) has a significant effect on the amount and structure of the adsorbed SDS layers. In this article, we investigated the adsorption of SDS to a polar but almost electrically neutral solid surface of Al2O3 by measuring the forces between the surfaces using the SFA technique. This technique measures the force over large areas (500 nm2) and so can be used to infer properties over the same range. Wetting tests of Al2O3 surfaces with adsorbed surfactant were also conducted as a complement to the force measurements. The implications of the results for improving our understanding of surfactant adsorption mechanisms, flotation, and ceramic processing are discussed. Experimental Methods Preparation of Alumina Surfaces. Single-crystal aluminum oxide substrates (sapphire crystals) were prepared in the same batch as those used in previous SFA experiments.12-14 They were grown by vapor-phase condensation from a solution of aluminum oxide in lead fluoride. The crystals were several micrometers in thickness and up to a square centimeter in area. The exposed surface was identified as a single-crystal basal plane (0001) using electron diffraction and was confirmed to be Al2O3 by XPS characterization.14 Over areas of ∼10 µm2, the surfaces were found to be as smooth as mica with a peak-to-peak roughness of 0.2 nm when imaged by an AFM.14 No steps were found over any of the regions between which forces were measured. Similar observations were reported by Horn et al.12 The sapphire samples were first examined with a normal optical microscope to select pieces that appeared to be free of defects and that adhered spontaneously to mica. The selected samples were left in fuming nitric acid for 30 min to oxidize organic contaminants and to etch the surface. After being thoroughly rinsed in distilled water and ethanol and blown-dried with dry nitrogen gas, the thin sheets were placed on a large, freshly cleaved mica sheet to which they spontaneously adhered across the whole area of contact. The exposed surfaces were coated with a silver layer 53 nm thick to form a highly reflecting surface, required for viewing the surfaces using fringes of equal chromatic order (FECO).12 Two such sapphire sheets were glued onto curved silica disks with their silvered sides facing the disks, and the exposed surfaces were cleaned with UV irradiation for 30-40 min prior to mounting into the SFA. The UV radiation was found to reduce the water contact angle from 30° to 0°, presumably by decomposing any residual adsorbed organic contaminants. (10) Bitting, D.; Harwell, J. H. Langmuir 1987, 3, 500. (11) Cross, W. M.; Kellar, J. J.; Miller, J. D. Proceedings: XVII International Mineral Processing Congress, Dresden, Germany, 1991; Vol. 2, p 319. (12) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J. Mater. Res. 1988, 3, 413. (13) White, E. A. D.; Wood, J. D. C. J. Mater. Sci. 1974, 9, 1999. (14) Ducker, W.; Xu, Z.; Clark, D. R.; Israelachvili, J. N. J. Am. Ceram. Soc. 1994, 77, 437.
Xu et al. Table 1. Literature Values for the Minimum Surfactant Coverage Density (Expressed as Area per Molecule) Required To Develop a Hydrophobic Surfactant Monolayer
system SDS-Al2O3 SDS-sapphire CTAB-mica
bulk surfactant surface area concentration per molecule (mM) (nm2) 0.05 0.05 0.10
83 111 92
technique solution uptake9 FTIR/ATR11 SFA5
Force Measurements. Forces between alumina surfaces were measured using a SFA Mark III.15 This device enabled the force (F) to be measured between two curved surfaces of radius R ∼ 2 cm as a function of surface separation (D), which was measured using FECO interferometry. Under most conditions, the error in the distance measurement D was (0.2 nm and that in the force F was (0.3 µN. The separation between the surfaces was controlled to within 0.1 nm using micrometers, springs, and a piezoelectric transducer, as previously described.15 At the beginning of each series of measurements, the sapphire surfaces were first brought into adhesive contact in an atmosphere of dry nitrogen gas. The separation between the two reflecting sapphire surfaces (i.e., twice the sapphire sheet thickness) was measured, and this position was used to define the zero of separation, or “contact”, in air. The apparatus was then filled with deionized water, and the contact position was again checked. In water, a slight outward shift of the contact position by about 0.2 nm was generally observed, and this was taken as the reference for the contact position in water, which defines D ) 0 in the subsequent force runs in water and aqueous surfactant solutions. After each force run, a preprepared amount of SDS stock solution (1 mM) was injected through a microsyringe into the SFA chamber and then mixed to attain the next (higher) SDS concentration. To reach even higher SDS concentrations near the cmc (>1 mM), the apparatus was totally drained and refilled with bulk solution at the appropriate concentration, without changing the optical alignment. Each force run was conducted 30 min after the solution concentration was changed. All measurements were made at a temperature of 22 ( 1 °C. The thicknesses of the adsorbed SDS layers and their refractive index16 were also determined with an accuracy of ∼0.2 nm and (3%, respectively. Contact Angle Measurements. Contact angles were measured using sapphire surfaces treated in the same way as those used in the surface force measurements. Sapphire surfaces were immersed in SDS solution and allowed to equilibrate for 30 min. An air bubble, generated at the tip of a microsyringe, was then pushed against the sapphire surface from below, and the contact angle at the three-phase contact line was observed with an optical microscope. The SDS concentration was then increased by adding the required amount of stock solution into the contact angle cell, and the measurements were repeated. Chemicals and Preparation of Solutions. SDS was obtained from Sigma (99.99%) and used without further purification. KOH (99.999%) from Aldrich was used as received for adjusting solution pH. The water was purified by reverse osmosis, followed by ion exchange and activated charcoal, and then by double distillation. It had a typical conductivity of 18 Ω.
Results Adhesion in Air. First, the adhesion forces between two sapphire surfaces were measured in dry nitrogen gas. The pull-off force (F/R) was found to be in the range 6001000 mN/m, corresponding to a surface energy of γ ) 64110 mJ/m2 according to the Johnson-Kendall-Roberts (JKR) theory (γ ) F/3πR).17 This value is slightly higher than the value of γ ) 51 mJ/m2 reported by Horn et al.,12 (15) Israelachvili, J. N.; McGuiggan, P. M. J. Mater. Res. 1990, 5, 2223. (16) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (17) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301.
Forces between Crystalline Alumina Surfaces
Figure 1. Comparison of measured forces between two sapphire basal planes in water (open circle) with those measured in 0.01 mM SDS solutions (solid circles). The shaded band in the inset shows the theoretical van der Waals force curves: the upper and lower limits correspond, respectively, to Hamaker constants of 0.7 × 10-20 J (for hydrocarbon across water) and 6.7 × 10-20 J (for sapphire across water); the dotted line is the classical DLVO force for sapphire in water at a constant surface potential of ψ0 ) +18 mV.
suggesting that UV irradiation may have resulted in a cleaner and hence higher energy surface. With these measured surface energies, Hamaker constant (A11) values of 16 × 10-20 to 27 × 10-20 J were obtained using the equation A11 ) 24πγD02, where D0, the separation at contact, is set as 0.165 nm.18 These values are in the range of Hamaker constants expected for common metal oxides and are comparable to a reported value of 15.6 × 10-20 J for Al2O3,19 suggesting that the adhesion between two sapphire surfaces in inert air is determined mainly by van der Waals forces. Colloidal Forces in Water (No Added Surfactant). The forces measured in purified water (pH ) 7.2) are shown in Figure 1 (open circles). As shown in the inset of this figure, a weakly repulsive double-layer force was measured at large separations, and an attractive van der Waals force was found to dominate at separations less than 25 nm. At a separation of about 15 nm, the gradient of the attractive surface force exceeded the stiffness of the force-measuring spring, and at this point the surfaces jumped into adhesive “primary minimum” contact at D ) 0.2 nm. Horn et al.12 reported a similar force curve between sapphire surfaces in 10-3 M NaCl solution at pH 6.7. Forces in SDS Solutions (10 nm2) than in a micelles (typically 90%) of the counterions remain bound to adsorbed layers, thereby also neutralizing most of the surface charge. Criterion for Adsorption of a Hydrophobic Monolayer. Over the whole SDS concentration range studied, neither significant long range attraction nor strong short range adhesion was observed, although they have been reported between other adsorbed monolayers in aqueous electrolyte solutions. That hydrophobic SDS monolayers were not attained in this study was further ascertained from contact angle measurements. This seems to be inconsistent with a previous adsorption study9 and insitu surface characterization with FTIR/ATR.11 To understand these discrepancies, it is instructive to examine some previous work on SDS adsorption on alumina. In an adsorption study of SDS onto alumina powder,9 it was found that substantial association of adsorbed SDS molecules does not occur until a bulk SDS concentration of 5 × 10-5 M is reached, which is followed by a step change in the adsorption isotherm to a new surface coverage of 1.2 × 10-12 mol/cm2, corresponding to one molecule per 100 nm2. A similar stepped adsorption to a density of 1.1 × 10-12 mol/cm2 occurring at 8 × 10-5 M SDS was reported in a spectroscopic study of flat sapphire in SDS solutions.11 These coverages correspond to a SDS surface concentration of 8 mM (dcmc) on alumina surfaces, which suggests that the formation of a full hydrophobic monolayer will not occur unless association of the adsorbed surfactant is enhanced by electrostatic binding or specific interactions between the surfactant and surface ionic groups. The requirement of preadsorption of surfactant to a critical surface coverage for the development of a hydrophobic monolayer can be further illustrated using the mica-CTAB aqueous system, as discussed below. In dilute surfactant solutions (much below the cmc) where surfactants usually adsorb as individual molecules, the concentration of surfactant at the surface can be calculated using the Stern-Grahame equation:23
number of adsorbed molecules Γ ) ) volume of interfacial region r c exp(-zFψ0/RT) (1) where Γ is the adsorption density of surfactant molecules per unit area, r the effective length of the molecules, z the number of charges per headgroup, c the bulk surfactant concentration, F the Faraday constant, ψ0 the electric potential of the surface, R the gas constant, and T the temperature. Equation 1 gives the magnification of the (23) Fuerstenau, D. W. In Principles of Flotation; King, R. P., Ed.; South Africa IMM: 1982; p 31.
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surfactant concentration at the surfaces over that in the bulk due to electrostatic effects. Using a surface potential of -122 mV for bare mica in distilled water as derived from a direct force measurement,3 the molecular length of r ) 1.6 nm for CTAB, and a bulk concentration of c ) 0.1 mM (,cmc), at which hydrophobic or submonolayer formation was previously observed, eq 1 predicts a surface concentration of 22 mM, which is 2-3 times the cmc. This value corresponds to an adsorption density of Γ ) 1.8 × 10-12 mol/cm2, which is in excellent agreement with the value of 1.2 × 10-12 mol/cm2 quoted above for Al2O3-SDS aqueous systems,9,11 illustrating the requirement of a minimum initial adsorption density, close to the cmc, for developing a dense hydrophobic surfactant monolayer. A similar calculation using eq 1 for the present system results in an adsorption density of 2 × 10-14 mol/cm2, corresponding to a surface concentration of only 1 mM (
