Langmuir 1997, 13, 7301-7303
Ellipsometric Study of Adsorption of Three Surfactants on a Titania Thin Film L. Luciani and R. Denoyel* Centre de Thermodynamique et Microcalorime´ trie du CNRS, 26 rue du 141e` me RIA, 13003 Marseille, France Received August 4, 1997. In Final Form: October 22, 1997
The adsorption of surfactants at the solid/liquid interface has been the subject of numerous studies in the last twenty years. In the case of hydrophilic minerals, and for single chain surfactants (ionic or nonionic), most authors have shown the formation of aggregates limited in size (called surface micelles or fragmented bilayers) at least at the level of the plateau of adsorption isotherms. Microcalorimetry has shown indeed that the energetics of these aggregates have similarities with that of bulk micelles1-4 whereas the aggregation numbers deduced from fluorescence decay experiments show that the aggregates have a limited size.5,6 More recently, neutron experiments (scattering7 or reflectivity8) and ellipsometry9 also lead to the picture of fragmented bilayers, since most results are well fitted by a model of the adsorbed phase having the thickness of a bilayer but with a lower density. Finally surface micelles were recently “seen” by AFM in the case of cationic surfactant adsorption on silica or mica.10 Only in the case of a double-chain surfactant, was a complete bilayer observed.10 Nevertheless AFM being not easily used in all conditions due to adsorption on both surface and tip, a technique like ellipsometry is useful for in situ experiments. Most conclusive results obtained in this way correspond to the adsorption of surfactants on the silica layer grown on a silicon wafer.9,11,12 With silica being negatively charged in the pH range where experiments are easy to carry out without too much risk of chemical reactions (say pH 2-10), the number of suitable adsorptives is then limited. The aim of the present work is to get an insight on the structure of surfactants adsorbed on a titania-covered silicon wafer. When in powder, titania has indeed a middle range isoelectric point (PIE around 5-6), which makes it a good sample for studying adsorption of anionic, cationic, and nonionic surfactants.13,14 Experimental Section Three surfactants were used in this work: a nonionic one (hexaethylene glycol monododecyl ether, purchased from NIKKO), an anionic one (sodium 4-dodecylbenzenesulfonate, pro* To whom correspondence should be sent. (1) Denoyel, R.; Rouquerol, F.; Rouquerol, J. In Adsorption from Solution; Rochester, C., Ed.; Academic Press: London, 1983. (2) Denoyel, R.; Rouquerol, J. J. Colloid Interface Sci. 1991, 143, 555. (3) Mehrian, T.; De Keizer, A.; Korteweg, A. J.; Lyklema, J. J. Colloid Surf. A 1993, 73, 133-143. (4) Lindheimer, M.; Keh, E.; Zaini, S.; Partika, S. J. Colloid Interface Sci. 1990, 138, 83. (5) Levitz, P.; El Miri, A.; Keravis, D.; Van Damme, H. J. Colloid Interface Sci. 1984, 99, 484. (6) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1988, 117, 31. (7) Cummins, P. G.; Staples, E.; Penfold, J. J. Phys. Chem. 1990, 94, 3740. (8) Mc Dermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas R. K.; Rennie A. R. Langmuir, 1992, 8, 1204. (9) Tiberg, F.; Jo¨nsson, B.; Tang, J.-A.; Lindman, B. Langmuir, 1994, 10, 2294. (10) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (11) Wa¨ngnerud, P.; Olofsson, G. J. Colloid Interface Sci. 1992, 153, 392. (12) Luciani, L.; Denoyel, R. J. Colloid Interface Sci. 1997, 188, 7580. (13) Fukushima, S.; Kumagai, S. J. Colloid Interface Sci. 1973, 42, 539. (14) Bohmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2649.
S0743-7463(97)00873-1 CCC: $14.00
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vided by IRCHA), and a cationic one (tetradecyltrimethylammonim bromide, provided by SIGMA). They will be called C12E6, ABS, and TTAB, respectively. The critical micelle concentrations (cmc’s) are 80, 1500, and 3300 µmol‚kg-1. C12E6 and ABS were used as received whereas TTAB was recrystallized from ethanol/acetone mixtures. Adsorption was studied on a thin film of titania (10 nm) which was deposited by sputtering on the native silica layer of a silicon wafer (sample prepared by Y. Wang and I. Rivory, Laboratoire d'Optique des Solides, Universite´ Pierre et Marie Curie, Paris). The wafer was cleaned by a UV ozone treatment. Ellipsometric measurements were made with a Rudolph apparatus type 43603-200E, already often described in numerous papers.9,11,16 This is a null ellipsometer with a horizontal polarizer, compensator, sample, and analyzer (PCSA) arrangement. All details on the null procedure can be found in the book by Azzam and Bashara.15 The sample was maintained vertically in a trapezoid quartz cell (Thuet et Bichelin, Strasbourg), allowing experiments at an incidence angle of 68°. The wavelength was 632.8 nm. The thickness of the oxide layer (TiO2) was determined in air or solvent by the four-zone method.15 After the measurement in solvent, which gives initial ∆ and Ψ values, one zone is chosen and the polarizer and analyzer are set in the null conditions. A stock solution of surfactant is then added, and the intensity of light detected by the photomultiplier tube is recorded as a function of time. The next injection is made when equilibrium is achieved. It may be either another injection of surfactant or an injection of acid (HCl) or base (NaOH) to modify the pH. When needed, null conditions are again established to calculate final ∆ and Ψ values. The refractive index and the thickness of the surfactant layer are then calculated by using a four-layer model, since the adsorbed layer is formed on a layer of titania deposited on the native silica of a silicon wafer. The surface concentration can be calculated by the following equation:16
Γ)
d(n - n0) dn dc
where n0 is the solvent refractive index and dn/dc is the refractive index increment estimated from data in solution. In some cases, the surface concentrations were calculated by using the “offnull” method,12,17 which considers that for small thicknesses the surface concentration is proportional to the square root of the reflected intensity. All experiments are performed at 25 °C.
Results and Conclusions Examples of the adsorption isotherms of the three surfactants on the titania layer are given in Figure 1. The concentration axis is normalized toward the cmc. The adsorption isotherm of C12E6 is S-shaped, as is often the case with nonionic surfactants having a short ethoxy chain.12 The usual interpretation is that a few molecules are anchored by H-bonds with OH surface groups at low equilibrium concentration whereas just as the concentration is increased before the critical micelle concentration, micelle-like aggregates (or fragmented bilayers) are formed around the anchored molecules. For the two ionic surfactants the adsorption isotherms are very different, especially in the low concentration range where the slope is much higher. Here, neutralization of the surface charge by the ionic head of the surfactant can be invoked whereas an aggregation process occurs close to the cmc. This is clear in the case of the cationic surfactant TTAB, whose adsorption isotherm shows two clear steps, but for the anionic one, the steps of neutralization and aggregation are probably superimposed. (15) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; Elsevier Science Publishers B. V.: Amsterdam, 1989. (16) De Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759. (17) Arwin, H.; Welin-Klintstro¨m, S.; Jansson, R. J. Colloid Interface Sci. 1993, 156, 377.
© 1997 American Chemical Society
7302 Langmuir, Vol. 13, No. 26, 1997
Notes
Figure 1. Adsorption isotherms of C12E6, TTAB, and ABS on titania at pH 5.5, 6.5, and 3.3, respectively.
Figure 2. Surface concentrations at the plateau for the three surfactants as a function of pH.
As a whole, these results are consistent with standard descriptions found in the literature for adsorption either on divided solids14,18 or on surfaces,11 although the adsorption of pure ethoxy surfactants on divided TiO2 was not observed to our knowledge. This last point is an indication that the surface chemistry of the titania thin film, despite numerous similarities shown in the following, is not exactly that of crystalline powder samples, which are usually the rutile or anatase form of titania. What is interesting here is that, thanks to in situ ellipsometry, it is possible to study on the same sample the adsorption of three surfactants and to allow a parameter (here the pH) to be varied in a large range. (18) Koopal, L.; Lee, E. M.; Bo¨hmer, M. R. J. Colloid Interface Sci. 1995, 170, 85.
This is illustrated in Figure 2, where the surface concentration at the plateau is now plotted for each surfactant as a function of pH in the range 2-9. In the pH range 5-6, the anionic surfactant surface concentration decreases, the cationic one increases, and the nonionic one shows a maximum. This is the typical behavior of a surface having an isoelectric point (IEP) in that pH range. Indeed, close to the IEP, the surface density of neutral OH groups is maximum (allowing nonionic surfactant adsorption) whereas at lower pH the surface is positively charged (allowing anion adsorption) and at higher pH it is negatively charged (allowing cation adsorption). Thicknesses, when measurable, are, as expected, close to that of a bilayer but slightly higher (5-6 nm) whereas surface concentrations, at least for the cationic and nonionic surfactants, are too small to correspond to a
Notes
bilayer, which leads to the conclusion of finite size aggregates. In the case of the anionic surfactant, the high surface concentration at low pH and the presence of two chains in the hydrophobic tail may justify the formation of a full bilayer in the low pH range. These branched surfactants are also known to form lamellar phases in solution due to their middle range packing ratio.19 At this point it is interesting to discuss the meaning of the ellipsometrically measured thicknesses. It is difficult to get a reproducibility better than 1 nm, and these thicknesses are calculated by assuming a model for the adsorbed phase (isotropic and homogeneous), which is an approximation difficult to quantify. Here, they range between 5 and 6 nm, which is somewhat high as compared with those for the two surfactant molecules in an extended conformation. (For example, we would get 4 nm for the cationic surfactant20). Nevertheless AFM images of TTAB adsorbed on silica show aggregates whose diameter is (19) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1525-1568.
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around 5.5 nm.10 A possible explanation is that surface aggregates are spherical, as an average, but can have more complex shapes like micelles, which can be ellipsoidal. The measured thickness is then an average of the various conformations. Finally, as underlined in recent papers,9,11 ellipsometry is a good tool for getting a better understanding of surfactant adsorption at the solid/liquid interface. The good correlation obtained here between the evolution of surface concentrations with pH and the IEP expected value is likely to be due to the fact that we are in the case where interactions between surfactant polar heads and the surface are mainly nonspecific, which corresponds to “weakly bound adsorbate-adsorbent systems” in the classification of Cases and Villieras.21 LA970873F (20) Van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier Science Publishers B. V.: Amsterdam, 1993. (21) Cases, J. M.; Villieras, F. Langmuir, 1992, 8, 1251.
