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On the Consequences of Surface Treatment on the Adsorption of Nonionic Surfactants at the Hydrophilic Silica-Solution Interface J. Penfold,*,† E. Staples,‡ and I. Tucker‡ ISIS Facility, CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K., and Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, U.K. Received October 22, 2001. In Final Form: February 5, 2002 Specular neutron reflectivity has been used to study the effects of different surface treatments on the adsorption of nonionic surfactants at the hydrophilic silica-solution interface. The consequences of different surface treatments, variations in solution pH, and the exposure to cationic surfactants illustrate the delicate nature of the cooperativity of the adsorption of nonionic surfactants at the liquid-solid interface.
As a result of the evident practical applications and the developments in experimental techniques, such as neutron and X-ray reflectivity,1-3 ellipsometry,4,5 and atomic force microscopy,6,7 there is a continued interest in the adsorption of surfactants at the solid-solution interface. A large number of studies, using a variety of experimental techniques, have been made on the adsorption of both ionic and nonionic surfactants, and there is good qualitative agreement about the main features, such as the cooperative nature of the adsorption isotherm. There is, however, a large variation in the absolute values of the adsorbed amounts, and this has been particularly commented on in a number of studies. In part, this is due to the large range of different surface treatments that are used. As part of a study of mixed ionic-nonionic surfactants adsorbed at the liquid-solid interface, we have made some systematic measurements of the effect of different conditions on the adsorption of the nonionic surfactant hexaethylene monododecyl ether, C12E6, at the hydrophilic silica-solution interface. The consequences of different surface treatments, variations in solution pH, and exposure to cationic surfactants illustrate the delicate nature of the cooperativity of the adsorption. The dominant interaction between nonionic surfactants and the hydrophilic surface of silica is due to hydrogen bonding of the ether oxygens of the ethylene oxide group and the surface OH groups. It is well established8,9 that the formation of silanol groups (Si-OH), when a surface is exposed to water, makes the surface hydrophilic. In contrast siloxane groups render the surface hydrophobic. The isoelectric point of silica occurs at a pH in the range 1-2. Above pH 2.0 the surface is net negatively charged, and below 1.0 it is either net positively charged8 or has zero surface potential, as there is no conclusive evidence † ‡
Rutherford Appleton Laboratory. Unilever Research.
(1) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162 196. (2) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 12, 325. (3) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. J. Langmuir 1997, 13, 6638. (4) Tiberg, F.; Johnsonn, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (5) Brink, J.; Tiberg, F. Langmuir 1996, 12, 5042. (6) Manne, S.; Gaub, H. E. Science 1995, 220, 1480. (7) Patrick, H. N.; Warr, G. S.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349. (8) Iler, R. K. The chemistry of silica; Wiley InterScience Publishers; New York, 1979 (9) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204.
that the silica surface is amphoteric. Above 9.0 the surface begins to dissolve. Reducing pH will reduce the surface charge density and control the density of hydroxyl groups on the surface, and we have previously shown that the adsorption of nonionic surfactants is highly pH dependent.1,3 At high pH (∼9.0) desorption occurs,1 and the adsorption at pH 2.4 is significantly greater than that at pH 7.0.3 A variety of different surface treatments are reported in the literature.1,9 Fragneto10 has described in detail the controlled formation of a thin oxide layer in hydrogen peroxide solution, following the passivating and etching of the surface in HF, the subsequent formation of a hydrophilic surface (using either Piranha solution or RCA1 and RCA2 solutions), and the removal of organic contaminations using a UV ozone treatment (see footnote 11 for details of the different surface treatments). Atkin et al.12 describe a different strategy for surface preparation in their study of the effect of pH, salt, and surface preparation on the adsorption of hexadecyltrimethylammonium bromide, C16TAB, at the silica/solution interface using optical reflectometry. They show that varying pH from 6.5 to 10.0 has little effect on the C16TAB adsorption, even though a greater proportion of the surface hydroxyl groups are ionized. Comparison of hydroxylated and pyrogenic surfaces showed similar adsorption at low surfactant concentrations but greater adsorption for the pyrogenic surface at higher concentrations. This was attributed to a reduced density of hydroxyl groups but with a greatly increased degree of ionization. Measurements of adsorption isotherms4,5 and of the structure of the adsorbed layer1-3 confirm the cooperative nature of the adsorption. There is little or no adsorption below the solution critical micellar concentration, cmc, and above the cmc saturation adsorption is reached very rapidly with increasing concentration. The structure of the adsorbed layer resembles an aggregated structure. (10) Fragneto, G. D. Phil Thesis, Oxford, 1996. (11) Details of some of the different surface treatments reported are as follows. Piranha treatment:9 7:1 H2SO4/H2O2 (30%) at 80 °C, but can lead to sulfur contamination. RCA1:9 NH4OH (40%)/H2O2 (30%)/H2O in the ratio 1:1:5 at 70 °C, is designed to remove organic and metallic contamination. RCA2:9 HCl/H2O2 (30%)/H2O in ratio 1:1:5 at 70 °C will remove alkali ions and cations. Mild Piranha treatment: 4:1:5 H2SO4/ H2O2 (30%)/H2O at 80 °C. Pyrogenic surface:10 Baking at 100 °C in oxygen atmosphere, then soaking in water. Baking forms siloxane bonds and lower density hydroxyl groups. It is more highly charged when immersed in water, as the reduced hydroxyl group density is more greatly ionized. Surface is rehydroxylated by soaking in water, followed by cleaning in 10% NaOH and rinsing in water and ethanol. (12) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2000, 16, 9374.
10.1021/la011575s CCC: $22.00 © 2002 American Chemical Society Published on Web 03/19/2002
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Figure 1. Specular neutron reflectivity for 4.2 × 10-4 M h-C12E6/D2O/pH 2.4 at the hydrophilic silica-solution interface (4). The solid line is a fit to the model described in the text.
Figure 2. Specular neutron reflectivity for 4.2 × 10-4 M h-C12E6/D2O at the hydrophilic silica-solution interface: (b) at pH 2.4 and (O) at pH 7.0.
From neutron reflectivity measurements,1-3 it has been described as a “fragmented bilayer” or “flattened micelles”, and this is consistent with the lateral structure observed by atomic force microscopy (AFM).6,7 Lee et al.1 interpreted neutron reflectivity data for C12E6 adsorption on quartz as a bilayer structure with partial coverage (∼75%) at pH 5-6 and showed that desorption occurred at higher pH. Bohmer et al.13 used the same model to interpret the adsorption of a range of nonionic surfactants (from C12E6 to C12E25) at the hydrophilic silica-solution interface and obtained an adsorbed amount of 3.2 µmol m-2 for C12E6. McDermott et al.9 compared the adsorption of C12E6 onto different hydrophilic substrates, crystalline quartz and crystalline and amorphous silicon, and although the bilayer thickness was ∼50 Å and the area/molecule was ∼44 Å2, the coverage varied in the range 40-60% (amorphous quartz, 40% (1.36 mg m-2), 75% (2.44 mg m-2); (13) Bohmer, M. R.; Koopal, L. K.; Jansenn, R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 2228.
crystalline quartz, 60% (2.04 mg m-2); crystalline silicon, 60%). Penfold et al.,3,14,15 in neutron reflectivity studies on surfactant mixtures of C12E6/C16TAB and C12E6/SDS adsorbed at the hydrophilic silica/solution interface, have observed similar variations in C12E6 adsorption, dependent on pH, surface treatment, and surface history. At pH 7 fractional coverages of 0.15 (0.77 × 10-10 mol cm-2) and 0.09 (0.62 × 10-10 mol cm-2) were obtained for different surfaces, and 0.5 (4.88 × 10-10 mol cm-2) was obtained at pH 2.4. In contrast the fractional coverage for C16TAB was 0.73 (6.2 × 10-10 mol cm-2). Thirtle et al.16 have demonstrated the delicate nature of the adsorption of nonionic surfactants by comparing the adsorption of C12E6 on surfaces with different functionalities. For the hydro(14) Penfold, J.; Staples, E. J.; Tucker, I.; Thomas, R. K. Langmuir 2000, 16, 8879. (15) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. J.; Thomas, R. K. Int. J. Thermophys. 1999, 20, 19. (16) Thirtle, P. N.; Li, Z. X.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Sung, L. P. Langmuir 1997, 13, 5451.
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Figure 3. Variation of adsorbed amount, Γ (×10-10 mol cm-2), as a function of solution pH: (b) after “mild Piranha” treatment and pH from 2.4 to 7.0; (4) after “mild Piranha” treatment and pH from 7.0 to 2.4; (0) after “mild Piranha” treatment, pH from 7.0 to 2.4, and pH 2.4 to 7.0; (O) postexposure to sub-cmc C16TAB.
philic silica surface they obtain a bilayer structure with 25% coverage, whereas on a self-assembled monolayer of undecenyltrichlorosilane a monolayer with ∼60% coverage is observed. Hydroxylating that surface produced an extremely thin surface adsorbed layer, consistent with the molecules lying flat on the surface. In this paper we report some measurements, using specular neutron reflectivity, on the adsorption of the nonionic surfactant C12E6 at the hydrophilic silicasolution interface. We have made measurements that address the effects of pH and surface treatment on the adsorption. Measurements were made for h-C12E6 (CH3(CH2)11(OCH2CH2)6OH) at 2 × 10-4 M in D2O, The silica
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surface was prepared using a “mild Piranha” treatment (see footnote 11), and measurements were made for decreasing and increasing solution pH, in the range 7.02.4. Additional measurements were made as a function of pH, following exposure of the surface to a sub-cmc C16TAB solution. The specular neutron reflectivity measurements were made on the SURF reflectometer17 at the ISIS pulsed neutron source, using the “white beam time-of-flight” method. Measurements were made in the Q range 0.0120.4 Å-1, using a wavelength band of 1-7 Å, and three different glancing angles of incidence, 0.35, 0.8, and 1.8°. A sample geometry, which is now well established for studies at the liquid-solid interface, was used18 and where the neutron beam is incident at the liquid-solid interface by transmission through the crystalline silicon upper phase. Specular neutron reflection gives information about the concentration or composition profile in a direction perpendicular to the surface or interface on molecular scale, and the reflectivity is directly related to the Fourier transform of the neutron scattering length density of refractive index distribution. The data for the liquidsolid interface is analyzed by assuming a structural model and calculating the reflectivity using, as an alternative to the Fourier transform description, the exact optical matrix method.19 Details of the “bilayer” model used have been described elsewhere in detail,1-3,14,15 and the important parameters for this discussion are the fractional coverage and the adsorbed amount. All the reflectivity profiles measured were well described by the bilayer model used previously,1-3,14,15 and typical data and model fit at pH 2.4 are shown in Figure 1. At pH 7.0 the fractional coverage is ∼20-30%, within the range of values discussed earlier, and at pH 2.4 the fractional coverage is ∼40-50%, consistent with the previously reported values for pH 2.4.3 Repeated treatments with the “mild Piranha” solution produced reproducible levels of adsorption but not the highest amounts observed for hydrophilic silica. Consistent with our previous observations3,14,15 reducing the solution pH from 7.0 to 2.4 produced enhanced adsorption, and this is clearly evident from the increased visibility of the interference fringe, arising from the ∼50 Å adsorbed layer, in the reflectivity data (see Figure 2).
Figure 4. Specular neutron reflectivity for 4.2 × 10-4 M h-C12E6/D2O/pH 2.4 at the hydrophilic silica-solution interface: (b) preand (O) postexposure of surface to sub-cmc C16TAB solution.
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The variations in the adsorbed amount and its variation with pH depend very much upon the history of the exposure of the surface to the different solution pH values, and some typical trends are summarized in Figure 3. Starting at pH 2.4 and increasing the pH to 7.0 result in marked decrease in the adsorbed amount (from 3.43 × 10-10 to 1.12 × 10-10 mol cm-2). In a separate sequence of measurements, as a function of pH (also shown in Figure 3), a similar trend is seen from a series of measurements decreasing the pH from 7.0 to 2.4. However, subsequent measurements with increasing pH show a marked hysteresis, and the decrease in adsorbed amount with increasing pH is much less pronounced. At pH 7.0 the adsorbed amount is 50% greater than its previous (or initial) value at pH 7.0 (1.9 × 10-10 mol cm-2 instead of 1.5 × 10-10 mol cm-2), and at all pH values the adsorption is higher than its value measured during the initial sequence. Exposure of the surface to solutions of low pH has produced a greater affinity of the nonionic surfactant for the surface than at the higher pH. Following the earlier arguments of Atkin et al.,12 we attribute this to a higher degree of ionization of the hydroxyl groups on the silica (17) Penfold, J.; et al. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (18) Penfold, J.; Staples, E. J.; Tucker, I.; Fragneto, G. Physcia B 1996, 221, 325. (19) Heavens, O. S. Optical properties of thin films; Butterworths: London, 1953.
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surface and not to any changes in the density of surface groups. Finally the surface that was subjected to a sequence of solutions of decreasing and increasing pH was exposed to a sub-cmc C16TAB solution. The concentration of C16TAB was such that no adsorption of the C16TAB was detectable by the neutron reflectivity measurements (consistent with a fractional coverage of less than a few %). However, the effect of the C16TAB exposure was dramatic, and this is illustrated in Figure 4, where the reflectivity profiles for 2 × 10-4 M h-C12E6 in D2O are shown for a solution pH of 2.4, before and after exposure to the sub-cmc C16TAB solution. At pH 2.4 the adsorbed amount is dramatically reduced, and the variation with increasing pH is not so pronounced (see Figure 3). At all pH values (between 2.4 and 7.0) the adsorbed amount is noticably reduced. We attribute this change in the affinity of C12E6 for the surface as arising from the blocking of sufficient charged hyroxyl groups by the submonolayer coverage of the C16TAB such that the cooperative adsorption is partially inhibited. This is clear evidence of the delicate nature of the cooperative adsorption of nonionic surfactants at the hydrophilic silica surface. Furthermore, it is clear from these measurements that the exact nature and details of the surface treatment and its history will critically affect its adsorption properties. Hence it is important to take such factors into account when comparing data from different studies. LA011575S
