J. Phys. Chem. 1996, 100, 18133-18137
18133
Adsorption of Nonionic Surfactants on Silica Sol Particles: The Effects of Sol Type and Concentration, Surfactant Type, Concentration, and Temperature J. Penfold* ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom
E. Staples, I. Tucker, and P. Cummins UnileVer Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, United Kingdom ReceiVed: April 24, 1996; In Final Form: July 23, 1996X
Small angle neutron scattering has been used to investigate the nature of the adsorption of the alkyl polyoxyethylene ether (CnEOm) nonionic surfactants on Ludox silica sols. Variations in the adsorbed layer thickness and volume fraction of surfactant in the adsorbed layer with sol type, concentration, temperature, and surfactant type are observed. In particular, the structure of the adsorbed layer of the C16EO6 and C16EO8 surfactants (for which the bulk micellar phase is rodlike micelles) is similar to that found for the C12EO6 and C12EO5 surfactants.
Introduction The nature of the adsorption of surfactants, and especially nonionic surfactants, on solid surfaces is of considerable interest because of their importance to a number of industrial and technological processes associated with colloidal stability and detergency. As a result, there has been much research activity in the investigation of nonionic surfactant adsorption onto different sol materials1 and planar surfaces.2,3 Furthermore, a number of different of experimental techniques, which include small angle neutron scattering (SANS),4 neutron reflectivity,2,3 ellipsometry,5 fluorescence decay spectroscopy,6 and atomic force microscopy (AFM),7 have been used. Previous SANS studies on the adsorption of the nonionic surfactants C12EO6 and C12EO5 onto Ludox silica sols have confirmed the cooperative nature of the adsorption process.4 We have investigated the temperature and pH dependence of the adsorption process4 and demonstrated that the adsorbed layer remains intact with increasing temperature. The surfactantcoated particles have a lower consolute boundary, which is coverage dependent.8 The changes in the scattering data with increasing temperature were consistent with a rearrangement of the surface layer with temperature, and other differences in the adsorption behavior between C12EO6 and C12EO5 (9) were also reported. Neutron reflectivity has emerged as a powerful technique for the study of surfactant adsorption at planar liquid-solid interfaces.2,3 The results on the adsorption of C12EO6 at the quartz-water interface2 were consistent with SANS data4 but provide much more detailed information on the structure of the adsorbed layer. More recently, the pioneering work of Tiberg and co-workers5 has established ellipsometry as an important complementary technique for the study of adsorption at such interfaces. The technique is very sensitive to small changes in adsorption and, in particular, offers the possibility of measuring time-dependent or kinetic processes. In their most recent publication,10 they have made a systematic study of the effect of nonionic surfactant type on adsorption. The results on coverage, adsorbed layer thickness, and detailed structure from SANS,4,8,9 neutron reflection,2,3 and ellipsometry5,10 are in good agreement. Rutland and Senden7 X
Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)01183-5 CCC: $12.00
have, however, rationalized their AFM results for C12EO5 adsorption onto silica in terms of a detailed five-step process from single monomers to dense bilayers. Such details are not apparent from the other more direct methods. The techniques that study adsorption at the planar interface are able to provide either sensitive time-dependent information10 or to probe the structure of the adsorbed layer in some detail.2,3 However, SANS provides important complementary information, in that the effects of sol size, surface curvature, and sol concentration (the contribution from interparticle interactions) can be investigated. In real applications these are of course important issues. The nature of the surface is also crucial to the adsorption and has given rise to some variablity in the results obtained by many of the techniques used. In this study, we have used SANS to investigate the effects of sol size and surfactant type on the adsorption of the polyoxyethylene nonionic surfactants (for different alkyl chain and ethylene oxide chain lengths which have micelle geometry ranging from spherical to rodlike) onto Ludox silica sols in aqueous solution. Experimental Details The Ludox HS and TM silica sols were obtained from DuPont. The poly(oxyethylene glycol) nonionic surfactants C12EO6, C12EO8, C16EO6, and C16EO8 were obtained from Nikkol and were used without further purification. The pH of the dispersions was adjusted (by the addition of HCl) to a value of 8.0. The samples were contained in standard 1 mm path length quartz cells and were thermostated to an accuracy of (1 °C. The SANS measurements were made on the LOQ diffractometer11 at the ISIS pulsed neutron source, RAL, U.K. The white beam (broad range of incident neutron wavelengths, λ, of 2-10 Å) “time-of-flight” method was used to cover a scattering vector, Q, range (where Q ) 4π/λ sin θ/2 and θ is the scattering angle) of 0.007-0.2 Å-1. The data were corrected for instrumental factors, background, and adsorption at each temperature and were reduced to an absolute scattering cross section (dσ/dΩ, in cm-1) by reference to a standard scatterer.11 SANS data from the bare silicon particles (in the absence of surfactant) in H2O were used to obtain the particle size and polydispersity. The SANS measurements used to characterize the adsorbed layer were made with the Ludox silica sols and © 1996 American Chemical Society
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Penfold et al.
Figure 1. Scattering cross section, dσ/dΩ (in cm-1), for 20 mg/mL (b) C16EO8 and (O) C12EO8 on 3% TM Ludox silica sol. The solid lines are model fits described in the text.
surfactant dispersed in water which had a neutron refractive index matched to silica [H2O (0.39)/D2O(0.61)]. The scattering then arises only from the adsorbed layer of protonated surfactant and its associated hydration and any free surfactant micelles present in solution.4 The scattering data were analyzed using a standard and wellestablished model,4 and the important features are briefly described here. For a solution of globular polydisperse interacting particles, the coherent scattering cross section can be written by the approximation (assuming that there are no correlations between position, orientation, and size)12,13
dσ/dΩ|coh ) Np[S(Q)〈|F(Q)|2〉 + |〈F(Q)〉|2 - 〈|F(Q)|2〉] (1) (the so-called “decoupling approximation”), where the averages denoted by 〈 〉 are averages over particle size and orientation. Np is the particle number density, S(Q) the structure factor, and F(Q) the particle form factor. The adsorbed layer is modeled as a layer of uniform density,4 and hence the form factor is that of a sphere plus an outer shell
F(Q) ) V1(F1 - F2)F0(QR1) + V2(F2 - Fs)F0(QR2) (2) where Vj ) 4πRj3/3, F0(QR) ) 3j1(QR)/QR, and F1, F2, and Fs are the scattering densities of the core, shell, and sovent. The data are evaluated for this simple model by a least-squares criteria using eqs 1 and 2. A scale factor (data/theory) has been used to adjust the absolute scale of the theory (model) to the data and the systematic errors in the data and model dictate that the acceptable deviation from unity is (10%. Instrumental resolution (using the known combinations for ∆λ/λ and ∆θ/θ) and polydispersity (using form factors calculated for a Schultz distribution of particle sizes12) have been included in the model calculations. The structure factor, S(Q), is modeled using the analytic solution in the mean spherical approximation for a screened Coulombic potential.13 This is used just as a parmetrization of a hard core plus soft repulsive tail interaction or an attractive interaction. Measurements of the bare HS and TM sols in H2O were used to obtain the particle radius, R1, and the level of polydispersity and to verify the density (and hence the neutron refractive index matching condition of the solvent) of the sol particles. The
values of particle radius, R1 (78 Å for HS and 138 Å for TM sols), polydispersity, σ (15% for HS and 12% for TM sols), and density (3.4 g cm-3) were used throughout the modeling of the adsorbed layer. The scattering profiles, measured in the presence of excess surfactant, have an additional scattering (evident at high Q) due to the existence of “free” micelles in solution. At low micellar concentrations, we have simply approximated this additional scattering as a second-order polynomial. In the Q range measured this is a good approximation.4 A more complete treatment is required at higher micellar concentrations. This analysis is still in progress and is not reported here. The main parameters obtained from the model fits (see Tables 1-3) are the thickness of the adsorbed layer, d (where d ) R2 - R1) and its scattering density. From the thickness and scattering density obtained from the model fits (and the known surfactant and solvent molecular volumes and scattering lengths), the volume fraction of surfactant in the adsorbed layer (the fraction of the surface layer covered by surfactant) can be estimated.4 Results and Discussion The concentration dependence and temperature dependence of the adsorption of C12EO8 and C16EO8 onto TM Ludox sols at a fixed sol concentration of 3% were measured. The maximum concentrations of surfactant are such as to ensure saturation coverage in the absence of free micelles. Figure 1 shows the scattering data for the same surfactant concentration (20 mg/mL) of C12EO8, and C16EO8; the data and model fits are typical. The adsorbed layer parameters for both C12EO8 and C16EO8 are summarized in Table 1. The adsorbed layer thickness is, within error, independent of surfactant concentration and temperature, and the mean thickness for both C12EO8 and C16EO8 is 58 ( 5 Å. At the lower surfactant concentrations, there is no marked change in the adsorbed amount with temperature or surfactant type. This is consistent with the volume fraction of surfactant in the adsorbed layer being limited by the amount of added surfactant. At higher concentrations, the volume fraction of surfactant in the adsorbed layer is seen to increase with temperature. This indicates that at these concentrations and at the lower temperature, the adsorbed layer is in equilibrium with free micelles. As the critical micellar concentration, cmc, of these surfactants decreases with increas-
Adsorption of Nonionic Surfactants
J. Phys. Chem., Vol. 100, No. 46, 1996 18135
TABLE 1: Adsorbed Layer Parameters for C12EO8 and C16EO8 on 3% TM Ludox Sol
surfactant concn (mg/mL)
temp (°C)
6.0
25 40 50 25 40 50 25 40 50 25 40 50 25 40 50 25 40 50 25 40 50 25 40 50
10.0 19.0 26.0
5.0 10.0 20.0 25.0
adsorbed layer thickness, d (Å)
scattering density (×10-6, Å-2)
vol fraction of surfactant in the layer
(a) C12EO8 61.0 68.0 61.0 58.0 60.0 60.0 54.0 55.0 55.0 51.0 53.0 53.0
0.322 0.324 0.321 0.293 0.293 0.293 0.285 0.275 0.265 0.25 0.25 0.23
0.19 0.20 0.20 0.38 0.41 0.41 0.40 0.48 0.55 0.58 0.62 0.73
(b) C16EO8 50.0 55.0 54.0 59.0 59.0 60.0 58.0 65.0 66.0 53.0 60.0 62.0
0.31 0.31 0.31 0.285 0.285 0.285 0.257 0.235 0.235 0.23 0.22 0.22
0.20 0.22 0.22 0.40 0.41 0.42 0.63 0.86 0.89 0.67 0.88 0.92
TABLE 2: Adsorbed Layer Parameters for C12EO6, C12EO8, C16EO6, and C16EO8 on TM and HS Sols scattering vol fraction density of surfactant concn surfactant adsorbed layer (mg/mL) type thickness, d (Å) (×10-6, Å-2) in the layer 8.5
17.0
25.0
50.0
C12EO6 C12EO8 C16EO6 C16EO8 C12EO6 C12EO8 C16EO6 C16EO8 C12EO6 C12EO8 C16EO6 C16EO8 C12EO6 C12EO8 C16EO6 C16EO8
(a) 3.3% TM Sol 54.0 41.0 56.0 58.0 60.0 71.0 63.0 (b) 2.5% HS Sol 33.0 29.0 48.0 45.0 40.0 32.0 40.0 38.0
0.294 0.275 0.27 0.292 0.25
0.42 0.33 0.55 0.37 0.89
0.29 0.235
0.56 0.40
0.225 0.265 0.24 0.26 0.255 0.29 0.19 0.255
0.68 0.33 0.85 0.57 0.69 0.28 0.93 0.47
ing temperature, the observed temperature dependence of the adsorption implies a yet more rapid decrease in the free energy of the formation of the adsorbed species. This further suggests that the ethylene oxides adjacent to the surface become more “dehydrated” with temperature than those associated with free micelles. At the higher surfactant concentrations, there is a particularly marked increase in the volume fraction of surfactant in the adsorbed layer for C16EO8 compared to C12EO8. Table 2 summarizes the dependence on surfactant type (C12EO6, C12EO8, C16EO6, and C16EO8) of the adsorbed layer parameters for both HS and TM Ludox silica sols. There is an increased adsorption with increasing ratio n/m for the nonionic surfactants CnEOm such that the volume fraction of surfactant in the adsorbed layer for C16EO6 is significantly larger than for C12EO8. This is in agreement with the recent ellipsometry
Figure 2. Scattering cross section, dσ/dΩ (in cm-1), for 25 mg/mL (b) C12EO8, (O) C12EO6, (2) C16EO8, and (4) C16EO6 on 2.5% HS Ludox silica sol.
results10 at the planar interface. This is shown clearly in Figure 2, where the scattered intensity increases in the order C12EO8 f C12EO6 f C16EO8 f C16EO6 (the marked increase in scattered intensity at low Q in Figure 2 for C16EO6 is due to sol aggregation.). This adsorption sequence is observed for measurements made on both the TM and HS sols and indicates that the free energy of formation of the surface bound micelles follows that of the micelles in solution (that is, the cmc decreases with increasing n/m ratio). It is noteworthy that in the case of C16EO6, which has a cloud point of 32 °C, the surfactant-coated sol shows evidence of aggregation up to T ∼ 45 °C, while the adsorbed layer maintains its integrity. Figures 3 and 4 show a comparison of the scattering curves for C12EO6 and C12EO8 on a 2.5% HS sol and for C12EO6 and C16EO6 on a 3.3% TM sol. They illustrate (through the increases in scattered intensity) the changes in the volume fraction of surfactant in the adsorbed layer with surfactant type and further demonstrate the quality of the model fits obtained throughout this work. The increase in the volume fraction of surfactant in the adsorbed layer with increasing n/m ratio for the nonionic surfactants is also observed from neutron reflection experiments at the planar silica-water interface.14 In Table 2a for adsorption on the TM sol we see that the volume fraction of surfactant on the adsorbed layer of C12EO6 increases with added surfactant, while that of C16EO6 and C16EO8 is invariant, consistent with the coexistence of free micelles. This coexistence is observed in all cases for the HS sol (Table 2b). It appears that the C16EO6 adsorbs to a greater extent onto the HS than the TM sol. This comparison is under conditions in which there is sufficient surfactant available for complete coverage on both the HS and TM sols. Despite these measurements being made at a constant pH, it is likely that this difference reflects differences in the nature of the surface (for example, the charge density) rather than geometric factors associated with the sol. Within error, there are no systematic variations in adsorbed layer thickness with surfactant type on both the HS and TM sols. However, the adsorbed layer thicknesses are markedly larger on the TM sols, and this variation in thickness will be considered in more detail later in the discussion. Table 3 summarizes the dependence on sol and surfactant concentration of the adsorption of C16EO6 on the HS sol. The mean thickness of the adsorbed layer is 48 ( 5 Å and is, within
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Figure 3. Scattering cross section, dσ/dΩ (in cm-1), for 17 mg/mL (b) C12EO6 and (O) C16EO6 on 3.3% TM Ludox silica sol. The solid lines are model fits described in the text.
TABLE 4: Comparison of Adsorbed Layer Thicknesses from SANS, Neutron Reflection, and Ellipsometry SANS this study surfactant neutron type HS sol8 TM sol4,9 HS sol TM sol reflctn2,3,16 ellips10 C12EO5 C12EO6 C12EO8 C16EO6 C16EO8
Figure 4. Scattering cross section, dσ/dΩ (in cm-1), for 25 mg/mL (b) C12EO6 and (O) C12EO8 on 2.5% HS Ludox silica sol. The solid lines are model fits described in the text.
TABLE 3: Adsorbed Layer Parameters for C16EO6 on HS Sols, Variation of Sol and Surfactant Concentration sol concn (mg/mL) 0.55 0.78 0.78 1.25 1.85 2.5 2.5
scattering adsorbed surfactant vol fraction density layer thickness, concn of surfactant (×10-6, Å-2) in the layer d (Å) (mg/mL) 11.0 5.7 7.8 12.5 25.0 25.0 50.0
46.0 47.0 50.0 49.0 56.0 48.0 40.0
0.20 0.22 0.26 0.26 0.25 0.24 0.19
1.0 0.98 0.77 0.74 1.00 0.85 0.93
error, independent of sol or surfactant concentration. For the HS sol, higher volume fractions of coverage are achieved. Table 4 shows a comparison of the adsorbed layer thicknesses measured by different techniques (SANS, neutron reflectivity, and ellipsometry). Given the different experimental circumstances and the errors associated with the various measurement techniques, the agreement is remarkably good. There are some
44.0
45.0 40.0
38.0 31.0 48.0 41.0
57.0 58.0 63.0 58.0
40.0 50.0
42.0 41.0 44.0 52.0
unexplained discrepancies and the SANS data for C12EO6 here show the largest variation or inconsistency. Indeed, in ref 8 the thickness of the adsorbed layer of C12EO6 on the TM Ludox sols was systematically lower (∼30 Å) than in all the other reported measurements.4,9 Furthermore, in that case the adsorbed layer was thicker for the HS sols than for the TM sols and is at variance with the data in Table 2. In this series of measurements there is, however, a marked trend toward an increase in the layer thicknesses for all of the nonionic surfactants on TM Ludox sols compared to the HS Ludox sols. The different thicknesses of the adsorbed layers are all consistent with a bilayer structure on the sol surface. The origin of the increased thickness for the measurements on the TM sols and the variation in some of the measured values for the same systems is at present uncertain. The variations in thickness are not consistent with any contribution from changes in curvature of the sols (from HS to TM) or with any changes in geometry of the coexisting surfactant micelles. That is, it would be difficult to reconcile these differences with fundamentally different structural changes on the surface. Although the measurements were made at a controlled value of pH, some of the differences observed may well reflect changes in the surface properties of the different sols. In a previous study8 there was some evidence in the SANS data for the rearrangement of C12EO6 on the surface of Ludox sols. This appeared to be sol size, pH, and coverage dependent and consistent with the discrete surfactant patches becoming better defined. Subsequently, we made a direct comparison of the pH and temperature dependence of the adsorption of C12EO6 and C12EO5,9 and some clear differences were found. It was concluded that the more labile arrangement of C12EO5 on the surface of the silica sol with increasing temperature and the
Adsorption of Nonionic Surfactants enhanced adsorbed layer thickness were consistent with a smaller number of EO groups being in contact with the surface. There were no significant changes in the pattern of adsorption with temperature for C12EO6, and this is consistent with our observations here for C12EO8 and C16EO8. However, there is, as previously stated, an increase in volume fraction of surfactant in the adsorbed layer with temperature at the higher surfactant concentrations. At the higher surfactant concentrations, there is an additional contribution to the scattering profiles from “free” micelles in solution. At low micellar concentrations it is sufficient, as described earlier and in ref 4, to characterize this additional scattering in terms of a simple polynomial expansion. At higher micelle concentrations, the contribution is more profound and will require detailed modeling of both parts of the complex colloidal solution. This is not important in the context of the data presented here and will be the subject of a separate study and analysis.15 Summary In this paper, we have demonstrated the effects of temperature, sol type, and surfactant type and concentration on the adsorption of nonionic surfactants onto solid surfaces. It is not possible to obtain such detailed structural information on the adsorbed layer as is possible at planar interfaces (using reflectivity techniques). However, it has been demonstrated that aspects of the adsorption process not apparent at planar interfaces can be investigated. The different natures of the sols,
J. Phys. Chem., Vol. 100, No. 46, 1996 18137 their surface and temperature, and the type of surfactant are all shown to contribute to the adsorption process. References and Notes (1) Clune, J. S.; Ingram, B. T. In Adsorption from Solution at the Liquid-Solid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; pp 105-152. (2) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. (3) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204. (4) Cummins, P. G.; Staples, E.; Penfold, J. J. Phys. Chem. 1990, 94, 3740. (5) Tiberg, F.; Landgren, M. Langmuir 1993, 9, 927. (6) Levitz, P.; Damne, H. V. J. Phys. Chem. 1986, 90, 1302. (7) Rutland, M.; Senden, T. J. Langmuir 1993, 9, 412. (8) Cummins, P. G.; Staples, E.; Penfold, J. J. Phys. Chem. 1991, 95, 5902. (9) Cummins, P. G.; Staples, E.; Penfold, J. J. Phys. Chem. 1991, 96, 8092. (10) Tiberg, F.; Jonsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (11) Heenan, R. K.; Osborn, R.; Mildner, D.; Furusaka, M.; King, S.; Stanley, H. J. Appl. Crystallogr. 1996, to be submitted for publication. (12) Hayter, J. B. In Physics of Amphiphiles, Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M. V., Eds.; North-Holland: Amsterdam, 1988; p 59. (13) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. Hayter, J. B.; Hansen, J. P. Mol. Phys. 1982, 42, 651. (14) Penfold, J.; Staples, E.; Tucker, I.; Fragnetto, G. Phys. B 1996, 221, 325. (15) Penfold, J.; Staples, E.; Tucker, I. In preparation.
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