3740
J . Phys. Chem. 1990, 94. 3740-3145
Study of Surfactant Adsorption on Colloidal Particles P. G.Cummins,+ E. Staples,+and J . Penfold*$* Unileuer Research, Port Sunlight Laboratory, Bebington, Wirral, England, and Neutron Science Division, Rutherford Appleton Laboratory, Chilton, Oxon, England (Received: July 25, 1989; In Final Form: November 15, 1989) Surface tension and small-angle neutron scattering have been used to study the nature of surfactant adsorption on silica sols. This paper presents results on the characterization of the ludox silica sol and adsorbed layers of hexaethylene glycol monododecyl ether (C12Es). Preliminary results are presented that demonstrate the presence of a lower consolute boundary for the composite system.
( I ) Introduction The adsorption of surfactants onto colloidal particles is normally associated with enhanced dispersion stability.’ Exceptions to this general observation can of course occur with the reversal of the polarity of the particles’ charge, even when the adsorbed surfactant is nonionic.2 In this paper, we present aspects of the adsorption of hexaethylene glycol monododecyl ether (C&,) onto a ludox silica sol. The system shows a range of aggregation effects that are a function of the surface coverage of the surfactant, surfactant concentration, temperature, and pH, and in which, the electrostatic potential of the silica particle plays a minor role. With an absorbed surfactant layer, the silica sols display a temperature-dependent aggregation, consistent with the existence of a lower consulate boundary, where the clouding temperature is dependent upon the degree of surface coverage. Such dispersions provide a suitable model system that is “geometry invariant” for the investigation of the profile of the lower consolute boundary in nonionic surfactants. In particular, the role of micelle geometry in critical opalescence, that has been discussed for certain nonionic ~ u r f a c t a n t s ,may ~ be confirmed. Although small-angle neutron scattering (SANS) is a wellestablished technique for determining the nature of polymer ad,~ few measurements have been resorption in s o l ~ t i o nrelatively ported on surfactant adsorption. Harris et al.s have reported measurements on dodecanoic acid on polystyrene latex, and more recently, Ottewill et aL6 have carried out a detailed study of the nature of the adsorption of an alkyl aryl sulfonic acid on a calcium carbonate sol in nonaqueous media. In this paper, we concentrate on the characterization of the bare sol and its adsorbed layer from partial to saturation coverage by SANS and surface tension measurements. The temperature and concentration dependence of the interparticle interactions have been observed. A detailed analysis of the scattering data at lower momentum transfers, Q,in terms of interparticle interactions will be the subject of a future paper. (2) Experimental Section The hexaethylene glycol monododecyl ether (C12E6)was obtained from Nikkol Chemicals and was used without further purification. The silica sol, ludox TM, was supplied by Du Pont, and the pH of the dispersion was modified by the addition of 0.1 M HCI. The density of the silica particles and the water content of the silica stock were determined gravimetrically. The surface tension measurements were carried out at 25 f 0.1 “C with a thermostatically controlled Kruss KIOT digital tensiometer with a De Nouy ring attachment. The procedure recommended by the manufacturer was followed with the surfactant-silica system being equilibrated for a few minutes prior to measurements. The SANS measurements were made at the ISIS pulsed neutron source with the LOQ diffractometer.’ With use of the ‘Port Sunlight Laboratory. 3 Rutherford Appleton Laboratory,
0022-36S4/90/2094-3740$02.S0/0
white-beam (neutron wavelengths, A, of 2-10 A) time-of-flight (TOF) method and a large area detector, a wide Q range (where Q = 47r/X sin (0/2) and 0 is the scattering angle) is simultaneously covered from 0.007 to 0.2 A-I. The samples were contained in standard 1-mm path length quartz cells, thermostated to an accuracy of f l OC. The data were corrected for instrumental factors, appropriate backgrounds were subtracted, corrections were made for beam adsorption and the data were reduced to an absolute cross section (du/dQ in cm-I) by reference to standard scatterers by established procedures.* After background subtraction, the statistical quality of the data restricts the Q range to 0.007-0.07 A-1. The Q resolution is approximately constant for Q > 0.05 A-I ( A Q / Q (fwhm) 7%), but for Q 0.05 A-I, it is a strongly varying function of Q and is dominated by the contribution due to A0.8 The resolution is shown in Figure 1 and has been included in the subsequent data analysis.
-
(3) Neutron Scattering Theory For a solution of globular polydisperse interacting particles, the coherent scattering cross section can be written in the approximation that there are no correlations between position, orientation, and size as9910 = N,[S(Q)(IF(Q)I2)+ I(F(Q))I2- (IF(Q)IZ)l (1) dQ coh where the averages denoted by ( ) are averages over particle size and orientations. N is the particle number density, S ( Q ) is the structure factor a n t F(Q) is the particle form factor. A number of different distribution functions have been used to describe colloidal systems, and in general the data is not too sensitive to the detailed form of these functions. A convenient form is the Schultz distribution, which has been used by Hayterg and Kotlarchyk and Chen’O to calculate analytically { lF(Q)I2) and I(F(Q))IZ. The normalized Schultz distribution (in our case with respect to number) can be conveniently written as f ( r ) = ( Z + l)z+lP exp[-(z I ) x ] / r r ( z 1) (2) where r is the mean particle radius, x = r / r , z = (1 - s2)/s2, and s = u / r , u2 being the variance of the distribution. The Schultz distribution is also similar to the log normal distribution, which
+
+
( 1 ) Adsorption from solution; Ottewill, R. H., Rochester, C. H., Smith, A. L., Eds.; Plenum Press: London, 1983. (2) Van Rybuski, B.; Schwinger, M. J. In Nonionic Surfactants, Physical Chemistry, Schick, M. J., Ed.; Dekker: New York, 1987; Chapter 2. (3) Evans, H.; Tildesley, D. J.; Leng, C. A. J . Chem. Soc., Faraday Trans. 1 1987,83,1525. (4) Cosgrove, T.; Crowley, T. L.; Vincent, B. Symposium on Adsorpfion from solution; Plenum Press: London, 1982. (5) Harris, N. M.; Ottewill, R. H.; White, J. W. In ref 1. (6) Markovic, I.; Ottewill, R. H.; Cebula, D. J.; Field, I.; Marsh, J. F. Colloid Polym. Sci. 1984, 262,648. Markovic, I.; Ottewill, R. H. Ibid. 1986, 264, 65. Markovic, 1.; Ottewill, R. H. Ibid. 1986, 264, 454. (7) Neutron scattering instruments at the ISIS facility, user guide, Rutherford Appleton Laboratory, 1987. (8) Heenan, R. K. To be published. (9) Hayter, J. B. In Physics of Amphiphiles: Micelles, vesicles and microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1958; p 59. (10) Kotlarchyk, M.; Chen, S. H. J . Chem. Phys. 1983, 79, 2461.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 9, I990 3741
Study of Surfactant Adsorption on Colloidal Particles
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LOQ diffractometer. has been proposed as a suitable distribution for many colloidal systems. For a sphere plus an outer shell, the particle form factor is F(Q) = VI(PI - PZ)FO(QRI)+ V7.h - PJFo(QRz)
(4) Results and Discussion ( a ) Bare Silica Sol. The requirement to both minimize the complications of interactions and to retain sufficient signal strength leads to a choice of sol concentrations of the order of 4%. The SANS data from such a sol in H 2 0 (0.187), D 2 0 (0.183), and 0.2 M electrolyte at pH 9.4 is shown in Figure 2; this data is representative of a number of measurements. The dotted-dashed line is a model fit for polydisperse spheres of uniform density interacting through a screened Coulombic potential. The main ~~~~
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(1 1) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (12) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651. (13) Hayter, J. B. In Solid Stare Division Progress Report; Green, P. H., Watson, R. M., Eds.; Oak Ridge National Laboratory Report ORNL-6306; Oak Ridge National Laboratory: Oak Ridge, TN. 1986, p 153. (14) Hayter, J. B.; Penfold, J. J . Chem. Soc., Furaduy Trans. I 1981, 77,
1851. (15) Hayter, J. B.; Penfold, J . Colloid Polym. Sci. 1983, 261, 1022. (16) Bunce, J.: Ramsay, J. D. F.; Penfold, J. J . Chem. Soc., Faraday Trans. I 1985, 81, 2845. (17) Penfold, J.; Ramsay, J. D. F. J . Chem. Soc., Faruday Trans. 1 1984, 81. 117.
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Figure 2. Neutron scattering cross section for 4% silicon sol in H 2 0 (0.187)/D20 (0.813) at pH 9.4. The dotted-dashed line is a model fit to the data as described in the text; shown also are the individual contributions from P ( Q ) (dashed line) and S ( Q ) (solid line).
(3)
where u, = 4rRj3/3, Fo(QR) = 3jl(QR)/QR and p l , p2, and ps are the scattering densities of the core, shell, and solvent. (IF(Q)I2) and I(F(Q))I* can be evaluated analytically9 for F(QR) andf(R) defined by eqs 2 and 3. The structure factor, S(Q), is modeled with the analytic solution of the mean spherical approximation, MSA," for a screened Coulombic potential, with refinements for low volume fractions, rescaling12 and penetrating background13 included. The parameters that determine the form of S(Q) are the volume fraction 4 = rN,d3/6 (where d is the particle diamter and N p the particle number density), the surface charge Q, and the Debye screening length, K. This model of the structure factor has been successfully applied to a range of colloidal system^,^ including micellar solut i o n ~ ' ~and * ' ~silica S O I S . ' ~ J ~ The neutron scattering data has been reduced to an absolute cross section, da/d!l (cm-I). the data have been evaluated by a least-squares criterion with a model based on eqs 1-3. A scale factor (=data/theory) has been used to adjust the theory to the data; systematic errors in the data and the theoretical model dictate that the acceptable deviation of the scale factor from unity is f 10%. Although in all the data presented in this paper well-defined oscillations are observed in the form factor, they are more heavily damped than would be anticipated for a monodisperse system of spheres. The damping cannot be attributed solely to polydispersity, because if the minima at low Q are well fitted by adjusting the polydispersity, then the higher order oscillations are totally damped. As indicated earlier, the Q-dependent instrumental resolution has been used to "smear" all model calculations.
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H 2 0 fraction
Q 0.04
0.04 0.04 0.02
in s o h 0.187 0.187 0.187
0.09
conditions pH = 6.5 pH = 9.4 0.2 M NaCl pH = 9.4
R, A
152.6 153.6 152.4 152.5
scale factor 0.97 1.03 0.972 0.925
'Symbols: Q = volume fraction: R = particle radius. fitting parameters are summarized in Table I. The sol is well described by spheres with a polydispersity of 12% (variance) and a mean radius of 152.8 f 1 A. The observed radius of the silica particles is in good agreement with values reported elsewhere in the literature.I6J7 The scale factor in Table I is the ratio between the measured and calculated cross sections; the values indicate that not only is the shape of the scattered intensity profile well described by the model but also importantly its absolute value. The model fitting has been carried out under a variety of conditions, and the spread in values in Table I is indicative of the errors in the fitting procedure. The contribution from interparticle interactions, S( Q), is small but not insignificant, especially at low-Q. S(Q) is well described by the MSA calculation, and the relative contributions from S(Q) and P ( Q ) to the total cross section are shown in Figure 2. The data fitting has also been carried out with the high-Q data (Q > 0.02 kl) only and so essentially removing the added complication of interactions. The quality of the fit is equally good, and the fitted parameters are well within the range of errors in Table I. ( b ) Adsorbed Layer Characterization. The surface tension of the ludox sols was measured to be that of water, and as a result, the concentration of surfactant monomer in equilibrium with the adsorbed layer can be determined by a simple surface tension measurement. The measurements were made from dispersions with their solvent composition optimized for neutron scattering
3742 The Journal of Physical Chemistr,y, Vol. 94, No. 9, 1990
Cummins et al.
TABLE 11"
9 0.031 0.033 0.034 0.036 0.038 0.039
surfactant phase vol anhydrous hydrated' 0.023 0.018 0.014 0.010 0.0057 0.0029
d, A
0.033 0.026 0.020 0.0146 0.0083 0.0042
36.7 40.4 37.3 42.2 31.2 18.2
b2,b
A-2
0.195 0.22 0.24 0.285 0.295 0.28
adsorbed
% ' saturation
layer phase void 0.028 0.0335 0.0318 0.0391 0.0282 0.0157
isotherm IO0 75 57 38 21 10
coverage ex
fraction of surface 'Overed surfactant ()' to depth phase vol anhydrous c abs intensed (ex abs intens) 1 I8 78 63 37 29 26
71 66 52 34 28 21
phase separation temp, "C
0.015 0.015 0.012 0.0 105 0.0058 0.0034
54 58 63.5 65 69
"Symbols: 4 = silica sol phase volume; d = adsorbed layer thickness. bAdjusted to give a scale factor of unity f 5%. CFromcolumns 3 and 6 . dFrom columns 1 and 4. CFrom column 5. /Hydration assumed to be two water molecules per EO.
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Figure 4. Neutron scattering cross section for a 4% silica sol in H20 (0.39)/D20 (0.61) for 10% to saturation coverage of CIZE6surfactant: (+) 95%, (*) 80%. ( 0 )60%, ( X ) 40%, (B) 20%, (A) 10%.
from the adsorbed layer (61% D20/39% H,O), and at the appropriate points during the addition of surfactant small values (0.3 mL compared to a total of 10 mL) were removed as scattering samples. The resulting sample silica sol and surfactant phase volumes are as given in Table 11. The adsorption isotherm calculated from the surface tension measurements is shown in Figure 3 and is similar to the shape of other surfactant is0therms.l From such measurements, it is not possible to confirm that saturation coverage has been achieved M), but the at the critical micellar concentration, cmc, (9 X measured maximum adsorption and the geometical sol area are consistent with an area/molecule of 23 A2,suggesting a complete bilayer coverage. It should be noted however that at a higher value of pH 9.2 a substantial reduction in adsorption is observed (the maximum coverage corresponds to only 500 A2/molecule); however, the cooperative nature of the isotherm is retained. The neutron scattering measurements have been carried out in circumstances where the scattering patterns have the maximum sensitivity to the structure of the adsorbed layer. This is achieved when the solvent scattering density is matched to the silica (61% D20/39% H20), and the scattering arises from the adsorbed layer only. The solvent composition was selected with a silica density of 2.34 g/cm3, and the scattering from the bare sol at this solvent composition was verified to be negligible. Indeed, measurements where there was contrast between the silica, the adsorbed layer, and the solvent give rise to scattering patterns dominated by the contribution due to the silica core. Figure 4 shows such SANS data for the same 4% ludox silica sol system in H 2 0 (0.39)/D20 (0.61) at pH 8.5 from 10% saturation to saturation coverage with C,,E, surfactant (for partial coverage, the nomenclature used, i.e., percent coverage, refers to the percent of the saturation or maximum coverage, where saturation will not necessarily imply complete coverage). The solvent is now matched to the silica, and hence, the scattered intensity arises from the adsorbed surfactant layer only. In Figure 5, the data for the different coverages are represented as ratios to the saturation coverage data. These results show that, except at the lower coverages, the thickness of the adsorbed layer is constant and that the only significant change with coverage is the mean density of the layer. I n fact the ratios in Figure 5 gave a clear
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Figure 5. Ratio of partial coverage scattering to saturation coverage from data in Figure 4: ( 0 )40%, (A)60%, (0) 8076, ( X ) 95%.
indication of the way the density changes with coverage. Compared to the scattering from the bare sol (Figure 3), the higher order oscillations in the form factor are more pronounced, indicating an apparent decrease in the polydispersity. As we will see from the detailed model fits, this is a feature of the vesicular nature of the form factors and not an indication of a change in polydispersity. SimulationsLshave shown that the general form of the data is only achieved when the core is closely matched to the solvent. The A-*; this solvent scattering length density, Nb,, is 0.366 X implies for the silica particles a density of 2.34 g/cm3, compared with a Dow-quoted value of 2.28 g/cm3.I9 As previously discussed, in order to obtain acceptable counting statistics, the measurements have been made at a concentration where interparticle interactions are present, and although the addition of the nonionic surfactant partially suppresses the effects of interactions, they still contribute at low Q. However, although observable, the interactions are not significant, and we will comment no further at this stage on the interactions, except to mention that they do not affect the values of the fitted parameters. We have chosen to restrict the Q range over which to evaluate the data to the region in which the statistical accuracy of the data is acceptable and the model (treating the adsorbed layer as a uniform layer) is applicable. Instead of using radius of gyration measurements to obtain dimensions, and extrapolations to zero Q to determine anhydrous masses, we have adopted the procedure of modeling the entire scattering profile. The model is taken as a uniform shell of surfactant with a density corresponding to the amount of surfactant and solvent both bound and free in the shell. The inner radius of the shell is taken as the mean silica radius (152.8 A); polydispersity, instrumental resolution, and interparticle interactions are included as previously described. The main refinable parameters are the thickness and density of the adsorbed surfactant layer. The thickness of the layer determines principally the shape of the scattering profile, whereas the density of the layer determines the absolute scattered intensity and has been adjusted to provide acceptable scaling. The individual fits for the data or ( I 8) Penfold, J . Unpublished work. (19) Jennings, B. R.; Jerrard. H. G. J . Poly" Sci., Purr A 1964, 2, 2025.
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3743
Study of Surfactant Adsorption on Colloidal Particles a
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Figure 6. Neutron scattering cross section for 4% silica sol in H20(0.39)/D20 (0.61). The solid line is a model fit as described in the text, for (a) saturation coverage, (b) 80%, (c) 60%, (d) 40%, (e) 2070, and (f) 10%.
figure are shown in Figure 6a-f, and the fitted parameters are summarized in Table 11; the quality of the fits is excellent. For 240% coverage, the mean thickness of the adsorbed layer is 39 f 5 A and is within that error constant, consistent with bilayer formation on the surface of the silica. For coverages
