6330
Langmuir 2005, 21, 6330-6336
Adsorption of Nonionic Surfactant Mixtures at the Hydrophilic Solid-Solution Interface J. Penfold,† I. Tucker,‡ and R. K. Thomas§ ISIS, Rutherford Appleton Laboratory, CCLRC, Chilton, Didcot, Oxon, UK, Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, UK, and Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, UK Received February 10, 2005. In Final Form: April 12, 2005 The adsorption of the mixed nonionic surfactants, monododecyl triethylene glycol (C12EO3) and monododecyl octaethylene glycol (C12EO8), at the hydrophilic silica-solution interface has been studied by specular neutron reflectivity. The adsorption at the solid-solution interface is compared with that previously measured at the air-solution interface. The marked differences that are observed are explained in terms of the different packing constraints or preferred curvature arising from the disparity in the respective headgroup dimensions.
Introduction The nature of the adsorption of surfactant mixtures at interfaces is important because most domestic, industrial, and technological applications of surfactants involve mixtures.1 In spite the importance of these applications, there are many aspects of surfactant mixing that are not well understood at a molecular level.2 This has stimulated a resurgence of interest in recent years and has resulted in the application of new experimental techniques, such as neutron reflectivity,3 optical probes such as ellipsometry,4 second harmonic generation (SHG),5 sum frequency spectroscopy, SFS,6 and atomic force microscopy, AFM,7 to augment more classical techniques, such as surface tension. Furthermore, there have been developments in the applications of the pseudophase approximation or regular solution theory,8 which have provided much of the thermodynamic and theoretical basis of our understanding of surfactant mixing, and the development of molecular thermodynamic theories.9-11 In particular, we have demonstrated that neutron reflectivity is a powerful tool for studying surfactant adsorption at interfaces, providing information about adsorbed amounts and the structure of the adsorbed layer.3 Isotopic labeling, through deuterium/ hydrogen substitution, makes the technique particularly powerful for the study of mixtures, where the contribution to the surface adsorption and structure of each component of a surfactant mixture can be straightforwardly evaluated.12 This has now been extensively applied to a range † ISIS, Rutherford Appleton Laboratory, CCLRC, Chilton, Didcot, Oxon. ‡ Unilever Research Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral. § Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford.
(1) Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Surfactant Science Series, Vol. 46; Marcel Dekker, 1993. (2) Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society: Washington, DC, 1986. (3) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143. (4) Tiberg, F.; Jonsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 19, 2294. (5) Bain, C. D.; Davies, P. B.; Ward, R. N. Langmuir 1994, 10, 2000. (6) Richmond, G. L. Appl. Spectrosc. 2001, 55, 321A. (7) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (8) Holland, P. M. Colloid Surf., A 1986, 19, 171. (9) Puvvada, D.; Blankschtein, D. J. Phys. Chem. 1992, 96, 5567. (10) Nagarjan, R.; Ruckenstein, E. Langmuir 1991, 7, 2934. (11) Hines, J. D. Langmuir 2000, 16, 7575.
of ionic/nonionic and nonionic/nonionic surfactant mixtures at the air-solution interface,3,13-16 and this has revealed some of the limitations of the pseudophase approximation and demonstrated the importance of structural changes on mixing. Applications to the study of surfactant and mixed surfactant adsorption at the solid-solution interface are less extensive, but the applicability of the technique for the study of surfactant adsorption at the solid-solution interface has nevertheless been convincingly demonstrated.17-19 The cooperative nature of surfactant adsorption at the hydrophilic solid-solution interface has been confirmed,17 and the adsorptions of a range of cationic and nonionic surfactants have been characterized in terms of adsorbed amounts and of the structure of the adsorbed layer.18,19 More recently we have shown how polyelectrolytes can be used to manipulate the adsorption of ionic surfactants,20 and the adsorption at different hydrophobically modified silica surfaces has been studied.21 The studies of single surfactants at the hydrophilic solid-solution interface have now been extended to surfactant mixtures.22-24 In particular, in studying the adsorption of the cationic/ nonionic mixture of hexadecyl trimethylammonium bromide (CTAB)/hexaethylene glycol monododecyl ether (C12EO6) and the anionic/nonionic mixture of sodium (12) Staples, E. J.; Thompson, L.; Tucker, I.; Penfold, J.; Thomas, R. K. Langmuir 1993, 9, 1651. (13) Penfold, J.; Staples, E.; Cummins, P. G.; Tucker, I.; Thomas, R. K.; Simister, E. A.; Lu, J. R. J. Chem. Soc., Faraday. Trans. 1996, 92, 403; 1173. (14) Staples, E.; Thompson, L.; Tucker, I.; Penfold, J. Langmuir 1994, 10, 4136. (15) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I. Colloid Surf., A 1995, 102, 127. (16) Penfold, J.; Staples, E.; Tucker, I.; Thompson, L.; Thomas, R. K. J. Colloid Interface Sci. 2001, 246. (17) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J. Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. (18) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 12, 6036. (19) Fragneto, G.; Lu, J. R.; McDermott, D. C.; Thomas, R. K.; Rennie, A. R.; Galagher, P. D.; Satija, S. K. Langmuir 1996, 13, 477. (20) Penfold, J.; Tucker, I.; Staples, E.; Thomas, R. K. Langmuir 2004, 20, 7177. (21) Thirtle, P. N.; Li, Z. X.; Thomas, R.; Rennie, A. R.; Satija, S. K.; Sung, L. P. Langmuir 1997, 13, 5451. (22) Penfold, J.; Staples, E.; Tucker, I.; Thompson, L. Langmuir 1997, 13, 6638. (23) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. Langmuir 2002, 18, 5752. (24) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. Langmuir 2000, 16, 8879.
10.1021/la0503799 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/27/2005
Surfactant Adsorption at the Solid-Solution Interface
dodecyl sulfate (SDS)/C12EO6 we have demonstrated how the specific interaction of the surfactants with the hydrophilic solid surface can modify the adsorption behavior. The focus of this paper is the study of the adsorption of the nonionic surfactant mixture of C12EO3/C12EO8 at the hydrophilic silica-solution interface. The adsorption behavior of such nonionic surfactant mixtures should be relatively straightforward to understand because surface tension measurements25 at the air-water interface are consistent with the expected ideal mixing behavior. However, neutron reflectivity studies on C12EO3/C12EO8 mixtures at the air-solution interface15,16,26 at higher concentrations (well above the critical micelle concentration (CMC)) show an adsorption behavior that appears not to be quantitatively consistent with ideal mixing. Thus, at concentrations well in excess of the mixed CMC the surface composition is still relatively rich in C12EO3 compared with the solution. This was explained qualitatively, but not quantitatively, in terms of the packing constraints due to the different headgroup geometries and the resulting different contributions to the surface pressure.15,16,26 Measurements of the structure of the mixed monolayer, using partial isotopic labeling of the alkyl and ethylene oxide chains of the C12EO3 and C12EO8,26 has provided information about the distributions and relative positions of those labeled fragments at the interface. The frustration caused by the packing of the triethylene and octaethylene glycol headgroups results in changes in the structure of the mixed monolayer compared with that of the corresponding pure C12EO3 and C12EO8 monolayers. In particular the EO3 group is less hydrated and the EO8 group is more hydrated and less extended. As such, these structural measurements confirm the importance of packing constraints on surface mixing behavior. Measurements of adsorption isotherms and of the structure of the adsorbed layer by neutron reflectometry17-19 and ellipsometry27 confirm the expected cooperative nature of nonionic surfactant adsorption onto hydrophilic silica. There is little or no adsorption below the CMC, and above the CMC, saturation adsorption is rapidly achieved with increasing surfactant concentration. The structure of the adsorbed layer resembles the solutionaggregated structure of the surfactant. From our neutron reflectivity results17 we have described that surface structure as a “fragmented bilayer” or as a “flattened micellar” structure, and this is broadly consistent with the observations from AFM7 and ellipsometry.4 Neutron reflectivity measurements have been made on a range of different nonionic surfactants, from C12EO4 to C12EO25,28 and ellipsometry measurements have been reported on the adsorption of C10EO6 to C16EO6, and for C12EO5 to C12EO8.27 The dominant interaction between the nonionic surfactants and the hydrophilic silica surface is due to hydrogen bonding of the ether oxygens of the EO groups to surface OH groups, and has been demonstrated to be sensitive to the surface treatment.29 Such a surface interaction suggests that surface adsorption should become stronger for higher EO groups. However, ellipsometry measurements by Tiberg et al.27 and others28 show that adsorption is stronger for shorter EO chains. This would be consistent with a combination of more efficient packing of the shorter-chain EO molecules and a favored (25) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1982, 86, 164. (26) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. J. Colloid Interface Sci. 1998, 201, 223. (27) Brinck, J.; Tiberg, F. Langmuir 1990, 12, 5042. (28) Bohmer, M. R.; Koopal, L. K.; Janssen, R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 2228. (29) Penfold, J.; Staples, E.; Tucker, I. Langmuir 2002, 18, 2967.
Langmuir, Vol. 21, No. 14, 2005 6331
planar aggregate structure. Hence, for example, Tiberg et al.27 observed a higher adsorption for C12EO5 than for C12EO8. This is likely to have important consequences for mixed nonionic surfactants, just as described above for the nonionic mixture C12EO3/C12EO8 at the air-solution interface.15,16,26 However, there are few reported studies on the adsorption of nonionic surfactant mixtures at the hydrophilic solid-solution interface. One such study, by Brinck and Tiberg,27 investigated mixtures of C14EO6/C10EO6 and C12EO5/C12EO8 adsorbed at the hydrophilic silica surface. The adsorption of the individual surfactants shows the expected behavior with alkyl and ethylene oxide chain length, and the surface compositions, inferred from the variation of total adsorption with solution composition, implies ideal mixing. Although the surfactant pairs investigated show different relative surface activities, their preferred curvatures and, hence, packing constraints at the interface are not dramatically different. In this paper we report the use of specular neutron reflectivity to investigate the adsorption of the nonionic surfactant mixture of C12EO3/C12EO8 at the hydrophilic silica solution interface. Measurements as a function of solution composition and concentration are presented, and the use of deuterium/hydrogen isotopic substitution enables the surface composition, adsorbed amount, and structure to be determined directly. The results are compared with equivalent data for the adsorption at the air-solution interface. The contrasting surface behavior at the two interfaces is discussed in the context of the differing packing constraints or preferred curvatures arising from the different headgroup dimensions in these two surfactants. Experimental Details Specular neutron reflectivity gives information about the composition or concentration profile in the direction perpendicular to the surface or interface on a molecular length scale, and the technique is described in detail elsewhere.30 The specular reflectivity, R(Q), (where Q is the wave-vector transfer normal to the surface and is defined as Q ) 4π/λ sin θ, where θ is the grazing angle of incidence, and λ is the neutron wavelength) is given by the kinematic approximation as,31
R(Q) )
16π2 |F(Q)|2 Q2
(1)
where F(Q) is the one-dimensional Fourier transform of F(z), the average scattering length density distribution in a direction normal to the interface, such that,
F(Q) )
∫
+∞
-∞
F(z) eiQzdz
(2)
and
F(z) )
∑n (z)b i
i
(3)
i
ni(z) is the number density distribution of species i, and bi is its neutron scattering length. Different isotopes have different neutron scattering lengths, but notably, D and H have a particularly large difference and are of opposite sign. Hence, D/H isotopic substitution can be used to manipulate the neutron refractive index, where the neutron refractive index is defined as
n(z) ) 1 -
λ2 F(z) 2π
(4)
and is related to the reflectivity through eq 1. It is this selectivity (30) Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369. (31) Crowley, T. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Physica B 1991, 173, 143.
6332
Langmuir, Vol. 21, No. 14, 2005
Penfold et al.
Figure 1. Specular reflectivity for 50/50 h-C12EO3/h-C12EO8 at the silicon/D2O interface, for concentrations 6 × 10-5 M (b), 10-4 M (O), and 2 × 10-4 M (4), (b) calculated reflectivity for 40 Å thick uniform layer of different scattering length densities in the range 10-6 M to 4 × 10-6 M at the silicon/D2O interface, including 15 Å oxide layer at the interface (see figure for annotation). that makes neutron reflectivity such a powerful tool for surface adsorption studies, especially for the study of the adsorption of surfactant mixtures. The neutron reflectivity measurements were made on the SURF reflectometer32 at the ISIS pulsed neutron source using the “white beam time-of-flight” method. Measurements were made in the Q range 0.012-0.4 Å-1, using the wavelength range 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,33 was used, where the neutron beam is incident at the liquid-solid interface by transmission through a crystalline upper silicon phase. The cell uses a thin solvent gap (
