Polyelectrolyte Modified Solid Surfaces: the Consequences for Ionic

Nov 3, 2005 - Unilever Research and Development, Port Sunlight, Quarry Road ... and Physical and Theoretical Chemistry Laboratory, Oxford University,...
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Langmuir 2005, 21, 11757-11764

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Polyelectrolyte Modified Solid Surfaces: the Consequences for Ionic and Mixed Ionic/Nonionic Surfactant Adsorption J. Penfold,*,† I. Tucker,‡ and R. K. Thomas§ ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom, Unilever Research and Development, Port Sunlight, Quarry Road East, Bebington, Wirral, United Kingdom, and Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, United Kingdom Received July 25, 2005. In Final Form: September 21, 2005 This paper describes how the cationic polyelectrolyte, polyDMDAAC (poly(dimethyl diallylammonium chloride)), is used to manipulate the adsorption of the anionic surfactant SDS and the mixed ionic/nonionic surfactant mixture of SDS (sodium dodecyl sulfate)/C12E6 (monododecyl hexaethylene glycol) onto the surface of hydrophilic silica. The deposition of a thin robust polymer layer from a dilute polymer/surfactant solution promotes SDS adsorption and substantially modifies the adsorption of SDS/C12E6 mixtures in favor of a surface relatively rich in SDS compared to the solution composition. Different deposition conditions for the polyDMDAAC layer are discussed. In particular, at higher solution polymer concentrations and in the presence of 1 M NaCl, a thicker polymer layer is deposited and the reversibility of the surfactant adsorption is significantly altered.

Introduction Understanding the nature of surfactant and mixed surfactant adsorption at interfaces is important in the context of the widespread domestic, technological, and industrial applications of surfactants, in detergency, surface modification or conditioning, colloidal stability, dyeing, lubrication, adhesion, and mineral flotation.1 Of particular importance in many of these application areas is the solid-solution interface, and this encompasses hydrophilic, hydrophobic, and other modified surfaces.2 It is only in the past decade or so that detailed information at a molecular level has become available. This has been in part due to the development of a range of new surface sensitive techniques: these include neutron3 and X-ray4 reflectometry, ellipsometery5 and other optical techniques,6 and force probe methods, such as atomic force microscopy, AFM,7 and surface force microscopy, SFM.8 The availability of more detailed information on adsorption, and the current interest in manipulating surface adsorption properties, for the development of surfaces with specific functionality, “smart surfaces”, has provided considerable recent impetus in the study of the adsorption at the solid-solution interface. The specific interaction with the solid surface has a profound effect on the adsorption of surfactants compared to that observed at the simpler air-solution interface. At the hydrophobic solid-solution interface, the adsorption * To whom correspondence should be addressed. † Rutherford Appleton Laboratory. ‡ Unilever Research and Development. § Oxford University. (1) Scamehorn, J. F. Phenomena in mixed surfactant systems; ACS Symp Ser 311; American Chemical Society: Washington, DC, 1986; p 324. (2) Tiberg, F.; Brinck, J.; Grant, L. Curr. Opin. Colloid Interface Sci. 2000, 4, 411. (3) Penfold, J.; Thomas, R. K. J. Phys.: Condens Matter 1990, 2, 1369. (4) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171. (5) Tiberg, F. R. J. Chem. Soc., Faraday Trans. 1996, 92, 531. (6) Richmond, G. L. Appl. Spectrosc. 2001, 55, 321A. (7) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (8) Klein, J.; Perahia, D.; Warburg, S.; Fetters, L. J. Nature 1991, 352, 143.

is of the form of a monolayer9 and similar to that at the air-solution interface.10 In contrast, at the hydrophilic solid-solution interface, the adsorption is cooperative, and this results in the development of an aggregated structure at the interface which can resemble the micellar aggregates in solution.11 Other patterns of adsorption have been observed at hydroxylated surfaces,12 cellulosic surfaces,13 and a variety of polymeric surfaces.14 There is much current interest, and a considerable amount of recent literature developing on the adsorption of polyelectrolytes15 and polyelectrolyte/surfactant mixtures16-19 at the solid-solution interface. In this area, to date much of the interest has been on the deposition of polymeric multilayers of controlled thickness20-23 and in the modification or manipulation of surface properties.24-26 This paper falls into the latter category and is an extension of previously reported work which used primarily neutron reflectivity to characterize the adsorption of surfac(9) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Inteface Sci. 1994, 162, 304. (10) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143. (11) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. (12) Thirtle, P. N.; Li, Z. X.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Sung, L. P. Langmuir 1997, 13, 5451. (13) Rundlof, M.; Karlsson, M.; Wagberg, L.; Poptoshev, E.; Rutland, M.; Claesson, P. J. Colloid Interface Sci. 2000, 230, 441. (14) Turner, S. F.; Clarke, S. M.; Rennie, A. R.; Thirtle, P. H.; Cooke, D. J.; Li, Z. X.; Thomas, R. K. Langmuir 1999, 15, 1017. (15) Meszaros, R.; Thompson, L.; Bos, M.; Groot, P. Langmuir 2002, 18, 6164. (16) Zimin, D.; Craig, V. S. J.; Kunz, W. Langmuir 2004, 20, 2282. (17) Terada, E.; Samoshina, Y.; Nylander, T.; Lindman, B. Langmuir 2004, 20, 1753. (18) Windsor, R.; Neivandt, D. J.; Davies, P. B. Langmuir 2002, 18, 2199. (19) Meszaros, R.; Thompson, L.; Varga, I.; Gilanyi, T. Langmuir 2003, 19, 9977. (20) Losche, M.; et al. Macromolecules 1998, 31, 8893. (21) Klitzing, R. V.; Mohwald, H. Langmuir 1995, 11, 3554. (22) Steitz, R.; et al. Langmuir 2001, 17, 4471. (23) Glinel, K.; et al. Langmuir 2002, 18, 1408. (24) Dedinaite, A.; Claesson, P. M.; Bergstrom, M. Langmuir 2000, 16, 5257. (25) Schouten, S.; Stroeve, P.; Longo, M. L. Langmuir 1999, 15, 8133. (26) El Khouri, R. J.; Johal, M. S. Langmuir 2003, 19, 4880.

10.1021/la052012+ CCC: $30.25 © 2005 American Chemical Society Published on Web 11/03/2005

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tants11,12,27,28 and mixed surfactants29-31 at the solidsolution interface and to use polyelectrolytes to manipulate those surface adsorption properties.32 Other related studies reported in the recent literature include the adsorption of cationic cellulose derivatives and SDS onto silica by Tarada et al.,17 the adsorption of modified hydroxylethyl cellulose and SDS by Zimin et al.,16 the adsorption of SDS/ PEI (poly(ethyleneimine)) mixtures onto silica by Meszaros et al.,19 and the adsorption of SDS and SDS/polyelectrolyte mixtures onto PEI modified surfaces.24 In this paper, we report an extension of some preliminary measurements on the adsorption of SDS and C16TAB (hexadecyl trimethylammonium bromide)) onto hydrophilic silica modified by the cationic and anionic polyelectrolytes, polyDMDAAC and PSS (poly(styrene sulfonate)).32 Here we explore how different deposition conditions can produce polyDMDAAC surfaces with different adsorption properties for SDS and how the polyDMDAAC surface modifies the adsorption behavior of the anionic/nonionic surfactant mixtures of SDS/C12E6. Experimental Details The specular neutron reflectivity measurements were made on the SURF reflectometer33 at the ISIS pulsed neutron source, using the white beam time-of-flight method. Measurements were made in the Q range (where Q, the wave vector transfer normal to the surface of interface, z direction, is defined as Q ) 4π sin θ/λ, λ is the neutron wavelength and θ is the grazing angle of incidence) 0.012-0.4 Å-1, using the neutron wavelength range 1-7 Å, and three different grazing angles of incidence, 0.35, 0.8, and 1.8°. The resolution in Q, ∆Q/Q, was ∼ 4%. The neutron beam is incident at grazing incidence at the solid-solution interface by transmission through the upper crystalline silicon phase. The surface of the silicon, 〈111〉 , supplied by CRYSTRANS, was polished to a surface roughness e5Å rms and the illuminated area was ∼30 × 60 mm2. The cell uses a thin solvent gap (e10 µm) and a sample volume of ∼0.2 mL Solutions are exchanged using a Merck LaChrome Chromatography HPLC mixing pump, and exchange of ∼20 mL of sample provides efficient sample changing and cleaning. The silicon surface was made hydrophilic by storage under water, following a “mild piranha” surface treatment, as described elsewhere.35 The data were normalized for the incident beam spectral distribution, run time, and detector efficiency and established on an absolute scale by reference to the direct beam intensity.35 The d,h-SDS and d,h-C12E6 were synthesized and purified as described elsewhere. The cationic polymer, polyDMDAAC, was synthesized as described by Warren36 and had a molecular weight of ∼100k. All of the measurements were made in D2O and 0.1 M NaCl and at a pH of 2.4 (established by the addition of HCl), unless otherwise stated. Specular neutron reflectivity provides information about the composition and concentration profile in the direction perpendicular to the surface or interface on a molecular scale, probing distances of ∼ a few Å’s to ∼3000 Å, and is described in detail elsewhere.3,10 The specular reflectivity, R(Q), can be described using the kinematic approximation37 or in terms of the optical description of reflectivity in thin films.38 The latter approach is (27) Fragneto, G.; et al. Langmuir 1996, 12, 6036. (28) Fragneto, G.; et al. Langmuir 1996, 13, 477. (29) Penfold, J.; et al. Langmuir 1997, 13, 6638. (30) Penfold, J.; et al. Langmuir 2002, 18, 5755. (31) Penfold, J.; et al. Langmuir 2000, 16, 8879. (32) Penfold, J.; Tucker, I.; Staples, E.; Thomas, R. K. Langmuir 2004, 20, 7177. (33) Penfold, J.; et al. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (34) Penfold, J.; Staples, E.; Tucker, I. Langmuir 2002, 18, 2967. (35) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 16, 6036. (36) Warren, N. Ph.D. Thesis, University of Oxford, Oxford, U.K., 1999. (37) Crowley, T. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Physica B 1991, 173, 143. (38) Heavens, O. S. Optical properties of thin films; Butterworths: London, 1953.

Penfold et al. used in the modeling of the reflectivity data reported in this paper. A key feature of the technique that is exploited here is that for neutrons the refractive index can be manipulated using H/D isotopic substitution, where H and D have vastly different scattering powers for neutrons. This gives rise to a difference in the scattering length density F(z) or refractive index, n(z), where n(z) is defined as

n(z) ) 1 -

λ2 F(z) 2π

(1)

H/D isotopic substitution can be applied to the adsorbate and solvent, and modeling the reflectivity data from the different combinations simultaneously can minimize uncertainties in the model.29 It relies on there being no significant isotopic dependence on the adsorption and the structure of the adsorbed layer, and this is well established for these systems.10 The initial polyDMDAAC coated surface (surface A) was deposited from 5 × 10-4 M 6:94 mole ratio SDS/C12E6/D2O/0.1 M NaCl/pH 2.4/50 ppm polyDMDAAC. The hydrophilic silica surface was exposed to the dilute polymer/surfactant solution for up to 90 min, and the reflectivity profile was measured every 30 min. Further deposition of polyDMDAAC was attempted using the same solution, but with an increased polymer concentration of 150 ppm, at higher electrolyte concentrations, 0.5 and 1.5 M NaCl, and in a concentrated polyDMDAAC solution (2000 ppm polyDMDAAC in D2O/1 M NaCl/pH 2.4). Between each distinct sequence of depositions onto the initially coated surface, the surface was washed with an excess D2O. The SDS adsorption isotherm was measured at the end of this entire sequence for h-SDS/D2O/0.1M NaCl/pH 2.4 in the SDS concentration range 10-4-10-2 M. A second polyDMDAAC coated surface (surface B) was established on a freshly prepared hydrophilic silica surface by deposition from a concentrated polyDMDAAC solution at a high electrolyte concentration (2000 ppm polyDMDAAC/1 M NaCl/ D2O/pH 2.4). Following washing in D2O, measurements of the SDS adsorption were made at 10-4 and 10-2 M SDS (for h-SDS/ D2O/0.1 M NaCl/pH 2.4), and subsequently, the entire SDS isotherm was measured from 10-4 to 7.5 × 10-3 M SDS. Measurements of the adsorption of the SDS/C12E6 surfactant mixture were made on a block coated with polyDMDAAC from a dilute solution (surface type A) as initially described here and used previously.32 Measurements were made at 10-3 M and for the isotopic combinations h-SDS/h-C12E6 in D2O/0.1 M NaCl/pH 2.4, and for the solution compositions of 20/80, 30/70, 50/50, and 70/30 mole ratio SDS/C12E6.

Results and Discussion In a previous study32 we had demonstrated how a thin polyDMDAAC layer can be deposited from a dilute polymer/surfactant solution to form a thin robust layer (5 × 10-4 M is consistent with the adsorption of a layer of SDS ∼35 to 40 Å thick at the interface. The

SDS conc (M)

A (Å2)

fc

d1 (Å)

d2 (Å)

d3 (Å)

Γ (×10-10 mol cm-2)

5 × 10-4 10-3 2 × 10-3 5 × 10-3 10-2

40 40 40 40 40

0.15 0.39 0.39 0.40 0.44

5.8 3.3 4.3 6.9 9.8

16.1 37.2 36.7 31.7 28.7

3.5 5.4 3.8 5.7 10.1

1.25 3.15 3.15 3.32 3.65

solid lines in Figure 2 are model calculations for a model which includes the thin oxide and polymer layers and a surfactant layer, and the key model parameters are summarized in Table 1. The quantitative analysis of the reflectivity data arising from the adsorbed surfactant layers has been made using a simple “three layer” model to describe the adsorbed layer. This has been extensively used and is described in detail elsewhere.11,29 It describes effectively the “flattened micelle” or “fragmented bilayer” structure that is adsorbed at the interface. As previously reported,11,29 it is the simplest model that is consistent with the type of data reported here. The basis of the model is that the adsorbed surfactant layer is described as three layers; a layer of thickness d1 adjacent to the solid surface containing surfactant headgroups and their associated hydration, a layer of thickness d2 containing hydrocarbon chains interpenetrating or overlapping from both sides of the layer, and a layer of thickness d3 adjacent to the solvent phase containing headgroups and hydration. To accommodate disorder and edge effects an additional paremeter, fc, is included to account for some intermixing between the headgroups and alkyl chains. The model is then described by three thicknesses, d1, d2, and d3, the area/ molecule in the layer, A, the fractional coverage, f, and fc. From known scattering lengths and molecular volumes the scattering length density of each layer can be calculated. The total adsorbed amount, Γ, in Table 1, is estimated from

Γ)

2f 1 Na A

()

(2)

In the analysis of the data in Figure 2, the contribution from the oxide layer and the polyDMDAAC layer are incorporated into a single layer in the model calculations (∼35 Å thick and a scattering length density slightly less than that of SiO2). The SDS adsorption is broadly similar to that previously reported.32 The significant difference is that the adsorbed amount at saturation (c > cmc) is less, 3.7 × 10-10 compared to 4.8 × 10-10 mol cm-1 previously measured.32 Although some variability should be expected, this difference could be attributed to the attempts to adsorb further polyDMDAAC onto the initial polymer layer, resulting in less charge sites being available for the SDS adsorption. An important feature that is retained in comparison to the previous measurements32 is that the SDS adsorption is entirely reversible, and washing the surface in D2O removes all the adsorbed SDS. (ii) SDS Adsorption onto PolyDMDAAC Modified Surface (Surface B). It is expected from previous studies of polyelectrolyte multilayers22 that deposition in the presence of high salt concentrations will produce thicker polymer layers. Substantial changes were not observed in the sequence of measurements reported in the previous section, and this is attributed to the surface adsorption being already dominated by the initial adsorption step (from dilute solution). Subsequent polymer adsorption

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Figure 3. Specular reflectivity for h-SDS/D2O/0.1 M NaCl/pH 2.4 onto polyDMDAAC coated silica surface (surface B), (red) 10-4 M, (blue) 10-2 M, and (green) 10-4 M (post washing in D2O) h-SDS. The solid lines are the fitted model reflectivity curves for 10-2 and 10-4 (post washing in D2O) M SDS, using the parameters in Table 2 and the model described in the text.

Penfold et al.

Figure 5. Adsorbed amount of SDS onto polyDMDAAC coated silica surface, Γ × 10-10 mol cm-2, as a function of SDS concentration for (b) original study,32 (O) surface A, and (∆) surface B. Table 2. Model Parameters for h-SDS/D2O/0.1 M NaCl/pH 2.4 onto PolyDMDAAC Coated Silica Surface (Surface B) SDS conc (M)

A (Å2)

fc

d1 (Å)

d2 (Å)

d3 (Å)

Γ (×10-10 mol cm-2)

10-2 10-4 2 × 10-4 5 × 10-4 10-3 2 × 10-3 5 × 10-3

40 40 40 40 40 40 40

0.45 0.35 0.42 0.65 0.66 0.65 0.66

5.5 1.3 2.5 8.2 4.4 2.3 1.9

27.7 26.4 20.2 26.7 31.9 28.7 28.4

4.6 5.2 5.6 3.0 5.7 5.1 4.9

3.8 2.9 3.5 5.4 5.5 5.4 5.5

Table 3. Model Parameters for Multilayer Fits for 7.5 × 10-3 M h-SDS/D2O/0.1M NaCl/pH 2.4 onto PolyDMDAAC Coated Silica Surface (Surface B)

Figure 4. As in Figure 2, except for surface B, (purple) 10-4 M, (yellow) 2 × 10-4 M, (red) 5 × 10-4 M, (green) 10-3 M, (blue) 2 × 10-3 M, (cyan) 5 × 10-3 M, and (grey) 7.5 × 10-3 M h-SDS. For clarity, the model fits are not shown but are comparable to those plotted in Figures 2 and 3.

steps then only modified that initially formed surface, and this is in part confirmed by the different SDS adsorption observed. To address this issue in more detail, measurements were made on a fresh hydrophilic silicon surface. Measurements in D2O prior to any adsorption were consistent with an oxide layer ∼20 Å, similar to that for the previous surface. PolyDMDAAC was deposited onto the surface from a 2000 ppm polyDMDAAC/1 M NaCl/D2O solution, and the resulting reflectivity (not shown) was similar to that shown in Figure 1. This implies that the adsorbed layer of polyDMDAAC is not substantially thicker. However, the adsorption of SDS onto this surface shows some interesting differences compared to the previous study32 and those reported in the previous section. This is illustrated in Figure 3, which shows the reflectivity for h-SDS adsorbed at the Silicon/polyDMDAAC/ D2O interface. The initial measurement at 10-4 M SDS shows no adsorption (the reflectivity is similar to that for the bare interface), consistent with previous measurements on surface A. At 10-2 M SDS, strong adsorption, characterized by a pronounced interference fringe, is observed. However,

d (Å)

F (× 10-6 Å-2)

21.8 26.8 40.5

4.0 1.3 3.9

when washed in D2O and exposed again to 10-4 M SDS, there is still substantial SDS adsorption remaining. Figure 4 shows the adsorption pattern for SDS in the concentration range 10-4-7.5 × 10-3 M, following the initial adsorption and rinsing. The key model parameters for the model fits to the data in Figure 4 (except for the highest concentration of 7.5 × 10-3 M) are summarized in Table 2. For clarity, the model fits are not shown in Figure 4 but are of similar quality to those included in the other figures (see Figures 2 and 3). Compared to the previously measured isotherms (see Figure 5), the onset of adsorption now occurs at a lower SDS concentration, and the amount adsorbed at saturation is higher. This is a direct consequence of the SDS that was initially adsorbed and which is now irreversibly bound to the surface. The initial adsorption at 10-2 M SDS gave a coverage of ∼3.8 × 10-10 mol cm-1, which is similar to that reported in the previous section but less that that previously reported.32 At the highest surfactant concentration measured, 7.5 × 10-3 M, the interference fringe pattern in the specular reflectivity changes markedly, consistent with the formation of a much thicker layer at the interface. The data are no longer consistent with a simple three layer model as described earlier. An unconstrained multilayer model fits the data for the thicknesses and scattering length densities in Table 3.

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Figure 6. Specular reflectivity for 10-3 M SDS/C12E6 adsorbed at the polyDMDAAC (surface A type) coated silica surface in D2O/0.1 M NaCl/pH 2.4, (b) h-SDS/h-C12E6, (O) d-SDS/h-C12E6, (a) 20/80, (b) 30/70, (c) 50/50, and (d) 70/30 SDS/C12E6. The solid lines are model fits to the data using the parameters in Table 4 and the model described in the text. Table 4. Model Parameters for SDS/C12E6 (10-3 M) Adsorbed onto PolyDMDAAC Coated Silica (Surface A Type), from D2O/0.1 M NaCl/pH 2.4 solution comp (mol fraction SDS) 20/80 30/70 50/50 70/30

contrast

Asds (Å2)

Ac12e6 (Å2)

fc

d1 (Å)

d2 (Å)

d3 (Å)

hh dh hh dh hh dh hh dh

49 50 48 49 46 46 44 44

64 65 96 96 150 150 228 228

0.52 0.44 0.51 0.49 0.58 0.50 0.38 0.40

13.5 13.2 11.5 13.4 11.6 12.6 10.9 12.6

16.1 18.6 17.2 16.2 15.8 15.2 18.2 16.2

12.3 10.4 11.9 12.1 11.6 12.2 14.9 11.9

In broad qualitative terms, the initial layer corresponds to the oxide layer at the interface, the second layer is a layer of polyDMDAAC swollen by solvent and/or surfactant, and the third layer is equivalent to the normal surfactant micellar layer. (iii) SDS/C12E6 Adsorption onto a PolyDMDAAC Modified Surface. We have previously reported30 some preliminary measurements on the adsorption of SDS/C12E6 surfactant mixtures at the silicon -solution interface and how an adsorbed layer of polyDMDAAC can substantially alter that adsorption due to the strong interaction between the SDS and the polyDMDAAC.32 Here we have extended those initial observations, and the specular reflectivity for 10-3 M SDS/C12E6/D2O/0.1M NaCl/pH 2.4 is shown in Figure 6 for solution compositions (mole fraction of SDS) of 0.2, 0.3, 0.5, and 0.7. The two reflectivity profiles correspond to the isotopic combinations h-SDS/h-C12E6/D2O and d-SDS/h-C12E6/D2O, where for the former combination the reflectivity is a

surface comp (mol fraction SDS)

Γ (×10-10 mol cm-2)

0.56

5.57

0.67

5.29

0.76

5.1

0.84

3.5

measure of the total adsorption and in the latter case the reflectivity is dominated by the C12E6 component (the d-SDS is effectively matched to the D2O solvent). The solid lines in the figures correspond to model fits, using the three layer model described earlier and adapted to incorporate the binary mixture (defined by the area/ molecule for each component, ASDS and AC12E6). The key model parameters are summarized in Table 4. The variation of the surface composition (expressed as mole fraction of SDS) with the solution composition is plotted in Figure 7 and shows that the surface is rich in SDS compared to the solution. This illustrates very clearly the role of the polyDMDAAC in modifying the pattern of adsorption of the SDS/C12E6 mixture, due to the strong interaction between the SDS and the polyDMDAAC. Plotted also in Figure 7 are the data for the solution compositions of 40/60 and 60/40 at 10-3 M SDS/C12E6 measured previously32 on a surface prepared in the same way, but coated with polyDMDAAC on a different occasion.

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Penfold et al.

Figure 7. Variation in the surface composition for 10-3 M SDS/C12E6 adsorbed at the solid-solution interface as a function of solution composition (mole fraction SDS) for (b) onto polyDMDAAC coated silica (surface A type), this study, (O) from previous measurements onto a similar polyDMDAAC surface (partially reported in ref 32), and (2,∆) at the air-solution interface with/without polyDMDAAC.39,40

Figure 8. Adsorbed amount of SDS/C12E6 as a function of solution composition for 10-3 M SDS/C12E6, (b) onto polyDMDAAC coated silica (surface A type), this study, (O) onto polyDMDAAC coated silica (surface A type) from ref 32, and (∆) onto hydrophilic silica.30

The results are broadly consistent, but quantitatively different, reinforcing previous discussions about the variability of such surfaces. Plotted also in Figure 7 are some corresponding data for SDS/C12E6 mixtures at the air-water interface, in the presence and absence of polyDMDAAC;30,32 where the enhanced adsorption of the SDS due to complexation with the polyDMDAAC is clearly seen but is not as pronounced as at the solid-solution interface. Figure 8 shows the variation in adsorbed amount with solution composition, where for the SDS/C12E6 mixture the adsorbed amount is evaluated from

Γ)

(

)

1 2f 1 + Na ASDS AC12E6

(3)

The total adsorption decreases for solution increasingly rich in SDS. This general trend has also been observed for the adsorption of SDS/C12E6 mixtures at the surface of hydrophilic silica (in the absence of polymer).

However, the trend here is much less pronounced. Plotted also in Figure 8 are the data measured at 10-3 M SDS/C12E6 and compositions 40/60, 60/40, and 80/20 on a different polyDMDAAC surface as discussed earlier30 (the equivalent surface compositions are shown in Figure 7). The trends with solution composition are similar, but the total adsorption is higher. Figure 8 also shows the variation in adsorbed amount with composition for SDS/C12E6 adsorbed onto silica (in the absence of polymer), from ref 30. This exhibits a much stronger decrease in adsorption for solutions increasingly rich in SDS, consistent with the lack of affinity of the SDS for the anionic silica surface. (iv) General Discussion. We have demonstrated how the adsorption of a thin layer of the cationic polyelectrolyte, polyDMDAAC, onto the surface of hydrophilic silica can substantially modify the adsorption of SDS and of SDS/ C12E6 mixtures to that interface. This arises from the strong electrostatic interaction between the SDS and the polyDMDAAC. Adsorption of the polyDMDAAC from dilute solution results in a thin robust layer, consistent with the polymer adsorbed in a flat extended conformation on the surface in a way that reverses the surface charge from anionic to cationic, as observed in other studies.24,32 Different measurements on separately prepared surfaces, but using nominally the same procedures, reported both here and in a previous paper32 show the same qualitative trends but some variation in quantitative amounts. This variation is similar to that observed at the bare hydrophilic silica surface for the adsorption of other surfactants.34 However, variations depending upon the experimental route have been reported.24 A number of different approaches were used in order to produce a thick polyDMDAAC layer at the interface, and so affect the pattern of surfactant adsorption. These included exposure of the surface to dilute polymersurfactant solution for longer times, adsorption from dilute polymer/surfactant solution in the presence of high salt concentrations (up to 1.5 M NaCl), and adsorption from polymer solution at high concentrations and high salt concentrations (2000 ppm polymer, 1 M NaCl). It has been previously demonstrated in the deposition of polyelectrolyte multilayers that deposition in the presence of strong electrolyte results in substantially thicker layers.17 When applied to an existing thin polymer layer (established from dilute solution), none of these approaches produced a substantially thicker layer which was irreversibly attached to the surface. Indeed the adsorption of SDS showed a qualitatively similar trend to that previously reported for adsorption onto a polyDMDAAC surface established from dilute polymer/surfactant solution.32 Quantitatively the adsorption was slightly less (as discussed in the previous paragraph) but was within the range of variation encountered in surfactant adsorption onto hydrophilic silica. Hence, it is difficult again to conclude whether this is due to differences in the initial silica surface or to the polyDMDAAC layer. Establishing the polyDMDAAC layer onto a fresh hydrophilic silica surface from a polymer solution at a higher concentration and a higher salt concentration (2000 ppm polyDMDAAC, 1 M NaCl) resulted in a polymer layer that was superficially similar to those established from dilute solution but with a significantly different behavior. In this case it was shown that the SDS adsorption is only partially reversible, whereas for surfaces established from dilute polymer/surfactant mixtures, the SDS adsorption is entirely reversible. Furthermore, at the higher surfactant concentrations, it is evident from the data that SDS and solvent penetrate and swell the polymer layer and results in substantially more surfactant adsorption.

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Table 5. Parameters Used in RST Calculation41 for 70/30 SDS/C12E6 Mixtures at the Air-Solution Interface, with and without PolyDMDAAC SDS

no polymer 50 ppm poly-DMDAAC

C12E6

mixture

cmc (M)

π (mNm-1)

cmc (M)

π (mNm-1)

cmc (M)

π (mNm-1)

2 × 10-3 7 × 10-4

38 40

8 × 10-5 8 × 10-5

38 40

1.2 × 10-4 7 × 10-5

38 35

Other studies,24,42 using AFM, ellipsometry, and XPS, have considered the reversibility of the adsorption and desorption of polyelectrolyte and surfactant at the solid-solution interface. In a study of cationic derivatives of cellulose with SDS, Rojas et al.42 demonstrated that surfactant induced desorption of the polyelectrolyte had a concentration dependence that was related to the charge on the polyelectrolyte. Furthermore, they were able to show that, for strongly charged polyelectrolytes, desorption was preceded by swelling, whereas for weakly charged polyelectrolytes, this was not the case. In general, we do not observe desorption of the polyDMDAAC, except in the situation where additional polymer has been adsorbed on top of the initially compact layer of polymer. However, for polymers deposited from higher salt concentrations (surface B) swelling of the polymer layer at high SDS concentrations (>7.5 × 10-3 M) is observed. Dedinaite et al.2 have suggested, from their study of adsorbed layers of a highly charged cationic polyelectrolyte (a cationically modified cellulose derivative) and SDS on mica, that the adsorbed layers are trapped in a metastable state, where true equilibrium is not established quickly. They observed swelling of a preadsorbed polyelectrolyte layer with the addition of SDS. De-swelling of the polyelectrolytesurfactant layer by the removal of SDS resulted in some SDS remaining trapped in the layer. They observed a different behavior between surfactant associated with a preadsorbed layer and those established from polyelectrolyte-surfactant aggregates in solution. That is, the nature of the adsorbed layer depends on the experimental route, and this is broadly consistent with our observations. The adsorption of the thin polyDMDAAC polymer layer at the surface of the hydrophilic silica is shown to have a profound effect on the adsorption of the SDS/C12E6 mixture to the surface. At 10-3 M (>mixed cmc) and over a wide range of solution compositions, from 20/80 to 70/30 mole ratio SDS/C12E6, the surface is preferentially rich in SDS compared to the solution composition. A similar but less pronounced enhancement of the surface composition, in favor of the anionic surfactant, is also observed at the air-solution interface in the presence of polyDMDAAC.39,40 Figure 9 shows the evolution of the surface composition at the air-water interface for SDS/C12E6 in 0.1 M NaCl for a solution composition of 70/30, with and without 50 ppm polyDMDAAC. The solid lines are calculated from Holland’s extension of regular solution theory, RST,41 to surface mixing. The parameters used in the calculation are summarized in Table 5. To first order the effect of the polyDMDAAC on the adsorption behavior can be quantitatively accommodated by assuming that the polyDMDAAC acts like an electrolyte, reducing the cmc of the SDS and of the mixture (as listed in Table 5). A reduction in the SDS cmc from 2 × (39) Creeth, A. M.; Staples, E.; Thompson, L.; Tucker, I.; Penfold, J. J. Chem. Soc., Faraday Trans. 1996, 92, 589. (40) Creeth, A. M.; Cummins, P. G.; Staples, E. J.; Thompson, L.; Tucker, I.; Penfold, J.; Thomas, R. K.; Warren, N. Faraday Discus. 1997, 104, 245. (41) Holland, P. M. Colloids Surf. A 1986, 19, 171. (42) Rojas, O. J.; Neumann, R. D.; Claesson, P. M. J. Colloid Interface Sci. 2001, 237, 104.

Figure 9. Variation of surface composition for 70/30 SDS/ C12E6/0.1 M NaCl/pH 2.4 with concentration at the air-solution interface, (b) SDS, (O) C12E6, (red) without, (blue) with 50 ppm polyDMDAAC (from ref 40). The solid lines are RST calculations as described in the text and using the parameters in Table 5.

10-3 to 7 × 10-4 is equivalent to an addition of 0.2 M NaCl. The corresponding reduction in the mixed cmc results in a surface interaction parameter, βs, which is less negative, consistent with the addition of electrolyte.41 The equivalent number of moles of charge in solution from the polyDMDAAC is by comparison