Competition between Polymers for Adsorption on Silica: A Solvent

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Competition between Polymers for Adsorption on Silica: A Solvent Relaxation NMR and Small-Angle Neutron Scattering Study Catherine L. Cooper,*,† Terence Cosgrove,† Jeroen S. van Duijneveldt,† Martin Murray,‡ and Stuart W. Prescott*,§ †

School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. AkzoNobel, Wexham Road, Slough, Berkshire SL2 5DS, U.K. § School of Chemical Engineering, University of New South Wales, Kensington NSW 2052, Australia ‡

ABSTRACT: The competition between poly(vinylpyrrolidone) and poly(ethylene oxide) for adsorption at the silica surface was studied by solvent relaxation nuclear magnetic resonance and small-angle neutron scattering. The additive nature of the NMR relaxation rate enhancement was used to observe changes in the train layer when the two polymers were in direct competition for an increasing weight percentage of silica. PVP is shown to displace preadsorbed PEO from the particle surface, and this was observed for a range of PVP molecular weights. SANS measurements were found to give detailed information on the adsorption of the polymers in the separate systems; however, only qualitative information on the adsorption of the polymers could be obtained from the mixed samples. At a total polymer concentration of 0.4% w/v with 1.1% w/v silica, the SANS data were consistent with PVP adsorbing at the surface and dPEO remaining in solution, in agreement with the NMR data.

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INTRODUCTION Water-based commercial formulations tend to contain a complex mixture of particles and stabilizers. Interactions or competition between the adsorbing species is therefore highly influential on the stability and properties of the final product. Adsorbed polymers are frequently used to provide steric stabilization,1 and a range of techniques can be used to study the adsorbed layer, such as light scattering2−4 and small-angle neutron or X-ray scattering.5,6 Competition between polymers for adsorption at a surface can be complicated, with factors such as molecular weight and surface chemistry causing large differences in the adsorption properties due to the cumulative effect from the adsorption of multiple segments.7 Longer chains in solution have less translational entropy per monomer than shorter chains of the same polymer; however, they gain approximately the same energy upon adsorption.8 It is therefore more favorable for higher molecular weight polymer to adsorb from solution, although shorter chains are more mobile and may adsorb preferentially before equilibrium is reached.9−11 Competitive adsorption between poly(ethylene oxide) (PEO) and poly(4-vinyl-N-n-propylpyridinium bromide) (PVPB) onto silica at pH 4 was observed by Kawaguchi et al.12 When the polymers were adsorbed individually, PVPB was found to give a plateau adsorbed amount that was 1.5 times greater than that of PEO. It was therefore expected that PVPB would adsorb preferentially; however, PEO at molecular weights above and below that of the PVPB was both found to displace PVPB chains from the silica surface. This was © XXXX American Chemical Society

rationalized by the strength of the hydrogen bonds between PEO and silica in comparison with the hydrophobic interactions between PVPB and siloxane groups. The adsorption of PEO13−19 or poly(vinylpyrrolidone) (PVP)20−24 onto silica has been studied in some detail, providing a useful model system in which to observe competition effects. The adsorption mechanism for both polymers involves the formation of hydrogen bonds between surface silanols and oxygen atoms on the polymer,25−27 leading to direct competition for adsorption sites upon the silica. Cohen Stuart et al. used low molecular weight proton acceptors such as dimethyl sylfoxide and pyridine to displace PVP from the silica surface.28,29 A segmental adsorption energy in the region of 4 kBT was calculated for PVP adsorbing from water, indicative of a strong interaction with the silica surface. Poly(ethylene oxide) was found by Trens and Denoyel to have a much weaker adsorption to silica.30 Microcalorimetry was used to obtain a segmental adsorption energy of 1.2 kBT for a range of PEO molecular weights. Polymer layers on particle surfaces can be studied indirectly using the mobility of the solvent, measured by relaxation nuclear magnetic resonance.31,32 The adsorption of PEO to silica was found to give an enhancement to the relaxation rate,33 due to an increase in the proportion of adsorbed water molecules at the surface and/or an increased residence time of Received: July 5, 2013 Revised: September 20, 2013

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spectrometer, using the Carr−Purcell−Meiboom−Gill (CPMG) pulse sequence.39,40 A spacing of 2 ms between the 90° and 180° pulse was used, and a recycle delay of at least 5 times the spin−lattice relaxation time between consecutive scans was necessary to ensure full recovery of the magnetization between acquisitions. The single data point at the top alternate echoes are collected to reduce errors from phasing offsets.41 Typically, 8192 data points were collected for each scan, and the signal was averaged over eight scans for each sample. The spin−spin relaxation rate, R2, was found by fitting each magnetization decay curve, My(t), versus time t to eq 1 using a nonlinear least-squares algorithm:

each solvent molecule. At higher polymer molecular weights, loops and tails are formed in the adsorbed layer and the train segment density is only weakly affected. The relaxation rate enhancement was found to be independent of the molecular weight, indicating that the technique is sensitive only to the train segments of the adsorbed layer. The desorption of poly(vinylpyrrolidone) with increasing pH was also shown, with complete desorption occurring above pH 11.33 Solvent relaxation NMR has recently been used to investigate competition effects in colloidal systems involving polymers and surfactants34,35 or competition arising from multiple particle types.36 Competition between PVP and PEO for adsorption onto silica was studied by Nelson et al.37 The rate enhancement from adsorption was found to be different for each polymer, and as the solvent is still in fast exchange between the surface and the bulk, a weight-averaged relaxation rate was measured for the system. This allowed the rate enhancements for each component to be added, so comparisons could easily be made between a competitive system and samples containing only one polymer. At low silica concentrations, the relaxation rate of the PVP/PEO/silica system was the same as when only PVP was adsorbed, indicating that PVP completely displaced the preadsorbed PEO. At higher silica concentrations, once all the PVP had adsorbed to the surface, a relaxation rate consistent with simultaneous adsorption of PVP and PEO was measured; however, it was unknown if the polymers coadsorbed in a mixed layer or onto separate particles. Here we extend the work by Nelson et al. to investigate the region of the PVP/PEO/silica system where there is no longer sufficient polymer to cover the entire available surface. In addition to NMR measurements, we also study polymer competition using small-angle neutron scattering (SANS) by matching the contrast of the solvent to that of silica and observing changes in the polymer layer volume fraction profile.

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My(t ) = My(0)e−R2t

(1)

where My(0) is the transverse magnetization at the beginning of the pulse sequence. All the relaxation decay curves measured here could be fitted to a single-exponential decay curve. Small-Angle Neutron Scattering. Measurements were carried out at 25 °C on the SANS 2d instrument at the ISIS Pulsed Neutron Source (STFC Rutherford Appleton Laboratory, Didcot, U.K.). SANS 2d is a “white beam” time-of-flight instrument which uses incident wavelengths from 2.2 to 10.0 Å, giving a Q range from 0.002 to 3 Å−1.42 The samples were placed in Hellma rectangular quartz cells with a 2.0 mm path length. The solvent for the contrast matched samples contained 58.6% w/v D2O and 41.2% w/v H2O in order to match the scattering length density (SLD) to that of silica (3.51 × 10−6 Å−2), and hence we observe only the scattering from the adsorbed polymer layer. The SLD of d-PEO is 6.88 × 10−6 Å−2, and that of 4 × 104 g mol−1 PVP is 1.39 × 10−6 Å−2, giving different contrast conditions depending upon which polymer is adsorbed at the particle surface. A sample containing 1.99% w/v silica in 42.5% w/v D2O was also run at the Institut Laue-Langevin in Grenoble, on the instrument D22. This gave a scattering length difference of 1.09 × 10−6 Å−2, allowing the particle to be visible.

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THEORY Relaxation NMR. The spin−spin relaxation rate, R2, of protons within the solvent molecules is inversely proportional to the mobility of the solvent in the system. The relaxation rate of protons within free water, R2f, is approximately 0.4 s−1. Protons within water molecules bound to a surface have a faster relaxation rate, R2b, as their motion becomes anisotropic and more restricted, improving the efficiency of the relaxation process.43 When there is fast exchange of the molecules between the two environments, the technique will detect an average rate, weighted by the fraction of time that a proton spends in each environment:

EXPERIMENTAL SECTION

Materials. The colloidal silica used in this study was Bindzil 40/ 220, kindly provided by Eka Chemicals in the form of an aqueous dispersion, stabilized by a small amount of sodium hydroxide to give a pH of around 10. The particle surface area was stated by the manufacturer as being 220 m2 g−1, measured by Sears titration.38 The silica dispersion was extensively dialyzed against Milli-Q plus water prior to use, resulting in a pH between 7 and 8. The number-average diameter of the particles was found to be 18.2 nm from transmission electron microscopy (TEM) measurements. Poly(vinylpyrrolidone) with weight-average molecular weights of 1 × 104, 4 × 104, and 7 × 105 g mol−1 were obtained from SigmaAldrich, Polysciences Inc., and BDH, respectively, and used as supplied. Poly(ethylene oxide) with a molecular weight of 1 × 106 g mol−1 and a low dispersity (Mw/Mn = 1.06) was obtained from Polymer Laboratories. Deuterated PEO was used for the small-angle neutron scattering experiment; this had a molecular weight of 1.49 × 105 g mol−1 and was purchased from Polymer Source Inc. Stock polymer solutions were prepared using Milli-Q water and agitated on a roller mixer overnight, before further dilution for use in samples. A known quantity of dialyzed particle stock was added to the dilute polymer solutions to give a specific final polymer and particle concentration. An equilibration time of at least 12 h was allowed before measurement. The samples containing both polymers were prepared by adding the silica stock to a dilute PEO solution first and then allowing 12 h equilibration before adding the necessary quantity of PVP stock solution to give the appropriate final concentration. The samples were rotated on a roller mixer for a day before measurement. Relaxation NMR. Spin−spin relaxation time measurements of the water 1H nuclei were performed on a Bruker 400 MHz NMR

R 2 = (1 − pb )R 2f + pb R 2b

(2)

where pb is the time-independent probability of finding a randomly chosen solvent molecule bound to the silica surface. In the absence of a polymer or surfactant, the relaxation rate for a solvent in a particulate dispersion scales linearly with the available particle surface area due to the increase in pb. Solvents associated with free polymer or within polymer loops and tails have no significant change in relaxation rate as they are still highly mobile.33 However, when a polymer is adsorbed as a train layer at the particle surface, there is an enhancement in the overall relaxation rate due to an increased proportion and/or residence time of water molecules in the near-surface regions.34,37 The specific relaxation rate constant, R2sp, is a useful way of presenting solvent relaxation data in complex system, defined as B

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R2 −1 R 2°

∞

δrms =

(3)

where R2° is the relaxation rate of a suitable background sample, usually a sample of pure solvent. Small-Angle Neutron Scattering. The theory of scattering from polymer-coated particles has been covered by previous publications, so here only a brief description is given.34,44−46 The scattering cross section was measured as a function of the momentum transfer, Q, which is related to the scattering angle θ, and the wavelength of the incident neutrons, λ: Q=

⎛θ⎞ 4π sin⎜ ⎟ ⎝2⎠ λ

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Figure 1. Enhancement in R2sp when different molecular weights of PVP are adsorbed onto silica: bare silica (○); 0.1% w/v 10K PVP (■); 0.1% w/v 40K PVP (green ■); 0.2% w/v 40K PVP (green ▲); 0.2% w/v 700K PVP (red ▲); 1.0% w/v 700K PVP (red ●).

dispersions with and without PVP. The R2sp was calculated using the relaxation rate of Milli-Q water as the normalization factor, as described in eq 3. The relaxation rate from samples containing an increasing concentration of silica in the absence of polymer can be fitted with a straight line through the origin. This is as predicted from eq 2, where as we increase the amount of surface in the system, the proportion of bound water molecules, pb, will also increase. This also indicates that there is fast exchange occurring between the water at the surface and the highly mobile water in the bulk. The solid line in Figure 1 is a linear fit showing the enhancement in the relaxation rate when there is excess polymer in the system. Here there is 1.0% w/v PVP, which corresponds to a concentration of between 0.9 and 4.5 mg m−2 in the range studied; at these concentrations the surface is saturated with polymer train segments. When such saturation occurs, the measured R2sp is independent of the amount of nonadsorbing polymer in the solution, as these chains do not sufficiently reduce the mobility of the bulk water molecules.33 When the polymer adsorbs to the silica, the segments adsorbed as trains at the surface further reduce the mobility of the nearby water molecules, while still retaining the fast exchange requirements, therefore giving an enhancement in relaxation. Transition points occur in the data for the systems containing 0.1 and 0.2% w/v PVP, where the gradient switches from that of the enhanced relaxation rate to that of the bare silica dispersion. These occur at the point where there is no longer sufficient polymer to cover the entire surface area in the system. Upon addition of further silica, the proportion of the total surface area that is unoccupied by adsorbed polymer increases, giving the gradient characteristic of the bare silica.

(5)

(6)

When modeling on-contrast layer scattering, it has previously been found that a good fit to the data can be obtained by assuming that the volume fraction of polymer, φ(z), decays exponentially from the particle surface.34,50,51 Hence, in the model used here ⎛ z⎞ φ(z) = φs exp⎜ − ⎟ ⎝ z0 ⎠

(8)

(4)

In the contrast-matched samples in this study, the solvent is chosen such that Ipp(Q) and Ipl(Q) terms are zero and only the scattering from the layer and the incoherent background is observed. The layer scattering can be subdivided into two components: scattering resulting from the average structure of the layer, I(̅ Q), and scattering arising from the concentration fluctuations within the layer, I(̃ Q):45 Ill(Q ) = I ̅(Q ) + I (̃ Q )

∞

∫z = 0 φ(z) dz

RESULTS AND DISCUSSION NMR Studies of Polymer Adsorption. Figure 1 shows the specific relaxation rate (R2sp) for a range of aqueous silica

Initially, the data were modeled using a core−shell form factor model with polydispersity and a structure factor based on the Hayter−Penfold model. We have previously shown that the particle and adsorbed layer can be modeled as two concentric spheres,44 with a log-normal distribution of core size and a constant shell thickness.47 The Hayter−Penfold model adds an interparticle screened Coloumbic structure factor, S(Q).48,49 The drawback of this approach is the assumption that the scattering length density within the layer is uniform, leading to a block profile that does not satisfactorily describe the adsorption of a polymer layer. A diffuse layer model was therefore used to treat only the form factor, P(Q), of the polymer layer. The scattering from the polymer-coated particle, I(Q), can be separated into four parts:50 scattering from the particle, Ipp(Q), scattering from the layer, Ill(Q), interference between the particle and the layer, Ipl(Q), and an incoherent background, Iinc, which depends upon the concentration of hydrogen atoms in the sample. I(Q ) = Ipp(Q ) + Ipl(Q ) + Ill(Q ) + Iinc

∫z = 0 φ(z)z 2 dz

(7)

where φs is the volume fraction of the polymer in direct contact with the surface and z0 is the decay length controlling the extent of the profile. The adsorbed amount of polymer can be obtained from the integral of the volume fraction profile multiplied by the polymer density, and we define the bound fraction as the fraction of polymer within 10 Å of the polymer surface. The root-mean-square (rms) layer thickness, δrms, is given by C

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the particle interface to a greater extent than poly(ethylene oxide), despite the difference in molecular weight. Polymer Competition Studied by NMR. Figure 4 shows the results of an experiment in which 0.1% w/v PEO was

The black, green, and red points in Figure 1 correspond to PVP molecular weights of 1 × 104, 4 × 104, and 7 × 105 g mol−1, respectively. It was found that the R2sp was independent of molecular weight across the range studied, in agreement with the predictions of van der Beek et al. following measurement of a range of PVP and PEO molecular weights.33 The relaxation rate enhancement is related to the number of surface bound polymer segments, which is molecular weight independent. Figure 2 shows an analogous system for the adsorption of poly(ethylene oxide) on the colloidal silica particles. The

Figure 4. Displacement of 0.1% w/v preadsorbed 1000 kg mol−1 PEO by 0.1% w/v of different molecular weights of PVP: 10K (◆); 40K (green ◆); 700K (red ◆). Also shown are the fitted lines from Figures 1 and 2 for 0.1% w/v PVP adsorption, 0.1% w/v PEO adsorption, and bare silica. The solid line was calculated using the gradients of the individual polymer adsorption data to match the measured R2sp.

Figure 2. Enhancement in R2sp when 1000K PEO is adsorbed onto silica at different initial polymer concentrations: bare silica (○); 0.1% w/v (green ■); 1.0% w/v (▼).

initially adsorbed onto silica, and 0.1% w/v PVP was added after an equilibration time of 12 h. The lines of best fit from the single polymer adsorption experiments (Figures 1 and 2) are also included for comparison. The initial gradient of the experimental points in Figure 4 is identical to that observed for the pure PVP system, for all the molecular weights studied. This observation is the same as seen by Nelson et al. for 0.2% w/v 4000K PEO and 0.2% w/v 700K PVP in competition.37 The similarity to the single polymer samples in this region indicates that the train density of PVP at the silica surface is the same in both the presence and absence of additional PEO. We assume that the water molecules are in fast exchange between different relaxation environments, so the contributions from each relaxation rate in the system are additive. A mixed adsorption layer of PVP and PEO would therefore have a relaxation rate equal to the weighted average of the individual components and give a transition point at a distinctly different silica concentration. For this experiment, PEO had been preadsorbed, so the presence of only PVP at the surface after equilibration indicates that the PEO has been displaced. This situation is described in Figure 5 and is a consequence of the difference in adsorption energy parameters for the two polymers.28,30 After the transition point for the PVP on silica system, the gradient of the competition points switch to that of excess PEO adsorbed upon the colloidal silica, although the absolute R2sp is shifted to a higher value. This shift indicates that the adsorbed PVP is still present at the surface, and now that PVP has adsorbed to the maximum extent, the PEO is able to adsorb onto the excess bare silica surface. The additive nature of the relaxation rate allows us to see clearly that both polymers are adsorbed simultaneously; however, it is not possible to

transition point can be seen at 4.1% w/v silica. This compares to an equivalent transition point of 1.0% w/v in Figure 1 for the same percentage weight of poly(vinylpyrrolidone), highlighting the stronger affinity that PVP has for silica, with an adsorption energy parameter, χs of approximately 4kBT28 as opposed to only 1.2kBT for PEO adsorption.30 The adsorption of PVP also leads to a greater enhancement in the relaxation rate, as shown in Figure 3. For an equivalent surface area of silica, the adsorption of poly(vinylpyrrolidone) restricts the mobility and/or increases the amount of water at

Figure 3. Comparison between the enhancement in R2sp upon adsorption of 1.0% w/v 700K PVP (red ●) or 1.0% w/v 1000K PEO (▼) onto bare silica (○). D

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Figure 5. Schematic representation illustrating the displacement of PEO, with a radius of gyration, Rg, of approximately 36 nm, from the surface of silica particles by the addition of 40 kg mol−1 PVP (Rg ≈ 3 nm).

determine the exact locations of the two polymers in this system. A second transition point can also be seen at approximately 5.1% w/v silica, equal to the sum of the concentrations of silica at the transition points for pure PVP and PEO adsorption. This is predicted in the work by Nelson et al. but was not seen due to the instability of samples above about 6.2% w/v silica. Here we used smaller polymer concentrations to reduce the risk of flocculation and to lower the predicted position of the transition point. At 5.1% w/v silica with 0.1% w/v of each polymer, there is a total polymer concentration of 0.17 mg m−2, which corresponds to the point where there is no longer a sufficient amount of either polymer to coat all the available surface area. The gradient of the line returns to that of bare silica, but the absolute R2sp values are the sum of the enhancement of both PEO and PVP for these silica concentrations, indicating that both polymers are still adsorbed to the surface. The similarity of the experimental points using 10K, 40K, and 700K PVP indicates the strength of the adsorption in comparison to that of PEO. It might be expected that a PVP molecule with a sufficiently small chain length might fail to displace the 1000K PEO molecule from the silica surface; however, the production of low molecular weight PVP chains with a low dispersity is difficult, and the presence of even a small number of larger chains may also contribute to PEO desorption. SANS Studies of Polymer Adsorption. Small-angle neutron scattering experiments were conducted on samples containing 1.1% or 2.1% w/v SiO2 and combinations of PVP and PEO. The initial polymer concentrations used in the SANS experiments correspond to just below train layer saturation for PVP at 0.4 mg m−2 and substantial coverage at a concentration of 0.8 mg m−2. Relatively low concentrations were chosen to limit the scattering arising from free polymer, which could obscure the scattering from the diffuse polymer layer. Free polymer was found to require an extra Guinier−Debye model to fit SANS data at concentrations of 1.5 mg m−2 PVP in earlier work done by this group.34 Figure 6 shows the scattering data from a sample containing 2.1% w/v Bindzil 40/220 silica in 42.5% w/v D2O and the fit to a hard-sphere model with polydispersity and a Hayter−Penfold structure factor. The parameters used to produce this fit are outlined in Table 1, and these were used as a starting point to fit the remainder of the samples. The particle size and polydispersity as measured by SANS are consistent with that measured by TEM. Figure 7 shows the scattering measured on SANS2D at ISIS, Didcot, arising from adsorbed PVP in a system where the solvent has been contrast matched to the scattering length density of the silica particles, as determined from a plot of 5.4 ± 0.1% w/v silica in varying H2O/D2O ratios. The data in Figure

Figure 6. SANS from 2.1% w/v Bindzil 40/220 silica in 42.5% w/v D2O, fitted to a hard-sphere model with particle size polydispersity and a Hayter−Penfold model for the S(Q). The inset shows the separate form and structure factors and the resulting combined fit.

7 are fitted to a diffuse core−shell model 44,45 with polydispersity of the particle size and the Hayter−Penfold structure factor model.48,49 The fit was carried out with the fitting program Playtime, using a nonlinear least-squares routine with the Levenberg−Marquardt algorithm.52−54 Figure 8 shows analogous systems with adsorbed deuterated PEO instead of PVP. There is a greater intensity from the layer scattering than seen for the PVP samples due to the larger contrast between the deuterated layer and the solvent. The diffuse layer model allowed volume fraction profiles to be obtained, as shown for both PVP and PEO in Figure 9. Additional parameters that can be obtained from the fitting, such as the adsorbed amount, the bound fraction of the polymer, and the rms layer thickness, δrms, are also shown. The volume fraction profiles highlight very different adsorption characteristics for the two polymers; PEO gives an extended layer with a lower bound fraction at the surface, whereas PVP forms a compact layer with a higher proportion of polymer train segments. This corresponds well with the NMR data, which detects the train layer adsorption rather than the entire polymer layer. The adsorbed amount of PEO calculated from the volume fraction profiles corresponds well with previous work by Flood et al., who found an adsorbed amount of 0.71 mg m−2 when 1 mg m−2 PEO was added to 25 nm diameter silica.55 The adsorbed amount of PVP found from the SANS data fits well with the adsorption isotherm obtained from higher molecular weight PVP on the same silica particles.36 The adsorbed amounts are higher than that found recently by Cattoz et al.;34 however, the Bindzil silica in that paper had not been dialyzed so differences in the pH of the system and changes in the surface chemistry of the particle can explain the difference. E

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Table 1. Parameters Used To Generate the Fit in Figure 6 SLD difference particle−solvent/Å−2 volume fraction radius/Å polydispersity Q resolution total surface charge/no. electrons Debye length/Å S(Q) scale factor background

2% SiO2 off contrast

fitting variable

1.06 × 10−6 ± 0.34 × 10−8 0.009 05 94.4 ± 0.26 0.154 0.006 29 29.9 ± 0.63 213 ± 28 1.00 (0.637 ± 0.31) × 10−3

Y

Figure 7. SANS from 0.2% w/v PVP physisorbed onto 1.1% w/v (□) and 2.1% w/v (○) Bindzil 40/220 silica, fitted to a diffuse layer core− shell model with particle size polydispersity and a Hayter−Penfold potential. The solvent is contrast matched to the silica, giving scattering only from the polymer layer.

Y

Y Y Y

Figure 9. Volume fraction profiles for polymer adsorbed on silica, acquired from analysis of SANS data using the diffuse layer model.

some form of exchange or combination does occur, agreeing with the observations from the NMR measurements. The complexity of these samples provides a challenge for fitting the data to a diffuse layer model in the same manner as the single polymer data above. Figure 11 shows a diagram of the predicted polymer locations and the resulting differences in contrast between the layer and the solvent. However, this gives an oversimplification as free polymer in solution, particularly

Figure 8. SANS from 0.2% w/v PEO physisorbed onto 1.1% w/v (■) and 2.1% w/v (●) contrast matched Bindzil 40/220 silica, fitted to a diffuse layer core−shell model as described above.

Polymer Competition Studied by SANS. Samples containing 1.1 or 2.1% w/v silica and 0.2% w/v of each polymer were prepared in a similar manner to the NMR samples, with 24 h equilibration time between addition of the two polymers and longer before measurement. The order of polymer addition was investigated; however, the resulting scattering was identical, as shown for the 1.1% w/v silica system in Figure 10. The similarity between the samples confirms that

Figure 10. Scattering arising from 1.1% w/v silica with 0.2% w/v dPEO and 0.2% w/v PVP both present in the system. The solvent is contrast matched to the silica, so scattering only arises from the polymer. Green points indicate that the dPEO was preadsorbed before addition of PVP (green ■), and gray points indicate samples where the PVP was adsorbed first (gray ■). F

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Without knowing the exact composition of the layer it becomes difficult to measure the adsorbed amount of polymer. Multicomponent samples such as these are challenging to analyze, but it is hoped that the combination of polymer layers and polymer in solution could provide a starting point for the analysis of other complex SANS data from colloidal systems.

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CONCLUSIONS Solvent relaxation NMR was used to observe the displacement of PEO from silica on addition of PVP. The mobility of the water molecules is influenced by the polymer adsorption, leading to a different enhancement in relaxation when each of the polymers adsorb at the silica surface. It was observed that the relaxation rate enhancement was independent of the PVP molecular weight due to the sensitivity only to the train layer of the adsorbed polymer.33 Competition experiments between the two polymers were performed with 0.1% w/v of each polymer for the NMR experiments. This allowed the observation of an additional transition point in the graph, where the change in the gradient shows the region at high silica concentration where both polymers are adsorbed to their maximum extent and there is also bare silica. The competition experiment also supported the conclusions from the earlier work by Nelson, as the displacement of preadsorbed PEO was seen on the addition of PVP to the system. This was also independent of the PVP molecular weight, despite the PEO having a significantly higher molecular weight. Small-angle neutron scattering was also used to investigate the adsorption of PVP and PEO onto silica. Samples where the polymer was adsorbing individually were fitted to diffuse layer core−shell models with particle size polydispersity and the Hayter−Penfold structure factor model. This allowed the calculation of volume fraction profiles, which were seen to give different adsorption characteristics for each polymer. At an initial concentration of 0.81 mg m−2, adsorption of dPEO led to an extended layer of 30 Å with a low volume fraction at the surface and adsorbed amount of 0.52 mg m−2. PVP adsorption, however, gave an adsorbed amount of 0.64 mg m−2 with a

Figure 11. Schematic diagrams to show the scattering length densities of the different components in the mixed polymer systems. A 50:50 mixture of polymers is assumed for the mixed layer sample, and the numbers given for the SLD are represented as factors of 1 × 10−6 Å−1.

dPEO, will lead to different scattering across the Q-range measured. Initial fits for the mixed polymer samples can be estimated from the addition of polymer layer scattering to that of the calculated scattering arising from nonadsorbing polymer in the solution. It can be seen in Figure 12 that the scenario in which only the PVP is adsorbed upon the silica gives the best fit to the scattering data across the entire Q-range, both for the sample in which it had been added before dPEO and the sample where it was added after dPEO was preadsorbed. This means that in combined samples, similar to those used in Figure 4, the preferential adsorption of PVP on the silica surface leads to the displacement of PEO. The possible presence of a mixed polymer layer significantly complicates the analysis of the samples containing 2.1% w/v silica. The calculated fits arising from the combination of one polymer on the surface and one polymer in solution were unable to give a satisfactory match to the measured data. However, comparison with the NMR data indicates that at a total polymer concentration of 0.81 mg m−2 we expect both polymers to be adsorbed either as a mixed layer or separate polymers on each particle. The assumption of an equal ratio mixed layer with an SLD of 4.13 × 10−6 Å−1 gives a very small contrast with the solvent, so the scattering is dominated by any free polymer in solution.

Figure 12. SANS from 1.1% w/v silica with 0.2% w/v dPEO and PVP, with symbols as in Figure 10. Also shown are two polymer adsorption scenarios obtained by combining the fit for the polymer on the particle (green line) with the polymer in solution (gray line). The black lines show the sum of the two contributions, and it can be seen that the situation in which the PVP is adsorbed to the silica, on the left, gives a better fit than adsorbed dPEO. G

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much larger fraction of polymer at the surface and root-meansquare layer thickness of 11 Å. These observations agreed well with previous measurements on both polymers.34,55 The samples that contained a mixture of polymers competing for adsorption required a more complex analysis. The scattering was independent of the sample preparation order, indicating exchange or association between the polymers at the surface. The systems containing an excess of polymer (with 1.1% w/v silica) were fitted to a model with PVP at the silica surface and dPEO present as nonadsorbed coils in solution. It was expected that PVP would be primarily at the surface in these samples, as this corresponded well with the NMR data. The samples with 2.1% w/v silica and 0.2% w/v of each polymer required the combination of a fit for a mixed layer of polymer at the surface and both polymers in solution; however, the resulting fit was dominated by the calculated scattering so no information about the polymer layer could be extracted. The NMR data suggest that either a mixed layer is formed in these systems or there is a mixture of silica particles with individual polymers adsorbed. Although small-angle neutron scattering is a powerful technique for characterizing polymer adsorption on silica nanoparticles, the data for complex adsorption scenarios are much more difficult to fit. However, the NMR technique can be shown to be informative for multiple adsorption systems due to the additional nature of the relaxation rate at the fast exchange limit and the distinct difference in the enhancement for each polymer.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (C.C.). *E-mail [email protected] (S.W.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The EPSRC and AkzoNobel are gratefully acknowledged for funding this research, and Eka Chemicals is also thanked for providing the colloidal dispersions. The U.K. Science and Technology Facilities Council (STFC) and the Institut Laue− Langevin (ILL) are thanked for allocation of beamtime and grants towards travel and accommodation. The authors also thank Steve King, Isabelle Grillo, and Emma KastrisianakiGuyton for their help during experiments.

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