Langmuir 1998, 14, 997-1001
997
Solvent Relaxation NMR Measurements on Polymer, Particle, Surfactant Systems S. J. Mears,† T. Cosgrove,*,† L. Thompson,‡ and I. Howell‡ School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, U.K., and Unilever Port Sunlight Laboratory, Quarry Road East, Bebington, The Wirral Received July 7, 1997. In Final Form: November 3, 1997 1H NMR solvent relaxation has been used to probe the effect of SDS on the adsorption of poly(ethylene oxide) at the silica-water interface. In the absence of SDS an enhancement in the water relaxation rate resulting from the adsorption of PEO was observed while for PEO in the absence of particles only a negligible enhancement was observed. However, upon addition of SDS to the PEO/silica system a sequential reduction in the relaxation rate with increasing SDS was found. The results are discussed in terms of the levels of adsorbed polymer and its conformation at the interface.
Introduction There are many techniques which may be used to investigate polymer adsorption, each sensitive to a particular facet of the adsorption process. For example, static and dynamic light scattering may be employed to extract hydrodynamic data whereas contrast variation techniques in small-angle neutron scattering measurements allow components in a mixture to be ‘viewed’ individually. In this paper, we investigate the dynamics of the solvent using pulsed NMR techniques to examine an adsorbed polymer layer in the presence of a surfactant. In previous papers we have investigated the particle/ polymer/surfactant system using adsorption isotherms, photon correlation spectroscopy (PCS),1 and small-angle neutron scattering (SANS).2 For poly(ethylene oxide) (PEO) and silica, it was shown that the addition of even small quantities of sodium dodecyl sulfate (SDS) ( 99%) and was used as received. Adsorption. The adsorption isotherm for PEO was determined using traditional depletion methods. The excess adsorbed amount of polymer, Γ, is determined by measuring the change in bulk polymer solution from its initial value (Cinit) to the equilibrium concentration (Ceqm) on exposure to a known surface area A of adsorbent. The variation of Γ with Ceqm gives the adsorption isotherm. In these techniques free polymer is separated from the adsorbed polymer by centrifugation (14 000 rpm for 30 min). For PEO, the concentration of polymer remaining in the supernatant was detected by complexation with tannic acid using UV/visible spectroscopy.7 Unfortunately, the addition of SDS causes the nature of the PEO/tannic acid complex to be affected. The result is a loss of linearity of the calibration plot which renders this method useless. NMR. The relaxation measurements were carried out using a JEOL FX-200 spectrometer upgraded with a SMIS (Surrey Medical Imaging Systems) console which replaces the computational and RF parts of the JEOL system. The standard CarrPurcell-Meiboom-Gill (CPMG) sequence was employed, and the spin-spin relaxation times were extracted by a nonlinear least-squares analysis of eq 3:
My(τ) ) My(0) exp(-τ/T2) + B
(3)
where My(τ) is the instantaneous signal intensity between even pairs of 180° pulses separated by a time τ. B is a baseline offset.
Results and Discussion In the fast exchange limit all the components of the system will make an additive contribution to the solvent relaxation rate.3 The behaviors of bare silica, SDS, and PEO were thus first examined. The 1H spin-spin (5) Meiboom, S.; Gill, D. Rev. Sci. Instrum. 1958, 29, 688. (6) Snowtex Data Sheets; Nissan Chemical Industries Ltd., Specitality Chemicals Division: Tokyo, Japan. (7) Nuysink, J.; Koopal, L. K. Talanta 1982, 29, 495.
Figure 1. Specific transverse (R2sp) relaxation rates of the solvent for aqueous dispersions of the silicas Snowtex 50 and Snowtex 40 as a function of solids concentration in H2O. The relaxation rates have been normalized against H2O for all of these measurements. Snowtex50 has a diameter of 20-30 nm, and Snowtex 40 has a diameter of 11-14 nm.
relaxation times were determined from the amplitudes of the spin-echoes produced by a CPMG sequence.5 Figure 1 shows the transverse (R2sp) relaxation rates of aqueous dispersions of the silica Snowtex 50 as a function of solids concentration. As expected, within the range examined, there is a linear relationship between the available surface area and the relaxation rate which indicates that there is fast exchange8 occurring between water molecules in the bound state and highly mobile free water molecules. The slope of this line is proportional to R2spbsthe averaged specific relaxation rate of protons near the surface:
R2sp ) PbR2spb
(4)
The fraction of these protons Pb is proportional to the amount of silica per unit solvent volume. Also shown in Figure 1 is the transverse relaxation rate for Snowtex 40, which is identical in composition to Snowtex 50 but with a much smaller radius (and hence increased surface area). This is reflected in the much greater specific relaxation rate for a given solids concentration. Cosgrove et al.8 measured the specific relaxation rate of a series of different sized silica particles as a function of silica concentration, and in all cases with increasing surface area per volume, the amount of bound water increased accordingly. A linear relationship was obtained between relaxation rate enhancement and surface area which is also confirmed by the data shown in Figure 2 for Snowtex 40 and Snowtex 50 dispersions. Figure 3 shows the transverse specific relaxation rates of 114 000 Mw PEO as a function of polymer concentration. These data have been normalized with respect to pure water. It can be observed that the values of R2sp show an increase, albeit relatively small, with concentration. This increase was not observed in earlier studies3 but is most likely either a result of the higher molecular weight polymer or the increased resolution of the instrument used (8) Cosgrove, T.; Griffiths, P. C.; Lloyd, P. M. Langmuir 1995, 11 (5), 1457.
Solvent Relaxation NMR Measurements
Figure 2. Specific surface area versus relaxation rate enhancement for two different sized silica particles.
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Figure 4. Specific relaxation rate data for the adsorbed polymer from Figure 3 versus the adsorbed amount of the polymer.
of the polymer segments did not contribute equally to the relaxation rate enhancement. It was suggested that this enhancement was proportional to the adsorbed amount in trains (Γtrains) according to eq 5
Γtrains ) k-1R′2sp
(5)
where R′2sp is the specific relaxation rate normalized against silica rather than water and k is a proportionality constant. Furthermore, the bound fraction 〈p〉 may be calculated:
〈p〉 )
Figure 3. Specific transverse relaxation rate of the solvent for 114 000 molecular weight PEO in H2O as a function of polymer concentration (b). Also shown is the corresponding data for PEO (O) in the presence of 5.0% w/w Snowtex 40 as a function of initial polymer concentration. The data for the adsorbed PEO have been normalized against those of the silica dispersion.
in this study. Also shown in Figure 3 is the corresponding data for adsorbed PEO. Since relaxation rates are additive,3 the enhancement from bare silica has been subtracted from these data in order to investigate the effect of adsorption directly. Evidently, from Figure 3, an enhancement is produced which is in excess of that for PEO in solution, suggesting either that the mobility of the water has been substantially constrained or that its residence time in the bound environment has increased. Another factor is that the actual amount of bound water may have changed. The filled circles (PEO in the absence of silica) indicate this effect is not due to free polymer. These effects are in good agreement with those of previous studies.3,8 van der Beek et al.3 found the increase in solvent binding was not proportional to molecular weight, which inferred that all
Γtrains Γtotal
(6)
Therefore, a plot of R′2sp against the total adsorbed amount (Γtotal) (Figure 4) at low coverages will yield a straight line with a slope of k-1, since at low coverages all of the polymer segments lie in trains. Taking the first four data points from Figure 4, k was obtained as 0.145 mg-1 m-2, and this value was inserted into eqs 5 and 6 to obtain values of 〈p〉 as a function of adsorbed amount. This is shown in Figure 5. This figure shows that at low coverages the bound fraction approaches unity, while at higher coverages where longer loops and tails are formed the bound fraction is reduced until at full coverage it is only about 0.15. These data agree very well with those obtained previously by van der Beek et al.3 on a similar system. Figure 6 shows the specific transverse relaxation rates for the surfactant SDS as a function of concentration. The data are in contrast to those in Figure 1 in that, across the whole concentration range under investigation, there is very little enhancement in the relaxation rate. The results may suggest some increase with concentration beyond the cmc of SDS (0.23% w/w), but since this is within the experimental error of these measurements, it is difficult to determine at this stage whether this is real. Since these effects are so small, they would not be noticeable in systems which have very short relaxation times compared to that of water, such as the silica case reported in Figure 1. Also given in Figure 6 are the specific relaxation rates of the solvent in the presence of silica
1000 Langmuir, Vol. 14, No. 5, 1998
Figure 5. Bound fraction versus adsorbed amount for 114 000 molecular weight PEO adsorbed onto 5.0% w/w Snowtex 40.
Mears et al.
Figure 7. Effect of mixtures of PEO and SDS on the specific transverse relaxation rate of the solvent (b). The polymer concentration is maintained at 0.5% w/w, and the SDS concentration is varied. Also shown are the data for pure SDS (O).
Figure 6. Specific relaxation rate of the solvent for SDS with 5.0% w/w silica as a function of SDS concentration (b). Also shown are the data for pure solutions (O).
and SDS. As for PEO in Figure 3, the enhancement from bare silica has been subtracted in order to investigate the effect of silica on SDS directly. SDS does not adsorb onto silica, and therefore no enhancement in the relaxation rate is observed in excess of that for the bulk SDS. This demonstrates beautifully the additive effect of relaxation rates in non-interacting systems. Figure 7 shows the effect of mixtures of PEO and SDS on the solvent relaxation rate. For all of these data the initial PEO concentration is maintained at 0.5% w/w and the SDS concentration is varied in the range 0-1.0% w/w. From the phase diagrams of Cabane,9 the cac for SDS with PEO at this concentration is 0.08% w/w. Therefore, at all of the concentrations measured, SDS and PEO are interacting. Evidently from this figure, there is very little enhancement in the relaxation rate on mixing SDS and PEO. This suggests only a weak interaction between the (9) Cabane, B. J. Phys. Chem. 1977, 81, 169.
Figure 8. Specific relaxation rate of the solvent for mixtures of PEO, SDS, and silica as a function of SDS concentration (O). In all of these samples the polymer concentration is maintained at 0.5% w/w, the silica concentration is maintained at 5.0% w/w, and the SDS concentration is varied from 0 to 1.0% w/w.
two species incorporating little bound water or water that has only a very short residence time when bound to the polymer. Figure 8 shows the effect of SDS on an adsorbed PEO layer. In this instance the silica concentration is maintained at 5.0% w/w and the initial PEO concentration is maintained at 0.5% w/w. This corresponds to an adsorbed amount of 0.6 mg m-2 which is not quite at full coverage (full coverage . 0.8 mg m-2). However, this concentration of polymer was chosen so that the surface was saturated with trains and there would be very little free polymer in the bulk (
