Sodium Polyacrylate Adsorption onto Anionic and Cationic Silica in

Apr 18, 2007 - School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, U.K. ... In contrast, NaPA adsorbs very strongly on catio...
0 downloads 0 Views 125KB Size
Download PDF
Langmuir 2007, 23, 6191-6197

6191

Sodium Polyacrylate Adsorption onto Anionic and Cationic Silica in the Presence of Salts Charlie Flood,* Terence Cosgrove,* and Youssef Espidel School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.

Ian Howell and Patricia Revell UnileVer Research, Port Sunlight Laboratory, Quarry Road East, Bebington, The Wirral CH63 3JW, U.K. ReceiVed January 8, 2007. In Final Form: March 5, 2007 Sodium polyacrylate is well known for its application as a scale inhibitor in common household products, and the effects of both monovalent and divalent metal cations on its structure have been covered by a range of previous publications. In the present article, we extend this work by using solvent relaxation NMR to look at the adsorption of the polyelectrolyte onto both positively and negatively charged silica and how this is altered by calcium chloride. In the anionic case, we found that polyacrylate adsorption was predictably very weak, and interestingly, perhaps counterintuitively, it was further reduced by calcium ions. This is probably linked to NaPA-Ca2+ binding, which changes the conformation and charge of the polyelectrolyte. In contrast, NaPA adsorbs very strongly on cationic silica, to the point that precipitation often occurs, particularly on addition of salt.

Introduction Sodium polyacrylate (NaPA) has a range of uses in industrial, household, and personal care products. Although it has been known for some time that NaPA inhibits calcium carbonate precipitation, a mechanism has only recently been proposed by Fantinel et al.1 By using an infrared technique to study complexation between NaPA and Ca2+ ions, the authors were able to look at the kinetics of the interaction and identify several intermediates prior to formation of the final bidentate chelating complex, where both oxygen atoms in the polyacrylate carboxylate group bond to a single Ca2+ ion. It was also suggested that the strength of binding increases with the molecular weight of NaPA. The proposed mechanism is consistent with the findings of several other studies, including several light scattering and smallangle neutron scattering (SANS) experiments by Schweins et al.2-4 The results highlight the difference between NaCl, which is capable of inducing only a simple counterion condensation of NaPA via its ionic strength5 and CaCl2, which can induce conformational changes via additional specific ion effects attributed to binding between stoichiometric quantities of NaPA and Ca2+. Results showed that varying the solvency from good to theta conditions by increasing the NaCl concentration from 0.1 to 1.5 M results in a decrease in both the hydrodynamic radius, Rh, and the radius of gyration, Rg, as seen with neutral polymers. The ratio of Rg/Rh also decreased, indicating a transition from an extended good solvent configuration to a more collapsed poor solvent one. Furthermore, the molecular weight dependencies of Rg and Rh both obeyed a power law, with the exponent increasing with the NaCl concentration.2 The situation on addition of Ca2+ is more complicated, with precipitation occurring when the Ca2+/NaPA ratio is sufficiently high. As the phase boundary was approached, SANS results exhibited power laws characteristic * Corresponding authors. E-mail: [email protected], [email protected]. (1) Fantinel, F.; Rieger, J.; Molnar, F.; Hubler, P. Langmuir 2004, 20, 2539. (2) Schweins, R.; Hollmann, J.; Huber, K. Polymer 2003, 44, 7131. (3) Schweins, R.; Lindner, P.; Huber, K. Macromolecules 2003, 36, 9564. (4) Schweins, R.; Huber, K. Eur. Phys. J. E 2001, 5, 117. (5) Schiessel, H.; Pincus, P. Macromolecules 1998, 31, 7953.

of objects with sharp boundaries, implying that the extended polyelectrolyte chains collapse in the presence of sufficient Ca2+. Results for partially collapsed polyelectrolyte solutions were best fitted using a model including a mixture of spheres and dumbbells, backing up the hypothesis of pearl-necklace-shaped intermediates.3 One of the most interesting findings was that adding sufficient NaCl to collapsed NaPA-Ca2+ aggregates resulted in a re-expansion of the polyelectrolyte, showing that excess NaCl displaces Ca2+ from the polyacrylate chains and that this effect more than outweighs the additional screening caused by the increased ionic strength.4 A further contribution by Sinn et al. uses isothermal titration calorimetry to offer some insight into the energetics of binding.6 The interaction of NaPA with Ca2+ was compared to that of sodium poly(styrene sulfonate), a polyelectrolyte with a more hydrophobic backbone. Despite the difference, the dilution curves of the two polyelectrolytes were similar, and both are close to ideal. This implies that an entropy gain, specifically due to the removal of water from the hydration shells of NaPA and Ca2+, rather than enthalpy is the driving force for binding. In the case of NaPA, binding to Ca2+ is actually endothermic, but the entropy gain is still sufficient for the process to be spontaneous. Another paper by Goerigk and Schweins used anomalous smallangle X-ray scattering to show that another divalent cation, strontium, can bind in a similar way to Ca2+.7 They were able to study the distribution of Sr2+ ions around polyacrylate chains and conclude that the results corroborated chain collapse and the pearl-necklace model. The binding of simple ions to polyelectrolytes in general was recently reviewed by Borkovec et al. The authors commented that mean-field theory was not sufficient to describe the binding between multivalent ions and polyelectrolytes due to both strong electrostatic interactions and the occurrence of multivalent binding.8 (6) Sinn, C. G.; Dimova, R. D.; Antonietti, M. Macromolecules 2004, 37, 3444. (7) Goerigk, G.; Schweins, R.; Huber, K.; Ballauff, M. Europhys. Lett. 2004, 66, 331. (8) Borkovec, M.; Koper, G. J. M.; Piguet, C. Curr. Opin. Colloid Interface Sci. 2006, 11, 280.

10.1021/la070047z CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007

6192 Langmuir, Vol. 23, No. 11, 2007

In summary, it is clear that NaPA has a specific interaction with Ca2+ ions and can remove them from solution, hence preventing the precipitation of calcium carbonate. However, only a relatively small amount of previous work has considered the effect of added particulates in the solution.9-15 Cationic substrates studied include polystyrene latex with surface quaternary ammonium groups, where it was found that adsorption was low at high pH but increased as the pH was reduced and the polyacrylate chains became more protonated, with not so many Ca2+ ions accumulating around the particle. Varying the ionic strength with inert salts had little effect.9 Vermohlen et al. studied the adsorption of polyelectrolytes onto oxides in the presence of salts and found that both NaCl and CaCl2 increased the adsorbed amount. The adsorbed layer was found to have a flat conformation at low ionic strength, with loops and tails becoming more stable as added salts reduced repulsions between the polyacid chains. It was suggested that calcium ions are more effective at increasing adsorption than sodium ions because of their ability to form bridges between the carboxylate groups of the polyacid.10 Friedsam et al. compared the adsorption energies of polyelectrolytes on different substrates using atomic force microscopy (AFM). By varying the inert salt concentration, they were able to find that the nonelectrostatic contribution to the adsorption energy was dominant even for highly charged substrates, although they did not include divalent cations.11 Also in the AFM field, Stamm et al. have carried out a great deal of work on systems including the aggregation of oppositely charged polyelectrolytes,16 the effects of pH and solvent quality on the adsorption of diblock polyampholytes,17-19 and the counterion distribution within polyelectrolyte brushes.20 Substrates studied include TiO2,12 alumina,13 and silicon, on which Prabhu et al. measured the counterion distribution within swollen polyelectrolyte films, finding counterion depletion near the surface of the substrate.14 In this study, we use solvent relaxation NMR to study the addition of both anionic and cationic silica particles to NaPA and poly(acrylic acid)-salt mixtures in an attempt to discover how the functionality of the polyelectrolyte is affected. The technique is sensitive to the restricted solvent motion that is characteristic of silica-solution interfaces, and the associated water structure has been studied previously using infrared spectroscopy by Hair et al.21,22 In summary, it was found that single water molecules tended to bridge between two surface hydroxyl groups in a way that was analogous to the behavior of dichlorosilanes. Experiments carried out at 150 °C showed that increasing the pressure of water vapor gave rise to shoulders in the adsorption isotherm corresponding to the adsorption of one, three, and six water molecules per pair of surface hydroxyl groups, (9) Blaakmeer, J.; Bohmer, M. R.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1990, 23, 2301. (10) Vermohlen, K.; Lewandowski, H.; Narres, H.-D.; Schwuger, M. J. Colloids Surf., A 2000, 163, 45. (11) Friedsam, C.; Gaub, H. E.; Netz, R. R. Europhys. Lett. 2005, 72, 844. (12) Bohmer, M. R.; El Attar Sofi, R.; Foissy, A. J. Colloid Interface Sci. 1994, 164, 126. (13) Dupont, L.; Foissy, A.; Mercier, R.; Mottet, B. J. Colloid Interface Sci. 1993, 161, 455. (14) Prabhu, V. M.; Vogt, B. D.; Wu, W.; Douglas, J. F.; Lin, E. K.; Satija, S.; Goldfarb, D. L.; Ito, H. Langmuir 2005, 21, 6647. (15) Joppien, G. R. J. Phys. Chem. 1978, 82, 2210. (16) Kiriy, A.; Yu, J.; Stamm, M. Langmuir 2006, 22, 1800. (17) Mahltig, B.; Jerome, R.; Stamm, M. J. Polym. Res. 2003, 10, 219. (18) Mahltig, B.; Gohy, J.-F.; Jerome, R.; Pfutze, G.; Stamm, M. J. Polym. Res. 2003, 10, 69. (19) Walter, H.; Harrats, C.; Muller-Buschbaum, P.; Jerome, R.; Stamm, M. Langmuir 1999, 15, 1260. (20) Tran, Y.; Auroy, P.; Lee, L-T.; Stamm, M. Phys. ReV. E 1999, 60, 6984. (21) Hair, M. L. J. Non-Cryst. Solids 1975, 19, 299. (22) Herty, W.; Hair, M. L. Nature 1969, 223, 5250.

Flood et al.

indicating the formation of ordered H2O layers on the surface. Clearly, the situation here in solution at ambient temperature is somewhat different, but the papers give an idea of the fast exchange of hydrogen-bonded water structures that give rise to the observed enhancement in the solvent relaxation rate constant. Although the structures were not studied as a function of pH, it is clear that they would be very sensitive to protonation or deprotonation of the surface hydroxyl groups. Experimental Section Materials. The two silica samples were kindly provided by Clariant. Klebosol 30R25 is anionic (pH 9) and has a particle diameter of 25 nm and a surface area of 120 m2 g-1. Klebosol 30CAL25 has the same size and surface area but is dispersed in a solution of pH 4 and is cationic by virtue of its surface alumina layer. The two polyelectrolyte samples were poly(acrylic acid) (PAA) from Aldrich (Mw 2000) and its conjugate base NaPA (Mw 4300, polydispersity 1.13, Polymer Source). The NaPA sample was analyzed using MALDI-TOF mass spectrometry, which gave a polydispersity of 1.21, Mn 2265, and Mw 2743. CaCl2 was purchased from Aldrich. Sample Preparation. All of the components were added in the form of stock solutions. First, polyelectrolyte was added to CaCl2. This resulted in initial polymer flocculation where the local CaCl2/ NaPA ratio was high, but the precipitate redissolved with more thorough mixing at all but the very lowest NaPA concentrations with g2.5 mM CaCl2. Silica stock was then added, which in the case of Klebosol 30R25 did not lead to any further flocculation at the concentrations used. Klebosol 30CAL25 addition did, however, give cloudy white solutions with a tendency to phase separate after a period of time, as discussed later. Solvent Relaxation NMR. In summary, the technique takes advantage of the fact that protons within water molecules bound at an interface have a shorter spin-spin nuclear magnetic relaxation time than those that are free in solution and hence yields information on the polymer train layer. Results are commonly expressed in terms of the relaxation rate constant, R2, which is the reciprocal of the relaxation time, T2, or the specific relaxation rate constant, R2sp, which is the relaxation rate relative to the relaxation rate, R 02, of a suitable reference sample, which in this case is the pure solvent, water. R2sp )

R2 R02

-1

(1)

An increase in R2sp implies that there is either more solvent or more strongly bound solvent at the interface and hence that there is a higher number of polymer segments close to the interface. A more complete description of the technique can be found elsewhere.23 A Bruker MSL 300 MHz NMR spectrometer, using a Carr-PurcellMeiboom-Gill (CPMG) pulse sequence, was used to obtain a relaxation decay curve for each sample.24,25 The time between each pulse, or τ spacing, was between 2 and 6 ms depending on the sample, and the recycle delay was 15 s. A total of 4096 data points were collected in each scan, and the signal was averaged over 32 scans for each sample. A computational method was then used to obtain the T2 value for each relaxation decay curve, My(t). This involved using a nonlinear least-squares algorithm to fit each curve to a single-exponential decay as described by eq 2, where My(0) is the transverse magnetization immediately after the 90° pulse and t represents the relaxation intervals, which are multiples of τ. My(t) ) My(0)e-t/T2

(2)

Results and Discussion NaPA and Anionic Silica with CaCl2. Solvent relaxation NMR was used to determine the level of NaPA adsorption on (23) Nelson, A.; Jack, K. S.; Cosgrove, T.; Kozak, D. Langmuir 2002, 7, 2750. (24) Carr, H. Y.; Purcell, E. M. Phys. ReV. 1954, 94, 630. (25) Meiboom, S.; Gill, D. ReV. Sci. Instrum. 1958, 29, 688.

Sodium Polyacrylate Adsorption onto Silica

Langmuir, Vol. 23, No. 11, 2007 6193

Figure 1. Specific relaxation rate against anionic silica concentration for NaPA/CaCl2 samples: (×) no NaPA, no salt; (4) 0.15 wt % (0.25 mg m-2) NaPA, no salt; (0) no NaPA, 5 mM CaCl2; and (]) 0.15 wt % NaPA, 5 mM CaCl2. The solid lines were found by linear regression with the intercept fixed at zero.

Klebosol 30R25 with and without CaCl2. The polyelectrolyte and silica are both negatively charged, so the adsorption detected by this method was very weak, as shown in Figure 1; adding NaPA gives very little enhancement to the bare silica relaxation rates. This result was not unexpected, but the aim was to determine whether adsorption could be mediated by the cations found in hard water. When the experiment was repeated in the presence of CaCl2, the results with and without NaPA were still almost identical to each other; in fact, the trend lines shown in Figure 1 can be virtually superimposed. The only real observed effect of salt was to increase the relaxation rate constant of each bare silica dispersion, as has been observed previously.26 This effect originates from Ca2+ ions either bound or electrostatically attracted to the silica surface, which in turn affects the solvent structure near the interface. Previous work by Schindler suggests that calcium ions can bond to either one or two surface hydroxyl groups.27 This binding involves the release of protons and is therefore very pH-dependent, with adsorption increasing with pH. Furthermore, Janusz et al. have looked at the competitive adsorption of divalent ions at oxide surfaces.28,29 For Ca2+ with silica, results show that adsorption is undetectable below ∼pH 5 but then increases to its maximum within the space of ∼2 pH units. This is important here because it implies that Ca2+ ions will bind to anionic silica at pH 9 but not to the cationic particles at pH 4 discussed later. The next step was to carry out the experiments at increased salt concentrations of up to 20 mM and with 0.5 wt % NaPA in order to accentuate any effects; these results are shown in Figure 2. This approach again shows how salt affects silica in the absence of polymer; the relaxation rate initially increases steadily with salt concentration before the silica eventually gels, which gives a sharp increase in R2sp, as seen with 20 mM CaCl2. When NaPA is added at the higher concentration, shown in Figure 2, some (26) Flood, C.; Cosgrove, T.; Howell, I.; Revell, P. Langmuir 2006, 22, 6923. (27) Schindler, P. W. In Adsorption of Inorganics at Solid-Liquid Interfaces,; Ann Arbor Scientific: Ann Arbor, MI, 1981; pp 1-49. (28) Janusz, W.; Patkowski, J.; Chibowski, S. J. Colloid Interface Sci. 2003, 266, 259. (29) Chibowski, S.; Janusz, W. Appl. Surf. Sci. 2002, 196, 343.

adsorption can be observed as shown by the jump in the relaxation rate from ∼2.75 to ∼3.3 in the absence of salt (shown by the points that lie on the y axis). Background measurements for NaPA alone in solution (shown in Figure 3) gave an R2sp enhancement of ∼0.1 at this concentration, confirming that the increase of ∼0.5 is mainly due to adsorption. On increasing the salt concentration, it is clear that the level of adsorption does not increase significantly because the relaxation rate constant does not increase any faster in the presence of polyelectrolyte. The results of another more detailed experiment on anionic silica with varied NaPA concentration are shown in Figure 3. First, it is clear that in the absence of salt R2sp increases linearly as NaPA is added, as shown by the solid lines in Figure 3 found by linear regression. The background measurements of NaPA in the absence of silica give a relatively small R2sp enhancement, confirming that the increase with silica is mainly due to adsorbed polyelectrolyte. It is also evident that 5 mM CaCl2 has no effect on the NaPA background, so any effects in the presence of silica can be attributed to changes at the particle surface. With reference to the samples containing silica, if salt had no effect at all on the adsorption we would expect a series of lines parallel to the no-salt data, with R2sp increasing only in line with the effect of salt on bare silica, as for the points on the y axis. Instead we see two distinct regions, highlighted in Figure 3 by a pair of straight lines at each salt concentration. It should be noted that the lines are added as a guide to the eye and are not found via any particular fitting process. Hence, because R2sp initially remains constant, it appears that CaCl2 actually hinders adsorption up to a point, before allowing it at higher NaPA concentrations, albeit with a decreased affinity (gradient). The likely explanation is that NaPA acts as a “Ca2+ sponge”, binding to the cations as described in the literature.1-7 Polyelectrolyte is then able to adsorb only after the break point, which corresponds to the point where a sufficient proportion of the salt is bound or perhaps the point at which enough salt has been removed from the surface. This may seem counterintuitive if Ca2+ is expected to mediate an interaction between the negatively charged particles and polyelectrolyte but concurs with the suggestion that the

6194 Langmuir, Vol. 23, No. 11, 2007

Flood et al.

Figure 2. Specific relaxation rate against salt concentration for anionic silica/NaPA samples: (×) 5 wt % silica, the solid line is a guide to the eye and (4) 5 wt % silica + 0.5 mg m-2 (0.5 wt %) NaPA, the solid line is found by linear regression.

Figure 3. Specific relaxation rate against NaPA concentration for 5 wt % anionic silica samples: (×) no salt, (4) 1 mM CaCl2, (0) 2.5 mM CaCl2, (]) 5 mM CaCl2, and (O) 10 mM CaCl2. Background measurements are also shown for (3) NaPA alone and (1) NaPA with 5 mM CaCl2. The background measurements use samples containing no silica, so the maximum adsorbed amount is calculated as if 5 wt % silica were present, as it is for the other samples.

electrostatic contribution to adsorption may not be the most important factor.9,11 This hypothesis is illustrated for anionic silica in the lower half of Figure 8. Looking again at Figure 3, it is interesting that as the salt concentration increases the break point also increases, in terms of the NaPA concentration. This relationship is plotted in Figure 4 with the break points converted to the concentration of monomer units in order to give more quantitative insight into NaPA-Ca2+ binding. Because the break points are determined by eye from Figure 3, the error bars in Figure 4 are chosen to represent a pessimistic estimate of their possible range. The solid curve is added purely to highlight the way in which the data levels off. At the low salt level of 1 mM CaCl2, around 15 mM monomer units are required before adsorption takes place. In other words,

if the salt concentration is equivalent to less than 1 cation bound to every 15 monomer units of the polyelectrolyte, then adsorption will occur. Above this level of salt, it seems that the cations are bound to the polyelectrolyte too densely for adsorption to be possible, and of course if the Ca2+/NaPA ratio is sufficiently high, then the polyelectrolyte coil collapses and will precipitate.3,4 Interestingly, the relationship is not linear, but instead the increase in the break point levels off as higher salt concentrations are reached, to the point that at 10 mM CaCl2 the onset of adsorption occurs at a ratio of just 4.5 mM NaPA monomers per mM Ca2+ ions, rather than the initial 15. This is probably a function of the adsorption equilibrium for NaPA on silica, with higher polymer levels being able to force some adsorption despite the presence of Ca2+. Even above the break point, adsorption is not as strong

Sodium Polyacrylate Adsorption onto Silica

Langmuir, Vol. 23, No. 11, 2007 6195

Figure 4. Relationship between the break point and salt concentration.

Figure 5. Specific relaxation rate against NaPA/PAA added per unit area of cationic silica: (×) 3 wt % silica + NaPA, (4) 5 wt % silica + NaPA, and (0) 5 wt % silica + PAA.

in the presence of CaCl2, as evidenced by the reduced gradients with salt in Figure 3. Hence even a small proportion of Ca2+ bound to the polyelectrolyte is able to hinder adsorption. We also attempted to study NaPA adsorption onto anionic silica using adsorption isotherms and small-angle neutron scattering (SANS), but the low adsorbed amounts rendered both methods unworkable. In the case of the isotherms, the differences in polyelectrolyte concentration were too low to measure accurately by titration or even NMR, whereas in the SANS experiments the adsorbed layer scattering was weak and swamped by scattering from the free polyelectrolyte so extremely long run times would be required to obtain accurate data. NaPA and PAA with Cationic Silica and CaCl2. Whereas the interaction between NaPA and anionic silica is weak, the polyelectrolyte has been found to adsorb strongly on positively charged particles.9 Two different polyelectrolyte samples were

used with Klebosol 30CAL25: the same NaPA (Mw 4300) used previously and also PAA in the protonated form (Mw 2000). The results are plotted in Figures 5-7 with the solid lines in each case included simply to emphasize the trends and not to represent any particular physical model. The initial solvent relaxation NMR results shown in Figure 5 demonstrate that adsorption is strong because the R2sp values go up to almost 30. Adsorption is very strong initially, particularly in the case of PAA. R2sp values for the 3 and 5 wt % silica samples with NaPA differ by a factor of around 3/5, as expected given the different surface areas present. After the initial increase, the adsorption “isotherm” levels off gradually in the case of PAA. With NaPA, however, a break point is reached at which further polyelectrolyte addition perhaps surprisingly causes a decrease in R2sp. The only difference is effectively the substitution of PAA, which has a pKa similar to the pH of the initial dispersion;

6196 Langmuir, Vol. 23, No. 11, 2007

Flood et al.

Figure 6. (×) Specific relaxation rate, (4) pH, and (0) relative level of flocculation against NaPA added per unit area of cationic silica.

Figure 7. Effect of CaCl2 on the 5 wt % cationic silica-NaPA/PAA system: (×) NaPA, no salt; (4) NaPA + 2.5 mM CaCl2; (0) PAA, no salt; and (]) PAA + 2.5 mM CaCl2.

Figure 8. Summary of the conclusions.

for its conjugate base, therefore, this is almost certainly a pH effect. Although previous results show that the R2sp of silica is fairly constant in the pH range of 4-10,30,31 it is possible that (30) van der Beek, G. P.; Cohen Stuart, M. A.; Cosgrove, T. Langmuir 1991, 7, 32. (31) Belton, P. S.; Hills, B. P.; Raimbaud, E. R. Mol. Phys. 1988, 63, 825.

polyelectrolyte adsorption could still be affected. However, a more likely cause of the R2sp decrease is probably flocculation; as neutral pH is approached, an increasing level of flocculants drop out of the solution and hence the measured R2sp is reduced because it accounts only for particles in the remaining diluted dispersion. These stability problems, not observed with anionic silica, were evidenced by the fact that a few hours after measurement all of the samples had settled into two layers: a white lower layer containing the flocculated silica and a clear, or at least translucent, upper layer containing predominantly solvent. The height of the flocculated phase in the NMR tubes was measured relative to the total height and is plotted in Figure 6 for the 3 wt % silica data, along with the pH of each sample and the corresponding relaxation data from Figure 5. Figure 6 shows that at around the break point (peak) in the relaxation rate the pH jumps from about 4, the pH of the silica solution, to

Sodium Polyacrylate Adsorption onto Silica

approximately 8. With excess NaPA addition, it then approaches the pKa value of the polyelectrolyte (9.7). The peak in the level of flocculants at around pH 7 confirms that precipitation occurs most quickly when the solution is approximately neutral. Considering all of the evidence, it is likely that most of the first 0.5 mg m-2 of NaPA added is strongly adsorbed, at which point the silica is effectively neutralized. Above this point, although more PAA could be adsorbed as a result of hydrophobic interactions, most additional NaPA appears to go into solution because the pH increases sharply. The level of flocculants decreases again as the silica becomes overcharged. pH measurements of the PAA samples were also made but are not shown; there was a smooth decrease from ∼4 initially to ∼3.5 after the addition of 1.5 mg m-2 of the polyacid. The addition of CaCl2 had little effect; the pH values were just fractionally reduced, as was the case in the presence of NaPA. Figure 7 shows the effect of adding polyelectrolyte in the presence of salt. The results show that at a concentration of 2.5 mM, CaCl2 slightly increases the level of adsorption, at least in terms of the train density as studied by solvent relaxation NMR. This is the opposite to the effect seen with anionic silica, where adsorption was reduced by CaCl2, and implies that NaPA-Ca2+ complexes adsorb more readily onto the cationic substrate. This may be linked to a lack of Ca2+ ions adsorbed on the silica surface as discussed earlier,28,29 which is evidenced here by the fact that salt alone does not increase the relaxation rate constant (i.e., all of the points on the y axis of Figure 7 are approximately superimposed). This result for a divalent cation adds to the findings of Blaakmeer et al.,9 where the ionic strength of a monovalent salt

Langmuir, Vol. 23, No. 11, 2007 6197

was shown to have little effect on the cationic polystyrene latexNaPA system. Even with divalent salt, the interaction between the oppositely charged polymer and the substrate is so strong that the more subtle effects of salt are difficult to distinguish and the data cannot really be used to analyze the salt effects quantitatively, as it was in the case of anionic silica. The conclusions for both substrates are illustrated in Figure 8.

Conclusions NaPA-Ca2+ interactions have been studied in the presence of both anionic and cationic silica. In the anionic case, polyelectrolyte adsorption onto the particles is very weak and is hindered even further by calcium ions. This is attributed to binding between NaPA and the cations, which changes the conformation and charge of the polyelectrolyte. In contrast, cationic silica interacts very strongly with NaPA, and in many cases, precipitation occurs. The more subtle effects of added salt at the concentrations used were therefore difficult to distinguish, although a small increase in adsorption was detected. In terms of preventing calcium carbonate precipitation, future work could include using NMR to determine why NaPA is not as effective when sodium dodecyl sulfate is present and determining the optimum molecular weight for the polyelectrolyte. Acknowledgment. We thank Ralf Schweins of the Institut Laue-Langevin, Grenoble, France, for useful discussions and also Unilever and the EPSRC for funding. LA070047Z