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Adsorption of partially hydrolyzed polyacrylamides on titanium dioxide
Adsorption of Partially Hydrolyzed Polyacrylamides on Titanium Dioxide G. GIROD, J. M. LAMARCHE, AND A. FOISSY Laboratoire Electrochimie des Solides, CNRS 04436, Facultd des Sciences, 25042 Besancon Cedex, France Received November 5, 1986; accepted February 17, 1987 The adsorption ofpartiaUy hydrolyzed (0, 4, 16%) polyacrylamides on titanium dioxide in an aqueous medium was studied to analyze the influence ofpH, ionic strength, and the presence of Ca 2+ complexing ions. Experimental results cannot be explained solely by considering the variation of the polymer's and surface's charge densities. An important parameter concerning the polymer's affinity for the surface could be the variation of its solubility in the medium. © 1988AcademicPress,Inc. INTRODUCTION
Most recent studies of polyelectrolyte adsorption stress the influence of parameters such as surface charge density, polymer charge, pH, ionic strength, and solvent quality. Few studies, however, give experimental support to theoretical developments and many of them do not supply sufficient information on the experimental procedure, the nature of the substrate, or the polymer characteristics. As a continuation of previous investigations on the adsorption of polyacrylic acid on titanium dioxide (1) and calcium carbonate (2), this study will be concerned with the adsorption of partially hydrolyzed polyacrylamides on titanium dioxide. Our objective is to measure the influence of the polyelectrolyte charge (degree of ionization) and other parameters (pH, polymer concentration, molecular conformation, etc.) on the amount adsorbed. An industrial application of this study is to improve the practice of selective flocculation of mineral oxides. Titanium dioxide was chosen as a substrate for adsorption since its surface charge can be measured as a function of pH and ionic strength using potentiometric titration. The material used here was characterized in our laboratory (3). Some studies on the adsorption of hydrolyzed polyacrylamides in aqueous media are
already available. Chong and Curthoys (4) have shown how the uptake on titanium dioxide decreases when the pH increases from 1 to 3. They correlated their results to the variations of charge density and molecular conformations. Other studies pointed out the increase in adsorption with ionic strength (5) and the influence of both a complexing ion (Ca2+) and a surfactant (6). Many industrially motivated studies, such as the adsorption on cellulose and titanium dioxide for papermaking (7), on asbestos for flocculation and filtration applications (8), and on silica and iron oxide for flocculation in the presence of divalent ions (9), have been carried out. More recent studies were made on the adsorption of pure potyacrylamide on silica and silicon carbide (10). In these two cases, some insight concerning the adsorption mechanism and the binding of the polymer was obtained. MATERIALS
Titanium Dioxide
The titanium dioxide used by us is a commercial product prepared by oxidation of titanium chloride (P 25, Degussa, Germany). The crystalline form is anatase (95%). The specific area is measured by N2 adsorption (BET) and by immersion calorimetry (Harkins, Jura method) 53 + 2 m 2 g-1. In spite of
265 Journal of Colloid and Interface Science, Vol. 121, No. 1, January 1988
0021-9797/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
266
GIROD, L A M A R C H E , A N D FOISSY
this high value the grain size obtained by light scattering after 5 min sonication in a solution of sodium polyacrylate as a dispersing agent is about 0.4 #m. The particles are then probably partly aggregated. The surface charge density was measured potentiometrically in sodium chloride solutions between 10 -3 and 5 X 10 -1 M. Details on the procedure and results were described elsewhere (1). Figure 1 shows a zero point of charge at pH 6.6 and the usual increase in the surface charge with the electrolyte concentration. The measurement of electrophoretic mobility gave an isoelectric point at the same pH, indicating no specific effect due to dissolved or surface impurities.
Polymers Neutral and hydrolyzed linear polyacrylamides are prepared and commercialized by Hoechst (Germany). The molecular weight was 2 X 106 and the degree of hydrolysis was controlled potentiometrically. We used three samples, the raw material which is actually 0.3% hydrolyzed (thereafter called PAM) and two products, polyelectrolytes hydrolyzed respectively by 4% (HPAM 4) and 16% (HPAM
+18
Cm - 2 L•TTo/•C.
+14 +10 +6 +2 -2
:3
4
5
6
'
'
'
H
-6 -10
FIG. 1. Surface charge of TiO2 = f(pH): (A) NaCI 10-3 M, ( e ) NaC1 10-2 M, (O) NaC1 10-I M, (v) NaC1 5 × 10-1 M. Journal of Colloid and Interface Science, VoL 121,No. 1, January 1988
[~/] Cm3G - 1
v7300 - --v-..
/ /
•
2000
1000
4
6
8
10
PH
FIG. 2. Intrinsic viscosity as a function of p H (HPAM 16): (V) 0 M NaC1, (0) 10 -1 M NaC1, (O) 5 X 10-1 M NaC1, (A) 1 M NaC1, (O) 2 M NaC1.
16). The polymers are very sensitive to aging and mechanical degradation. We then prepared a 1 g/liter solution before each experiment by slowly pouring the concentrate obtained from Hoechst into the chosen sodium chloride solution under gentle magnetic stirring. The solution was kept still for about 24 h and slowly agitated for 10 min just before use. We measured a very strong decrease in viscosity during the 24-h rest period; this is attributed to a disaggregation of the polyelectrolyte chains. Solvent quality and molecular conformations were evaluated by the measurements of intrinsic viscosity as a function ofpH and ionic strength. The intrinsic viscosity was obtained by extrapolation of the reduced viscosity to zero polyelectrolyte concentration. Viscosity was measured with a capillary tube apparatus. No important viscosity variation with pH or salt concentration was measured for PAM. HPAM 4 and HPAM 16 showed similar variations of viscosity with a maximum around pH 8 and a strong decrease with an increase in ionic strength (Fig. 2). At high salt concentrations the hydrolyzed polymers showed viscosity behaviors similar to those of the neutral polyacrylamides. These results are typical of
ADSORPTION ON TITANIA polyelectrolyte solutions, reflecting a screening of the polymer electrical charge by the electrolyte (Na +) and a resulting decrease in molecular size. The same effect was measured at a constant salt concentration by decreasing the solution's pH, this is due to an ionization decrease in the carboxylic functional groups of the chain. See Table I.
267
_Q ads mg/g
0
{J
10
0
Y f III
•
EXPERIMENTAL PROCEDURE AND RESULTS Adsorption Measurements
The amount of polymer adsorbed is obtained by calculating the difference in concentration between the initial solution of polyelectrolyte and that measured in the supernatant. The concentration measurement is carried out by viscosimetry, which is especially suited here since adsorption isotherms are of the high-affinity type. Thus, for low polyelectrolyte concentrations, adsorption is total. Moreover, the concentration is relatively low when the isotherm's plateau is reached (Fig. 3). The error due to a modification of the distribution of molecular masses during adsorption of polydispersed polymers is then minimal. The order of adding reagents, addition speed, concentration of added solutions, and especially stirring have all been investigated and showed some inconsistency in results. Consequently, we employed the following measuring procedure: 25 ml polymer solution TABLE I Intrinsic Viscosity[n] (cm3 g-l) pH
[NaCI] in M polymer
0.1
0.5
1
9
HPAM 0.3 HPAM 4 HPAM 16
300 800 2000
250 740 900
-660 450
HPAM 0.3 HPAM 4 HPAM 16
420 760 1850
380 700 900
-640 440
H P A M 0.3 HPAM 4 HPAM 16
380 500 800
300 440 700
-400 300
7
5
2
600 370 _ 580 38o -
-
320 250
Ceq mg/L ,
1~0
FIG.3. Adsorptionisothermof HPAM 16 on TiO2(0.1% solid): (e) pH 12, (A) pH 7, (O) pH 5.
at the desired pH and ionic strength is slowly added, with magnetic stirring to a 25-ml suspension of particles adjusted to the same pH and salinity. The analysis of polymer concentration as a function of time shows that adsorption remains unchanged after 2 h of contact. To be certain, analyses were carried out after 24 h of slow stirring. Adsorption measurements on suspensions containing high solid concentrations (2 to 4 wt%) gave inconsistent results. This phenomenon is probably due to problems of accessibility of the surfaces of the particles by the polymer. In these systems, flocs, whose sizes vary according to the quantity of polymer added, adhere to the walls. Experiments were therefore carried out in suspensions containing only 0.1 wt% particles. Polymer adsorption, expressed per gram of solid, is independent of mineral concentration in suspensions between 0.1 and 0.5 wt%. Duplicated experiments gave a reproducibility on the order of 5%. Influence o f p H
Figure 3 shows typical isotherms at different pH values (pH 5, 7, and 12) and Fig. 4 shows the pseudo-plateau adsorption of the three polymers as a function of pH. Under pH 6 the adsorption of the three isoJournal of Colloid and Interface Science, Vol. 12 l, No. 1, January 1988
268
GIROD, LAMARCHE, AND FOISSY creases. Results are summarized in Table II for two pH values with HPAM 16. Comparing these results with those obtained in a NaC1 medium, this increase cannot be explained only by the effect of ionic strength. This phenomenon can be attributed to the complexation of the functions of carboxylic acids ionized by the C a 2+ cation which would bring about a modification of the polymer conformation (contractions of the macromolecule) and especially a decrease in solubility.
mg/g~.. 15 10
DISCUSSION
Influence of pH
6
;
8
9
I0
I'I pH
FIG. 4. Adsorbedquantity at the plateau as a function ofpH without NaCI: (O) HPAM 16, (A) HPAM 4, (0) HPAM 0.3.
mers increases with decreasing pH. Over pH 6 the adsorption of unhydrolyzed polymers varies little with pH while the adsorption of hydrolyzed polymers diminishes slightly as pH increases. In addition, between pH 6 and 4 adsorption decreases as the hydrolysis rate increases. For lower pH values adsorption becomes practically equal for both hydrolyzed polymers but remains lower than that of neutral polyacrylamide.
Influence of Salinity The isotherm's adsorption plateau is shown in Figs. 5, 6, and 7 as a function of pH and for different sodium chloride concentrations from 0 to 2 M. Salinity has only a weak influence on the adsorption of the neutral polymer. The adsorption for hydrolyzed polyelectrolytes increases considerably with an increase in ionic strength at any pH.
Influence of Calcium Adsorption increases in the presence of calcium chloride as the concentration of salt inJournal of Colloid and Interface Science, Vol. 121, No. 1, January 1988
The pH has the same influence on the adsorption of PAM and the two HPAMs. Adsorption rapidly decreases between pH 4 and pH 6 or 7; then it is constant (PAM) or continues to slowly decrease. Other authors have measured an increase in adsorption with a decrease in pH in the case of the HPAMs and polyacrylic acid. In the case of a negative substrate (silica and silicon carbide) Chauveteau and Lecourtier attribute the increase in adsorption to the decrease in the polymer charge
30/~Q adsmg/g 20~ e A •
o e ~_o
I0
~
~
8
~
1'o
pH
F)G.5. Adsorptionof HPAM 0.3 at the isothermplateau as a function of pH: (A) without salt, (©) 10-' M NaC1, (O) 0.5 M NaC1, (¢) 1 MNaC1, (~) 2 M NaC1.
269
A D S O R P T I O N ON TITANIA
(10). In the case of positively charged surfaces (hematite and ruffle in an acid medium) Gebhardt et al. attribute an increase in adsorption to the growth of electrostatic attractive forces between the mineral and the polymer (11). In this example, however, electrostatic interactions alone cannot account for the experimental results since at very acidic pH, near or less than 3, the polymer is no longer charged and adsorption should decrease. For the adsorption ofpolyacrylic acid on titanium dioxide (anatase) Foissy suggested that a continuous increase in adsorption with the acidity of the medium results from a combined influence of charge variations and a decrease in the size of the polymer (1). All these arguments cannot be used for the adsorption of PAM because it is practically neutral. Neither its charge nor its size varies between pH 7 and 4. The increase in electrostatic forces with acidity of the medium should be demonstrated experimentally. Most arguments in this direction are based on the variation of the substrate and polymer charge densities independently
Q ads
mg/g
"-,•a ds
rng/g
! 30
20
5
6
7
8
9
10
PH
FIG. 7. Adsorption of H P A M 16 at the isotherm plateau as a function o f p H : (A) without NaC1, (O) 10-1 MNaC1, (O) 5 × 10- 1 M NaC1, (0) 1 M NaC1, (V) 2 M NaC1.
and suppose that they are not modified by adsorption. Yet Joppien (12) shows that the charge density of silica decreases as polyoxyethylene is adsorbed. In another study, Foissy et al. (13) suggest the deionization of the interface wfaen polyacrylic acid is adsorbed on anatase in an acidic medium.
Influence of Salinity 30
The adsorption of PAM is not very sensitive to the medium's salinity, which is consistent with a lack of electrical interactions between the surface and an uncharged polymer or between adsorbed molecules. At any pH between 4 and 11 adsorption of HPAM 4 and 16 increases considerably when NaC1 concentration goes from 10 i to 2 M.
20
t0 TABLE II Adsorption of H P A M 16 (mg g 1) 5
6
7
8
9
10
PH
FIG. 6. Adsorption of H P A M 4 at the isotherm plateau as a function o f p H : (A) without NaC1, (O) 10-1 MNaC1, (©) 5 × 10- 1 M N a C 1 , (0) 1 M N a C I , (v) 2 M NaC1.
CaCI2(M):
10-4
10-3
10-2
10-1
pH 7
11
13
19
32
pH 10
7
15
24
9.5
Journal of Colloid and Interface Science, Vol. 121,No. 1, January 1988
270
GIROD,
LAMARCHE,
This result cannot be explained by the sole effect of salinity on the electrical interactions between the substrate and the polyelectrolyte. On the one hand, in alkaline media, the mineral and the polymer are negatively charged and the electrolyte promotes adsorption by suppressing the electrostatic repulsion. On the other hand, below pH 6, both entities have no charges, or opposite charges and electrical interactions should be zero or attractive; the electrolyte would then have no effect on, or hinder, the adsorption. A limiting example of this phenomenon is shown in Fig. 5 in the case of an uncharged polymer. As shown by Bonekamp et al. (14) in the case of polylysine on AgI and polystyrene latex, an electrolyte can drastically promote the adsorption by reducing the mutual repulsion between similarly charged groups, thus favoring the development of loops and a high molecular density on the surface. This effect still occurs at high molar ionic strength. The decrease in intramolecular electrostatic repulsions can be tentatively evaluated by the variation of Rg with salt concentration. Indeed for HPAM 16 a good correlation is found between adsorption and Rg calculated from intrinsic viscosity (Fig. 2) by the Flory-Fox equation [~/] = 2.1 × 1021(Rg-2)2/3M -1. At pH 10 for instance Rg varies from 1180 to 640 ~, as the adsorption increases from 11.5 to 24 mg/g when the salt concentration increases from 10-1 to 2 M. Nevertheless this reasoning no longer holds true for HPAM 4 since for an identical pH its adsorption doubles under similar conditions while its Rg decreases only from 875 to 800 ~. If the electrical repulsions between charged groups and/or the size of the molecule were the main parameter, the adsorption level of the HPAM at strong salt concentrations should be the same as that of the neutral polymer. In fact, in an alkaline medium, the adsorption of hydrolyzed polymers is almost twice that of PAM. Journal of Colloid and InterfaceScience, Vol. l 21, No. 1, January 1988
AND FOISSY
Influence of Polymer Solubility and Adsorption Energy The two main theoretical models of polyelectrolyte adsorption, that of Hesselink (15) and more recently that of Van der Schee and Lyklema (16) clearly show that the quantity adsorbed also depends on the nonelectric (chemical) interaction energy between the surface and the polymer on the one hand and the interaction energy between the polymer and the solution on the other hand. The first term is represented by e (Hesselink) or Xs (Van der Schee) and the second is measured by the Flory-Huggins parameter. Very few experimental certitudes exist concerning the nature of the bond between more or less hydrolyzed polyacrylamides and mineral oxides. Nevertheless, it can be assumed that a hydrogen bond is established more easily with a carboxylic rather than with an amide group. This hypothesis is confirmed by Espinasse (17) for the adsorption of HPAM on montmorillonite. In our study the greater adsorption of HPAM with respect to PAM can also be explained in this way. Concerning interaction energies between the polymer and the surface, we showed a decrease in integral enthalpy of adsorption of polyacid on anatase as pH increases and ionic strength decreases (13). This is rather unexpected since these experimental conditions correspond to an increase in adsorption. We explained this phenomenon by the fact that polymer adsorption is associated with an exothermic reaction. The hydrogen bond between the surface and a polymer segment is stronger than that between the surface and solvent molecule. Using a simple enthalpic balance we showed that the endothermic contribution was the deprotonation of the surface sites. The adsorption reaction can be summarized in the following way: SOH~- • • • C1- + HOOC - polymer SOH- - • HOOC - polymer + H + + C1-.
271
ADSORPTION ON TITANIA
It is evident that the endothermic contribution takes on a more important role in a more acidic medium and at a higher ionic strength since in these two cases the density of charged surface sites is greater. This hypothesis concerning the reactional mechanism of adsorption was confirmed by an analysis of the surface density of chloride ions as a function of the polymer adsorption rate. A confirmation of this result using independent techniques would be desirable because it would invalidate all explanations of an electrostatic nature on the increase in adsorption of these polyacids on mineral oxides in an acid medium. On the other hand, in an alkaline medium we showed that the adsorption of polyacrylic acid was accompanied by the adsorption of Na + counter ions. The nature of the bond has not yet been elucidated (hydrogen bond on the nonionized hydroxyl sites, or bridging by the Na + ions) but this result suggests that the increase in adsorption with ionic strength could result, at least partially, from a better screening of the charge of adsorbed molecules. Van der Schee and Lyklema's (16) calculations and more recently those by Evers et al. (18) show the considerable influence of the solubility parameter X on the amount of uptake polymer. In the second reference, the adsorption of a polymer resembling HPAM 16 ( p o l y a c i d , pKin t = 4, mol wt 2,000,000) is calculated as a function of p H and the FloryHuggins parameter x. The influence of X is particularly sensitive when pH is near or less than pK. Since information in this area is lacking, we measured the variation of the solvent strength of the medium by calculating the Huggins constant as a function o f p H and ionic strength from the polymer solution's viscosity (19). Figure 8 shows that at pH 4 the polymer solubility greatly decreases when salt concentration increases from 10 -~ to 1 M. This result is then appropriate to explain the adsorption increase in an acid medium when ionic strength increases from 0 to 2 M. In conclusion, it is always possible to qual-
PHA4 1
0,5
0.1
,
,
0.5
1
Cs
M.L- !
2
FIG. 8. Huggins constant k as a function of the NaC1 concentration: (A) pH 4, (O) pH 10.
itatively explain the variations in adsorption ofpolyelectrolytes because the parameters are numerous and the systems are not sufficiently defined. Moreover, in certain cases, arguments of an electrostatic nature are insufficient or unjustified. Given the most recent theoretical analyses and our evaluation from viscosity measurements, it seems important to take into account the solubility variation as a function of solvent strength and the type of bond or adsorption reaction mechanism. Experimental knowledge still seems insufficient for hydrolyzed polyacrylamides and polyacrylic acids to corroborate available theoretical models. ACKNOWLEDGMENTS This work was supported by the CNRS through its PIRSEM project on the conservation of natural resources. T. X. Scott is acknowledged for the translation, adaptation, and typing of the report.
REFERENCES 1. Foissy, A., Thesis, Besan~on, France, 1985. 2. Lamarche, J. M., Persello, J., and Foissy, A., Ind. Eng. Chem. Prod. Res. Dev. 22, 1, 125 (1983). 3. Foissy, A., M'Pandou, A., Lamarche, J. M., and Jaffrezic Renault, N., Colloids Surf. 5, 363 (1982). 4. Chong, P., and Curthoys, G., Int. J. Miner. Process. 5, 335 (1979). 5. Dodson, P. J., and Somasundaran, P., J. Colloid Interface Sci. 97, 481 (1984). 6. Bocquenet, Y., and Siffert, B., Colloids Surf. 9, 147 (1984).
Journal of Colloid and Interface Science, Vol. 121, No. 1, January 1988
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GIROD, LAMARCHE, AND FOISSY
7. Howard, G. J., Lyth Hudson, F., and West, J., J. Appl. Polym. Sci. Polym. Sympl 61, 389 (1977). 8. Klimenko, O. I., and Lapin, V. V., Kolloid Z. 42, 5, 968 (1980). 9. Slater, R. W., Clark, J. P., and Kitchener, J. A., Brit. Ceram. Soc. Proc. 13, 1 (1969). 10. Chauveteau, G., and Lecourtier, J., Colloque RCP "Rh~ologie des solutions de polym~res utilis6s en r6cuperation assist6 du p6trole," Sainte-Crain-auxMines, May, 1985. 11. Gebhardt, J. E., and Fuerstenau, D. W., Colloids Surf. 7, 221 (1983). 12. Joppien, G. R., J. Phys. Chem. 82, 20, 2210 (1978). 13. Foissy, A., Lamarche, J. M., Fahys, B., and Partyka,
Journal of Colloid and Interface Science, Vol. 121, No. 1, January i988
14. 15. 16. 17. 18.
19.
S., AFCAT Conference, "Joum~es de Calorim6trie et d'analyse thermique," Montpellier, France, May 20-22, 1985. Bonekamp, B. C., Van der Schee, H. A., and Lyklema, J., Croat. Chem. Acta 56, 4, 695 (1983). Hesselink, F. Th., J. Colloid Interface Sci. 60, 448 (1977). Van der Schee, H. A., and Lyklema, J., J. Phys. Chem. 88, 6661 (1984). Espinasse, P., Thesis, Mulhouse, France, 1979. Evers, O. A., Fleer, G. J., Scheutjens, J. M. H. M., and Lyklema, J., J. Colloid Interface Sci. 111, 2, 446 (1986). Yamakawa, H., J. Chem. Phys. 34, 4, 1360 (196I).
Most recent studies of polyelectrolyte adsorption stress the influence of parameters such as surface charge density, polymer charge, pH, ionic strength, and solvent quality. Few studies, however, give experimental support to theoretical developments and many of them do not supply sufficient information on the experimental procedure, the nature of the substrate, or the polymer characteristics. As a continuation of previous investigations on the adsorption of polyacrylic acid on titanium dioxide (1) and calcium carbonate (2), this study will be concerned with the adsorption of partially hydrolyzed polyacrylamides on titanium dioxide. Our objective is to measure the influence of the polyelectrolyte charge (degree of ionization) and other parameters (pH, polymer concentration, molecular conformation, etc.) on the amount adsorbed. An industrial application of this study is to improve the practice of selective flocculation of mineral oxides. Titanium dioxide was chosen as a substrate for adsorption since its surface charge can be measured as a function of pH and ionic strength using potentiometric titration. The material used here was characterized in our laboratory (3). Some studies on the adsorption of hydrolyzed polyacrylamides in aqueous media are
already available. Chong and Curthoys (4) have shown how the uptake on titanium dioxide decreases when the pH increases from 1 to 3. They correlated their results to the variations of charge density and molecular conformations. Other studies pointed out the increase in adsorption with ionic strength (5) and the influence of both a complexing ion (Ca2+) and a surfactant (6). Many industrially motivated studies, such as the adsorption on cellulose and titanium dioxide for papermaking (7), on asbestos for flocculation and filtration applications (8), and on silica and iron oxide for flocculation in the presence of divalent ions (9), have been carried out. More recent studies were made on the adsorption of pure potyacrylamide on silica and silicon carbide (10). In these two cases, some insight concerning the adsorption mechanism and the binding of the polymer was obtained. MATERIALS
Titanium Dioxide
The titanium dioxide used by us is a commercial product prepared by oxidation of titanium chloride (P 25, Degussa, Germany). The crystalline form is anatase (95%). The specific area is measured by N2 adsorption (BET) and by immersion calorimetry (Harkins, Jura method) 53 + 2 m 2 g-1. In spite of
265 Journal of Colloid and Interface Science, Vol. 121, No. 1, January 1988
0021-9797/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
266
GIROD, L A M A R C H E , A N D FOISSY
this high value the grain size obtained by light scattering after 5 min sonication in a solution of sodium polyacrylate as a dispersing agent is about 0.4 #m. The particles are then probably partly aggregated. The surface charge density was measured potentiometrically in sodium chloride solutions between 10 -3 and 5 X 10 -1 M. Details on the procedure and results were described elsewhere (1). Figure 1 shows a zero point of charge at pH 6.6 and the usual increase in the surface charge with the electrolyte concentration. The measurement of electrophoretic mobility gave an isoelectric point at the same pH, indicating no specific effect due to dissolved or surface impurities.
Polymers Neutral and hydrolyzed linear polyacrylamides are prepared and commercialized by Hoechst (Germany). The molecular weight was 2 X 106 and the degree of hydrolysis was controlled potentiometrically. We used three samples, the raw material which is actually 0.3% hydrolyzed (thereafter called PAM) and two products, polyelectrolytes hydrolyzed respectively by 4% (HPAM 4) and 16% (HPAM
+18
Cm - 2 L•TTo/•C.
+14 +10 +6 +2 -2
:3
4
5
6
'
'
'
H
-6 -10
FIG. 1. Surface charge of TiO2 = f(pH): (A) NaCI 10-3 M, ( e ) NaC1 10-2 M, (O) NaC1 10-I M, (v) NaC1 5 × 10-1 M. Journal of Colloid and Interface Science, VoL 121,No. 1, January 1988
[~/] Cm3G - 1
v7300 - --v-..
/ /
•
2000
1000
4
6
8
10
PH
FIG. 2. Intrinsic viscosity as a function of p H (HPAM 16): (V) 0 M NaC1, (0) 10 -1 M NaC1, (O) 5 X 10-1 M NaC1, (A) 1 M NaC1, (O) 2 M NaC1.
16). The polymers are very sensitive to aging and mechanical degradation. We then prepared a 1 g/liter solution before each experiment by slowly pouring the concentrate obtained from Hoechst into the chosen sodium chloride solution under gentle magnetic stirring. The solution was kept still for about 24 h and slowly agitated for 10 min just before use. We measured a very strong decrease in viscosity during the 24-h rest period; this is attributed to a disaggregation of the polyelectrolyte chains. Solvent quality and molecular conformations were evaluated by the measurements of intrinsic viscosity as a function ofpH and ionic strength. The intrinsic viscosity was obtained by extrapolation of the reduced viscosity to zero polyelectrolyte concentration. Viscosity was measured with a capillary tube apparatus. No important viscosity variation with pH or salt concentration was measured for PAM. HPAM 4 and HPAM 16 showed similar variations of viscosity with a maximum around pH 8 and a strong decrease with an increase in ionic strength (Fig. 2). At high salt concentrations the hydrolyzed polymers showed viscosity behaviors similar to those of the neutral polyacrylamides. These results are typical of
ADSORPTION ON TITANIA polyelectrolyte solutions, reflecting a screening of the polymer electrical charge by the electrolyte (Na +) and a resulting decrease in molecular size. The same effect was measured at a constant salt concentration by decreasing the solution's pH, this is due to an ionization decrease in the carboxylic functional groups of the chain. See Table I.
267
_Q ads mg/g
0
{J
10
0
Y f III
•
EXPERIMENTAL PROCEDURE AND RESULTS Adsorption Measurements
The amount of polymer adsorbed is obtained by calculating the difference in concentration between the initial solution of polyelectrolyte and that measured in the supernatant. The concentration measurement is carried out by viscosimetry, which is especially suited here since adsorption isotherms are of the high-affinity type. Thus, for low polyelectrolyte concentrations, adsorption is total. Moreover, the concentration is relatively low when the isotherm's plateau is reached (Fig. 3). The error due to a modification of the distribution of molecular masses during adsorption of polydispersed polymers is then minimal. The order of adding reagents, addition speed, concentration of added solutions, and especially stirring have all been investigated and showed some inconsistency in results. Consequently, we employed the following measuring procedure: 25 ml polymer solution TABLE I Intrinsic Viscosity[n] (cm3 g-l) pH
[NaCI] in M polymer
0.1
0.5
1
9
HPAM 0.3 HPAM 4 HPAM 16
300 800 2000
250 740 900
-660 450
HPAM 0.3 HPAM 4 HPAM 16
420 760 1850
380 700 900
-640 440
H P A M 0.3 HPAM 4 HPAM 16
380 500 800
300 440 700
-400 300
7
5
2
600 370 _ 580 38o -
-
320 250
Ceq mg/L ,
1~0
FIG.3. Adsorptionisothermof HPAM 16 on TiO2(0.1% solid): (e) pH 12, (A) pH 7, (O) pH 5.
at the desired pH and ionic strength is slowly added, with magnetic stirring to a 25-ml suspension of particles adjusted to the same pH and salinity. The analysis of polymer concentration as a function of time shows that adsorption remains unchanged after 2 h of contact. To be certain, analyses were carried out after 24 h of slow stirring. Adsorption measurements on suspensions containing high solid concentrations (2 to 4 wt%) gave inconsistent results. This phenomenon is probably due to problems of accessibility of the surfaces of the particles by the polymer. In these systems, flocs, whose sizes vary according to the quantity of polymer added, adhere to the walls. Experiments were therefore carried out in suspensions containing only 0.1 wt% particles. Polymer adsorption, expressed per gram of solid, is independent of mineral concentration in suspensions between 0.1 and 0.5 wt%. Duplicated experiments gave a reproducibility on the order of 5%. Influence o f p H
Figure 3 shows typical isotherms at different pH values (pH 5, 7, and 12) and Fig. 4 shows the pseudo-plateau adsorption of the three polymers as a function of pH. Under pH 6 the adsorption of the three isoJournal of Colloid and Interface Science, Vol. 12 l, No. 1, January 1988
268
GIROD, LAMARCHE, AND FOISSY creases. Results are summarized in Table II for two pH values with HPAM 16. Comparing these results with those obtained in a NaC1 medium, this increase cannot be explained only by the effect of ionic strength. This phenomenon can be attributed to the complexation of the functions of carboxylic acids ionized by the C a 2+ cation which would bring about a modification of the polymer conformation (contractions of the macromolecule) and especially a decrease in solubility.
mg/g~.. 15 10
DISCUSSION
Influence of pH
6
;
8
9
I0
I'I pH
FIG. 4. Adsorbedquantity at the plateau as a function ofpH without NaCI: (O) HPAM 16, (A) HPAM 4, (0) HPAM 0.3.
mers increases with decreasing pH. Over pH 6 the adsorption of unhydrolyzed polymers varies little with pH while the adsorption of hydrolyzed polymers diminishes slightly as pH increases. In addition, between pH 6 and 4 adsorption decreases as the hydrolysis rate increases. For lower pH values adsorption becomes practically equal for both hydrolyzed polymers but remains lower than that of neutral polyacrylamide.
Influence of Salinity The isotherm's adsorption plateau is shown in Figs. 5, 6, and 7 as a function of pH and for different sodium chloride concentrations from 0 to 2 M. Salinity has only a weak influence on the adsorption of the neutral polymer. The adsorption for hydrolyzed polyelectrolytes increases considerably with an increase in ionic strength at any pH.
Influence of Calcium Adsorption increases in the presence of calcium chloride as the concentration of salt inJournal of Colloid and Interface Science, Vol. 121, No. 1, January 1988
The pH has the same influence on the adsorption of PAM and the two HPAMs. Adsorption rapidly decreases between pH 4 and pH 6 or 7; then it is constant (PAM) or continues to slowly decrease. Other authors have measured an increase in adsorption with a decrease in pH in the case of the HPAMs and polyacrylic acid. In the case of a negative substrate (silica and silicon carbide) Chauveteau and Lecourtier attribute the increase in adsorption to the decrease in the polymer charge
30/~Q adsmg/g 20~ e A •
o e ~_o
I0
~
~
8
~
1'o
pH
F)G.5. Adsorptionof HPAM 0.3 at the isothermplateau as a function of pH: (A) without salt, (©) 10-' M NaC1, (O) 0.5 M NaC1, (¢) 1 MNaC1, (~) 2 M NaC1.
269
A D S O R P T I O N ON TITANIA
(10). In the case of positively charged surfaces (hematite and ruffle in an acid medium) Gebhardt et al. attribute an increase in adsorption to the growth of electrostatic attractive forces between the mineral and the polymer (11). In this example, however, electrostatic interactions alone cannot account for the experimental results since at very acidic pH, near or less than 3, the polymer is no longer charged and adsorption should decrease. For the adsorption ofpolyacrylic acid on titanium dioxide (anatase) Foissy suggested that a continuous increase in adsorption with the acidity of the medium results from a combined influence of charge variations and a decrease in the size of the polymer (1). All these arguments cannot be used for the adsorption of PAM because it is practically neutral. Neither its charge nor its size varies between pH 7 and 4. The increase in electrostatic forces with acidity of the medium should be demonstrated experimentally. Most arguments in this direction are based on the variation of the substrate and polymer charge densities independently
Q ads
mg/g
"-,•a ds
rng/g
! 30
20
5
6
7
8
9
10
PH
FIG. 7. Adsorption of H P A M 16 at the isotherm plateau as a function o f p H : (A) without NaC1, (O) 10-1 MNaC1, (O) 5 × 10- 1 M NaC1, (0) 1 M NaC1, (V) 2 M NaC1.
and suppose that they are not modified by adsorption. Yet Joppien (12) shows that the charge density of silica decreases as polyoxyethylene is adsorbed. In another study, Foissy et al. (13) suggest the deionization of the interface wfaen polyacrylic acid is adsorbed on anatase in an acidic medium.
Influence of Salinity 30
The adsorption of PAM is not very sensitive to the medium's salinity, which is consistent with a lack of electrical interactions between the surface and an uncharged polymer or between adsorbed molecules. At any pH between 4 and 11 adsorption of HPAM 4 and 16 increases considerably when NaC1 concentration goes from 10 i to 2 M.
20
t0 TABLE II Adsorption of H P A M 16 (mg g 1) 5
6
7
8
9
10
PH
FIG. 6. Adsorption of H P A M 4 at the isotherm plateau as a function o f p H : (A) without NaC1, (O) 10-1 MNaC1, (©) 5 × 10- 1 M N a C 1 , (0) 1 M N a C I , (v) 2 M NaC1.
CaCI2(M):
10-4
10-3
10-2
10-1
pH 7
11
13
19
32
pH 10
7
15
24
9.5
Journal of Colloid and Interface Science, Vol. 121,No. 1, January 1988
270
GIROD,
LAMARCHE,
This result cannot be explained by the sole effect of salinity on the electrical interactions between the substrate and the polyelectrolyte. On the one hand, in alkaline media, the mineral and the polymer are negatively charged and the electrolyte promotes adsorption by suppressing the electrostatic repulsion. On the other hand, below pH 6, both entities have no charges, or opposite charges and electrical interactions should be zero or attractive; the electrolyte would then have no effect on, or hinder, the adsorption. A limiting example of this phenomenon is shown in Fig. 5 in the case of an uncharged polymer. As shown by Bonekamp et al. (14) in the case of polylysine on AgI and polystyrene latex, an electrolyte can drastically promote the adsorption by reducing the mutual repulsion between similarly charged groups, thus favoring the development of loops and a high molecular density on the surface. This effect still occurs at high molar ionic strength. The decrease in intramolecular electrostatic repulsions can be tentatively evaluated by the variation of Rg with salt concentration. Indeed for HPAM 16 a good correlation is found between adsorption and Rg calculated from intrinsic viscosity (Fig. 2) by the Flory-Fox equation [~/] = 2.1 × 1021(Rg-2)2/3M -1. At pH 10 for instance Rg varies from 1180 to 640 ~, as the adsorption increases from 11.5 to 24 mg/g when the salt concentration increases from 10-1 to 2 M. Nevertheless this reasoning no longer holds true for HPAM 4 since for an identical pH its adsorption doubles under similar conditions while its Rg decreases only from 875 to 800 ~. If the electrical repulsions between charged groups and/or the size of the molecule were the main parameter, the adsorption level of the HPAM at strong salt concentrations should be the same as that of the neutral polymer. In fact, in an alkaline medium, the adsorption of hydrolyzed polymers is almost twice that of PAM. Journal of Colloid and InterfaceScience, Vol. l 21, No. 1, January 1988
AND FOISSY
Influence of Polymer Solubility and Adsorption Energy The two main theoretical models of polyelectrolyte adsorption, that of Hesselink (15) and more recently that of Van der Schee and Lyklema (16) clearly show that the quantity adsorbed also depends on the nonelectric (chemical) interaction energy between the surface and the polymer on the one hand and the interaction energy between the polymer and the solution on the other hand. The first term is represented by e (Hesselink) or Xs (Van der Schee) and the second is measured by the Flory-Huggins parameter. Very few experimental certitudes exist concerning the nature of the bond between more or less hydrolyzed polyacrylamides and mineral oxides. Nevertheless, it can be assumed that a hydrogen bond is established more easily with a carboxylic rather than with an amide group. This hypothesis is confirmed by Espinasse (17) for the adsorption of HPAM on montmorillonite. In our study the greater adsorption of HPAM with respect to PAM can also be explained in this way. Concerning interaction energies between the polymer and the surface, we showed a decrease in integral enthalpy of adsorption of polyacid on anatase as pH increases and ionic strength decreases (13). This is rather unexpected since these experimental conditions correspond to an increase in adsorption. We explained this phenomenon by the fact that polymer adsorption is associated with an exothermic reaction. The hydrogen bond between the surface and a polymer segment is stronger than that between the surface and solvent molecule. Using a simple enthalpic balance we showed that the endothermic contribution was the deprotonation of the surface sites. The adsorption reaction can be summarized in the following way: SOH~- • • • C1- + HOOC - polymer SOH- - • HOOC - polymer + H + + C1-.
271
ADSORPTION ON TITANIA
It is evident that the endothermic contribution takes on a more important role in a more acidic medium and at a higher ionic strength since in these two cases the density of charged surface sites is greater. This hypothesis concerning the reactional mechanism of adsorption was confirmed by an analysis of the surface density of chloride ions as a function of the polymer adsorption rate. A confirmation of this result using independent techniques would be desirable because it would invalidate all explanations of an electrostatic nature on the increase in adsorption of these polyacids on mineral oxides in an acid medium. On the other hand, in an alkaline medium we showed that the adsorption of polyacrylic acid was accompanied by the adsorption of Na + counter ions. The nature of the bond has not yet been elucidated (hydrogen bond on the nonionized hydroxyl sites, or bridging by the Na + ions) but this result suggests that the increase in adsorption with ionic strength could result, at least partially, from a better screening of the charge of adsorbed molecules. Van der Schee and Lyklema's (16) calculations and more recently those by Evers et al. (18) show the considerable influence of the solubility parameter X on the amount of uptake polymer. In the second reference, the adsorption of a polymer resembling HPAM 16 ( p o l y a c i d , pKin t = 4, mol wt 2,000,000) is calculated as a function of p H and the FloryHuggins parameter x. The influence of X is particularly sensitive when pH is near or less than pK. Since information in this area is lacking, we measured the variation of the solvent strength of the medium by calculating the Huggins constant as a function o f p H and ionic strength from the polymer solution's viscosity (19). Figure 8 shows that at pH 4 the polymer solubility greatly decreases when salt concentration increases from 10 -~ to 1 M. This result is then appropriate to explain the adsorption increase in an acid medium when ionic strength increases from 0 to 2 M. In conclusion, it is always possible to qual-
PHA4 1
0,5
0.1
,
,
0.5
1
Cs
M.L- !
2
FIG. 8. Huggins constant k as a function of the NaC1 concentration: (A) pH 4, (O) pH 10.
itatively explain the variations in adsorption ofpolyelectrolytes because the parameters are numerous and the systems are not sufficiently defined. Moreover, in certain cases, arguments of an electrostatic nature are insufficient or unjustified. Given the most recent theoretical analyses and our evaluation from viscosity measurements, it seems important to take into account the solubility variation as a function of solvent strength and the type of bond or adsorption reaction mechanism. Experimental knowledge still seems insufficient for hydrolyzed polyacrylamides and polyacrylic acids to corroborate available theoretical models. ACKNOWLEDGMENTS This work was supported by the CNRS through its PIRSEM project on the conservation of natural resources. T. X. Scott is acknowledged for the translation, adaptation, and typing of the report.
REFERENCES 1. Foissy, A., Thesis, Besan~on, France, 1985. 2. Lamarche, J. M., Persello, J., and Foissy, A., Ind. Eng. Chem. Prod. Res. Dev. 22, 1, 125 (1983). 3. Foissy, A., M'Pandou, A., Lamarche, J. M., and Jaffrezic Renault, N., Colloids Surf. 5, 363 (1982). 4. Chong, P., and Curthoys, G., Int. J. Miner. Process. 5, 335 (1979). 5. Dodson, P. J., and Somasundaran, P., J. Colloid Interface Sci. 97, 481 (1984). 6. Bocquenet, Y., and Siffert, B., Colloids Surf. 9, 147 (1984).
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GIROD, LAMARCHE, AND FOISSY
7. Howard, G. J., Lyth Hudson, F., and West, J., J. Appl. Polym. Sci. Polym. Sympl 61, 389 (1977). 8. Klimenko, O. I., and Lapin, V. V., Kolloid Z. 42, 5, 968 (1980). 9. Slater, R. W., Clark, J. P., and Kitchener, J. A., Brit. Ceram. Soc. Proc. 13, 1 (1969). 10. Chauveteau, G., and Lecourtier, J., Colloque RCP "Rh~ologie des solutions de polym~res utilis6s en r6cuperation assist6 du p6trole," Sainte-Crain-auxMines, May, 1985. 11. Gebhardt, J. E., and Fuerstenau, D. W., Colloids Surf. 7, 221 (1983). 12. Joppien, G. R., J. Phys. Chem. 82, 20, 2210 (1978). 13. Foissy, A., Lamarche, J. M., Fahys, B., and Partyka,
Journal of Colloid and Interface Science, Vol. 121, No. 1, January i988
14. 15. 16. 17. 18.
19.
S., AFCAT Conference, "Joum~es de Calorim6trie et d'analyse thermique," Montpellier, France, May 20-22, 1985. Bonekamp, B. C., Van der Schee, H. A., and Lyklema, J., Croat. Chem. Acta 56, 4, 695 (1983). Hesselink, F. Th., J. Colloid Interface Sci. 60, 448 (1977). Van der Schee, H. A., and Lyklema, J., J. Phys. Chem. 88, 6661 (1984). Espinasse, P., Thesis, Mulhouse, France, 1979. Evers, O. A., Fleer, G. J., Scheutjens, J. M. H. M., and Lyklema, J., J. Colloid Interface Sci. 111, 2, 446 (1986). Yamakawa, H., J. Chem. Phys. 34, 4, 1360 (196I).