Liquid Interfaces

Journal of Colloid and Interface Science 216, 197–220 (1999) Article ID jcis.1999.6312, available online at http://www.idealibrary.com on

FEATURE ARTICLE Polyacrylamide at Solid/Liquid Interfaces Emile Pefferkorn Institut Charles Sadron, 6 rue Boussingault, 67083 Strasbourg Cedex, France Received February 26, 1999; accepted April 27, 1999

foundly modifies the nature of the interaction forces. The specific interaction typical of neutral polyacrylamide progressively disappears with increasing hydrolysis until for polyacrylic acid the surface coverage corresponds to nonselective interfacial deposition. Extremely slow displacement of hydrophilic and hydrophobic groups are found to occur within the adsorbed layer.

In the literature, the term polyacrylamide refers to neutral, hydrolyzed, and chemically modified polyacrylamide and in this sense polyacrylic acid may likewise be considered to consist of hydrolyzed acrylamide groups. These hydrosoluble polymers are employed in wide range of applications where the polymer characteristics at solid–liquid interfaces play an important role. The interaction with solid adsorbents is very complex since electrostatic, hydration, van der Waals, and other forces operate simultaneously. As in most applications the polyacrylamide is brought into contact with oxides, clays or soils, synthetic alumino-silicates, and aluminum oxide may be used as model adsorbents. Thus, with reference to the two systems polyacrylamide/alumino-silicate and polyacrylamide/aluminum oxide, the author attempts to define some of the interfacial processes affecting the adsorbed macromolecules and to describe certain interfacial characteristics of these complex systems. The main results are as follows.

Despite the promising use of macromolecules in environmental strategies, a consideration of the complexity of the underlying interfacial phenomena points to the difficulties which may arise at short or long terms in such applications. © 1999 Academic Press Key Words: polyacrylamide; adsorption at solid–liquid interface; interfacial reconformation; interfacial mobility; interfacial exchange.

—Neutral polyacrylamide. Hydrogen bonding between acrylamide and neutral aluminol groups is responsible for adsorption and the amount of polymer adsorbed parallels the surface density of aluminol sites. At ambient temperature, the kinetics of layer formation is governed initially by the random deposition of solution macromolecules and finally by tunneling. Thermodynamic and kinetic considerations lead to a model where the majority of the adsorbed macromolecule share identical dynamic characteristics under equilibrium conditions. Departure from equilibrium induced by changing the polymer concentration in the supernatant phase greatly modifies the dynamic features of the adsorbed macromolecules. —Hydrolyzed polyacrylamide. Charge– charge interactions superimpose on the persistent effects of hydrogen bonding to alter the amount of polymer adsorbed and the adsorption kinetics. The strong affinity of hydrolyzed polyacrylamide for positively charged surfaces induces interfacial spreading of the adsorbed macromolecules and in some instances desorption of part of this population (overshoots). —Complexed polyacrylamide. The presence of positively charged, negatively charged, and neutral chain segments confers amphoteric character on the polymer chain. Strongly complexed polymers display fast adsorption, while initially weakly complexed polymers show delayed adsorption. In addition, under certain circumstances, the presence of hydrophobic microdomains confers an amphiphilic character on the macromolecules, which pro-

INTRODUCTION

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The interfacial behavior of macromolecules adsorbed at solid–liquid interfaces has aroused interest since 1950 (1, 2) and theoretical studies immediately accompanied the early experimental investigations (3). The first reference book in this domain was the Lipatov and Sergeeva Adsorption of Polymers (4), which was published in 1974. The major developments concerned adsorption kinetics and equilibrium characteristics. In thermodynamic terms, polymer adsorption was treated like that of low-molecular-weight solutes, for which Langmuir-type kinetics and equilibria had been found to apply (5, 6). Adsorption enthalpy was for example derived from the temperature dependence of adsorption isotherms (7–9). Killmann pointed out that this method might cause problems insofar as the conditions of isostericity and reversibility were not fulfilled in the case of polymer adsorption and showed that contradictory results were obtained using the Clausius–Clapeyron equation or direct measurements of adsorption enthalpy (10). This observation demonstrated that the kinetics of polymer adsorption and the equilibrium characteristics of adsorbed polymers must involve very complex phenomena. The aim of the present paper is to present some (early, recent, and unpublished) experimental results revealing the complex behavior of a typical class of water-soluble polymer 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.

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(polyacrylamide) at solid–water interfaces and to briefly discuss the possible influence of polymer conformation on the stability of disperse media. References to the literature are by no means exhaustive and citations mainly refer to results which drew the author’s attention. In the author’s opinion, two domains of investigation deserve particular attention: —The establishment of adsorption equilibrium. An equilibrium situation appears to be established within hours or days for certain polymers, whereas adsorption has been found to increase over months in other systems. Since the stability of suspended colloids may change dramatically in the presence of polymer molecules, the mechanism of initial surface coverage will also be considered. —Reconformation processes in the adsorbed state. Such processes have been detected in calorimetric studies and could be correlated with the progress of surface coverage. In the case of poly(ethylene glycol) adsorbed on silica from carbon tetrachloride solutions, Killman showed that one-half of the chain segments were in close contact with surface silanol groups at low coverage but only 0.2 to 0.25% chain segments were at saturation (10). Among the different polymer systems which have been synthesized, characterized, and employed for industrial, agricultural, and environmental purposes, polyacrylamide and hydrolyzed polyacrylamide occupy a unique position. Use of polyacrylamide is well known in enhanced oil recovery, where polymer flooding is considered to be the most attractive chemical process (12–14). Salinity and temperature constraints have nevertheless strongly limited fundamental investigations in this area (15–20). In environmental applications, polyacrylamide is employed to reduce erosion and sealing of cultivated soil and to improve soil stability and clay flocculation (21–32), while addition of inorganic and organic polymers with inorganic flocculents in water treatment can serve to reduce the residual concentration of inorganic salts in drinking water (33– 40).

In order to present a general description of solute transfers to and from a solid–liquid interface, we will use the development of de Gennes (41– 43) to interpret the experimental results. The chemical potential m s in solution is given by

[1]

where P is the polymerization index and C b the bulk concentration. The chemical potential m i in the adsorbed layer is a function of the coverage G (41):

m i~G! 5 m i~G e! 1 m 9 3 ~G 2 G e!.

J si 5 K si 3 C b,

[2]

[3]

with K si ; D/R, where D is a diffusion coefficient characterizing the chain transfer within a layer of thickness R. The inverse flux is given by J is 5 K is

G . R

[4]

Defining standard conditions such that m i(G 0) 5 m s0, the adsorption isotherm is Ge 5 G0 1

kT C b,eR , ln Pm9 G0

[5]

and the rate of establishment of adsorption equilibrium is described by (43)

F

G

dG 1 N~ m i 2 m s0! . 5 F~G! K si C b 2 exp dt Ge kT

[6]

Kinetics of Polymer Adsorption De Gennes proposed the adiabatic growth model for polymer adsorption under the following conditions: a very dilute ambient solution (C b 3 0), so that growth is slow and allows the adsorbed layer to maintain internal equilibrium. Equation [6] is simplified by the additional limitation that the transfer of adsorbed molecules from interface to bulk solution may be neglected in comparison to the inverse transfer. The adsorption rate is then described by (43) dG 5 C bF~G! K~G!. dt

INTERFACIAL RATE EQUATIONS

kT m s 5 m s0 1 ln C b, P

The flux of chains from solution to interface J si is

[7]

In the absence of a glassy layer, K(G) is assumed to be constant and the rate of adsorption to be limited by the term F(G). Defining the relative degree of surface coverage as

u5

G , Ge

[8]

F(G) may be expressed by the relationships [9] and [10] using the mean field and scaling approaches, respectively: F~G! } 1 2 u F~G! } ~1 2 u ! 3/ 2 .

[9] [10]

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The relaxation time t is defined by 1 Nm9 5 K siC b . t kT

[11]

Interfacial Exchange The establishment of adsorption equilibrium requires the polymer coverage to be maximal and constant and the polymer to have adopted an “equilibrium” conformation. Analogy with the Langmuir model shows that transfers from solution to adsorbent and adsorbent to solution cancel each other out. As pointed out by de Gennes, an important feature of the adsorbed layer is the strong repulsion between adjacent chains. On completion of the coverage by successive addition of polymer chains, there is, however, a strong restoring force tending to maintain G e constant. This means that chains can escape from an equilibrated adsorbed layer only if some of the chains leaving the surface are replaced by new incoming chains from the bulk solution (42). The interfacial exchange can be calculated at adsorption equilibrium for a system in which J is refers to adsorbed labeled macromolecules present at a ratio of G*/G and macromolecules in solution are nonlabeled at time 0. At small values of C b, the characteristic exchange time t* is given by (43) dG* G* 52 , dt t*

[12]

1 K siF~G e!C b 5 . t* Ge

[13]

with

As noted by de Gennes, the main conclusion is that t* is finite and inversely proportional to C b. The relaxation time t during adsorption is relatively short in comparison with the characteristic exchange time t* and is estimated to vary like t*/P. Experimental System and Methodology Adsorption experiment (44). Simultaneous determination of the amount of polymer adsorbed and its concentration in solution is difficult since the rate of adsorption on a bare surface is rapid and usually cannot be recorded. The adsorption experiment was therefore performed in such a manner as to enable the rate of adsorption to be determined with precision as a function of time. A reactor of volume V (48.83 ml) contained the polymer solvent, in which the adsorbent was suspended and degassed. The reactor was then completely filled with solvent and closed. A solution of radioactive polymer of concentration C 0 (2020 cpm/ml) was injected through the inlet aperture at constant rate J V with an automatic syringe. At the outlet aperture, the effluent was filtered and collected in fractions of

0.5 6 0.04 ml (corresponding to time intervals Dt of 1 min and an error margin of one liquid drop). Since a fixed number of 12 drops was collected over each time interval, the volume of the fractions increased slowly with time (typically from 0.532 to 0.543 ml) due to the increase in polymer concentration which could be expected to modify the air/water interface of the drops. Therefore, the effluent fractions were weighed and their radioactivity content, corresponding to the polymer content, was reported as a function of the cumulative volume. The value of the radioactivity was interpolated at intervals of volume increment v of 0.5 ml. This procedure enabled us to characterize to entire adsorption domain as the polymer concentration as close to zero at the start when the initial adsorption could be expected to be fast, but relatively high at the end when the final adsorption could be expected to be slow. The mass conservation equation gives the average increase in adsorption DG during the time interval Dt, at the adsorption time (n 1 1), taking into account the increase in mean effluent concentration [(C b) n12 2 (C b) n ] during collection of the fraction (n 1 1): ~DG! n11 5 J VC 0 Dt 2 v~C b! n11 2

Vv @~C b! n12 2 ~C b! n #. 2J VDt [14]

Since at time t 5 0 n is equal to zero, n gives the adsorption time in minutes. Figure 1 presents (C 0 2 v(C b) n11)/C 0 as a function of time on a log-normal plot. Straight lines are observed as expected in the absence of adsorbent or in the presence of nonadsorbing beads. In the presence of adsorbing beads, deviation from a straight line is expected to occur up to the end of adsorption, where the variation with time of the effluent concentration is given by

S

D

C 0 2 v~C b! n11 JV 5 exp ~n 2 f !Dt . C 0 2 v~C b! f11 V

[15]

The existence of superimposed straight lines in Fig. 1 for dilution experiments in the presence and absence of nonadsorbing beads leads us to conclude that a stagnant liquid layer on the bead surface does not significantly limit solution to surface transfer. Therefore, variations in the polymer concentration of the effluent must be attributed to surface area effects. Exchange experiment at constant bulk solution concentration (45). Adsorption of radioactive polymer was carried out as above and the equilibrium concentration of nonadsorbed polymer was fixed at a value of C *b. The beads were then allowed to settle and the supernatant was carefully collected and immediately replaced with a nonlabeled solution at the same concentration C b. The interstitial solution was weighted to precisely calculate the remaining bulk radioactivity. After filling of the cell with the same nonlabeled solution, this solution was injected at 0.05 ml/min into the closed vessel. The

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effluent at the outlet aperture B was collected in fractions of 1 ml corresponding to time increments of 20 min and the radioactivity of each fraction was determined as previously. Once again, the rate limiting effect of the stagnant layer was neglected and Eq. [16] provides the radioactivity balance for J is transfer as a function of elution time t: 2

dG* dC *b 5 C *b 2 V . dt dt

[16]

The existence of an equilibrium situation was confirmed in an inverse experiment where J si transfer characteristics were determined for the exchange of nonlabeled adsorbed polymer with labeled bulk solution polymer. In this situation,

[17]

FIG. 2. Representation of the synthetic alumino-silicate surface containing –Si–OH and AAl–OH surface groups and the hydrogen bonds (- - -) established on adsorption of neutral polyacrylamide with the chemical formula (–CH 2 –CH–CAO–NH 2 ) n .

holds, where C *0 is the concentration of the injection solution. Exchange experiment at increasing bulk solution concentration (46). In this experiment, labeled polymer was adsorbed onto the beads and at adsorption equilibrium the supernatant was removed and replaced with solvent, after which the cell was filled with solvent. The polymer concentration in the cell

was slowly increased by immediate injection of nonlabeled polymer at concentration C 0 and desorption was measured by collecting and analyzing the effluent as a function of time. The desorption rate is given by Eq. [16].

dG* d~C *0 2 C *b! 5 ~C *0 2 C *b! 2 V dt dt

Neutral Polyacrylamide Adsorbed on Synthetic Alumino-silicate Beads Adsorbents. Nonporous glass beads (Verre et Industrie) of 34 mm diameter were soaked for 24 h in hot hydrochloric acid solution to exchange and extract heavy metal ions and make the surface fully hydrophilic. The adsorbent was then washed with water until free of acid and dried under reduced pressure at 25°C. This sample, resembling silica beads, provided the nonadsorbing beads. Adsorbing (AAl–OH) sites protruding from the silica surface were generated by treating the beads with aluminum chloride in dry chloroform, followed by immersion in water before drying. The resultant surface contains AAl–OH groups as shown in Fig. 2. The adsorbent is characterized by the degree of surface grafting (DSG) defined by (44) DSG 5

FIG. 1. Representation as a function of time of the specific radioactivity of the reactor content C b (cpm/ml) according to Eq. [14], resulting from the injection of a radiolabeled solution of concentration C 0 equal to 2020 cpm/ml at the rate J V of 0.5 ml/min for experiments carried out without adsorbent (E) (cell volume, 48.89 ml), and with 1 g of nonadsorbing beads (F) and 1 g of adsorbing beads (■) (cell volume, 48.38 ml). The lines determine the domains of variation of the specific radioactivity resulting from dilution of the injected solution as derived from Eq. [15].

@AAl–OH# . @™Si–OH#

[18]

Polyacrylamide. Polyacrylamide was prepared by radical polymerization in the presence of traces of acrolein. After purification and fractionation, the polymer was treated in solution with tritiated potassium borohydride to yield the following chemical structure: @–CH2 –CH–# m – @–CH2 –CH–# n u u CAO CH–OH , u u 3 NH2 H

[19]

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

201

FIG. 4. Representation of the polypeptidic hydrogen bond (- - -) linking two acrylamide groups in concentrated solutions or gels at temperatures below 40°C. FIG. 3. Representation of the maximal adsorption of polyacrylamide (mg/m 2) as a function of temperature (°C) on alumino-silicate beads (DSG 5 0.05) suspended in water at pH 4.5.

with m 1 n 5 1 and n of the order of 0.001. The characteristics of the sample used in this study were M w 5 1.2 3 10 6 , M w/M n 5 1.25 (GPC), and specific radioactivity of 1.12 3 10 8 cpm/g. Since the detection limit of sample counting (Tricarb Spectrometer, Packard) was close to 10 cpm, 0.1 mg of polymer could be detected in 0.5 ml of effluent. The usual concentration C 0 injected into the reactor was of the order of 10 13 polymer molecules or 25 mg/ml. Adsorption isotherms. The amount of polymer adsorbed at equilibrium was found to depend strongly on pH. Acid– base titration and electrophoretic mobility measurements led to the conclusion that interactions between neutral AAl–OH surface groups and acrylamide groups of the polymer were responsible for this effect (45, 32). Correlation of the variation with pH of the number of surface sites and the amount of polymer adsorbed on adsorbents having different DSGs gave a constant average number of about 70 6 10 chain segments linked to each AAl–OH surface group in all situations (32). The hydrogen bonding of aluminol and acrylamide groups is schematized in Fig. 2. Figure 3 shows the temperature dependence, where a strong decrease in G max near 40°C may be attributed to disappearance of the intermolecular polypeptidic hydrogen bonds represented in Figure 4 (47–50). The temperature range in which such bonds are disrupted is relatively small for natural proteins but is large for synthetic polypeptides due to their molecular weight polydispersity. Finally, the coverage of 0.2 mg/m 2 corresponds to surface filling with individual polyacrylamide molecules, whereas at lower temperature the layer resembles a dense network of chains where some segments are directly connected to the adsorbent but the majority are held near the surface by intermolecular links. The difference in adsorption was indirectly determined by Griot and Kitchener who found that a silica suspension (Aerosil) at 40°C could not be flocculated by polyacrylamide, although on subsequent cooling to room temperature the system flocculated normally

(51). In fact, the small surface coverage established above 40°C was unable to sterically stabilize the suspended colloid and to achieve thermodynamic equilibrium and colloid stability at 25°C would have required a 10-fold increase in adsorption. Figure 5 shows the adsorption isotherm at pH 4.5 (semilogarithmic plot). The isotherm is sharp, full coverage is established for equilibrium concentrations exceeding 0.5 mg/ml (60 cpm/ml), and the amount of polymer adsorbed at the plateau is equal to 1.84 mg/m 2. Determination of the hydrodynamic characteristics by viscosimetry and dynamic light scattering techniques showed the hydrodynamic thickness of the adsorbed layer to be close to 109 nm, twice the radius of gyration of the macromolecule in solution. Taking into account all these data, we calculated the average polymer density to be 44 and

FIG. 5. Reduced adsorption isotherms (semi-log plot) determined at 25°C for polyacrylamide adsorbed on beads (DSG 5 0.05) suspended in water at pH 4.5 (E) and 4.0 (F).

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4.6 mg/ml in the interfacial layer and in the solute coil volume, respectively (46). Kinetics of adsorption. The rate of establishment of the adsorbed polymer layer of Fig. 6 has previously been interpreted on the basis of the Langmuir model assuming two types of kinetic to be operative during the adsorption process (44). However, no clear explanation was proposed to account for the experimental results and only an additional resistance to interfacial transfer offered by the diffuse layer at high segment density was considered to further limit the adsorption rate. The kinetic model (43) of de Gennes will here be employed to validate this approach. Figure 7 shows the average rate of adsorption (1/C b)(D u /Dt) as a function of (1 2 u) for one typical experiment at pH 4.5. Open symbols were obtained by a polynomial fit of the variation of the sample radioactivity with time and filled symbols by simple optical interpolation. G e was taken for simplicity to be equal to G max over the full range of coverage, and in the range 0.65 , u , 0.99 where G e is strictly equal to G max (see Fig. 5), F(u) is determined to vary like (1 2 u) 3/2. The interfacial layer can be established through two processes. A first rapid process corresponds to a diffusion-limited process of surface filling, while a second slower process corresponds to the formation of interpolymer polypeptidic bonds, where there is interfacial exchange between incoming chain segments and adsorbed segments already linked to aluminol groups.

FIG. 6. Rate of surface coverage for polyacrylamide during adsorption on beads (DSG 5 0.12) suspended in water at pH 4.5 and 25°C as derived using polynomial (E) and optical fitting (F) for interpolation of the temporal variation of the specific radioactivity C b of the reactor effluent.

FIG. 7. Representation of the instantaneous rate of adsorption as a function of the fraction of free surface area according to Eqs. [7] and [10]. The dashed line of slope 5 1 corresponds to the random sequential adsorption model and is valid for u , 0.65 (dotted line) while the solid line of slope 5 1.5 determines the domain of validity of the theory of de Gennes. The two sets of experimental points were calculated using polynomial (E) and optical fitting (F) for interpolation of the temporal variation of the specific radioactivity C b of the reactor effluent. The experimental conditions are given in the legend of Fig. 6.

Kinetics of Interfacial Exchange At constant solution concentration. Desorption of radioactive polymer did not occur in the presence of polymer-free solvent within the limits of experimental precision. Conversely, at finite solution concentrations of polymer, a unidirectional net flux of labeled polymer from surface to solution set in. Extrapolation of the radioactivity to time zero revealed a rapid, increasingly important initial drop in surface radioactivity with increasing bulk solution concentration. Figure 8 shows the variation with time of C *b for solution concentrations C b of 2, 5, 10, and 17 mg/L at pH 4.0 and the lines represent the best polynomial fits for the different experiments. Similar experiments were carried out at varying pH and the expected correlation (Eq. [13]) between 1/t*, C b, and G e was obtained in all cases (Fig. 9). At pH 4.0, there was linear dependence of 2d ln G*/dt on C b, while the influence of surface coverage was apparent in experiments at different pH. These results demonstrate the existence of two populations and led us to conclude that the slowly exchanging polyacrylamide adsorbed in the plateau region of the isotherms displays interfacial characteristics which are independent of the amount of polymer adsorbed. Finally, experiments using nonlabeled adsorbed polymer and radioactive bulk solution polymer showed K si to be equal to K is. At increasing solution concentration. Since the amount of radioactive polymer desorbed during the initial fast exchange

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FIG. 8. Representation as a function of time of the specific radioactivity C *b of the reactor content originating from interfacial exchange with nonlabeled polyacrylamide of radiolabeled polyacrylamide adsorbed on beads (DSG 5 0.05) suspended in water at pH 4.0 and 25°C. In the different experiments, the constant total concentration (mg/L) of polyacrylamide in the reactor was fixed at 2 (‚), 5 (E), 10 (h), and 17 ({).

process was found to increase with bulk solution concentration, the interfacial desorption was determined as a function of polymer solution concentration C b under different pH conditions. Figure 10 shows the variation with time of C *b in experiments carried out a different pH. No desorption is observed at time zero, when the supernatant is water at the given pH. As

FIG. 10. Representation of the specific radioactivity C *b of the reactor effluent as a function of time originating from interfacial exchange with nonlabeled polyacrylamide of radio-labeled polyacrylamide adsorbed on beads (DSG 5 0.05) suspended in water at 25°C and pH 3.0 (h), 4.5 (1), 4.7 ({), 5.5 (‚), and 7.0 (E). The rate of supply ( J VC 0 ) of nonlabeled polyacrylamide was 1.4 3 10 23 mg/min. The lines are polynomial fits which were employed to derive the rate of variation of C *b.

the polymer concentration progressively increases, we calculate from Fig. 10 the reduced variation of 2(C b/C *b) 3 (dG*/ dt) as a function of time given in Fig. 11. Considering the methodology of our experiments (concentration C 0 of 28 mg/L injected at constant J V of 0.05 ml/min) and assuming the existence of a fast interfacial exchange between labeled and nonlabeled polymer, Eq. [20] describes the expected variation of the reduced variable (46): 2

FIG. 9. Representation of the specific exchange rate of radio-labeled polyacrylamide as a function of the total polymer concentration in the reactor. The beads (DSG 5 0.05) were suspended in water at 25°C and pH 4.0 (F). For the other experiments carried out at the equilibrium concentration given on the abscissa the pH is indicated near the symbol.

S DS D Cb C *b

F G

dG* d ln C *b 5 C 0 J V 1 VC < C 0 J V. dt dt C b

[20]

In fact, 2(C b/C *b) 3 (dG*/dt) is close to 1.4 3 10 23 mg/min, as is represented by the straight line of Fig. 11. Since in these exchange experiments the polymer distribution between interface and solution is taken to be the same at all times, the adsorbed polymer molecules appear to engage in a relatively fast exchange with bulk solution molecules with increasing polymer concentration in the supernatant. Interfacial exchange between adsorbed and bulk solution polymer has thus revealed the existence of two populations of adsorbed polymer molecules when the situation is frozen at constant concentration of the polymer in the bulk. Nevertheless, adsorbed molecules belonging to one population may exchange with molecules of the second when the situation thaws with increasing polymer solution concentration. A con-

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25 mg/L hydrolyzed polyacrylamide in the supernatant. Figure 12 represents the adsorption as a function of pH on synthetic alumino-silicate of polyacrylamide having different values of t. If one refers to the adsorption peak at pH 4.5 determined for the neutral polyacrylamide—illustrated by the dashed line— one observes that the other peaks are progressively shifted to lower pH for polymers of a higher degree of hydrolysis (20). Moreover, the adsorption varies differently as a function of t below and above pH 4.5. In the domain where adsorption increases with t, charge effects and hydrogen bonding cooperate, while in the region where adsorption decreases with t, charge effects oppose the adsorption by hydrogen bonding. The levels of adsorption measured after an incubation time of 12 h did not correspond to plateau values in all situations and very different adsorption patterns (Fig. 13) were determined as a function of the polymer concentration in the supernatant (46). Three situations may be distinguished:

FIG. 11. Representation as a function of time of the specific exchange rate of polyacrylamide evidenced by the desorption of radio-labeled polyacrylamide in the presence of increasing total concentrations of polyacrylamide in the reactor, for experiments carried out at the different pH values of 3.0 (h), 4.0 (1), 4.5 ({), 4.7 (‚), and 7.0 (E). The experimental conditions are given in the legend of Fig. 10.

stant finding is that an absence of macromolecules in the supernatant precludes the desorption of adsorbed macromolecules. Hydrolyzed Polyacrylamide Adsorbed on Synthetic Alumino-silicate Beads

—At low pH, the net charge of the adsorbent is positive and in addition to the usual AAl–OH– acrylamide hydrogen bond responsible for the adsorption of neutral polyacrylamide (51), electrical forces between AAl–OH 21 and dissociated acrylic acid and acrylate groups contribute to increase the amount of polymer adsorbed. These electrical interactions promote the adsorption of hydrolyzed polyacrylamide of higher t and adsorption equilibrium is rapidly established under such experimental conditions. —At high pH, the net charge of the adsorbent is negative and electrical forces between AAl–OH 2 and acrylate groups tend to decrease the amount of polymer adsorbed. This leads to less adsorption of hydrolyzed polyacrylamide of higher t, but

Hydrolyzed polyacrylamide. Polyacrylamide was synthesized and radiolabeled as described above. A sample of molecular weight 8.3 3 10 5 (M w/M n 5 1.3) was selected and hydrolyzed to different degrees in aqueous NaOH at 50°C for periods of 15 min to 4 h. Polyacrylamide and NaOH concentrations were 3.5 g/L and 0.1 M, respectively, and samples taken at different times displayed the following chemical structure: @–CH2 –CH–# x –@–CH2 –CH–# y u u CAO CAO . u u NH2 OH

[21]

The degree of hydrolysis, t 5 100y/( x 1 y), was determined by acid– base titration. t 5 1 defines the chemical formula of polyacrylic acid. Adsorption isotherms. Amounts of polymer adsorbed after 12 h were determined for a fixed equilibrium concentration of

FIG. 12. Representation as a function of pH of the amount (mg/m 2) of hydrolyzed polyacrylamide adsorbed after a period of 12 h on beads (DSG 5 0.17) suspended in water at 25°C. The degree of hydrolysis t of the different hydrolyzed polyacrylamide was 0.054 (h), 0.15 (■), 0.22 (E), and 0.24 (F).

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

205

where only a small proportion of the injected polymer is adsorbed, at a given moment all the polymer remaining in solution is adsorbed, leaving the supernatant free of polymer. This fast adsorption regime ends like that corresponding to the presence of attractive forces. On the two curves of Fig. 16, the line predicts the expected variation of the effluent radioactivity at the end of adsorption. In the case of the polymer of t 5 15% (curve a), this line fits the experimental variation of the radioactivity, which indicates the arrest of all interfacial net transfer processes. In the case of polymer of t 5 24% (curve b), the radioactivity of the supernatant is found to be greater than that corresponding to dilution of the injected radioactivity as represented by the line. One has to conclude that part of the adsorbed polymer must desorb and contribute to increase the radioactivity of the supernatant beyond its expected levels.

FIG. 13. Representation as a function of the polymer concentration C b in the supernatant of the amount of hydrolyzed polyacrylamide (t 5 0.15) adsorbed after a period of 12 h for experiments carried out at the different pH of 3.26 (■), 4.21 (‚), 4.71 (E), 5.05 (F), and 5.26 (h).

This two-step adsorption process is attributed to interfacial ion-exchange reactions involving polyions and small counterions of the adsorbent. In order to demonstrate this effect, an experiment was carried out using the radiolabeled polyion (b emitter) and radioisotope 125I 2 (g emitter) as tracer counterions. At time zero, the suspension at pH 5.0 contained a mixture of Cl 2 and 125I 2, with the following stoichiometry: @Cl 2 # 1 @ 125 I 2 # > mX 5 @AAl–OH 1 2 #.

adsorption equilibrium is still rapidly established under these conditions. —Near the point of zero charge, the more or less equal densities of positive and negative groups generate conflicting interactions which strongly retard or inhibit the establishment of thermodynamic equilibrium (Fig. 14).

[22]

Quantities in brackets are ionic concentrations in eq/ml of solution, m is the concentration of adsorbent in g/ml, and X is the number of Al groups per gram of adsorbent. The experi-

Kinetics of Adsorption The kinetics of adsorption of polymers having different degrees of hydrolysis t from solutions of varying pH were determined using the apparatus and methodology described above. The basic information was the temporal variation of the radioactivity in the supernatant and we compared the different patterns obtained at low and high pH and near the point of zero charge of the adsorbent (46). —When attractive forces are operative, the radioactivity of the effluent remains close to zero for about 20 min (Fig. 15, curve a), since all the polymer (t 5 24%) being injected is immediately adsorbed. Thereafter, the rate of rate decreases sharply to reach zero after 25 min. In the region of the exponential behavior, the polymer adsorption amounts to 6.16 mg/m 2. —When repulsive forces are operative, only a small part of the radioactive polymer (t 5 24%) injected is adsorbed (Fig. 15, curve b) and when the curve fits exponential behavior the level of adsorption is 0.8 mg/m 2. —Near the point of zero charge of the adsorbent, the radiogram is complex as a result of the successive development of different kinetics. After an initial period of slow adsorption

FIG. 14. Representation as a function of the polymer concentration C b in the supernatant of the amount of hydrolyzed polyacrylamide (t 5 0.15) adsorbed after different periods of incubation of 12 h (‚), 60 h (E), and 84 h (h) for experiments carried out at pH 4.2 on beads (DSG 5 0.17) suspended in water at 25°C.

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step to undergo gradual interfacial spreading (52–57). Our first model of interfacial spreading (flattening) is based on the ideas to be outlined below. INTERFACIAL RECONFORMATION OF ADSORBED POLYELECTROLYTE

When Attractive Forces Are Operative

FIG. 15. Representation of the specific radioactivity C *b of the supernatant as a function of time. Curve (a) corresponds to adsorption of hydrolyzed polyacrylamide (t 5 0.24) following the injection of a solution of C 0 5 1894 cpm/ml at J V 5 0.429 ml/min: the polymer adsorbed amounts to 6.16 mg/m 2. Curve (b) corresponds to adsorption of hydrolyzed polyacrylamide (t 5 0.15) following the injection of a solution of C 0 5 1380 cpm/ml at J V 5 0.68 ml/min: the polymer adsorbed amounts to 0.8 mg/m 2. The solid curves determine the period corresponding to the pure dilution regime.

ment was performed as usual by controlled injection of radiolabeled polymer and the concentrations of b and g emitters were determined in the effluent as a function of time (Fig. 17). Using 125I 2 as tracer, it was not possible to quantify the exchange process since the coefficient of selective Cl 2/I 2 ion exchange is unknown. Nevertheless, it can be seen in Fig. 17 that the solution concentration of 125I 2 decreases strongly during the initial slow adsorption of polyelectrolyte, remains constant during the fast adsorption regime and slowly increases as the adsorption slows down. This result may be interpreted as follows. In the initial adsorption step, the negatively charged adsorbing polymer interacts with existing AAl–OH 21 groups, while the “wall” of the excess negatively charged carboxyl groups induces enhanced protonation of the neutral AAl–OH groups. The surface excess of positive charge is neutralized by rapid transfer of Cl 2 and 125I 2 from the solution to the electrical double layer, as reflected by the decrease in the 125I 2 concentration of the bulk solution. Due to the preferential adsorption of polyelectrolyte, a supplementary transfer of polyions from solution to surface later leads to inverse transfer of the small ions. It was of particular interest to determine the behavior of the adsorbed layer when the polymer in solution was present at very low concentration or injected very slowly. In fact, when the supplementary transfer of polymer was prevented or delayed, we expected the molecules adsorbed during the slow

First, we consider the situation where the net charge of the adsorbent is positive. When polymers approach the surface very slowly, the adsorbed macromolecules are subject to the strong attraction of the adsorbent for a relatively long time before a new adsorbing solute macromolecule competes for the same surface sites. The area S(0) occupied by the macromolecules at the impact point may be considered to correspond to the cross-sectional area of the solute macromolecule and continuously increases with adsorption time t due to the slow interfacial reconformation. Recalling that macromolecules are deposited on the surface at time intervals of Dt and must be adsorbed without delay since no polymer is detected in the effluent, we calculate the number of polymers which can be deposited on unit surface area, where the adsorption stops after the N sth polymer brought to the surface has been adsorbed (46, 52). Comparison of the adsorption per unit area in model and

FIG. 16. Specific radioactivity of the supernatant C*b as a function of time. Curve (a) corresponds to adsorption of hydrolyzed polyacrylamide (t 5 0.15) following the injection of a solution of C 0 5 1859 cpm/ml at J V 5 0.425 ml/min: the polymer adsorbed amounts to 2.53 mg/m 2. Curve (b) corresponds to adsorption of hydrolyzed polyacrylamide (t 5 0.24) following the injection of a solution of C 0 5 1840 cpm/ml at J V 5 0.68 ml/min: the polymer adsorbed amounts to 3.92 mg/m 2. The solid curves indicate the radioactivity corresponding to the dilution of the injected polymer solution according to Eq. [15].

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

FIG. 17. Representation as a function of time of the specific radioactivity (arbitrary unit) of the reactor content for the hydrolyzed polyacrylamide (t 5 0.15) (F) and the iodine ion (E) during adsorption on beads (DSG 5 0.26) suspended in water at 25°C and pH 5.05.

experiments gives the following equations for the initial (0) and equilibrium (eq) situations. (eq) means that the relaxation of one adsorbed polymer has not been hindered by the presence of its nearest neighbors: ~N s 2 1!Dt 5 C S/J VC 0 ,

207

Relative values of C S/C S(0) are expressed as a function of J VC 0 t R/C S(0) in Eq. [27] in order to illustrate the interdependence of experimental parameters and polymer characteristics. The results of experiments carried out at pH 4.4 using the polyelectrolyte of t 5 24% are reported in Fig. 18, where the line corresponds to Eq. [27] assuming C S(0) and t R values of 12 mg/m 2 and 700 min, respectively. In Fig. 19, superposition of the experimental points on the different representations of C S/C S(0) as a function of J VC 0 t R/C S(0) allows estimation of C S(eq)/C S(0). If one ignores the open symbols, correlation of the experimental points represented by the black symbols with the theoretical fit leads to C S(eq)/C S(0) 5 0.02. A final important result of Fig. 20 is the rate of reconformation (flattening) of the adsorbed macromolecule (curve a) as derived from Eq. [26]. Clearly, different pairs of values of C S(0) and t R may serve to fit the experimental results and thus lead to different values of C S(eq)/C S(0). The variation of S(t)/S(0) at short times (t , 200 min), however, does not depend on the initial choice of parameters and only the long-term behavior is affected. In previous work, we proposed taking into account the fact that the injected polymer may not always be immediately adsorbed and some adsorption attempts may remain unsuccessful. This is similar to allowing the macromolecules already adsorbed to continue flattening so long as no new chain succeeds in adsorbing. It is then necessary to modify the first assumption (Eq. [23]) and assume that

[23]

S~0! 5 @N S~0!# 21 5 M W/C S~0!

[24]

S~eq! 5 @N S~eq!# 21 5 M W/C S~eq!.

[25]

If the flattening is as usual expected to stop after a given adsorption period as due to the limited size of a polymer chain, the variation with time of the surface area S(t) is given by

S D

S~t! 2 S~eq! t , 5 exp 2 S~0! 2 S~eq! tR

[26]

where S(eq) corresponds to the area of an adsorbed polymer when no neighbor prevents full flattening. The final surface coverage C S (mg/m 2) is related to the experimental parameters J V (ml/min 3 m 2) and C 0 (mg/ml) and the polymer characteristics C S(0), C S(eq) (mg/m 2), and t R (min) are obtained through C S~eq! CS 2 C S~0! C S~0! J VC 0 t R C S/C S~0! 5 1 2 exp 2 C S~0! J VC 0 t R/C S~0! C S~eq! 12 C S~0!

F

S

DG

.

[27]

FIG. 18. Representation as a function of the rate of polymer supply J VC 0 of the amount of hydrolyzed polyacrylamide (t 5 0.24) adsorbed on beads (DSG 5 0.26) suspended in water at 25°C and pH 4.4. The solid line corresponds to the fit derived from Eq. [26] using C S(0) 5 12 mg/m 2 and t R 5 700 min.

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EMILE PEFFERKORN

The general conclusions are as follows: —C S increases with J VC 0 t R. In a given experiment (constant rate of polymer supply J VC 0 ), C S increases with t R. Hence, if the reconformation is slow, the area available for further adsorption decreases slowly. Similarly, for a given polymer (constant rate of reconformation), C S increases with J VC 0 and maximal coverage is obtained with concentrated solutions. —C S increases as S(eq)/S(0) decreases. A reconformation of small amplitude leads to relatively slow surface saturation, whereas wide spreading of the macromolecules results in rapid surface blocking. This model has been used to quantitatively interpret the reconformation of polyvinylpyridine during its adsorption on polystyrene latex from acidic aqueous solutions (53). Near the Point of Zero Charge FIG. 19. Representation of the reconformation C S/C S(0) of the adsorbed polymer as a function of the reduced variable J VC 0 t R/C S(0). The different curves are generated using C S(eq)/C S(0) 3 `, (dashed curve) and the values of C S(eq)/C S(0) indicated on the solid curves according to Eq. [26]. Black and open circles are the experimental values shown in Fig. 18, black squares are the experimental values of curve (b) in Fig. 21.

~N s 2 1!Dt , C S/J VC 0 .

[28]

In order to evaluate the correcting time and coverage factors which have to be applied when a certain proportion of the injected macromolecules are not immediately adsorbed and have to make further attempts, we assumed (i) the rate of adsorption to be limited by the surface occupation as derived by the random sequential adsorption model and (ii) the limitation in surface coverage resulting from polymer reconformation to be defined by the following growing disc model. Flattening of a macromolecule begins at the time of its contact with the solid surface. In simulation, the flattening of the ith adsorbed polymer (disc) is expressed in terms of its radius (r(i, t9) by r~i, t9! 5 r

H

F

G

S DJ

S~eq! S~eq! t9 1 12 3 exp 2 S~0! S~0! ntR

Limitation of the surface coverage at different rates of polymer supply was investigated at pH 5 for hydrolyzed polyacrylamide (t 5 0.15), where slow and fast adsorption regimes are observed (Fig. 16). The amounts of polymer adsorbed calculated after the slow and fast regimes are represented in Fig. 21 for the different supply rates, while the reduced variables C S/C S(0) and J VC 0 t R/C S(0) calculated from the values of C S at the end of fast adsorption and for C S(0) 5 5 mg/m 2 and t R 5 100 min are reported in Fig. 19. Comparison of the calculated and experimental data shows C S(eq)/C S(0) to be of the order of 0.04. Thus, the reconformation would appear to be of smaller amplitude when the adsorbent bears a net negative charge. Equation [26] provides the rate of the interfacial reconformation (curve b in Fig. 20), which is initially faster than that when

1/ 2

, [29]

where t9 is zero at the moment the disc comes into contact with the surface. In the algorithm, t9 is incremented by one unit when n successively injected discs have attempted to adsorb on the surface, n representing the number of macromolecules supplied to the surface per unit time t9. This is similar to the adsorption of one macromolecule with relaxation time n t R. Thus, by varying the parameter n, it is possible to define different rates of polymer supply or relaxation times (53).

FIG. 20. Representation of the reconformation rate (according to Eq. [24]) of hydrolyzed polyacrylamide (t 5 0.24) adsorbed at pH 4.4 on beads (DSG 5 0.26) using C S(0) 5 12 mg/m 2, t R 5 700 min, and C S(eq)/C S(0) 5 0.02 (curve a) and hydrolyzed polyacrylamide (t 5 0.15) adsorbed at pH 5.0 on the same beads (DSG 5 0.26) using C S(0) 5 5 mg/m 2, t R 5 100 min, and C S(eq)/C S(0) 5 0.04 (curve b).

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

209

absence of complexing ions, the degree of dissociation a of the carboxylic acid groups is defined by

a5

@–COO 2 # @–COO 2 # 1 @–COOH#

[30]

and is determined by the progress of the following chemical reaction: –COOH 7 –COOH 2 1 H 1 .

FIG. 21. Representation as a function of the rate of polymer supply J VC 0 of the amount of hydrolyzed polyacrylamide (t 5 0.15) adsorbed on beads (DSG 5 0.26) suspended in water at 25°C and pH 5.0. Curves (a) and (b) show the adsorption at the end of the slow and fast adsorption regimes, respectively.

only attractive forces are operative (curve a). Differences in the ionic composition of the complex diffuse layer may possibly be responsible for these different kinetics of interfacial reconformation. POLYELECTROLYTES IN THE PRESENCE OF SOLUBLE ADSORBENTS

Taking into account the above considerations and complementary studies, the adsorption of polyelectrolytes at solid– aqueous interfaces would appear to combine a number of complex phenomena (58 – 61). The situation is expected to be of even higher complexity when the suspending liquid medium contains components able to modify the polyelectrolyte characteristics, such as complexing ions and surfactants. We note that the present situation is different from that where multivalent ions act as chelating agents for polyanions and the negatively charged adsorbents (62– 64). In the present work, we examined the situation where complexing aluminum ions were formed by partial dissolution of the colloidal adsorbent aluminum oxide. These investigations focused mainly on the modifications of the solution and of the interfacial behavior of polyelectrolytes resulting from the presence of complexing ions (38, 65, 66). —Polyacrylamide (M W 5 1.1 3 10 6 ) was chosen as the neutral reference to assess the role of hydrogen bonding in adsorption. —Hydrolyzed polyacrylamide of different degrees of hydrolysis (t 5 16 and 30%) and polyacrylic acid ( t 5 1, M W 5 9.6 3 10 5 ) were employed to determine the progressive influence of complexation on adsorption. Let us now consider the ionic characteristics of the polyelectrolyte in two situations. In aqueous solution and in the

[31]

Polyelectrolyte titration in very dilute solutions showed a to lie within 0.15– 0.20. In the presence of aluminum ions, the degree of complexation a comp of the carboxylic acid groups is defined by:

a comp 5

@Al~–COO! 3 # @Al~–COO! 3 # 5 3@N –COOH# @N –COOH# ~a! ~b!

[32]

where N –COOH represents the associated, dissociated, and complexed acid groups, and the concentrations of the complexed species are expressed by N, volume normality (equiv./L), and M, molarity (mol/L) of Al, respectively. Formation of Aluminum—Polycarboxyl Complexes Complex formation was found to depend on pH and on the concentrations of the different constituents. When AlCl 3 was diluted in 0.001 M KCl at pH 5.0, the degree of hydrolysis increased with aluminum chloride concentration (Fig. 22). The centered symbol at n 1 5 2.23 corresponds to the solvent (10 24 M AlCl 3 and 10 23 M KCl at pH 5.0). Since the same average charge of the aluminum ion was observed at higher salt con-

FIG. 22. Representation of the average positive charge n 1 of aluminum ion as a function of the concentration of aluminum ion in 0.001 N KCl solution at pH 5.0. (J) Average net charge n 1 5 2.3 corresponding to the present experimental condition: 0.0001 M AlCl 3.

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EMILE PEFFERKORN

TABLE 1 Complexation Characteristics of Polycarboxylic Acid 10 24 M

5 3 10 25 M

[AlCl 3] Polymer (t 5 )

b max

a comp

b max

a comp

0.16 0.30 1

0.53 0.43 0.29

0.66 0.59 0.51

0.57 0.44 0.26

0.67 0.58 0.48

with FIG. 23. Representation of the hydrogen ion concentration resulting from the interaction of AlCl 3 (1.5 3 10 24 N, solid symbols) and (3 3 10 24 N, open symbols) with carboxylic acid groups as a function of the concentration of carboxylic acid groups for addition of hydrolyzed polyacrylamide (t 5 0.16; ‚, Œ) and (t 5 0.30; ƒ, ), and polyacrylic acid ({, }) to the system.

centration and lower pH (4.6), the different species Al(OH) 30, Al(OH) 21, Al(OH) 121, and Al 31 may be assumed to still represent 0, 25, 8, and 67%, respectively (67) Addition of hydrolyzed polyacrylamide and polyacrylic acid induces a pH drop where the amplitude depends on the aluminum ion and carboxylic concentrations. The hydrogen ion production during complex formation may be interpreted on the basis of Al~OH! ~32m!1 1 n~1 2 a !~–COOH! 1 n a ~–COO 2 ! 7 m n~1 2 a !H 1 1 @~–COO! n Al~OH! m # ~32m2n!1 ,

[33]

where m and n vary from 0 to 2 and from 1 to (3 2 m), respectively. Reaction [33] is too general to give simple information concerning a comp insofar as certain thermodynamic equilibrium constants are unknown. However, as mixing of aluminum ions and polyelectrolyte induced a maximal drop in pH and no free aluminum ions were detected in solution, we assumed the polyion complexation to be maximal and obtained estimates of, respectively, 1.7 and 0.77 and 0.53 for n, m, and the positive charge of the complex. Taking n 5 1.7, one deduces a comp to be close to 0.56. Experimentally, one may assume that the increase in [H 1] shown in Fig. 23 combines the ions produced by complexation of the associated acidic groups and those required to bring the aluminum initially present as di- and monovalent ions to the trivalent form. Thus, from the average value of m and the slope (0.34) of the maximal hydrogen ion production as a function of the total carboxylic concentration, Equation [34] provides the experimental value of a comp (68): ~1 2 a !@H 1 # 5 a compt @N –COOH# 2 m@N AlCl3# 5 $ a comp 2 b max% 3 t @N –COOH#,

[34]

b max 5 @M AlCl3#/ t @N –COOH#.

[35]

The results for b max and a comp are given in Table 1. In very dilute solution, the interaction of negatively charged polyelectrolytes with aluminum ions produces complexes bearing a net positive charge. The different polymers are expected to exhibit complex adsorption behavior since neutral, negative, and positive groups are randomly distributed along the polymer chain (69 –72). Moreover, ion-pairing between carboxylic acid groups and multivalent cations is known to decrease the hydrophilic nature of these polyacids and to induce hydrophobic microdomains (73). Titration of hydrolyzed polyacrylamide (t 5 30%) and polyacrylic acid with KOH in the presence of various aluminum ion concentrations yields typical Katchalsky–Spitnik curves (74) for ion complex formation (Figs. 24 and 25, respectively) which reveal the presence of such hydrophobic domains. The two linear plots of pH vs log(1 2 a)/a were obtained even at low degrees of complexation and demonstrate the existence of stable coordinated complexes. At maximal complexation, the curves nevertheless show some analogy with the pH jump observed in the titration of polymethacrylic acid, where the supplementary energy is derived mainly from the balance of hydrophobic and electrostatic re-

FIG. 24. Representation of pH as a function of log[(1 2 a)/a] corresponding to the titration with KOH of complexed hydrolyzed polyacrylamide (t 5 0.30) for different relative concentrations of the two constituents –COOH and AlCl 3: 1.12 (h), 0.88 (E), and 0.99 (ƒ) in 0.001 N KCl.

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

FIG. 25. Representation of pH as a function of log[(1 2 a)/a] corresponding to the titration with KOH of complexed polyacrylic acid for different relative concentrations of the two constituents –COOH and AlCl 3: 1.08 (h), 1.36 (E), and 0.87 (‚) in 0.001 N KCl.

pulsive forces (75). This characteristic shape could result from the conformational transition from a compact chain containing only complexed groups at low pH to the extended chain containing both dissociated and complexed acrylate groups at higher pH. Therefore, aluminum complexed moieties may be expected to be characterized by a lower solubility in aqueous solution at pH 5. CHARACTERISTICS OF THE POLYMER ADSORPTION

Since the polyelectrolyte chain could no longer be represented as a random distribution of associated R–COOH and dissociated R–COO 2 groups without ignoring complexation phenomena, we determined its solution characteristics by measuring changes in the total and relative concentrations of associated, dissociated, and complexed acidic groups in the supernatant and its interfacial characteristics by determining the electrophoretic mobility of the colloid/polymer complexes. Two types of adsorbent were employed, alumina and polystyrene latex. Materials and Methods Alumina. The a-alumina samples of industrial origin (Aluminum-Pe´chiney) were partially soluble at pH 5 and contained various major impurities such as Na 2O, SiO 2, Fe 2O 3, and CaO. The evolution with time of the pH of suspensions in 10 23 M KCl solutions initially at pH 3.5, each at an oxide concentration ensuring a constant surface area to volume ratio, was found to be greatly sample dependent. However, the final hydrogen and aluminum ion concentrations in the supernatant depended only slightly on the sample. In the case of the a-alumina used in this study, the characteristics of the supernatant different little after an incubation period of 2 or 24 h, despite the fact that this did not strictly correspond to an equilibrium situation. Two successive rates of dissolution were measured in acidic solution

211

(pH 2), indicating the existence of a more soluble layer at the alumina surface. This was shown by ESCA to correspond to alumina hydrates, while the underlying slowly soluble material was aluminum oxide. ESCA also gave information about the nature of the surface impurities. Thus Ca (0.1%) could be fully extracted, whereas Mg (0.3%) persisted after treatment in acid or salt media (68). When 0.4 g of oxide powder was suspended in 50 ml of water containing 10 23 N KCl at pH 4.06, the instantaneous dissolution of the aluminum oxide provided 3 3 10 24 N aluminum ions at pH 5.0. These concentrations of hydrogen and aluminum ions were established instantaneously when the oxide powder was suspended in water containing 10 23 N KCl, 3 10 24 N AlCl 3 at pH 4.06. Therefore, our system comprised a swollen surface network of hydrophilic aluminum hydrates and a supernatant containing 10 23 N KCl, 3 10 24 N AlCl 3, and 10 25 N HCl. Acid– base titration of the colloid suspended in electrolyte solutions of different concentration showed the point of zero charge to lie at pH 9.1. The alumina particles had a mean size of 1.54 mm and a specific surface area of 3.0 m 2/g (BET) and the zeta potential at pH 5.0 and 10 23 N KCl was 58 mV. Polystyrene latex. Spherical polystyrene latexes bearing amidine surface groups (4.28 mC/cm 2) were purchased from IDC (Portland, OR) and used without further purification. The mean particle radius was 198 nm and the specific surface area was 29.5 m 2/g, while the positively charged surface groups gave a constant surface potential of 155 mV over the pH range 2–7. Complex characterization. Concentrations of free and complexed carboxyl groups were determined by acid– base titration using the correlation (Fig. 26) determined between the amount of NaOH required to ionize the carboxylic acid groups (pH 4 to 9) and the initial aluminum ion concentration. As the concentration of reagent required to titrate the free acid groups (for [Al] 5 0) was equal to the concentration of aluminum ions necessary to complex the polymer (no free polymer), this led us to deduce 0.6 as the maximal value of a comp. Concentrations of aluminum ion–polyelectrolyte complexes were also obtained by titration of the ionic constituents, between pH 8 and 10 in the case of excess of polymer and between 7 and 10 in the case of excess free aluminum ions. Ionization of the neutral form was found to be negligible in this pH range. The correlation between the amount of NaOH required to obtain a given pH shift and the initial concentration of aluminum ions in the polyelectrolyte solution is shown in Fig. 26. In order to take into account the coexistence of complexed and free polyelectrolyte segments, we established linear correlations between [NaOH] and the concentrations [–COO 2-f] and [–COO 2-c] of free and complexed acidic groups, which were then used to derive the unknown concentrations of these constituents. The concentration of free polyelectrolyte was calculated from the concentration of complexed aluminum ions

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EMILE PEFFERKORN

Donnan potential in the polyelectrolyte layer, thus overestimating the surface potential at the boundary between the interfacial layer and the surrounding solution (76). Nevertheless, to obtain information concerning the variation of the electrical characteristics with the amount of polymer adsorbed and as a function of time, we used the classical equation to relate the electrophoretic mobility U e and zeta potential z, Ue 5

«zE , 4 ph

[36]

where E is the applied field strength and « and h the dielectric constant and viscosity of the medium, respectively. Kinetics of Adsorption

FIG. 26. Representation of the correlation existing between the initial concentration of aluminum ions and the concentration of sodium hydroxide added to the solution to titrate-free (1.414 3 10 23 N (E) and 2.228 3 10 23 N (h)) and complexed carboxylic acid groups (1.414 3 10 23 N (F) and 2.228 3 10 23 N (■)).

and the total concentration of polymer (68). This latter quantity was also obtained from the total carbon content of the suspension. These titration techniques provided reliable values for the concentrations of all ionic constituents of the supernatant liquid phase in the cases of polyacrylamide and hydrolyzed polyacrylamide of t 5 0.30, but failed to do so for the hydrolyzed polyacrylamide of lower t. Electrophoretic mobility. Measurements of the electrophoretic mobility of the bare aluminum oxide and oxide– polymer complexes were carried out in a Malvern Zeta Sizer III. In order to obtain reliable values of the mobility using the cylindrical AZ4 cell, the oxide suspensions were conditioned to remove large sedimenting particles. The suspension was left standing for a given time to allow settling of the large particles, after which a small portion of the supernatant phase was separated as the sample. The remaining suspension was centrifuged at 15,000 rpm for 45 min and the clear supernatant was used to dilute the sample to the level required by the Malvern method. In the case of a mixture of latex and alumina particles, the sample was centrifuged to separate latex from oxide, an operation which is facilitated by the large difference in density between the latex (1.045) and the aluminum oxide (3.97). This procedure enabled determination of the electrophoretic mobility of the latex–polymer complexes. The electrophoretic mobility of a polyelectrolyte coated particle cannot be interpreted simply on the basis of a rigid particle model. Calculation of the zeta potential in this way ignores the effects of the Debye–Hu¨ckel parameter and the

In kinetic experiments, HCl and AlCl 3 were added to the alumina suspension to establish a stable pH of 5.0, after which a small volume of polyelectrolyte solution at pH 5.0 was added rapidly with stirring. A sharp initial decrease in pH decrease was followed by a slow increase. When the polymer was added to the clear supernatant, the pH remained constant after the fast initial decrease resulting from polyelectrolyte complexation. Figure 27 shows the hydrogen ion concentration during mixing of alumina and polymer as a function of the amount of polymer added to the suspension per square meter of the area developed by the aluminum oxide suspension. The characteristics of the solution (mol/L) and suspension (mg/m 2) can be related through the fact that the mass of colloid suspended in 1 L develops an area of 24 m 2. In the present experiments, the hydrogen ion concentration is an extensive parameter and the increase observed in Fig. 27 depends on all parameters of the suspension including the initial pH, the area

FIG. 27. Representation of the hydrogen ion concentration (mol/L) resulting from the addition of polymer to 0.4 g of oxide suspended 50 ml of (0.001 N KCl 1 0.0003 N AlCl 3) at the initial pH 5.0, for hydrolyzed polyacrylamide (t 5 0.16, Œ) and (t 5 0.30, F) and polyacrylic acid (■). [Pol] is the amount of polymer added per square meter of the area developed by the suspended oxide at time zero.

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

213

—The second case corresponds to the situation of excess polyelectrolyte, where maximal complexation is not attained during the initial mixing of colloid suspension and polyelectrolyte solution. In Figs. 29 and 30, the concentrations of the constituents, free polymer (–COO 2-f), total polymer (–COO 2), and complexed aluminum ion (Al-c), are expressed in terms of the normality of the supernatant on the left ordinate and of the corresponding polymer dosage in mg/m 2 on the right ordinate.

FIG. 28. Representation as a function of time of the free aluminum ion concentration [Al-f] in the supernatant resulting from the addition to the oxide suspension of different amounts expressed in mg/m 2 of hydrolyzed polyacrylamide (t 5 0.30) (0 (h), 2 (F), 4 (■)) and polyacrylic acid (0 (h), 0.4 (E), 0.95 (‚), and 1.7 (ƒ)).

developed by the oxide, and the volume of the liquid phase. It is worthy of note that modification of any one of these parameters would induce a different change in hydrogen ion concentration. The classical expression of the polymer concentration in mol/L is here not adequate to describe the system, since the true equilibrium characteristics depend on the area developed by the aluminum colloid, the volume of the liquid phase, and the initial polymer concentration. Clearly, the degree of complexation is fixed by the solution characteristics, while the degree of surface coverage is related to all characteristics of the system. The polymer concentration in the liquid phase is therefore better defined by the initial or remaining mass of polymer per square meter of oxide surface area (mg/m 2). On the other hand, in order to obtain information concerning the nature (complexed or not) of the nonadsorbed polymer, we chose to consider separately the results for the two situations of excess aluminum ions and excess polymer. The threshold polymer concentrations were close to 2.5 and 4 mg/m 2 for polyacrylamide and hydrolyzed polyacrylamide (t 5 0.30), respectively. —The first case corresponds to situations where the aluminum ions initially present in solution induce total complexation of the added polyelectrolyte, which is thus rapidly and fully adsorbed. Figure 28 shows the residual concentration of free aluminum ions [Al-f] as a function of time for different doses of polyacrylamide and hydrolyzed polyacrylamide (t 5 0.30). After an initial sharp drop, [Al-f] remains constant at a value determined by the polymer concentration. The existence of such constant concentrations of free aluminum ions at levels lower than the initial value of 10 24 M demonstrates the strong protective effect of the adsorbed polyelectrolyte layer against further oxide dissolution.

Figures 29a and 30a show the temporal variation of the concentrations of free polyacrylic acid and the free hydrolyzed polyacrylamide in the liquid phase and Figs. 29b and 30b the corresponding variation of the total concentration of free and complexed polyelectrolyte in the supernatant. In Figs. 29c and 30c, the temporal variation of the level of complexed aluminum ions is also shown. A number of general conclusions may be drawn from these results. In Figs. 29a and 29b and 30a and 30b we note that the concentration of free polyelectrolyte decreases more rapidly than the total concentration of free and complexed polyelectrolytes, which demonstrates (i) the occurrence of complexation prior to adsorption and (ii) the validity of our assumption of the coexistence of polyelectrolytes with different degrees of complexation. It is assumed that the hydrogen ions associated with the polyions dissociate and are slowly replaced by aluminum ions. The time required for complete exchange to provide polyelectrolytes with a maximal degree of complexation increases with the amount of excess polymer, indicating that the aluminum ions originate from the dissolution of alumina. The total polyelectrolyte concentration in the supernatant decreases continuously for relatively small amounts of added polyelectrolyte, less than 4 mg/m 2. A nonmonotonic variation is observed at higher doses, where the added polyelectrolyte is in excess. At relatively low polyelectrolyte concentrations (Figs. 29a and 30a, the complexed polymer is fully adsorbed and aluminum ions could not be detected in the supernatant after 4 to 6 h. In the presence of large amounts of polyelectrolyte, the concentration of complexed aluminum ions increases to values well above the initial value of 3 3 10 24 N, which indicates that the added polyelectrolyte induces a supplementary dissolution of aluminum oxide. Interfacial Characteristics of the Colloid–Polyelectrolyte Complexes The zeta potentials of the bare colloid and polyelectrolyte– colloid complexes were calculated from the electrophoretic mobilities and Eq. [36]. Hence, one must bear in mind that the absolute value of the zeta potential of polyelectrolyte coated alumina particles may have been overestimated as these complexes were treated like rigid particles. Figure 31a shows the zeta potential of alumina particles as a function of the dose of polyacrylic acid. The potential of bare alumina remains constant, thus confirming existence of an equilibrium situation.

214

EMILE PEFFERKORN

increases slowly after a period of 5 to 10 h and seems to become stable only after 24 h. This long transition period may reflect a modification of the distribution of free and complexed carboxylic groups along the polymer chain. —Above 4 mg/m 2, the initial drop leads to a strongly negative potential which remains more or less constant for 24 h. The temporal dependence of adsorption appears to correlate with that of the zeta potential. Thus, adsorption of strongly

FIG. 29. Representation as a function of time of the concentration of the constituents of the supernatant solution resulting from the initial addition of different amounts of polyacrylic acid (mg/m 2): 3.1 (h), 3.8 (E), 4.3 (‚), and 6.7 (ƒ). (a) Concentration of noncomplexed polyacrylic acid expressed in terms of equivalent/L (left ordinate) and mass facing 1 m 2 of the adsorbent area (right ordinate). (b) Concentration of free and complexed carboxylic acid groups (left ordinate) expressed in terms of equivalent/L (left ordinate) and mass facing 1 m 2 of the adsorbent area (right ordinate). (c) Concentration of complexed aluminum ion (equivalent/L).

—Below 4 mg/m 2, the extent of the initial rapid drop in zeta potential increases with the initial polymer dose. However, this does not correspond to the equilibrium value as the potential

FIG. 30. Representation as a function of time of the concentration of the constituents of the supernatant solution resulting from addition of different amounts (mg/m 2) of hydrolyzed polyacrylamide (t 5 0.30): 5 (‚) and 7 (ƒ). Symbols as in Fig. 29.

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

215

Neutral polyacrylamide. The mobilities of bare alumina and latex particles were equal, but were reduced to different extents in the presence of adsorbed polyacrylamide (Fig. 32). This different behavior could result from differences in the interfacial characteristics of the polymer layers, the ionic clouds at the oxide and latex surfaces, and/or the hydrophilic nature of the adsorbents. On the other hand, the observation that the electrophoretic mobility of a colloid–polymer complex depends on the nature of the colloid raises a point of considerable importance in the present context. Since the surface characteristics of the inert adsorbent immersed in the oxide suspension remain distinct from those of the oxide surface, we may conclude that the ionic species released during initial or progressive dissolution of the oxide are not adsorbed on the latex surface. Hydrolyzed polyacrylamide (t 5 0.16). Figure 33 shows the reduction of the electrophoretic mobilities of alumina and latex particles as a function of the dose of hydrolyzed polyacrylamide. The adsorption is once again different on the two colloids and must therefore involve interactions with specific surface sites on the adsorbents. Comparable surface properties were nevertheless obtained for the latex as for the oxide by slightly increasing the polymer dose.

FIG. 31. Representation as a function of time of (a) the zeta potential of the bare and polymer-coated colloidal aluminum oxide resulting from the addition of different amount of polyacrylic acid (mg/m 2): 0 (h), 2.0 (E), 4.0 (‚), 6.0 (ƒ), and 8.0 ({) and (b) the pH of the aluminum oxide suspension (symbols as in (a)).

complexed polyelectrolytes gives rise to positively charged polymeric interfaces, whereas adsorption of weakly complexed polyions gives interfaces characterized by negative values of the zeta potential. The variation of the pH as a function of time is represented in Fig. 31b. At polymer doses of less than 4 mg/m 2, the time required to reach a quasi-equilibrium situation parallels that of the zeta potential in Fig. 31a. Above the threshold polymer dose, the pH increases during the period corresponding to the polyacid induced dissolution of aluminum oxide (Figs. 29c and 30c). Calculations showed that the variations in hydrogen ion concentration cannot originate solely from modifications of the charge characteristics of the oxide surface, but must also involve changes in the degree of ionization and/or complexation of adsorbed carboxylic groups and therefore alterations in the structure of the adsorbed polymer layer.

Polyacrylic acid. Figure 34 shows that the reduction in electrophoretic mobility as a function of the dose of polyacrylic is the same for latex and aluminum oxide particles. In the apparent absence of specific interactions, solution parameters seem to play a predominant role in this deposition process, where the inert or partially soluble adsorbent acts merely as a nonspecific collector for the complexed polyacrylic acid. The establishment of interfacial layers of neutral polyacrylamide proceeds differently from the deposition of layers of

Selective or Nonselective Adsorption (Deposition)? Information concerning the selective or nonselective nature of the adsorption was obtained by compared the electrophoretic mobilities of alumina–polyelectrolyte and polystyrene latex– polyelectrolyte complexes.

FIG. 32. Representation of the reduced electrophoretic mobility of the alumina/polyacrylamide complexes (h) and latex/polyacrylamide complexes (■) as a function of the initial dosage of polyacrylamide in the suspension. The electrophoretic mobility is the value determined after an incubation period of 24 h.

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after which flocculation was initiated by stirring. In the case of a relatively low-molecular-weight polymer (M w 5 40,000), the following observations were made.

FIG. 33. Representation of the reduced electrophoretic mobility of the alumina/hydrolyzed polyacrylamide complexes (h) and latex/hydrolyzed polyacrylamide complexes (■) as a function of the initial dosage of hydrolyzed polyacrylamide (t 5 0.16) in the suspension. The electrophoretic mobility is the value determined after an incubation period of 24 h.

polyacrylic acid from its solutions in media containing complexing ions. The former process resulting from specific interactions may be termed adsorption, while the latter nonspecific process resembles the interfacial deposition of poorly soluble species on inert adsorbents. An intermediate situation combining specific and nonspecific interactions arises in the case of hydrolyzed polyacrylamide, where the large hydrosoluble chain segments ensure an enhanced solubility in the liquid medium.

—If the suspension was gently stirred after addition of polymer, only weak flocculation was obtained as almost no polymer has had time to adsorb and the average size of the flocs was 1.6 mm. —After 5 min, there was much more extensive flocculation since the polymer has had time to adsorb and at least some of the adsorbed molecules were in a conformation favoring particle aggregation. The average size of the flocs was 3.17 mm. —At longer times, 15 or 30 min, most of the polymer had adsorbed and also had time to adopt a flat equilibrium conformation on the particle surface. Consequently, there was again only weak flocculation and the average sizes of the flocs were 2.2 and 1.7 mm, respectively. The efficient flocculation of neutral polymers at low surface area coverage (79) requires the layer thickness to be larger than the Debye–Hu¨ckel length, since electrostatic stabilization is always active at these short distances (77, 80). Experiments have shown that poly(ethylene oxide) molecules of high molecular weight rearrange their configuration on the surface of latex and that this rearrangement occurs on a time scale of a few seconds (81, 82). At very short times, the adsorbed molecule has a diameter larger than the double layer thickness of the particle and can act as molecular bridges, causing flocculation. At longer times, the polymer as flattened its configuration so that electrostatic stabilization is operative and impedes flocculation. An analytical version of the growing disc model has been applied to poly(ethylene oxide) by van Eijk et al. (57). The

ADSORPTION AND RELATED PROCESSES IN COLLOIDAL SYSTEMS

Neutral Polymers and Polyelectrolytes The present studies were focused on water-soluble polyelectrolytes, which are widely employed in industrial, environmental, and agricultural applications. In fact, at the moment a polymer is added to a suspension, several processes are initiated (77): —mixing of polymer molecules with the particles, — diffusion of polymer within the liquid phase and its attachment to the particles, —rearrangement of adsorbed polymer chains to give an equilibrium conformation, — collisions between coated particles which may result in the formation of aggregates. The first manifestation of polymer reconformation was described by Wagberg et al. (78). These authors conducted experiments in which the polymer was simply mixed into the suspension and the adsorption left to take place by diffusion,

FIG. 34. Representation of the reduced electrophoretic mobility of the alumina/polyacrylic acid complexes (h) and latex/polyacrylic acid complexes (■) as a function of the initial dosage of polyacrylic acid in the suspension. The electrophoretic mobility is the value determined after an incubation period of 24 h.

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

model was found to be capable of describing the adsorption and further the simple spreading model of exponential relaxation could be justified by molecular dynamic simulations of low-molecular-weight poly(ethylene oxide) near a graphite surface. Finally, using polarized infrared spectroscopy in attenuated total internal reflection, Enriquez and Granick directly observed chain flattening of poly(ethylene oxide) during its adsorption on germanium from aqueous solutions (81). Reconformation produced a population of adsorbed molecules which could be washed off by replacing the supernatant with solvent. In charged systems, reconformation leads to canceling of surface charges by the adsorbed polymer and this is expected to fully destabilize the coated colloid particles and change the aggregation mechanism. Thus, in the case of polyvinylpyridine adsorbed on polystyrene latex particles bearing surface carboxylic groups, reconformation of the polymer layer has been shown to influence the mechanism of aggregation of the latex (32, 83). The transition from reaction-limited to diffusionlimited aggregation was found to depend on the interfacial relaxation of the adsorbed polymer. At a given concentration corresponding to the initial coverage of 12.5%, where the polymer extended loops into the solution, interparticle bridging was operative. After reconformation, when the surface was fully covered with flat macromolecules, the aggregation rate increased due to total destabilization of the latex particles which then bore a net zero surface charge. Polyampholytes The adsorption on charged colloids of polyelectrolytes bearing positive and negative groups has recently gained new interest on account of the potential use of these polymers as universal flocculents. Joanny and Dobrynin et al. studied the solution conformation of polyampholytes carrying a given fraction f 1 and f 2 of positive and negative charges and their adsorption on a surface of specific charge s (70, 72). In solution, the overall size and shape is determined by the relative contributions of chain entropy, fluctuation induced attraction between charges, and Coulombic repulsion between excess charge to the free energy, which depends on the values of f 1 and f 2 and on the degree of polymerization. The reference situation corresponds to the adsorption of a charged polyampholyte chain on a similarly charged surface. Attractive interactions are induced by electrostatic polarization of the chain which acquires a dipole moment in the surface electric field. Repulsive interactions arise from the repulsive potential experienced by the chain as it approaches the surface and from the appearance of an image polyampholyte chain of similar charge located symmetrically with respect to the adsorbing surface. The polyampholyte adsorbs when the net interaction energy is attractive. When the surface charge density exceeds a critical value, the electrical field of the adsorbent is sufficiently strong to deform the polyampholyte chain. The charged chain segments then redistribute so as to decrease the electro-

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static energy and the chain binds to the adsorbent. Such segregation of polyampholyte chain segments at the interface has been observed experimentally as has typical polyelectrolyte and typical polyampholyte behavior (71). Amphiphilic Polymers In the presence of complexing ions like aluminum polyelectrolytes such as hydrolyzed polyacrylamide and polyacrylic acid in fact combine amphiphilic and amphoteric characteristics since the ion-pairing induces hydrophobic microdomains. The hydrolyzed polyacrylamide chain contains neutral acrylamide at a relative concentration (1 2 t), together with negative acryl groups and positive complexes of the form [(–COO) n Al(OH) m ] (32m2n)1 at a relative total concentration t. Polyacrylic acid has only free and complexed carboxyl groups distributed along the chain. The destabilization by aluminum chloride of rigid colloidal particles bearing surface carboxyl groups was investigated by Ikegami and Imai, who observed that the stability of the colloid–aluminum ion complexes depended on the degree of dissociation a (84). At low a, relatively high concentrations of AlCl 3 were required to induce aggregation, whereas at higher a, very small amounts of salt destabilization the suspension. This was attributed to the presence of two different mechanisms of colloid destabilization. Below a 5 0.2, a high concentration of salt ions would render the suspension unstable by changing the water structure, while above a 5 0.2 cation binding would make the polymer surface hydrophobic. The latter situation was indeed found to arise at very low salt concentrations, where one could expect ion pairing to create hydrophobic microdomains along the polymer chain (84). Long-term behavior has to be considered separately insofar as the short-term behavior discussed above can be mainly attributed to the progress of adsorption. A redistribution of hydrophilic and hydrophobic groups within the adsorbed polymer layer has, on the other hand, been proposed to account for the fragmentation of colloidal aggregates occurring over the periods of weeks (68, 85, 86). In fact, among all the present experimental systems sustaining successive aggregation and fragmentation, no changes in the concentrations of polyacrylic acid and aluminum ions in the supernatant were observed at 6 weeks as compared to 24 h. These constant solution characteristics would indicate that the fragmentation developed without interfacial transfer of polyelectrolyte or aluminum (85). Figure 35 shows the pH of a suspension at 24 h and 6 weeks. It can be seen that pH 5 does not correspond to a true equilibrium situation even in the absence of polymer and there is a shift toward pH 4.6 over 6 weeks. However, addition of polyelectrolyte to the suspension would normally be expected to lead to higher pH values. The hydrogen ion concentration is in fact only slightly modified below a polymer dose of 4 mg/m 2, but above this dose the curve increases more strongly. These variations in pH may be attributed to changes in the ion

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FIG. 37. Schematic representation of the anisotropic distribution of the hydrophobic (F) and hydrophilic (E) chain segments at the surface of stabilized colloid (left) and the “random” distribution of the same groups for chains linking together two colloids (right).

FIG. 35. Representation of the pH of the oxide suspension in the presence of polyacrylic acid as a function of the polymer dosage for incubation periods of 24 h (h) and 6 weeks (E).

distribution within the polymer layer. Figure 36 shows that the zeta potential of colloid–polyelectrolyte complexes decreases with time only for polymer doses of less than 5 mg/m 2, while for higher doses the potential is stable. Once again, the results would suggest a modification of the ion distribution in the

polyelectrolyte layer. This could lead to increases in concentrations of (i) aluminum ion-complexed groups (hydrophobic groups) near the oxide surface where the high chain segment density favors segment–segment contacts and (ii) carboxyl and carboxylic acid groups (hydrophilic groups) in the outer zone where polymer loops protrude into the liquid phase and segment–solvent contacts are more probable. As a result, the distributions of aluminum, potassium, chloride, and hydrogen ions would differ within polymer layers belonging to grain agglomerates (during the initial period of aggregation) or to isolated particles (at the end of the dispersion process). The layer is expected to be more isotropic when the polyelectrolyte chains are initially confined between two colloid surfaces, as in schematically represented in Fig. 37. The relative isotropic distribution results from the random complexation of carboxyl and acidic groups along the adsorbed polyelectrolyte chain. On the contrary, segregation corresponds to the establishment of an anisotropic distribution of hydrophobic and hydrophilic chain segments during stabilization of the colloid–polyelectrolyte complexes. An anisotropic distribution is expected to be favored by enthalpic forces insofar as hydrophobic groups are then concentrated near the adsorbent surface where chain– chain contacts are predominant and hydrophilic groups in the outer zone where chain–solvent contacts are important. CONCLUSION

FIG. 36. Representation of the zeta potential of the aluminum oxide/ polyacrylic acid complexes as a function of the polymer dosage for incubation periods of 24 h (h) and 6 weeks (E).

Progress in our understanding of the adsorption characteristics of hydrosoluble polymers and polyelectrolytes requires more theoretical, numerical, and experimental studies insofar as real systems combine various interactions not only between adsorbent and polymers but also between the polymer molecules. Adsorption of a polymer at a solid–liquid interface can be a very efficient strategy in many industrial, environmental, and agricultural domains.

POLYACRYLAMIDE AT SOLID/LIQUID INTERFACES

One of the major points concerns the reconformation of polymers at plane solid–liquid interfaces. Nothing is known for instance about systems where the “solid” substrate, as in the case of platelets of nanometric thickness, is extremely thin and able to respond to the adsorption of a single polymer molecule by sustaining some local curving. Studies of adsorption, polymer reconformation, and solid curving could lead to the design of new colloidal materials and powders with original properties. In addition, polymer adsorption and interfacial behavior can be monitored at short or long terms by setting the molecular weight of the polymer (or polymer mixture) and the ionic composition. Hydrolyzed polyacrylamide complexed with a conventional mineral flocculant like alum (37) may act as mixed flocculent of increased efficiency in colloid aggregation and aggregate fragmentation. One major advantage is that the aggregate fragmentation and resultant dewatering of the system is expected without release of the additive into the liquid effluent. Finally, the adsorption of polymers on clays and clay minerals may improve the stability of agricultural soils. Natural organic substances constitute a class of macromolecules of very complex chemical structure and conformation due to the coexistence of different amphiphilic and amphoteric groups. Since carboxylic acid functions seem to play a predominant role in cooperative interactions with clays and oxides, one might envisage use of synthetic polycarboxylic acids as substitutes when organic matter is lacking and/or the irrigation water is of poor quality (87). ACKNOWLEDGMENTS This research was supported by the Institut Franc¸ais du Pe´trole (IFP) and the Centre National de la Recherche Scientifique (CNRS). G. Chauveteau and R. Varoqui are acknowledged for stimulating discussions. The author is particularly indebted to A. Carroy, A.-C. Jean-Chronberg, and E. Ringenbach, who have helped to progressively improve his understanding of these complex adsorption phenomena.

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