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Copolymer polyelectrolyte adsorption onto titanium dioxide
Colloids and Surfaces A: Physicochemical and Engineering Aspects 158 (1999) 375 – 384 www.elsevier.nl/locate/colsurfa
Copolymer polyelectrolyte adsorption onto titanium dioxide Annabelle Pina a, Evelyne Nakache a,*, Bruno Feret b, Pierre Depraetere c a
Laboratoire de Chimie Mole´culaire et Thio-organique, groupe ‘Polyme`res et Interfaces’, UMR CNRS 6507, 6, B6d Mare´chal Juin, F 14050 Caen Cedex, France b Elf-Atochem, Centre d’Applications de Le6allois, 95, rue Danton, F 92300 Le6allois Perret, France c Laboratoire de Pharmacie Gale´nique, B6d Henry Becquerel, He´rou6ille-Saint-Clair, F 14032 Caen Cedex, France Received 12 June 1998; accepted 24 March 1999
Abstract We study the adsorption of a polyelectrolyte, which is a hydrolysed styrene maleic anhydride (hSMA) copolymer, on titanium dioxide. When dissolved in aqueous solutions, at basic pH, this polyelectrolyte presents an apolar part, styrene, and a polar part, two carboxylic functions. Our experiments show that the adsorption isotherms are dependent on the pH and on the total ionic strength of the solution which can be varied both by increasing the concentration of the polyelectrolyte and by addition of an external monovalent salt-potassium nitrate. The adsorption isotherms display a plateau followed by an increasing adsorption at small salt concentrations. A detail study of the ionic strength effect show that this increasing adsorption cannot be attributed to polydispersity nor to successive adsorption layers. At high salt concentrations, the adsorption model is Langmuirian. The observed adsorption may be explained by interactions taking place: between the substrate and the copolymer segments. inside the adsorbed layer. We proposed that the interactions near the solid substrate are electrostatic between the discrete positive sites of the surface and the hSMA copolymers and nonelectrostatic such as hydrogen bonds. The interactions inside the adsorbed layer are nonelectrostatic corresponding to hydrophobic bonds between the preadsorbed hSMA copolymers and the free hSMA copolymers in solution via their phenyl groups. The interactions near the solid surface are predominant for small ionic strengths, whereas the interactions inside the adsorbed layer are predominant for high ionic strength as the repulsion between the hSMA copolymer segments is quantitatively diminished. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Polyelectrolyte; Copolymer; Styrene; Maleic anhydride; Ionic strength
1. Introduction Polyelectrolyte adsorption is a significant phenomenon at solid/liquid interfaces, which is * Corresponding author. Tel.: +33-231-452842; fax: + 33231-452877. E-mail address: [email protected] (E. Nakache)
widely studied owing to its relevance in many industrial applications: painting, wastewater treatment, flotation separation… Adsorption of polyelectrolytes have been experimentally [1–4] and theoretically [5] studied during the last few years. Most of the studied polyelectrolyte are homopolymers [2–4] which are strongly or weakly
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dissociated. In the first case, the charge of the polymer does not depend on pH whereas, in the second case, the polymer charge may depend on it. Moreover, the solid substrate charge can be constant or determined by pH or ions in solution. In this study, an hydrolysed styrene maleic anhydride (hSMA) copolymer was chosen. The styrene part is non-polar, while the hydrolysed maleic anhydride monomer gives two carboxylic functions to the polyelectrolyte. The adsorption of hSMA is performed on titanium dioxide at basic pH. Consequently, the copolymer and the substrate are pH dependent and negatively charged. Thus, the studied system is relatively complex. In the literature, a few charged copolymers have been investigated in terms of adsorption capacity [6–8,18] on pH dependent surfaces while a wellknown weak polyelectrolyte, the polyacrylic acid (PAA), has been studied on different solids [9,10] and especially on titanium dioxide [11 –14]. On this substrate, the literature gives some principal tendencies for PAA adsorption. For instance, at a given pH value, Chibowski [13] and Foissy et al. [12] noted that PAA adsorption increased as the molecular weight of the polymer rose as predicted theoretically for neutral polymers in poor solvent [5]. Nevertheless, there are some discrepancies between the shapes of the different adsorption isotherms. Chibowski [13] and Heijman et al. [14] obtained isotherms without saturation level contrary to Foissy et al. [12] and Gebhardt et al. [11]. These authors discovered that the adsorption of PAA increased as the solution pH decreased. They assumed [11 – 13] that the adsorption observed at acidic pH was due to the positive charges on titanium dioxide surface. Therefore, adsorption took place via electrostatic or hydrogen bonding. Nevertheless, PAA adsorption is observed [12,13] or not [11] at point of zero charge or at a more basic pH value. In this case, Foissy et al. [12] proposed that nonelectrostatic interaction exists between the surface and the PAA, thus allowing adsorption. Whereas Chibowski [13] supposed an interaction between the surface and the polyelectrolyte via hydrogen bonding and specific adsorption of the carboxylic groups. It is noteworthy that, for another system, Cosgrove et al. [18] found also an adsorption
between similarly charge polymer chains and surfaces. The studied hSMA copolymer is a commercial product used as a dispersing agent. Aqueous solutions of this kind of copolymer have been studied [15–17] and it has been shown that hydrophobic linkage via phenyl groups took place inside a copolymer chain. The last few years, Argillier et al. [19] and Volpert et al. [20] have shown that hydrophobic bonds took place in the adsorbed layer of hydrophobically modified copolymers. The aim of our study is to investigate the hydrolysed SMA copolymer adsorption on titanium dioxide for basic pH value, in order to state: if the adsorption of this negative hSMA copolymer is possible onto a negatively charged surface in spite of the electrostatic repulsion between the surface and the copolymer and between the copolymer segments. if the styrene groups of the copolymer play a role in the adsorption mechanism.
2. Materials and methods
2.1. Materials 2.1.1. Titanium dioxide Titanium dioxide P25 was supplied by Degussa France (anatase 80%, rutile 20%). The average primary particle size is given by Degussa as 20nm. Its BET surface area from N2 adsorption was 5292 m2 g − 1 [21]. The point of zero charge (pzc) of this solid support was measured by several authors [21,22] and equals 6.5, corresponding to its isoelectric point (iep) in presence of indifferent electrolytes. Consequently, the titanium dioxide surface charge is positive when pHB 6.5 and negative when pH\ 6.5. 2.1.2. Styrene maleic anhydride copolymer The styrene maleic anhydride copolymer (SMA3000F, which involves three styrene groups) was obtained from Elf Atochem. Its average molecular weight, Mw, was about 9300 with a polydispersity of 3. The molar ratio styrene to maleic anhydride was 3 according to the manufacturer. Its solid form was hydrolysed by sodium
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hydroxide (1N) stoechiometrically. The structural formula of the hSMA copolymer is given in Fig. 1. The hSMA stock solutions were filtered through a filter paper to remove any polystyrene impurities. Before use, the exact concentration of the hSMA stocks solutions was determined by taking a given volume of solution and evaporating off the water at 90°C under reduced pressure. A series of solutions (1250 – 10 000 mg/l) were prepared from stock solutions. These solutions were heated at 80°C during 10 h in well-closed flasks and were used after cooling. This operation allowed us to obtain equilibrated solutions from the stock solutions. It was observed that, if this step was not respected, the solutions obtained from stock solutions contained copolymer aggregates formed by hydrophobic bonding between polymer chains which broke up partially with time and interfered with adsorption.
2.2. Methods 2.2.1. Adsorption isotherms Titanium dioxide suspensions were stirred in aqueous solutions containing a potassium nitrate salt during 24 h. After adjusting their pH, a same volume (50 ml) of a known hSMA solution at the same pH and ionic force was added to stop fitted erlenmeyers. The amount of solid was 5 or 10% w/w. The suspensions were magnetically stirred at 25°C for 6 days to reach equilibrium. Then, the suspensions were centrifuged at 25 000 r.p.m. for 30 min at 25°C. The sampled supernatants were filtered through a 0.45 mm filter to remove any TiO2 traces. The adsorbed amounts were calculated from the difference between initial and equilibrium concentrations. No variation of pH value was observed during the adsorption of hSMA
Fig. 1. Structural formula of hydrolysed styrene maleic anhydride copolymer.
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copolymer on TiO2 particles. All adsorption isotherms were at least duplicated.
2.2.2. Analytical determination of hSMA copolymer The non-adsorbed hSMA concentration in the supernatant was determined by acid-base back titration using a Titroline titrator (Schott). The maximum error on the adsorbed quantities was 90.75 mg/g TiO2 and indicated in the figures if necessary. 2.2.3. Microelectrophoretic measurements For electrokinetic studies by microelectrophoresis, a zetaphoremeter II (Sephy Society) was used. The samples were diluted in a given volume of the initial suspension in the corresponding supernatant. The amount of TiO2 in the electrophoretic samples was about 5× 10 − 5% w/w. The cleanliness of the zetaphoremeter cell was verified by realising a measurement profile. The electrophoretic mobilities of the particles were calculated from the average times of ten measurements, and the polarity of the field was reversed after each measurement. Electrokinetic zeta potential was calculated by the Henry equation using a particle diameter of 0.5 mm which we measured by quasi-elastic light scattering [23]. In all experiments, the solutions and salts were of analytical grade. Water was obtained from a MILLI Q (Millipore Society) and immediately used. This apparatus supplied water free of carbon dioxide with a resistivity of 18.2 MV cm.
3. Results and discussion
3.1. Effects of pH on hSMA copolymer adsorption 3.1.1. Adsorption isotherms In order to maintain a constant ionic strength, KNO3 was added to the solutions. In Fig. 2, adsorption isotherms were performed at pH 9 and 10.2 for two salt concentrations [KNO3]= 10 − 3 M (Fig. 2a) or 10 − 2 M (Fig. 2b). The two figures show that adsorption exists although the surface and the hSMA copolymer are negatively charged
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until a narrow plateau which is less obvious at pH 10.2, because of the small adsorbed quantities and the experimental error. Then, the adsorbed amount rises again. In Fig. 2b, the two isotherms have a typical shape. The adsorbed quantity increases with the hSMA copolymer concentration until the saturation level is reached. The effect of the ionic strength is similar in the two figures: the adsorbed amount increases with increasing salt concentration. As it is largely accepted, increasing the salt concentration induces two effects: a decrease in the electrostatic repulsion between the negatively charged surface and copolymer and a decrease in the electrostatic repulsion between the copolymer segments. This explains the rise of the adsorbed amount observed.
Fig. 2. The adsorbed amounts of hSMA reduced to 5% solid weight versus the equilibrium concentration of hSMA at two salt concentrations, (a) [KNO3] = 10 − 3 M and (b) [KNO3]= 10 − 2 M and two pH values: ", pH 10.2 and , pH 9. The continuous curves in Fig. 2b were calculated from Langmuir adsorption equation [1].
at the studied pH, as observed by others authors on negatively charged polyelectrolyte and similar pH value [12,13]. In the two graphs, the best adsorption and the highest initial slope are obtained at pH 9. Indeed, the surface charge of titanium dioxide becomes less negative with decreasing pH. Thus, the electrostatic repulsion decreases between the surface and the hSMA copolymer and the adsorbed amounts are the more significant at pH 9. In these two figures, the isotherms behavior differs with the salt concentration. In Fig. 2a, the two isotherms show an unusual shape. At the beginning, the adsorption increases with increasing equilibrium hSMA copolymer concentrations
3.1.2. Langmuir isotherms In the literature, several models are used to fit the experimental values of adsorption isotherms. The self consistent field theory gives an achieved model for polyelectrolytes adsorption [5]. Nevertheless, other simple models by Langmuir or Freundlich [24,25] can fit adsorption isotherms of polyelectrolytes. Some authors obtained langmuirian adsorption isotherms with sodium polyacrylate on hydroxyapatite [4] or sodium salt of an acrylic and maleic acid copolymer on clay minerals [8]. These last models may apply to our experiments. But, the Langmuir model is the one retained because the other one applies more particularly to adsorption on microporous solids. Langmuir equation is the following: G=
GmaxKCe 1+ KCe
Where G and Gmax are, respectively, the adsorbed surface amount of hSMA copolymer and the saturation surface amount of hSMA copolymer. Both amounts are reduced to the solid weight. Ce is the equilibrium concentration of hSMA copolymer and K is the equilibrium constant of the adsorption process. Gmax and K values obtained from 1/G versus 1/Ce plot are displayed in Table 1. The two isotherms of Fig. 2b show a good fitting to this model.
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Table 1 K and Gmax values from Langmuir equation Solid weight (%) 5 10
[KNO3] (mol/l)
pH
K (10−3 l/mg)
Gmax (mg/g of TiO2)
10−2 M 10−2 M 10−2 M 3×10−2 M 5×10−2 M
9 10.2 10.2 10.2 10.2
2.9 9 0.4 1.8 90.2 1.6 90.1 3.49 0.2 5.09 0.4
25.19 0.8 20.19 0.9 20.99 0.4 27.1 9 0.3 29.7 9 0.4
Although this approach allows to predict the polymer adsorption at a given concentration, one must bear in mind that the Langmuir model has no basis in theory for the adsorption at the solid/ liquid interface, especially when a polydisperse polymer is used. At [KNO3]= 10 − 3 M, the isotherms show an unusual shape of isotherms. Let us consider the different mechanisms which may be involved in this original behavior: 1. It is known that the shape of an adsorption isotherm could be affected by the polydispersity of the polymer used [5]. In a polydisperse polymer solution, the entropy of mixing decreases with the increasing chain length. For this reason, at equilibrium, the high molecular weight polymers adsorb preferentially to the small ones. Thus, adsorption of a polydisperse polymer onto a surface is a function of the solid percentage in solution. Fleer et al. [5] predicts that for the smaller solid percentage, the saturation level is reached more rapidly, as observed experimentally [26]. 2. The last increase in adsorption observed in Fig. 2a could be due to an increase in the ionic strength coming from the polyelectrolyte itself carrying two ionic counter-ions. In this study, the surface and the polyelectrolyte are charged negatively. An increase in the ionic strength could induce, as was noticed before, a decrease in the electrostatic repulsion between the surface and the hSMA copolymer and between the segments of the copolymer, and at the same time an increase in the adsorbed amounts on titanium dioxide. 3. The shape could be due to two successive adsorption layers as Bijsterbosch [27] observed
for the adsorption of dodecyltrimethylammonium bromide on silica. In order to find which adsorption mechanism is determinant, the polydispersity effect was first tested by performing adsorption isotherms for two solid percentages.
Fig. 3. The adsorbed amounts of hMSA reduced to the solid weight as a function of the equilibrium concentration of hMSA at two salt concentrations, (a) [KNO3]= 10 − 3 M and (b) [KNO3]= 10 − 2 M and two introduced solid weights: ", 5% and×, 10%. Adsortion isotherms are obtained at pH 10.2.
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3.2. Effects of solid percentage on hSMA copolymer adsorption The plotted isotherms in Fig. 3 show the adsorbed amounts of hSMA reduced to the solid weight, as a function of the equilibrium concentration of hSMA. Two solid percentages are studied: 5 and 10%. The chosen pH is 10.2 and two salt concentrations are tested: 10 − 3 M (Fig. 3a) and 10 − 2 M (Fig. 3b) as before. In Fig. 3b, the two isotherms fit together for all the equilibrium hSMA copolymer concentrations. Moreover, in this graph, the two saturation levels begin at an equilibrium concentration of hSMA copolymer greater than in Fig. 3a. We can assume that the mechanisms involved are different at [KNO3]= 10 − 3 M and 10 − 2 M. In Fig. 3a, the isotherms show the same behaviour until Ce =1000 mg/l. At the beginning, the adsorption increases with increasing hSMA copolymer concentration until a narrow plateau, less obvious at pH 10.2 owing to experimental error. Then, the isotherms do not fit together from Ce = 1000 mg/l: the isotherm corresponding to 10% of titanium dioxide introduced is above the one obtained for 5% of solid. A polydispersity effect should give an inversed order of the curves. Consequently, the unusual shape is not due to the polydispersity effect, probably because the molecular mass of the copolymer is too small [28]. In order to verify the second mechanism, i.e. that the effect of ionic strength of the hSMA copolymer is the determining parameter for adsorbed amounts increasing from Ce =1000 mg/l, the adsorbed amount of hSMA copolymer is plotted versus the initial concentration of hSMA copolymer, Cinitial, before adsorption (Fig. 4). In this case, the initial concentration of hSMA copolymer will be equivalent to the total ionic strength of the solution. It can be observed that the two isotherms fit together from Cinitial =1500 mg/l, corresponding to Ce =1000 mg/l in Fig. 3a. Consequently, the rise in the adsorbed amount is correlated to an increase in the total ionic strength which can be attributed to hSMA copolymer. Then, it becomes obvious that if no effect of the polyelectrolyte ionic strength at [KNO3]= 10 − 3 M can be observed for Cinitial smaller than 1500
Fig. 4. The adsorbed amounts of hMSA reduced to the solid weight versus the initial concentration of hMSA at two introduced solid weights: 2, 5% and+ , 10%. Adsorption isotherms were performed at pH 10.2 and at [KNO3]= 10 − 3 M.
mg/l, no effect at [KNO3]= 10 − 2 M should be observed for Cinitial smaller than 15 000 mg/l. This latter concentration is not reached in the adsorption isotherms, in Fig. 3b. The validity of this approach is confirmed since Fig. 3b shows two very similar isotherms for 5 and 10% of solid. The study of the hSMA copolymer adsorption at two solid percentages and two salt concentrations has allowed us to show an ionic strength effect of the hSMA copolymer. But, if it is undoubtedly that the second step increase observed in Fig. 2a and 3a is the result of ionic strength due to the polyelectrolyte, the hSMA copolymer adsorption mechanism is not so clear. In order to go further in examining the charge effects, we are going to investigate first the variation of zeta potential of the covered particle with the hSMA copolymer adsorbed amounts and then the detailed influence of the ionic strength on hSMA copolymer adsorption that has been evidenced in this chapter.
3.3. Zeta potential and adsorbed amounts In Fig. 5, the zeta potential (dotted curve) and the adsorbed amounts are plotted versus the equilibrium hSMA concentration at pH 10.2 and [KNO3]= 10 − 2 M.
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A decrease in zeta potential only takes place for the small values of equilibrium hSMA concentration. From Ce =1500 mg/l, the zeta potential value remains constant while the adsorbed amount increases. From these results, it can be assumed that the first decrease in zeta potential is related to the adsorption of the negatively charged hSMA copolymer. Two mechanisms may explain the constant value of the zeta potential. First, adsorption of the copolymer involves simultaneously a charge regulation process which enhances adsorption without charge variation. Hooggeven et al. [29] working on the adsorption of a positive polyelectrolyte onto a positive surface as titanium dioxide involves proposed this mechanism: in the presence of the positive polyelectrolyte, the dissociation of the anionic surface groups is stimulated Eq. (2) and the protonation of the cationic groups is prevented Eq. (3). TiOH TiO- +H+ TiOH TiOH + H + 2
(2) +
(3)
In our experiments, a reverse trend should be observed-the protonation of the cationic surface groups stimulated and the dissociation of the anionic sites prevented leading to a pH increase. In fact no pH variation was observed experimentally during the hSMA copolymer adsorption. Thus, we can assume that no charge regulation takes place.
Fig. 6. Adsorption isotherms obtained from 10% solid suspensions at pH 10.2. The adsorbed amounts of hMSA reduced to the solid weight as function of the equilibrium concentration of hMSA at different nitrate potassium concentrations, (a) ×, 10 − 3 M; , 3× 10 − 3 M; , 5× 10 − 3 M;2, 10 − 2 M and (b) 2, 10 − 2 M;+, 3 ×10 − 2 M; , 5 ×10 − 2 M. The plotted curves in Fig. 5b were calculated with Langmuir equation [1], K and Gmax values are displayed in Table 1.
The second mechanism is that adsorption of the copolymer increases the charge of the particle but is offset by the decrease in zeta potential induced by the higher electrolyte concentration generated by the polyelectrolyte.
3.4. Effect of ionic strength on hSMA copolymer adsorption
Fig. 5. Adsorption isotherm obtained from 5% solid suspensions at pH 10.2. The zeta potential and the adsorbed amounts of hMSA reduced to the solid weight are plotted versus the hMSA equilibrium concentration at [KNO3] =10 − 2 M.
Fig. 6a and 6b display isotherms performed at pH 10.2 and 10% of introduced solid. In Fig. 6a, the salt concentration increases from 10 − 3 M to 10 − 2 M and in Fig. 6b from 10 − 2 M to 5 ×10 − 2 M.
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In Fig. 6a, the adsorption isotherms obtained from [KNO3]= 10 − 3 M to 5 ×10 − 3 M have an unusual shape: a transition to a plateau appears, whose concentration threshold and length increase with increasing salt concentration. This experimental result can be interpreted, as previously, as a predominant role of the ionic strength of the polyelectrolyte. Indeed, when the initial salt concentration rises, the effect of the ionic strength of the hSMA copolymer appears for higher and higher initial concentrations. Thus, the plateau of the first step becomes larger and larger as observed. The maximum adsorbed amount at the plateau increases with increasing initial salt concentration. Surprisingly, the curve obtained at 10 − 2 M of potassium nitrate does not obey this rule. In fact the saturation adsorbed amount is inferior to the one obtained at [KNO3]= 5× 10 − 3 M. Then, the maximum adsorbed amounts enhances again with increasing salt concentration (Fig. 6b). As far as we know, this behaviour has never been observed in literature. The decrease in the maximum adsorbed amount at the plateau is probably relevant of an electrostatic interaction between the surface and the polymer as predicted theoretically [5]. The surface and the hSMA copolymer are both negatively charged at pH 10.2, but, the titanium dioxide surface is a net negative surface which also carries discrete neutral and positive sites. The electrostatic attraction may exist between the positive sites of the surface and the carboxylic groups of the hSMA copolymer. This assumption is reported by other authors [29]. Indeed, they observed that a positive polyelectrolyte, a quaternized dimethylaminoethyl methacrylate, adsorbed onto titanium dioxide surface at pHB pzc, i.e. on a net positive surface. They proposed that, for oxidic surfaces like titanium dioxide, the net charge is positive far from the surface but heterogeneous closer to the surface due to negative, positive and neutral sites. Adsorption, which is a short-range process, is influenced by this heterogeneity. Thus, in Fig. 5a, the decrease in zeta potential may be explained by an electrostatic adsorption process between the discrete positives sites of titanium dioxide and the negative segments of hSMA copolymer.
When an electrostatic attraction exists between the surface and the hSMA copolymer, one could assume that as the salt concentration increases, on the one hand, the repulsion between copolymer segments decreases, which promotes adsorption. On the other hand, the attraction between the surface and the copolymer diminishes which allows salt ions to compete with the polyelectrolyte to adsorb on the surface. Therefore, the copolymer chains could be desorbed from the surface, especially the smallest ones when the salt concentration increases which could explain the decrease in adsorption at [KNO3]= 10 − 2 M. Nevertheless, this interpretation is not relevant for higher concentrations. Indeed, the decrease does not reach a zero adsorption as observed experimentally [29]. This can be explained by the electric barrier due to the adsorbed layer [11] (chains of the hSMA copolymer can be displaced by the salt ions and have to pass through the electric barrier before to diffuse from the surface to the bulk) and by the existence of nonelectrostatic interactions between the surface and the copolymer. Tjipangandjara et al. [30] proposed that hydrogen bondings exists between polyacrylic acid segments and negatively charged surface at similary basic pH owing to unionised carboxylic acid functions. We assumed that this kind of hydrogen bondings exist either between COOH groups and TiO− surface sites or, more probably, between COO− groups and TiOH surface sites. After the observed decrease at [KNO3]= 10 − 2 M explained before, the adsorption of hSMA copolymer increases dramatically with increasing salt concentration (Fig. 6b). This second increase cannot be related to the electrostatic and nonelectrostatic processes described before. It is likely that a second nonelectrostatic mechanism interferes with the electrostatic and nonelectrostatic interactions between the surface and the hSMA copolymer. The problem is to know if this nonelectrostatic affinity occurs between the surface and the polyelectrolyte or between the polyelectrolyte chains. In most cases, a nonelectrostatic affinity usually takes place between the surface and the polymer [11–13,29]. In the studied system, a nonelectrostatic interaction can also occur between the adsorbed chains of hSMA and
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the chains in solution. This adsorption could result from hydrophobic linkage between the phenyl groups of hSMA copolymers, a mechanism which is encountered in the literature [15 – 17]. It can be concluded that the different processes involved in the adsorption phenomenon of hSMA on titanium dioxide are the following: Electrostatic and nonelectrostatic interactions between the solid and the copolymer. Hydrophobic interaction between the apolar groups of the polyelectrolyte. In conclusion, the competition between these three processes may explain the observed results: the first increase of the adsorption with increasing salt concentrations is mainly due to the electrostatic and nonelectrostatic interaction and the adsorption decrease when the salt concentrations increase may be explained in terms of competition of the polyelectrolyte with the salt ions. But, this decrease does not reach a zero adsorption because an electric barrier due to the adsorbed layer which prevents more desorption of the polyelectrolyte and the nonelectrostatic interactions between the surface and the copolymer. Then the increase in adsorption with increasing salt concentrations observed in Fig. 6b could be essentially due to the nonelectrostatic hydrophobic interaction enhanced by the screening of the repulsion. Briefly, the electrostatic and nonelectrostatic interactions give to the adsorption capacity of the copolymer a rich behaviour dependent on the ionic strength as observed in Fig. 6a and 6b.
4. Conclusions This investigation allows us to answer the two questions initially asked. Adsorption of hSMA copolymer at 9 and 10.2 pH values is possible contrary to what could be expected from the negatively charged surface and polyelectrolyte. Moreover, the adsorption mechanism of the copolymer is highly dependent on the net ionic strength. The shape of adsorption isotherms and the hSMA copolymer adsorption capacity vary with this latter parameter. The hSMA copolymer itself can modify the net ionic strength. A plateau followed by an adsorp-
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tion increase occurs when a monovalent salt is introduced at a concentration smaller than 10 − 2 M. For the hSMA concentrations studied, this effect is no more observed for higher salt concentration. In a concentration range near 10 − 2 M salt concentration, another ionic strength effect is noted. The hSMA adsorbed amounts decrease with increasing salt concentration. This behaviour may be generated by an electrostatic interaction between the surface and the copolymer. At higher salt concentrations, the hSMA adsorbed amounts increase. This last result can only be explained by a second nonelectrostatic interaction dependent on the ionic strength, since the adsorbed amounts increase with increasing salt concentration. This nonelectrostatic interaction is assumed to take place by hydrophobic bonds between the styrene groups of the preadsorbed hSMA chains and these of the free hSMA chains in solution. As far as we know, this investigation is the first one concerning the adsorption of this copolymer. It presents a rich and non continuous adsorption behaviour as a function of the ionic strength of the solution. This behaviour could be interesting for its industrial use. Acknowledgements This work was carried out with the support of Elf Atochem. We are indebted to Degussa Society for providing us with titanium dioxide and to Dr Halim Mesbah for his help in the model calculations. References [1] W. Norde, J. Lyklema, J. Colloid Interface Sci. 66 (1978) 266 – 284. [2] G. Girod, J.M. Lamarche, A. Foissy, J. Colloid Interface Sci. 121 (1988) 265 – 272. [3] L. Jarstrom, P. Stenius, Colloids Surfaces 50 (1990) 47– 73. [4] D.N. Misra, J. Colloid Interface Sci. 181 (1996) 289–296. [5] G.J. Fleer, M.A. Cohen Stuart, J.M.H.M. Scheutjens, T. Cosgrove, B. Vincent, Polymers at Interfaces, Chapman & Hall, London, 1993, p. 343.
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[19] J.F. Argillier, A. Audibert, J. Lecourtier, M. Moan, L. Rousseau, Colloids Surfaces 113 (1996) 247 – 257. [20] E. Volpert, J. Selb, F. Candau, N. Green, J.F. Argillier, A. Audibert, Langmuir 14 (1998) 1870 – 1879. [21] A. Foissy, A.M. Pandou, J.M. Lamarche, Colloids Surfaces 5 (1982) 363 – 368. [22] M.J.G. Janssen, H.N. Stein, J. Colloid Interface Sci. 109 (1986) 508 – 515. [23] M. Kerker, Scattering of light and other electromagnetic radiation, Academic Press, London, 1969. [24] A.W. Adamson, Physical Chemistry of Surfaces, 5th edition, Wiley, New York, 1990, p. 421. [25] W. Stumm, Chemistry of the Solid – Water Interface, Wiley Interscience, New York, 1992, p. 87. [26] A. Hild, J.M. Sequaris, H.D. Narres, M. Schwuger, Colloids Surfaces 123 – 124 (1997) 515 – 522. [27] B.H. Bijsterbosch, J. Colloid Interface Sci. 47 (1974) 186 – 198. [28] B. Cabot, A. Deratani, A. Foissy, Colloids Surfaces 139 (1998) 287 – 297. [29] N.G. Hoogeven, M.A. Cohen Stuart, G.J. Fleer, J. Colloid Interface Sci. 182 (1996) 133 – 145. [30] K.F. Tjipangandjara, P. Somansundaran, Advanced Powder Technol. 3 (1992) 119 – 127.
Copolymer polyelectrolyte adsorption onto titanium dioxide Annabelle Pina a, Evelyne Nakache a,*, Bruno Feret b, Pierre Depraetere c a
Laboratoire de Chimie Mole´culaire et Thio-organique, groupe ‘Polyme`res et Interfaces’, UMR CNRS 6507, 6, B6d Mare´chal Juin, F 14050 Caen Cedex, France b Elf-Atochem, Centre d’Applications de Le6allois, 95, rue Danton, F 92300 Le6allois Perret, France c Laboratoire de Pharmacie Gale´nique, B6d Henry Becquerel, He´rou6ille-Saint-Clair, F 14032 Caen Cedex, France Received 12 June 1998; accepted 24 March 1999
Abstract We study the adsorption of a polyelectrolyte, which is a hydrolysed styrene maleic anhydride (hSMA) copolymer, on titanium dioxide. When dissolved in aqueous solutions, at basic pH, this polyelectrolyte presents an apolar part, styrene, and a polar part, two carboxylic functions. Our experiments show that the adsorption isotherms are dependent on the pH and on the total ionic strength of the solution which can be varied both by increasing the concentration of the polyelectrolyte and by addition of an external monovalent salt-potassium nitrate. The adsorption isotherms display a plateau followed by an increasing adsorption at small salt concentrations. A detail study of the ionic strength effect show that this increasing adsorption cannot be attributed to polydispersity nor to successive adsorption layers. At high salt concentrations, the adsorption model is Langmuirian. The observed adsorption may be explained by interactions taking place: between the substrate and the copolymer segments. inside the adsorbed layer. We proposed that the interactions near the solid substrate are electrostatic between the discrete positive sites of the surface and the hSMA copolymers and nonelectrostatic such as hydrogen bonds. The interactions inside the adsorbed layer are nonelectrostatic corresponding to hydrophobic bonds between the preadsorbed hSMA copolymers and the free hSMA copolymers in solution via their phenyl groups. The interactions near the solid surface are predominant for small ionic strengths, whereas the interactions inside the adsorbed layer are predominant for high ionic strength as the repulsion between the hSMA copolymer segments is quantitatively diminished. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Polyelectrolyte; Copolymer; Styrene; Maleic anhydride; Ionic strength
1. Introduction Polyelectrolyte adsorption is a significant phenomenon at solid/liquid interfaces, which is * Corresponding author. Tel.: +33-231-452842; fax: + 33231-452877. E-mail address: [email protected] (E. Nakache)
widely studied owing to its relevance in many industrial applications: painting, wastewater treatment, flotation separation… Adsorption of polyelectrolytes have been experimentally [1–4] and theoretically [5] studied during the last few years. Most of the studied polyelectrolyte are homopolymers [2–4] which are strongly or weakly
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dissociated. In the first case, the charge of the polymer does not depend on pH whereas, in the second case, the polymer charge may depend on it. Moreover, the solid substrate charge can be constant or determined by pH or ions in solution. In this study, an hydrolysed styrene maleic anhydride (hSMA) copolymer was chosen. The styrene part is non-polar, while the hydrolysed maleic anhydride monomer gives two carboxylic functions to the polyelectrolyte. The adsorption of hSMA is performed on titanium dioxide at basic pH. Consequently, the copolymer and the substrate are pH dependent and negatively charged. Thus, the studied system is relatively complex. In the literature, a few charged copolymers have been investigated in terms of adsorption capacity [6–8,18] on pH dependent surfaces while a wellknown weak polyelectrolyte, the polyacrylic acid (PAA), has been studied on different solids [9,10] and especially on titanium dioxide [11 –14]. On this substrate, the literature gives some principal tendencies for PAA adsorption. For instance, at a given pH value, Chibowski [13] and Foissy et al. [12] noted that PAA adsorption increased as the molecular weight of the polymer rose as predicted theoretically for neutral polymers in poor solvent [5]. Nevertheless, there are some discrepancies between the shapes of the different adsorption isotherms. Chibowski [13] and Heijman et al. [14] obtained isotherms without saturation level contrary to Foissy et al. [12] and Gebhardt et al. [11]. These authors discovered that the adsorption of PAA increased as the solution pH decreased. They assumed [11 – 13] that the adsorption observed at acidic pH was due to the positive charges on titanium dioxide surface. Therefore, adsorption took place via electrostatic or hydrogen bonding. Nevertheless, PAA adsorption is observed [12,13] or not [11] at point of zero charge or at a more basic pH value. In this case, Foissy et al. [12] proposed that nonelectrostatic interaction exists between the surface and the PAA, thus allowing adsorption. Whereas Chibowski [13] supposed an interaction between the surface and the polyelectrolyte via hydrogen bonding and specific adsorption of the carboxylic groups. It is noteworthy that, for another system, Cosgrove et al. [18] found also an adsorption
between similarly charge polymer chains and surfaces. The studied hSMA copolymer is a commercial product used as a dispersing agent. Aqueous solutions of this kind of copolymer have been studied [15–17] and it has been shown that hydrophobic linkage via phenyl groups took place inside a copolymer chain. The last few years, Argillier et al. [19] and Volpert et al. [20] have shown that hydrophobic bonds took place in the adsorbed layer of hydrophobically modified copolymers. The aim of our study is to investigate the hydrolysed SMA copolymer adsorption on titanium dioxide for basic pH value, in order to state: if the adsorption of this negative hSMA copolymer is possible onto a negatively charged surface in spite of the electrostatic repulsion between the surface and the copolymer and between the copolymer segments. if the styrene groups of the copolymer play a role in the adsorption mechanism.
2. Materials and methods
2.1. Materials 2.1.1. Titanium dioxide Titanium dioxide P25 was supplied by Degussa France (anatase 80%, rutile 20%). The average primary particle size is given by Degussa as 20nm. Its BET surface area from N2 adsorption was 5292 m2 g − 1 [21]. The point of zero charge (pzc) of this solid support was measured by several authors [21,22] and equals 6.5, corresponding to its isoelectric point (iep) in presence of indifferent electrolytes. Consequently, the titanium dioxide surface charge is positive when pHB 6.5 and negative when pH\ 6.5. 2.1.2. Styrene maleic anhydride copolymer The styrene maleic anhydride copolymer (SMA3000F, which involves three styrene groups) was obtained from Elf Atochem. Its average molecular weight, Mw, was about 9300 with a polydispersity of 3. The molar ratio styrene to maleic anhydride was 3 according to the manufacturer. Its solid form was hydrolysed by sodium
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hydroxide (1N) stoechiometrically. The structural formula of the hSMA copolymer is given in Fig. 1. The hSMA stock solutions were filtered through a filter paper to remove any polystyrene impurities. Before use, the exact concentration of the hSMA stocks solutions was determined by taking a given volume of solution and evaporating off the water at 90°C under reduced pressure. A series of solutions (1250 – 10 000 mg/l) were prepared from stock solutions. These solutions were heated at 80°C during 10 h in well-closed flasks and were used after cooling. This operation allowed us to obtain equilibrated solutions from the stock solutions. It was observed that, if this step was not respected, the solutions obtained from stock solutions contained copolymer aggregates formed by hydrophobic bonding between polymer chains which broke up partially with time and interfered with adsorption.
2.2. Methods 2.2.1. Adsorption isotherms Titanium dioxide suspensions were stirred in aqueous solutions containing a potassium nitrate salt during 24 h. After adjusting their pH, a same volume (50 ml) of a known hSMA solution at the same pH and ionic force was added to stop fitted erlenmeyers. The amount of solid was 5 or 10% w/w. The suspensions were magnetically stirred at 25°C for 6 days to reach equilibrium. Then, the suspensions were centrifuged at 25 000 r.p.m. for 30 min at 25°C. The sampled supernatants were filtered through a 0.45 mm filter to remove any TiO2 traces. The adsorbed amounts were calculated from the difference between initial and equilibrium concentrations. No variation of pH value was observed during the adsorption of hSMA
Fig. 1. Structural formula of hydrolysed styrene maleic anhydride copolymer.
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copolymer on TiO2 particles. All adsorption isotherms were at least duplicated.
2.2.2. Analytical determination of hSMA copolymer The non-adsorbed hSMA concentration in the supernatant was determined by acid-base back titration using a Titroline titrator (Schott). The maximum error on the adsorbed quantities was 90.75 mg/g TiO2 and indicated in the figures if necessary. 2.2.3. Microelectrophoretic measurements For electrokinetic studies by microelectrophoresis, a zetaphoremeter II (Sephy Society) was used. The samples were diluted in a given volume of the initial suspension in the corresponding supernatant. The amount of TiO2 in the electrophoretic samples was about 5× 10 − 5% w/w. The cleanliness of the zetaphoremeter cell was verified by realising a measurement profile. The electrophoretic mobilities of the particles were calculated from the average times of ten measurements, and the polarity of the field was reversed after each measurement. Electrokinetic zeta potential was calculated by the Henry equation using a particle diameter of 0.5 mm which we measured by quasi-elastic light scattering [23]. In all experiments, the solutions and salts were of analytical grade. Water was obtained from a MILLI Q (Millipore Society) and immediately used. This apparatus supplied water free of carbon dioxide with a resistivity of 18.2 MV cm.
3. Results and discussion
3.1. Effects of pH on hSMA copolymer adsorption 3.1.1. Adsorption isotherms In order to maintain a constant ionic strength, KNO3 was added to the solutions. In Fig. 2, adsorption isotherms were performed at pH 9 and 10.2 for two salt concentrations [KNO3]= 10 − 3 M (Fig. 2a) or 10 − 2 M (Fig. 2b). The two figures show that adsorption exists although the surface and the hSMA copolymer are negatively charged
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until a narrow plateau which is less obvious at pH 10.2, because of the small adsorbed quantities and the experimental error. Then, the adsorbed amount rises again. In Fig. 2b, the two isotherms have a typical shape. The adsorbed quantity increases with the hSMA copolymer concentration until the saturation level is reached. The effect of the ionic strength is similar in the two figures: the adsorbed amount increases with increasing salt concentration. As it is largely accepted, increasing the salt concentration induces two effects: a decrease in the electrostatic repulsion between the negatively charged surface and copolymer and a decrease in the electrostatic repulsion between the copolymer segments. This explains the rise of the adsorbed amount observed.
Fig. 2. The adsorbed amounts of hSMA reduced to 5% solid weight versus the equilibrium concentration of hSMA at two salt concentrations, (a) [KNO3] = 10 − 3 M and (b) [KNO3]= 10 − 2 M and two pH values: ", pH 10.2 and , pH 9. The continuous curves in Fig. 2b were calculated from Langmuir adsorption equation [1].
at the studied pH, as observed by others authors on negatively charged polyelectrolyte and similar pH value [12,13]. In the two graphs, the best adsorption and the highest initial slope are obtained at pH 9. Indeed, the surface charge of titanium dioxide becomes less negative with decreasing pH. Thus, the electrostatic repulsion decreases between the surface and the hSMA copolymer and the adsorbed amounts are the more significant at pH 9. In these two figures, the isotherms behavior differs with the salt concentration. In Fig. 2a, the two isotherms show an unusual shape. At the beginning, the adsorption increases with increasing equilibrium hSMA copolymer concentrations
3.1.2. Langmuir isotherms In the literature, several models are used to fit the experimental values of adsorption isotherms. The self consistent field theory gives an achieved model for polyelectrolytes adsorption [5]. Nevertheless, other simple models by Langmuir or Freundlich [24,25] can fit adsorption isotherms of polyelectrolytes. Some authors obtained langmuirian adsorption isotherms with sodium polyacrylate on hydroxyapatite [4] or sodium salt of an acrylic and maleic acid copolymer on clay minerals [8]. These last models may apply to our experiments. But, the Langmuir model is the one retained because the other one applies more particularly to adsorption on microporous solids. Langmuir equation is the following: G=
GmaxKCe 1+ KCe
Where G and Gmax are, respectively, the adsorbed surface amount of hSMA copolymer and the saturation surface amount of hSMA copolymer. Both amounts are reduced to the solid weight. Ce is the equilibrium concentration of hSMA copolymer and K is the equilibrium constant of the adsorption process. Gmax and K values obtained from 1/G versus 1/Ce plot are displayed in Table 1. The two isotherms of Fig. 2b show a good fitting to this model.
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Table 1 K and Gmax values from Langmuir equation Solid weight (%) 5 10
[KNO3] (mol/l)
pH
K (10−3 l/mg)
Gmax (mg/g of TiO2)
10−2 M 10−2 M 10−2 M 3×10−2 M 5×10−2 M
9 10.2 10.2 10.2 10.2
2.9 9 0.4 1.8 90.2 1.6 90.1 3.49 0.2 5.09 0.4
25.19 0.8 20.19 0.9 20.99 0.4 27.1 9 0.3 29.7 9 0.4
Although this approach allows to predict the polymer adsorption at a given concentration, one must bear in mind that the Langmuir model has no basis in theory for the adsorption at the solid/ liquid interface, especially when a polydisperse polymer is used. At [KNO3]= 10 − 3 M, the isotherms show an unusual shape of isotherms. Let us consider the different mechanisms which may be involved in this original behavior: 1. It is known that the shape of an adsorption isotherm could be affected by the polydispersity of the polymer used [5]. In a polydisperse polymer solution, the entropy of mixing decreases with the increasing chain length. For this reason, at equilibrium, the high molecular weight polymers adsorb preferentially to the small ones. Thus, adsorption of a polydisperse polymer onto a surface is a function of the solid percentage in solution. Fleer et al. [5] predicts that for the smaller solid percentage, the saturation level is reached more rapidly, as observed experimentally [26]. 2. The last increase in adsorption observed in Fig. 2a could be due to an increase in the ionic strength coming from the polyelectrolyte itself carrying two ionic counter-ions. In this study, the surface and the polyelectrolyte are charged negatively. An increase in the ionic strength could induce, as was noticed before, a decrease in the electrostatic repulsion between the surface and the hSMA copolymer and between the segments of the copolymer, and at the same time an increase in the adsorbed amounts on titanium dioxide. 3. The shape could be due to two successive adsorption layers as Bijsterbosch [27] observed
for the adsorption of dodecyltrimethylammonium bromide on silica. In order to find which adsorption mechanism is determinant, the polydispersity effect was first tested by performing adsorption isotherms for two solid percentages.
Fig. 3. The adsorbed amounts of hMSA reduced to the solid weight as a function of the equilibrium concentration of hMSA at two salt concentrations, (a) [KNO3]= 10 − 3 M and (b) [KNO3]= 10 − 2 M and two introduced solid weights: ", 5% and×, 10%. Adsortion isotherms are obtained at pH 10.2.
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3.2. Effects of solid percentage on hSMA copolymer adsorption The plotted isotherms in Fig. 3 show the adsorbed amounts of hSMA reduced to the solid weight, as a function of the equilibrium concentration of hSMA. Two solid percentages are studied: 5 and 10%. The chosen pH is 10.2 and two salt concentrations are tested: 10 − 3 M (Fig. 3a) and 10 − 2 M (Fig. 3b) as before. In Fig. 3b, the two isotherms fit together for all the equilibrium hSMA copolymer concentrations. Moreover, in this graph, the two saturation levels begin at an equilibrium concentration of hSMA copolymer greater than in Fig. 3a. We can assume that the mechanisms involved are different at [KNO3]= 10 − 3 M and 10 − 2 M. In Fig. 3a, the isotherms show the same behaviour until Ce =1000 mg/l. At the beginning, the adsorption increases with increasing hSMA copolymer concentration until a narrow plateau, less obvious at pH 10.2 owing to experimental error. Then, the isotherms do not fit together from Ce = 1000 mg/l: the isotherm corresponding to 10% of titanium dioxide introduced is above the one obtained for 5% of solid. A polydispersity effect should give an inversed order of the curves. Consequently, the unusual shape is not due to the polydispersity effect, probably because the molecular mass of the copolymer is too small [28]. In order to verify the second mechanism, i.e. that the effect of ionic strength of the hSMA copolymer is the determining parameter for adsorbed amounts increasing from Ce =1000 mg/l, the adsorbed amount of hSMA copolymer is plotted versus the initial concentration of hSMA copolymer, Cinitial, before adsorption (Fig. 4). In this case, the initial concentration of hSMA copolymer will be equivalent to the total ionic strength of the solution. It can be observed that the two isotherms fit together from Cinitial =1500 mg/l, corresponding to Ce =1000 mg/l in Fig. 3a. Consequently, the rise in the adsorbed amount is correlated to an increase in the total ionic strength which can be attributed to hSMA copolymer. Then, it becomes obvious that if no effect of the polyelectrolyte ionic strength at [KNO3]= 10 − 3 M can be observed for Cinitial smaller than 1500
Fig. 4. The adsorbed amounts of hMSA reduced to the solid weight versus the initial concentration of hMSA at two introduced solid weights: 2, 5% and+ , 10%. Adsorption isotherms were performed at pH 10.2 and at [KNO3]= 10 − 3 M.
mg/l, no effect at [KNO3]= 10 − 2 M should be observed for Cinitial smaller than 15 000 mg/l. This latter concentration is not reached in the adsorption isotherms, in Fig. 3b. The validity of this approach is confirmed since Fig. 3b shows two very similar isotherms for 5 and 10% of solid. The study of the hSMA copolymer adsorption at two solid percentages and two salt concentrations has allowed us to show an ionic strength effect of the hSMA copolymer. But, if it is undoubtedly that the second step increase observed in Fig. 2a and 3a is the result of ionic strength due to the polyelectrolyte, the hSMA copolymer adsorption mechanism is not so clear. In order to go further in examining the charge effects, we are going to investigate first the variation of zeta potential of the covered particle with the hSMA copolymer adsorbed amounts and then the detailed influence of the ionic strength on hSMA copolymer adsorption that has been evidenced in this chapter.
3.3. Zeta potential and adsorbed amounts In Fig. 5, the zeta potential (dotted curve) and the adsorbed amounts are plotted versus the equilibrium hSMA concentration at pH 10.2 and [KNO3]= 10 − 2 M.
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A decrease in zeta potential only takes place for the small values of equilibrium hSMA concentration. From Ce =1500 mg/l, the zeta potential value remains constant while the adsorbed amount increases. From these results, it can be assumed that the first decrease in zeta potential is related to the adsorption of the negatively charged hSMA copolymer. Two mechanisms may explain the constant value of the zeta potential. First, adsorption of the copolymer involves simultaneously a charge regulation process which enhances adsorption without charge variation. Hooggeven et al. [29] working on the adsorption of a positive polyelectrolyte onto a positive surface as titanium dioxide involves proposed this mechanism: in the presence of the positive polyelectrolyte, the dissociation of the anionic surface groups is stimulated Eq. (2) and the protonation of the cationic groups is prevented Eq. (3). TiOH TiO- +H+ TiOH TiOH + H + 2
(2) +
(3)
In our experiments, a reverse trend should be observed-the protonation of the cationic surface groups stimulated and the dissociation of the anionic sites prevented leading to a pH increase. In fact no pH variation was observed experimentally during the hSMA copolymer adsorption. Thus, we can assume that no charge regulation takes place.
Fig. 6. Adsorption isotherms obtained from 10% solid suspensions at pH 10.2. The adsorbed amounts of hMSA reduced to the solid weight as function of the equilibrium concentration of hMSA at different nitrate potassium concentrations, (a) ×, 10 − 3 M; , 3× 10 − 3 M; , 5× 10 − 3 M;2, 10 − 2 M and (b) 2, 10 − 2 M;+, 3 ×10 − 2 M; , 5 ×10 − 2 M. The plotted curves in Fig. 5b were calculated with Langmuir equation [1], K and Gmax values are displayed in Table 1.
The second mechanism is that adsorption of the copolymer increases the charge of the particle but is offset by the decrease in zeta potential induced by the higher electrolyte concentration generated by the polyelectrolyte.
3.4. Effect of ionic strength on hSMA copolymer adsorption
Fig. 5. Adsorption isotherm obtained from 5% solid suspensions at pH 10.2. The zeta potential and the adsorbed amounts of hMSA reduced to the solid weight are plotted versus the hMSA equilibrium concentration at [KNO3] =10 − 2 M.
Fig. 6a and 6b display isotherms performed at pH 10.2 and 10% of introduced solid. In Fig. 6a, the salt concentration increases from 10 − 3 M to 10 − 2 M and in Fig. 6b from 10 − 2 M to 5 ×10 − 2 M.
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In Fig. 6a, the adsorption isotherms obtained from [KNO3]= 10 − 3 M to 5 ×10 − 3 M have an unusual shape: a transition to a plateau appears, whose concentration threshold and length increase with increasing salt concentration. This experimental result can be interpreted, as previously, as a predominant role of the ionic strength of the polyelectrolyte. Indeed, when the initial salt concentration rises, the effect of the ionic strength of the hSMA copolymer appears for higher and higher initial concentrations. Thus, the plateau of the first step becomes larger and larger as observed. The maximum adsorbed amount at the plateau increases with increasing initial salt concentration. Surprisingly, the curve obtained at 10 − 2 M of potassium nitrate does not obey this rule. In fact the saturation adsorbed amount is inferior to the one obtained at [KNO3]= 5× 10 − 3 M. Then, the maximum adsorbed amounts enhances again with increasing salt concentration (Fig. 6b). As far as we know, this behaviour has never been observed in literature. The decrease in the maximum adsorbed amount at the plateau is probably relevant of an electrostatic interaction between the surface and the polymer as predicted theoretically [5]. The surface and the hSMA copolymer are both negatively charged at pH 10.2, but, the titanium dioxide surface is a net negative surface which also carries discrete neutral and positive sites. The electrostatic attraction may exist between the positive sites of the surface and the carboxylic groups of the hSMA copolymer. This assumption is reported by other authors [29]. Indeed, they observed that a positive polyelectrolyte, a quaternized dimethylaminoethyl methacrylate, adsorbed onto titanium dioxide surface at pHB pzc, i.e. on a net positive surface. They proposed that, for oxidic surfaces like titanium dioxide, the net charge is positive far from the surface but heterogeneous closer to the surface due to negative, positive and neutral sites. Adsorption, which is a short-range process, is influenced by this heterogeneity. Thus, in Fig. 5a, the decrease in zeta potential may be explained by an electrostatic adsorption process between the discrete positives sites of titanium dioxide and the negative segments of hSMA copolymer.
When an electrostatic attraction exists between the surface and the hSMA copolymer, one could assume that as the salt concentration increases, on the one hand, the repulsion between copolymer segments decreases, which promotes adsorption. On the other hand, the attraction between the surface and the copolymer diminishes which allows salt ions to compete with the polyelectrolyte to adsorb on the surface. Therefore, the copolymer chains could be desorbed from the surface, especially the smallest ones when the salt concentration increases which could explain the decrease in adsorption at [KNO3]= 10 − 2 M. Nevertheless, this interpretation is not relevant for higher concentrations. Indeed, the decrease does not reach a zero adsorption as observed experimentally [29]. This can be explained by the electric barrier due to the adsorbed layer [11] (chains of the hSMA copolymer can be displaced by the salt ions and have to pass through the electric barrier before to diffuse from the surface to the bulk) and by the existence of nonelectrostatic interactions between the surface and the copolymer. Tjipangandjara et al. [30] proposed that hydrogen bondings exists between polyacrylic acid segments and negatively charged surface at similary basic pH owing to unionised carboxylic acid functions. We assumed that this kind of hydrogen bondings exist either between COOH groups and TiO− surface sites or, more probably, between COO− groups and TiOH surface sites. After the observed decrease at [KNO3]= 10 − 2 M explained before, the adsorption of hSMA copolymer increases dramatically with increasing salt concentration (Fig. 6b). This second increase cannot be related to the electrostatic and nonelectrostatic processes described before. It is likely that a second nonelectrostatic mechanism interferes with the electrostatic and nonelectrostatic interactions between the surface and the hSMA copolymer. The problem is to know if this nonelectrostatic affinity occurs between the surface and the polyelectrolyte or between the polyelectrolyte chains. In most cases, a nonelectrostatic affinity usually takes place between the surface and the polymer [11–13,29]. In the studied system, a nonelectrostatic interaction can also occur between the adsorbed chains of hSMA and
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the chains in solution. This adsorption could result from hydrophobic linkage between the phenyl groups of hSMA copolymers, a mechanism which is encountered in the literature [15 – 17]. It can be concluded that the different processes involved in the adsorption phenomenon of hSMA on titanium dioxide are the following: Electrostatic and nonelectrostatic interactions between the solid and the copolymer. Hydrophobic interaction between the apolar groups of the polyelectrolyte. In conclusion, the competition between these three processes may explain the observed results: the first increase of the adsorption with increasing salt concentrations is mainly due to the electrostatic and nonelectrostatic interaction and the adsorption decrease when the salt concentrations increase may be explained in terms of competition of the polyelectrolyte with the salt ions. But, this decrease does not reach a zero adsorption because an electric barrier due to the adsorbed layer which prevents more desorption of the polyelectrolyte and the nonelectrostatic interactions between the surface and the copolymer. Then the increase in adsorption with increasing salt concentrations observed in Fig. 6b could be essentially due to the nonelectrostatic hydrophobic interaction enhanced by the screening of the repulsion. Briefly, the electrostatic and nonelectrostatic interactions give to the adsorption capacity of the copolymer a rich behaviour dependent on the ionic strength as observed in Fig. 6a and 6b.
4. Conclusions This investigation allows us to answer the two questions initially asked. Adsorption of hSMA copolymer at 9 and 10.2 pH values is possible contrary to what could be expected from the negatively charged surface and polyelectrolyte. Moreover, the adsorption mechanism of the copolymer is highly dependent on the net ionic strength. The shape of adsorption isotherms and the hSMA copolymer adsorption capacity vary with this latter parameter. The hSMA copolymer itself can modify the net ionic strength. A plateau followed by an adsorp-
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tion increase occurs when a monovalent salt is introduced at a concentration smaller than 10 − 2 M. For the hSMA concentrations studied, this effect is no more observed for higher salt concentration. In a concentration range near 10 − 2 M salt concentration, another ionic strength effect is noted. The hSMA adsorbed amounts decrease with increasing salt concentration. This behaviour may be generated by an electrostatic interaction between the surface and the copolymer. At higher salt concentrations, the hSMA adsorbed amounts increase. This last result can only be explained by a second nonelectrostatic interaction dependent on the ionic strength, since the adsorbed amounts increase with increasing salt concentration. This nonelectrostatic interaction is assumed to take place by hydrophobic bonds between the styrene groups of the preadsorbed hSMA chains and these of the free hSMA chains in solution. As far as we know, this investigation is the first one concerning the adsorption of this copolymer. It presents a rich and non continuous adsorption behaviour as a function of the ionic strength of the solution. This behaviour could be interesting for its industrial use. Acknowledgements This work was carried out with the support of Elf Atochem. We are indebted to Degussa Society for providing us with titanium dioxide and to Dr Halim Mesbah for his help in the model calculations. References [1] W. Norde, J. Lyklema, J. Colloid Interface Sci. 66 (1978) 266 – 284. [2] G. Girod, J.M. Lamarche, A. Foissy, J. Colloid Interface Sci. 121 (1988) 265 – 272. [3] L. Jarstrom, P. Stenius, Colloids Surfaces 50 (1990) 47– 73. [4] D.N. Misra, J. Colloid Interface Sci. 181 (1996) 289–296. [5] G.J. Fleer, M.A. Cohen Stuart, J.M.H.M. Scheutjens, T. Cosgrove, B. Vincent, Polymers at Interfaces, Chapman & Hall, London, 1993, p. 343.
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