polyacrylic acid solution interface – Temperature impact

Microporous and Mesoporous Materials 175 (2013) 92–98

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Effect of the solid pore size on the structure of polymer film at the metal oxide/polyacrylic acid solution interface – Temperature impact Małgorzata Wis´niewska a,⇑, Agnieszka Nosal-Wiercin´ska b, Iwona Da˛browska a, Katarzyna Szewczuk-Karpisz a a b

Department of Radiochemistry and Colloid Chemistry, Faculty of Chemistry, Maria Curie Sklodowska University, Maria Curie Sklodowska Sq. 3, 20-031 Lublin, Poland Department of Analytical Chemistry and Instrumental Analysis, Faculty of Chemistry, Maria Curie Sklodowska University, Maria Curie Sklodowska Sq. 3, 20-031 Lublin, Poland

a r t i c l e

i n f o

Article history: Received 18 November 2012 Received in revised form 5 February 2013 Accepted 9 March 2013 Available online 29 March 2013 Keywords: Metal oxide pore size Structure of polymer film Polyacrylic acid functional groups Macromolecule conformation Polymer adsorption in solid pores

a b s t r a c t The goal of the study was to determine the effects of adsorbent type and its pore size on the polyacrylic acid (PAA) adsorption mechanism on the solid surface in the temperature range 15–40 °C. The following metal oxides were applied: aluminum (III) oxide, silicon (IV) oxide, zirconium (IV) oxide and controlled porosity glass (CPG). PAA adsorption characteristics at the metal oxide-aqueous solution interface was proposed based on the data obtained from the techniques: spectrophotometry, viscometry and potentiometric titration. They allowed the determination of the following parameters: the amount of adsorbed polymer, the linear dimensions of polymer chains in the solution, the thickness of the polymer adsorption layer, as well as the surface charge density of the adsorbent in the absence and presence of PAA. It was shown, that the adsorption of polyacrylic acid on the metal oxide surface proceeds through both electrostatic and hydrogen bridge interactions. Moreover, the temperature increase leads to the increase of macromolecule size, which influences the polymer adsorbed amount and the structure of its adsorption layer. It results in the increase of polymer adsorption with the temperature rise in the case of alumina, silica and CPG. For zirconia the adsorption minimum in the temperature range 25–30 °C was obtained. Such behaviour of the investigated systems is caused by various pore size of the applied metal oxides. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction The basic research concerning the adsorption process of various substances (simple ions, organic compounds, natural and synthetic polymers) at the solid–liquid interface is very important owing to significant practical application of such systems in many fields of human activity [1–4]. Ability to influence the conformation of macromolecules by temperature change is closely related to the stability of colloidal systems containing a polymer. Stabilization-flocculation properties of polymers are used in many industrial and environmental processes, i.e. cosmetics, pharmaceuticals, paint, paper production and food processing, as well as in drinking water purification and wastewater treatment [5–9]. Achieving the desired effect of stability of the suspension or emulsion in the process of its production, requires the determination of the adsorption mechanism of a macromolecular compound at the solid–liquid interface. Therefore, the basic research is very important because it will enable prediction of the behaviour of colloidal suspension in the presence of the polymer.

⇑ Corresponding author. Tel.: +48 81 5375622; fax: +48 81 5332811. E-mail address: [email protected] (M. Wis´niewska). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.03.032

One of the factors that affects the conformation of the polymer chains, both in the bulk solution and adsorbed on the solid surface, is temperature. Temperature changes cause changes in linear dimensions of macromolecules, which is associated with the degree of their development. Linear dimensions of macromolecules in the solution are characterized by the following parameters: the expansion coefficient (aexp), the root mean square chain endto-end distance ðr 2 Þ1=2 and the hydrodynamic radius of polymer coil (Rh). These parameters are explained in detail in the experimental part. Temperature effect on the macromolecule conformation results from the changes of the interaction between the polymer segments and the solvent molecules [10]. For each polymer – solvent system there exists such value of temperature at which the polymer chains form the so-called statistical coils. They are characterized by the Gaussian distribution of polymer segments as a function of distance from their mass centre. Moreover, their size is proportional to ðr 2 Þ1=2 . The temperature at which this phenomenon occurs is called h temperature (theta temperature). The polymer–solvent system at h temperature is in the so-called undisturbed state and the solvent in this state is referred to as the theta solvent or the ideal solvent. In this case, the polymer–solvent interaction is exactly balanced by the long-range one between the polymer segments. For this reason, the expansion coefficient is equal to one. Furthermore, in the

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undisturbed state the osmotic pressure of the polymer solution satisfies the van’t Hoff law for the ideal solution. This is due to the fact that the second virial coefficient of osmotic pressure, connected with the interaction potential of polymer segments in the solution, assumes the value zero (B2 = 0) [11]. In most cases, the temperature rise improves the quality of the solvent. This is due to the fact that the interactions of polymer segments with the solvent molecules are more favourable than the segment-segment and solvent–solvent ones. This is manifested by the development of polymer coils and increase of macromolecule linear dimensions in the solution. The expansion coefficient, defining the degree of this development under given conditions in relation to that in the undisturbed state at the theta temperature, assumes the value greater than 1 (aexp > 1). The solvent in which the polymer exhibits such behaviour is referred as a good solvent. After lowering the temperature below the theta temperature, the polymer–polymer and solvent–solvent interactions dominate over the polymer– solvent ones. As a consequence, polymer coils shrink, thereby reducing their size in comparison to the undisturbed size (aexp < 1). This is manifested by significant deterioration of the polymer solubility in the solvent, which is called a bad solvent. Therefore, it becomes evident that by changing temperature one can effectively influence the behaviour of the colloidal suspension, without changing any other parameters in the colloidal system. For this reason, the temperature is a very important factor for stability of solid particles dispersed in the polymer solution. Although the world literature reports are scarce concerning this problem [12]. Thus, the main goal of this paper is determination of temperature impact (15–40 °C) on the polyacrylic acid (PAA) conformation in the metal oxide water suspension. PAA is a very popular component of industrial stabilizers and flocculants, widely used in many fields of human activity (i.e. production of paint, varnish, paper, drugs, body creams and emulsions). Moreover, PAA is very well soluble in water and that is why it can be safely used in many everyday products such as foods, cosmetics or pharmaceuticals. The applied adsorbents have well defined physicochemical properties, i.e. the specific surface area, pore size and solubility. Besides, they are the subject of model research and their properties are very well documented in the literature [13,14]. Additionally, porous adsorbents find a wide usage in a variety of practical applications. The most important of them are: purification of drinking water and wastewater (removal of detergents, pesticides, bacteria, viruses and substances responsible for taste, color and smell of the water) [15], purification of air by adsorption of industrial waste gases (SOx, NOx) [16], recovery of organic solvents (benzene, toluene, acetone, alcohols, esters) [17], as well as energy storage (adsorption of natural gas, hydrogen and methanol) [18]. In medicine these materials are used in food poisoning, the blood clearance (dialysis) and as bone implants [19]. Due to the fact that polyacrylic acid is widely used in many industrial brunches, its certain quantities may be found in the industrial wastewaters. The good idea of PAA removal from technological water seems to be application of porous materials. Very often technological operations are carried out at different temperatures. For this reason our studies concerning the effects of temperature and pore size of the solid on the polyacrylic acid adsorption may contribute to the development of effective methods of water purification.

2. Experimental The following metal oxides: aluminum (III) oxide (Merck) – alumina, silicon (IV) oxide (Merck) – silica, zirconium (IV) oxide

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(Aldrich) – zirconia and controlled porosity glass (CPG) were applied in the studies. CPG was prepared using Vycor glass according to the procedure described elsewhere [20,21]. This solid has the following percentage composition: SiO2 55%, B2O3 35% and Na2O 10%. Before the experiments all solids were washed to achieve the supernatant conductivity of less than 2 lS/cm. The physicochemical characteristics of the applied adsorbents is given in Table 1. Their specific surface areas and mean pore diameters were determined by the BET method using the Accelerated Surface Area and Porosimetry apparatus (Micromeritics Instruments), whereas the average grain sizes were obtained from the dynamic light scattering technique using Zetasizer 3000 (Malvern Instruments). Anionic polymer – polyacrylic acid – PAA (Fluka), with the two weight average molecular weights (M w ): 2000 and 240,000 was used in the study. To eliminate inorganic contamination and lower polymer fractions (decrease of polymer polydispersity), the PAA samples were filtered through the cellulose membranes (Millipore). These membranes were characterized by NMWL (nominal molecular weight limit) equal to 1000 and 100,000, respectively. It means that the membrane with NMWL 1000 represents its ability to retain globular macromolecules larger than those of the molecular weight 1000. Each segment of polyacrylic acid (PAA) contains a carboxyl group. These groups undergo dissociation with the increasing pH. The pKPAA pKPAA = log[KPAA] – the negative decimal logarithm of the dissociation constant of polyacid) value is 4.5 [22]. The characteristics of polyacrylic acid functional groups dissociation is presented in Table 2 [23]. Adsorption, viscosity and solid surface charge density measurements were carried out in the temperature range 15–40 °C. Such range of temperatures was chosen, because the h temperature for the aqueous polyacrylic acid solutions equals 14 °C [24]. NaCl (1  102 mol/dm3) was used as the supporting electrolyte. The adsorption measurements were made by the static method in the polymer concentration range 10–200 ppm at pH = 6. The appropriately chosen weight of the solid sample (0.025 g – ZrO2, 0.04 g – Al2O3, 0.06 g – CPG, 0.03 g – SiO2) was added into the Erlenmeyer flask containing 10 cm3 of polymer solutions of known concentration. Then the solution pH was adjusted. Such prepared suspension was shaken in a water bath for 24 h and its pH was monitored. Then the sediment was centrifuged and 5 cm3 of supernatant was taken for analysis. The amount of adsorbed PAA was

Table 1 Adsorbents characteristics. Metal oxide

Specific surface area (BET) [m2/g]

Mean pore diameter [nm]

Mean grain diameter [nm]

ZrO2 Al2O3 CPG SiO2

32 155 58.4 326

20 6.1 72.4 13.6

440 470 304 350

Table 2 Characteristics of the PAA chains dissociation in the solution at different pH values (adiss – degree of dissociation, [COOH]/[COO] – ratio of number of undissociated groups to dissociated ones). pH

adiss

[COOH]/[COO]

3 4.5 6 7.5 9 10

0.03 0.5 0.96932 0.999001 0.99998 0.999997

32 1 0.032 0.001 0.000032 0.0000032

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obtained from the difference between the polymer concentration in the solution before and after the adsorption process. For this purpose, the reaction of polyacrylic acid with hyamine proposed by Crummet and Hummel [25] was used. The obtained turbidity was measured with the UV–VIS Specord M42 spectrophotometer (Carl Zeiss Jena) at 500 nm. All obtained results were the mean values of five independent measurements (the measurement error in all cases did not exceed 5%). The thickness of the polymer adsorption layer (d) was determined from the viscosity measurements [26] using a CVO 50 rheometer (Bohlin Instruments).The d values were obtained from the relation:

" d¼r

/p /o

1=3 # ð1Þ

where: r – radius of the solid particle [nm], Uo - volume fraction of the solid in the absence of the polymer [dimensionless value], Up volume fraction of the solid in the presence of the polymer [dimensionless value].The Einstein equation connects the volume fraction of the dispersed solid with the suspension viscosity in the following way:

g ¼ 1 þ k/ gl

ð2Þ

where: g - viscosity of the suspension [Pa  s], gl - viscosity of the liquid phase [Pa  s], k - Einstein coefficient (for instance for the alumina suspension k = 8.73).The viscosity measurements in the presence of polymer were made with the volume fractions equal to: 8.73  103 – ZrO2, 12.6  103 – Al2O3, 5.34  103 – CPG, 9.44  103 – SiO2, respectively. The following parameters: aexp – the expansion coefficient, ðr 2 Þ1=2 - the root-mean-square chain end-to-end distance and Rhthe hydrodynamic radius of a polymer coil in the solution were calculated from the viscosity data. These parameters were obtained from the expressions:

aexp



½g ¼ ½gh

vessel. The added values of the polymer and electrolyte solutions gave their required final concentrations (CPAA = 100 ppm and CNaCl = 1  102 mol/dm3). The initial pH of the solution (3–3.5) was adjusted by the use of HCl (1  101 mol/dm3). Then the solid was added in such prepared solutions and the obtained suspension was titrated by the NaOH solution (1  101 mol/dm3). The following instruments were used for the potentiometric titrations: Teflon vessel, burette Dosimat 665 (Methrom), thermostat RE204 (Lauda), pH-meter – 71 pH meter (Beckman) connected with the computer and the printer. The solid surface charge density was calculated with the special program Titr_v3 elaborated by W. Janusz. 3. Results and discussion

1=3 ð3Þ

where: [g] - intrinsic viscosity of the polymer solution at a given temperature [dm3/g], [g]h - intrinsic viscosity of the polymer solution at h temperature [dm3/g].

 1=2 ½gM1=3 r2 ¼ F

Fig. 1. Schematic representation of chain end-to-end distance and radius of gyration of polymer coil.

ð4Þ

where: F - Flory-Fox constant approximately equal to 2.1  1021, M - polymer molecular weight [g/mol].

Figs. 2 and 3 present the adsorbed amounts of polyacrylic acid with two investigated molecular weights as a function of temperature. Their analysis indicates that in the case of the zirconia–PAA solution system the adsorption minimum in the temperature range 25–30 °C was obtained [28]. For the other oxides (alumina, silica and porosity glass) the polymer adsorption increases with the temperature rise [29–31]. It should be noted that adsorption measurements were performed at pH 6. The value of solution pH is very important because

 1=2 r2 Rh ¼ fRg ¼ f

61=2

ð5Þ

where: f - constant value irrespective of the polymer molecular weight (for PAA f = 0.66 [27]), Rg – radius of gyration [nm]. The expansion coefficient (aexp) determines how many times the linear dimensions of macromolecule in a given solvent are larger than the undisturbed linear dimensions of the statistical coil of the same molecular weight. The schematic representation of chain end-to-end distance and radius of gyration of polymer coil is presented in Fig. 1. The surface charge density of the solid in the absence and presence of the polymer was determined from the potentiometric titrations of the systems containing NaCl or NaCl + PAA solutions and appropriately chosen weight of the solid sample (0.5 g – ZrO2, 0.2 g – Al2O3, 0.3 g – CPG, 0.1 g – SiO2). For this purpose, 50 cm3 of the polymer solution in the supporting electrolyte (or only the supporting electrolyte solution) was introduced into the Teflon

Fig. 2. Adsorbed amount of PAA 2000 on the surface of various oxides as a function of temperature; CPAA = 100 ppm, pH = 6.

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Fig. 3. Adsorbed amount of PAA 240,000 on the surface of various oxides as a function of temperature; CPAA = 100 ppm, pH = 6.

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Fig. 4. Thickness of PAA 2000 adsorption layer on the surface of various oxides at different temperatures; CPAA = 100 ppm, pH = 6.

Table 3 Points of zero charge (pHpzc) of adsorbents at different temperatures. Metal oxide

ZrO2 Al2O3 CPG SiO2

pHpzc 15 °C

25 °C

35 °C

7.3 8.4 6.7 3.0

7.0 8.0 7.0 <3.0

7.1 7.1 7.4 <3.0

it influences both the charges of anionic PAA chains and the solid surface. The characteristics of the PAA chains dissociation in the solution at different pH values is placed in Table 2. Additionally, Table 3 contains the information about the points of zero charge (pHpzc) of the adsorbents at different temperatures. As can be seen in Table 2 the degree of PAA chains dissociation (adiss) at pH 6 is equal to 0.96932, whereas the ratio of the number of undissociated groups to the dissociated ones ([COOH]/[COO]) is 0.032. It means that practically all carboxyl groups in the polyacrylic acid macromolecules are dissociated and thus the polymer chains are negatively charged. On the other hand, only silica surface is negatively charged at pH 6 (Table 3, pHpzc  3). Therefore, under these conditions there is a strong electrostatic repulsion between the negatively charged silica surface and the negatively charged PAA macromolecules. The evidence for this is the lowest adsorption of polymer on the SiO2 surface (Fig. 2 and 3) and the thickest adsorption layer (Figs. 4 and 5). The other oxides at pH 6 are still positively charged (Table 3, pHpzc > 6 for zirconia, alumina and CPG). Thus, in the case of these oxides the electrostatic attraction between their positive surface and negative polymer chains occurs. It results in higher adsorption of PAA on the surfaces of these oxides in comparison to that of silica (Fig. 2 and 3). Furthermore, the presence of PAA adsorption under the electrostatic silica-polymer repulsion indicates that binding of polyacrylic acid with the metal oxide surface proceeds through hydrogen bridges. These bridges can be formed in all examined systems between the solid surface groups (both neutral and charged: „MeOH, „MeO, „MeOH2+, Me: metal ion – Al, Si, Zr, B, Na) and the PAA carboxyl groups (both dissociated and undissociated). Our previous investigations indicate that among all types of solid surface groups, the neutral ones („MeOH) have the greatest affinity for the polymer functional groups, mainly due to their highest concentration [32]. Their concentration is of the order of tens (and even hundreds) of lC/cm2, whereas the concentration of the charged groups is of the order of a few lC/cm2.

Fig. 5. Thickness of PAA 240,000 adsorption layer on the surface of various oxides at different temperatures; CPAA = 100 ppm, pH = 6.

Probably for this reason, the adsorption of polyacrylic acid is the greatest on the zirconia surface in comparison to that, for example, on the alumina surface. The concentration of „ZrOH groups at pH 6 equals 191.5 lC/cm2 [33], whereas that of „AlOH is 121.7 lC/ cm2 [34]. The analysis of the adsorbed amounts of polyacrylic acid on the surfaces of the examined oxides as a function of temperature (Fig. 1 and 2) leads to the conclusion that only in the ZrO2–PAA system the adsorption minimum in the temperature range 25–30 °C is observed. The other systems: Al2O3–PAA, SiO2–PAA, CPG–PAA show similar adsorption behaviour, that is the adsorption increase with the temperature rise. There may be two reasons for such phenomena: conformational changes of adsorbing macromolecules with the temperature rise and different pore size of the applied adsorbents. Description of the conformational changes of the adsorbing polymer chains as a function of temperature can help explain the above dependences. These changes can be analyzed based on the values of the parameters characterizing the linear sizes of macromolecules in the solution and those adsorbed on the solid surface. Conformation of macromolecules adsorbed on the solid surface can be characterized by thickness of the polymer adsorption layer (d). It was obtained from the viscosity data and are shown in Figs. 4 and 5. In turn, conformation of polymer chains in the bulk solution can be described by the following parameters: the root-mean-square

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96 1 2

chain end-to-end distance – ðr 2 Þ , the hydrodynamic radius of a polymer coil in the solution – Rh and the expansion coefficient – aexp. They were also calculated from the viscosity data and are presented in Figs. 6–8. The analysis of the parameters characterizing the conformation of the PAA macromolecules in the bulk solution shows that their values increase with the increasing temperature. This means that the conformation of the polymer chains becomes more and more stretched, causing the increase of their linear dimensions. This follows from the fact that the temperature increase makes the polymer – solvent interaction more energetically favoured compared to that between the polymer segments. Therefore, at 15 °C, which is very close to the theta temperature of the PAA solution (Th = 14 °C), the polymer macromolecules are coiled. The expansion coefficient is nearly equal to 1. The polymer coils develop as temperature rises. Thus, at 35 °C the conformation of the PAA macromolecules is characterized by some stretching. The expansion coefficient achieves the values greater than 1 i.e. 1.37 and 1.7 for PAA 2000 and 240,000, respectively. It means that the macromolecule size is 1.37 or 1.7 times greater at 35 °C compared to that at 15 °C. On the other hand, the polymer adsorption layer becomes thicker with the increasing temperature. Therefore, the linear dimension of macromolecules in the adsorption layer change in the same way as it takes place in the solution. The conformational changes of the polyacrylic chains in the solution with the increasing temperature are the same for all examined adsorbents. Nevertheless, in the case of zirconia the adsorption minimum at mean values of the studied temperatures appears. It seems to be caused by the varying pore size of the applied oxides. Zirconia was characterized by the mean pore diameter equal to 20 nm (Table 1). The SAXS (Small Angle X-ray Scattering) measurements performed in the ZrO2–PAA system [35] have shown that at 25° C only adsorption of PAA 2000 on the pore surface is possible. The polyacrylic acid chains with high molecular weight are not able to penetrate into the pores and to form a uniform polymer layer on the pore surface. Therefore, it is highly likely that at 15 °C it is possible to adsorb strongly coiled polymer macromolecules on the surface of both the adsorbent particles and in their pores. It results in higher PAA adsorption. In the temperature range 25–30 °C adsorption possibility in the adsorbent pores is significantly reduced due to the more developed conformation of macromolecules, leading to the decrease in the polymer adsorption. As the temperature rises, further development of the polymer chains takes place and their adsorption in the pores is no longer possible. However, they form a densely packed adsorption layer

Fig. 6. The root mean square of chain end-to-end distance of the polyacrylic macromolecules in the solution at different temperatures; CPAA = 100 ppm, pH = 6.

Fig. 7. The hydrodynamic radius of the polyacrylic acid coils in the solution at different temperatures; CPAA = 100 ppm, pH = 6.

Fig. 8. The expansion coefficient of the polyacrylic acid chains in the solution at different temperatures; CPAA = 100 ppm, pH = 6.

on the solid surface. A significant contribution of polymer segments in the tail and loop structures causes that, despite the absence of the polymer adsorption in the pores, the adsorbed amount of PAA is large. The increase of polymer linear dimension promote its adsorption on metal oxide. At 15 °C (close to h conditions) PAA macromolecules assume on the solid surface more coiled conformation (similarly to that in the solution). The adsorption layer consists of loosely tangled polymer coils which leads to blockade of solid active sites and limitation of other PAA chains adsorption on the adsorbent surface. As a result at 15 °C low adsorption and thin adsorption layer were obtained. The increase of temperature causes polymer coils developing (increase of linear sizes of PAA chains) resulting in more stretched conformation of macromolecules. As a consequence, at 35 °C the polymer adsorption layer are composed of long tail and loop structures, expanding perpendicularly to the solid surface towards the bulk solution. Such stretched conformation of PAA chains does not consume many solid active sites, because a few single segments undergo bonding with the solid surface. For this reason the greater number of polymer chains can adsorb on the same surface unit in comparison to coiled conformation at 15 °C. It results in high adsorption and thick adsorption layer. Taking into account such adsorption mechanism, it can be assumed that in the case of aluminum (III) oxide and silicon (IV) oxide, characterized by the pore diameters 6.1 and 13.6 nm, respectively (Table 1), the adsorption of macromolecules in the pores is not possible in the whole range of the studied temperatures and occurs only on the surface of solid particles. However,

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bent surface charge in the polymer presence compared to that in its absence. Changes in the surface charge density of applied adsorbents can be explained on the basis of the analysis of both adsorbed amount of PAA and also thickness of its adsorption layer. The thicker the adsorption layer is, the smaller number of unadsorbed dissociated carboxylic groups in compact Stern layer are present. These polymer charges influences the solid surface charge density. The others located in diffusion layer affect the electrokinetic potential of solid particles. Moreover, one should noticed that adsorption of anions causes increase of the solid surface charge. In the case of polyanion (PAA) not all carboxyl groups can possibility to adsorb directly on the solid active sites. The others are located in surface area in the solution. The first ones cause increase of solid surface charge and the latter ones cause its decrease. The sum of these two effects leads to the obtained values of surface charge density. Thus, the greater number of solid active sites is, the smaller decrease of the solid surface charge should be obtained, even in the case of the greatest polymer adsorption. For this reasons it is very difficult to compare these data for different adsorbents, that is why each system should be analyzed independently of one another. The analysis of the data in Table 4 concerning the zirconia-polyacrylic acid system leads to the conclusion that the smallest changes of the solid surface charge density in the PAA presence in comparison to that without PAA are obtained at 25 °C. It may be a result of small adsorption of polymer (adsorption minimum) and specific structure of polymer adsorption layer on the zirconia surface under such temperature conditions. Thus, the smaller number of the dissociated carboxyl groups of the adsorbed polymer chains is present in the liquid phase with the tail and loops structures. As a consequence, the effect of lowering of the zirconia surface charge in the polymer presence is the smallest at 25 °C. For the other applied oxides the following tendency can be noticed: the higher the temperature is, the greater changes of the solid surface charge density in the PAA presence in comparison to

in the systems containing the porous glass with the pore size of 72.4 nm, the adsorption of macromolecular compound may take place on the surfaces of solid particles and their pores in the whole range of the studied temperatures [36,37]. As a result, in the systems in which no adsorption occurs in the pores or vice versa it takes place in an unlimited way (due to the pore size), the increase in the amount of the adsorbed polymer in the whole temperature range is observed. The schematic representation of the adsorption possibility of the PAA macromolecules of different sizes in the solid pores of various diameters is presented in Fig. 9. The case (a) concerns the systems characterized by the large pore diameter in which adsorption of all polymer chains (strongly and loosely coiled) is possible, i.e. in the CPG–PAA system. The case (b) refers to the solid with the mean pore diameter (that is zirconia) for which adsorption of only strongly coiled macromolecules takes place. On the other hand, the case (c) shows adsorption behaviour of metal oxides with small pore diameters, that is alumina and silica for which polymer adsorption in the pores is impossible irrespective of macromolecule conformation. The additional information about the adsorption mechanism and the structure of the polymer adsorption layer can be obtained from the analysis of the changes in the surface charge density of the adsorbent occurring in the polymer presence. The difference between the surface charge density of the solid without PAA and that with PAA (Dr0 = r0(without PAA)  r0(with PAA)) at pH 6 at different temperatures is presented in Table 4. For all studied systems the polyacrylic acid adsorption results in significant reduction of the surface charge density of the metal oxide and the shift of pHpzc of the adsorbent towards lower pH values. This is due to the presence of the dissociated carboxyl groups in the PAA macromolecules. Only a part of these groups undergoes adsorption on the solid surface, whereas the others are in the loop and tail structures of the adsorbed macromolecules. These unadsorbed –COO– groups are responsible for reduction of the adsor-

(b)

(a)

(c)

Fig. 9. The schematic representation of the adsorption possibility of PAA macromolecules of different sizes in the solid pores of various diameters: (a) large pore diameter (CPG) – the adsorption of all polymer chains (strongly and loosely coiled), (b) mean pore diameter (zirconia) – adsorption of only strongly coiled macromolecules), (c) small pore diameter (alumina, silica) – polymer adsorption impossible.

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Table 4 Difference between surface charge density of the solid without PAA and that with PAA (r0 = r0(without PAA)  r0(with PAA)) at pH 6 at different temperatures. Metal oxide

PAA molecular weight

|Dr0| [lC/cm2] 15 °C

25 °C

35 °C

ZrO2

2000 240,000

3.81 13.98

2.32 7.57

6.28 17.27

Al2O3

2000 240,000

0.66 8.07

1.03 9.45

2.08 10.66

CPG

2000 240,000

14.67 18.50

20.54 26.51

23.18 28.48

SiO2

2000 240,000

1.99 3.07

2.29 4.95

3.91 6.89

macromolecules in the pores is not possible in the whole range of the studied temperatures and occurs only on the surface of solid particles. On the contrary, the adsorption of PAA may take place on the surface of the solid particles and their pores in the whole range of the studied temperatures for the systems containing the porous glass (pore size 72.4 nm). In the case of zirconia (pore size 20 nm), the polymer adsorption in the solid pores proceeds in an unlimited way probably at 15 °C, whereas in the temperature range 25–30 °C it is significantly limited (adsorption minimum). On the other hand, at 35 °C polyacrylic acid adsorbs only on the surface of the solid particles forming the densely packed layer composed mainly of the polymer segments present in the tail and loops structures. References

those without PAA are obtained. One of the reasons of such behavior may be the increasing polymer adsorption with the increasing temperature. Moreover, very interesting is the fact that in the case of the systems containing the controlled porosity glass, the changes of the solid surface charge density in the PAA presence in comparison to that without PAA are the greatest. This may be the evidence for the unlimited adsorption of the polymer in the pores. As a consequence, the available surface area for the adsorbing macromolecules is large and thus the changes of CPG surface charge are significant. Another reason for this may be a relatively small number of surface active sites on which PAA carboxyl groups undergo adsorption. The unlimited adsorption in the CPG pores means that there is no limitation through the size of the macromolecules (rising with temperature) and such limitation are present in the relation of PAA adsorbed amount in the pores. 4. Conclusions The effect of the adsorbent type (alumina, silica, zirconia and porosity glass) on the polyacrylic acid (PAA) adsorption mechanism at the solid-polymer solution interface in the temperature range 15–40 °C was examined. It was indicated that only in the ZrO2–PAA system the adsorption minimum in the temperature range 25–30 °C is obtained. In the other systems: Al2O3–PAA, SiO2–PAA, CPG–PAA, the polymer adsorption increases with the temperature rise. Such adsorption behaviour of the studied system may result from both conformational changes of adsorbing macromolecules with the temperature rise and different porosity of the applied adsorbents. The temperature increase causes that the conformation of the polymer chains becomes more and more stretched, leading to the increase of their linear dimensions in the solution and in the adsorption layer. In the case of aluminum (III) oxide and silicon (IV) oxide, characterized by the pore diameters 6.1 and 13.6 nm, the adsorption of

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