Positively Charged Polymers (Polycations)

Chapter 5

Positively Charged Polymers (Polycations) 5.1. INTRODUCTION Polycations have long been used as aggregating agents of negatively charged colloids in water and wastewater treatment (Bolto and Gregory, 2007; Bolto et al., 2001; Cohen et al., 1958; Dixon, 1967; Maximova and Dahl, 2006; Pressman, 1967). Nevertheless, the interactions of clay minerals with positively charged polymers have not received as much attention as those involving uncharged and negatively charged polymers. More recent research has focused on the use of clay–polycation complexes as sorbents of organic pollutants, controlled release formulation of herbicides and catalysts for organic reactions (Akelah et al., 1999; Breen, 1999; Breen and Watson, 1998a,b; Churchman, 2002; Ganigar et al., 2010; Radian and Mishael, 2008; Rebhun et al., 1969; Rehab et al., 2002; Yue et al., 2007; Zadaka et al., 2009; Zhang et al., 2006). A relatively novel development in the clay–polycation interaction relates to the synthesis of thin films consisting of alternating layers of polycations and smectites having interesting (nonlinear) optical and electronic properties (Fan et al., 2002; Glinel et al., 2001; Hammond, 2004; Kleinfeld and Ferguson, 1994; Kotov et al., 1997; Lvov et al., 1996; Ras et al., 2007; Schoonheydt, 2002; Schoonheydt and Johnston, 2006; Starodubtsev et al., 2009; Tang et al., 2003). The destabilization of dilute clay mineral suspensions by positively charged polymers has been described by numerous authors (Black and Vilaret, 1969; Black et al., 1965, 1966; Chang et al., 1997; Kimura et al., 1976; Narkis et al., 1968; Pressman, 1967; Ruehrwein and Ward, 1952; Shyluk, 1964; Ueda and Harada, 1968a; Yorke, 1973). Polycations have also served as aggregants of suspended silica and metal hydr(oxide) particles (Bolto et al., 2001; Dixon et al., 1967; Iler, 1971; Kane et al., 1964a,b,c; Li et al., 1998; Linke and Booth, 1960; Posselt et al., 1968; Shin et al., 2002; Shubin, 1997; Shyluk, 1964; Wang and Audebert, 1988), as well as polymer fibres and latex particles (Goossens and Luner, 1976; Gregory, 1969, 1973; Lindstro¨m and So¨remark, 1976; Petzold and Lunkwitz, 1995; Swerin et al., 1997; Tanaka et al., 1999), and humic substances (Bratskaya et al., 2004; Narkis et al., 1968; Rebhun et al., 1969). Developments in Clay Science, Vol. 4. DOI: 10.1016/B978-0-444-53354-8.00005-0 # 2012 Elsevier B.V. All rights reserved.

129

130

Formation and Properties of Clay-Polymer Complexes

Increase in aggregation relative to control (%)

Since positively charged polymers can function as both coagulants and flocculants (cf. Chapter 2), they can act as effective aggregants even at low concentrations (<5 ppm) and in the absence of neutral electrolytes. The coagulating effect is exerted through charge neutralization and compression of the diffuse double layers around clay particles, while flocculation is effected through interparticle bridging. For polycations of high charge density and low-to-moderate molecular weight, charge neutralization is much more important than interparticle bridging (Baran and Gregory, 1996; Durand-Piana et al., 1987; Gill and Herrington, 1987a; Petzold et al., 2004; Ueda and Harada, 1968a; Yu et al., 2006). Indeed, Goossens and Luner (1976) found little evidence for interparticle bridging when they added an ionene polymer (molecular weight
6104 Da) to a suspension of microcrystalline cellulose. In many instances, however, charge neutralization alone cannot adequately account for colloid destabilization. For polycations of high molecular weight, however, interparticle bridging often becomes the dominant mechanism or, at least, plays an important part in particle aggregation (Black et al., 1965, 1966; Gill and Herrington, 1987a,b; Iler, 1971; Narkis et al., 1968; Petzold et al., 2004; Shyluk, 1964; Yu et al., 2006). As might be expected, positively charged polymers are generally more effective than uncharged and anionic polymers in destabilizing suspensions of negatively charged particles. Using silica suspensions and different polyelectrolytes, Kane et al. (1964a,b,c) and Dixon et al. (1967), for example, found that particle aggregation by polycations occurred for molecular weights and dosages that were at least one order of magnitude smaller than those shown by anionic polymers. Flaig et al. (1962) have reported similarly for the destabilization of kaolinite suspensions by various polymers of comparable molecular weight. Thus, at low polymer dosages (<0.01 g/100g) aggregating efficiency decreased in the order: (cationic) polyethyleneimine> (nonionic) polyvinyl pyrrolidone>(anionic) polymethacrylic acid (Figure 5.1).

A

0

B 50 C 100 10-4

10-3 10-2 10-1 1 Polymer concentration (g/100 g)

FIGURE 5.1 Aggregation of kaolinite suspensions by a polycation, a nonionic polymer and a polyanion of comparable molecular weight. Curve A, polymethacrylic acid; curve B, polyvinyl pyrrolidone; curve C, polyethyleneimine. From Flaig et al. (1962).

Chapter

5

Positively Charged Polymers (Polycations)

131

The same sequence was also observed by Ben-Hur et al. (1992) for the adsorption of cationic, nonionic and anionic polysaccharides (guar) by illite and montmorillonite, and by Besra et al. (2002) for polyacrylamides adsorbing to kaolinite.

5.2. FORMATION AND PROPERTIES OF COMPLEXES Early on, Ruehrwein and Ward (1952) proposed that the aggregation of suspended clay particles by polymethacrylate (a polyanion) and b-dimethylaminoethylmethacrylate hydroacetate (a polycation) was effected through interparticle bridging, that is, by adsorption of the polymer to two or more particles at the same time. In line with this proposal, aggregation was promoted by the presence of NaC1 since an increase in ionic strength would decrease interparticle repulsion, bringing the clay particles close together. For moderately to highly charged polycations, increasing the ionic strength would also reduce repulsion between charged segments, giving rise to a less extended chain conformation and enhanced adsorption (Durand et al., 1988). The positive effect of ionic strength on adsorption, however, may be offset by the reduction in surface accessibility due to interlayer contraction and particle aggregation (Bailey et al., 1994; Yue et al., 2007). Ruehrwein and Ward (1952) also noted that b-dimethylaminoethylmethacrylate hydroacetate was capable of intercalating into Naþ-montmorillonite, and replacing the interlayer Naþ ions. The basal spacing of the resultant interlayer complex increased with the amount adsorbed. Lagaly and Ziesmer (2006), Lockhead et al. (2006) and Huskic´ et al. (2009) have observed similarly for the intercalation of different polycations by montmorillonite. As mentioned in Chapter 3, this observation is indicative of random interstratification of sodium- and polycationrich layers within a single particle (cf. Figure 3.4). The existence of polycation-rich and polycation-poor (i.e. inorganic cation-rich) interlayers has been identified by Breen et al. (1996). X-ray diffraction analysis further indicates that the proportion of polycation-rich layers increases with polymer loading. Ruehrwein and Ward (1952) measured a maximum basal spacing of 1.44 nm for the interlayer complex of montmorillonite with b-dimethylaminoethylmethacrylate hydroacetate, indicating intercalation of a single polycation layer in a flat (extended) conformation. Similar findings have been reported by Breen et al. (1996), Fournaris et al. (1999), Inyang and Bae (2005), Deng et al. (2006) and Huang et al. (2009) for different polycations. On the other hand, Churchman (2002) measured basal spacings of 1.92.8 nm for montmorillonite complexes with poly(diallyldimethylammonium chloride), indicative of multilayer intercalation. Layer thicknesses deduced from basal spacing measurements, however, may not apply to that portion of the polymer that adsorbs to external particle surfaces (Szczerba et al., 2010). Even if the polymer used were monodisperse, it may form longer loops or thicker layers on external basal than on interlayer surfaces. Tian et al. (2006), for example,

132

Formation and Properties of Clay-Polymer Complexes

inferred from atomic force microscopy observations that poly(diallyldimethylammonium chloride) was attached with long loops and tails to the (external) surface of mica particles. The investigation by Ueda and Harada (1968a) provides strong, albeit indirect, evidence to indicate that polycations adsorb to clay minerals by a cation exchange process (i.e. charge neutralization), giving rise to an extended surface conformation. Using Naþ-bentonite (montmorillonite) with a cation exchange capacity (CEC) of 62.4 cmol/kg and a cationic diallyldimethylammonium chloride-SO2 copolymer with a molecular weight of 167,000 Da (copolymer I) and 14,000 Da (copolymer II), these workers followed the changes in both the CEC and anion exchange capacity (AEC) of the complex as adsorption progressed. The results for copolymer I (Table 5.1) show that with rising amounts adsorbed, the CEC (column B) steadily declines, while the AEC (column C) increases. The decrease in CEC may be ascribed to neutralization of the negative surface charge by train segments (cf. Figure 2.1), while the development of an AEC is due to the formation of (positively charged) loops and tails (since the parent bentonite does not possess a measurable AEC). On this premise, the difference between the amount adsorbed (column A) and the AEC represents the quantity of polymer in contact with the surface (column D). The ratio of D/A therefore gives the proportion of segments in trains (p). The sum of (BþD), which is practically constant and close to the total CEC of the clay, lends further support to the proposed mode of interaction. The development of an AEC (at high polycation loadings) in both montmorillonite and palygorskite has also been reported by Churchman (2002) and Yue et al. (2007). In this connection, we might also add that montmorillonite– polycation complexes can develop an AEC by other means than what Ueda and Harada (1968a) have proposed. Darder et al. (2003), for example, were able to intercalate a single or a double layer of chitosan chains into montmorillonite, depending on the ratio of polycation dosage to the CEC of the clay mineral. They proposed that the first chitosan layer was intercalated by cation exchange, while the second chitosan layer was taken up as the acetate salt (cf. Figure 11.9). As a result, the bilayer complex could act as an anion exchanger. Thus, up to a surface coverage of
0.5, practically all of the polycation segments were attached to the montmorillonite surface (p
1), while at full coverage about 75% of the segments were in trains, with the remainder 25% being present as loops and tails. Because of the numerous segmentsurface contacts (“octopus” effect), the adsorbed polymer could not be removed by washing the montmorillonite–polycation complex with water and electrolyte solutions (up to 1.5 M CaC12). Ueda and Harada (1968a), however, did not carry out basal spacing measurements in order to establish that the polycation was intercalated in an extended surface conformation. At comparable concentrations in solution, more of the low molecular weight copolymer II was taken up relative to copolymer I. Further, copolymer

Amount Adsorbed (cmol/kg) A

AEC (cmol/kg) C

Amount in Trains (cmol/kg) D (¼A–C)

Sum of BþD (cmol/kg)

Proportion of Segments in Trains (p) D/A

0

0

0

62.4

0

0

62.4

1

2.67

1.93

8.56

49.5

0

8.56

58.1

1

5.33

4.18

18.5

44.6

0

18.5

63.1

1

10.0

7.57

33.6

30.7

0.97

32.6

63.3

0.97

13.3

9.18

40.7

24.5

3.85

36.8

61.3

0.90

20.0

9.82

43.5

23.9

5.34

38.2

62.1

0.88

26.7

10.8

47.9

21.4

6.68

41.2

62.6

0.86

33.3

10.5

46.6

23.7

6.37

40.2

63.9

0.86

43.3

13.0

57.7

15.8

13.2

44.5

60.3

0.77

56.7

13.2

58.5

14.4

15.1

43.4

57.8

0.74

73.7

14.2

63.0

12.8

15.5

47.5

60.3

0.75

93.3

14.5

64.3

11.1

18.2

46.1

57.2

0.71

Positively Charged Polymers (Polycations)

(g/100g)

CEC (cmol/kg) B

5

Amount Added (g/100g)

Chapter

TABLE 5.1 Data for the Adsorption of Diallyldimethylammonium Chloride-SO2 Copolymer I by Montmorillonite at a Suspension Concentration of 1.5g/100mL and 303K.

From Ueda and Harada (1968a).

133

134

Formation and Properties of Clay-Polymer Complexes

II was adsorbed with more than 90% of its segments in trains (p>0.9), even at high surface coverage, presumably because the relatively small molecules of copolymer II can readily intercalate into montmorillonite and establish numerous polymer-surface contacts as compared with its high molecular weight counterpart. Similarly, Lindstro¨m and So¨remark (1976) reported that adsorption of cationic polyacrylamides to cellulose increased with a decrease in polymer molecular weight and charge. Ueda and Harada (1968a) also found that polycation adsorption by montmorillonite increased in the presence of sodium chloride, reaching a maximum at a concentration of 0.15 M NaCl (Table 5.2). This finding is contrary to expectation since, as remarked on above, an increase in ionic strength would suppress interlayer swelling (of montmorillonite) and hence diminish surface accessibility (Bailey et al., 1994). In explanation, these workers suggested that the presence of NaCl caused the polycation chain to coil as Durand et al. (1988) have proposed. As a result, the surface requirement of the polycation would decrease, allowing an increased amount to be adsorbed. The decrease in the fraction of segments in trains (p) with increasing NaCl concentrations (Table 5.2) is consistent with this suggestion. The positive effect of salt addition on adsorption was also reported by Peyser and Ullman (1965) for the interaction of poly-4-vinylpyridinium chloride with (negatively charged) porous glass surfaces, and by Tekin et al. (2005) for the adsorption of cationic polyacrylamide by kaolinite.

TABLE 5.2 Adsorption of Diallyldimethylammonium Chloride-SO2 Copolymer I by Montmorillonite in the Presence of Sodium Chloride Solutions of Different Concentrations. NaCl conc. (M)

AEC Amount Adsorbeda CECb Amount (cmol/kg) (cmol/kg) in Trains (g/100g) (cmol/kg) B C (cmol/kg) A D

Proportion of Segments in Trains (p) D/A

0

9.66

42.8

24.6

5.03

37.8

0.88

0.02

11.4

50.4

25.3

13.3

37.1

0.74

0.05

12.2

54.1

26.0

17.7

36.4

0.67

0.15

13.5

59.8

24.4

21.8

38.0

0.63

0.50

13.5

59.8

25.6

23.0

36.8

0.62

1.25

13.4

59.5

28.6

21.4

34.4

0.57

From Ueda and Harada (1968a). a Based on a suspension concentration of 1.5 g clay per 100mL solution and an amount of polymer added of 20.8 g per 100g clay. b Calculated from 62.4C, where 62.4 is the cation exchange capacity (CEC) of the parent montmorillonite sample.

Chapter

5

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Positively Charged Polymers (Polycations)

The influence of molecular size and charge as well as clay concentration (surface accessibility) on the montmorillonite–polycation interaction has been systematically investigated by Durand-Piana et al. (1987), Wang and Audebert (1988) and Denoyel et al. (1990). By polymerizing acrylamide (AM) and N,N, N-trimethylaminoethylchloride acrylate (CMA) monomers (Scheme 5.1), they were able to obtain copolymers (PCMA) varying in molecular weight and cationicity (molar percentage of CMA). As might be expected for a cation exchange process—for which the net segment-surface interaction energy is much larger than 1 kT—the adsorption of PCMA copolymers by Naþ-montmorillonite was rapid (60 min), and the isotherms were of the H-type (cf. Chapter 2). High-affinity type isotherms generally obtain for the adsorption of positively charged polymers by clay minerals (Alemdar et al., 2005; Baran and Gregory, 1996; Breen et al., 1996; Deng et al., 2006; Ganigar et al., 2010; Tekin et al., 2005, 2006a; Ueda and Harada, 1968a). For a given molecular weight (
106 Da), the maximum extent of adsorption decreased with an increase in cationicity, t (Figure 5.2).

-----(CH2

CH)m -----(CH2

CH)n ------

C O

C O

NH2

O CH2

CH3 CH2

+

N

Cl–

CH3 CH3

(AM)

(CMA) SCHEME 5.1

Amount adsorbed (g/m2)

t 1% 3

5%

2

13% 1 23% 30% 0

0

1500 500 1000 Equilibrium concentration (mg/L)

2000

FIGURE 5.2 Isotherms for the adsorption of PCMA copolymers with different cationicities (t) by Naþ-montmorillonite. From Denoyel et al. (1990).

136

Formation and Properties of Clay-Polymer Complexes

Thus, the plateau value for PCMA with t¼1% was an order of magnitude larger than that of its counterpart with t¼100%. Durand-Piana et al. (1987) suggested that like uncharged polymers, the low-charge (t5%) PCMA copolymers were intercalated with a large proportion of their segments in loops and tails. In keeping with this suggestion, the basal spacing of the montmorillonite increased to 3.5 nm (for t¼5%). As cationicity increased (t>5%), however, the polycation chains became more stretched and rigid due to increased electrostatic repulsion. As a result, the fraction of segments in trains (p) rose until at t¼100% the polymer was adsorbed in a highly extended (flat) conformation (p
1) to give a basal spacing of 1.8 nm, indicating intercalation of a bilayer of PCMA chains. An and Dultz (2007) have observed similarly for the montmorillonite–chitosan interaction. Denoyel et al. (1990) also found that the uptake (of low-charge PCMA copolymers) rises with an increase in molecular weight, in common with the behaviour of uncharged polymers (cf. Figure 3.7). At the same time, the fraction of adsorbed segments (p) declined, indicating increased formation of loops and tails. Polycation uptake was also influenced by surface accessibility in that the maximum extent of adsorption decreased with an increase in clay concentration (cf. Table 5.3). Black et al. (1966), Gill and Herrington (1986) and Kislenko and Verlinskaya (2001) have observed similarly for the adsorption of different cationic polymers by kaolinite, montmorillonite and palygorskite. TABLE 5.3 Some Data on the Interactions of a Polycation (14C-Labelled N-Substituted Piperidinium Chloride) with Dilute Suspensions of Kaolinite and Montmorillonite. Optimum Electrophoretic Pm/Solid Polymer Mobility at Pm Content Dosage, (mms1/Vcm1) (103) Pm (mg/L) 103

Clay Species

Initial Suspension Strength or Solid Content (mg/L)

Kaolinite

73.2

8.1

143

29.8

12.0

14.9

Extent of Polycation Adsorptiona (mg/g)

0.7

1.95

62

0.0

2.08

18.4

30

0.4

2.01

18.5

890

þ0.5

6.18

66.8

29.2

670

þ0.7

10.03

34.4

30.4

147

0.3

4.27

Montmorillonite 144.0

From Black et al. (1966). a Estimated from applying the linear transform of the Langmuir equation to the adsorption data.

Chapter

5

137

Positively Charged Polymers (Polycations)

As alluded to at the beginning of this chapter, research into the clay- and mineral-polycation interaction was stimulated by the propensity of positively charged polymers for destabilizing aqueous particulate dispersions. An early systematic attempt in this direction was that by Black et al. (1966) who studied the destabilization of dilute (<150 mg/L) suspensions of kaolinite and montmorillonite by 14C-labelled N-substituted piperidinium chloride polymer of molecular weight >50,000 Da. The approach used was to mix a solution of the polycation with the clay suspension under controlled conditions of pH, ionic strength and duration and intensity of agitation. The extent of aggregation (destabilization) and adsorption was assessed by measuring the residual turbidity and polymer concentration in the supernatant solution, respectively, after a given time of settling. Concurrently, the electrophoretic mobility of the clay–polymer system was determined. The results for kaolinite are shown in Figure 5.3 where the electrophoretic mobility of the particles is plotted against polymer dosage for three levels of (initial) clay suspension strength. For mobility we may substitute zeta (z) potential (cf. Equation 8.3). The corresponding adsorption isotherms (not shown) have the same appearance as the mobility curves, indicating that adsorption increases with polymer concentration until a point is reached at which the residual turbidity of the supernatant solution is at a minimum. This point is assumed to coincide with that of maximum aggregation, and the corresponding polycation concentration is referred to as the optimum

A

+1 Electrophoretic mobility (mm s–1/V cm–1)

FIGURE 5.3 The electrophoretic mobility of kaolinite suspensions as a function of polycation (N-substituted piperidinium chloride) dosage for three different initial clay concentrations: 14.9 mg/L (A); 29.8 mg/L (B); 73.2 mg/L (C). Arrows indicate the optimum polymer dosage at which maximum aggregation occurred. From Black et al. (1966).

0 –1

B

+1 0 –1

+1

C

0 –1 0

0.2

0.4 0.6 0.8 Polymer dosage (mg/L)

1.0

138

Formation and Properties of Clay-Polymer Complexes

(polymer) dosage. The approximate position of this optimum (Pm) is indicated by arrows in Figure 5.3. The observation that the electrophoretic mobility (z potential) of the particles becomes less negative as adsorption progresses is of general applicability to the clay–polycation interaction. As loading increases beyond the optimum polymer dosage (or the CEC of the clay mineral), the negative surface charge is reversed, causing the clay–polymer system to restabilize (Alemdar et al., 2005; Baran and Gregory, 1996; Chang et al., 1997; Churchman, 2002; Durand-Piana et al., 1987; Gill and Herrington, 1987b; Tekin et al., 2006a, b; Tian et al., 2006; Yu et al., 2006). Similarly, Billingham et al. (1998) have reported that floc size (of Csþ-montmorillonite) increases with polycation loading until charge reversal occurs, and the particles redisperse. With low molecular weight polycations (and cationic surfactants), minimum residual turbidity (maximum aggregation) commonly occurs at a z potential close to zero (Bolto and Gregory, 2007; Chang et al., 1997; Kane et al., 1964c; Mabire et al., 1984; Narkis et al., 1968; Ottewill and Rastogi, 1960; Wang and Audebert, 1987). On this basis, Dixon et al. (1967) have suggested that aggregation of negatively charged particles by polycations is the result of charge reduction, followed by interparticle bridging. With high molecular weight polycations, however, Pm is often less than that required to bring the z potential to zero (Tian et al., 2006; Vincent, 1974; Yu et al., 2006) (Figure 5.3). In this case, interparticle bridging probably occurs concurrently with charge reduction, that is, before the negative surface charge has been completely neutralized (Gregory, 1969; Heller, 1966). Black et al. (1966) also observed that Pm broadens as the initial (clay) suspension concentration is increased. That is to say, there is a range of polymer concentrations—rather than a single value—over which turbidity removal is most effective. This effect is more pronounced with montmorillonite than with kaolinite (Table 5.3), reflecting the larger accessible surface area of the former mineral. The width of this aggregation “zone” also tends to increase with polymer molecular weight and ionic strength (Gregory, 1969, 1978; Yu et al., 2006). In reacting a high-charge polycation with Csþ-montmorillonite, Billingham et al. (1997) also noted a rapid accumulation of positive charges on the external particle (“tactoid”) surface, causing aggregation and a reversal of charge much below the CEC of the clay mineral. The above observations may be explained in terms of the “electrostatic patch” mode of aggregation that operates when there is a mismatch in charge separation between the cationic polymer and the adsorbing surface (Gregory, 1973, 1978). It would be difficult, for example, for a positively charged segment of a high-charge polycation to neutralize each negatively charged surface site since the average distance between charged polymer segments is much smaller than that between negatively charged surface sites. As a result, polycation adsorption gives rise to “patches” of positive and negative charge although the amount adsorbed may be sufficient to neutralize all of

Chapter

5

Positively Charged Polymers (Polycations)

139

the surface negative charge (cf. Figure 2.7C). Here aggregation occurs through electrostatic attraction between positively charged patches on one particle and negatively charged (polycation-free) regions of another particle when the particles collide. Such interparticle contacts may be broken under shear and re-establish afterward. Aggregates formed by the electrostatic patch mechanism tend to be weaker than those produced by polycation bridging but stronger than those formed by (salt) coagulation (Bolto and Gregory, 2007; Gregory, 1978). The effects of molecular weight and cationicity on the electrostatic patch flocculation of silica, kaolinite, and montmorillonite suspensions have been described by several authors (Bratskaya et al., 2005; Durand-Piana et al., 1987; Mabire et al., 1984). The formation of polycation patches on negatively charged latex particles has been demonstrated by Akari et al. (1996) and Popa et al. (2009), using atomic force microscopy. The importance of surface area is also illustrated by the data of Black et al. (1965, 1966). Thus, for kaolinite (where adsorption is confined to external particle surfaces), the electrophoretic mobility corresponding to the optimum polymer dosage (Pm) ranges from zero to 0.7 mms1/Vcm1, whereas for montmorillonite with its larger accessible surface area, this range is between 0.3 and þ0.7 mms1/Vcm1 (Table 5.3). As would be expected, Pm increases with an increase in initial clay concentration. Indeed, there is a strict proportionality between suspended solid content and Pm in the case of kaolinite as Baran and Gregory (1996) have also noted. Again, such a correlation is not so well displayed by montmorillonite even if (polycation) intercalation were absent. A linear relationship between solid content (particle surface area) and optimum polymer dosage was also observed by Linke and Booth (1960) and Kane et al. (1964a) for the aggregation of silica suspensions by polyacrylamide and polyethylenimine, respectively. Similarly, Iler (1971) found that the amount of a polycation required to aggregate a unit weight of silica varies inversely with particle diameter when this is smaller than 40 nm. The relationship between optimum polymer dosage and particle surface area has been further investigated by Black and Vilaret (1969) using kaolinite, latex and silica with different particle size distributions and polydiallyldimethylammonium chloride as the aggregant. The results for kaolinite (Figure 5.4) clearly indicate that the dosage required for maximum aggregation is closely correlated with particle size (i.e. surface area). Further, the rate of aggregation and the size of the flocs formed increase with a decrease in mean particle size. Since the effects of particle size (surface area) and polymer molecular weight on flocculation may not operate in the same direction, a balance must be struck between these two parameters if polycationic aggregants are to be applied to full advantage. Yorke (1973), for example, found that the effectiveness of poly(diallyldimethylammonium chloride) fractions in aggregating kaolinite samples of different particle size decreased below and above a certain polymer molecular weight.

Optimum polymer dosage (mg/L)

140

Formation and Properties of Clay-Polymer Complexes FIGURE 5.4 Relationship between kaolinite suspension concentration and optimum dosage of poly(diallyldimethylammonium chloride). Upper curve: particles with an average equivalent spherical diameter (e.s.d.) of 0.26 mm. Lower curve: particles of 0.69 mm e.s.d. From Black and Villaret (1969).

500

400

300

200

100

0

0

50 150 100 200 Suspension concentration (mg/L)

Earlier, Shyluk (1964) observed that at low shear rates and when the molecular weight of the polymer (poly(1,2-dimethyl-5-vinylpyridinium methyl sulphate)) used was sufficiently high, the rate of floc formation was initially greater than that of floc disruption. As a result, large (kaolinite) flocs were formed enclosing an appreciable amount of active (adsorbing) sites. When agitation was prolonged, however, the rate of floc formation declined because the polycation supply was depleted, while the previously “hidden” surface sites became exposed, giving rise to relatively small and dense flocs. Similarly, Goossens and Luner (1976) reported that the polycation concentration required to aggregate cellulose suspensions increased with the duration and intensity of agitation. The sensitivity of polymer adsorption and aggregate formation to mixing and stirring conditions is a major factor affecting the reproducibility of experimental results (Chaplain et al., 1995). Most studies on the clay–polycation interaction have been carried out at more or less constant, near neutral pH conditions, using polymers with a somewhat wide molecular weight and charge density distribution. The degree of polymerization (n) and degree of charge substitution are, of course, intrinsic properties of the polymer concerned, and some of their effects on dispersion stability have already been described. On the other hand, the pH of the medium (as well as its ionic strength) influences clay colloid stability by modifying the charge characteristics of both the polycation (through its effect on the degree of dissociation) and the clay mineral. In order to separate these effects, Ueda and Harada (1968b) have examined the behaviour of different cationic polymers towards kaolinite suspensions as a function of n and solution pH. To this end, they prepared

Chapter

5

Positively Charged Polymers (Polycations)

141

copolymers of a secondary amine hydrochloride (diallylamine HCl-SO2), a tertiary amine hydrochloride (diallylmethylamine HCl-SO2) and a quaternary ammonium chloride (diallyldimethylammonium-SO2 and diallyldiethylammonium-SO2), ranging in molecular weight from 14,000 to 330,000 Da. The extent of flocculation—more precisely aggregation—was assessed by measuring the change in transmittance of the supernatant liquid. Aggregating efficiency was expressed in terms of an “aggregation value”, defined as the amount of polymer required to increase the transmittance (at 660 nm) of a 2% (w/v) suspension by 50%. Figure 5.5 shows that the aggregation value for poly(diallyldimethylammonium-SO2) copolymer decreased (exponentially) with an increase in the degree of polymerization. Thus, the longer the chain (or the higher the molecular weight), the less polymer was needed to achieve a comparable extent of aggregation. Entirely parallel results were obtained by Durand-Piana et al. (1987) for the aggregation of montmorillonite suspensions by low-charge polycations (cationicity<15%). For polymers with a cationicity in excess of 25%, however, charge neutralization (probably through the electrostatic patch mechanism) becomes dominant, and the optimum polymer dosage is no longer dependent on molecular weight. The effect of pH on aggregation is shown in Figure 5.6 for different cationic polymers with the same mean degree of polymerization (n¼224). Below pH 7, all four polycations were equally effective in aggregating kaolinite suspensions. Differences in aggregating ability, however, emerged under alkaline conditions,

10.0 Aggregation value (meq/L ⫻ 102)

FIGURE 5.5 Aggregation of kaolinite suspensions by poly(diallyldimethylammonium-SO2) copolymer (pH 7.1) as a function of degree of polymerization. From Ueda and Harada (1968b).

5.0

2.0

1.0

0.5

101

103 102 Degree of polymerization

104

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Formation and Properties of Clay-Polymer Complexes

8 A

Aggregation value (meq/L ⫻ 102)

7 6 5

FIGURE 5.6 Effect of suspension pH on the aggregating ability of some cationic copolymers with an average degree of polymerization of 224. Curve A, poly (diallylmethylamine HC1-SO2); curve B, poly(diallylamine HCl-SO2); curve C, poly(diallyldimethylammonium-SO2 and poly(diallyldiethylammonium-SO2). From Ueda and Harada (1968b).

B 4 3 2 1

C

4

5

6

7

8 9 pH

10

11

becoming more pronounced above pH 9. This observation may be ascribed to the influence of pH on the degree of dissociation (a) of the polycation, on the one hand, and the dispersibility of the clay mineral, on the other hand. Since poly(diallyldimethylammonium-SO2) and poly(diallyldiethylammonium-SO2) are quaternary ammonium derivatives, their degree of dissociation is essentially independent of solution pH. Thus, their effectiveness as aggregants remains high over the pH range of 4.5–10.5. Linke and Booth (1960) have reported similarly for the aggregation of silica by polyacrylamide containing 11% quaternary ammonium groups. On the other hand, titration studies indicate that the degree of dissociation of poly(diallylamine HClSO2) and poly(diallylmethylamine HCl-SO2) falls off sharply as the solution pH increases from 6 to 8. Indeed, at pH>9, these polymers exist as uncharged, randomly coiled molecules in solution. As a result, their effectiveness as an aggregant would be greatly diminished. It is interesting to note that although the effective charge of these two polycations at pH 7 is about half that at pH 5, their aggregating ability is only slightly decreased as the pH increases from 5 to 7 (Figure 5.6, curves A and B). This observation argues for the importance of interparticle bridging. Variations in pH also affect clay particle dispersibility and accessibility. As mentioned in Chapter 1, the edge surface of kaolinite particles becomes increasingly negative as the solution pH is raised above 7 (cf. Figure 1.6). In line with this behaviour, Tekin et al. (2005) found that the adsorption of

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a cationic polyacrylamide by kaolinite increased more than twofold when the suspension pH was increased from 7 to 10.5. A small increase in uptake with an increase in pH (from 3 to 6) was also reported by Gill and Herrington (1986). Besides disrupting any edge-to-face association, alkaline pH conditions would enhance interparticle repulsion and increase the mean distance between suspended particles, leading to a reduction in the rate and extent of interparticle bridging and collision. In this instance, the effect of pH on particle dispersion is apparently less important than that on the charge characteristics of the polymer (Figure 5.6, curve C). Positively charged polymers have been used to good effect in removing turbidity and “colour” from surface waters (Cohen et al., 1958; Dixon, 1967; Pressman, 1967). These characteristics arise from the presence in the water of suspended clay and mineral particles, together with humic substances in solution or associated with particle surfaces (cf. Chapter 12). Such organic materials and their mineral complexes tend to “interfere” with the aggregating action of polycations in that the clay and the organic acids compete with each other for the polymer (Narkis et al., 1968). Thus, when a polyethyleneimine solution is added to a clay or organoclay suspension containing (free) humic acid in solution, the polycation apparently reacts with the highly mobile, negatively charged humic acid before acting on the clay. Since a portion of the added polymer is used to neutralize the charge on the free or adsorbed humic acid, larger concentrations or dosages of the polycation are required for aggregation to occur. By the same token, the high adsorptive capacity of montmorillonite for a wide variety of organic compounds (Theng, 1974), including the reaction products of polycations with dissolved organics, makes montmorillonite well suited for removing contaminants from municipal wastewater. The application of montmorillonite in combination with cationic polymers to secondary effluents therefore lifts the efficiency of turbidity removal and decreases the optimum polycation dosage (Rebhun et al., 1969). “Competition” of a different kind has been described by Roberts et al. (1974) in the aggregation of kaolinite suspensions by a cationic polyacrylamide. In this instance, the competing or interfering substance is the positively charged hydroxy-aluminium ion. When this polymeric inorganic ion is added to the system in sufficiently large amounts (
30 ppm), its adsorption by kaolinite reverses the negative surface charge of the clay particles. The polycation is now effectively repelled from the particle surface, and its flocculating efficiency is thereby markedly reduced. To complete this chapter, we will briefly describe the formation and properties of clay–polycation multilayer films. These “hybrid” nanostructural assemblies may be obtained through essentially two different approaches: self-assembling (“layer-by-layer”) and Langmuir–Blodgett (LB) techniques. The former method has the advantage of being simple and rapid, while the latter offers a relatively high degree of control over film organization. This is because LB films are formed at the gas/liquid interface, allowing the lateral distribution of clay platelets and organic cations to be controlled as the hybrid

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monolayer is compressed. Since the LB method commonly uses long-chain n-alkylammonium ions—rather than positively charged polymers—we will only sketch out the basic elements. Self-assembling (“fuzzy-assembling”) refers to the alternate deposition of layers of positively charged polymers and smectite on a suitable substrate, such as glass and mica. van Duffel et al. (1999), for example, were able to construct the initial poly(diallyldimethylammonium) (PDDA) layer by spreading a few drops of the polycation solution over the negatively charged mica surface, washing away the excess polymer with water and allowing the water to evaporate. This was followed by depositing a few drops of the clay mineral suspension on the polycation layer, washing and drying as before. Multilayers were obtained by repeating the polycation/clay monolayer deposition cycle, and the overall film thickness increased linearly with the number of cycles (Figure 5.7). Atomic force microscopy (AFM) indicated that the clay platelets in each clay layer were separated by empty space, forming a “sub-monolayer” of randomly oriented and partially overlapping platelets (Tang et al., 2003; van Duffel et al., 1999). As a result, extrapolation of film thickness to zero layers did not go through the origin, while film “roughness” increased with the number of cycles and was proportional to the polycation concentration. Schoonheydt (2002) has suggested that at low coverage by PDDA, the polymer chains were adsorbed in an extended conformation to give a relatively smooth film surface. At high coverages, however, the adsorbed polycations would adopt a conformation with long loops and tails, giving rise to a rough film surface. Using AFM,

n

*

*

N+ Cl– Clay platelet

PDDA

PDDA solution

Clay suspension

FIGURE 5.7 Diagram showing the formation of a multilayer film, using the self-assembling method. The film is made up of alternating poly(diallyldimethylammonium) (PDDA) polymers and clay platelets deposited on a mica substrate (hatched rectangle). From Schoonheydt (2002).

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Tang et al. (2003) have demonstrated that the preferred conformation of PDDA (molecular weight¼200,000 Da) in densely packed films at high concentrations was that of a tightly coiled chain. Besides being dependent on surface coverage, film roughness is also influenced by the size and shape of the clay particles. Thus, for films of approximately 80 nm in thickness, a roughness value of 10–12 nm was measured with hectorite but only 4–5 nm in the case of Laponite. This is because hectorite particles are heterogeneous in size and shape, whereas Laponite consists of very small disc-shaped particles showing minimal overlap (Schoonheydt and Johnston, 2006; van Duffel et al., 1999). Structural order has also been observed by Glinel et al. (2001) for multilayer films of Laponite with different poly(ammonium) salts and by Kim et al. (2002) for films of ultrathin saponite platelets with substituted ionic polyacetylenes. Functional films may be prepared by introducing compounds with the desired properties. Using layer-by-layer deposition, Kleinfeld and Ferguson (1995), for example, have prepared multilayer films of Laponite and PDDA capable of taking up water vapour rapidly and reversibly. By incorporating an anionic dye, NAMO (4-[4-(n-allyl,N-methylamino) phenylazo] benzenesulphonic acid), into self-assembling smectite/PDDA multilayers, van Duffel et al. (2001) have obtained films with non-linear optical properties. When illuminated with a Nd: YAG laser, the film generated light (at 532 nm) whose intensity depended not only on the content of PDDA but also on the type of clay mineral, in the film. It would therefore appear that the amount and orientation of dye molecules in the film depended on the PDDA loading. The layer-by-layer method was also used by Mamedov and co-workers (Mamedov and Kotov, 2000; Mamedov et al., 2000) to prepare PDDA/smectite/magnetite (Fe3O4) films showing magnetic properties. Similarly, Eckle and Decher (2001) were able to synthesize organic light-emitting diodes based on self-assembling poly(p-phenylenevinylene) and poly(methacrylic acid) including montmorillonite as an isolating layer. As already remarked on, organized films with minimal overlap between clay platelets may be obtained using the LB technique. Such films may be prepared in one of two ways. Early attempts involve suspending an organically modified clay mineral (cf. Chapter 7) in a volatile organic solvent (e.g. chloroform), spreading the dilute suspension over the water surface in an LB trough, evaporating the solvent, compressing the clay film and transferring it to a suitable substrate, such as a hydrophobic glass plate (Hotta et al., 1997a,b). In the alternative approach, a solution of an amphiphilic organic cation (e.g. n-octylammonium) in an organic solvent is spread over the surface of a dilute aqueous suspension of Naþ- or Liþ-exchanged smectite in the LB trough (Umemura et al., 2001a,b). The Naþ or Liþ counterions on the elementary smectite layers are instantaneously replaced by the amphiphilic cations (at the air/water interface), giving an organoclay monolayer that can be compressed and then transferred to a glass substrate by either vertical or horizontal deposition. If the substrate is hydrophilic, vertical deposition is preferred,

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whereas horizontal pick-up is performed if a hydrophobic glass plate is used. In the first case, the sequence is substrate/clay platelet/amphiphilic cation, whereas in the second instance, the sequence of substrate/amphiphilic cation/clay platelet applies. Multilayered films are formed by repeating the above process. Umemura et al. (2001a,b) found that both the film thickness and amount of amphiphilic organic cations in each layer increased linearly with the number of layers deposited, indicating that the overall layer composition was practically invariant. Here again, molecules with some desired functionality may be inserted to obtain functional films (Ras et al., 2007).

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