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Stability versus flocculation of particle suspensions in water—correlation with the extended DLVO approach for aqueous systems, compared with classical DLVO theory
Colloids and Surfaces B: Biointerfaces 14 (1999) 47 – 55 www.elsevier.nl/locate/colsurfb
Stability versus flocculation of particle suspensions in water —correlation with the extended DLVO approach for aqueous systems, compared with classical DLVO theory W. Wu a, R.F. Giese b, C.J. van Oss b,c,d,* a
Department of Chemistry, State Uni6ersity of New York at Buffalo, New York, NY 14214, USA Department of Geology, State Uni6ersity of New York at Buffalo, New York, NY 14214, USA c Department of Chemical Engineering, State Uni6ersity of New York at Buffalo, New York, NY 14214, USA d Department of Microbiology, State Uni6ersity of New York at Buffalo, New York, NY 14214, USA b
Abstract Stability versus flocculation is studied for aqueous suspensions of a variety of mineral particles (e.g. clay, asbestos, glass), via the extended DLVO (XDLVO) approach (which includes Lewis acid – base interactions in addition to van der Waals and electrostatic interactions), as well as via classical DLVO theory, as a function of absence or presence of plurivalent counterions. Also discussed are XDLVO and DLVO analyses of polymers, biopolymers, cells and phospholipids, in aqueous media, under similar conditions. It is concluded that, in aqueous media, XDLVO analysis practically always describes the interactions of immersed or dissolved particles, cells, vesicles, polymers, biopolymers or phospholipids more accurately than classical DLVO theory. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Particle suspensions; Stability; Flocculation
1. Introduction In water (w), completely apolar materials (i) undergo a mutual hydrophobic attraction which = − 102 mJ m − 2 (at amounts to DG hydrophobic iwi 20°C). In comparison with this attractive energy, the Lifshitz – van der Waals (LW) attraction between biological or mineral particles immersed in water usually only varies between DG LW iwi : 0.5 to : −7.0 mJ m − 2. Electrostatic repulsive energies, LW DG EL iwi , tend to be in the same range as DG iwi (but * Corresponding author. Tel.: +1-716-29-2900; fax: + 1716-829-2158.
of the opposite sign) or lower, for most biological molecules, particles or cells. The hydrophobic attraction is driven by the hydrogen-bonding (i.e. in more general terms, Lewis acid–base, or AB) free energy of cohesion of the water molecules of the liquid medium. In other words [1–3] DG hydrophobic = DG AB iwi ww
(1)
Eq. (1) indicates that the hydrophobic effect is always present in all interactions taking place in aqueous media, because the free energy of cohesion of water is always there. Thus, a net hydrophilic repulsion between particles or molecules immersed in water must have a (repulsive) free
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energy (i.e. a positive DG hydrophilic ) which exceeds iwi 2 the value of DG AB iwi = −102 mJ m (at 20°C). In practice, when a net hydrophilic repulsion occurs between particles or molecules in water, it is ascribed to (see [4], p. 44): DG hydrophilic =4 g iwi w gi ,
(2)
where g w is the electron-accepting parameter of the polar surface tension component of water, which, at 20°C, is established as equal to 25.5 mJ m − 2 [5], and g i is the electron-donating parameter of the polar surface tension component of hydrophilic molecule or particle, i, and is also equal to 25.5 mJ m − 2 at 20°C. Mutual hydrophilic repulsion of particles or molecules, immersed in water, generally occurs with strongly electron-donating (and close to zero electron-accepting) monopolar materials [5]. To achieve actual hydrophilic repulsion, g must not only be i greater than 25.5 mJ m − 2, but it usually must be at least about 28.4 mJ m − 2 to be able to surmount not only the always-present hydrophobic attraction caused by the aqueous medium, but also to overcome the small, but not negligible, Lifshitz–van der Waals attraction ([4], p. 209). Thus, in all cases of non-covalent interactions between particles and/or molecules, immersed in water, one must take the net DG AB iwi -values into account, whether attractive or repulsive. In addition, even when particles are exceedingly close to being hydrophobically/hydrophilically neutral, only a DLVO-approach, extended to include Lewis acid – base interactions (XDLVO-approach), can describe the stability or instability of their suspension behavior with some degree of accuracy. Cases in point have been: the stability versus flocculation behavior of suspensions of the smectite clay mineral, hectorite, as a function of ionic strength [6], and; the stability versus flocculation of another smectite clay mineral, Na-montmorillonite, coated with organic matter from river water, as a function of the degree of ozonation [7]. The stability of suspensions of blood erythrocytes and leukocytes only correlates with XDLVO analysis, on account of the predominant role of AB repulsion in that stability [4,8]. Similarly, the aqueous solubility of proteins, e.g. fibrinogen
closely correlates with XDLVO analysis, whereas classical DLVO would predict insolubility [9]. Finally, the degree of adhesion of the bacterium, Pseudomonas aeruginosa at different growth phases, onto dolomite particles, correlates with XDLVO, but not with DLVO analysis [10]. The bacteria showed very little adhesion when taken at the logarithmic or decay growth phases, but manifested strong adhesion onto the dolomite particles when harvested at the stationary growth phase [10]. The reason for this is that these bacteria are very hydrophilic at the logarithmic and decay growth phase, but less hydrophilic at the stationary phase. Dolomite particles are of intermediate hydrophobicity.
2. Materials and methods In this paper the stability versus flocculation is studied for a number of well-characterized mineral particles [11], suspended in a 10× diluted physiological saline buffer and analyzed via the XDLVO as well as the classical DLVO approach. The particles include a smectite clay mineral, hectorite, as well as a number of asbestos particles, i.e. samples of chrysotile from three different localities (‘white’ or serpentine asbestos) and one sample of crocidolite (‘blue’ or amphibole asbestos). (The latter type of asbestos particle is of the category held to be among the most dangerous, in causing severe pulmonary pathology in man, in the form of mesothelioma [12]). In addition, the change brought about in the stability of an aqueous suspension of glass particles upon addition of 0.47 mM LaCl3 is analyzed via the XDLVO and the classical DLVO approach [1,13]. The surface tension components and parameters of the various particles studied, as well as their z-potentials, were measured earlier, and were part of a long list of surface and electrokinetic properties of clays and other mineral particles previously published [11]. The z-potentials were measured in a 10 × diluted physiological saline buffer, pH 7.4, ionic strength m =0.015 (PBS/10); the experiments portrayed here were all done in the same buffer, with Na + as counterion, except in those cases where small amounts of
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 14 (1999) 47–55 Table 1 Surface tension components and parameters of mineral particles from contact angle measurements (reported in mJ m−2) and z-potentials in PBS/10 (in mV) Minerala
g LW
g+
g−
z
Hectorite Glass b Glass+La3+ Crocidolite (Cape Town, SA) Chrysotile (Globe, AZ) Chrysotile (Salt River, AZ) Chrysotile (Zimbabwe)
39.9 31.5 30.3 37.4
0 0.4 0.2 0.9
23.7 37.1 20.9 23.9
−37.3 −52.7 −16.4 −40.8
35.1 42.7 37.7
0 0 0
31.2 5.0 6.2
−31.0 −34.2 −46.7
a
Glass particle suspension flocculated in the presence of 0.47 mM LaCl3; see [13]. b See [10].
La3 + ions were added, in these cases the ionic strength being kept at m= 0.015 by concomitantly reducing the Na + concentration. The surface ten sion components (g Lw i , g i , g i ) and the z-potentials of the particles used are shown in Table 1. The equations used for translating the g- and z-values, given in Table 1, to obtain the DGiwi-values shown in Table 2, can be found in [4], pp. 14–15 (Lifshitz – van der Waals interactions); pp. 20, 23 (Lewis acid – base interactions); and pp. 46, 48 (electrostatic interactions). The DGiwi-values given in Table 2 are the values at the minimum equilibrium distance, l0, between non-covalently interacting particles or molecules (l0 :0.157 nm), AB EL cf. [4], pp. 76 ff. The DG LW iwi , DG iwi and DG iwi -values at l\l0 are obtained by means of equations given in [4], pp. 75, 80, 82. The particles discussed
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here have been assumed, for calculation purposes, to be close to spherical, with a radius R= 1.0 mm, but it should be noted that these mineral particles are not really ideally spherical. (However, the asbestos fibers were reduced to a fair degree by first cutting them into small pieces (about 0.1 mm long), using a sharp blade. Then the fibers were placed in a commercial blender (1 wt% w/v in water) and blended at moderate speed for 5 min). The close encounter between particles, when flocculated, usually takes place between protrusions and/or edges with a radius rBR. Nonetheless, as all interaction energies (DG LW, DG AB and DG EL) are equally proportional to the radius, their final values in energy units remain proportional to R= 1.0 mm (cf. Table 2). In reality these values may be slightly smaller, when the radii of contacting protrusions are somewhat smaller than R. However, the proportionality between the DG LW, DG AB and DG EL values remain preserved (cf. Table 3), so that the DLVO and XDLVO diagrams remain the same except for the DG-scale, which may be somewhat reduced. Thus, the irregular shape of the particles does not influence the outcome in terms of flocculation versus stability.
3. Results and discussion
3.1. Hectorite particles Fig. 1 shows the stability versus flocculation of 0.5% (w/v) suspensions of the smectite clay mineral, hectorite: (A) in 0.015 M NaCl; (B) in 1/10
Table 2 The free energy (in kT) of interaction between particles (1) immersed in aqueous medium (PBS/10, w) at minimum equilibrium separation distance l0
Hectorite Glass a Glass+La3+ Crocidolite (Cape Town, SA) Chrysotile (Globe, AZ) Chrysotile (Salt River, AZ) Chrysotile (Zimbabwe) a
DG LW 1w1 (l0)
DG AB 1wl (l0)
DG EL lw1 (l0)
DG TOT lw1 (l0)
Photos
−660 −220 −170 −510 −380 −850 −530
−2800 +14 000 −6700 −2000 8400 −44 000 −40 000
850 +3000 +770 1300 1000 1300 2400
−2000 +17 000 −6100 −740 +9000 −43 700 −38 000
(A), (B), (C) – – (E) (F) (G) (D)
Glass particle suspension flocculated in the presence of 0.47 mM LaCl3; see [13].
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Table 3 Free energies of interaction (DGiwi) between spherical particles, i, of radius, R, immersed in water, w, as a function of distance, l Configuration
Equation
Unretarded Lifshitz–van der Waals (LW) energies of interaction, expressed as a function of the 2a,b Hamaker constant, A = −DG LW%% iwi ×12pl0
DG LW iwi =−(AR/12l)
Lewis acid–base (AB) energies of interactiona (l is the decay length of water at 20°C, = 1.0 nm) DG AB iwi =pRlDG AB%% iwi exp[(l0−l)/l] Electrostatic (EL) energies of interaction (c0 is the surface potential at the particle’s surface; o is the dielectric constant and k is the inverse thickness of the diffuse ionic double layer, cf. [4], p. 82.)
2 DG EL iwi =(1/2)oRc 0 ln [1+exp(−kl)]
DG LW%% and DG AB%% indicate the free energies of interaction between two plane parallel plates, at the minimum equilibrium iwi iwi distance, l = l0. b l0 =0.157 nm; cf. [4], pp. 75 and 80. a
physiological saline buffer, pH 7.4, m = 0.015; (C) in 0.026 M LaCl3, pH 7.4, m= 0.015. Fig. 1, top, shows the suspensions immediately after preparation; middle, after 10 min; bottom, after 40 min. Clearly, the admixture of La3 + causes flocculation in less than 10 min (Fig. 1, middle) and at 40 min flocculation begins at just m =0.015 with NaCl, buffered or not. Fig. 2 shows that classical DLVO theory predicts total stability (at m= 0.015, without La3 + ), which is clearly not the case, although the 0 and 10 min photographs do show that a certain degree of (early) metastability is in evidence. This agrees well with the XDLVO graph also shown in Fig. 2; there is a secondary maximum of repulsion of about + 400 kT, at a distance of l : 2.5 nm. For monosized, completely spherical particles this secondary maximum of repulsion might cause a fairly long-lasting stability, but with more irregularly shaped particles localized sites (e.g. edges of some particles) can approach each other more closely, so that, as in this case, flocculation commences in less than 40 min, succumbing to a deep trough of attraction of at least −1000 kT. The influence of La3 + ions is discussed in more detail in Section 3.3.
3.2. Asbestos particles Shown in Fig. 3 are 0.5% (w/v) suspensions of asbestos particles (reduced to fairly symmetrical
particles from the original fibers): (E) is crocidolite; (D), (F), and (G) are chrysotile particles from, respectively, Zimbabwe, Globe, AZ, and Salt River, AZ. Initially, all asbestos particle suspensions were stable (Fig. 3, top) but within 30 min (Fig. 3, middle) chrysotiles (D) and (G) had flocculated ((G) even more vigorously than (D)), and crocidolite (E) also started to flocculate. The classical DLVO plots (Fig. 4) wrongly predict total stability of all the asbestos particles. Table 2 makes it quite clear that in these aqueous asbestos suspensions the strong hydrophobic attractions occurring with the chrysotile from Salt River, AZ and Zimbabwe ((G) and (D)), cannot be neglected. The hydrophobic attraction between crocidolite particles (E) is somewhat smaller, but still not negligible, as seen in Fig. 3, bottom, taken after 4 h. Thus again, in water, one has to revert to XDLVO plots (which include the Lewis acid– base interactions, which are, inter alia, responsible for the hydrophobic effect) to obtain a good correlation between prediction and experiment. From Fig. 5, showing the XDLVO plots for the four asbestos particles, it can be seen that the only really stable suspension should be obtained with the chrysotile from Globe, AZ ((F), in Fig. 3), which is indeed the case. Next, crocidolite (E) which has a sizable secondary maximum of repulsion of about + 600 kT, at l: 1.5 nm should be fairly stable for some time, as is indeed the case,
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 14 (1999) 47–55
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as it only shown incipient flocculation at 4 h. Finally, the strongly hydrophobic chrysotiles from Salt River, AZ and Zimbabwe ((G) and (D)) should have only faint metastability, from the XDLVO plot (Fig. 5) and indeed flocculate in 30 min (Fig. 3, middle).
Fig. 2. DLVO and XDLVO plots of the hectorite suspension shown in Fig. 1B.
3.3. Glass particles, hydrophobized with LaCl3
Fig. 1. Suspensions of 0.5% (w/v) of the smectite, hectorite, in: (A) 0.015 M NaCl; (B) PBS/10; (C) 0.026 M LaCl3. Top, t =0 min; middle, t= 20 min; bottom, t= 40 min.
It has been noted earlier [1,13] that low concentrations of plurivalent counterions induce flocculation in aqueous suspensions of charged particles [14] by making them hydrophobic and not, as was thought before, because these counterions reduce LW DG EL iwi to a lower value than DG iwi . A case in point is the influence of minute amounts of La3 + on the stability of glass particles. Admixture of 0.47 mM LaCl3 in a buffer of pH 7.4, m =0.015, sufficed to flocculate a glass particle suspension which, when suspended at pH 7.4, m= 0.015 with just NaCl, was totally stable (photographs not shown here). As can be seen from Table 2, whereas La3 + clearly reduced DG EL iwi for the glass particles from + 3000 to + 770 kT, the latter repulsive energy was still significantly higher than the van der Waals attraction, DG LW iwi = − 170 kT. What clearly happened was than concomitantly to the admixture of La3 + the particles’ surfaces switched from being very hydrophilic (DG AB iwi = + 14 000 kT) to quite hydrophobic, at DG AB iwi =
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W. Wu et al. / Colloids and Surfaces B: Biointerfaces 000 (1999) 000–000
− 6700 kT (cf. Table 2). The reason for this is that when plurivalent cations, which act as electron-acceptors (Lewis acids), combine with negatively charged particles which also are electron-donors (Lewis bases; cf. the high value for g of 37.1 mJ m − 2 for untreated glass partii cles in Table 1), the surfaces of such particles become hydrophobic through the decrease in g i
Fig. 4. DLVO plots of the asbestos particles shown in Fig. 3.
(to 20.9 mJ m − 2) caused by the La3 + –glass interaction. Fig. 6 shows the classical DLVO treatment of glass particles+La3 + , wrongly suggesting complete stability. Fig. 7 comprises the
Fig. 3. Suspensions of 0.5n˜5 (w/v) asbestos particles in PBS/10. (D) Chrysotile, Zimbabwe; (E) crocidolite; (F) chrysotile, Globe, AZ; (G) chrysotile, Salt River, AZ.
Fig. 5. XDLVO plots of the asbestos particles shown in Fig. 3.
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 14 (1999) 47–55
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4.1. Hydrophobic particles or macromolecules In aqueous suspensions of all types of hydrophobic particles, neglecting DG AB iwi (which is always negative, i.e. attractive) almost invariably leads to a (DLVO-based) prediction of stability, except when the particles have an unusually low (or zero) z-potential, which is fairly rare. Examples of this are shown in Fig. 3, with the very hydrophobic chrysotiles, (G) and (D), (see also Figs. 4 and 5) but also with the more mildly hydrophobic hectorite (Figs. 1 and 2) and crocidolite (E) (Figs. 3–5) (cf. Sections 3.1 and 3.2, above).
4.2. Particles that are close to the hydrophobic/hydrophilic transition
Fig. 6. DLVO plots of glass particles suspended in PBS/10, with 0.047 mM LaCl3.
XDLVO plot of glass particles + La3 + correctly predicting the strong flocculation which was observed experimentally [1,13]. The hydrophobizing interaction of Ca2 + on negatively charged phospholipid surfaces was first noted by Ohki in 1982 [15]. The mechanism of this interaction, as described above, was first proposed in 1988 [16]. However, although other mechanisms for the influence of Ca2 + on phospholipids, involving their orientation and/or conformation have been proposed [17], the hydrophobizing effect of plurivalent cations on hard mineral particles which are negatively charged [1,13] would argue that the same hydrophobizing effect of Ca2 + and La3 + is also operative on negatively charged (but not on neutral) phospholipids [18].
Particles which are hydrophobically/hydrophilically close to the borderline of transition between the two states are exceedingly sensitive to the ionic strength of the liquid medium. In such cases DLVO analysis is less sensitive to changes in NaCl-concentrations (e.g. 16, 82 and 410 mM [7])
4. Conclusions The effectiveness of XDLVO versus DLVO analyses of four different categories of aqueous suspensions can be distinguished.
Fig. 7. XDLVO plots of glass particles suspended in PBS/10, with 0.047 mM LaCl3.
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W. Wu et al. / Colloids and Surfaces B: Biointerfaces 000 (1999) 000–000
than the XDLVO approach, which conforms closely to the experimental results [7].
4.3. Hydrophilic particles or macromolecules with a low z-potential One example of these is mammalian (e.g. human) peripheral blood cells [8], especially leukocytes, which would not be able to form a lasting stable suspension (as in blood) according to classical DLVO theory, but are predicted to form perfectly stable suspensions by the XDLVO approach [4,8]. With the zero z-potential [19] biopolymer, dextran [4], DLVO analysis would, ipso facto, entail only a van der Waals attraction, which, for a large molecule, would of necessity mean total insolubility. XDLVO analysis, on the other hand, shows a strong mutual repulsion between the dextran molecules of DG AB iwi : + 18 kT, (at l=l0) which correlates with total aqueous solubility, which is indeed the case.
4.4. Hydrophilic particles or macromolecules with a high z-potential, subject to admixture of pluri6alent counterions or pH changes A typical example is glass powder (cf. Section 3.3, above), which, as is, would be predicted to form an exceedingly stable suspension in water, by classical DLVO as well as by XDLVO analysis (due to the high DG EL iwi and the very high DG AB -values; cf. Table 2). Upon admixture of a iwi small amount of LaCl3, DG EL iwi is reduced about four-fold, but is still positive, i.e. repulsive (cf. Table 2), so that via classical DLVO analysis, stability should still prevail (Fig. 6). In reality LaCl3 admixture also causes a severe hydrophobization (together with a significant but not fatal decrease in z-potential) of the glass particles, which changes DG AB iwi from +14 000 to −6700 kT, resulting in an XDLVO prediction of total instability (Fig. 7), in agreement with the experimental observation. Lowering the pH to approach the isoelectric point of polymers [20], biopolymers [4], or phospholipids [18] immersed in water also causes hydrophobization [18,20]
(well before the point of zero charge is reached). Here also XDLVO plots offer the best correlation with experimental results, and they are liable to differ from DLVO theory.
4.5. Summarizing conclusion In aqueous media it is always essential to incorporate Lewis acid–base interactions in energy versus distance analyses, i.e. it is safest for the study of particles, cells, phospholipids membranes and vesicles, and macromolecules immersed in water to use the XDLVO approach, as outlined above.
References [1] W. Wu, Linkage between z-potential and electron donicity of charged polar surfaces, Ph.D. SUNY/Buffalo, (1994) 185. [2] C.J. van Oss, Colloids Surfaces B: Biointerfaces 5 (1995) 91. [3] C.J. van Oss, Curr. Opin. Colloid Interface Sci. 2 (1997) 503. [4] C.J. van Oss, Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994. [5] C.J. van Oss, M.K. Chaudhury, R.J. Good, Ad. Colloid Interface Sci. 28 (1987) 35. [6] C.J. van Oss, R.F. Giese, P.M. Costanzo, Clays Clay Miner. 38 (1990) 151. [7] P. Chheda, D. Grasso, Langmuir 10 (1994) 1044. [8] C.J. van Oss, Cell Biophys. 14 (1989) 1. [9] C.J. van Oss, in: H. Visser (Ed.), Protein Interactions, VCH, Weinheim, New York, 1992, p. 25. [10] D. Grasso, B.F. Smets, K.A. Strevett, B.D. Machinist, C.J. van Oss, R.F. Giese, W. Wu, Environ. Sci. Tech. 30 (1996) 3604. [11] R.F. Giese, W. Wu, C.J. van Oss, J. Disp. Sci. Tech. 17 (1996) 527. [12] C.J. van Oss, J.O. Naim, R.F. Giese, W. Wu, A.F. Sorling, Clays Clay Miner. (1999) (in press). [13] W. Wu, R.F. Giese, C.J. van Oss, Colloid Surf. A 89 (1994) 241. [14] J.Th.G. Overbeek, in: H.R. Kruyt (Ed.), Colloid Science, Elsevier, Amsterdam, 1952, p. 194. [15] S. Ohki, Biochim. Biophys. Acta 689 (1982) 1. [16] C.J. van Oss, M.K. Chaudhury, R.J. Good, in: S. Ohki, D. Doyle, T.D. Flanagan, S.W. Hui, E. Mayhew (Eds.), Molecular Mechanisms of Membrane Fusion, Plenum Press, New York, 1988, p. 113.
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 000 (1999) 000–000 [17] D. Papahadjopoulos, W.J. Vail, C. Newton, S. Nir, K. Jacobson, G. Poste, R. Lazo, Biochim. Biophys. Acta 465 (1977) 579. [18] M. Mirza, Y. Guo, K. Arnold, C.J. van Oss, S. Ohki, J. Disp. Sci. Tech. 19 (1998) 951.
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[19] S.C. Edberg, P.M. Bronson, C.J. van Oss, Immunochemistry 9 (1972) 273. [20] S.R. Holmes-Farley, R.H. Reamey, T.J. McCarthy, J. Deutch, G.M. Whitesides, Langmuir 1 (1985) 725.
. .
Stability versus flocculation of particle suspensions in water —correlation with the extended DLVO approach for aqueous systems, compared with classical DLVO theory W. Wu a, R.F. Giese b, C.J. van Oss b,c,d,* a
Department of Chemistry, State Uni6ersity of New York at Buffalo, New York, NY 14214, USA Department of Geology, State Uni6ersity of New York at Buffalo, New York, NY 14214, USA c Department of Chemical Engineering, State Uni6ersity of New York at Buffalo, New York, NY 14214, USA d Department of Microbiology, State Uni6ersity of New York at Buffalo, New York, NY 14214, USA b
Abstract Stability versus flocculation is studied for aqueous suspensions of a variety of mineral particles (e.g. clay, asbestos, glass), via the extended DLVO (XDLVO) approach (which includes Lewis acid – base interactions in addition to van der Waals and electrostatic interactions), as well as via classical DLVO theory, as a function of absence or presence of plurivalent counterions. Also discussed are XDLVO and DLVO analyses of polymers, biopolymers, cells and phospholipids, in aqueous media, under similar conditions. It is concluded that, in aqueous media, XDLVO analysis practically always describes the interactions of immersed or dissolved particles, cells, vesicles, polymers, biopolymers or phospholipids more accurately than classical DLVO theory. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Particle suspensions; Stability; Flocculation
1. Introduction In water (w), completely apolar materials (i) undergo a mutual hydrophobic attraction which = − 102 mJ m − 2 (at amounts to DG hydrophobic iwi 20°C). In comparison with this attractive energy, the Lifshitz – van der Waals (LW) attraction between biological or mineral particles immersed in water usually only varies between DG LW iwi : 0.5 to : −7.0 mJ m − 2. Electrostatic repulsive energies, LW DG EL iwi , tend to be in the same range as DG iwi (but * Corresponding author. Tel.: +1-716-29-2900; fax: + 1716-829-2158.
of the opposite sign) or lower, for most biological molecules, particles or cells. The hydrophobic attraction is driven by the hydrogen-bonding (i.e. in more general terms, Lewis acid–base, or AB) free energy of cohesion of the water molecules of the liquid medium. In other words [1–3] DG hydrophobic = DG AB iwi ww
(1)
Eq. (1) indicates that the hydrophobic effect is always present in all interactions taking place in aqueous media, because the free energy of cohesion of water is always there. Thus, a net hydrophilic repulsion between particles or molecules immersed in water must have a (repulsive) free
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energy (i.e. a positive DG hydrophilic ) which exceeds iwi 2 the value of DG AB iwi = −102 mJ m (at 20°C). In practice, when a net hydrophilic repulsion occurs between particles or molecules in water, it is ascribed to (see [4], p. 44): DG hydrophilic =4 g iwi w gi ,
(2)
where g w is the electron-accepting parameter of the polar surface tension component of water, which, at 20°C, is established as equal to 25.5 mJ m − 2 [5], and g i is the electron-donating parameter of the polar surface tension component of hydrophilic molecule or particle, i, and is also equal to 25.5 mJ m − 2 at 20°C. Mutual hydrophilic repulsion of particles or molecules, immersed in water, generally occurs with strongly electron-donating (and close to zero electron-accepting) monopolar materials [5]. To achieve actual hydrophilic repulsion, g must not only be i greater than 25.5 mJ m − 2, but it usually must be at least about 28.4 mJ m − 2 to be able to surmount not only the always-present hydrophobic attraction caused by the aqueous medium, but also to overcome the small, but not negligible, Lifshitz–van der Waals attraction ([4], p. 209). Thus, in all cases of non-covalent interactions between particles and/or molecules, immersed in water, one must take the net DG AB iwi -values into account, whether attractive or repulsive. In addition, even when particles are exceedingly close to being hydrophobically/hydrophilically neutral, only a DLVO-approach, extended to include Lewis acid – base interactions (XDLVO-approach), can describe the stability or instability of their suspension behavior with some degree of accuracy. Cases in point have been: the stability versus flocculation behavior of suspensions of the smectite clay mineral, hectorite, as a function of ionic strength [6], and; the stability versus flocculation of another smectite clay mineral, Na-montmorillonite, coated with organic matter from river water, as a function of the degree of ozonation [7]. The stability of suspensions of blood erythrocytes and leukocytes only correlates with XDLVO analysis, on account of the predominant role of AB repulsion in that stability [4,8]. Similarly, the aqueous solubility of proteins, e.g. fibrinogen
closely correlates with XDLVO analysis, whereas classical DLVO would predict insolubility [9]. Finally, the degree of adhesion of the bacterium, Pseudomonas aeruginosa at different growth phases, onto dolomite particles, correlates with XDLVO, but not with DLVO analysis [10]. The bacteria showed very little adhesion when taken at the logarithmic or decay growth phases, but manifested strong adhesion onto the dolomite particles when harvested at the stationary growth phase [10]. The reason for this is that these bacteria are very hydrophilic at the logarithmic and decay growth phase, but less hydrophilic at the stationary phase. Dolomite particles are of intermediate hydrophobicity.
2. Materials and methods In this paper the stability versus flocculation is studied for a number of well-characterized mineral particles [11], suspended in a 10× diluted physiological saline buffer and analyzed via the XDLVO as well as the classical DLVO approach. The particles include a smectite clay mineral, hectorite, as well as a number of asbestos particles, i.e. samples of chrysotile from three different localities (‘white’ or serpentine asbestos) and one sample of crocidolite (‘blue’ or amphibole asbestos). (The latter type of asbestos particle is of the category held to be among the most dangerous, in causing severe pulmonary pathology in man, in the form of mesothelioma [12]). In addition, the change brought about in the stability of an aqueous suspension of glass particles upon addition of 0.47 mM LaCl3 is analyzed via the XDLVO and the classical DLVO approach [1,13]. The surface tension components and parameters of the various particles studied, as well as their z-potentials, were measured earlier, and were part of a long list of surface and electrokinetic properties of clays and other mineral particles previously published [11]. The z-potentials were measured in a 10 × diluted physiological saline buffer, pH 7.4, ionic strength m =0.015 (PBS/10); the experiments portrayed here were all done in the same buffer, with Na + as counterion, except in those cases where small amounts of
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 14 (1999) 47–55 Table 1 Surface tension components and parameters of mineral particles from contact angle measurements (reported in mJ m−2) and z-potentials in PBS/10 (in mV) Minerala
g LW
g+
g−
z
Hectorite Glass b Glass+La3+ Crocidolite (Cape Town, SA) Chrysotile (Globe, AZ) Chrysotile (Salt River, AZ) Chrysotile (Zimbabwe)
39.9 31.5 30.3 37.4
0 0.4 0.2 0.9
23.7 37.1 20.9 23.9
−37.3 −52.7 −16.4 −40.8
35.1 42.7 37.7
0 0 0
31.2 5.0 6.2
−31.0 −34.2 −46.7
a
Glass particle suspension flocculated in the presence of 0.47 mM LaCl3; see [13]. b See [10].
La3 + ions were added, in these cases the ionic strength being kept at m= 0.015 by concomitantly reducing the Na + concentration. The surface ten sion components (g Lw i , g i , g i ) and the z-potentials of the particles used are shown in Table 1. The equations used for translating the g- and z-values, given in Table 1, to obtain the DGiwi-values shown in Table 2, can be found in [4], pp. 14–15 (Lifshitz – van der Waals interactions); pp. 20, 23 (Lewis acid – base interactions); and pp. 46, 48 (electrostatic interactions). The DGiwi-values given in Table 2 are the values at the minimum equilibrium distance, l0, between non-covalently interacting particles or molecules (l0 :0.157 nm), AB EL cf. [4], pp. 76 ff. The DG LW iwi , DG iwi and DG iwi -values at l\l0 are obtained by means of equations given in [4], pp. 75, 80, 82. The particles discussed
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here have been assumed, for calculation purposes, to be close to spherical, with a radius R= 1.0 mm, but it should be noted that these mineral particles are not really ideally spherical. (However, the asbestos fibers were reduced to a fair degree by first cutting them into small pieces (about 0.1 mm long), using a sharp blade. Then the fibers were placed in a commercial blender (1 wt% w/v in water) and blended at moderate speed for 5 min). The close encounter between particles, when flocculated, usually takes place between protrusions and/or edges with a radius rBR. Nonetheless, as all interaction energies (DG LW, DG AB and DG EL) are equally proportional to the radius, their final values in energy units remain proportional to R= 1.0 mm (cf. Table 2). In reality these values may be slightly smaller, when the radii of contacting protrusions are somewhat smaller than R. However, the proportionality between the DG LW, DG AB and DG EL values remain preserved (cf. Table 3), so that the DLVO and XDLVO diagrams remain the same except for the DG-scale, which may be somewhat reduced. Thus, the irregular shape of the particles does not influence the outcome in terms of flocculation versus stability.
3. Results and discussion
3.1. Hectorite particles Fig. 1 shows the stability versus flocculation of 0.5% (w/v) suspensions of the smectite clay mineral, hectorite: (A) in 0.015 M NaCl; (B) in 1/10
Table 2 The free energy (in kT) of interaction between particles (1) immersed in aqueous medium (PBS/10, w) at minimum equilibrium separation distance l0
Hectorite Glass a Glass+La3+ Crocidolite (Cape Town, SA) Chrysotile (Globe, AZ) Chrysotile (Salt River, AZ) Chrysotile (Zimbabwe) a
DG LW 1w1 (l0)
DG AB 1wl (l0)
DG EL lw1 (l0)
DG TOT lw1 (l0)
Photos
−660 −220 −170 −510 −380 −850 −530
−2800 +14 000 −6700 −2000 8400 −44 000 −40 000
850 +3000 +770 1300 1000 1300 2400
−2000 +17 000 −6100 −740 +9000 −43 700 −38 000
(A), (B), (C) – – (E) (F) (G) (D)
Glass particle suspension flocculated in the presence of 0.47 mM LaCl3; see [13].
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 000 (1999) 000–000
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Table 3 Free energies of interaction (DGiwi) between spherical particles, i, of radius, R, immersed in water, w, as a function of distance, l Configuration
Equation
Unretarded Lifshitz–van der Waals (LW) energies of interaction, expressed as a function of the 2a,b Hamaker constant, A = −DG LW%% iwi ×12pl0
DG LW iwi =−(AR/12l)
Lewis acid–base (AB) energies of interactiona (l is the decay length of water at 20°C, = 1.0 nm) DG AB iwi =pRlDG AB%% iwi exp[(l0−l)/l] Electrostatic (EL) energies of interaction (c0 is the surface potential at the particle’s surface; o is the dielectric constant and k is the inverse thickness of the diffuse ionic double layer, cf. [4], p. 82.)
2 DG EL iwi =(1/2)oRc 0 ln [1+exp(−kl)]
DG LW%% and DG AB%% indicate the free energies of interaction between two plane parallel plates, at the minimum equilibrium iwi iwi distance, l = l0. b l0 =0.157 nm; cf. [4], pp. 75 and 80. a
physiological saline buffer, pH 7.4, m = 0.015; (C) in 0.026 M LaCl3, pH 7.4, m= 0.015. Fig. 1, top, shows the suspensions immediately after preparation; middle, after 10 min; bottom, after 40 min. Clearly, the admixture of La3 + causes flocculation in less than 10 min (Fig. 1, middle) and at 40 min flocculation begins at just m =0.015 with NaCl, buffered or not. Fig. 2 shows that classical DLVO theory predicts total stability (at m= 0.015, without La3 + ), which is clearly not the case, although the 0 and 10 min photographs do show that a certain degree of (early) metastability is in evidence. This agrees well with the XDLVO graph also shown in Fig. 2; there is a secondary maximum of repulsion of about + 400 kT, at a distance of l : 2.5 nm. For monosized, completely spherical particles this secondary maximum of repulsion might cause a fairly long-lasting stability, but with more irregularly shaped particles localized sites (e.g. edges of some particles) can approach each other more closely, so that, as in this case, flocculation commences in less than 40 min, succumbing to a deep trough of attraction of at least −1000 kT. The influence of La3 + ions is discussed in more detail in Section 3.3.
3.2. Asbestos particles Shown in Fig. 3 are 0.5% (w/v) suspensions of asbestos particles (reduced to fairly symmetrical
particles from the original fibers): (E) is crocidolite; (D), (F), and (G) are chrysotile particles from, respectively, Zimbabwe, Globe, AZ, and Salt River, AZ. Initially, all asbestos particle suspensions were stable (Fig. 3, top) but within 30 min (Fig. 3, middle) chrysotiles (D) and (G) had flocculated ((G) even more vigorously than (D)), and crocidolite (E) also started to flocculate. The classical DLVO plots (Fig. 4) wrongly predict total stability of all the asbestos particles. Table 2 makes it quite clear that in these aqueous asbestos suspensions the strong hydrophobic attractions occurring with the chrysotile from Salt River, AZ and Zimbabwe ((G) and (D)), cannot be neglected. The hydrophobic attraction between crocidolite particles (E) is somewhat smaller, but still not negligible, as seen in Fig. 3, bottom, taken after 4 h. Thus again, in water, one has to revert to XDLVO plots (which include the Lewis acid– base interactions, which are, inter alia, responsible for the hydrophobic effect) to obtain a good correlation between prediction and experiment. From Fig. 5, showing the XDLVO plots for the four asbestos particles, it can be seen that the only really stable suspension should be obtained with the chrysotile from Globe, AZ ((F), in Fig. 3), which is indeed the case. Next, crocidolite (E) which has a sizable secondary maximum of repulsion of about + 600 kT, at l: 1.5 nm should be fairly stable for some time, as is indeed the case,
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 14 (1999) 47–55
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as it only shown incipient flocculation at 4 h. Finally, the strongly hydrophobic chrysotiles from Salt River, AZ and Zimbabwe ((G) and (D)) should have only faint metastability, from the XDLVO plot (Fig. 5) and indeed flocculate in 30 min (Fig. 3, middle).
Fig. 2. DLVO and XDLVO plots of the hectorite suspension shown in Fig. 1B.
3.3. Glass particles, hydrophobized with LaCl3
Fig. 1. Suspensions of 0.5% (w/v) of the smectite, hectorite, in: (A) 0.015 M NaCl; (B) PBS/10; (C) 0.026 M LaCl3. Top, t =0 min; middle, t= 20 min; bottom, t= 40 min.
It has been noted earlier [1,13] that low concentrations of plurivalent counterions induce flocculation in aqueous suspensions of charged particles [14] by making them hydrophobic and not, as was thought before, because these counterions reduce LW DG EL iwi to a lower value than DG iwi . A case in point is the influence of minute amounts of La3 + on the stability of glass particles. Admixture of 0.47 mM LaCl3 in a buffer of pH 7.4, m =0.015, sufficed to flocculate a glass particle suspension which, when suspended at pH 7.4, m= 0.015 with just NaCl, was totally stable (photographs not shown here). As can be seen from Table 2, whereas La3 + clearly reduced DG EL iwi for the glass particles from + 3000 to + 770 kT, the latter repulsive energy was still significantly higher than the van der Waals attraction, DG LW iwi = − 170 kT. What clearly happened was than concomitantly to the admixture of La3 + the particles’ surfaces switched from being very hydrophilic (DG AB iwi = + 14 000 kT) to quite hydrophobic, at DG AB iwi =
52
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 000 (1999) 000–000
− 6700 kT (cf. Table 2). The reason for this is that when plurivalent cations, which act as electron-acceptors (Lewis acids), combine with negatively charged particles which also are electron-donors (Lewis bases; cf. the high value for g of 37.1 mJ m − 2 for untreated glass partii cles in Table 1), the surfaces of such particles become hydrophobic through the decrease in g i
Fig. 4. DLVO plots of the asbestos particles shown in Fig. 3.
(to 20.9 mJ m − 2) caused by the La3 + –glass interaction. Fig. 6 shows the classical DLVO treatment of glass particles+La3 + , wrongly suggesting complete stability. Fig. 7 comprises the
Fig. 3. Suspensions of 0.5n˜5 (w/v) asbestos particles in PBS/10. (D) Chrysotile, Zimbabwe; (E) crocidolite; (F) chrysotile, Globe, AZ; (G) chrysotile, Salt River, AZ.
Fig. 5. XDLVO plots of the asbestos particles shown in Fig. 3.
W. Wu et al. / Colloids and Surfaces B: Biointerfaces 14 (1999) 47–55
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4.1. Hydrophobic particles or macromolecules In aqueous suspensions of all types of hydrophobic particles, neglecting DG AB iwi (which is always negative, i.e. attractive) almost invariably leads to a (DLVO-based) prediction of stability, except when the particles have an unusually low (or zero) z-potential, which is fairly rare. Examples of this are shown in Fig. 3, with the very hydrophobic chrysotiles, (G) and (D), (see also Figs. 4 and 5) but also with the more mildly hydrophobic hectorite (Figs. 1 and 2) and crocidolite (E) (Figs. 3–5) (cf. Sections 3.1 and 3.2, above).
4.2. Particles that are close to the hydrophobic/hydrophilic transition
Fig. 6. DLVO plots of glass particles suspended in PBS/10, with 0.047 mM LaCl3.
XDLVO plot of glass particles + La3 + correctly predicting the strong flocculation which was observed experimentally [1,13]. The hydrophobizing interaction of Ca2 + on negatively charged phospholipid surfaces was first noted by Ohki in 1982 [15]. The mechanism of this interaction, as described above, was first proposed in 1988 [16]. However, although other mechanisms for the influence of Ca2 + on phospholipids, involving their orientation and/or conformation have been proposed [17], the hydrophobizing effect of plurivalent cations on hard mineral particles which are negatively charged [1,13] would argue that the same hydrophobizing effect of Ca2 + and La3 + is also operative on negatively charged (but not on neutral) phospholipids [18].
Particles which are hydrophobically/hydrophilically close to the borderline of transition between the two states are exceedingly sensitive to the ionic strength of the liquid medium. In such cases DLVO analysis is less sensitive to changes in NaCl-concentrations (e.g. 16, 82 and 410 mM [7])
4. Conclusions The effectiveness of XDLVO versus DLVO analyses of four different categories of aqueous suspensions can be distinguished.
Fig. 7. XDLVO plots of glass particles suspended in PBS/10, with 0.047 mM LaCl3.
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W. Wu et al. / Colloids and Surfaces B: Biointerfaces 000 (1999) 000–000
than the XDLVO approach, which conforms closely to the experimental results [7].
4.3. Hydrophilic particles or macromolecules with a low z-potential One example of these is mammalian (e.g. human) peripheral blood cells [8], especially leukocytes, which would not be able to form a lasting stable suspension (as in blood) according to classical DLVO theory, but are predicted to form perfectly stable suspensions by the XDLVO approach [4,8]. With the zero z-potential [19] biopolymer, dextran [4], DLVO analysis would, ipso facto, entail only a van der Waals attraction, which, for a large molecule, would of necessity mean total insolubility. XDLVO analysis, on the other hand, shows a strong mutual repulsion between the dextran molecules of DG AB iwi : + 18 kT, (at l=l0) which correlates with total aqueous solubility, which is indeed the case.
4.4. Hydrophilic particles or macromolecules with a high z-potential, subject to admixture of pluri6alent counterions or pH changes A typical example is glass powder (cf. Section 3.3, above), which, as is, would be predicted to form an exceedingly stable suspension in water, by classical DLVO as well as by XDLVO analysis (due to the high DG EL iwi and the very high DG AB -values; cf. Table 2). Upon admixture of a iwi small amount of LaCl3, DG EL iwi is reduced about four-fold, but is still positive, i.e. repulsive (cf. Table 2), so that via classical DLVO analysis, stability should still prevail (Fig. 6). In reality LaCl3 admixture also causes a severe hydrophobization (together with a significant but not fatal decrease in z-potential) of the glass particles, which changes DG AB iwi from +14 000 to −6700 kT, resulting in an XDLVO prediction of total instability (Fig. 7), in agreement with the experimental observation. Lowering the pH to approach the isoelectric point of polymers [20], biopolymers [4], or phospholipids [18] immersed in water also causes hydrophobization [18,20]
(well before the point of zero charge is reached). Here also XDLVO plots offer the best correlation with experimental results, and they are liable to differ from DLVO theory.
4.5. Summarizing conclusion In aqueous media it is always essential to incorporate Lewis acid–base interactions in energy versus distance analyses, i.e. it is safest for the study of particles, cells, phospholipids membranes and vesicles, and macromolecules immersed in water to use the XDLVO approach, as outlined above.
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[19] S.C. Edberg, P.M. Bronson, C.J. van Oss, Immunochemistry 9 (1972) 273. [20] S.R. Holmes-Farley, R.H. Reamey, T.J. McCarthy, J. Deutch, G.M. Whitesides, Langmuir 1 (1985) 725.
. .