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Stability of Copper Phthalocyanine Dispersions in Organic Media D. F. K. Hughes,† Ian D. Robb,*,† and Peter J. Dowding‡ Centre for Water Soluble Polymers, North East Wales Institute, Mold Road, Wrexham LL11 2AW, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. Received October 6, 1998. In Final Form: March 30, 1998 This paper deals with the stability of dilute dispersions of copper phthalocyanine in organic solvents, a subject of importance to the paint and ink industries. It has been found that quite small surface potentials (a few millivolts) are sufficient to stabilize the dilute dispersions of the particles in solvents of low dielectric constant (possibly because this would be a constant charge system) though insufficient to stabilize particles in solvents with dielectric constants above about 20. Addition of organic acids enhanced the particles’ stability, probably by a proton-exchange mechanism, and addition of an organic-soluble electrolyte, tetrabutylammonium bromide, caused all systems to flocculate. The presence of a polymer at concentrations above about 1% caused flocculation to occur, probably by a depletion mechanism. The attractive depletion energy increased with polymer concentration, resulting in the sedimentation volume increasing with polymer concentration.
Introduction The dispersion and stability of colloidal particles in organic solvents has received much less attention1 than that in aqueous systems, partly because of experimental difficulties and partly because of the difficulty in establishing the usually small but finite surface charge on particles and the ionic concentrations in solution. The notion that a particle surface charge in nonaqueous media did not exist or that it was too small to be significant was thought to be true2 though electrophoretic and electrodeposition measurements3-5 showed that ionic species did exist in hydrocarbon media. van der Minne and Hermanie6 found that carbon black particles in benzene developed appreciable ζ potentials (∼25 mV) and hence colloidal stability in the presence of calcium soaps (which gave positive ζ potentials) or ammonium picrates (which gave negative ζ potentials); mixtures7-9 of these systems resulted in colloidal instability. Dispersions of R-alumina and carbon black (≈1 µm diameter) in p-xylene with the surfactant sodium bis(2-ethylhexyl sulfosuccinate) (AOT) developed ζ potentials of 50-70 mV, which gave longterm (>24 h) colloidal stability.10 It is generally accepted that colloidal particles acquire a charge in any solvent by two general mechanisms, viz., dissociation of surface groups and adsorption of ionic species.1,11 Surface group dissociation is energetically less favorable in low dielectric solvents though one likely mechanism has been suggested to be by transfer of protons * To whom corespondence should be addressed. † North East Wales Institute. ‡ University of Bristol. (1) Lyklema, J. Adv. Colloid Interface Sci. 1968, 2, 65. (2) Feat, G. R.; Levine, S. J. J. Colloid Interface Sci. 1976, 54, 34. (3) La Mer, V. K.; Downes, H. C. J. Am. Chem. Soc. 1931, 53, 888; 1933, 55, 1840; Chem. Rev. 1933, 13, 47. (4) Fuoss, R. M. Chem. Rev. 1935, 17, 27. (5) Strong, L. E.; Kraus, C. A. J. Chem. Soc. 1950, 72, 166. (6) van der Minne, J.; Hermanie, P. H. J. J. Colloid Sci. 1952, 7, 600. (7) Napper, D. H. J. Colloid Interface Sci. 1977, 58, 390. (8) Napper, D. H. In Colloidal Dispersions; Goodwin, J. W., Ed.; Royal Society of Chemistry Special Publication No. 43; Royal Society of Chemistry: Herts, U.K., 1982; Chapter 5. (9) Croucher, M. D. Macromolecules 1978, 11, 874. (10) McGown, D. N. L.; Parfitt, G. D.; Willis, E. J. Colloid Sci. 1965, 20, 650.
between the particle and solvent.1 Whether protons are removed from or adsorbed onto the particle depends on the relative Brønsted acidity and basicity of the solvent and particle. This helps explain why trace amounts of water can have a considerable influence on the sign and magnitude of the surface charge.12-15 In addition to proton exchange, electron donor-acceptor is another mechanism13 for charging the surface. Dispersions in organic solvents have not only the problem of whether a surface charge develops but what the solubility of electrolyte in the solvent is and hence the extent of any diffuse double layer. The very low values of the ionic strength mean that any surface potential will decay slowly with distance, giving a weak interparticle force that may be effective at large16 distances. The majority of studies of colloidal dispersions in organic solvents have concerned particles with polar or highenergy surfaces where water has been shown12-15 to have a significant influence on the stability of the dispersions, sometimes via surface adsorption effects and sometimes via bulk (ionic strength) effects. However, there are few reports16,17 on the origin of the stability of hydrophobic colloids in organic solvents. The van der Waals attraction18 is related to the differences in dielectric permittivities and/or the refractive indices, which tend to be smaller for hydrophobic particles in low dielectric solvents than with polar materials. (11) Fowkes, F. M.; Jinnai, H.; Mostafa, M.; Anderson, F. W.; Moore, R. J. Colloids and Surfaces in Reprographic Technology; Goddard, E. D., Vincent, B., Eds.; ACS Symposium Series 200; American Chemical Society: Washington, DC, 1982; p 307. (12) Malbrel, C. A.; Somasundaran, P. J. Colloid Interface Sci. 1989, 133, 404. (13) Labib, M. E.; Williams, R. J. Colloid Interface Sci. 1987, 115, 330. (14) Siffert, B.; Jada, A.; Eleli-Letsango, J. J. Colloid Interface Sci. 1994, 167, 281. (15) Miller, J. F.; Clifton, B. J.; Benneyworth, P. R.; Vincent, B. Colloids Surf. 1992, 66, 197. (16) van der Hoeven, Ph. C.; Lyklema, J. Adv. Colloid Interface Sci. 1992, 42, 205. (17) McKay, R. B. In Interfacial Phenomena in Apolar Media; Eicke, H., Parfitt, G. D., Eds.; Pigment Dispersion in Apolar Media 361; Dekker: New York, 1987; Chapter 9. (18) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1985.
10.1021/la981389v CCC: $18.00 © 1999 American Chemical Society Published on Web 07/08/1999
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In this work the stability of copper phthalocyanines (CuPc) in organic solvents has been investigated. CuPc pigments are the most ubiquitous and commercially valuable class of organic pigments17 used in the paint and ink industries, and concentrated dispersions of the pigment can be quite viscous, even at solid-phase volumes as low as 10%. To help understand this, the behavior of dilute dispersions of the pigment was investigated. The R and β forms of CuPc are the only polymorphs widely used in commercial products17 though other forms are known. The crystal structure of R-CuPc has been characterized by Ogorodnik19 and Honingman20,21 using IR absorption and reflection spectra in polarized light and X-ray diffraction, respectively. The crystal has two planes: the basal plane, in which the aromatic ring systems and copper atoms are exposed, and the prismatic plane, in which the hydrogen atoms of the aromatic rings are exposed. Depending on the exact shape and size of the crystal, many parallel stacks of CuPc molecules run along the basal plane (also called the b axis or acicular axis). This asymmetric nature of the surface can influence the particle’s charge distribution, which will not be uniform but will differ from face to face. Experimental Section Materials. Two types of CuPc pigments were used: Monastral Blue CSN supplied by Zeneca (Z-CuPc) and Heliogen Blue L7080 supplied by BASF (B-CuPc). Z-CuPc was an R-stabilized crystal form with a BET surface area of 58 m2 g-1 and a density22 of 1.6 g cm-3. Its surface treatment, if any, was unknown. B-CuPc was a β-crystal form with a BET surface area of 63 m2 g-1 and a density22 of 1.6 g cm-3 but had no surface treatment. The pigments were oven dried at 120 °C until a constant mass was achieved and were then stored in a desiccator. They were not subjected to further treatment apart from size reduction as part of the dispersion process. The solvent used was 1-methoxypropyl 2-acetate (MPA). It had a density of 0.97 g cm-3 and a bp range of 144-147 °C. MPA was distilled and stored over anhydrous magnesium sulfate drying agent (BDH, GPR). Distillation removed ethanoic acid (1%) impurity from the solvent. A polymer (essential to increase the viscosity of the MPA solvent) was used to aid milling of the powdered pigment. The polymer, Laropal A81, was supplied by BASF and was a polydisperse hexahydropyrimidine with a molecular mass distribution from 500 to 35 000. Six organic acids were used: ethanoic acid (EA), 99%; propanoic acid (PA), 99%; 3-methoxypropanoic acid (3-MPA), 96%; benzoic acid (BA), 99.5%; 1-octanoic acid (OA), 99%; 1-dodecanoic acid (DA), 99%. The acids were obtained from Sigma-Aldrich and were used as received. Tetrabutylammonium bromide (TBAB; 98% pure), an electrolyte soluble in organic solvents, was obtained from Lancaster Synthesis Ltd. and was used as received. Other organic solvents used were ethyl acetate from SigmaAldrich (99.8%), methanol and xylene from BDH (AnalaR grade), and 1-octanol from Sigma-Aldrich (99%). The water used was double distilled. The pigment powders were dispersed by milling a mixture of the CuPc powders (∼14% w/w), MPA solvent (31% w/w), and polymer (55% w/w) using a Netsch Zeta mill for nine passes. These concentrated dispersions were usually diluted to give dispersions containing 0.28% w/w CuPc and 1% polymer in MPA. Experimental Methods. Dilute dispersions of CuPc (0.28% w/w in MPA) were prepared by sonication with a Soniprobe for 30 s/20 g of dispersion at room temperature (∼20 °C), in cylindrical glass vials. Sedimentation volumes were measured from the height of the sediment. The state of flocculation of the dispersion (19) Ogorodnik, K. Z. Opt. Spektrosk. 1974, 37, 600. (20) Honigmann, B. J. Paint Technol. 1966, 38, 77. (21) Honigmann, B. Ber. Bunsen-Ges. Phys. Chem. 1967, 71, 239. (22) Data from the manufacturer.
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Figure 1. Particle size distribution of Z-CuPc after milling and additional sonication was examined by light microscopy, using a droplet of dispersion suspended beneath a supported glass cover slip. Particle size measurements were made on a Malvern 3000 zetasizer. Electrophoretic mobilities were measured using the phase analysis light scattering (PALS) technique. This is an extension of laser Doppler electrophoresis, which involves the detection of light scattered by colloidal particles which move relative to an interference fringe pattern. This fringe pattern is generated by two laser beams intersecting in the gap between the electrodes (used to induce the electrophoretic motion in the colloidal sample). It is necessary for electrophoretic motion to occur over several fringe spaces during a single field pulse, and so for samples of low mobility, large voltages are required that can result in heating of the sample. PALS removes such restrictions by the use of moving fringes for phase demodulation of the laser Doppler signal and allows the determination of very low mobilities23 with good precision (down to 10-12 m2 s-1 V-1.
Results Particle Size. Examination of the dispersed pigment using both light and electron microscopy showed the particle size to be in three broad ranges: ∼0.02 µm, which corresponded to the primary particles having a surface area of 50-60 m2/g; these particles were usually agglomerated into particles between 0.5 and 2.0 µm. The size distributions of the pigment particles after grinding and sonication were measured by photon correlation spectroscopy and are shown in Figure 1. These micronsized particles were capable of forming large flocs up to about 50 µm depending on solution conditions. Sedimentation of the colloidally stable, micron-sized particles will be opposed by Brownian motion. The distance (lt) fallen by a particle, radius r, in time t at the terminal velocity can be written as lt ) 2(Fs - Fl)gr2t/9η, where Fs and Fl are the densities of the solid (1.6 g cm-3) and solvent (0.97 g cm-3), respectively, η is the viscosity of the solvent, and g is the gravitational force. The distance (lBt ) moved by a particle because of the Brownian displacement over the same time can be written as lBt ) {2kTt/6πηr}1/2. Equating lt and lBt gives an estimate of the time taken for particles of various sizes to sediment. This is shown in Figure 2. Differences in densities between the various solvents have a negligible effect on the result, but the time taken to sediment a given distance is inversely (23) Miller, J. F.; Schatzel, K.; Vincent, B. J. Colloid Interface Sci. 1991, 143, 532.
Stability of Copper Phthalocyanine Dispersions
Langmuir, Vol. 15, No. 16, 1999 5229 Table 1. Electrophoretic Mobilities and ζ Potentials for B-CuPc Invarious Solvents system B-CuPc in ethyl acetate B-CuPc in ethyl acetate + 410 mM benzoic acid B-CuPc in methanol B-CuPc in xylene B-CuPc in ethyl acetate + 1% water
Figure 2. CuPc particle diameters as a function of the time at which their Brownian and gravity displacements are equal.
Figure 3. Sedimentation volumes after 30 days of Z-CuPc dispersions in MPA solvent as a function of added organic acid concentration.
proportional to the solvent viscosity. The results indicate that any particles sedimenting in less than about 10 h must be larger than about 1 µm; i.e., in this system they were flocculated. Effect of Added Acids. After 3 days the sedimentation volume of the Z-CuPc pigment at 0.28% w/w in distilled MPA solvent was 18% but 85% in undistilled MPA. The sedimented particles in the distilled solvent were in large flocs, about 5-20 µm in diameter, but those in undistilled MPA were largely unflocculated. The difference in sedimentation volumes was attributed to the presence of ethanoic acid. In contrast, B-CuPc showed no sedimentation in either distilled or undistilled solvent, and the particles remained dispersed with particles sizes between 0.5 and 2.0 µm. To determine if the effect of ethanoic acid on Z-CuPc was general to other organic acids, sedimentation volumes of the pigment (0.28% w/w) were measured in solutions of distilled MPA containing a number of acids. The data in Figure 3 show that the stability increases with molar acid concentration, essentially independent of the chain length of the acid. Stability is attained after about 100 mM acid, as confirmed by optical microscopy. The fact that quite short organic acids were able to prevent flocculation indicated that the stability did not arise from any steric mechanism. As the dispersion was dilute, the other obvious mechanism was electrostatics, even though
electrophoretic mobility m2 (V s)-1
ζ-potential (mV)
3.4 × 10-10 6.2 × 10-10
7.7 14.0
2.5 × 10-9 -6.9 × 10-11 -7.43 × 10-11
13.0 -3.4 -1.7
the solvent was a dried ester having a low dielectric constant (∼6). To determine if added organic acid could alter the surface charge of the particles, electrophoretic mobility measurements were made using the PALS technique. The system chosen was not Z-CuPc, as this was flocculated in the absence of additives, but B-CuPc. The solvent chosen was ethyl acetate, as this was available in a pure form with a known dielectric constant. Electrophoretic mobilities of B-CuPc with various additives are shown in Table 1. Dilute dispersions (∼0.05% w/w) were required to avoid problems with multiple scattering. Effect of Added Electrolyte. The small increase in positive charge found after the addition of organic acid may have been the result of exchange of protons from the acid to the particle surface and may have been sufficient to stabilize the particles. If this were so, then addition of electrolyte would have a destabilizing effect. To study this, the organic-soluble electrolyte tetrabutylammonium bromide (TBAB, 0.1%; 3 × 10-3 M) was added to both pigments (0.28% w/w dispersed in MPA and MPA + 675 mM propanoic acid). Both pigments were stable in the propionic acid/MPA solution and were flocculated by addition of TBAB. Flocculation and sedimentation were nearly complete after 1 h, and the sedimentation volumes, recorded after 14 days, were between 12 and 14% for both pigments. Flocculation also occurred on addition of TBAB (0.1%) to dispersions of both pigments in other organic acids. This confirmed the importance of electrostatic repulsion in achieving the stability of the pigment dispersion. Effect of Solvent Type. The sedimentation of the 0.28% dispersions of both pigments, made from the 14% w/w dispersions, were measured in methanol, octanol, xylene, and acetone, and the data are shown in Figure 4. Optical microscopic examination showed the particles in methanol and acetone were flocculated, whereas those in octanol and xylene remained as separate micron-sized particles. Nonsedimentation was also found in solvents of decanol and hexadecanol. The effect of adding propanoic acid on the sedimentation of B-CuPc in acetone and xylene was measured, and the data are shown in Figure 5, with the acid clearly reducing flocculation and retarding sedimentation over 12 days. Effect of Added Polymer. The viscosities of concentrated dispersions of the Z-CuPc pigment (14% w/w Z-CuPc in 60% polymer and 36% MPA) were high enough to prevent pouring, even at particle volume fractions as low as 10%. As the presence of the polymer may have contributed to this, the effect of added polymer on the stability of the pigment in dilute dispersions was studied. Dispersions of Z-CuPc (0.28% w/w) in MPA solvent containing 200 mM 2-methoxypropanoic acid were prepared containing amounts of polymer from 1-60%. The sedimentation volumes after 1, 168, and 1032 h are shown in Figure 6. Flocculation and sedimentation were most rapid at intermediate polymer concentrations of 10-20%, with all systems containing added polymer eventually
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Figure 6. Sedimentation volumes of B-CuPc dispersions as a function of added polymer concentration after 1, 168, and 1032 h. Figure 4. Sedimentation volumes of 0.28% w/w Z- and B-CuPc dispersions in various solvents after 125 days.
Figure 5. Sedimentation volumes of Z- and B-CuPc in xylene and acetone as a function of added propanoic acid concentration.
flocculating. Below 5% added polymer, the floc sizes decreased with decreasing polymer concentration but were about 50-100 µm above 5% polymer. Effect of Added Water. Dispersions of both Z-CuPc (0.28% in MPA solvent with 5% propanoic acid; 670 mM) and B-CuPc (0.28%) were prepared containing 0.25-5% water. For both systems the particles remained stable with 1% added water but flocculated to volumes of 20% and 48%, respectively, with 5% added water after 1 h. B-CuPc in MPA solvent containing 5% propanoic acid and 5% water was colloidally stable after 14 days, having a sedimentation volume of 95%. To estimate the effect of water on the surface charge, the electrophoretic mobility of B-CuPc was measured in ethyl acetate containing 1% water, and the result is shown in Table 1. Mobilities of Z-CuPc were not measured because of its flocculated state. Discussion Stabilization of colloidal particles in solvents of low dielectric constant () by electrostatic repulsion has already
been demonstrated.16 The energy of van der Waals attraction, often less in organic than aqueous solvents, decreases with (distance of separation)-2, whereas the electrical repulsion decreases with (distance of separation)-1, so that at large separations the electrical repulsion can dominate. At 0.28% pigment in solution, the average distance between the particles is 6.5 µm, the same order of magnitude as 1/κ in these systems.16 Calculation of the van der Waals attraction was not possible as the Hamaker constant is unknown. However, the stability of the pigment particles in xylene ( ) 2.3) contrasts with the conclusions from earlier studies,16 summarized by Morrison,24 that electrostatic stabilization is unlikely in such solvents. The observed electrostatic stabilization in xylene may well be accounted for by the fact that the repulsion between particles having constant charge at the surface (rather than constant potential) increases significantly25,26 as the separation decreases, particularly at distances below 1/κ. In low solvents containing no added ions, it would be expected that discharge of approaching surfaces would be difficult. Thus, although the surface potential of B-CuPc is quite small, it is sufficient to keep the dilute dispersions stable. In the absence of added electrolyte, the interparticle repulsive force was calculated from Coulomb’s law to be 2 × 10-17 N. This compares with a sedimentation force (mg∆F) of 3 × 10-15 N. Thus, the electrical repulsion between particles in a vertical plane is insufficient on its own to prevent sedimentation. In these systems sedimentation is mainly opposed by Brownian motion so that rapid sedimentation arises from flocculation (which reduces Brownian diffusion). The lower stability of Z-CuPc is presumably a result of even lower surface potentials, though no measurement of this was possible because of its flocculated state. The importance of electrostatic repulsion to the stability of the dispersions was emphasized by the effect of additives. Organic acids enhanced the stability of the dispersion, probably by raising the surface potential by a proton-exchange mechanism, as has been previously demonstrated.16 Addition of TBAB (3mM), an organic(24) Morrison, I. D. Colloids Surfaces A 1993, 71, 1. (25) Weise, G. R.; Healy, D. W. Trans. Faraday Soc. 1970, 66, 949. (26) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, U.K., 1987.
Stability of Copper Phthalocyanine Dispersions
soluble electrolyte, caused flocculation of both pigments in all solvents, including those with added organic acids. The effect of water on the stability of polar solids in organic solvents is well-known, though there is much less work on its effect on dispersions of low energy solids. Small amounts of added water (up to 1%) had no observable effect on particle stability, though the surface potential of B-CuPc was reduced. Addition of water would be expected to allow more surface groups to dissociate, though the different chemical nature of the surfaces precludes any prediction of changes in size or sign of the surface potential. Higher water concentrations (5%) caused flocculation of both pigments to occur, and adding 5% propionic acid restored the stability of B-CuPc again, indicating that the effect of water is to reduce the surface charge. Dispersions of both pigments in organic solvents methanol and acetone were flocculated, whereas those in xylene and octanol were stable. The origin for these differences in behavior probably lies with the dielectric constants (). Those of methanol (32.6) and acetone (20.7) are relatively high compared to those of xylene (2.3) or octanol (10.3). The effect of on the stability arising from electrostatic repulsion has two contributions, the change in dissociation of ionic species at the colloid surface affecting the surface potential and the ionic concentrations influencing 1/κ, the Debye length. For low (2-5) solvents, low ionic strengths produce large values of 1/κ, and even small surface potentials will produce a net repulsion at large distances. For a constant charge system, this repulsion will increase significantly below 1/κ in contrast to a constant25 potential system. For solvents with above about 15, 1/κ approximates to aqueous systems16 and may be only about a few nanometers. Thus, even though the electrophoretic mobility of B-CuPc was greater in methanol than in xylene, it was less stable in the former. The particles in xylene sedimented but remained unflocculated. Sedimentation in octanol was much slower, because of its greater viscosity (16 mPa‚s) than xylene (0.7 mPa‚s). Added polymer caused flocculation of the pigments, probably by a depletion mechanism. The point of interest is why the sedimentation volume increases with polymer concentration. The attractive depletion potential, Vdep, is given27 by (27) Vincent, B.; Edwards, J.; Emmett, S.; Jones, A. Colloids Surf. 1986, 18, 261.
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Vdep ) 2πr([µ1 - µ01]/V01)(∆ - H/2)2(1 + 2∆/3r + H/6r) where r is the particle radius, ∆ is the depletion layer thickness (assumed to be equal to the polymer’s radius of gyration), H is the interparticle separation, V01 is the molar volume of the solvent, and µ1 and µ01 are the chemical potentials of the polymer solution and solvent, respectively. This shows that increasing concentrations of polymer increase the depletion potential and hence the attractive energy holding the particles together. At polymer concentrations up to 5%, flocculation was quite slow, and very weak, open flocs were formed. Above 5% added polymer, Vdep was sufficient to cause larger flocs to form though small enough to allow the flocculating particles to roll around the floc, finding the lowest energy position. Thus, between 5 and 20% polymer, a floc of three particles is likely to form a triangular rather than a linear floc. At higher polymer concentrations (i.e., attractive energies), particles may not be able to move after collision with the growing floc and so form more open structures. Thus, the sedimentation volume increased with polymer concentration. At higher polymer concentrations the increased viscosity caused the rate of flocculation to decrease, with the system with 60% added polymer taking over 700 h to flocculate. Conclusions The colloid stability of two samples of copper phthalocyanines dispersed in 1-methoxypropyl 2-acetate has been studied. The stability of the dispersions increased with added organic acids. Electrophoretic mobility data showed an increase in the small positive surface potential on addition of organic acids, suggesting that charge repulsion is the origin of the stability. This was confirmed by addition of an organic-soluble electrolyte, which caused flocculation of all stable systems. Addition of water tended to flocculate the particles, though concentrations as high as 5% were needed to have this effect. Again electrophoretic mobility data showed that water lowered the surface potential. Increasing the concentration of polymer caused the dispersion to flocculate, probably by a depletion mechanism. Acknowledgment. The authors thank one of the reviewers for pointing out the likely importance of the constant charge model for the systems studied here. LA981389V
