Electrical charges in nonaqueous media

Co/lords and Surfaces A: Physicochemical and Engineering Aspects, 71 (1993) 1-37 Elsevier Science Publishers B.V., Amsterdam

Review

Electrical charges in nonaqueous

media

Ian D. Morrison Xerox Corporation, Webster, NY 14580, USA (Received 2 September 1992; accepted 2 December 1992) Abstract From an explosion at Shell’s refinery at Pernis to the megahertz oscillations of an electrically charged micelle in hexane to the most advanced of electronic imaging processes, electric charges in nonaqueous media make their presence known. The study of these electric charges has slowly and steadily increased over the last 50 years or so, but the answers to the elementary questions about how charged species are created and how they remain charged are still hotly debated. This review concentrates on the low conductivity solutions and dispersions typical of hydrocarbon media. The model generally accepted for nonaqueous electrolyte solutions is that the electric charges are stabilized against neutralization by being held separate in large structures, such as micelles or complex macroions. Electrical conductivity arises by the field-induced motion of these charged species. The electric-field and concentration dependence of the conductivity depend strongly on their size and structure. Particles acquire electrical charges either by preferentially adsorbing the ion of one sign or the other, possibly still associated with its stabilizing structure, or by an ion dissociating from its surface to be held in some lyophilic structure in the nonaqueous medium. Many aspects of this model are not universally accepted. The influence of water on the creation and stabilization of electrical charges is reviewed. Water plays a key role in the properties of nonaqueous electrolyte solutions and dispersions but its behavior is complex because it influences both the formation of structures such as micelles, the dissociation of ionic molecules, and reactions on particle surfaces. The current theories of the physics of nonaqueous electrolyte solutions and dispersions are reviewed. This review is presented in the form of discussions of twenty-two related topics. Keywords:

Charge-to-mass ratio; electrical charges; electrophoretic mobility; inverse micelles; nonaqueous dispersions; nonaqueous

media.

1. Existence of electrical charges in nonaqueous media The initial studies of electrolytes in nonaqueous media came from two sources, the electric power industry and the petroleum processing industry. Formation and accumulation of electrolytes change the capacitance or insulating ability of materials and so are a concern for the control of electric power. In the petroleum industry the generation of large electrostatic fields accompanying the flow of insulating liquids is an explosion hazard. The addition of electrolytes to the fluid raises its conductivity and provides a means for the dissipation of electric charge. 0927-7757/93/$06.00

0 1993 -

Early work in the electric power industry showed that a likely source of unwanted dielectric loss is the build-up of metal soaps produced by corrosion reactions [ 11. The metal soaps were in the colloidal state, insoluble but stable. Piper et al. [l] also showed that the dielectric loss was a critical phenomenon in that abrupt transitions in dielectric constants occurred at specific temperatures. They showed that these temperatures corresponded to phase changes in the metal soap solutions. Various phase changes were proposed, including transitions from two-phase systems to micellar solutions to gels. These phase changes are certainly accompanied by changes in the concentration and size of soluble and mobile ionic species. Having discov-

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I.D. MorrisonlColloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

ered the types of compounds responsible for unwanted dielectric changes, the problem then was to avoid their formation. The situation is somewhat different in the petroleum processing industry. Klinkenberg and van der Minne [2] showed that trace compounds present in oil products are responsible for the build-up of unwanted electrostatic potentials (occasionally leading to explosions). The mechanism was preferential adsorption of ions of a particular sign on metal surfaces. The flow of liquid separated the two types of charges, building up electric fields to such an extent that they would discharge by sparking. Klinkenberg and van der Minne showed that these same species at higher concentrations increased the conductivity of the oil sufficiently to bleed off accumulated charges and thus prevent the build-up of large electric potentials. Various materials, such as tetraisoamylammonium picrate, calcium diisopropylsalicylate, and cetylpyridinium bromide, were found that increase the conductivity of petroleum products. The specific conductivities of these (and other materials) as a function of concentration show gradual changes, often with a minimum. A new principle was also discovered, namely that certain metal salts, when used in combinations with other compounds, impart a strongly enhanced conductivity, sometimes as much as several thousand times as high as the sum of the individual components. The enhanced conductivity must be due to an association of the electrolytes. Nelson and Pink [3,4] showed by ebullioscopic methods (boiling point elevations) that the chargecarrying species were inverse micelles and that some differences between materials could be ascribed to differences in micelle size and the rate at which they equilibrate with changes in solution. The more polar soaps form larger micelles and an increase in chain length in soaps of the same metal leads to a decrease in the degree of aggregation. One point should be noted here. Evidence for the existence of electrical charges in nonaqueous media does not primarily come from studies of the stability of nonaqueous colloids and arguments about whether they are electrocratic or not. The

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role of electrostatics in the colloidal stability of nonaqueous dispersions is a derivative topic (see Section 13). In fact, electrostatic interactions are often simply assumed to be absent because the dispersion is prepared in a hydrocarbon [ 178,179]. The importance of the presence of electric charges was sometimes only discovered later [lSO]. The early observations tell us that one of the keys to understanding the behavior of electric charges in nonaqueous media will be to understand the nature of micelles. The question is, why micelles? A consideration of the physics of charge separation in liquids will help. 2. On the stability of ions in liquids Consider a dilute aqueous solution of NaCl. The solution contains completely dissociated and uniformly distributed ions. The ions are constantly diffusing and must therefore be colliding with each other frequently. Half of the collisions will be between oppositely charged ions (if the collisions are random). During each collision a number of forces come into play, one of which is coulombic attraction between oppositely charged ions. The attraction will vary with distance and can be calculated reasonably well by

where e is the charge per ion, 1.602. lOpi9 C, D is the dielectric constant of the medium, about 80 for water, about 2 for hydrocarbons, t,, is the permittivity of free space, 8.85*10-‘2 C V-i m-‘, and d is the center-to-center distance of separation. The coulombic energy is attractive and increases as the ions approach each other. Since salt solutions do not spontaneously coalesce, thermal motion must be great enough to prevent the ions from coalescing. This can only be true if the ions never get any closer to each other than that distance where kinetic energy, kT= 4.11-l O- 21 J (at room temperature), can overcome the coulombic attraction. For water this distance of closest approach is approximately 0.7 nm. (The center-to-center distance in

I.D. MorrisonlColloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

the NaCl crystal is about 0.28 nm.) Ions in water must be held apart by tightly bound water molecules. Salt solutions are sterically stabilized dispersions! If this calculation is repeated for salt solutions in hydrocarbons, the distance of closest approach before the ions are bound is about 28 nm. For salt to stay ionized in hydrocarbons, the hydrocarbon molecules would have to form a tightly bound shell around the ions 14 nm thick. Evidently this does not happen. A more thorough analysis of the equilibrium between symmetric ion pairs and free ions in solution is due to Fuoss [S]. He analyzed the dissociation as an equilibrium between two phases, one of pairs of bound ions and one of free ions. The difference between the phases is the extra energy due to the electrostatic attraction. The degree of dissociation a is given implicitly by

(2)

cr=l-

where d is the center-to-center distance of the ion pair, n, is the number of ions per unit volume, z is the valency of the ion, andfis the Debye-Hiickel activity coefficient which approaches unity for dilute solutions. Taking the activity coefficient to be unity since we are considering low ionic strengths and neglecting u when compared to unity, Eqn (2) can be rearranged to give 2un, =

/s

exp(gT)

(3)

where 2an, is the number of free ions per unit volume. (The conductivity due to these ions is estimated in Section 5.) Taking 0.5 nm for the radius of a l-l ion pair, the estimated number of ion pairs is 2.10-r1 ions 1-l at a concentration of solute of 10m3 M [6]. That is to say, that even if the charge-carrying species is less than the critical size estimated above, 28 nm, there will be some equilibrium distribution of ions between pairs of charged ions and free ions. Certainly the larger the ion, the more the equilibrium is shifted to free charges. A common approximation [7,8] to calculate the

3

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number of ions per unit volume is to use Walden’s hypothesis which states that for any electrolyte the product of the limiting equivalent conductance and the viscosity of the liquid is a constant, independent of the solvent [9] /I;$+, = /i”oa)lna

(4)

If the limiting ionic conductance for an electrolyte &, the viscosity of water qw and the viscosity of the nonaqueous solvent q”” are known, then the limiting ionic conductance for the electrolyte in the nonaqueous solvent /jr can be calculated. If nonaqueous electrolytes are structures whose existence or size depends on concentration, /1”,”cannot be obtained by extrapolation [lo]. Walden’s rule is best obeyed by electrolytes consisting of large symmetrical ions, and is exemplified by the experimental values [I l] of the equivalent conductivity of tetraethylammonium picrate in water, methyl cyanide, the lower aliphatic alcohols, acetone, and ethylene dichloride. Walden’s rule is not obeyed for electrolytes containing small ions, such as ammonium and the elementary ions [12]. The conductivity is related to the limiting conductance by the relation n,zeA,

A=----

F

(5)

where F is Faraday’s constant. Considering solutions of the same concentration but in different solvents, Walden’s law implies that

which is true when the ratio of the degree of dissociation of the electrolyte to the ion size is the same in water as in the nonaqueous solvent (see Eqn (9) below). This seems improbable. Returning to the question of the stability of ions in nonaqueous media, the conclusion is that the ions only remain dissociated in low dielectric media if they are large or contained in some large structure. Micelles are one such structure and large polymers are another. The understanding of what structure in the nonaqueous dispersion is available

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I.D. MorrisonjColloids

to hold an ion is the key to understanding any effects of electrical charges in nonaqueous media. If the structure is larger than some critical diameter, approximately 28 nm, then all the ions will remain separate. If the structure is smaller than this, an equilibrium will be set up between bound and free ions; the larger the ion, the greater the fraction that will be free. We use the term “micelle” to mean generally the association structures present in nonaqueous media. Of course, the association structures can be larger, certainly to the point of becoming microemulsions. Hence, Bradley and Jaycock [13] can talk about the solubilization of various acids from the oxidation of oil into the core of succinimide “micelles.” The complexity of multicomponent systems where all kinds of normally insoluble materials can be in the core of micelles or suspended as microemulsions makes up the challenge of understanding nonaqueous systems. 3. Micelle formation in nonaqueous media In 1953 Mathews and Hirschhorn reviewed the evidence for micelle formation in hydrocarbon media, in particular low concentration data [14]. Typical colloidal phenomena such as abnormally low osmotic pressure, indicating a reduction in the number of molecules per unit volume of solution, light scattering, indicating large structures, and solubilization, indicating regions of water-like properties, had been observed. From measurements of viscosity and flow birefringence on solutions of sulfonates in hydrocarbons, van der Waarden [15] concluded that the micelles possessed a plate-like shape with axial ratio between 10 and 50. Mattoon and Mathews [16] concluded from conductance measurements that AOT (di-(2ethylhexyl) sodium sulfosuccinate) micelles behaved as positively charged particles in an electric field. The principles of micelle formation in nonaqueous media have been reviewed elsewhere [ 173, and depend, as all physical processes do, on a balance of intermolecular forces and changes in entropy.

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In water the two dominant factors determining whether a solute will form a micelle or not are the interaction of the hydrophilic portion of the solute with the solvent and the change in entropy as water molecules become oriented at the molecular surface [ 181. The hydrophobic portions of the solute are not expelled from water by any repulsive force as the name hydrophobic seems to imply, but rather the hydrophobic portions of the solute are excluded from water solution because their presence causes the hydrogen bonding network of water to rearrange and thus decrease entropy. An aqueous micelle will form when the strength of interaction of the hydrophilic “head” groups with water is strong enough to pull the solute into solution, and the disruptive nature of the “tail” group is minimized by forming a closed structure. The balance of the energies of attraction and entropic constraints leads to the rather complex association structures of amphipathic solutes [19-211. The balance between intermolecular forces and changes in entropy is quite different for solutes in nonaqueous media. The primary difference is in the fact that most nonaqueous solvents, especially ones with low dielectric constants, do not selfassociate. Therefore the introduction of a solute generally increases the entropy of the system and is a favorable process. The primary driving force for forming a nonaqueous micelle is therefore the strong intermolecular forces between the hydrophilic portions of the molecule. If these interactions are sufficiently greater than interactions with the solvent, the solute will form micelles. The decrease in entropy caused by this self-association is not as large as the decrease in entropy caused by the introduction of “hydrophobic” molecules into water. The various combinations of intermolecular interactions of solvent with the parts of the amphipathic molecule and the amphipathic molecule with itself are dominated by donor-acceptor or acid-base interactions [17]. It is easy to see that the presence of water is critical in this micelleforming process since the donor-acceptor inter-

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actions in the core of a nonaqueous micelle can be greatly enhanced on the addition of a molecule like water capable of acting as both an acid and a base. Furthermore, with some water present in the core of the micelle, the possibility of the core containing an ion or stabilizing a net electronic charge is greatly enhanced. In 1967 Becher reviewed micelle formation in nonaqueous media and found little information on the micellar properties of nonionic solutes (a total of only two pages out of 35!) [22]. He reported considerable discrepancies between light scattering and vapor pressure data in the critical micelle concentration (CMC) for the few compounds studied. It may be that considering only nonionic solutes, considerable information about other amphipathic, nonaqueous solutes was missed. In the more recent literature Verbeeck et al. [23] report studies on the aggregation behavior of the cationic surfactant didodecyldimethylammonium chloride and its derivatives in toluene by means of UV absorbance and fluorescence decay. Water is less easily solubilized in the quaternary ammonium cores of these micelles than in the cores of anionic solutes. The solubilization of ions in the core of nonaqueous micelles forms a unique, high ionic strength environment, which has been exploited for various novel reactions [24]. Kitahara has reviewed solubilization and catalysis in nonaqueous micelles [25]. Micelles offer just the structure necessary for stabilizing charge separation in low dielectric media; a place for the ion to be solubilized, and a large separation between ions in different micelles or between a micelle and a particle. Singleterry [26] lists two score surface-active molecules that form inverse micelles in nonaqueous media. The micelle size is small because the binding energy comes from the association of the hydrophilic head groups and is limited by steric interactions of the lyophilic tails. The role of water is quite important as hydrogen bonding can be a major component of the head group association forces. Kon-no and Kitahara [27] describe how the size of the micelle varies with molecular structure.

5

Anionics generally form larger micelles than cationits because of the smaller size of typical anionic head groups. The anionics generally form micelles of constant size but the size of micelles formed from cationics varies with concentration. The effect of succinimide structures (OLOA 1200 from Chevron is the common example) on micelle formation has been studied by Fedorov et al. [28]. Petit et al. [29] report on their studies of the shape of bimetallic AOT solutes, Co(AOT),, Cu(AOT),, and Cd(AOT),, as a function of water content. Na(AOT) forms spherical micelles that simply grow with the addition of water. These bimetallic materials form spherical micelles at low water content but become cylindrical as the water content is increased. The size and concentration dependence of micelles in nonaqueous media was examined by Muller [30] theoretically. He concluded that two distinct patterns of aggregation are available. When the sum of the radii of the ionic head groups is large (e.g. most cationics), the micelle size is small and increases slowly with concentration of solute and no critical transition is seen. When the sum of the radii of the ionic head groups is small (e.g. most anionics) micelles form at the CMC; the micelle size is large and is independent of the concentration of solute above that concentration. The micelle size is determined by a competition between structure forming by the head groups and steric hindrance of the tails. Eicke [31] in a comprehensive review in 1980 describes aggregation and micellization in nonaqueous media. CMCs in nonaqueous media are generally less than critical while aggregation numbers are generally less than in water. The formation of inverse micelles is dominated by hydrogen bonding. The quantity of water needed to stabilize the micelles is undetectably small. The inverse micelles can have a wide variety of shapes as opposed to aqueous micelles which tend to be spherical (because of electrostatic repulsive forces of charged ionic groups at the surface of the micelles). He also describes various experimental methods and micelle-formation models.

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4. Experimental techniques to study nonaqueous micellization Micelle formation in aqueous media can often be detected by a sudden change in the slope of specific conductivity vs concentration plots. Below the CMC the solute is molecularly dispersed; above the CMC micelles form and the mobility of the ions decreases. While micelles may be charged in nonaqueous media, and therefore contribute to electrical conductivity, they are weak electrolytes. For micelles in nonaqueous media other techniques have been found to be applicable. For instance, if iodine is added to a nonaqueous solution containing micelles, it shows a distinctive color as iodine becomes complexed in the micelle center [32]. The advantage of using iodine is that its molecular size is small and hence iodine is less likely to induce the formation of micelles. Water-soluble dyes can be added to nonaqueous solute solutions to check for the presence of micelles [26,33]. The usual concern is that the dyes themselves will initiate the formation of micelles in the same way that water does [34]. Particularly interesting is the use of fluorescent dyes since a careful examination of the fluorescent spectrum gives information about the microenvironment [35]. The rate of depolarization of fluorescent dyes has been used as an indicator of the size of inverse micelles [36]. Dye molecules in the core of a micelle move at a rate equal to that of the micelle so that the rate of depolarization is a measure of the diffusion rate of the micelle. From the diffusion constant of the micelle the equivalent spherical diameter can be calculated from the Einstein equation [37]. Another technique sensitive to the formation of micelles is proton NMR. By this technique Fendler et al. [38,39] determined the CMC, the aggregation number, and the equilibrium constant for micelle formation for a series of alkylammonium propionates and carboxylates in benzene. In Eicke’s 1980 review of aggregation and micellization in nonaqueous solvents [31], he lists methods based on colligative properties, classical light

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scattering, ultracentrifugation, viscometry, photoncorrelation spectroscopy, fluorescence spectroscopy, NMR, dielectric measurements, and positron annihilation. Klinkenberg and van der Minne have shown that the cause of static build-up during the processing of petroleum products is the separation of charged oil-soluble materials from the container with which they had exchanged charge [2]. Besides the fundamental work they did on understanding the physics and chemistry of the process, they developed a simple test to quantify the “charging tendency” of oil. The experimental apparatus consists of an electrically isolated container into which they poured the oil to be tested. On the bottom of that container is a thin tube through which the oil could drain. The oil that fell out of the capillary was collected in an electrically isolated cup. An electrometer was connected between the container holding the oil and the cup catching it. The charging tendency of the oil is measured by the potential build-up between the two containers due to the separation of the oil-soluble charged species from their countercharges on the metallic surface of the container. If the oil is pure, then there is no buildup of potential. If the oil is sufficiently conductive, then again there is no build-up of potential. It is only in some intermediate range of conductivities that the danger of static build-up exists. Heffner and Marcus [40] describe an apparatus with automatic dilution and measurement features which expedite the determination of conductivity vs concentration profiles in low conductivity systems although the system they studied, AOT in dichloroethane, showed no CMC. 5. Electrical conductivity of nonaqueous solutions Measurements of the conductivity of a solution are usually used to study the nature of electrically charged species. The mechanisms of electrical conduction were also examined by Alj et al. [41]. They found, agreeing with Muller [30], that those additives that increase the conductivity of cyclohexane could

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MorrisonlColloids

Surfaces

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Eng. Aspects

71 (19931

be divided into two classes. The first type consists of molecules like triisoamylammonium picrate which do not have any CMC and whose association number is small and somewhat concentration dependent. These materials adsorb on electrodes, where there is both dissociation and electron exchange with the electrode, and one sign of charge (generally the anion) escapes its image charge and diffuses toward the opposite electrode. The current is carried by the injected ions. The mobility of those ions is typically >lO-a m2 V-r s-l, implying that they are quite small. The second type consists of molecules like AOT which has a CMC and whose association number is large [30,34] and generally concentration independent. These materials dissociate spontaneously in solution to give rise to a volume conductivity. No charge is injected from the electrodes, possibly because of the large size of the charge-carrying entity which prevents close approach to the electrode. (Chemical differences, such as the ability of an ion to be oxidized or reduced, are also important.) The mobility of the charge carriers is small, (6-7). 1O-9 m2 V-i s-r, which is consistent with the large size of the charge carrier. The mobility, and hence the size of the charge carrier, is the same at concentrations appreciably less than the CMC as it is above the CMC which implies the presence of structures at as much as 3 orders of magnitude below the CMC. The conductivity J. of a dilute electrolyte is (7) where ni is the number of charged species of type i per unit volume, pi is the electrophoretic mobility, and zi is the valence of the ion. For the present analysis assume that the only charge-carrying species are the micelles and also make the reasonable approximation that the micelles are spherical with diameter d. The mobility of the micelle is ze

P = 37cdq where q is the viscosity of the liquid [42]. Assuming that the positive and negative ions are the same

I

l-37

size and that they are at equal concentrations, I A

2m2e2n 0 =

3rcdr/

then

(9)

where c1is the fraction of charged micelles and no is the total number of micelles per unit volume. Equation (9) has been used to estimate the degree of dissociation of lecithin micelles in a synthetic alkane, Isopar H (from Exxon) [43]. By measuring the micelle diameter by light scattering, 7.0 nm, and assuming that all the lecithin formed into monodisperse, spherical micelles each micelle having only a single charge, Davis et al. [43] estimate that about 0.29% of the lecithin micelles are charged. Since the lecithin micelle diameter of 7.0 nm is less than the critical size estimated above for stable ionic species, 28 nm, then only a small fraction of the lecithin micelles would be expected to carry a charge. The data Davis et al. used for their analysis comprised the concentration dependence of the conductivity. What is surprising is that they find (as is also found for AOT in benzene [44]) that the conductivity is linear over quite a range of concentration. For weak electrolytes, the degree of dissociation varies with concentration and the conductivity should vary (at low concentrations) with the square root of concentration. When the degree of dissociation is independent of concentration, as it is for bimolecular reactions, the conductivity will vary linearly with concentration. That is, one explanation for the linear dependence of conductivity on concentration is that the charged lecithin micelles are not formed by the dissociation of an uncharged micelle, but by the bimolecular collision of two uncharged micelles, ion exchange, and thus the creation of two charged micelles of opposite sign. Another possible explanation is that these materials contain a small fraction of completely ionized (strong electrolyte) component [44,49]. If the strong electrolyte component exists it must be associated with some of the micelles since to exist as an isolated species it would have to be large and hence detectable.

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I.D. Morrison/Colloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

If we assume the Fuoss equilibrium between bound ion pairs and free ions, then Eqn (3) can be substituted into Eqn (9) to give

(10) which implies that the conductivity should vary as the square root of the electrolyte concentration. Equation (10) is the weak electrolyte model. The data of Davis et al. seem to imply that the charged micelles are formed by bimolecular collisions. Conductivity can only be measured by applying an electric field. If the solute contains weakly bound charge pairs, then the applied electric field can induce the formation of more charged species. The indication of field-induced conductivity is a deviation from Ohm’s law. This phenomenon was first analyzed by Onsager [45] who showed that the increase in the equilibrium constant is proportional to the electric field and inversely proportional to the dielectric constant. In nonaqueous media of low dielectric constant, this means a larger effect than in water [41,44,49]. The behavior of nonaqueous electrolytes is obviously only poorly understood. Materials like the anionic surface-active solute AOT clearly form inverse micelles above some critical concentration. However, the conductivity of solutions of AOT in alkanes does not show an abrupt change in behavior at the CMC [44,49]. A possible explanation is that conductivity measurements are sensitive to the most mobile carriers, which are necessarily the smallest. The presence of inverse micelles may not be important for electrical conductivity because they move slowly. However, the concentration of charged micelles may be important to the ionic strength and hence the Debye length. The application of more modern mathematical techniques that have been applied to aqueous solutions [46] may prove valuable in the study of nonaqueous solutions. 6. AOT, di-(Zethylhexyl) sodium sulfusuccinate Certainly the most studied nonaqueous micelleforming solute is Aerosol OT (from American

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Cyanamid). One reason may be that AOT is soluble in both water and in alkanes and forms micelles in both. Kitahara et al. [47] showed that the size of AOT micelles is strongly dependent on the solvent, ranging from a molecular weight of 10 000 in water to 1800 in ethanol to 25000 in cyclohexane. The micelles in cyclohexane are inverted in the sense that the hydrophilic head groups are internal. Mathews and Hirschhorn [ 143 report the dependence of conduction upon total solute concentration for AOT solutions in dodecane containing solubilized water from about 0.03 to 30% solute and volume ratios of water to AOT of 0.000,0.114, 0.228, 0.570 and 1.14. At the lower concentrations, the addition of water increased the conductance by orders of magnitude. At the higher concentrations, the changes in viscosity make the interpretation of changes in conductance difficult. These authors do conclude that the low apparent specific volume of solubilized water may be interpreted as indicating that the initial increments of water strongly hydrate the hydrophilic groups of the AOT. Furthermore they report that the equivalent spherical diameters of the AOT micelles vary from about 3.4 to 9.9 nm as the water content is increased. Since this size is less than the critical size of about 28 nm, then only a fraction of the AOT micelles carry a charge (see Section 2). Eicke and Christen [48] also find that AOT has an apparent CMC in cyclohexane above which the micelles are nearly monodisperse. Furthermore, they find evidence for premicelle nuclei. The existence of these structures may be significant in explaining the conductivity of dilute solutions of AOT by Eicke and Arnold [34]. The conductivity data of Eicke and Christen [48] showed a sublinear concentration dependence near the CMC but linear (with the same proportionality constant) above the CMC. They interpreted these data to mean that the appearance of inverse micelles at the CMC had a significant effect on the conductivity. Later data [14,41,49,50] (discussed below) show the conductivity to be linear through the CMC concentration range.

I.D. Morrison/Colloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

Randriamalala et al. [50] studied the fieldinduced dissociation of ions in AOT solutions. They found that the conductance increases with field as predicted by Onsager’s theory up to 5.5~10~ V m-r over a wide range of AOT concentrations both above and below the apparent CMC! This means that the presence of inverse micelles does not influence the field-induced dissociation processes. Their low field conductivity data (less than the threshold field for any significant production of new free carriers) for the most anhydrous solutions they prepared is consistent with a model of (i) small charge carriers at concentrations ~10~~ M; (ii) a steady increase in the size of these carriers up to lo- 3 M; and (iii) no change in size of charge carriers >10e3 M. When water is intentionally added to the solutions, the conductivities are substantially increased. The increase in conductivity is either due to an increase in the number of charge carriers or an increase in their rates of diffusion. Water increases the size of inverse micelles, hence their diffusion rates would drop. The conductivity increases because water causes more charge carriers to be formed. Therefore adding water to solutions of AOT increases the conductivity by creating a larger number of charged species. The mechanisms of electrical conduction of hydrocarbon solutions of AOT were examined by Denat et al. [44] under two conditions, the first a low voltage a.c. field and the second a continuous d.c. field over a wide range of voltages. The first set of conditions approximates equilibrium and in this case the equivalent conductivity is nearly constant from lo-’ to 10-l M. They found no change in equivalent conductivity around 10m3 M where AOT micelles are thought to form. They explain their data with a model in which AOT molecules associate into a small structure (presumably the trimer) even at the lowest concentration and some of these trimers exchange charge to produce an equilibrium ionic concentration that is the source of the conductivity. Furthermore they claim that the trimers are the predominant species and that essentially no free AOT molecules are

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present. When constant d.c. fields are applied to the solutions, the current transient decreases gradually from an initially high value to some lower value that depends on the electric field. Their analysis leads them to claim that the source of the ions at steady state is the dissociation of AOT molecules and not due to the charge exchange between trimers as the former rate would be linear in AOT concentration, which is what is found, and the latter would be quadratic. AOT behaves differently from other nonaqueous conducting species in that there is no evidence of charge injection at the electrodes. This may be due to the fact that even the small structures of AOT formed prevent a close enough approach to the electrode for charge injection. Nevertheless this study on the concentration dependence of the conductivity shows that the charge carrying species are association structures but not necessarily micelles. Denat et al. [49] considered this apparent dichotomy between the data presented by those studying association structures like micelles where, for molecules like AOT, there is a fairly welldefined CMC, and those studying electrical properties where, for molecules like AOT, there is no change in the nature of the charge carriers at equilibrium over a wide concentration range including the CMC. They explain that conductivity measurements are most sensitive to the charge carriers with the highest mobilities. Therefore the conclusion is that the majority of charge carriers are small association structures, which form at low concentrations and are present even at the highest concentrations (above the CMC). The micelles that form above the CMC may be charged, but they are so much larger, and thus their mobilities so much smaller, that they do not contribute significantly to electrical conduction. Heffner and Marcus [40] studied the conductivity of AOT in dichloroethane and found a powerlaw dependence of the conductivity on AOT concentration over more than four decades of AOT concentration with an exponent of 0.73 f 0.02. The data do not indicate any sign of a CMC. The authors submit that the data are in contradiction

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I.D. MorrisonlColloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

to the usual models associated with conductivity and call for further experiments into the mechanisms of conduction and the microstructure of the system. No micelles were detected by light scattering although the lack of scattering may be due to index matching between the micelles and the solvent. 7. Particles dispersed in nonaqueous media without added electrolyte It is quite clear that various hydrocarbon-soluble molecules are capable of forming solutions with free ions. The free ions are stabilized by small association structures formed by those ions at concentrations well below any CMC, if the solute has one. The question is whether or not solid particles dispersed in nonaqueous media (with no added molecular solute) can be electrically charged. In aqueous dispersions many particles are charged either by the adsorption or desorption of hydroxyl or hydronium ions [Sl]. This mechanism is less likely in nonaqueous media because the desorbed protons or hydroxyls are not complexed by the solvent. Several investigations of the sign of the charge of various particles in a variety of nonaqueous solvents have been reported. Damerell et al. have found carbon to be negative [52], calcium carbonate to be positive [53] and silica to be positive, all in xylene [54]. Dawson and Oei [55] found CuS and As& sols in N-methylacetamide to be negative. In the latter case, one can imagine that the N-methylacetamide can complex positive ions desorbed from the particle surface, but for the dispersions in xylene, it is difficult to see how small ions can be stabilized. Tamaribuchi and Smith [56] studied the combinations of carbon black, toluidine red, and titanium dioxide in 2,2,4_trimethylpentane, benzene, methyl ethyl ketone, and ethyl acetate and concluded that the charge-determining step was the exchange of protons because the sign of the solid particles changed from positive to negative as the base strength of the solvent increased. All of these investigations used electrodeposition

1-37

to measure the sign of the particle. One wonders whether the electrodeposition might be measuring some electrode reaction, possibly at high field if the particles come close to the electrode, rather than some equilibrium charge distribution. In any case the simple interpretation of the data stands in opposition to the idea that the separation of charge depends on a sizable structure in which each ion must be bound. Koelmans and Overbeek [57] showed that electrodeposition is a consequence of electrolysis in organic solvents with higher dielectric constants (e.g. methanol and acetone.) Model colloids prepared from polymethyl methacrylate by Kitahara et al. [SS] showed no electrophoretic mobility when dispersed in cyclohexane. These particles were stabilized with covalently attached butyl, lauryl, and glycidyl methacrylate copolymers and could be charged by the addition of metal salts of alkyl sulfonic acids. Early measurements by electro-osmosis established that there exists a regular sequence in the charge on oxide particles in various media, i.e. SiOz > TiOz > ZrO, and water > acetone > ethanol>methanol [59]. In other words, in every combination, the point of zero charge caused by a distribution equilibrium of H+ and OH- as potential-determining ions, is such that in each medium SiO, is more negative than TiO,, and TiOz more negative than ZrO,, and that each substance is more negative in water than in acetone, in acetone more negative than in ethanol, and so on [59]. SiOz is weakly acidic, TiOz intermediate, and ZrO, is basic. The solvent series becomes increasingly more acidic going from water to methanol. Water added to nonaqueous dispersions of these metal oxides is expected to be adsorbed at the surface and affect its acid or base strength. The change in charge of the particles with change in acid or base strength of the solvent is analogous to the change in the point of zero charge of the oxides in water as a function of pH [60]. The acid-base or donor-acceptor interactions of materials provide a powerful model in the understanding of the interactions at interfaces [61,62]. Labib and Williams [63,64] have applied

I.D. Morrison/Colloids Surfaces A: Physicochem. Eng. Aspects 71 (19931 l-37

the donor-acceptor scales of Gutmann [6S] to the problem of characterizing SiOz, TiOz, A1203, and MgO particles in nonaqueous liquids. They define the donicity of the solid by setting it equal to the donicity of a solvent with which it neither donates nor accepts charge. They determine the charge on the particle by using the Laser Zee Meter Model500 microelectrophoresis apparatus from Pen Kern Co. The cell is stored at 110°C to keep it dry. Furthermore, by knowing the donicity of the solid by electrophoretic measurements in nonaqueous liquids and the point of zero charge in water, they have an experimental alignment of the donicity scale with the pH scale. They make quantitative the earlier work of Verwey [59] (see above). Furthermore, Labib and Williams have studied the effect of added water on the donicity of the oxides [66]. They consistently find that moisture weakens interactions, making the solids less acidic. While it seems easy to understand that different solvents can exchange charge with a solid surface in a manner analogous to the matching of Fermi levels of electrons across metal junctions, what needs explaining is how this exchange of charge can be over long enough distances to give a measurable zeta potential. One possibility is that the solvents tested by Labib and Williams can all form reasonably sized ion-solvent complexes. The solvents were 1,2-dichloroethane, nitromethane, nitrobenzene, acetic anhydride, acetonitrile, ethyl acetate, tetrahydrofuran, dimethylformamide, dimethylacetamide, dimethylsulfoxide, ethylene diamine, and trimethylamine in ascending donicity number. This kind of information gives clues as to possible methods for charging particles. A small amount of one of these solvents added to (or present as an impurity in) a noncomplexing solvent may be capable of reacting with ions on the solid surface and forming lyophilic ions. 8. Electrodynamics

of nonaqueous colloids

The motion of particles in nonaqueous dispersions when electric fields are applied obeys the same laws of physics as the motion of particles in

11

aqueous dispersions (of course!) However, because the number of charge carriers is lower (low conductivity) and the electric fields used to study them are often larger, four important effects are frequently encountered in the study of nonaqueous dispersions that are rarely encountered in the study of aqueous dispersions. The first of these is the long time needed for the relaxation of electric fields created by charge separation, the second is the build-up of space-charge layers at electrodes, the third is the formation of nonuniform electric fields, and the fourth is electrohydrodynamic instability. Relaxation times

If the discharge of the electric field across a fluid is modeled as the discharge of a capacitor, the characteristic time is [67] 7=.

DC, A

(11)

where t is called the relaxation time and 1 is the specific conductivity (R-i m- ‘). The half-time for discharge is 69% of the relaxation time. Half-times for hydrocarbons vary from about 12 000 s for highly purified hydrocarbons to a few seconds for light distillates [2]. (As a reference, distilled water has a half-time of a few microseconds.) Any fluid motion that occurs on time scales shorter than relaxation times will produce charge separation. Space-charge eflects

With the application of an electric field to a dispersion of charged particles, the particles (and ions) diffuse toward the electrode of opposite polarity. This motion continues, of course, until the particles (and ions) reach the electrode. If charged particles (or ions) collect on the electrode and are not neutralized by the electrode, they will form what is called a space-charge layer. This layer of charged species reduces the electric field between the electrodes. If the electric field is reduced, then the motion of the remaining particles (or ions) will

12

I.D. MorrisonlColloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

be reduced. To the observer it appears as if the remaining particles have less charge. If the effect is rapid, then the dispersion appears to be of uncharged particles, because when an electric field is applied, no motion is apparent. It is possible that the (apparent) lack of motion of particles in a nonaqueous dispersion has led some to conclude that the particles are uncharged, especially if the observations were not made promptly or if what appeared to be some initial instabilities were ignored. This phenomenon is not seen in measurements of aqueous dispersions for two reasons: (i) the bulk of the current is carried by ions and the ions are more likely to exchange charge at the electrode than are particles and (ii) even if the electric field builds up at the electrode, it can only do so until some component of the dispersion, usually water, begins to be oxidized or reduced. Nonuniform electric$elds

Nonuniform electric fields are generated when space-charge layers build up around electrodes. Nonuniform electric fields can also develop when the materials between two electrodes have a large difference in dielectric strength. This phenomenon is particularly acute in the design of some electrophoretic cells. In the usual microelectrophoresis cell, the two electrodes are separated by a capillary containing the dispersion. When the dispersion is an aqueous dispersion, the electric field lines pass from one electrode to the other through the conductive dispersion. However, when the dispersion is a low conductivity dispersion, the cell walls or, even worse, the constant temperature bath surrounding the cell may provide a more conductive path between the electrodes. In this case the electric field lines pass through the cell walls and into the constant temperature bath (which is certainly at ground potential) and not through the dispersion. If the electric field lines do not pass through the dispersion, then there will be no electrophoretic motion even if the particles are charged. It is possible that some investigators, on seeing no motion in their electrophoretic cells, have con-

l-37

cluded that their particles were uncharged, when in fact they might have been highly charged but did not move because the electric fields were outside the cell [68,69]. Electrohydrodynamic

instabilities

Almost all fluids are populated by a variety of charge carriers. When an electric field is applied, the fluid will move. If the motion of the charge carriers is small then (it is easy to imagine) the motion can be treated simply as the motion of particles and the countermotion of the liquid is ignored. However, this is an approximation whose limits must be estimated. The general analysis has been presented as an analogy to Benard instability [70], which is the instability in a fluid caused by density gradients (sometimes caused by thermal gradients). For uniform conduction, the threshold voltage V, from laminar into the turbulent regime is approximately [71] 3Orl IG = (p&J*

(12)

where p is the density. Note that the limit is a voltage, not an electric field. Since electrophoresis is a field-dependent measurement, the smaller the gap between the electrodes, the higher the field can be before the electrohydrodynamic instability sets in. For typical values of viscosity and density, the critical voltage is about 100 V. This limit of voltage conflicts with the requirements for higher voltages in the measurements of electrical and optical transients and in electrophoresis of nonaqueous dispersions. The saving factor is that the electrohydrodynamic instabilities take some time to develop. A more thorough discussion of the various limits in these measurements is given by Novotny [71]. 9. Principles of particle charging When a solution of a nonaqueous electrolyte in a solvent is considered, then the charge-exchange reactions are symmetric, and the solution has as

I.D.

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many anions as cations [2]. If we consider just the interaction of a solvent with a particle surface, then whatever the sign of the association structure that carries the countercharge in solution, the particle carries the opposite charge [17]. (It is possible for a single nonaqueous electrolyte to react with different portions of the particle surface producing both signs of charge: see Section 14 below.) The sign of the charge depends on the relative acidity or basicity of the core of the micelle compared with the surface with which it is interacting. Basic micelles are positively charged and acidic micelles negatively charged. This means, of course, that the particles they have interacted with are of opposite sign. The basic micelles have produced negatively charged particles and acidic micelles have produced positively charged particles. This is exactly opposite to aqueous solutions where acidic surfaceactive solutes produce negative particles and basic surface-active solutes produce positive particles. Kitahara et al. [SS] studied the charging effect of sodium and calcium 1,2-bis(2-ethylhexyloxycarbonyl)- 1-ethane sulfonic acid, dodecylammonium propionate, and dodecylammonium butyrate on sterically stabilized poly(methacrylate) particles and on carbon black and titanium oxide in cyclohexane. They postulate the rule that the sign of the particle charge is determined by the preferential adsorption of the more hydrophobic ions on the hydrophobic surfaces of the polymer colloid and the carbon black and the adsorption of the more hydrophilic ions on the hydrophilic surface of titanium oxide. They report other data on the charging by Mn(OT), and Co(OT), but concluded that irreversible chelating interactions with the surfaces complicated the simple rule. This rule is difficult to understand in the sense that no surface could be more hydrophobic than the cyclohexane solvent so that preferential adsorption of the hydrophobic ion is surprising. Fowkes and co-workers [72-741 propose quite a different mechanism. They maintain that the degree of dissociation of nonaqueous electrolytes is so small that preferential adsorption cannot account for particle charging. Instead they propose

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13

that the charging of particles comes about by the adsorption of uncharged electrolyte, followed by ion exchange with surface groups and then desorption of charged electrolyte. The most common ion exchanged is a proton, hence the mechanism is an acid-base interaction. This mechanism explains why in nonaqueous media cationics charge particles negatively and anionics charge particles positively, exactly the opposite of what is found in aqueous media. The most usual mechanism for ion exchange is proton (or hydroxyl) transfer between micelles and particles or container walls [56]. Certainly, transfer of larger ions is possible when free ions such as calcium or sodium are available. Adsorption of water at the surface of particles is a likely prerequisite for the formation of mobile ions. It is not surprising that the addition of water to a nonaqueous dispersion of hydrophilic particles has a significant effect on the sign and magnitude of the charges on those particles. Experimental verification of this mechanism was provided by Fowkes et al. [75]. Carbon particles in white oil stabilized with 14C labeled copolymers of long chain methacrylates with vinylpyridine were electrodeposited and the electrodes examined for 14C. The carbon particles were plated on the anode (they carried a negative charge) and the i4C labeled copolymer was deposited on the cathode. Note that the particles and the copolymer were plated on diflerent electrodes. If the charging mechanism was the ionization of the copolymer in solution followed by the preferential adsorption of the charged polymer, then the particles and the copolymer would have deposited on the same electrode. The electrocratic stability of these dispersions is not due to the adsorption of the basic copolymer but rather due to the extraction of a proton by micelles of the basic polymer to generate a negative particle. Experiments with the same set of materials using tagged and untagged polymer showed that while adsorbed polymer was so tightly held on the particle surface that it could not be washed off by solvent, polymer in solution replaced some of the

14

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adsorbed polymer in just a few minutes. The exchange reaction was detected by first allowing tagged polymer to be adsorbed and then resuspending the polymer-coated particles in a solution of untagged polymer. Some of the tagged polymer appeared in the solution within a few minutes. Indeed, the “equilibrium” between adsorbed and free polymer is an area of active research. Most examples of polymer adsorption appear to be irreversible in the usual sense that they are not washed off when the solution is replaced by the neat solvent [76]. By measuring the zeta potentials of a series of acidic and basic solids in solutions of acidic and basic polymers the following was observed [75]. The acidic polymer, post-chlorinated polyvinylchloride, generated positive particles of CaO and CaCO, in dioxane. The basic polycarbonate, Lexan 145 (General Electric), in methylene chloride generated negative particles of the acid-washed clays kaolin and bentonite. Additional work was done by varying the acid and base strength of the solvent. When the base strength of the solvent is high, then it effectively competes for any active acid sites on the polymer or on the solid and thus prevents particle charging. When the acid strength of the solvent is high, then it effectively competes for any active base sites on the polymer, or on the solid, and this prevents particle charging. In any case the sign of the charging of the particles can be understood in terms of a balance of acid and base strengths between the polymer, the solvent and the solid [61,75,77-813. A difficult conceptual point is the idea of charges being exchanged between the surface of a particle and the interior of a micelle, a transfer across several nanometers at least. This seems to present a considerable energy barrier. One possible explanation is that the micelle should be considered as a dynamic structure with individual molecules diffusing in and out at a rapid rate. The means by which an ion would be transported to the core of the micelle is on the back, as it were, of a new solute joining the micelle structure. This dynamic picture of the life of a micelle has been used to

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explain the fast diffusion of some micelles in aqueous solution, where the rate of diffusion of a single micelle is more easily explained in terms of individual molecules separating from the micelle, diffusing and reforming into new micelles [82]. In liquid solutions, the exchange of ions is the most likely means by which particles become charged but it is possible for two materials to exchange electronic charges [83,84]. Only by carefully designed experiments are the relative contributions of electron-charge exchange and ioncharge exchange to be understood [85,86]. 10. Particles dispersed in nonaqueous media with added electrolyte Damerell et al. [52-541 studied the effect of more than a score of solutes on the dispersion of carbon, calcium carbonate, and silica in xylene. Twenty-four solutes were tested on carbon, and lecithin, copper oleate, cobalt naphthenate, and AOT were found to be the most effective of those tested. Sols containing lecithin or AOT showed migration towards the anode (i.e. negative particles) at 100 V, whereas sols containing the soaps required 900 V before migration. Dryness or wetness did not seem to affect migration, and even a sol made under the driest of conditions showed migration. However, the less the water, the more stable the sols [52]. A similar study with calcium carbonate showed similar results except that the calcium carbonate particles were positively charged with the same solutes that charged the carbon negative and that the most effective solutes were the zinc, barium, and sodium salts of dioctyl sulfosuccinate, sodium dihexyl sulfosuccinate, lecithin and magnesium oleate. When lecithin was used as a dispersing aid, both positively and negatively charged particles were present. When a sol of carbon in xylene (negatively charged) was added to a calcium carbonate dispersion (positively charged), almost complete precipitation of both dispersed phases took place [53]. A third study was with silica dispersions in xylene. In all dispersions the silica particles were positive. The most

I.D. MorrisonlColloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

successful solutes were found to be the various metal sulfosuccinates, metal naphthenates, and lecithin [54]. Van der Minne and Hermanie [87] studied the action of two electrolytes: calcium diisopropylsalicylate (Ca-Dips) and tetraisoamylammonium picrate (Tiap) on carbon in benzene. The anionic CaDips charged the carbon particles positively, and the cationic Tiap charged the particles negatively. Again, this is opposite to what normally happens in aqueous solutions where anionic additives charge particles negatively and cationics charge particles positively. For the anionic Ca-Dips the adsorption was strong enough at low additions of agent to leave no free solute in solution. In this regime the particles remained uncharged. When the particles were nearly saturated with Ca-Dips, the electrophoretic mobility increased quickly with subsequent addition of Ca-Dips. Beyond a certain concentration, the particle mobility and the concentration of adsorbed solute decreased somewhat. This process is important to understand. When the adsorption is strong, as it is with Ca-Dips in benzene on carbon, then all polymer added to the dispersion is adsorbed by the particles until the polymer has covered a significant amount of the particle surface. No charged species are created. When the particle surface is substantially covered with polymer, a dynamic equilibrium is set up between adsorbed and free polymer. Some of the desorbing polymer has reacted with the particle surface and desorbs with a net electric charge. Since the Ca-Dips is an acidic polymer, it leaves its proton behind and desorbs as a negative ion. The carbon particle acquires a net positive charge. These authors [87] comment that no electrophoresis occurs unless a sufficient number of ions is present in solution. This is not to be interpreted to mean, as van der Minne and Hermanie claim, that there is some limitation to the experimental method in that free ions are required to establish an electric field in the electrophoretic cell. Electric fields are perfectly easily established across a vacuum! Rather the interpretation is that the particles have no charge until the free ions of the

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15

electrical double layer are produced. The particles cannot be charged without countercharges. The behavior of the cationic Tiap was fundamentally different. Adsorption of Tiap on the carbon was relatively small and followed a Freundlich isotherm, i.e. increasing linearly with the square root of the concentration of Tiap in solution. The particle charge became gradually more negative, reached a maximum negative charge, and then decreased towards zero, all while the adsorption was increasing. The tetraisoamylammonium cation is a large symmetric ion with the diameter necessary to lift free of its counterion on the surface of the carbon. Evidently this material is able to charge carbon particles by preferential adsorption. The decreasing negative charge at higher concentrations may be due to double-layer compression. Van der Minne and Hermanie continued their analysis with mixtures of the two electrolytes. The two electrolytes were antagonists with the presence of a small amount of either reducing the net charge and the stability of the system, adding credibility to the idea that these are electrocratic systems. We suggest that the difference between the behavior of the anionic Ca-Dips and the cationic Tiap can be included in the classification scheme of Muller [30] with Ca-Dips being Type II and Tiap Type I. Furthermore, the fact that the anionic additives charged the particles positively and the cationic charged the particles negatively implies that the ion preferentially adsorbed is the small polar ion. Therefore the sign of charge on the particle is due to the smaller polar ion having been adsorbed by the polar surface. This same conclusion was reached by Koelmans and Overbeek [57]. They derive the theoretical basis for the claim of van der Minne and Hermanie that these nonaqueous carbon dispersions are electrostatically stabilized and show that quite modest zeta potentials are sufficient. They provide data showing the correlation between stability and zeta potential (about 30 mV for a 1 urn particle). The stability analysis described below, Eqn (17), predicts that a potential of 32 mV is adequate. McGown et al. [SS] studied the variation of zeta

16

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potential with concentration of AOT in xylene. They also find no change in equivalent conductance from 0.5 to 50 mM which means that the presence of micelles, which should form at a concentration of a few millimolar, does not influence the conductivity of the system. Their zeta potential measurements on rutile also show AOT to charge the particles positively. All their zeta potential data are taken above the concentration of maximum adsorption, about 0.5 mM, so that they only see a decreasing potential just like that reported by van der Minne and Hermanie for Tiap. They were not able to make zeta potential measurements at lower concentrations because of relaxation effects at the low conductivities. This effect is due to having their cell in a water bath [68,69]. They also report substantial effects of water for the hydrophilic titanium oxide particles. Water can both be adsorbed at the particle surface and absorbed into micelles. They suggest that the sign of the charge on the particle is determined by the location of any water that will hydrate the small free ions. For instance water will accumulate at the surface of rutile so that rutile particles also adsorb sodium ions and in this case become positive. Water will not accumulate on carbon surfaces and will therefore be solubilized in the micelles which become positive. Adsorption of AOT anions makes the carbon particles negative. They assert that van der Minne and Hermanie found positively charged carbon particle because they used a highly oxidized carbon. The effect of solubilized water on the stability of carbon black in cyclohexane solutions of anionic and nonionic surfactants has been studied [SS]. The dispersions go through three regions as water is added: (A) at low concentrations of water, the particles are stable, presumably because of electric charges; (B) the dispersions become unstable at intermediate water concentrations because adsorbed water forms a hydrophilic surface on the carbon; (C) at high water concentrations, a waterin-oil microemulsion forms and the dispersion is stabilized by the network structure formed by the microemulsion drops. Two concentrations of solute

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(AOT) were used and the transition concentrations were found at the same water/AOT ratios. Kitahara et al. [7] measured the effect of water on the charging of hydrophobic particles such as carbon in nonaqueous media. They used a polycarbonate electrophoresis cell, air thermostated. When water is added to carbon dispersions, the zeta potential barely changes. Kitahara et al. explain the small changes in zeta potential with added water in terms of a general change in the partitioning of the water between the interface and the micelle, which then changes the partitioning of the sodium cation from the AOT. Surprisingly, even though the zeta potential changes only a small amount with the addition of water, Kitahara et al. find a large change in dispersion stability. All the data were taken above the concentration of AOT at which the surface is saturated and at which micelles would form. They concluded that the relation between zeta potential and stability is only true at higher water concentrations. Kandori et al. [90] find that AOT charges iron oxide and titanium oxide positively in dioxane and negatively in water, with a gradual transition in charge with change in composition from pure dioxane to pure water. The charged regions also correspond to the solvent compositions at which AOT is adsorbed on the solid. Kandori et al. explain that the sign of the particle charge is determined by preferential adsorption of ions from solution. The ion that adsorbs varies as the solvent changes from dioxane to water. Miller et al. [ 1773 report studies on calcium high base-number sulfonates which are colloidal dispersions of calcium hydroxide/calcium carbonate particles stabilized with alkylbenzene sulfonates. These materials are used in engine lubricating oils as bases to react with acidic residues (see Section 20 for more on petroleum sulfonates). The high viscosities of these dispersions required the development technique (see of a special electrophoretic Section 12 for phase-analysis light scattering). The variation in mobility with concentration was shown to be due to the change in viscosity and not to a change in particle charge. The particles

I.D. MorrisonlColloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

were positive and not sensitive to water in saturated hydrocarbons. In toluene, the particles were less positive but lost charge with time and were sensitive to water. These effects were explained in terms of the different solubilities of the sulfonates. A key parameter in understanding the behavior of dispersions is the thickness of the electric double layer around particles. This thickness depends inversely on the square root of the ionic strength of the solution. The ionic strength is the sum of the products of ion concentrations and the square of the ionic charge. The difficulty is that the ionic concentrations of most nonaqueous dispersions are unknown because the degree of dissociation of soluble species or surface groups is unknown. The charging of particles in nonaqueous media by added solutes at low concentration can therefore be by several mechanisms: (A) the preferential adsorption of either the anion or cation of a dissociated electrolyte; this could also be by the adsorption (and rearrangement?) of charged micelles; (B) the dissociation of a surface anion or cation and its stabilization in an association structure like a micelle; (C) the adsorption of a solute, followed by its complexing with a surface anion or cation and subsequent desorption as a charged complex [72]. As the concentration of the solute increases, the charge on the particle can be changed by the electrostatically assisted adsorption of solute ions. This effect manifests itself as a maximum in particle charge (zeta potential) with increasing solute concentration and has been reported for carbon black dispersed in a benzene solution of tetraisoamylammonium picrate [87], copper phthalocyanine dispersed in a heptane solution of AOT [88] and BaSO, dispersed in a cyclohexane or heptane solution of AOT [7]. It is generally assumed that the decrease in particle charge with solute concentration at high solute concentrations is not due to double-layer compression as the actual concentration of free ions in solution is small. The concentration dependence of solutes on the charge (zeta potential) of dispersed particles has been classified into three types [91]: (a) solutes of

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17

Type A show a maximum in zeta potential with solute concentration; (b) solutes of Type B show a steady decrease in zeta potential with concentration; (c) solutes of Type C show no change in zeta potential with concentration. For solutes of Type A the mechanism was proposed to be a twostep process. The first step is the preferential adsorption of either the cation or anion from the dissociated solute in solution, possibly on active sites of the particle. After the particle has acquired some charge by preferential adsorption, subsequent general adsorption of charges of both signs reduces the net particle charge. Simple equilibrium equations were proposed and equilibrium constants calculated. Another way to express this concept is that the particle charge is not directly related to the amount of solute in solution or on the surface, or at least not to the quantity derived from a simple adsorption experiment. The particle charge and solution conductivities are determined by the small fraction of solute that is dissociated [6]. Merinov and co-workers [92,93], in a series of experiments on the dispersion of polyvinyl chloride particles in the common plasticizer, dioctyl phthalate, stabilized with a sodium alkylsulfonate, find that the charge on the particles is due to the dissociation of adsorbed solute molecules from a saturated surface layer. The ions are either stabilized because of their size or because they are solubilized in the inverse micelles of the sodium alkylsulfonate. 11. Charged carbon particles in alkanes The prototypical nonaqueous dispersion is one comprising carbon particles dispersed in a saturated hydrocarbon liquid. It is almost self-evident that such a two-component dispersion should have no stable electric charge [94,95]. Van der Minne and Hermanie [68] observed, however, that suspensions of carbon in mineral oil could be stabilized by either calcium soaps or oil-soluble derivatives of the oil (called oxidation resins). However, when the two dispersions were mixed, the suspension flocculated quickly. In aqueous

18

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suspensions these observations could be explained by charge-neutralization reactions. In nonaqueous suspension, this explanation would not have been well accepted. After careful construction of a cell (described below) they were able to show that carbon particles could be charged either positively or negatively in both mineral oil and benzene [87]. Furthermore, they showed that the measured electric charges correlated in the expected way with the stability of the dispersion and with adsorption measurements. In a series of papers Fowkes and co-workers [77,79,96,97] described a study of the stabilization of carbon particles in a hydrocarbon by the addition of a polyisobutylene succinimide, OLOA 1200 from Chevron Chemical Co. This material, and others similar to it produced by Lubrizol and Amoco Chemicals, are common dispersants in the petroleum industry, particularly as additives for crankcase oil. The succinimide head groups are formed by a condensation of polyethyleneimines with succinic anhydride. The number of reaction products is manifold. In any case, the head groups can be tailored to have both acidic and basic sites, although they are predominantly basic. Therefore these head groups will attach themselves to carbon particles, particularly particles with acidic surfaces formed by the partial oxidation of oil and gasoline. The long polyisobutylene tails then form a steric layer around the particles. Fowkes and Pugh discovered that the stabilization of carbon particles by these materials was not just steric but also included a significant electrostatic component. They pointed out that the length of these molecules, about 5 nm, is too short to stabilize the carbon particles by simple steric repulsion only. The OLOA 1200 also produced significant charges on the suspended particles. The stability of the dispersions was a combination of the steric effect of the adsorbed layer and the electrostatic repulsion between particles generated by a charge exchange between the particle surface and OLOA 1200 micelles in solution. Parreira [98] studied the charging of carbon black (Sterling TP 138) in benzene solutions of the

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Mg, Ca, and Ba salts of dioctyl sulfosuccinates. Although the carbon is thought to be uncharged with no solute present, the maximum charge for all solutes, i.e. + 68 mV, was obtained at the lowest concentrations reported, lop4 M. The zeta potential decreased monotonically with concentration for both the Mg and the Ca salts. The Ba salt showed a minimum of +20 mV at about lo-’ M. Parreira argues that the data show that the particles are charged by preferential adsorption of soluble ions, but he presents no adsorption data nor solute conductivity data. The fact that the surface potential is monotonically decreasing (towards zero) with increasing solute concentration over the range of concentrations studied, means that arguments about preferential adsorption are based on phenomena out of the range of the data. 12. Experimental techniques for dispersions The classical experimental work to establish that some dispersions in hydrocarbons are stabilized by electric charge was by van der Minne and Hermanie [68,87]. First they designed a microelectrophoresis cell in which they could make reliable electrophoretic measurements on low conductivity dispersions. The cell was the normal capillary electrophoresis cell, two large volumes, each with an electrode, connected by a narrow capillary, except for two important design changes. The first was that the entire cell had to be constructed of quartz and carefully kept dry and/or coated with silicones. The second was that the capillary had to be surrounded by a vacuum or at least air and not held in a cooling bath. Both these precautions were to keep the electric field produced by the two electrodes focused down the capillary and not shorted to ground through a conductive capillary wall or through a cooling water bath. Parfitt [99] proposed using a rectangular cell for use with nonaqueous colloids. The advantage of the rectangular cell is that the optics are much simpler, especially when it is necessary to find an absolute position in the cell. The capillary tube design is popular because it is easy to cool. Joule

1.D. Morrison/Colloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

heating caused by the passage of current during the measurements must be eliminated with proper cooling. The current flow in nonaqueous measurements is so low that Joule heating is not a concern and cooling is not necessary, although temperature control is. However, Parfitt enclosed his cell in a water bath. As van der Minne and Hermanie point out, this can be a source of error. Since it is not necessary, the bath should be avoided for nonaqueous measurements [69]. Parreira also designed a rectangular cell for measurements of liquids at low conductivity [98]. Careful attention was paid to the location and design of joints, the inclusion of support structures, and the inclusion of narrow capillaries to equalize pressures. The cell was constructed of quartz and was not used in a constant temperature bath. The performance of the cell was checked with a carbon black (Sterling MT) dispersion in a benzene solution of calcium dioctyl sulfosuccinate. The electrophoretic motion in the cell was linear in the van Gils plot [ 169,170] and careful attention was paid to avoid any fieldinduced polarization due to the use of excessive applied voltages. Modern commercial equipment for measuring electrophoretic mobilities, which are designed to provide the uniform electric fields necessary for measurements in low conductivity liquids, are available (Coulter Electronics, Inc., Hialeah, FL and Malvern Instruments, Inc., Southborough, MA and Worcestershire, UK). These instruments use laser Doppler velocimetry (LDV) to measure the speed of the moving particles. A new optical technique [ 1761 called phase-analysis light scattering (PALS), is a variation on LDV and is capable of measuring electrophoretic mobilities down to 10-l’ m-* s-l V’. This allows the use of lower electric fields or the detection of smaller surface potentials. Fowkes et al. [75] used a simple electrical deposition technique where the nonaqueous dispersion is placed between two flat electrodes about 2 cm apart and a voltage of about 100 V applied for 16 h. Most experiments were done with electron microscope grids for electrodes and the mass

l-37

19

from transmission deposited was estimated electron micrographs. Stotz [171] was the first to study in detail the field dependence of the electrophoretic mobility of particles in low conductivity liquids. The low conductivity of the dispersions allowed the application of large electric fields, up to lo6 V m-‘, without excessive Joule heating. Electrokinetic phenomena vary between two limits, that of electrical double layers thin compared to the particle radius, the Smoluchowski limit, and that of electrical double layers thick compared to the particle radius, the Hiickel limit. In aqueous dispersions, changes in ionic strength of the medium cause the transition from one limit to the other. Stotz pointed out that in low conductivity media, the electric field strength can be increased to such an extent that particles can be stripped from their electrical double layers. In this limit their motion is described by a balance of coulombic attraction and viscous drag. Therefore a dispersion with the electrical double layers near the Smoluchowski limit can be made to behave like dispersions near the Hiickel limit by increasing the electric field strength. He demonstrated this phenomenon with 6 urn and 30 urn polystyrene particles dispersed in isodecane and charged positively with chromium anthranilate ionic additives. The cell he built consisted of two parallel, transparent electrodes, 1 mm apart. A sample of the dispersion was allowed to settle so as to obtain the clear serum between the electrodes then a drop of the suspension was added to the top of the cell and the particles allowed to sediment. As the particles settled between the electrodes, they were illuminated through the electrodes by a bright, narrow beam of light. Pulsed electric fields were applied during the sedimentation and the trajectory of the particles recorded on a photograph. From the velocity of the particles during the application of the electric field and the sedimentation velocity, both the electrophoretic mobility and the Stokes diameter were calculated. Stotz demonstrated the gradual stripping of the electrical double layers from the charged particles as the electric field was increased.

20

I.D. Morrisonlcolloids

Stotz’s studies of the field dependence of the electrophoretic mobility was extended by Vincett’s [172] study of the motion of pigmented polystyrene/n-butyl methacrylate spheres dispersed in a mixture of isoparaffins (average molecular weight near 150) at electric fields up to 3 - 10’ V m- ‘. No peptizing agents were added. The particles were plated against a metal electrode with the application of an electric field. The electric field was reversed and the particles, which had acquired electric charges during plating, diffused across the gap between the electrodes, usually 180 urn. The motion of the particles was reflected in the current transient as recorded on an oscilloscope. It was found that the motion of these particles corresponded to the limiting case of complete stripping of the counterions from the charged particles as the mobilities were independent of electric field. Subsequent work by Vincett [173,174] described in more detail how the current transients generated by the motion of charged particles and ions in nonaqueous dispersions, after the application of an electric field, could be used to obtain the particle mobility, particle charge, degree of aggregation, adhesion forces, and information on the high-field charging mechanisms. These studies were applied to systems that had no added nonaqueous electrolytes so that the electrical charges present in the dispersion were due to the interactions at the electrodes. These interactions are particularly evident at high electric fields. A simple, quantitative method to measure the average charge-to-mass ratio of suspended particles is to apply a constant potential between two parallel-plate electrodes for a known amount of time, calculate the total number of electric charges passed through the cell, and measure the mass of particles plated on the electrode. Since some of the current could have been carried by charged micelles, a second measurement of the current carrier with the supernatant only is made, the difference between the two measurements being the charge carried by the particles. In the author’s laboratory we use a high voltage (up to 2 kV) power supply and a simple cell with a 1 mm gap

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1-37

between two tared metal electrodes that can be removed easily. The low potential side of the cell is grounded through a coulometer. Quite often we are only interested in the sign of charge on the particles, or in establishing whether any particles are charged or not. Answers to these questions are apparent by observation. A similar experimental apparatus can be built where a high electric potential (e.g. 1 kV) is applied for sufficient time (about 3-6 s) to plate all the particles in the gap [loo]. The mass of particles plated is known from the dimensions of the cell and the particle concentration in the dispersion and therefore does not need to be measured. The current transient can be recorded. Comparisons of current transients on the dispersion and on the supernatant can be used to differentiate current due to the motion of particles and current due to the motion of ions. The motion of charged particles in a nonaqueous medium can be detected by measuring the intensity of backscattered light as the particles approach a transparent electrode. The measurement of the optical transient as well as the electrical transient caused by the motion of the particles is analogous to the time of flight measurements used in the study of carriers in semiconductors with the added advantage of being able to “see” at least some of the charge carriers, i.e. the charged particles [101,102]. The technique can be applied to either a uniform dispersion of particles or to a cell with all the particles initially plated against one electrode. Measurements can be carried out over a wide range of particle concentrations and electric fields (particularly, high electric fields). In principle the average particle sizes, mobilities, charges, and particle-substrate forces can be measured and the first moments of their distributions estimated. A variation on this backscattering detection is to illuminate the receding boundary of the dispersion as it is made to migrate away from the transparent electrode [103]. The backscattered light from the moving boundary will interfere with light reflected from the transparent electrode, going in and out of phase as the separation between the

I.D. MorrisonlColloids

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Eng. Aspects 71 (19931 I-37

two surfaces varies. The detection of the resulting optical signal with a photodiode yields an interference diagram. The electrophoretic mobility is calculated from the frequency of this interference diagram and the applied electric field. The observed motion was shown to be caused by pure electrophoresis and there were no indications of fieldinduced polarization effects up to fields of the order of 10’ Vm-‘. If the cell, described above for measuring the electrical and optical transients as particles are plated against a transparent electrode, is modified so that the particles plate against an electrode covered with a collection sheet, then the chargeto-mass ratio of the particles, the mobility of counterions, and the concentration of excess ions can also be determined [ 104,105]. The end of the optical transient indicates the end of particle motion and any further electrical transient is due to ionic species. The mass of the particles collected on the electrode and the measured charge they carried to the electrode give the average chargeto-mass ratio of the particle. From the charge-tomass ratio and the particle size the surface potential can be calculated. The experiment is run at sufficiently high applied electric fields or sufficiently low particle concentrations that space-charge effects at the electrodes are negligible. For measurements of electrical properties of nonaqueous dispersions, space-charge effects caused by the build-up of charges at an electrode can be substantial. An interesting variation of the electrical transient technique has been proposed by Novotny [106] who constructed a special cell with a series of parallel electrodes spaced at intervals down opposite walls so that the electrical properties at different heights in the cell can be measured independently and at various times. Initially all the electrodes would sense the same electrical conductivity because the composition of the dispersion is uniform. However, as the dispersion settles, each pair of electrodes would sense different conductivities depending on the particle composition at the height of the electrode. The results from this appa-

21

ratus for dispersions of titanium dioxide in xylene with two different types of solutes, one physically adsorbed and the other chemically attached, are dramatic, At low particle concentrations the dispersions have similar electrical conductivities to the supernatants but with a measurable component due to the particles. As the particle concentrations are increased, however, the dispersion conductivities can exceed the supernatant conductivities by two to three orders of magnitude! Some of the nonlinear effects could be due to nonuniform electric fields that can arise with particle motion. A somewhat similar technique is described by Ditter and Horn [103]. The pigment dispersion is placed in a cylindrically shaped brass electrode with a platinum center electrode suspended from the arm of a sensitive torsion balance. When an electric field is applied, the particles migrate to the wire electrode, and the increase in weight of the electrode is recorded. The zeta potential of the particles can be calculated with some simplifying approximations, but just taking the ratio of the mass collected per unit current was shown to be a satisfactory substitute for characterizing the extent of particle charge. That is, the mass/ampere ratio was proportional to deposition time, and the charge-to-mass ratio was independent of applied voltage time and proportional to pigment concentration. A new technique has been proposed to measure the average charge-to-mass ratio of particles suspended in low conductivity media [1073. This method is based on the idea behind the method Klinkenberg and van der Minne [2] proposed for measuring the charging tendency of oils. The counterions of particles dispersed in low conductivity media are, on the average, far from the particle surface; that is, the dispersion has a long Debye length. If the particles can be held, then a flow of liquid will separate the particles from their countercharges. Because the conductivity is low, a slow flow of liquid is capable of carrying the countercharges completely away from the particles and into a container where the total charge can be

22

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measured with a coulometer. The particles are held in position by putting the dispersion on the surface of a filter. As solvent is forced through the filter, it picks up the countercharges and carries them away. Since the number of countercharges is the same as the number of charges on all the particles, and the mass of all the particles is known from the sample size, the average charge-to-mass ratio is easily measured. The technique relies on finding a filter with sufficiently small pores to hold the particles, but sufficiently large to allow a rapid flow of liquid. Other methods particularly suitable for the study of the charges in nonaqueous dispersions are based on the electroacoustic effect. In the inaugural issue of the Journal of Chemical Physics, Debye described an idea he had for measuring the mass of hydrated ions [108]. The charge on the ions was known from their chemistry, but the degree of solvation was not known. Essentially he described a technique for measuring the charge-to-mass ratio of ions. The idea is to pass a supersonic wave through a solution. The different ions move depending on their size and mass. The differential motion of the ions produces local electric fields which Debye calculated would be large enough to be measurable. He provided a simple theory to calculate the size of the ions from the magnitude of the electric field. Rutgers pointed out in 1938 that the technique is even better suited for dispersions of particles as the mass differences between the particles and the counterions is large [109]. The theoretical derivations have been made more general and the reciprocal relations derived: the application of sound and the measurement of electric fields, and the application of electric fields and the measurement of sound [ 110-l 151. What makes these techniques particularly useful for nonaqueous dispersions is that the measurements can be made at high volume loadings, e.g. greater than 1 wt%. Published applications of this technique to nonaqueous dispersions are beginning to appear, the first being in the study of toners dispersed in hydrocarbons l-42,116,1173. The various techniques described here enable the experimenter to determine the charge-to-mass

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1-37

ratio of particles in nonaqueous media and the electrophoretic mobility. The particle size can be obtained in the usual ways [118-1211. These three values are not independent. If the dispersion comprises monodisperse spheres then the three measured quantities are related by [42]

(QIMV* p

1811

(13)

P2

where Q/M is the charge-to-mass ratio, d is the diameter of the particle, p is the electrophoretic mobility, q is the viscosity of the solvent, and p2 is the density of the particle. If the zeta potential is known rather than the mobility, then

(Q/MY* _ 12Dco i

(14)

P2

where D is the dielectric constant of the solvent and co is the permittivity of free space. Equations (13) and (14) are often used to calculate the third variable when only two are known. They can also be used to check for self-consistency if all three variables are known. A discrepancy between the measured quantities on the left-hand side of these equations and the expected quantity of the righthand side is a source of information about what assumption in a model may be inappropriate for one of the measurements. This is particularly useful since the characterization of nonaqueous dispersions, especially the characterization of electrical properties, is not nearly so advanced as the characterization of aqueous dispersions. Concentrated dispersions of some model systems have been shown to undergo “phase” transitions as a function of concentration and added electrolyte [122,123]. Charged nonaqueous dispersions may well be worth considering as model systems for they would have Debye lengths very much greater than interparticle distances without any need for dialysis. With the long Debye lengths, ordered transition might occur at much lower particle concentrations. Comparisons with some advanced concepts in solid-state physics [124] might be interesting.

I.D. Morrison/Colloids

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13. Electrostatic contributions to dispersion stability in nonaqueous media In aqueous lyophobic dispersions the electrostatic repulsion between dispersed particles is a dominant factor promoting stability [125]. The origin of the charge on the particles can be understood in terms of potential-determining electrolytes whose concentration can be measured and controlled. In nonaqueous media electrolyte concentrations are low and difficult either to measure or control. The electrostatic charging of particles in nonaqueous media is often affected substantially by trace impurities, especially water. These two difficulties have impeded the examination of the role of electrostatics in nonaqueous dispersion stability. On the theoretical side, the concentration of ions in nonaqueous dispersions is so low that the assumption of a uniform distribution of countercharges around any charged particle is tenuous. Lyklema [ 1261 examined the simple extension of DLVO theory to nonaqueous dispersions to identify the following areas needing further study. (A) The origin of the surface charge could be either by adsorption of ionic solutes or by the dissociation of surface groups and the most likely ion responsible for this would be the proton from an acid-base interaction. (B) The electrical double layers must be thick and this has some interesting consequences. (C) The surface potential and the potential at the plane of shear must be very nearly equal. (D) The slow decay of potential in the double layer is equivalent to a low field strength so that the repulsion between particles is low but long range (see below), hence electrostatic stabilization is important only at low concentrations. (E) The theories to estimate van der Waals’ attractions are adequate. In short, Lyklema claimed that, other than having difficulty knowing the actual physical parameters, nothing in DLVO theory would prevent its simple application (at least to first order) to nonaqueous dispersions, even though the ionic

l-37

23

strength is orders of magnitude lower than for aqueous dispersions. A key point in DLVO theory is that the repulsion between charged particles arises when the electrical double layers around those particles begin to interpenetrate. The repulsive force is equal to the gradient in that overlap potential [122]. Albers and Overbeek [127] argue that unless the concentration of particles (or droplets) in a nonaqueous suspension is low, then electrostatic repulsive forces are not sufficient to stabilize them. They reason that: (i) the electrical double layers in nonaqueous media are long because of the low ionic strength; (ii) since the double layers are long, the electric potential varies slowly with distance from the particle surface; (iii) since the electric potential varies slowly, the net potential as the double layers overlap varies slowly with distance. That is to say, the particles do not repel each other much. Van der Hoeven and Lyklema Cl753 follow essentially the same argument as Albers and Overbeek [127], maintaining that the ionic strength should be high enough for the potential decay around the particles to be steep, but that the concentration should not be so high as to compress the double layer. This leads them to the “intriguing conclusion . . . that there is a range of electrolyte concentrations, where the addition of ions improves colloidal stability.” They present extensive data primarily in liquids with dielectric constants between 5 and 11 (low-polarity media) and with solids used in detergent powders and some oxides. The classification of solvents based on dielectric constants is important in understanding the mechanisms of charge formation and electrostatic stabilization. Feat and Levine [ 1281 also conclude that double-layer effects alone will not stabilize a highly concentrated liquid hydrocarbon suspension. They model the electrostatic interaction between two particles in a nonaqueous dispersion as one with two interesting features. First, they assume that as the two particles approach each other, the counterions are contained in a volume equal to twice the

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I.D. Morrison/Colloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

volume per particle. Second, they assume that the counterions are at a uniform volume density (no discrete ion effects). From this model they conclude that at the mean separation between neighboring particles, the double layer force is attractive. The authors do stress that this result is only for concentrated dispersions. This model does seem to imply that as two particles collide, their electrical double layers collapse (so as to remain within the necessary volume per particle). If so, then the question is whether this is realistic or not. The distribution of counterions in a dispersion where the interparticle distance is less than the Debye length is an unsolved problem. Osmond objected to the direct application of DLVO theory to nonaqueous dispersions during the Faraday Society discussion on “Colloid stability in aqueous and nonaqueous media” in 1966 [129]. He reasoned that at the low ionic strengths of the systems being studied, the electrical double layers in the volume of medium surrounding these particles would contain only a few ions, perhaps ten at the maximum. To express the repulsion between the charged particles solely in terms of the repulsion between these ions would be an error. He proposed that the potential between two charged particles be expressed simply as the interaction through an inert medium, that is simple coulombic repulsion. He did not dispute the existence of charge repulsion in nonaqueous media, but just disagreed with the usual mathematical analysis. McGown and Parfitt [ 1303, apparently thinking along the same lines, calculated the total interaction between two particles in nonaqueous media as the sum of the Hamaker forces of attraction and the coulombic forces of repulsion. In essence this calculation ignores the electrical double layers. From this model they calculated the stability constant for a series of different particle sizes, Hamaker constants, and surface potentials. A comparison of the calculated stability constants with data taken in their laboratory revealed, in general, good agreement. Hair and Landheer also expressed the opinion

1-37

that the repulsion between the particles should be taken as coulombic and they made the suggestion, without explicit derivation, that the stability of nonaqueous dispersions should increase as the square of the measured zeta potential [ 1311. This alternative to DLVO theory applicable to nonaqueous media has been put in closed form [132]. As McGown and Parfitt suggested, the total energy of interaction between two charged particles is taken as the sum of the Hamaker energies of attraction and the coulombic energies of repulsion. The stability constant W, which is the factor by which the rate of rapid flocculation is reduced due to the interparticle forces, is calculated from the integration of these energies over all distances of separation of the particles [133]. Following an approximation used by Debye for a similar problem [134], the stability ratio is found to be

where A is the Hamaker constant for the particles in the medium, k the Boltzmann constant, T the absolute temperature, D the dielectric constant of the medium, c,, the permittivity of free space, a the particle radius, and @, the zeta potential. Note that the dependence of the stability on the Hamaker constant is weak, only as the fourth root. That is to say, if the dispersion has sufficient charge to stabilize it, then its rate of flocculation does not depend on the much shorter range dispersion interactions. If the total energy of interaction is taken to be coulombic repulsion only, the stability integral can be evaluated exactly and another expression for the stability ratio is obtained that turns out to be numerically equivalent (for dispersions at low ionic strength)

(16) If lo5 is taken as the stability ratio above which dispersions are stable, then for many common nonaqueous dispersions both these approxim-

I.D. MorrisoniColloids

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ations give at>%

(17)

as a good approximation relating the surface potential, in millivolts, needed to stabilize particles as a function of their radius in microns. As in aqueous dispersions, the larger the particle, the lower the electric charge need be for stable dispersions. The potentials calculated from Eqn (17) are quite comparable to those measured for stable nonaqueous dispersions [ 1321. Ignoring the electrical double layer and using Coulomb’s law for the repulsion between particles is one approximation. The other approximation is to assume that the particles are perfect insulators. Effects of the conductivity of the particles (in nonaqueous media) have been studied by Parfitt et al. [135]. They found that the differences are particularly significant for small particle separations. Serious errors are introduced for particles interacting at constant potential but, for particles interacting at constant charge, the errors are significantly less but usually >5%. If one wants to use DLVO theory to calculate the stability of a nonaqueous dispersion, three material constants are needed: the electric potential at the surface of the particle, the Hamaker or Lifshitz constant for the particles in the medium, and the ionic strength of the medium. Of these three, the value of the ionic strength is the most difficult to obtain for nonaqueous dispersions since the number and valency of all the different charged solutes must be known. Many conclusions in the literature about the relative contributions of electrostatic charges on particles to dispersion stability depend on the assumptions used to get the ionic strength. No method to measure ionic strength in nonaqueous media has been proposed (as far as is known by this author). The questionable use of Walden’s law is discussed in Section 2 above. 14. Particle charge as a function of shape Ditter and Horn [103] studied a series of copper phthalocyanine dispersions in toluene in which the

1-37

25

aspect ratio of the particles varied from 1 to 12. (The dispersions also contained a modified polyphenol resin, Albert01 Kp670 from Reichhold Albert Chemie AG.) Going from isometric to acicular particles, the electrophoretic mobility increased by a factor of forty. The yield point of the dispersions also increased from 0 to 40 Pa while the plastic viscosity remained the same. They interpret these data to mean that the copper phthalocyanine particles had a bipolar surface with the edges having a different charge from the tops and bottoms, similar in this respect to various clays. As the charge on a surface in nonaqueous media depends generally on some acid-base interaction such as proton exchange, the possibility of one particle having charges of different sign on different faces seems quite possible if it has quite different chemistries on different faces. Those nonaqueous solutes that are amphoteric in the sense of charging some particles positive and some particles negative are the very solutes that might actually enhance this effect, producing some unusual rheological and flocculation behaviors. Conferences on organic coatings are full of anecdotal comments (some proprietary!) about such peculiar behavior of pigment dispersions in organic media. 15. Role of water

Of itself, water cannot charge interfaces nor form the steric barriers that are required to stabilize dispersions in nonaqueous media. However, even at trace concentrations, water can have an enormous effect on the properties of nonaqueous solutions and dispersions. The reasons for this are fundamentally two-fold. First, the formation of nonaqueous micelles is greatly enhanced, if not enabled, by the presence of water. The water molecules form the links between the hydrophilic portions of the interior of the micelle. Some experimental evidence [136,137] indicates that a single water molecule is sufficient to cause the formation of a micelle. Second, water will adsorb on the surface of hydrophilic particles such as various mineral oxides. At this interface, water can loosen

26

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surface ions, change the acid-base character of the surface, and change the nature of adsorbed solute-surface interactions [ 1261. A comparison of these effects has been given by Kitahara [S]. If completely anhydrous nonaqueous dispersions could be prepared and studied, we might find that equilibrium electrical effects would be nonexistent. (Note: Electrical conductivity can always be attained by injecting free carriers, such as electrons or ions from an electrode.) 16. Role of water in micelles Early work [138] showed that the addition of water to carefully dried benzene solutions of sodium, lithium, and calcium soaps of xylyl- and xenyl-stearic acids transformed a high viscosity and flow-birefringent solution to a mobile liquid having a relative viscosity only slightly larger than that predicted by the Einstein equation for suspended spheres. These observations suggest that the addition of water causes a transition from thread-like aggregates in the anhydrous systems to a dispersion of isodimensional micelles, that is a phase change in the association structures of these soaps. Zhu et al. Cl393 studied solutions of Triton X- 100 (polyoxyethylene tert-octylphenyl ether) in cyclohexane as a function of water content, temperature, and CaCl, content by means of vapor pressure osmometry and light scattering. They concluded that, at the lowest concentrations of water, the initial micelles were composed of ten molecules of Triton X-100 and a single water molecule. As the water content was increased, a larger fraction of the Triton X-100 was associated into micelles and the number of water molecules per micelle increased. Photon correlation studies in H,O-AOT-isoC,H,, systems, pertinent IR investigations, and vapor pressure osmometric measurements with alkylated quaternary ammonium AOT strongly suggest that at least one water molecule is required for the formation of a micelle [ 1361. The water bonds to three AOT molecules to form a trimer.

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Variations in CMC values for AOT can be attributed to variations in water content. Proton NMR, 13C NMR, light scattering and density measurements showed that polyoxyethylene dodecyl ether did not form association structures in benzene at low water concentrations. (It should be noted that water is considerably more soluble in benzene than in saturated hydrocarbons.) At water concentrations between 10 and 20%, association structures started to appear, probably inverse micelles, in which the water-polyoxyethylene chain interaction is the predominant feature [140]. Micelles form in aqueous media so abruptly at a certain concentration of solute that the process can be reasonably expressed as a phase change. In nonaqueous media, the formation of micelles is more accurately described as an equilibrium [ 1413. In this equilibrium sense, water can be said to be necessary for the formation of the inverse micelles. Block copolymers, especially AB or (AB), block copolymers in which one of the two blocks is essentially insoluble in the medium, behave similarly to other amphipathic molecules in that they form association structures in solution and adsorb at interfaces [ 181. Block copolymers differ from most nonaqueous solutes in that they have a larger number of possible configurations. Nevertheless, trace quantities of water have been shown both experimentally and theoretically to have a profound effect on the formation of their association structures [137,142]. Water is readily absorbed into the core of nonaqueous micelles and in some cases there can be a gradual transition from water-imbibed micelles to microemulsions [ 141. The specific volume of water in the micelles is less than in bulk water but approaches it as the concentration in the micelle increases. The conductivity increases with solute concentration and water-to-solute ratio. The addition of water to the micelle tends to make it more spherical. Work by Eicke and Christen [136] strongly suggests that water is a prerequisite for micellization in nonaqueous media. The quantity of water

I.D. MorrisonlColloids Surfaces A: Physicochem. Eng. Aspects 71 (19931 l-37

necessary to stabilize micelles may be undetectably small and in fact water is probably ubiquitous [27]. That water plays a part in the formation of nonaqueous micelles implies that the process cannot be so well approximated as a phase change [ 1361. Eicke and Christen also provide a thermodynamic model for micelle formation [48]. The included water forms hydrogen bonds between the hydrophilic head groups of the nonaqueous surface-active solutes, providing a mechanism for micelle formation. Micelles in water are generally spherical because of the electrostatic repulsion between the ions on their outer surfaces. In nonaqueous media, the micelle shapes are generally much more varied. As water is added to the micelles they tend to become more spherical [31,143]. Nelson and Pink [3] showed that the more ionic the soap, the larger the micelle, while an increase in the chain length in soaps of the same metal leads to a decrease in the degree of aggregation. In spite of the foregoing, Yu et al. Cl443 find that water is not required for the formation of micelles of sodium bis(Zethylhexy1) phosphate (NaDEHP) in n-heptane but rather actually hinders the formation of the micelles. They report light-scattering results from both intensity and intensity autocorrelation data. They report no conductivity data. “Dry” solutions of NaDEHP form 50 nm micelles at 0.13 mA4. “Wet” (i.e. ambient) solutions of NaDEHP form smaller, 33 nm, micelles at a higher concentration, 2 mM. Yu et al. question the interpretation of the earlier data justifying the idea of water being a “glue” that helps create micelles. They also point out that almost all the data from which the “glue” model was derived were taken with AOT solutions. The differences in conclusions between this work and the previous work may be due to the same differences that are used to explain the difference between gas-phase and solution-phase acid and base strengths for organic compounds and therefore constitute not so much a contradiction as an advancement in the more general state of knowledge.

21

17. Role of water on particles Damerell and Urbanic [52] found that the lower the water content, the more stable were carbon dispersions in xylene solutions of lecithin, copper oleate, cobalt naphthenate, and AOT, although dryness or wetness did not seem to affect electrical migration. Even a sol made under the driest of conditions (with AOT) showed migration. Goodwin et al. [145] find that trace water has a significant effect on the charge of hydrophilic particles (silica) in a 4:l mixture of n-dodecane-nbutanol. The silica particles were first stabilized by adsorption of a nonionic solute. (The particles were actually weakly flocculated - possibly the molecular weight was too low for steric stabilization.) They also studied the effect of added hydrochloric acid and found that the zeta potential remained positive even with variations in added water. No comment was made about the ability of the nonionic hexaethylene oxide dodecyl ether to stabilize the countercharge. Removal of trace water caused a ten-fold increase in electrophoretic mobility of rutile in heptanol over the range 0.03-0.30 coverage [146]. Alumina goes from being negatively charged to positively charged in various low molecular weight alcohols with the addition of water, and the dispersions become less and less stable [ 1473. Aluminum hydroxide particles in the same alcohols become slightly more positive and the dispersions become more stable. Evidently the alumina surface becomes more basic with respect to the (acidic) alcohol solvents with the addition of water. The surface reactions are probably quite complex and certainly involve the formation of various aluminum ion complexes. One early postulate was that the water adsorbed at the surfaces of particles forms a polymolecular complex (polywater!) which aids in dispersion stability [148]. The presence of ordered layers of adsorbed solutes at the surfaces of dispersed particles even in the absence of water has been proposed [ 1491. The addition of water to the adsorbed

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layers is certainly expected to change the nature of the adsorbed layer structure. The difference in sensitivity to trace amounts of water between hydrophilic surfaces such as rutile and hydrophobic surfaces such as graphitized carbon black was shown in two papers by Parfitt and co-workers [95,150]. The charge of the rutile particles was a strong and complex function of the water content, while the charge on carbon particles was free of any complications due to the presence of trace water. While it is easy to see that water will be adsorbed at the surface of hydrophilic particles suspended in nonaqueous media, it is difficult to see how that adsorption produces charged particles. To produce charged particles, ions must be desorbed. The desorbed ions and their solvated layers, if any, must be large enough that they remain sufficiently far from the particle for thermal agitation to keep them free. The structures of such species remain a mystery. 18. Role of water on particles with adsorbed layers The addition of water to dispersions of carbon black and of barium sulfate in n-heptane, cyclohexane, and benzene solutions of AOT resulted in small changes in zeta potential [7]. With increasing water concentration, the authors claim a slight minimum for the negative carbon particles and a slight maximum for the positive barium sulfate particles which they explain by a partitioning of water between AOT micelles and the particle surface. This partition shifts with the ratio of water to AOT. The more striking observation, especially considering the difficulties of making accurate zeta potential measurements in low dielectric media, is that the charge on these hydrophobic particles is barely changed on the addition of water. In suspensions of carbon black and barium sulfate in the solvents without any AOT, no electrophoretic motion was observed at any concentration of added water. When the stability of the carbon dispersions as a function of water content in the presence of AOT

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is extended to higher concentrations of water, dramatic transitions in the sedimentation behavior are observed. The dispersions are stable at low and high concentrations of water, the ratio of water to AOT being the critical parameter, and unstable at intermediate concentrations. Presumably, as the ratio of water to AOT changes, the system passes through various phases. The stability has to be analyzed in terms of the partition of the water, the AOT, and the various possible ions present between micellar phases and the surface of the carbon. In dispersions of colloidal alumina in cyclohexane solutions of AOT, the addition of water has been shown to produce a succession of flocculated, dispersed, and again flocculated states as the ratio of water to AOT is increased [ 15 11. The water was shown by ESR studies to be adsorbed on the alumina surface, to form complexes with the adsorbed AOT, and to produce a fingering mechanism by which the water phase forms bridges between particles and causes them to flocculate. At very low concentrations, the AOT is adsorbed but not ionized. As the level of water is increased, surface charges are generated and the dispersion is stabilized electrostatically. At higher levels of water, the AOT becomes less strongly adsorbed and begins to form water-in-oil-like microemulsion structures. This confirms earlier work on the restructuring of monolayers of hexadecyltrimethylammonium bromide (CTAB) on mica surfaces in a low molecular weight silicone oil as measured in the surface forces balance [152]. Water can act as either an acid or a base depending on the other materials present. Therefore, in dispersions of alumina in tetrahydroforan (THF) and toluene, water decreases the stability of basic (OLOA 1200) stabilized alumina dispersions because it acts as a stronger acid than the alumina surface and forms complexes with the OLOA 1200. However, water does not decrease the stability of alumina dispersions stabilized by linolenic acid because it is not a strong enough base to disrupt the alumina-linolenic acid interaction [ 1531.

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Goodwin et al. [145] studied the effect of the addition of water on the rheology of concentrated suspensions of sterically stabilized silica particles. They showed that the stability and interparticle forces can be greatly affected by the presence of a charge repulsion term induced by addition of trace amounts of water. (This behavior is consistent with the hydrophilic nature of silica surfaces.)

19. Mixtures of colloidal electrolytes Nonadditive increases in the ratio of the conductivity of binary electrolyte solutions to the sum of the conductivities of solutions of the individual electrolytes, of up to 5 - lo3 in benzene [34] and lo4 in aliphatic hydrocarbons [2], have been reported. Klinkenberg and van der Minne [2] reported the conductivities of Ca-Dips, Tiap and their mixtures in benzene. The mixtures of the two electrolytes had up to 57 times the conductivity of the individual components. The interaction can not be explained on the basis of individual behaviors so the explanation lies in the effect of mixtures on the association structures. The authors suggest that the phenomenon has two prerequisites: (A) one of the components must be a divalent or polyvalent salt such as the copper, iron, manganese, nickel, cobalt, chromium, and thorium salts of various carboxylic and sulfonic acids; (B) the other component must be an electrolyte, such as AOT, alkylated ammonium salts, and lecithin, which imparts a certain conductivity of its own. Low molecular weight compounds including acids, alcohols, glycols, ketones, and amines are inactive. Eicke and Arnold [34] studied the enhancement of the conductivity of AOT and AAY (sodium di2-pentyl sulfosuccinate) solutions in benzene on the addition of metal and metal-hydroxy 3,5-diisopropylsalicylates. The conductivity plots of all binary mixtures are similar. All mixtures show a sharp CMC with this critical transition occurring far below the “real” CMC of the aerosol solute alone. The maximum value of the nonadditivity is

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observed when the mixture is approximately equimolar. This apparent CMC was called an “induced” CMC since the CMC is shifted to lower concentrations by two orders of magnitude. Eicke and Arnold found that the micelles formed in the mixed solute solutions are about the same size and have the same association number as in the neat solute solutions. These phenomena can be explained by the presence of dissociated metal chelates in the interior of the micelle. The presence of the micelles provides a mechanism for the stabilization of the ions of a metal chelate, and hence the increasing conductivity, while the association of metal chelates with the micelle core provides a free energy drive to lower the CMC. This same mechanism may be present in dispersions of solids where ionizable surface species of the solid act like metal chelates and can donate charged species to micelles. Furthermore, the presence of ionizable surface species on dispersed particles may enhance the formation of micelles at lower concentrations than without the dispersed solids. While the addition of metal chelates to solutions of AOT shows that breaks in the conductivity vs concentration curve occur where micelle formation may be expected, the conductivity vs concentration curve for pure AOT shows a break that is not anywhere nearly as sharp at what is thought to be the CMC. That is, CMCs for pure materials are clearly detected from vapor phase osmometry, but not so easily detected in conductivity measurements. Pearlstine and co-workers [ 116,117,154] report a similar phenomenon, the increase in surface charge of suspended particles, with the sorption of an acid or base on dispersed particles to provide an active site for soluble species to react.

20. Materials The role that small (trace) quantities of water can play in the charging process has already been discussed. In one sense water can be considered an impurity and hence we see how sensitive the charg-

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ing process is to a known impurity. The vast majority of the work reviewed here deals with relatively impure systems. It may well be that the difficulty in understanding the behavior of electrical charges in nonaqueous media follows directly from the lack of well-characterized systems to study. For a better understanding of these phenomena, well-characterized nonaqueous ionic compounds and solids with known surface properties in nonaqueous media would be invaluable. Damerell and co-workers [52-541 studied the effect of more than a score of solutes on dispersions of carbon, calcium carbonate, and silica in xylene. They found lecithin, copper oleate, cobalt naphthenate, and AOT to be the best for carbon; zinc, barium, and sodium salts of dioctyl sulfosuccinate, sodium dihexyl sulfosuccinate, lecithin, and magnesium oleate to be the best dispersants for calcium carbonate; and the various metal sulfosuccinates, metal naphthenates, and lecithin to be the best for silica. Van der Minne and Hermanie used calcium alkylsalicylate to charge carbon particles negatively in benzene, and tetraalkylammonium picrate to charge them positively [87]. Fowkes et al. [75] studied the charging of carbon particles in white oil using metal alkylaryl sulfonates (dinonylnaphthalene sulfonates, trihexylbenzene sulfonates, and petroleum sulfonates) and copolymers of longchain methacrylates or c+olefins with acidic or basic comonomers (vinyl acetate, vinyl alcohol, vinyl pyrrolidone, hydroxyethyl methacrylate, etc.). The u-olefin-vinyl acetate copolymers had molecular weights up to 2.5. lo4 and the methacrylates molecular weights greater than 105. Miller et al. [177] used alkylbenzene sulfonates to disperse calcium hydroxide-calcium carbonate particles. These materials are used in engine lubricating oils as bases to react with acidic residues. Electrophoretic display devices use AOT [ 1551, calcium dodecylbenzene sulfonate [ 1561 and calcium petroleum sulfonate [156]. Liquid immersion developers use iron naphthenate, zirconium octoate, cobalt octoate, calcium

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alkylbenzene sulfonates [ 1573, Li, Ca, Cd, Mn, Mg, and Zn salts of heptanoic acid, 2-ethylhexanoic acids, stearic acid, and naphthenic acids; metal dresinates (Hercules) and ammonium lauryl sulfate [158]. Bedenko and Balyasnikova [ 1591 list Span-80, OP-4 (an oxyethylated alkylphenol based on a polymer distillate with an average degree of oxyethylation of four), chromium stearate, and a technical alkenylsuccinimide made from diethylenetriamine and the alkenylsuccinic anhydride with the polyisobutylene radical of molecular weight about 1000, as dispersants for graphite in decane. Lee and Rives Cl603 show that both linolenic acid and OLOA 1200 will disperse alumina in THF and in toluene, although OLOA 1200 is the more effective. In toluene the acid charged the particle positively while in THF the particles were negative. The converse was true for OLOA 1200; the particles were negative in toluene and positive in THF. The effect of water is discussed above. Pearlstine reports the charging of liquid toners in hydrocarbons with zirconium octoate, manganese octoate, and Fe, Zn, Co, Mn, Ca, and Cu naphthenates [ 1171. Pearlstine and co-workers [116,154] report that particle charging can be controlled by the adsorption (or in their case, absorption) of an acid or base in the dispersed particle so as to provide an active site for soluble species to react. The results can be complicated by the exchange of material between particles and inverse micelles. In general, polymer dispersions are not significantly charged by soluble agents. This technique provides a means to produce highly charged polymer particles. Eicke and Arnold [34] show that the addition of metal and metal-hydroxy 3,5-diisopropylsalicylate chelates greatly enhance the conductivity of AOT and AAY solutions in benzene.

21. Explosion hazards in petroleum processing Static electrical effects are ubiquitous. In the petroleum processing industry they can be lethal.

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The reasons for this are twofold: (A) flammable vapors are often present and (B) the conductivity of petroleum products is low and thus permits the build-up of large potential gradients [ 1611. This topic was first given sound theoretical treatment by Klinkenberg and van der Minne [2]. The fundamental questions are: (i) where do the charges come from?; (ii) how are the charges separated to create large potentials?; (iii) what strategies are useful to prevent catastrophic static discharge? The ionic charges are generated by the interaction of minute concentrations of impurities with interfaces, primarily pipe and tank walls, but also water drops. It has been estimated that even at the highest levels of charge observed experimentally in commercial scale equipment that only one in 1.6.10” molecules carries a charge [161]. The charge exchange between some component of the petroleum and a wall is not significant until the liquid begins to move. The motion of the liquid carries the soluble ion away from the spot at which it exchanged charge. Of course, the electrostatic attraction will tend to pull the ion back. The key question is whether or not the liquid flow is quick enough to separate the two charges. In aqueous solutions the answer is that no significant charge separation can be generated since the equipment and fluid are conductive and conductors cannot support separated charges. The issue is quite different in the low conductivity hydrocarbons. For these there are not necessarily any conductive pathways for the charge separation to be neutralized. In fact, if the fluid is made to flow completely out of the container and into another container that is electrically isolated, then absolute charge separation is attained! This answers the question about whether a bottle of cations or a bottle of anions can be generated. The answer is “yes,” if the fluid is highly insulating. Unfortunately, when collected, all the ions will try to escape each other and end up on the bottle walls. That this phenomenon is real is easy to demonstrate when the petroleum fluid drains from one container into another and both containers are

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electrically isolated from each other. The potential difference between the two containers can be measured and used to quantify the “charging tendency” of the fluid [2]. The answers to the question about how to avoid the consequences of charge separation are three. (A) Keep all containers and pipes electrically connected. This is the root of the safety precaution that says ground metal containers that hold organic solvents to any container into which the solvent is being transferred. (B) Increase the conductivity of the fluid by the addition of nonaqueous electrolytes. The higher conductivity prevents the build-up of large potential gradients. (C) Keep the environment free of oxygen and ignore the sparking. This is essentially the procedure used in some oil tankers since the charge separation is often caused by the sedimentation of water drops through the crude oil. The drops carry one sign of ions down, so creating an internal charge separation which can lead (and has led) to disastrous sparking in the inside of oil tankers. Certainly, these same kinds of considerations are important in the handling of dusts and aerosols because air too has a very low conductivity.

22. Some applications of charged nonaqueous colloids Liquid immersion development (LID) is an alternative to the dry toning used in most xerographic copiers and printers. In this process, a dispersion consisting of charged pigment particles suspended in a dielectric liquid is passed between a grounded electrode and the charged surface of a photoconductor or dielectric receiver (sometimes coated paper) holding an electrostatic image [162,163]. The image is usually created by the exposure of a charged photoconductor to light. Laser printers use a scanning laser beam to discharge a photoconductor to form an image. The charged particles migrate by electrophoresis to neutralize the electrostatic image and thus develop a real image. This image can be dried to make it permanent or

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I.D. Morrison/Colloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

transferred to any other surface and then dried. The physicochemical properties necessary to make excellent images are manifold [ 157,158,164] but the important property in the context of this review is that the charge on the particles must be substantially greater than any charges of the same sign in solution. For example, if the particles are negatively charged, then there should be a minimum of negatively charged micelles. The reason for this is that the micelles of like sign will compete with the pigment particles in the development process and reduce the image density. Materials that will efficiently create charged particles in hydrocarbons can be found in the patent literature for LID. Electrostatic lithography [165] is a process that is based on a principle similar to offset lithography printing. Lithographic printing is accomplished by making a printing plate that has areas preferentially wetted by water and areas not wetted by water. First, water is spread across the plate, wetting only the nonimage areas. The ink is spread (by means of applicator rollers) across the wetted plate. The ink transfers only to image areas which are not wetted by water. An image is made by electrostatic lithography by forming an electrostatic image on a surface in any of the usual ways [ 1661, most commonly by discharging a photoconductor with light. A gravure roller is covered with a hydrocarbon-based ink that has a carefully conresistivity, usually in the range trolled lo*-10” ohm cm-‘. At this conductivity the electrostatic force on the ink from the image will pull ink from the gravure cells. Where there is no electrostatic image, no ink transfers. In this example, the important parameter is the resistivity of the ink and not the charges on individual pigment particles. Drop-on-demand ink jet printing also depends on producing an ink with a conductivity in the proper range. In this technology a stream of ink is expelled from an orifice and broken into small droplets by the application of an electric field. The charged ink drops thus produced can be guided either into a recirculating loop or towards the paper to make

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an image. Electrostatic spraying of organic-based coatings depends on similar control of conductivity. Sufficient conductivity is needed to be able to have a stream of liquid broken into small drops by an applied electrostatic field but not so much conductivity that the applied electric field is dissipated by conduction. Photoelectrophoresis is an imaging process that enables the formation of a full color image in one step [ 1671. The toner is a dispersion of photosensitive cyan, magenta, and yellow pigments dispersed in a hydrocarbon fluid. The particles are held against a transparent electrode by an electric field. When light illuminates the electrode, the various pigment particles react with the light they absorb, exchange charge with the electrode, and are carried away by the electric field. The migrating particles are collected on a back electrode. There they have formed a color image corresponding to the illumination. The charge-generation and chargeexchange processes in this nonaqueous process are complex in the extreme. Electrophoretic displays use changes in optical reflectivity at a transparent electrode to produce an image. The dispersions contain charged, highly scattering particles in a low conductivity suspending medium containing a dye [ 1681. The dispersion is contained in a thin cell consisting of two electrodes, one transparent, and at least one of the electrodes is divided into individually addressable elements. The particles can be made to move from front to back of the cell by application of an electric field. When an electric field is turned on, the particles move by electrophoresis to the transparent electrode where they reflect light brightly. When the field is removed (or biased slightly in the opposite direction) the particles move away from the transparent electrode and are replaced by the dyed solution which does not scatter light. When the electric field is applied, then the highly scattering particles will move to the viewer’s side so developing the image. This concept is appealing in its simplicity but a “well-behaved” materials package is a challenge to obtain [155].

I.D. MorrisonlColloids Surfaces A: Physicochem. Eng. Aspects 71 (1993)

Electrorheological jluids are suspensions whose viscosities increase significantly upon application of an external electric field. Two recent reviews of these fluids and their uses are available [181,182]. Electrorheological fluids are used in electrically triggered clutches, pumps, dampers, brakes and robotic joints. The electrorheological effect depends on a large dielectric mismatch between the particles and the medium. The media for these fluids are low dielectric, low conductivity fluids such as silicone oils, cooking oils, kerosene, mineral oils and halogenated hydrocarbons. The suspended particles can be flour, microcrystalline cellulose, ionexchange resins, metal soaps, silica, titania, and other metal oxides [ 1821. These combinations of solids and fluids are reminiscent of the types of dispersions discussed in this review where the primary interest has been in the effect of electric charges. The simplest models of electrorheological fluids describe the phenomenon in terms of uncharged, spherical, dielectric particles suspended in a non-conducting dielectric fluid. The dispersions must have low conductivity for obvious reasons. However, water and other high dielectric fluids are required to enhance or even provide the electrorheological effect. For instance Block and Kelly [ 1811 list 50 formulations for rheological fluids and only three do not require additives. The effects of mobile electric charges, or of the charges on the particles, have seen little investigation until recently. Adriani and Gast [ 1801 studied the behavior of electric-field induced chain formation in dilute suspensions of sterically stabilized, 1 urn poly(methy1 methacrylate) latices in hexane saturated with sodium acetate. They found that the charge on the latex particles had a profound influence on the field required to produce aggregates and a combination of coulombic repulsion and field-induced dipole attraction was necessary to explain the experimental data. They end with “This electrostatic repulsion in otherwise model nonaqueous suspensions may have profound implications for systematic studies.” This author could not agree more.

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Conclusions In general, the various hypotheses about the behavior of charges in nonaqueous media are based on data from too few materials. The most commonly studied electrolytes are AOT, and various salicylates and succinimides. The most commonly studied particle dispersions contain carbon blacks. This is not to criticize the quality of the effort already made, but to emphasize the opportunity for contributions in the future. Reasonably modern chemical analyses are being applied to define the chemistry of nonaqueous solutes and their structures. Modern experimental techniques to measure physical properties such as particle charge, bulk conductivity and electrode effects such as charge injection, need to be applied more often. The study of the charging of particles in nonaqueous media urgently needs model systems of well-defined surface chemistry. The general mechanisms proposed to explain the properties of electric charges in nonaqueous systems include nearly as many concepts as data to justify them. This is a sign of an area ripe for significant contributions. The need for better understanding stands in contrast to the apparent decline in the number of research programs, especially in academia, on the role of electric charges in nonaqueous media.

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