Electrokinetic Study of Hexane Droplets in Surfactant Solutions and

Only when a combination of DAH, SDS, NaPa, and inorganic electrolyte was added together could the measured electrokinetic behavior of hexane droplets ...
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Ind. Eng. Chem. Res. 2002, 41, 52-57

MATERIALS AND INTERFACES Electrokinetic Study of Hexane Droplets in Surfactant Solutions and Process Water of Bitumen Extraction Systems Jianjun Liu, Zhiang Zhou, and Zhenghe Xu* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6

The effect of surfactant and inorganic salts on the electrokinetics of bitumen and model oil (hexane) in aqueous solution was studied. The zeta potentials of the hexane droplets became less negative without changing the isoelectric point (iep) for increasing inorganic electrolyte concentration. The addition of dodecylamine hydrochloride (DAH) caused a drastic change in the zeta potentials, with a progressive shift of the iep toward a higher pH with increasing DAH concentration. With the addition of sodium dodecyl sulfate (SDS), the zeta potential of the hexane droplets became more negative without a measurable iep. Virtually no change in the zeta potentials of the hexane droplets was observed when palmitic acid was added. Only when a combination of DAH, SDS, NaPa, and inorganic electrolyte was added together could the measured electrokinetic behavior of hexane droplets in the resultant aqueous solution simulate that of hexane droplets in the process water from an industrial bitumen extraction process or bitumen droplets in simple electrolyte solutions. This finding revealed the presence of these three types of surfactants in bitumen and their release into the processing water or migration to and accumulation at bitumen/water interfaces. Introduction Bitumen in Athabasca oil sand deposits represents one of the major oil resources in Canada. The waterwettable nature of the associated sands in these deposits makes the extraction of bitumen from the oil sand ores economically attractive. Currently, more than 365 000 barrels of crude are produced daily through mining, extraction, and upgrading operations at Syncrude Canada Ltd. and Suncor Energy Ltd. The extraction of bitumen from Athabasca oil sand deposits is a complex physicochemical process. A prerequisite for bitumen extraction from Athabasca oil sands is detachment of the bitumen from the associated sand grains. Considerable research efforts over the past several decades have led to the commercial application of the hot-water extraction process for recovering bitumen from oil sand ores.1,2 In the commercial bitumen extraction process, elevated temperature and caustic addition have been employed to facilitate bitumen separation from sand grains. The process of detaching bitumen from sand grains can be considered similar to the removal of oily silt from fabrics in the detergency process. The use of surface-active chemicals such as soaps in detergency is well-documented.3 In these processes, surfactant plays a critical role in mediating interactions between oil (silt) and sand (fibers) in process water. It is well-known4 that the interactions between oil and sand grains are controlled by interfacial properties, such as chemical activity and electrokinetics, that are related to process water chemistry to a large extent. The release of different types and * To whom correspondence should be addressed. Phone: 1-780-492-7667. Fax: 1-780-492-2881. E-mail: [email protected].

concentrations of ionic species from bitumen/clays into the oil sand ore slurry makes the investigation of the extraction system more challenging,5 as the released surface-active species readily modifies the solids/bitumen/liquid/air interfacial properties and, hence, the extraction performance. Changing the electrokinetics of bitumen/water interface is just one example. Therefore, studying the electrokinetics of bitumen and model oil in process water provides an in-depth understanding of molecular interactions in bitumen extraction processes and, hence, better control of the bitumen extraction process. Extensive research has been conducted on examining the electrokinetic behavior of bitumen and crude oils in aqueous media. Bowman proposed6 that the bitumen-sand interaction could be explained in terms of the various forms of carboxylic groups of organic acids in bitumen. By assuming that the charge at the bitumen/water interface was derived from the ionization of carboxyl groups, Takamura and Chow7 applied the surface ionization model to explore the electrokinetic properties of the bitumen/water interface. From an application of the classical DLVO theory to bitumen extraction systems using the measured zeta potentials of both bitumen and sand grains individually, it was proposed8 that the presence of calcium ions in the solution would cause the attachment of bitumen to sand grains, thereby deteriorating the bitumen-sand separation and bitumen attachment to air bubbles. Brown and Neustadter9 suggested that the naturally occurring surfactants in crude oils in the form of protonated nitrogenous species might be responsible for the coagulation of fine silica with the oil over acidic pH ranges. More recent work by Dai and Chung10,11 further examined the electrokinetic properties of bitumen under

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different treatments. Unfortunately, the types of surfactants in bitumen and their relative abundances in the process water that would affect bitumen electrokinetics remain to be explored. To this end, we recently studied12 the effect of surfactant on bitumen-silica coagulation by using a model oil system. The results obtained shed more light on simulating complex bitumen behavior using a simple model system. In this communication, an attempt was made to simulate the eletrokinetic behavior of bitumen using model hydrocarbon oil (hexane) under different solution chemistry conditions. The experimental results obtained suggest that, with a suitable combination of various types of surfactants in water, the measured electrokinetic behavior of model oil can be made to simulate that of the model oil in the process water from an industrial bitumen extraction process or bitumen droplets in simple electrolyte solutions. Our study demonstrates that it is possible to establish the presence of various types of surfactant in oil sand ore slurries and their release from bitumen into these oil sand ore slurries. Experimental Section Materials. Hexane (>99%, Aldrich) was used as the model oil in this study. The surfactants examined included sodium dodecyl sulfate (SDS, >99.7%, Fisher Scientific), sodium palmitic acid (NaPa, >98%, Aldrich), and dodecylamine hydrochloride (DAH, >99%, Kodak). The first two are anionic surfactants, with SDS being a strong acid and NaPa a weak acid, whereas DAH is a cationic surfactant. Also examined were reagent-grade inorganic electrolytes such as CaCl2 and Na2SO4 (Alfa). Reagent-grade HCl and NaOH were used for pH adjustment, and ultra-high-purity KCl (>99.999%) was used as the supporting electrolyte. Unless otherwise stated, all experiments were carried out in deionized water with a resistivity of 18.2 MΩ cm, prepared with an E-lix 5 system followed by a Millipore ultra water-purification system (Millipore Canada, Ontario, Canada). Two types of process water from bitumen extraction were examined, both from Syncrude Canada Ltd. One is the existing tailings process water taken from Middle Lake Settling Basin (MLSB), and the other is the consolidated tailings (CT) process water containing 40 ppm Ca2+ taken after treating the MLSB water with gypsum by consolidate tailings process.13 The bitumen used in this study was Coker feed bitumen generously provided by Syncrude Canada Ltd. Zeta Potential Measurements. The oil droplets at ca. 0.2 vol % in surfactant solutions were prepared in an ultrasonic bath. The ultrasonication lasted for about 1-2 min in the presence and 4-5 min in the absence of surfactant to obtain oil droplets with an average size of around 1 µm. Similar procedures were used for the preparation of bitumen droplets in electrolyte solutions. Zeta potentials were measured with a Zetaphoremeter III (SEPHY/CAD), which consists of a rectangular electrophoresis cell, a pair of hydrogenated palladium electrodes, a laser illuminator, an optical microscope, and a digital video image capture (CCD camera)/viewing system. The computerized operating system allowed for the accurate positioning of the optical microscope view field at one of the two stationary layers for the accurate measurement of electrophoretic mobility. About 40 mL of the prepared oil droplet suspension was used to fill the electrophoresis cell. Through the laser-illuminating and video-viewing system, the movement of the droplets

Figure 1. Zeta potentials of hexane droplets as a function of pH in various electrolyte solutions.

in the stationary layer was traced, five times for each direction by alternating the positive/negative electrode potentials. The captured images were then analyzed using the built-in image-processing software. The distribution histogram of electrophoretic mobility and corresponding average values were determined and converted to zeta potential values as desired. The conductivity and pH of the suspension were continuously monitored during the measurements. In this study, the environmental temperature was maintained at 22 ( 0.1 °C. Each test was repeated several times, and the averages are reported. The measurement error was generally less than 5%, except at high electrolyte concentrations above 10-2 M, in which case the error could be as high as 10%. Results and Discussion Hexane in Electrolyte Solutions. The zeta potential values of hexane droplets in various electrolyte solutions as a function of pH are plotted in Figure 1. In 1 mM KCl solution (solid circles), hexane showed an isoelectric point (iep) of ca. pH 3. Similar iep values have been reported in the literature.14,15 The preferential adsorption of hydroxyl groups at the oil-water interface was considered responsible for the observed negative zeta potential of “pure” hexane droplets in supporting electrolyte aqueous solutions. This explanation is consistent with the weaker hydrated nature of hydroxide anions compared to hydrogen or hydronium cations. The zeta potential values of hexane droplets became less negative with increasing supporting electrolyte concentration of KCl (solid triangles) or Na2SO4 (solid squares), as anticipated from electrical double-layer compression. In this case, the iep remained unchanged at pH 3. In contrast, the addition of 1 mM calcium chloride (open triangles) caused a noticeable shift in iep to pH 4. The zeta potential values became much less negative, particularly at pH > 7. The speciation diagram of calcium16 shows an increased amount of calcium in the form of calcium monohydroxide (CaOH+) at pH > 9. It appears that the higher surface concentration of OH- at the hexane/water interface than in the bulk triggered the specific adsorption of Ca2+, forming positively charged calcium monohydroxide ions at the interface. As a result, these positively charged species partially neutralized the negative surface charges, making the overall zeta potential less negative.

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Figure 2. Zeta potentials of hexane droplets as a function of pH in 0.01 mM surfactant solutions containing 1 mM KCl as the supporting electrolyte. Solid symbols, without Ca2+; open symbols, with 1 mM Ca2+.

Effect of Surfactants. It is known17 that surfactant molecules preferentially adsorb at the oil/water interface. The amount and degree of ionization of the adsorbed surfactant molecules dictate the electrokinetic properties at oil/water interfaces. As shown in Figure 2, different types of surfactants affect the zeta potential of hexane droplets in aqueous supporting electrolyte solutions differently. With the addition of DAH (solid squares), hexane droplets became positively charged up to pH 9. In this case, a drastic shift of the measured iep to about pH 9 corresponded well with the anticipated precipitation of deprotonated amines at this pH.16 The addition of 1 mM calcium reduced the magnitude of the zeta potential (open squares) mainly by compressing the electrical double layers, with the shape of the curve and the iep remaining the same. With the addition of SDS (solid triangles), the hexane droplets became more negatively charged, particularly at pH below 6. The strongly acidic nature of the sulfate group from the SDS adsorbed at the hexane/water interface is responsible for the increased surface charge density and, hence, for the more negative zeta potential values. In this case, the iep could not be observed over the pH range studied, as anticipated for a surface of exposed sulfate groups. The addition of 1 mM calcium (open triangles) reduced the magnitude of the zeta potential of hexane droplets by 30-60 mV but did not cause charge reversal, suggesting a weak specific interaction of sulfate ions with calcium at this concentration level. In contrast, the addition of NaPa had a minimal effect on the electrokinetic properties of hexane droplets in electrolyte solutions (compare the solid circles in Figures 1 and 2). The adsorption of NaPa at the hexane/water interface has been well-established from the interfacial tension measurement. Because NaPa is a weak acid, the degree of ionization of carboxylic acid groups varies with pH over the range 4-6.16,18 At pH < 7, the carboxylic groups are predominantly in the neutral acid form. Adsorption at the hexane/water interface has a negligible effect on the surface charge density. Similarly, the presence of neutral palmitic acid molecules in solution has little impact on the distribution of supporting electrolyte in the electrical double layer. As a result, the zeta potential remains the same as in the absence of NaPa. At pHs higher than 7, the anionic form of NaPa

molecules might contribute to electrical double-layer compression, resulting in a reduced double-layer thickness and, hence, a reduced magnitude of zeta potential values. However, this contribution is unlikely to be significant, as the observed variation of the zeta potential with increasing NaPa concentration does not show any consistent trend and the observed variations are within the measurement errors (not shown in Figure 2). It is clear that the ionized carboxylic groups at hexane/water interfaces contribute to the net surface charge density in a manner similar to that of hydroxyl ions. It is interesting to note a significant change in the electrokinetic properties of hexane droplets in NaPa solutions upon the addition of 1 mM calcium (open circles). In this case, a finite shift in iep from pH 3 to pH 4.2 was observed, not only suggesting the strong specific adsorption of calcium through binding with the adsorbed carboxylic groups, but also confirming the accumulation of NaPa at the hexane/water interface. Clearly, the effect of calcium ion addition on the electrokinetic properties at the oil/water interface is far more complex than double-layer compression and is largely dependent on the type of the surfactant present at oil/ water interfaces. Mixed-Surfactant Systems. Because a collection of surfactants is anticipated to be present in the process and recycle water of the bitumen extraction process, the synergetic effects of the mixed surfactants have to be investigated to understand the role of surfactant in bitumen digestion and aeration. The zeta potentials of hexane droplets in the mixed-surfactant solutions with and without added calcium are given in Figure 3a and b, respectively. With a combination of DAH and NaPa, the zeta potential of the hexane droplets decreased from 60 to -80 mV with pH from 3 to 11, as shown in Figure 3a (circles). Upon the addition of DAH and SDS together, the zeta potentials of the hexane droplets all became negative, decreasing at pH’s above 9 (squares). The addition of NaPa to SDS aqueous solution had a marginal effect on the measured zeta potentials (upwardpointing triangles). When all three kinds of surfactant were added together (downward-pointing triangles), the zeta potential of the hexane droplets decreased from -10 to -90 mV with increasing solution pH. Compared with the case where the surfactants were added individually, it is clear that the effect of cosurfactant addition on the electrokinetics at an oil/water interface is not additive. The ionized component appears to dominate the electrokinetic properties of oil/water interfaces. For example, at low pH, cationic DAH molecules have a dramatic effect on the electrokinetics of SDScontaining hexane/water interfaces. The variation of the measured zeta potential resembles that for DAH added alone, although the absolute value varies substantially (solid squares). Similar results were found for NaPa at the interface between DAH-containing water and hexane (solid circles). The amphoteric nature of the hexane/ water interfaces in the mixed-surfactant aqueous solutions further confirms the coadsorption of surfactant at hexane/water interfaces. The addition of calcium ions had the most significant impact on the zeta potentials of hexane droplets in NaPa/SDS-containing solution and the least for DAH/ SDS-containing solution, as shown in Figure 3b. These observations further confirm the role of calcium in controlling interfacial electrokinetic properties through specific interactions with carboxylic headgroups at

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Figure 4. Zeta potentials of hexane droplets as a function of pH in MLSB water and CT process water.

Figure 3. Zeta potentials of hexane droplets as a function of pH in the mixed-surfactant (each at 0.01 mM) solutions containing 1 mM KCl: (a) without Ca2+ and (b) with 1 mM Ca2+.

hexane/water interfaces. The results obtained in this study clearly demonstrate the capability of fine-tuning the electrokinetics at oil/water interfaces by varying the composition of surfactant and inorganic electrolytes. The practical implication of this study is that the solution chemistry of industrial process water can be studied by mapping the electrokinetics of model oil in a variety of surfactant solutions and comparing the results with measurements in the industrial process water, as illustrated below. Whether the variation of the electrokinetics with the water chemistry of the mixedsurfactant system affects the interaction between bitumen and sand grains, and hence bitumen digestion from associated sands, remains to be examined in a separate communication. Simulating Process Water. To study the role of natural surfactant extracted from bitumen by caustic addition in bitumen digestion/recovery, one must identify the speciation and distribution of natural surfactants in bitumen extraction process water, which has been shown to be a challenging task.2 Our background study outlined above shows that mapping the electrokinetics of model oil in a collection of surfactant solutions is a powerful tool for this purpose. We have established that the type of surfactant and concentration of calcium are the sensitive parameters that affect the interfacial electrokinetics significantly. We also found that, in surfactant solution, the addition of KCl and Na2SO4 had a minimal impact on the interfacial electrokinetics at the hexane/water interface. These

electrolytes can therefore be used for the purpose of matching the electrical conductivity of the process water. To illustrate our approach, the water samples from MLSB and composite tailings (CT) process at Syncrude Canada were used to prepare hexane-in-water emulsions. The zeta potentials of the emulsified hexane droplets were measured as a function of the process water pH. As shown in Figure 4, the zeta potentials of the hexane droplets in the CT process water are less negative than those in MLSB water. The higher concentration of Ca2+ in the CT process water than in the MLSB water, measured by atomic absorption, appears to be the reason for the observed differences. By comparing the results in Figures 3 and 4, it is evident that the measured electrokinetics of hexane droplets in the CT process and MLSB water do not match with those measured in binary surfactant solutions, suggesting that the surfactant solution chemistry of the CT process water and MLSB water is more complex. This motivated us to map the electrokinetics of hexane droplets in a tertiary surfactant aqueous solution containing cationic DAH, anionic SDS, and weakly acidic NaPa surfactant with and without added calcium. In these tests, the conductivities of the solutions were adjusted using KCl and Na2SO4 to match those measured for the CT process water (6.4-6.6 mS/ cm) and MLSB water (3.7-3.9 mS/cm). The effect of the surfactant ratio on the zeta potentials of the hexane droplets in the aqueous surfactant solutions is shown in Figure 5. In this figure, level 1 represents a surfactant concentration of 0.01 mM, and the experiment was performed with 40 ppm (1 mM) added calcium. The choice of the calcium level was based on an analysis of the CT process water. When the ratio of SDS to DAH to NaPa was kept at 1:1:1 (with sets such as of 0.5:0.5:0.5, 1:1:1, 2:2:2), the measured zeta potential of the hexane droplets in surfactant solutions matched very well with those measured in the CT process water. At a higher SDS ratio, the measured zeta potentials were much too negative, whereas at a higher DAH ratio, the measured zeta potentials were much too positive. Varying the NaPa ratio, on the other hand, had a minimal impact on the zeta potential of the hexane droplets in the tertiary surfactant aqueous solutions, as was also observed in the binary systems. Mapping of the electrokinetics was also performed at varying calcium concentrations. As anticipated from the

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Figure 5. Zeta potentials of hexane droplets as a function of pH in 1 mM KCl and 1 mM CaCl2 solutions containing mixed surfactants each at the 0.01 mM level and in the CT process water.

Figure 6. Zeta potentials of hexane droplets as a function of pH in the process water best matched with the mixed-surfactant solution at a 1:1:1 molar ratio of DAH, NaPa, and SDS, each at the 0.01 mM level. All electrolyte concentrations cited are molar (M) concentrations.

results with binary systems, calcium had a significant impact on the zeta potential of the hexane droplets in aqueous solutions containing the three types of surfactants. Because the concentration of calcium ions can be determined rather accurately, its levels of 2.5 × 10-4 and 1 × 10-3 M as determined for the MLBS and CT process water, respectively, can be set unambiguously in the mapping tests. To reach the same electrical conductivities determined for the MLBS and CT process water, 2.5 and 10 mM sodium sulfate, as determined from the simulation of the zeta potentials of clay in the process water, were added, balanced by 22.5 and 40 mM KCl, respectively. The mapping was performed using tertiary surfactant systems, and the best-matched results are shown in Figure 6. Clearly, the electrokinetic properties of the hexane droplets in the process water can be simulated closely with a 1:1:1 molar ratio of the surfactants, each at the 0.01 mM level, confirming the presence of these surfactants at the cited ratio. Although we can determine the surfactant ratio in the process water by probing the electrokinetics of the model oil, it should be noted that the exact level of the surfactants in the process water cannot be determined, as a higher concentration of surfactants at the same molar ratio would give rise to the same electrokinetic properties of

Figure 7. Zeta potentials of hexane droplets in surfactantcontaining solutions at a 1:1:1 molar ratio, each at the 0.01 mM level (solid lines), and of bitumen in 1 mM KCl supporting electrolyte solution (symbols).

the model oil. In this respect, measurements of the interfacial tension, along with modeling of the surface site binding for the measured electrokinetics, would provide a more quantitative measure of the surfactant level in the process water, which, in turn, would help discern the role of electrokinetics in bitumen digestion. Finally, we illustrate the use of electrokinetic mapping to confirm the presence of the types and ratio of the natural surfactants in process water and at bitumen/ water interfaces. For this purpose, the zeta potentials of bitumen in electrolyte solutions were measured as a function of solution pH. As shown in Figure 7, the measured zeta potential of bitumen in an electrolyte solution can be closely mimicked by the electrokinetic behavior of hexane droplets only when all three surfactants are presented at a 1:1:1 molar ratio. This finding confirms the presence of all three types of surfactants at bitumen/water interfaces and in aqueous solution although their level might be low. Interestingly, this molar ratio is the same as determined for the surfactant in the bitumen extraction process water.) Our results further demonstrate that electrokinetic mapping is a sensitive method for determining the solution chemistry of a complex surfactant system. Summary Both inorganic electrolytes and surfactants have a significant effect on the zeta potentials of hexane droplets. Surfactants, and hence calcium, preferentially adsorb at oil/water interfaces, thereby controlling the interfacial electrokinetics. The molar ratio of various types of surfactants and their ionization characteristics are the controlling parameters in determining the electrokinetics of oil/water interfaces. The electrokinectics of hexane droplets in process water can be simulated only by considering all three kinds of surfactants at a 1:1:1 molar ratio, which confirms their presence and cooperative roles in the extraction process. The measured zeta potential of bitumen emulsions in electrolyte solution can be closely mimicked only when all three surfactants are presented at a 1:1:1 molar ratio. In conclusion, the measured electrokinetics of model oil or bitumen in process water allows us to evaluate

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the electrostatic contributions to bitumen/sand adhesion and hence their role in oil sand digestion. Literature Cited (1) Hepler, L. G.; Hsi, C. AOSTRA Technical Handbook on Oil Sands, Bitumen and Heavy Oils; AOSTRA Technical Publication Series #6; Alberta Oil Sands Technology and Research Authority: Edmonton, AB, Canada, 1989. (2) Hepler, L. G.; Smith, R. G. The Alberta Oil Sands: Industrial Procedures for Extraction and Some Recent Fundamental Research; AOSTRA Technical Publication Series #14; Alberta Oil Sands Technology and Research Authority: Edmonton, AB, Canada, 1994. (3) Kissa, E. Kinetics and mechanics of soiling and detergency. In Detergency: Theory and Technology; Cutler, W. G., Kissa, E., Eds.; Marcel Dekker: New York, 1987; Surfactant Science Series Vol. 20, pp 193-332. (4) Hunter, J. R. Introduction to Modern Colloid Science; Oxford University Press: New York, 1993. (5) Zhou, Z.; Xu, Z.; Masliyah, J. Effect of natural surfactants released from Athabasca oil sands on air holdup in a water column. Can. J. Chem. Eng. 2000, 78, 617-624. (6) Bowman, C. W. Molecular and Interfacial Properties of Athabasca Tar Sands. In Proceedings of the 7th World Petroleum Congress; World Petroleum Congress: London, 1967; Vol. 3, pp 583-604. (7) Takamura, K.; Chow, R. S. The electric properties of the bitumen/water interfaces. Part II: Application of the ionizable surface-group model. Colloids Surf. 1985, 15, 35-48. (8) Takamura, K.; Chow, R. S. A mechanism for initiation of bitumen displacement from oil sands. J. Can. Pet. Technol. 1983, 22, 22-30.

(9) Brown C. E.; Neustadter, E. L. The wettablity of oil /water/ silica systems with reference to oil recovery. J. Can. Pet. Technol. 1980, July-Sept., 100-110. (10) Dia, Q.; Chung, K. H. Bitumen-sand interaction in oil sand processing. Fuel 1995, 74, 1858-1864. (11) Dia, Q.; Chung, K. H. Hot water extraction process mechanism using model oil sands. Fuel 1996, 75, 220-226. (12) Zhou, Z.; Xu, Z.; Masliyah, J.; Czarnecki, J. Coagulation of bitumen with fine silica in model systems. Colloids Surf. A 1999, 148, 199-211. (13) Sheeran, D. E. Volume reduction of Clark hot water extraction fine tailings. In Advancing in Oil Sands Tailings Research; Alberta Department of Energy (Oil Sands Research Division): Edmonton, AB, Canada, 1995; Vol. III, pp 1-56. (14) Wen, W. W.; Sun, S. C. An electrokinetic study on the oil flotation of oxidized coal. Sep. Sci. Technol. 1981, 16, 1491-1521. (15) Wiacek, A.; Chibowski, E. Zeta potential: Effective diameter and multimodal size distribution in oil/water emulsion. Colloids Surf. A 1999, 159, 253-261. (16) King, R. P. Principles of Flotation; South African Institute of Mining and Metallurgy: Johannesburg, South Africa, 1982. (17) Becher, P. Emulsions: Theory and Practice; Reinhold Publishing Corporation: New York, 1965. (18) Laskowski, J. S. Weak electrolyte collectors. In Advances in Flotation Technology; Barekh, B. K, Miller, J. D., Eds.; SME Inc.: Littleton, CO, 1999; pp 58-82.

Received for review June 25, 2001 Revised manuscript received October 9, 2001 Accepted October 15, 2001 IE010543X