Soap: The Polymorphic Genie of Hierarchically Structured Soft

A bulk of these soap molecules are used in consumer goods such as bar soaps, ..... is more predominant along the straight edges as compared to the end...
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Ind. Eng. Chem. Res. 2008, 47, 6347–6353

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Soap: The Polymorphic Genie of Hierarchically Structured Soft Condensed-Matter Products Janhavi S. Raut* and Vijay M. Naik† Hindustan UnileVer Limited, Research Centre, 64 Main Road, Whitefield, Bangalore 560 066, Karnataka, India

Siddhant Singhal‡ and Vinay A. Juvekar‡ Indian Institute of Technology Bombay, Powai, Mumbai, 400 076 Maharashtra, India

More than 5 million tonnes of metal salts of fatty acids are manufactured and used worldwide every year, to create a range of soft condensed-matter products such as bar soaps, stick deodorants, personal care creams, toothpastes, and lubricant greases. These molecules, popularly known as soaps, are capable of forming a plethora of states and self-assembled aggregates such as micelles, liquid crystals, solid crystals, and gels, whose characteristic sizes or domain sizes can span from nanometers to centimeters. The type and mix of the phases formed, their morphologies, and their states of dispersion or the nature of their further supra-assemblies dictate the underlying micromechanical structures of products, which, in turn, are responsible for their optical, structural, and rheological properties. Developing processing guidelines to manipulate characteristic micromechanical structures is therefore key to obtaining the desired look, touch, feel, and function of these products. The article discusses a few illustrative examples of these structure-property relationships demonstrated by multiscale soap assemblies. Observations of some novel tertiary structures formed by crystallizing soap fibers at the air-water interface, serendipitously discovered by us in the recent past, are also discussed, to illustrate the richness and mysteries of the well-studied and so-called mature subject of soaps. 1. Introduction The term “soap” includes all compounds formed by reaction of either an inorganic or organic base with an organic fatty acid.1 An important subgroup of soaps is a class of molecules that are metal salts of linear fatty acids. These simple metal alkyl carboxylates are some of the oldest surfactants known. Today, more than 5 million tonnes of these molecules annually find their way into various products that fall in the class of soft condensed-matter composites. A bulk of these soap molecules are used in consumer goods such as bar soaps, liquid washes, personal care creams, and toothpastes. In almost all of these products, the soap molecules play the familiar role of a surface-active agent to deliver a variety of functions such as lather and emulsion stabilization, detergency, and surface wetting. What is relatively less appreciated is the ability of the soap molecules to selfassemble in numerous ways and form aggregates to reduce the system free energy, resulting in a range of different phases with unique molecular arrangements and physical characteristics. These aggregates and phases can, in turn, be engineered to take varied shapes and assemble into different tertiary structures through judicious use of field variables and processing conditions. The different equilibrium and nonequilibrium micromechanical structures thus produced can be employed to obtain a extensive spectrum of variations in physical properties and in-use characteristics of products, with minor changes in their mean bulk compositions. For example, by adjusting the size of soap crystallites and the refractive indices of various phases, one can produce an opaque white * To whom correspondence should be addressed. E-mail: [email protected]. † Current address: Indian Institute of Technology Bombay, Powai, Mumbai, 400 076 Maharashtra, India. ‡ Indian Institute of Technology Bombay.

bar of soap or a transparent bar of soap. Alternatively, by obtaining the right crystal morphology and state of dispersion, one can produce an extrudable/thermoplastic bar of soap at less than 15% water content or a rigid framework-structured nonextrudable bar containing as much as 90% water. This very ability of soap molecules to form self-assembled structures even in hydrophobic liquids makes them suitable for use as so-called low-molecular-mass organic gelators (LMOGs)2,3 in a diverse range of products. For example, lithium salts of 12-hydroxy stearic acid are employed4 to structure free-flowing lubricant liquids into viscous, gellike lubricant greases. Aluminum salts of naphthenic and palmitic acids can be used to structure free-flowing inflammable liquids, typically gasoline, to create the notorious flammable liquid napalm. Even simple products such as plasticine are

Figure 1. Simplified schematic T-x phase diagram of the aqueous soap system. L1, micellar solution; H1, hexagonal phase; LR, lamellar phase; S, solid phase; TK, Krafft boundary; dotted line, CMC curve; dashed line, molecular solubility curve for solid phase.

10.1021/ie0714753 CCC: $40.75  2008 American Chemical Society Published on Web 07/02/2008

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Figure 2. Polymorphism exhibited by soap molecules below the Krafft boundary: (a) plates, (b) rods, (c) ribbons, (d) twisted fibers, (e) C12 soap gel, and (f) C16/C18 soap gel formed in the presence of solvents such as mono-/dipropylene glycol (MPG/DPG). Plate b for 1% sodium myristate (NaMy) in water is from ref 17. Plates e and f are from ref 29.

Figure 3. (a) Phase composition of fresh-cast Pears soap as a function of temperature determined using wide-line NMR spectroscopy. (b) Small deformation oscillatory compression measurements as a function of temperature. Frequency ) 1 Hz. (c) Small deformation oscillatory shear measurements as a function of angular velocity. Amplitude of strain controlled at 0.05%.

formulated by employing suitable soaps to obtain their desired viscoplastic rheological behavior. In this article, we intend to illustrate the role played by typical phases exhibited by soap molecules and their supra-assemblies in the above categories of products, while “structuring” them to control their form, appearance, and other physical, mechanical, and sensory properties. In section 2, we briefly introduce commonly encountered primary phases and forms exhibited by soap molecules in aqueous environments and their dominant rheological behaviors. In sections 3 and 4, we highlight, through examples, the role of processing on the micromechanical superstructures formed by these phases and their influence on the end properties of finished products. Finally, we illustrate how soap molecules, despite centuries of technological

history, continue to surprise us with hitherto-unknown novel selfassembled structures and how the area is still an open arena for research. 2. Polymorphic Forms of Soap Molecules and Their Aggregates in Aqueous Systems For nearly a century, soap molecules have been the subject of a large number of scientific studies because of their ready availability and commercial importance. Driven by the need to shield their solvophobic part in a particular environment, soaps, like other amphiphilic molecules, organize themselves into a range of micellar, liquid-crystalline, and crystalline forms. Phase diagrams depicting these different transitions as functions of temperature, concentrations, presence of electrolytes, cosolvents,

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Figure 4. Effect of flash drying and shear working (below TK) on the mixed soap microstructure.

Figure 5. (a) Flow-direction-dependent transient behavior of platelike mica particles suspended in 5% sodium carboxymethyl cellulose (SCMC) solution with 20 wt % particle loading. Shear rate ) 1 s-1. The flow direction was reversed every 300 s. (b) Schematic depicting the effect of reversing the direction of shear on the suspension viscosity with anisotropic particles.

and so on have been determined for a large number of soap species and their mixtures.5–21 The typical phase diagram of a soap species, in the presence of a solvent system, has the general form depicted schematically in Figure 1. The diagram is characterized by the TK line corresponds to the Krafft boundary, which essentially gives the temperature above which the soap molecules exist in a molten but aggregated form. The Krafft point is defined as the point of intersection on the temperature axis between the curve of molecular solubility for the solid soap, depicted by the dashed-line extrapolation, and the curve of the critical micelle concentration (CMC), depicted by the dotted line. As the soap molecules are introduced into water, at very low concentration (to the left of the dotted line), they exist as individual molecules in solution, exhibiting preferential adsorption at surfaces or excess concentrations in the interfacial regions. They thus serve to stabilize interfaces by lowering the total free energy associated with maintaining a phase boundary. Upon an increase in concentration beyond the CMC curve, the molecules start forming aggregates, shielding their hydrophobic tails from the aqueous surroundings, thereby resulting in the formation of spherical micelles. With a further increase in

concentration, the spherical micelles deform to form rod-/ wormlike or disklike micelles. The individual micelles start packing together as the charge repulsion diminishes in the presence of higher counterion concentrations. This leads to the formation of liquid-crystalline phases in the molten state. Hexagonal phases (H1) are composed of rodlike micelles packed together side by side in a hexagonal pattern, whereas maximum packing of the molecules is achieved through the formation of lamellar structures (LR), wherein flattened disk micelles come together and form large stacks of double-layered sheets, as shown in Figure 1. Bicontinuous cubic phases (V1) having an aggregate curvature between those of rods and disks can also be present between the H1 and La phases. Below, the Krafft boundary, the soap molecules can form chain-frozen solid crystals, or gel phases, which consist of rigid lamellae of soap double layers with a disordered liquid solution sandwiched between them. It is well-documented that the chain-frozen solid soap crystals exhibit a number of forms such as fine fibers, ribbons, twisted fibers, rods, and flat platelets (Figure 2), depending on the type and mixture of soap molecules involved, as well as the crystallization conditions. The presence of nonsoap inclusions (cosurfactants, electrolytes, solvents, and so on) can modify the phase behavior, even at very low concentration levels. For instance, hydrotropes such as ethanol solubilize the liquid-crystalline phases, increasing the micellar solution region in the phase diagram. Minor levels of electrolytes can have a strong influence on both the CMC and the Krafft temperature through the common-ion effect, or they can produce insoluble soap salts.19 The rheological behavior of aqueous soap solutions is dependent on the nature of aggregation and the phases formed by the soap species.21,22 Whereas dilute spherical micellar solutions can exhibit Newtonian rheology, the rheological behaviors of solutions containing concentrated spherical micelles or anisotropic rodlike or disklike micelles are more complex. These micelles rotate in shear flow. The viscosity of solution increases sharply with increasing concentration of soap, when the rotating rods/disks start to overlap with each other, forming transient networks. The liquid-crystalline phases formed at higher concentrations exhibit even more complex nonideal rheological behaviors, as well as manifestations such as thixotropy, dilatency/pseudoplaticity, and shear banding. Peculiarly, the hexagonal and cubic phases have much higher viscosities than the more concentrated

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Figure 6. Stacking of (a) stearic acid platelets in a pearlescent skin cream and (b) aragonite platelets in a sea abalone shell.

Figure 7. Current yield stress versus concentration behavior for the solid structurant and the desired behavior through hierarchical structures.

lamellar phases, whose bilayers can slide more readily over one another. Most liquid-crystalline phases also exhibit a yield stress, with the cubic and hexagonal phases exhibiting pure elastic behavior under low deformation and at high oscillatory frequencies. Because of difficulties in obtaining single soap crystals of sufficiently large sizes, however, the mechanical properties of solid crystalline phases are not very well studied and reported in the open literature. 3. Soap as a “Material” One of the most common items of our daily use that consists almost entirely of soap molecules (as high as 85%) is a bar of soap. In this section, we use various case studies to illustrate how it is not the chemical composition alone but the nature and hierarchical superassembly of the coexisting phases that dictates the material properties of bar of soap. In the first case study, we discuss a historically well-known bar soap, namely, Pears. During the manufacturing process, it is cast as an approximately 60% solution of soap species in aqueous denatured spirit into molds and allowed to solidify as unmatured “green” Pears bars. At room temperature, these green Pears bars contain only about 20% solids as determined using low-field NMR techniques, with the remaining 80% being in liquid state. Yet, a green Pears bar exhibits a solidlike rheology (cf. Figure 3). However, if the bar is subjected to shear, it breaks down to form a flowable slurry. This is very similar to the rheological characteristics exhibited by fruits such as apples, which can be sheared to obtain a “juice”. The Pears slurry has the functionality of a bar of soap but lacks the structure, just as the crushed apple has the same taste but lacks the rigidity of a fruit. Analogously to the unique arrangement of lignocellulosic networks of turgid cells in fruits forming closed foam samplespanning structures, resulting in their rigidity, the green Pears formulation is also rendered into a rigid bar through the unique microstructural morphology of its solid crystallites, which form a sample-spanning load-bearing network. The commonality lies in the inability of both of the sample-spanning networks to withstand steady shear or large deformation. The desired morphology of the Pears bar, which is a nonequilibrium arrangement, is achieved through appropriate processing routes

and provides an excellent example of structure-property behavior manipulation through processing. However, the same is not true of all bar soaps, and a nonequilibrium structure is not always preferred. For a more conventional bar of soap (e.g., Lux), which is our next case study, the rules are completely different. These soaps, which contain approximately 85 wt % various soap species and 12-15% water, are processed via a high-shear milling and extrusion forming route, and their formulations are very different from that of a Pears bar. The structure of these soaps is described by the “brick-and-mortar” model. The bricks, which occupy about 60% of the phase volume, are composed of solid crystallites of long-chain insoluble soaps (e.g., sodium stearate, sodium palmitate) that serve the role of structurants, whereas the mortar consists of a mix of liquid-crystalline (LC) phases (30%) and isotropic liquid (10%) of short-chain and unsaturated soluble soaps (e.g., sodium laurate, sodium oleate) that provide the function of lathering and cleaning. In the initial stages of soap preparation, the neutralization of the fatty acids with caustic soda results in a soap mass with about 30-40% water in which both the insoluble and soluble soaps are held in a mixed lamellar phase. Downstream, this soap mass is subjected to flash drying to reduce the water content to the desired level of 12-15%. This rapid drying operation, which takes place on a time scale on the order of milliseconds, congeals the mixed lamellar phase into a nonequilibrium supersaturated state with the soluble and insoluble soap molecules trapped into a congealed mixedcrystalline phase called the kappa phase (cf. Figure 4) characterized by a unique X-ray pattern with peaks at 4.05, 3.20, and 2.97 Å, with the latter being used to characterize the phase. The trapped soluble soap is thus no longer readily available for providing rapid lathering or cleaning functions, and the resultant bar of soap has the structure of a bar but does not display the most desirable in-use characteristics of a bar soap. It lathers slowly and forms slimy mush in contact with a water layer. The bar, therefore, has to be shear-worked with the temperature maintained below the Krafft boundary to restore the equilibrium phase structure. The shear working is required to overcome the activation energy barrier for getting the soluble soap out of the nonequilibrium mixed soap crystals into the mortar phase. When the soluble soaps are extracted into the mortar phase, the longchain crystals left behind recrystallize into a new form called the zeta phase (characteristic X-ray peaks at 4.05, 3.50, and 2.75 Å). Another effect that has been observed after shear working of a bar of soap is a higher optical transparency for some specialty formulations. This is caused by a reduction of the crystal size (because of particle breakdown during shearing and recrystallization of the zeta phase, which is characterized by smaller crystallite sizes) and better refractive index matching between the solid and mortar phases because of the higher extents of soap solubilization in the mortar. The relative impact of both on the scattering behavior of the composite bar can be assessed by the Raleigh-Gans equation. Thus, in this case, in contrast to the case of a Pears bar, the nonequilibrium structure

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Figure 8. Different hierarchical structures observed in bar soaps.

formation of mush, which is associated with water ingress and subsequent swelling of the soap at the interface, is more predominant along the straight edges as compared to the ends, because of the alignment of the crystallites during extrusion. Additionally, controlled alignment can be exploited to create desirable optical effects in a bar of soap, such as color striping and pearlescence. The origin of soap pearlesence is similar to that observed, for instance, in a sea abalone shell (Figure 6), which occurs as a result of the interference of light reflected from the multilayer stacks of crystalline platelets of aragonite having a characteristic nanoscale thickness. Apart from a bar of soap, pearlesence can be seen in other soap-rich products such as liquid soaps or skin creams, wherein the crystallites of both sodium soaps and fatty acids (which are “protonated” soaps) are deployed to obtain the desired optical effects.23 Figure 9. Transition from (a) equilibrium platelike crystals to (b) straight fibers, (c) branched fibers, and (d) glassy/amorphous structures with increasing cooling rate.

is detrimental to the functioning of the bar soap. It is the restoration of a near-equilibrium structure that helps achieve the desired structure, function, and appearance of the bar. Whereas the applied shear provides the necessary energy input into the bar soap to bring about the phase transformations, it also interacts physically with the soap mix at various levels. Both the LC phases in the mortar and the solid platelike crystallites of soap are known to align along flow lines, leading to complex rheological behaviors that can pose significant challenges during processing. In Figure 5, we show results from transient viscosity measurements that were carried out employing a model suspension of mica platelets suspended in a 5% sodium carboxymethyl cellulose (SCMC) solution. This was used as a model system to obtain a qualitative understanding of the motion of the anisotropic platelike soap particles under shear, as similar studies with a fully formulated opaque soap system would not be feasible. The measurements were carried out on a CarriMed CSL 500 controlled-stress rheometer (TA Instruments, West Sussex, U.K.) using a parallel-plate geometry. It was observed that the suspension retained a memory of its past shear history. When the shearing was stopped and restarted in the opposite direction, the measured shear stress showed a sudden increase before falling off to the steady-state value. Alternatively, when the shearing was restarted in the same direction, there was no change in the measured shear stress. These measurements provide insight into the microstructural rearrangements taking place in the suspension of flat platelets of soap under shear flow. The shear flows thus result in product microstructures that have inherently anisotropic properties. It is well-known that the microstructural arrangement of the crystallites inside a bar of soap is anisotropic as a result of its flow history through the extruders, noodler plates, and dies. A typical bar of soap is thus more likely to crack along the length because the defect lines run in the direction of extrusion. The

4. Materials Science Challenges to Soap Technologists The ∼60% solid crytallites present in a typical milled and extruded bar of soap, which represents the majority of bar soaps produced today, are present only to give the bar its structural rigidity and do not contribute to its functionality in any way. The solid crystals are washed down the drain as the bar wears down during use and are a huge load on the environment. A challenge faced by the soap industry is, therefore, to reduce the level of structurants required in a bar to the minimum possible. A typical yield stress versus structurant concentration relationship for a milled and extruded brick-and-mortar bar of soap is shown in Figure 7. The question is: Can the shape of this curve be altered to achieve the same yield stress at much lower concentration of structurant? It is likely that multiscale hierarchical structures as seen in nature (e.g., bone) and as employed by man to create built environment (e.g., Eiffel tower) can also be deployed in soap to obtain rigidity at lower concentrations of structurant materials.24 We have made attempts to build these hierarchical structures inside a bar of soap through appropriate processing routes25 to achieve the required yield stress with a lower usage of insoluble soaps. Figure 8 shows some of the hierarchical structures achieved inside different soap systems obtained by casting, through appropriate manipulation of the processing conditions. The microstructures of these hierarchical structures can be controlled through variations in formulation, rates of cooling, solvent systems, supersaturation, electrolytes, and so on. Changing the rate of cooling and hence modifying the relative rates of nucleation vs crystal growth can result in very different structures from equilibrium platelets to straight fibers and branched fibers as illustrated in Figure 9. The mechanical properties of these structures are also very different. 5. Soap as a Source of Surprises and Inspiration in Materials Science Control of the cooling rate during the casting of soap systems also results in some very dramatic structure formation at the

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Figure 10. Spiral coiling of sodium myristate (NaMy) fibers and formation of 3D micropottery structures at the surface of the soap mass.

surface of the crystallizing soap mass. Figure 10 presents some of these fascinating structures formed by self-organization of sodium myristate (NaMy) soap crystallites at the air-liquor interface.26 The surface structures constitute planar spiral assemblies of soap fibers, as well as 3D “micropottery” structures emerging from the solidifying surface. Attempts were made to observe the actual dynamics of ring/structure formation through video microscopy. However, the dynamics of ring/ structure formation could not be resolved because the evolution/ development of the spiral structures was frozen into a latent pattern long before the filament thickness grew large enough for visibility. This indicates that the assembly process is initiated when the fibers are at an incipient stage in the crystallization. While the formation of the surface assemblies is broadly attributed to the Marangoni effect, i.e., motion and deformation of the crystallizing fiber owing to interfacial tension gradients in its vicinity;26 the exact origin of these fascinating assemblies is still unknown. Drawing from similar pattern formation in nature, we have multiple hypothesis ranging from (i) transient heat and mass fluxes at the interface leading to alternating depletion and restoration zones resulting in these patterns of nucleation and fiber growth; (ii) formation of precursor rings as a result of the bending of flexible filaments under the stress imposed by surface pressure gradients,16 followed by continual drawing of filaments from the crystallizing soup as a result of the cohesive forces between the loose filament and the filament bobbins, analogous to the coiling rope phenomenon described by Mahadevan et al.;27,28 and (iii) precursor rings causing flow patterns with roll cells normal to the interface, resulting in alignment of the nucleating fibers along the flow lines before they grow large enough to entangle. However, all of these hypotheses currently remain unsubstantiated, and further experimental and theoretical probing is warranted. 6. Concluding Remarks More than two centuries have elapsed since we first understood the molecular nature of soap. Nearly a century of research has been spent on understanding the intriguing phase behavior of soaps and the physical properties of individual phases. As this article illustrates, however, the consumer-relevant properties of most consumer goods containing soap as a functional or structuring ingredient are dependent on the mesoscopic and microscopic tertiary supra-structure formed by soap molecules in these products. What is fascinating is the myriad of different supra-structures formed by simple soap molecules to impart a range of desirable properties to the end products. What is bewildering is the ability of simple soap molecules to spring

surprises in the form of hitherto unseen, aesthetically charming, and entirely novel supra-structures such as micropottery. These bulk and surface structures bear testimony to the fact that the soap molecules still afford an extensive arena for research. We hope that the examples and challenges provided in this article more than justify its title. The polymorphic forms exhibited by simple soap molecules and the myriad of roles they play in a range of products are indeed no less bewildering than those of a mythological genie. Acknowledgment The authors acknowledge Dr. Hari Koduvely from Unilever Research India for useful discussions on crystal growth and the morphology of soap molecules. Literature Cited (1) Zhu, S.; Chambers, J. G.; Naik, V. M. Soap. In Kirk-Othmer Concise Encyclopedia of Chemical Technology; Wiley-Interscience: New York, 2007; Vol. 2, p 861. (2) Terech, P.; Rodriguez, V.; Barnes, J. D.; McKenna, G. B. Organogels and Aerogels of Racemic and Chiral 12-Hydroxyoctadecanoic Acid. Langmuir 1994, 10, 3406. (3) Terech, P.; Weiss, R G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. ReV. 1997, 97, 3133. (4) Witte, A. C. Grease Manufacture. In Encyclopedia of Chemical Processing and Design; McKetta, J. J., Cunningham, W. A., Eds.; Marcel Decker: New York, 1984; Vol. 25, p 88. (5) Darke, W. F.; McBain, J. W.; Salmon, C. S. The Ultramicroscopic Structure of Soaps. Proc. R. Soc. London, Ser. A 1920, 395. (6) McBain, J. W.; Sierichs, W. C. The Solubility of Sodium and Potassium Soaps and Diagrams of Aqueous Potassium Soaps. J. Am. Oil Chem. Soc. 1948, 221. (7) McBain, J. W.; Vold, R. D.; Frick, M. A Phase Rule Study of the System Sodium Stearate-Water. J. Phys. Chem. 1940, 44, 1013. (8) Madelmont, C.; Perron, R. Study of the influence of the chain length on some aspects of soap/water diagrams. Colloid Polym. Sci. 1976, 254, 581. (9) (a) Laughlin R. G. In AdVances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1978; Vol. 3, p 41. (b) Laughlin R. G. In Surfactants; Tadros, Th. F., Ed.; Academic Press: London, 1984; p 53. (c) Laughlin, R. G. The Aqueous Phase BehaVior of Surfactants; Academic Press: San Diego, CA, 1994. (10) Luzatti, V.; Mustacchi, H.; Skoulios, A. The Structure of the LiquidCrystal Phases of Some Soap + Water Systems. Faraday Soc. Discuss. 1958, 25, 43. (11) Buerger, M. J.; Smith, L. B.; Ryer, F. V.; Spike, J. E. The Crystalline Phases of Soap. Proc. Natl. Acad. Sci. U.S.A. 1945, 31 (8), 226– 233. (12) Ekwall, P.; Mandell, L.; Fontell, K. Ternary systems of potassium soap, alcohol, and water: I. Phase equilibria and phase structures; II. Structure and composition of the mesophases. J. Colloid Interface Sci. 1969, 31, 508.

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ReceiVed for reView October 31, 2007 ReVised manuscript receiVed May 6, 2008 Accepted May 16, 2008 IE0714753