CHAPTER 3 SYNTHESIS OF NANOPARTICLES: MICROEMULSION METHOD 3.1 Introduction Micro-emulsion method is one of the recent and ideal techniques for the preparation of inorganic nano-particles (Yu, et al., 2010). Oil and water are immiscible and they separate into two phases when mixed, each saturated with traces of the other component (Capek, 1999). An attempt to combine the two phases requires energy input that would establish water-oil association replacing the water-water/oil-oil contacts. The interfacial tension between bulk oil and water can be as high as 30-50 dynes/cm (Gelbart and BenShau, 1996; Bourrel and Schechter, 1988). This can be overcome by the use of surface-active molecules known as surfactants. Surfactants contain waterloving (hydrophilic) and oil-loving (lipophilic) moieties (Holmberg, 2002). Owing to this characteristic, they tend to adsorb at the water-oil interface. If enough surfactant molecules are present, they align and create an interface between the water and the oil by decreasing the interfacial tension (Gelbart and BenShau, 1996). An emulsion is formed when a small amount of an appropriate surfactant is mechanically agitated with the oil and water resulting in a two-phase dispersion where one phase exists as droplets coated by surfactant that is dispersed throughout the continuous, other phase. These emulsions are milky or turbid in appearance due to the fact that the droplet sizes range from 0.1 to 1 micron in (Holmberg, 2002). As a general rule, the type of surfactant used in the system determines which phase is continuous. If the surfactant is hydrophilic, oil will be emulsified in droplets throughout a continuous water phase. The opposite is true for more lipophilic surfactants. Water will be emulsified in droplets that are dispersed throughout a continuous oil phase in this case (Bancroft, 1913). Emulsions are kinetically stable, but are ultimately thermodynamically unstable, and will begin to separate back into their two phases. The droplets would merge together, while the dispersed phases will sediment (Holmberg, 2002). At this point, they degrade back into bulk phases of pure oil and pure water with some of the surfactant dissolved preferentially in one of the two (Gelbart and BenShau, 1996). 98
3.2 Characteristics of Microemulsions If a surfactant possessing balanced hydrophilic and lipophilic properties is used in the right concentration, a different oil and water system will be produced. The system remains an emulsion, but exhibits some characteristics that are different from the milky emulsions discussed earlier. These new systems are microemulsions. The interfacial tension between phases, amount of energy required for formation, droplet sizes, and visual appearance are only a few of the differences seen when comparing emulsions to microemulsions. Water-in-oil microemulsions are also known as reverse micelles. These systems have the ability to solubilise both hydrophilic and hydrophobic substances (Avramiotis, et al., 1997). Microemulsions usually exhibit low viscosities and Newtonian flow characteristics. Their flow remains constant when subjected to a variety of shear rates. Bicontinuous formulations may show some non-newtonian flow and plasticity (Moulik, and Paul, 1998). Microemulsion viscosity is close to that of water, even at high droplet concentrations. The microstructure constantly changes, making them very dynamic systems with reversible droplet coalescence (Capek, 1999). A variety of techniques are employed to characterize different properties of microemulsions. Light scattering, X-ray diffraction, ultracentrifugation, electrical conductivity, and viscosity measurements have been widely used (Singh, et al., 1983). 3.3 Types of Microemulsions Based on the phase equilibrium of microemulsions, they can be further classified into four different types. Winsor (1948) has developed a classification scheme for emulsions (micro- and macro-) illustrated in Figure 3.1. Oil-in-water (o/w) microemulsions are droplets of oil surrounded by a surfactant (and possibly co-surfactant) film that forms the internal phase distributed in water, which is the continuous phase, Winsor-I. The Winsor-II type is water-in-oil (w/o) microemulsion in equilibrium with excess water phase at the bottom. The o/w type microemulsion has generally a larger interaction volume than the w/o microemulsion (Lawrence, and Rees, 2000). Water-in-oil microemulsions are made up of droplets of water surrounded by an oil continuous phase. These are generally known as reverse-micelles. The middle phase bicontinuous microemulsions in equilibrium with excess oil phase at the top and excess water phase at the bottom have been classified as Winsor-III type. These may show non-newtonian flow and plasticity. 99
These properties make them especially useful for topical delivery of drugs or for intravenous administration, where upon dilution with aqueous biological fluids, form an o/w microemulsion (Liu, et al., 2009; Bolzinger, et al. 1998). Water-in-oil microemulsions are made up of droplets of water surrounded by an oil continuous phase. These are generally known as reverse-micelles. A diagrammatic representation of the three types of microstructures is shown in Figure 3.2. The fourth type, basically a macroemulsion, Winsor-IV, may exist in the form of one of three possible different microstructures: oil-in-water (o/w), water-in-oil (w/o), and bicontinuous. Generally, one would assume that whichever phase was in larger volume would be the continuous phase, but this is not always the case. The monolayer of surfactant forms the interfacial film that is oriented in a positive curve, where the polar head-groups face the continuous water phase and the lipophilic tails face into the oil droplets (Gelbart and Ben Shau, 1996). Figure 3.1 Classifications of Microemulsion (Winsor, 1948). 100
Figure 3.2 Three structures of microemulsions: Winsor-I, Winsor-III and Winsor-II. The main interest of the present study was focussed on the Winsor II Type microemulsion system, consisting of two phases in which water-in-oil droplets are in equilibrium with excess water phase at the bottom. 3.4 Microemulsion Formulation The properties of the surfactant-oil-water are important in determining the formation of microemulsions. Deviations from the actual formulation may cause the breaking of the microemulsion and formation of an unstable macro-emulsion. 3.4.1 Formulation Considerations A microemulsion generally consists of four different components, a lipophilic phase, a hydrophilic phase, surfactant and co-surfactant (Scriven, 1976). The nature of the components like the oil, surfactant, co-surfactant and water, as well as temperature and pressure which affect the microemulsion systems are known as the formulation variables. The quantities of different substances present, are also likely to change the properties, and are referred to as composition variables which can be expressed as weight, percentage or proportion (Salager, 2000). In order that the microemulsions attain low interfacial tension and good solubilisation ability, it is necessary that the microemulsions be formulated accurately. The formation of a microemulsion depends on factors such as: (1) oil/surfactant and surfactant/co-surfactant ratio; (2) nature and concentration of the oil, surfactant, co-surfactant and aqueous phase; (3) ph; (4) temperature; and (5) hydrophilicity/lipophilicity of polarity (Bolzinger, et al., 1998; 101
Salager, 2000). All these factors must be considered during the formulation of microemulsions. Moreover, it is important to consider the compatibility of the oil, surfactant or co-surfactant for the desired route of administration. To study the phase behaviour of simple microemulsion systems comprising of surfactant, oil and water at fixed pressure and temperature ternary phase diagrams are used. Each corner of the ternary phase diagram represents 100% concentrations of a particular component. When four or more components are used pseudo-ternary phase diagrams are used to depict these systems in which each corner represents binary mixtures of two components such as surfactant/co-surfactant, surfactant/water, oil/drug, and water/drug mixtures. A typical ternary phase diagram is shown in Figure 3.3 (Shaji, 2004). Figure 3.3 Ternary Phase Diagram showing different phases (Shaji, 2004). 3.4.1.1 Water Phase Depending upon the amount of water present in the system, water may form water pool or work as a dispersion medium in micro-emulsion systems (Chaparaba, 1991). 3.4.1.2 Oil Phase The oil phase must be chosen appropriately, since it governs the selection of the other ingredients for the microemulsion and there are two main factors that need be considered before selecting the appropriate oil phase. Firstly, the solubilising potential of the oil for the selected substance must be seen and secondly, the chosen must be 102
such that the microemulsion forming region is enhanced. Oils with shorter hydrocarbon chains are easier to micro-emulsify as compared to oils with long hydrocarbon chains. An oils ability to solubilise lipophilic groups is directly proportional to the chain length of the oil. Thus, the selected oil should be such that it is capable of solubilising the API, and facilitating the formation of microemulsions with desired characteristics (Warisnoicharoen, 2000). 3.4.1.3 Surfactants in Microemulsions Surfactants are molecules that typically contain a polar head group and an apolar tail. The schematic of surface active agent is shown in the Figure 3.4 (Evans and Wennerstrom, 1999). They are surface-active and microstructure-forming molecules with a strong chemical dipole (Holmberg, 2002). They can be ionic (cationic or anionic), nonionic, or zwitterionic. Surfactant molecules self-associate due to various inter- and intra-molecular forces as well as entropy considerations. The surfactant molecules can arrange themselves in a variety of shapes. They can form spherical micelles, rod-shaped micelles, a hexagonal phase (consisting of rod-shaped micelles), lamellar (sheet) phases, reverse micelles, or hexagonal reverse micelles (Lawrence, and Rees, 2000). The structure change of micelle with size is illustrated in Figure 3.5 (Lawrence and Rees, 2000). Figure 3.4 Spontaneous self-assembly of surfactants into micelles in aqueous solution. 103
Figure 3.5 Schematic representation of most occurred surfactant associates. Hydrophile Lipohile Balance (HLB) was a method proposed by Griffin, as a guide to select an optimal emulsifying agent (Garcia, et al., 1989). It has been shown by Sherman that HLB depends on both concentration as well as phase volumes of the oil and water. HLB is used to characterize most oil phases using solubility parameter (SP) values. The higher HLB is the higher solubility in water. The SP values may also be derived from basic physical properties (Vaughan and Dennis, 1990). The HLB value is derived from the following equation: HLB = 4 SP + 7 8 (3.1) This equation is used to predict the stability of emulsions and select optimal emulsifying agents. Therefore, in general a lower HLB number is used for water in oil (W/O) emulsifications while a higher HLB number is used for oil in water (O/W) emulsifications. The HLB value of the surfactant indicates the solubility of the surfactant. If the surfactant has lower HLB value then surfactant is more lipophilic or oil soluble or if the surfactant has higher HLB value then surfactant is more hydrophilic or water soluble. For surfactant system, several studies recommend the use of blend of at least two surfactants since mixtures of a low HLB and a high HLB surfactant gives better coverage at the interface (Murdan, et al., 1999). It is generally accepted that a surfactant with HLB in the range 3-6 will favour the formation of water-in-oil (w/o) 104
microemulsions, whereas surfactants with HLB from 8-18 are preferred for oil-in-water (o/w) microemulsions (Lawrence, and Rees, 2000). It is also pertinent to point out that microemulsions are obtained only under certain carefully defined conditions, and the HLB of the surfactant can, at the most be used as a starting point in the selection of components that will form a microemulsion. As mentioned above, surfactant can be ionic (cationic or anionic), nonionic, or zwitterionic. Most nonionic surfactants are structurally similar to ionic surfactants, except for the fact that with ionic surfactants, the headgroup is uncharged. Because there are no electrostatic charges from the headgroups, the interactions between these nonionic headgroups are dominated by steric and osmotic forces. Co-surfactants are generally not needed to form microemulsions with nonionics. This is due to the fact that pure specimens of nonionic s usually are made up of mixtures of slightly varying chain length (Myer, 2006). Ethoxylated alcohols are the most common nonionic surfactants. These alcohols contain a wide-ranging degree of ethoxylation, where ethylene oxide is added to fatty acids to make them more water-soluble. They are considered amphiphiles, with a lipophilic hydrocarbon tail group and water loving ethoxylated alcohol group (Holmberg, 2002). Examples of non-ionic surfactants include poly-oxyethylene surfactants, such as Brij 35, or sugar esters, such as sorbitan monooleate (Span 80). Polyoxyethylene sorbitan monooleate (Tween 80) and polyoxyethylene sorbitan monolaurate (Tween 20) appear safe and acceptable for oral and parenteral use (Kibbe, 2000; Lawrence, and Rees, 2000). A large majority of ionic surfactants do not form balanced microemulsions without the addition of another component. The salts or co-surfactants shift the overall HLB into the optimal range for microemulsion formulation (Holmberg, 2002). More often than not, one surfactant, whether nonionic or ionic, is not sufficient to form a microemulsion or does not result in an optimal microemulsion-forming region. Combinations of surfactants or sometimes co-surfactants are required for the optimal formation of a microemulsion. The term co-surfactant can describe any component that aids the primary surfactant in microemulsion formulation. Co-surfactant can refer to a second surfactant being used, but may also refer to a low-molecular-weight amphiphile, such as an alcohol (Holmberg, 2002). Two different nonionic surfactants can be mixed together. Mixing a more lipophilic nonionic surfactant with a more hydrophilic nonionic surfactant can result in the exact HLB needed to form a microemulsion. The two surfactants can be mixed in 105
varying ratios to determine the ideal combination of the two, which results in the largest microemulsion-forming region. Mixtures of nonionic surfactants can be seen in commercial products and can sometimes be regarded as a single component (a pseudocomponent) in the microemulsion system (Holmberg, 2002). Ionic surfactants can be combined with nonionic surfactants, or higher molecular weight ethoxylated alcohols. These mixtures have synergistic effects, which allow them to be applied to many things. The most popular advantage to these mixtures is the fact that they result in temperature insensitive microemulsions (Holmberg, 2002). Generally, ionic and nonionic surfactants react oppositely with increasing temperature. Ionic surfactants show a hydrophilic shift with increasing temperature, while nonionic surfactants exhibit a lipophilic shift. Therefore, when mixed together in a particular ratio, the two will cancel each other out, resulting in a temperature insensitive microemulsion formulation (Holmberg, 2002). Frequently, single chain surfactants are not able to reduce the surface tension to the ultralow levels required for microemulsion formulation. Short and medium chain alcohols, such as butanol, pentanol, ethanol, isopropanol, or propylene glycol, are commonly added as co-surfactants (Holmberg, 2002; Giustini, et al., 2004; Lawrence, and Rees, 2000). These co-surfactants help to further reduce the surface tension and fluidize the surfactant film, which increases the entropy of the system leading to its thermodynamic stability. Co-surfactants also increase the flexibility of the surfactant film around the microemulsion droplet (Maghraby, 2008; Junyaprasert, et al., 2008). As discussed earlier, surface active agent (surfactant) is one of the most important factors which can greatly influence on the formation of the emulsion systems and also the stability of these systems. In a research which was done by Porras, et al. (2004) on formation of water-in-oil nano-emulsions in water/mixed nonionic surfactant/oil system, several mixtures of Span 20, Span 80, Tween 20 and Tween 80 were studied. It was proved that the mixture of surfactants can provide better performance than pure surfactants. They also found that the best and most stable samples of emulsion, microemulsion and nanoemulsion can be formed in the formation of emulsion systems using of mixtures of Span-80 and Tween-80 surfactants in the ratio of 49/51, as shown in Figure 3.6. 106
Figure 3.6 Existence regions of microemulsion, nano-emulsion and emulsion: Span 80 Tween 80 (49:51)/decane/water (Porras, et al., 2004). In the present study, Span-80 and Tween-80 with the molecular structure shown in Figure 3.7 (Osseo-Asare, 1999) and with an HLB value less than 6 were used as surfactant. Figure 3.7 Molecular structures of two surfactants: (a) Tween-80: Sorbitan monooleate and (b) Span-80: Sorbitan oleate. Microemulsion use shows great potential in a wide variety of areas including enhanced oil recovery, cutting oils, drug delivery, detergency, and lubrication (Giustini, et al., 2004). Microemulsions can be applied in some industries as cleaners, hair products, perfumes, gels, and skin care products, to name only a few (Aikens, and 107
Friberg, 1999). The thermodynamically stable microemulsions prove to be useful media for chemical reactions, due to the fact that they can solubilise both oil and water components as well as have a large internal interface (Capek, 1999; Salem, 2006). They can serve as artificial blood substitutes, or can be used as models for biological membranes (Jadhav, et al., 2006). In short, microemulsions possess many characteristics that are ideal for multiple and versatile applications. As with all things, microemulsions have advantages and disadvantages. Microemulsions possess several advantages that make them suitable for making nanoparticles. These include the following factors. 1. Ease of Preparation: Microemulsions form spontaneously at room temperature, and are easy to manufacture, when compared to liposomes and macroemulsions which require high pressure homogenization during preparation (Salager, 2000). 2. Thermodynamic Stability: The stability and shelf life of the formulation is improved due to the thermodynamic stability of the microemulsions. 3. Ability to incorporate both hydrophilic and lipophilic therapeutic agents: Microemulsions can form diverse microstructures which enable them to solubilise both hydrophilic and hydrophobic drugs, either alone or in combination (Gallarat, et al., 2004). 4. As a template for the synthesis of nano-particles: Microemulsions are thermodynamically stable, and consist of small droplets which possess large interfacial area. These characteristics facilitate their use in nanoparticle synthesis (Sarciaux, et al., 1995). Microemulsions have some disadvantages as follows: Formation of microemulsions generally requires large amounts of surfactants and/or co-surfactants. All of these at high concentrations are generally irritating (Djordjevic, et al., 2004; Jadhav, et al., 2006). Many external factors, such as temperature and ph, influence the stability of microemulsions as well. 3.5 Reverse Micelle There are some factors that affect the stability of an emulsion and further affect the morphology and size distribution of produced particles. These factors include type and amount of surfactant and co-surfactant, the concentration of precursor solution, the kind of oil phase, and the water-to-oil ratio. Micro-emulsion is generated by gradual 108
addition of several drops of an agent as a non-continuous phase and the other material as a continuous phase and rapid mixing of these two phases. Oil drops in the water is a good example of micro-emulsions. A general schematic presentation of how the emulsion systems are generated is shown in Figure 3.8 (Hung-Jang, et al., 1992). Figure 3.8 Schematic of how emulsion systems are formed. (a) Two immiscible liquid phases, (b) An emulsion of B phase in the A phase, (c) Unstable emulsion which leads to phase s formation, and (d) Surface active agent in the interface of two phases makes the emulsion stable (Hung-Jang, et al., 1992). Reverse micelles provide an example of organized self assemblies of surfactants in solution and are most widely used as reaction media or templates for biomimetic synthesis of various inorganic nano-particles. The biomineralization process in nature uses organized aggregates of bio macromolecules to synthesize nano-particles with dimensional, morphological and architectural specificity and exercising full control over nucleation, growth and the patterns formed. The hydrophilic head and hydrophobic tail of surfactants in a polar solvent self assemble to give reverse micelles where the polar core contains the hydrophilic heads and the polar shell the hydrophobic chains, depicted in Figure 3.9. Water can be solubilized in the core forming water-inoil droplets (5 nm) which eventually become the w/o micro-emulsion as the water content increases (5 to 100nm). The water to surfactant molar ratio has a decisive influence on the diameter of the reverse micelles (Ramdas and Bhaskar, 2008). Reverse micelles are generally characterized by the molar ratio of water to surfactant, ω [ω = 109
(H 2 O)/ (surfactant)] (Capek, 2004). The shape can be spherical, rod-like or lamellar and depends on the concentration of surfactant, electrolyte, other additives, etc. The droplets undergo continuous collisions and exchange their contents (Ramdas and Bhaskar, 2008). Figure 3.9 Reverse micelle (Ramdas and Bhaskar, 2008). Three component systems are usually presented at constant temperature and pressure in a Gibbs phase-triangle. The composition of the mixture is specified by the composition variables ω, α, and γ. These variables were obtained by the following equations: ω = water surfactant (3.2) α = m oil m oil + m water (3.3) γ = m surfactant m surfactant + m oil + m water (3.4) where ω, α, and γ are, the water-surfactant molar ratio, weight fraction of oil in the mixture of oil and water, and the weight fraction of surfactant in the ternary mixture respectively (Lade, et al., 2000; Adityawarman, et al., 2005). 110
3.5.1 Formation of Nano-particles in W/O Microemulsion System In the present study pertaining to the synthesis of nano-particles (NPs), single phase w/o micro-emulsion with reverse micelles are required. Good dispersion of generated drops in the micro-emulsion is suitable for synthesis of nano-particles and it has good enough potential to control the chemical reaction that might occur in the micro-emulsion (Huang, et al., 2004). The basic idea of two-emulsion technique is illustrated in Figure 3.10 (Clifford, et al., 2001). Figure 3.10 Possible mechanisms of nano-particles formation by micro-emulsion route. 3.5.2 Mechanism of the Formation of Nano-particles The mechanism of nano-particle formation in the micro-emulsion has not yet been understood well. However, the suggested mechanism for the nano-particles synthesis within micro-emulsions can be explained by the results achieved from nanoparticulate material generation process in micro-emulsions. Mechanism for nanoparticles formation suggested by some researchers (Clifford, et al., 2001; Capek, 2004; Chen, et al., 2006) is shown schematically in Figure 3.10. When the microemulsions material including reactants are mixed together, reactants exchange takes place during the colliding of water droplets in microemulsion. The reactant exchange is too fast and precipitation reaction occurs in the nanodroplets, which is followed by nucleation, growth, and coagulation of primary particles, resulting in the formation of the final nanoparticles surrounded by water and/or stabilized by 111
surfactants. Because the exchange of aqueous contents between the microemulsion droplets or the intermicellar material exchange is closely related to the formation process of nanoparticles in reverse micelles, it is necessary to consider how the intermicellar exchange influences various aspects of the nanoparticle formation (Figure 3.9). It is generally accepted that the water contents of microemulsion droplets are exchanged rapidly through droplet collision and fusion, with the fusion step as the rate determining step (Qi, 2006). As shown in the Figure 3.10, at the first route, reactant diffusion is taken place through oily phase into aqueous droplets including the second reactant. While the particles achieve their final size, molecules of surfactant stick to particles surface, and cause their durability, stability and maintenance in a certain level and prevent more growing of particles. On the other hand, reactant ions exchange may be occurred because of coalescence of two droplets with each other (second route). In this case, the contact of reactants and subsequent reaction can be regarded as a number of sequential steps: 1. Diffusion and convection to bring the emulsion droplets together, 2. Surfactant layer opening and coalescence, 3. Diffusion of the solubilizate molecules in the temporary dimeric aggregate, 4. Reaction between solubilizate molecules, 5. Nucleation and crystal growth of precursor particles, and 6. Decoalescence to return as smaller droplets. 3.6 Factors Affecting the Size of Nano-particles in the Microemulsion The surface activated and stable micro-cavities produce cage-like effect and cause to limit growth nucleation and particles agglomeration (Lade, et al., 2000). The size of micro-emulsion drops has a clear effect on the particles size. On the other hand, the size of micro-emulsion drops in turn, depends on their collisions and created interactions. These interactions are dependent on the viscosity of mixture. For a diluted dispersion of spherical droplets without interactions, the relative viscosity, η r is expected to obey the Einstein-relation, equation 3.5: η r = η η o = 1 + 5 2 Φ (3.5) 112
Where η o is the viscosity of the solvent, η is the viscosity of the dispersion, and Φ the volume fraction of droplets. For droplet volume fractions up to 0.2, a maximum relative viscosity η r value of 1.5 is expected. A higher η r indicates structural changes of the micro-emulsion (Lade, et al., 2000). This is observed within the one-phase region approaching the lower phase boundary. Figures 3.11 and 3.12 show viscosity measurements within the one-phase region for micro-emulsions prepared from two different surfactants. In both systems a strong increase of viscosity is observed when decreasing the temperature and increasing the water concentration. This indicates stronger droplet interactions or a higher degree of structural transformation when approaching the lower phase boundary. If the critical micro-emulsion concentration (cµc) is known, it is possible to calculate the size of spherical droplets from a simple geometric model (Lade, et al., 2000). Figure 3.11 Relative viscosity of the one-phase region of water-cyclohexane- Marlipal O 13/50 at γ = 0.15 as a function of temperature and composition 113
Figure 3.12 Relative viscosity of the one-phase region of water-cyclohexane- Marlipal O 13/60 at γ = 0.15 as a function of temperature and composition When the solubilised water is assumed to be inside the spherical aggregates only, the average droplet core radius is given by: r = 3 V w S (3.6) Where V w is the volume of the dispersed water, and S is the total interfacial area covered by a monolayer of surfactant molecules in a unit mass of micro-emulsion. This area is calculated from the number of surfactant molecules, N s forming the interface and the area per surfactant molecule, a h : N S = γ cμc m N A MS (3.7) S = N S a h (3.8) N A is the Avogadro number, m is the mass of the micro-emulsion and M s is the molecular mass of the surfactant (Lade, et al., 2000). Further, the size of microemulsion drops can be controlled by changing ω (ratio of water to surface active agent), and therefore it is an important factor in this study. By increasing the water level, the size of particles become larger and leads to much broader particle size distribution. Liyi, et al. (1998) found that the water level in the micro-emulsion has significant effect on the size and particles distribution. They explained two main 114
reasons for interpretation of their results. The first is about the ratio greater than ω which in these circumstances, water drops in the micro-emulsion are larger and therefore micro-emulsion system has low stability due to less capability of surface active agent molecules for protecting of larger drops. Consequently, the particles may undergo slow coagulation and flocculation and this cause to broader distribution and particles size. The second reason is that soluble water in the surface active agent and non-ionic micro-emulsion is as free and surrounded water. At low ω ratios, more water molecules are surrounded with dissolution of the surface active agent and reactive components, while the number of free water molecules with the ω ratio increases. When the water ratio is good enough high, free water molecules in the hydrophilic region are present and they are the major part of product size in these conditions which may decrease the interface membrane efficiency and increase the exchange rate of water droplets that is in turn helpful in increasing the flocculation of nucleation (Liyi, et al., 1998). Lade, et al. (2000) reported since the particle size in the micro-emulsions are comparable with the size of micro-emulsion droplets, it can be found out that the level of water present in the micro-emulsion and the size of micro-emulsions droplets which is influenced by it, are controlling factor for particle size. Other factors that are believed to be involved in the control of particles size are the nature of the surface active agent and concentration of aqueous reactants. To investigate the effect of surface active agent on the particles, Liyi and his colleagues obtained the nano-particles of α-al 2 O 3 in different amounts of surfactant and mix, while the rate of water and concentration of aqueous reactant were constant. They reported that an increase in the level of the surface active agent decreases the particle size. Increasing the level of surface active agent causes decrease of ω ratio and this makes the micro-emulsion stronger (more stable). Therefore, the level of surface active agent affects both the stability of the micro-emulsion and control of the particle size control. Temperature is another effective factor on the particle size. As already mentioned, it can affect the viscosity of the solution and finally can influence the particle size. Increase in temperature causes increase in the growth rate of particles. Furthermore, at elevated temperatures, solubility of non-ionic surface active agent in water decreases due to less hydration of hydrophilic head-groups. In another way, at higher temperatures, solubility of surface active agent in oil phase increases, therefore, temperature management is necessary for particle size control. The particle size is also 115
influenced by the stirring rate. During micro-emulsion formation, stirring stalls further growth of the droplets and this effect increases with increasing the stirring rate. Therefore, the droplet size of micro-emulsion becomes small and consequently, the final produced particle size will be smaller (Xiang, et al., 2004). 3.7 Surface Treatment Surface treatment of calcium carbonate is an additional modification to enhance performance of the matrix-calcium carbonate interactions. It is also done to improve the fluidity and dispersion of the filler particles in polymer composites. Surface treatment of CaCO 3 reduces the inter particle interaction, enhances the polymer filler compatibility (Samsudin, et al., 2006; Deshmukh, et al., 2010). Pronounced effect of treatment is expected to be obtained by decreasing the filler size. Several methods can be used to surface-treat the CaCO 3 particles. In order to obtain the desired results, the type and mechanism of treatment must be chosen according to the chemical and physical properties of the components. The incompatibility of the high energetic hydrophilic surface of calcium carbonate with the low-energy surface of hydrophobic polymers is a particular problem that requires surface treatment of fillers (Rungruang, et al., 2006). Surface treatments can be reactive or non-reactive. Non-reactive surface treatment, the oldest and most used modification, covers the filler with a small molecular weight organic compound (surfactant). A typical example is the surface treatment of calcium carbonate with stearic acid. Stearic acid is the most common surface modifier for calcite because of its low cost (Rungruang, et al., 2006). Calcium carbonate surface adsorbs the polar group of stearic acid by the formation of ionic bonds between stearic acid and the surface of calcium carbonate. It is really important to know the right amount of surfactant to use in order to obtain the desired properties (Pukanszky, et al., 1995). As a result of treatment, surface energy of the fillers decreases dramatically (Fekete, et al., 1990). It is postulated that the stearic acid molecules interact with the calcium carbonate, with the carboxylate ion reacting with the surface and the organic chains sticking out normal from the surface (Ottewill & Tiffany, 1976). Reactive treatments assume a chemical reaction of the coupling agent with both of the components filler and matrix and create covalent bonds between the reactive groups of the polymer matrix and those of the filler (Ishida and Miller, 1984). Silane 116
coupling agents were proved successful with fillers that have reactive OH groups on their surface like glass fibre, mica and in recent years with CaCO 3 (Demjen and Pukanszky, 1997; Demjen, et al., 1997). It is difficult to bind fillers to the matrix polymer by covalent bonds, especially polyolefins, because these do not possess reactive chemical groups. Non-reactive surface treatment modifies only the secondary (van der Waals) forces between the surface of the filler and the matrix (Pukanszky, et al., 1989). The majority of treated calcium carbonates are post-treated on a separate production line at the end of the other processes. Surface treatment levels are usually determined by the mineral s surface area. Typically there will be a slight excess of treatment to insure complete encapsulation/reaction (Rungruang, et al., 2006). 3.8 Summary Microemulsions are a unique class of colloidal systems having novel properties because of their high degree of dispersion, their very low size and good enough potential to control the chemical reaction. Further, in the present study, calcium carbonate nano-powders were synthesized by a reverse micro-emulsion method at room temperature with Tween 80 and Span 80 as co-surfactant. To improve the fluidity and dispersion of the filler particles in polymer composites, surface-treatment of CaCO 3 nano-particles was done by stearic acid. An important operating variable in the Span 80-Tween 80/toluene/water reverse micro-emulsion system, the ω-value (water/surfactant molar ratio) was investigated. 117