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Stabile emulsion can be described as emulsion that can retain its initial characteristics either during storage or upon usage. The emulsion stability can be determines by rate of emulsion creaming, flocculates and coalescence. The oil droplets size and distribution in emulsion can be used to determine the emulsion stability. (Huang X. et al., 2001). One of the emulsion stability characterizations is creaming, and it can be determined by looking at its sedimentation rate. Sedimentation rate is related to Stoke's Law which it is states that the velocity at which a droplet moves is proportional to its square radius.
V = (2gr²)(d1-d2)/9µ where, (Equation 1)
V= velocity of fall (cm sec -1)
g= acceleration of gravity (cm sec -2)
r= radius of particle (cm)
d1=density of particles (g cm -3)
d2=density of medium (g cm -3), and
µ=viscosity of medium (dyne sec cm -2)
Most o/w emulsion is not stable without addition another agent. This is due to difference specific gravity between oil and water phase. The difference of specific gravity between these two phases can lead to emulsion instability such as gravitational separation, flocculation, and coalescence. According to Stoke's Law, o/w emulsion stability can be achieved by several ways which are by the effect of thickening agent which it can increase the viscosity of the water phase, by homogenization which can decrease the droplets size thus will produce the homogenous size distribution, and/or by decrease the density between the two phases involved. By altering these factors, the particles still can collide but it can help to reduce the frequency of collision between the particles, thus it can help to improve the emulsion stability. (Florence A.T. et al 2006).
According to Rajinder Pal, the mechanism of emulsion stability can be explained by emulsion rheology especially relates to viscosity and the droplet size distribution. Emulsion rheology may also relate to droplet size because the emulsion particle size can influence the rheological parameters especially the viscosity. The viscosity of the emulsion can be determined by emulsifier's material properties and the oil volume fraction but in case of low oil volume fraction, the emulsion viscosity is not depending on the droplet size. (McClements, 2005).
2.2 Emulsion Instability
Instability of emulsion can create a problem either to company or consumer. Emulsion instability can be divided into two types which are physical and chemical instability. For this study, physical instability is considered as the sedimentation rate value can be used to determine the stability of the emulsion. Physical instability of an emulsion can be divided into three types which are:
Creaming is a process that is also known as sedimentation where the emulsion droplets rise on top of the container but it is occurs on opposite direction of sedimentation. Creaming behavior is close related to emulsion composition where sometimes such in case of droplets that are monodisperse and highly coated with an effective surfactant will cause the droplets to repel each other and this will lead to creaming as they tend to become individual particles. (Robins M.M. 2000). Creaming is not a serious problem because it can be encounter by simply shaking the emulsion but it is undesirable because it will cause the patient to not take the enough or correct dose if they not shaking it properly. (Aulton M. E., 2007). According to Taherian Ali R., the cream and sedimentation values can be calculated from the ratio of cream and sediment volumes over total volume of emulsion samples upon on standing.
Flocculation is a process that related to van der Waals attraction that occurs in the disperse system. It involves the particles aggregation in loose clusters within the emulsion and this flocculation can be divided into two which are primary and secondary minimum. Problem with secondary minimum flocculation can be easily encounter rather than primary minimum flocculation. (Aulton M. E., 2007).
2.2.3 Coalescence (breaking or cracking)
Coalescence is the problem when the particles or droplets come close to each other as they approach one another. This situation can be encounter by addition of emulsifying agent that can form an interfacial film around the dispersed droplets and this existing charged film can contribute to repulsive forces that can lead to emulsion stability. (Aulton M. E., 2007).
2.2.4 Phase Inversion
A stable emulsion stated that emulsion that contain more than 50% dispersed phase
2.3 Emulsifying Agent
In order to retain emulsion stability, another agent known as emulsifying agent or surfactant should be added in the formulation. Emulsifying agent can be dividing in two general classes which are natural occurring materials and its derivatives and, synthetic and semi-synthetic surface active agents. (Gullapalli R.P and Sheth B.B., 1996). One of them is hydrocolloids. Hydrocolloids act as an emulsifying agent and it is used in emulsion preparation in order to control the emulsion shelf life. Emulsion preparations that need emulsifying agent included pharmaceutical products, cosmetics and also food industry. Hydrocolloids also can act either as stabilizers (stabilizing agent) or emulsifiers (emulsifying agent) in the oil in water emulsions based on the preparation. (Dickinson E. 2009).
In case of silica particles, to produce a stable o/w emulsion, the surface character of hydrophilic silica particles should be made somewhat hydrophobic in order to controlling the rheological properties. Hydrophobicity of colloidal particles can be modify through adsorption of low-molecular-weight charged surfactant such anionic sodium dodecyl sulfate (SDS) or cationic hexadecyltrimethylammonium bromide (CTAB). The balance of interaction is difficult to achieve as the surfactant can absorb at both liquid interface and solid particle surface. The attachment strength for the particle is depend on the extents of surfactant affects both the interfacial tension and the contact angle. (Binks, BP. 2007). Binks also stated that, the emulsion which can posses a gel-like consistency is considered as most stable emulsion with respect to creaming and coalescence.
Most common emulsifying agent that has been used by many researchers is hydrocolloids gums are xanthan gum, Arabic gum, fenugreek and carrageenan. These gums can help to stabilize the emulsion by prevent or reducing the rate of flocculation and coalescence instability due to formation or absorption of these gums onto the oil droplets surface. The precipitation or adsorption of the gums onto the oil droplets surface can occur due to characteristics of the gums itself which are water-soluble, rigid and hydrophilic. (Huang X. et al., 2001). Besides that, the way of mixing the surfactants, oil droplets and particles can affect the interaction between them thus can result in emulsion stability. (Whitby C.P., et al 2008).
2.4 Increasing Emulsion Stability Approaches
Stability of emulsion is determined by its ability to retain its initial characteristics which can be affected by several factors:
2.4.1 Effect of different emulsifying agent
Different emulsifying agents (surfactant) will have different psychochemical properties and this different characteristic will result in different emulsion stability. The emulsion stability depends on nature of the components and nature of surface active species. (Bibette J. et al 1996). The emulsion stability can also be achieved with mixed emulsifier system. According to Pichot R. which done a research about the effect of monoolein and hydrophilic silica particles mixture, with mixed emulsifiers system, the emulsion droplet size can be affected by concentration of both monoolein and silica particles. Under optimized conditions, the coarse emulsion can be produced with mixed emulsifiers systems. This emulsion is highly unstable to creaming but it posses long term stability against coalescence.
Emulsion can also be stabilized by other types of emulsifying agent which is wax particles. According to research that was done by Hodge SM and Rousseau D, both natural (beeswax) and synthetic (polyethylene-based) can stabilized oil-continuous emulsion. Hodge and Rousseau said that paraffin wax crystals can enhance the stability of water-in-oil (w/o) emulsion. The stability mechanism can be explained by absorption of wax particle and the formation of particle network in the continuous oil phase which the formation of this network particle immobilizes coarse water droplets thus prevent sedimentation.
2.4.2 Effect of different particle size
Different particle size will have different pattern of distribution in the emulsion. This distribution pattern will determined the emulsion stability. Theoretically, the nanoemulsion preparation with smaller particle size posses much better stability towards gravitational separation compared to conventional emulsion. It is because the smaller particle size has the Brownian motion that can prevent or reducing the tendency of emulsion to flocculate and coalesce. As the particle size decrease, the attractive forces between the droplets are also decrease. (McClements D.J and Qian C., 2010).
2.4.3 Effect of different temperature exposure and storage time
Different temperature exposure at specific time intervals to an emulsion leads to different emulsion stability. Different temperature is used to perform the proposed study about the emulsion stability. According to Ghassan Z. Abdullah study, the samples are exposed to either accelerated study by putting them in an oven with 40 0C or intermediate study by keeping them at room temperature which is 250C. He also had done the study by putting the sample at the same temperature but at different storage time. After the study was done, it represents different result on same or different temperature at specific time intervals or specific storage time. (Abdullah G.z. et al., 2011).
2.5 Rheology of emulsion
Rheology is a term that is related to flow properties. A rheological investigation of an emulsion is an important thing to consider as it can affect the stability of the products. The rheology studies was done by taking into consideration on numerous and multi-component materials on the emulsion formulation. Different types of emulsion will have different rheology properties. (Derkach S.R. 2009). In order to maintain the rheology of the emulsion, the rheology modifiers such as thickeners will be added.
Most emulsions, unless very dilute posses both plastic and pseudoplastic flow behavior rather than simple Newtonian flow. The flow properties of fluid emulsions should have little effect on their biological behavior as the characteristics of semisolid emulsion may affect their performance. The rheological properties can be used to determine the pourability, spreadability and syringeability of an emulsion. In some cases, the high viscosity w/o emulsion can create a problem especially with intramuscular injection preparation. This problems can be solved by decreasing the preparation's viscosity by replaced the external oil phase to an aqueous phase, thus can ease the injection. (Florence A.T. et al 2006).
Knowledge about the rheology properties is important in designing, processing and also handling the formulation. Besides that, a rheology property is one of the factors that formulators need to take into consideration mostly during transportation as the unstable emulsion will change its viscosity during the transportation. (Pal, R. 2010).
Emulsion rheology can also related to viscosity and droplet size distribution which can explain about the stability mechanism of the emulsion. The size of the droplets will have an effect on the emulsion rheology and also give rise on the droplet interaction. (Tipvarakarnkoon T., et al 2010). Droplets size will determines the shelf life of the emulsion as by reducing the droplets size will decrease the creaming rate of that emulsion. Droplets size reduction process can be achieved by homogenization. During homogenization, the turbulence effect during process will produce small droplets. This small droplet has an effect on rate of breakage and coalescence thus determined the shelf life of the emulsion. (Floury J., et al 2000).
2.5.1 Effect of temperature on emulsion rheology
Rheology properties can give the information about the physical stability and consistency of the products. Temperature is one of the parameters that can affect the rheology of the products. (Tharwat T. 2004). According to Nattapol Tangsuphoom and John N. Coupland, the effect of thermal treatment on emulsion stability was widely study now. As the temperature will decrease the viscosity of any preparation, thus it is one of the factors that can lead to emulsion instability.
2.6 Assessment of Emulsion Stability Approaches
Emulsions stability can be predicted by several ways which are by doing the accelerated study and extrinsic study. The objective of these studies is to predict the stability of the emulsion with some rheology measurements. Accelerated studies can be carried out under stressed condition in order to produce reliable and quick data.
2.6.1 Accelerated Stability Tests
Accelerated storage testing is useful in order to predict the products stability by means of changes of the preparation appearance with time. According to Tharwat Tadros, the stability of the products can be determined by subjecting the formulation to the different temperature, vibration and also subjecting the systems to centrifugation to detect any separation.
188.8.131.52 Storage at Adverse Temperature
Direct observation can be seen by directly observed the separation of the emulsion in graduated cylinder that is placed at constant temperature. The rate of sedimentation can be described as sediment volume. This value then will be compared to initial value or initial height of the emulsion. The separation will occur as the preparation is unstable. (Tharwat T. 2004).
Emulsion stability can also be assessing at elevated temperature and through this way, a quick indication of a rank order of degree of instability can be obtained. The result that is obtained by this way then is essential to correlate with the result that obtained from emulsion that stored at ambient temperature.
Centrifugation method is the method that is used to accelerate the rate of creaming or sedimentation. By using this method, the speed of centrifuge rotation can be set at specific speeds. (Tharwat T. 2004). Different speed of centrifuge can give different result on emulsion stability. Centrifugation method also is a suitable qualitative examination of Stoke's Law which is used to indicate the rate of sedimentation of a suspension and emulsion. Data that obtained through this type of accelerated testing is not always possible to predict accurately the behavior of a systems when stored at normal conditions.
As centrifugation process is done under stress condition which is vibration, this process may destroy the structure of flocculated system which it could remain intact under normal conditions. Usually, this centrifugation process speeds is no faster than 200 to 300 rpm. (Aulton M. E., 2007).
184.108.40.206 Rheological Measurement
Physical stability of the emulsion can be determined by viscosity measurement. The high shear rate involved may destroy the structure of suspension and emulsion. In order to preserve the structure of the suspension and emulsion, very low rates of shear can be applied such using the Brookfield viscometer with Helipath stand. By using this apparatus, it can give an indication of the change in the structure of the systems after various storage times. (Aulton M. E., 2007).
2.6.2 Extrinsic Study
Extrinsic study can be carried out by testing the emulsion stability after specified storage time which can be continuously checking after 15, 30, 45, 60 and finally 90 days which all of them is three month. The formulation is stored at same temperature which is in room temperature. The measurement or value that can be obtained after specified storage time then can be compared to initial value which then can give the information about the emulsion stability. (Abdullah G.z. et al., 2011). The parameters that are in concerned after these specified storage times are droplet size distribution, rheology and physical instability. Other methods that can be used to assessing the emulsion are:
220.127.116.11 Macroscopic Examination
The emulsion physical stability can be assessed by an examination of the degree of creaming or coalescence that occurs over period of time. This is carried by calculating the ratio of separated volume to the total volume of the emulsion. These values then can be compared for different products. (Aulton M. E., 2007).
18.104.22.168 Globule Size Analysis
Globule size analysis can indicate the coalescence as the mean globule size increases with time coupled with decrease in globule number. By knowing the globule size and number changes, it is therefore possible to compare the rates of coalescence on different emulsion formulation. The globule size measurement can be determined by using electronic particle counting devices such a Coulter Counter and Laser Diffraction Sizing. (Aulton M. E., 2007).
22.214.171.124 Viscosity Changes
Viscosity of an emulsion can be influenced by many factors. Any variation in globule size or number or migration of emulsifiers over a period time, may be detected by a change in apparent viscosity. (Aulton M. E., 2007).