INTRODUCTION:

The term “microemulsion” refers to a thermodynamically steady isotropically clear dispersion of two immiscible liquids, such as oil and water, stabilized by an interfacial film of surfactant molecules. A microemulsion is measured to be a thermodynamically or kinetically stable liquid dispersion of an oil phase and a water phase, in combination with a surfactant. The dispersed phase characteristically comprises small particles or droplets, with a size range of 5 nm-200 nm, and has very low oil/water interfacial tension. Because the droplet size is less than 25% of the wavelength of visible light, microemulsions are translucent. The microemulsion is formed readily and sometimes instinctively, generally without high-energy input. In many cases a cosurfactant or cosolvent is used in addition to the surfactant, the oil phase and the water phase.

Microemulsions are at present the subject of many investigations because of their wide range of possible and specific utilizations. The high capacity of microemulsions for drugs makes them attractive formulations for pharmaceuticals. These systems also offer some benefits for oral administration, including increased absorption, enhanced clinical effectiveness and decreased toxicity.[1] Microemulsions containing the oil and aqueous phase, surfactant and cosurfactant (cos), are optically translucent mixtures with a very small droplet size (G140 nm).3,4 Microemulsions have been increasing in popularity and garnering more consideration in recent years, because they may enhance the transdermal absorption of drug molecules by increasing drug solubilities and modifying their partition coefficients. A hydrogel base is used very often in topical formulations. The hydrogel formulation was ready and studied as a vehicle for its permeation potential.[2]

Advantages:

a. Thermodynamically steadiness and require minimum energy for formation.

b. compatibility in manufacturing.

c. Improved drug solubilization and improved bioavailability.

d. Micro emulsion are having extensive applications in colloidal drug delivery systems for the purpose of drug targeting and controlled release.[3]

Method of Preparation:

1. Phase Titration Method:

Microemulsions are prepared by the impulsive emulsification method (phase titration method) and can be depicted with the help of phase diagrams. Construction of phase diagram is a useful approach to study the compound series of interactions that can occur when different components are mixed. Microemulsions are formed along with a variety of association structures (including emulsion, micelles, lamellar, hexagonal, cubic, and various gels and oily dispersion) depending on the chemical composition and concentration of each component. The understanding of their phase equilibria and differentiation of the phase boundaries are essential aspects of the study As quaternary phase diagram (four component system) is time consuming and difficult to interpret, pseudo ternary phase diagram is often constructed to find the different zones including microemulsion zone, in which each corner of the diagram represents 100% of the particular component. The area can be alienated into w/o or o/w micro emulsion by basically considering the composition that is whether it is oil rich or water rich. Observations should be made carefully so that the metastable systems are not incorporated.

2. Phase Inversion Method:

Phase inversion of microemulsions occurs upon addition of excess of the dispersed phase or in response to temperature. During phase inversion extreme physical changes occur including changes in particle size that can affect drug release both in vivo and in vitro. These methods make use of changing the impulsive curvature of the surfactant. For non-ionic surfactants, this can be achieved by changing the temperature of the system, forcing a transition from an o/w microemulsion at low temperatures to a w/o microemulsion at higher temperatures (transitional phase inversion). During cooling, the system crosses a point of zero impulsive curvature and minimal surface tension, promoting the formation of finely discrete oil droplets. This method is referred to as phase inversion temperature (PIT) method. Instead of the temperature, other parameters such as salt concentration or pH value may be considered as well instead of the temperature alone. Additionally, a transition in the spontaneous radius of curvature can be obtained by changing the water volume fraction. By consecutively adding water into oil, initially water droplets are formed in a continuous oil phase. Increasing the water volume fraction changes the spontaneous curvature of the surfactant from initially stabilizing a w/o microemulsion to an o/w microemulsion at the inversion locus. A selection of mechanism for microemulsions suitable for pharmaceutical use involves a consideration of their toxicity and, if the systems are to be used topically, their irritation and sensitivity properties.10 The ionic surfactants are generally too toxic to be used for preparation of lipid emulsions; therefore, non ionic surfactants, such as the poloxamers, polysorbates, polyethylene glycol are preferred. Polysorbate 80 is widely applied to pharmaceutical preparations, including ophthalmic preparations, due to its history of convenience and safety, and it is listed in the United States Pharmacopoeia- National

Ternary and quaternary phase diagrams:

Winsor I: with two phases, the lower (oil/water, o/w) microemulsion phase in equilibrium with the upper excess oil;

Winsor II: with two phases, the upper microemulsion phase (water/oil, w/o) in equilibrium with excess water;

Winsor III: with three phases, middle microemulsion phase (o/w plus w/o, called bicontinuous) in equilibrium with upper overload oil and lower excess water.

Winsor IV: in single phase, with oil, water and surfactant consistently mixed. Inter conversion among the above-mentioned phases can be achieved by adjusting proportions of the constituents. Simultaneous presenceof two microemulsion phases, one in contact with water and the other in contact with oil is also possible. 15. This may be considered as an extension of Winsor's classification forming the fifth category. A microemulsion forming systems is shown in figure 1

Figure 1: Hypothetical phase regions of microemulsion system of oil (O), water (W), and surfactant + cosurfactant (S)

Another important consideration in the formation of microemulsions is related to the packing parameter, which is important for structures with high curvatures (Fig. 2).

Factors Affecting the Microemulsion:

The formation of microemulsion will depend on the following factors

a. Packing ratio:

The HLB of surfactant determines the type of microemulsion through its influence on molecular packing and film curvature. The analysis of film curvature for surfactant association's leadings to the formation of microemulsion.

b. Property of surfactant, oil phase and temperature: The type of microemulsion depends on the nature of surfactant. Surfactant contains hydrophilic head group and lipophilic tail group. The areas of these group, which are a assess of the differential propensity of water to swell head group and oil to enlarge the tail area are important for specific formulation when estimating the surfactant HLB in a particular system. When a high concentration of the surfactant is used or when the surfactant is in presence of salt, degree of dissociation of polar groups becomes lesser and resulting system may be w/o type. Diluting with water may increase dissociation and leads to an o/w system. Ionic surfactants are strongly influenced by temperature. It mainly causes increased surfactant counter ion dissociation. The oil constituent also influences curvature by its capability to penetrate and hence swell the tail group section of the surfactant monolayer.

Structure of Microemulsion:

Microemulsions or Micellar emulsion are active system in which the interface is incessantly and spontaneously fluctuating [31]. Structurally, they are divided in to oil in water (o/w), water in oil (w/o) and bi-continuous microemulsions. In w/o microemulsions, water droplets are dispersed in the continuous oil phase while o/w microemulsions are formed when oil droplets are dispersed in the continuous aqueous phase. In system where the amounts of water and oil are similar, the bi-continuous microemulsions may result [32]. The mixture oil water and surfactants are capable to form a extensive variety of structure and phase depending upon the proportions of component.

Fig. 3: Microemulsion Structure

Fig. 4: O/W, W/O and Bi-continuous Microemulsions

Theories of Microemulsion Formation

Historically, three approaches have been used to explain microemulsion development and stability. They are as follows-

· Interfacial or mixed film theories.

· Solubilization theories.

· Thermodynamic treatments.

The free energy of microemulsion formation can be considered to depend on the level to which surfactant lowers the surface tension of the oil water interface and change in entropy of the system such that,

Gf = γ a - T S

Where, Gf = free energy of formation A = change in interfacial area of microemulsion S = change in entropy of the system T = temperature γ = surface tension of oil water interphase. When microemulsion is formed the change in A is very large due to the large number of very small droplets formed. In order for a microemulsion to be formed(transient) negative value was required, it is predictable that while value of A is positive at all times, it is very small and it is offset by the entropic constituent. The dominant favourable entropic involvement is very large dispersion entropy arising from the mixing of one phase in the other in the form of large number of small droplets. However there are also predictable to be favourable entropic contributions arising from other dynamic processes such as surfactant diffusion in the interfacial layer and monomer-micelle surfactant exchange. Thus a negative free energy of formation is achieved when large reductions in surface tension are accompanied by significant favourable entropic change. In such cases, microemulsion is impulsive and the resulting dispersion is thermodynamically stable [35-37]

Pharmaceutical Formulation of Microemulsions:

Components of microemulsion formulations:

A large number of oils and surfactants are accessible which can be used as components of microemulsion system but their toxicity, irritation potential and unclear mechanism of action limit their use. One must choose formulation mechanism that are biocompatible, non-toxic, and clinically acceptable. Again the use of those formulation components is limited to suitable concentration range. The importance is, therefore, on the use of generally regarded as safe (GRAS) excipients.

1 Oils:

These constitute of the oil phase of the emulsions. Various externally applied emulsions, mineral oils, either only or in combination with soft paraffin or hard paraffin, are extensively used for their occlusive and sensory characteristics as well as used as vehicle for the drug. The non biodegradable mineral and castor oils are widely used in the oral preparations and these afford a local laxative effect, and fish liver oils or various fixed oils of vegetable origin (e.g., arachis, cottonseed, and maize oils) as nutritional supplements. Microemulsion for transdermal delivery of terbina fine was developed by employing oleic acid as oil phase (7).

2 Surfactants:

The quantity of surfactant in emulsions is very small, 0.1% to 1.0% of the total emulsion weight. The quantity of surfactant in a microemulsion is a minimum of 10% of the total ME weight. Such large surfactant levels are essential because of the large increase in interface area between the aqueous and oil phase. Selection of a proper surfactant is the key to the formation of any Microemulsion. In general, hydrophobic surfactants will be appropriate for the formation of w/o microemulsions (ME), and the hydrophilic surfactants will form o/w ME. In industrial applications, it is common to use inexpensive ionic surfactants but in food, pharmaceutical, and cosmetic applications, the ionic surfactant toxicity limits their use. The most common anionic surfactants are the sodium di-isooctylsulfosuccinate (AOT) and the sodium dodecylsulfate (SDS). Nonionic surfactants are very often used in pharmaceutical microemulsion formation. Tweens (ethoxylatedsorbitan esters) are well known and extensively used. They are water-soluble and have high HLB values and, therefore, are used mainly for making o/w microemulsions. Ethoxylated (with up to 40 EO units castor oil, ECO-40) and hydrogenated ethoxylated castor oil (HECO) with 8 to 40 EO group attached to the hydroxyl group on the side chain of the triglyceride are regarded a very efficient surfactants (8).

Characterization of Microemulsion:

a) Viscosity:

Viscosity is assess by Brookfield rotational viscometer. Viscosity provides an indication of rod like or worm like micelles and type of microemulsion.

b) Ph:

Digital pH meter is the instrument use for measurement of pH of microemulgel and results are taken into triplicate then averages of the results are taken into consideration. The pH of microemulsion is also required to because modify in pH may affect zeta potential and finally influence the stability of product.

c) Drug content determination:

Drug content in microemulgel will be measured by dissolving 1gm of microemulgel in solvent by sonication. Absorbance will be measured after dilution at λmax nm using UV Spectrophotometer.

d) Centrifugation:

Centrifugation will be measured to evaluated physical stability of microemulsion. Microemulsion will be centrifuged at ambient and 500 rpm for 10-60 min to evaluated the system for creaming pr phase separation. System will be observed visually for manifestation.

e) Conductivity:

Conductivity measurement using Digital conductometer provide a means of determining whether the microemulsion is oil continuous or water continuous. The results are taken into triplicate and average is taken [29].

f) Dilution test:

If continuous phase into microemulsion, it will not crack or separate into phases. A total inyo 50 to 100 aqueous dilution of microemulsion are carried out and visually checked for phase separation and clarity. Result are into triplicate and average is taken.

Applications of Microemulsion:

Microemulsion in pharmaceuticals:

Parenteral administration:

Parenteral administration (especially via the intravenous route) of drugs with limited solubility is a main problem in the pharmaceutical industry because of the very low amount of drug actually delivered to a targeted site. Microemulsion formulations have different advantages over macroemulsion systems when delivered parenterally because of the fine particle, microemulsion is cleared more gradually than the coarse particle emulsion and, therefore, have a longer residence time in the body. Both O/W and W/O microemulsion can be used for parenteral delivery.

Oral administration:

Oral administration of microemulsion formulations offer several benefits over predictable oral formulation including increased absorption, improved clinical potency, and decreased drug toxicity [27]. Therefore, microemulsion has been reported to be an ideal delivery of drugs such as steroids, hormones, diuretic and antibiotics.

Pharmaceutical drugs of peptides and proteins are highly potent and specific in their physiological functions. However, most are not easy to administer orally. With on oral bioavailability in conventional (i.e. non-microemulsion based) formulation of less than 10%, they are typically not therapeutically active by oral administration. Because of their low oral bioavailability, most protein drugs are only available as parenteral formulations. However, peptide drugs have an tremendously short biological half life when administered by parenteral route, so require multiple dosing [28].

Topical administration:

Topical administration of drugs can have compensation over other methods for several reasons, one of which is the avoidance of hepatic first pass metabolism of the drug and related toxicity effects. Another is the direct delivery and targetability of the drug to the affected area of the skin or eyes [29].

Ocular and pulmonary delivery:

Ocular and pulmonary delivery for the treatment of eye diseases, drugs are basically delivered topically. O/W microemulsions have been investigated for ocular administration, to dissolve poorly soluble drugs, to increase absorption and to attain extend release profile

Microemulsions in biotechnology:

Many enzymatic and biocatalytic reactions are conducted in pure organic or aqua-organic media. Biphasic media are also used for these types of reactions. The use of a pure polar media causes the denaturation of biocatalysts. The use of water-proof media is comparatively advantageous. Enzymes in low water content display and have:

1. Increased solubility in non-polar reactants.

2. Possibility of shifting thermodynamic equilibria in favour of condensations.

3. Development of thermal stability of the enzymes, enabling reactions to be carried out at higher temperatures.

Many enzymes, including lipases, esterases, dehydrogenases and oxidases often function in the cells in microenvironments that are hydrophobic in nature. In biological systems many enzymes operate at the interface between hydrophobic and hydrophilic domains and these usually interfaces are stabilized by polar lipids and other natural amphiphiles.

A microemulsion as fuels:

A microemulsion-based fuel in the existence of water is one of the advantages of stable microemulsion and they are successfully used to reduce stain formation. When the water is vaporized during the combustion, this will lower the heat released and the combustion temperature. As a direct consequence, the emission rate of gases like nitrogen oxides (NOx) and carbon monoxide (CO) will decrease.

Microemulsions as lubricants, cutting oils and corrosion inhibitors:

Microemulsions or reverse micellar solutions are in use as lubricants, cutting oils and corrosion inhibitors for several decades. The presence of surfactant in microemulsion causes corrosion inhibition and the increased water content compared to pure oil leads to higher heat capacity. On one hand the corrosive agents, because of solubilization in microemulsion cannot react with the metal surface and on the other, the metal surface is sheltered by the adsorbed hydrophobic surfactant film. However, solubilization is selective, and in some cases, other mechanisms might play a role in corrosion prevention. In microemulsions, water with much higher thermal conductivity, imparts higher heat capability to the system. Such formulations can be used in cutting oil; the oil lubricates the cutting surface, and the water helps to remove the frictional heat generated during the cutting process.

Microemulsions in cosmetics:

In many cosmetic applications such as skin care products, emulsions are extensively used with water as the continuous phase. It is believed that microemulsion formulation will result in a faster uptake into the skin. Cost, safety (as many surfactants are irritating to the skin when used in high concentrations), suitable selection of ingredients (i.e. surfactants, cosurfactants, oils) are key factors in the formulation of microemulsions [34].

Distinctive microemulsions as hair care products have been prepared. They contain an amino-functional polyorganosiloxane (a nonionic surfactant) and an acid and/or a metal salt. Solubilization of fragrance and flavored oils can be achieved in microemulsions. Cosmetic microemulsions (transparent and translucent) of silicone oils, produced by emulsion polymerization have been reported. They are, however, not thermodynamically stable products because of low solubility of silicone oil in the surfactants. Ultra fine emulsions prepared by condensation method have some advantages in cosmetic and medical products, as they have outstanding stability and safety and their droplet size can be readily controlled. Ultrafine emulsions can be regarded as thermodynamically unstable microemulsions, as they are O/W emulsions with droplet size similar to microemulsion.

CONCLUSION:

Microemulsions are optically isotropic and thermodynamically stable liquid solutions of oil, water and amphiphile. Microemulsions are readily distinguished from normal emulsions by their transparency, low viscosity and more essentially their thermodynamic stability. Drug delivery through microemulsions is a promising area for continued research with the aim of achieving controlled release with improved bioavailability and for drug targeting to various sites in the body. Furthermore, these formulations can be simply manufactured in term of the relative cost of commercial production. Topical products are now employing the microemulsion technology are likely to emerge. Microemulsions can also be used to achieve drug targeting however challenges remain, primarily because of the layers of barriers that these systems need to overcome to reach to the target. Recent research work is focused on the production of safe, competent and more compatible microemulsion constituents which will further improve the utility of these novel vehicles. Microemulsion in today's world can be accepted as full of potential in a novel drug delivery systems.

ACKNOWLEDGEMENT:

Author are thankful to Gourishankar college of D pharma, Limb Satara for providing valuable help and authors are also Thankful Mr. Raje V.N, Principal, Gourishankar college of D pharma, Limb, Satara for providing necessary guidance for this work.

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Received on 20.08.2018 Accepted on 24.10.2018

Asian J. Pharm. Res. 2019; 9(2): 90-96.

DOI: 10.5958/2231-5691.2019.00015.7

Emulsion	Microemulsion
1 Emulsions consist of approximately spherical droplets of one phase dispersed into the other.	1 They continuously evolve between various structures ranging from droplet like swollen micelles to bicontinuous structure.
2 Droplet diameter, 1 – 20 mm.	2 10 – 100 nm.
3 Most emulsions are opaque (white) because bulk of their droplets is greater than wavelength of light and most oils have higher refractive indices than water.	3 Microemulsions are transparent or transparent as their droplet diameter are less than ¼ of the wavelength of light, they scatter little light.
4 Ordinary emulsion droplets, however small exist as individual entities until coalesance or Ostwald ripening occurs.	4 Microemulsion droplet may disappear within a fraction of a second whilst another droplet forms spontaneously elsewhere in the system.
5 They may remain steady for long periods of time, will ultimately undergo phase separation on standing to attain a minimum in free energy. They are kinetically stable thermodynamically unstable.	5 More thermodynamically steady than macro emulsions and can have essentially infinite lifetime assuming no change in composition, temperature and pressure, and do not tend to separate.
6 They are lyophobic.	6 They are on the borderline between lyophobic and lyophilic colloids.
7 Require intense agitation for their formation.	7 Generally obtained by gentle mixing of ingredients.