INTRODUCTION:

Microemulsions form when a surfactant, or more commonly a mix of surfactants and cosurfactants lowers the oil/water interfacial natural phenomenon to ultra-low values. The review deals with the study of some important properties of surfactants that can stabilize water-oil films. The aqueous phase may contain salt (s) and/or other ingredients, and thus the "oil" could even be an elaborate mixture of various hydrocarbons and olefins. Microemulsions have advantage over the convention emulsion, it form upon simple mixing of the components and do not require the high-shear conditions. The two basic types of microemulsions are direct oil dispersed in water (o/w) and reversed water dispersed in oil (w/o). Microemulsion formulation stability and solubility can be improved with surfactant and Co-surfactant. This phenomenon, which involves incorporation of the solute by amounts of surfactant molecules, is thought as solubilization.

It occurs not just for small surfactant quantities, like spherical micelles, but also for larger aggregates, like microemulsion drops, vesicles, and cylindrical and plate like micelles. Lyotropic liquid crystals containing regular arrays of surfactant aggregates are effective media for solubilization and surfactant molecules have polar groups, like ions or ethylene oxide chains, and nonpolar groups like hydrocarbon or fluorocarbon chains¹. After they are added to water, aggregation normally occurs at fairly low concentrations to cut back the earth of contact between the nonpolar groups and water. But above a temperature of the Kraft point, which depends on surfactant structure, the hydrocarbon chains become more flexible and do not pack into regular crystalline structures. Hydrophilic surfactants can form small aggregates or micelles in aqueous phase, while the non polar groups conjure within and are largely shielded from water. The behavior of chains inside micelles is like that in liquid hydrocarbons, although their freedom of motion is restricted slightly by their attachment to the polar groups at the micelle surface². The composition of the microemulsion and the proportion of the oil phase, surfactant, and cosurfactant can be determined using the ternary phase diagram, Now a day’s its seems easy to get ternary phase diagram using freely available online tools like (http://www.chemix-chemistry-software.com). Once the proportion is found, the identical microemulsion could even be produced. Short oil hydrocarbon chains end in deep organic phase insertions into the interface membrane³. The surfactants utilized in microemulsions directly influence the microemulsion formation, size of droplet and influence incompatibility resultant into toxicity. Short- and middle-chain alcohols like ethanol, glycerol, and n-butyl alcohol are multi-selected as cosurfactants in microemulsions^3,4. In ternary systems like microemulsions, where two immiscible phases water and oil are present with a surfactant, the surfactant molecules may form a monolayer at the interface between the oil and water, with the hydrophobic tails of the surfactant molecules dissolved within the oil phase and thus the hydrophilic head groups within the aqueous phase⁵. As within the binary systems, surfactant self-assembled structures of varied types are often formed as an example from (inverted) spherical and cylindrical micelles to lamellar phases and bi-continuous microemulsions, which can coexist with predominantly oil or aqueous phases⁵. Lipophilic surfactants form larger, nonspherical micelles at low concentrations in water.

The formulation and development of novel drug delivery system, with the character of enhancing the effectiveness of existing of drug, is associate in progress method in pharmaceutical analysis⁶. Since there are many sorts of drug delivery systems, that are developed. The primary microemulsion by dispersing oil in associate binary compound surfactants resolution associated adding an alcohol as a co-surfactant, resulting in clear, stable formulation⁷. Different names for these systems are usually used, like swollen particle, transparent emulsion, solubilized oil, and micellar resolution. Microemulsions are bicontinuous systems that are primarily composed of bulk phases of water and oil separated by a surfactant/cosurfactant wealthy surface region. These systems have benefits over standard emulsions, thermodynamically stable liquid systems and are impromptu fashioned. The high capability of microemulsions for medication makes them enticing formulations for prescribed drugs⁸. These systems conjointly provide many edges for oral administration, as well as hyperbolic absorption, improved clinical efficiency, and attenuated toxicity⁹. Microemulsions might have an effect on the permeableness of medication within the skin. Many compounds utilized in microemulsions are rumored to enhance the transcutaneous permeation by sterilization the structure of the stratum corneum. Short-chain aliphatic alcohols (such as methanol) are widely used as permeation enhancers¹⁰. Nonionic surfactants utilized in topical formulations as solubilizing agents, however some recent results recommend that they will have an effect on conjointly the skin barrier operate¹¹. It’s of interest to explore the consequences of those parts within the organized microemulsion structures. Numerous theories regarding microemulsion formation, stability and section behavior are planned over the years. For instance, one rationalization of physics stability is that the oil/water dispersion is stabilised by the chemical agent gift and their formation involves the elastic properties of the chemical agent film at the oil/water interface. Calculations of the surface tension of the microemulsion with a synchronal oil or binary compound section also are usually of special focus during formulation¹².

Smaller microemulsion droplets bring about multiplied precise floor vicinity and multiplied membrane permeability of the drug through solubilization of positive membrane additives and pore formation. All these factors lead to enhanced contact with the gastrointestinal tract. Another important factor is the inner polarity of droplets, which is governed by the hydrophilic‐ lipophilic balance of surfactant. A change in droplet polarity may affect the arrangement of the drug and surfactant on the droplet interface and alter drug release. Microemulsions have also been considered as topical, transdermal, and parenteral drug delivery systems, and the HLB of the surfactant and the oil/surfactant ratio may influence microemulsion stability. So, most probably HLB run of 10–15 has been endorsed for constant emulsions with higher bead distance across⁹. Since the HLB of the surfactant is also expected to affect pellet formation and quality when added as self‐emulsifying wetting microemulsion. Sometimes to dissolve desirable drug concentration needs high oil content which may increase its content in the pellet, but there is a limit imposed on this because of the unfavorable impact on emulsion balance and droplet size¹³. Microemulsions are used as a potential drug delivery system for oral, topical and parenteral administration¹⁴. The purpose of this overview is to study and evaluate the effects of surfactant and co-surfactant concentrations on the phase behavior, physicochemical properties and drug release in respect of formulating microemulsion. The overview also shows the important property of surfactants, the stabilization of water and oil films and the formation of stable microemulsions. Compare to the conventional skin applications like cream and gel, Microemulsions are more effective topical vehicle. Microemulsions have number of applications and uses, such as in pharmaceuticals, agrochemicals, cutting oils, biotechnology, food, cosmetics, analytical applications, environmental detoxification¹⁵. The main goal of this review paper is to discuss the role of surfactants and cosurfactant to get proper stability in microemulsion base formulation.

MATERIAL AND METHODS:

Surfactants:

The term surfactant (surface-active-agent) denotes a substance that exhibits some superficial or surface activity and accustomed lower the surface or interface tension and it’s affinity for polar and nonionic solvents¹⁶. Surfactants are the molecules that contain a polar head cluster and a polar tail. Surfactant molecules self-associate because of numerous inter- and intra-molecular forces likewise as entropy concerns and there are types of spherical micelles, a polygon part, lamellar (sheet) phases, rodshaped micelles, reverse micelles, or polygon reverse micelles¹⁷. At low concentrations of spread (internal) part, spherical, isolated droplets area unit gift within the Microemulsions¹⁸.

Classifications of Surfactants:

Surfactants can be generally categories into four classes based on the charge of the water-soluble portion of the surfactant. The four classes include nonionic (no charge)¹⁹, anionic (negative charge), cationic (positive charge)²⁰, and amphoteric (containing a positive and a negative).

Table No 1. Classification of Surfactant²¹

Class	Example	Structures
Anionic	Na stearate	CH₃(CH₂)₁₆COO^-Na⁺
	Na dodecyl sulfate	CH₃(CH₂)11SO₄^-Na⁺
	Na dodecyl benzene sulfonate	CH₃(CH₂)₁₁C₆H₄SO₃^-Na⁺
Cationic	Laurylamilne hydrochloride	CH₃(CH₂)₁₁NH₃⁺CL^-
	Trimethyldodecylammonium chloride	C₁₂H₂₅N⁺(CH₃)₃CL^-
	CetylTriammonium bromide	CH₃(CH₂)₁₅N⁺(CH₃)₃BR^-
Non ionic	Polyoxyethylene alcohol	C_nH_2n⁺1(OCH₂CH₂)nOH
	Alkylphenolethoxylate	C₉H₁₉^-C₆H₄^-(OCH₂CH₂)nOH
	Propylene oxide-modified polymethylsiloxane (EO=ethyleneoxy, PO=propyleneoxy	(CH₃)₃SiO((CH₃)2SiO)₃(CH₃SiO)Si(CH₃)₃
Zwittorionic	Dodecyl betaine	C₁₂H₂₅N⁺(CH₃)₂CH₂COO^-
	Lauramidopropyl betaine	C₁₁H₂₃CONH(CH₂)₃N⁺(CH₃)₂CH₂COO
	Cocoamido-2-hydroxypropyl sulfobetaine	C₆H_2n⁺1CONH(CH₂)₃N⁺(CH₃)₂CH₂CH(OH)CH₂SO₃

Properties of Surfactants:

1. Wetting - Three types of phenomena are adhesional, spreading, immersional.

2. Emulsification - the property of surfactants to form a stable emulsion of two or more immiscible liquid. Emulsification has the following mechanism Interfacial tension, Interfacial double layer, Electrical double layer

3. Detergency - Detergents are surfactants used for removal of dirt by wetting of dirt particles and removing the insoluable dirt as a deflocculated particle.

4. Solubalization - A micelle is an aggregate of surfactant molecules dispersed in a liquid colloid. Micelle formation in polar and non polar solvent depends on the concentration of surfactant in the particular solvent.

5. Micellization - Process of preparing clear solution

Principle for use of Surfactants:²²

In ternary systems like microemulsions, the surface-active agent molecules could kind a monolayer at the interface between the oil and water, with the hydrophobic tails of the surface-active agent molecules dissolved within the oil section and thus the hydrophilic head teams within the liquid section. As within the binary systems (water/surfactant or oil/surfactant), self-assembled structures of various sorts will be shaped, ranging, for instance, from (inverted) spherical and cylindrical micelles to lamellar phases and bi-continuous microemulsions, which can be with preponderantly, oil or liquid phases. The three main factors deciding the transcutaneous permeation are drug within the vehicle, unleash of drug from the vehicle, and permeation of drug into the skin²³. These factors have an effect on either the thermodynamic activity that drives the drug into the skin or the permeability of drug within the skin significantly in stratum corneum²⁴.

Types of Surfactants:²⁵

1. Coexists of surfactant-poor oil phase within lower phase of surfactant-rich water phase.

2. Coexists of surfactant-poor water phase within the upper phase of surfactant-rich oil phase.

3. Coexists of surfactant rich middle-phase within both water as a lower phase and oil as a upper phase.

4. Single phase homogeneous mixture (Stable Microemulsion).

Advantages of Surfactants in Microemulsion²³

1. Increase the rate of absorption

2. Eliminates variability in absorption

3. Helps solubilise lipophilic drug

4. Increase bioavability

5. Less amount of energy requirement

6. Liquid dosage form increases patient compliance

Surfactants and HLB Scale:

The HLB takes under consideration the relative contribution of hydrophilic and hydrophobic fragments of the surfactant molecule. W/O microemulsions generally favors low HLB (3-6) value of surfactants, whereas O/W microemulsion systems favors with high HLB (8-18) value of surfactant. Ionic surfactants like

Table no. 2 various surfactants used in Microemulsion and their HLB values are as follows²⁶

Surfactant	Type	HLB	Properties	PH	Conc. range	Type of Micro-emulsion
Tween 80	non-ionic	15	Emulsifier and detergent	7.4	0.50%	o/w
Tween 40	non-ionic	15.6	Emulsifier and detergent, Miceller	7.4	1%	o/w
Triton-X 100	non-ionic	13.51	Emulsifier	6.0-8.01	0.10%	o/w
Brij 58	non-ionic	15.73	Emulsifier, Detergency,Solublizer	5.5	0.1-1.0%	o/w
Lacithin	Amphoteric	8	Emulsifier, Detergency,Solublizer	7	0.1-1.0%	o/w, w/o
Cremophorel	non-ionic	13.5	Emulsifier, Miceller, Solublizer	6,7,8	0.50%	w/o
PEG 400	Amphoteric	14	Emulsifier, Wetting	5,6,7	7.00%	w/o
Benzethonium	Cataonic	15	Miceller, Emulsifier	4.8-7.01	0.1-0.2%	o/w
Sodium dodecyl sulfate	Ionic	15.40	Detergent, wetting	6	0.1-0.5%	o/w
Sodium stearate	Ionic	8.3	Detergent, Wetting	10-11	0.40%	o/w

sodium dodecyl sulphate which have HLBs greater than 20, often require the presence of a co-surfactant to scale back their effective HLB to a worth within the range required for microemulsion formation.

Role of Surfactants in Microemulsions:

Surfactants are present in microemulsion with a prerequisite of large proportion along with co-surfactant in Microemulsion, if needed. It has been shown that the higher amount of surfactant in microemulsion produce lower droplet size of dispersed phase²⁷. Oil and surfactant ratio may affect the significant change in droplet formation, solubilization, drug permeability which greatly influences the safety and efficacy of microemulsion²⁸. Surfactants contribution on interfacial films formation, solubilzation is more important for to stabilize dispersed phase in microemulsion²⁹. Two distinct polar region of surfactant can favor to lowering interfacial tension of oil-water interface. The axiom behind its nanoscale stabilization is the formation of monomolecular layer around dispersed phase droplets and result in formation of several microstructures³⁰.

The surfactant film can be characterized by three phenomenological constants: tension, bending rigidity, and spontaneous curvature³¹. Surfactant films determine the static and dynamic properties of Microemulsions and these include phase behaviour and stability, structure, and solubilisation capacity³².

Co-surfactant:

Generally, co-surfactants are short-chain amines or alcohols. Cosurfactants can increase the solubility of drug by reducing the interfacial tensions, when used adjust at lower concentration with surfactant. Co-surfactant should be non-irritant to skin. But desirable concentration of co-surfactant should have capability produces more availability of drug across skin. Cosurfactants of ionic or nonionic nature help in the stabilization of a system by formation of dynamic micelles and further reducing the interfacial tension¹⁵.

Table No.3 List of commonly used cosurfactants and their characteristics:^{33, 34, 35}

Co surfactant	Classified	Preferred Surfactant	Solubility
Ethanol	Short chain alcohol	Ethoxylates	70%
Glycerol	Alkane diols and triols	Trisiloxane Surfactants	40%
Sodium deoxycholate	Bile salts	Tweeen 20 ,Tween 60	40%
Caprylic acid	Organic acids and salts	Polyglyceryl fatty acid esters	80%
Sodium Caprylate	Organic acids and salts	n-octanoate	62.40%
Pottasium sorbate	Organic acids and salts	Poly alcohol	58.20%
Propylene glycol	Alkane diols and triols	Ethoxylates	100%
2-Pyrrolidone	Pyrollidone derivative	Alkyl sulfates	100%
Butylene Glycol	Alkane diols and triols	Guanine crystalls	40%
Isopropanol	Short chain alcohol	Ethoxylates	70%

Role of a co-surfactant:³⁶

Co-surfactant plays crucial role to increase the fluidity, adjust HLB value and spontaneous curvature of the interface by changing surfactant partitioning characteristic.

Effect of Surfactant and co surfactant on various factors:³⁷

Property of surfactant: - surfactant contains two groups lipophilic and hydrophilic. Hydrophilic single chain surfactants such as cetyl ethyl ammonium bromide dissociate completely in dilute solution and have a tendency to form o/w microemulsion. Where as in case of w/o type of microemulsion there is need of surfactant is in salt form..

Property of Oil Phase: Oil phase also influence curvature by its ability to penetrate and Swell the tail group region of the surfactant monolayer, swelling of tail results into an increased negative curvature to w/o microemulsion.

Temperature: - The effect of temperature is extremely important in concern about surfactant.

At low temperature, Surfactants are hydrophilic and form normal o/w system.

At higher temperature, Surfactants are lipophilic and form w/o systems.

At an intermediate temperature – surfactants can forms bicontinuous structure.

Surfactant and cosurfactant effect on microemulsion stability:

The microemulsion is unstable due to insufficient surfactant, which is needful to reduce oil-water interfacial tension³⁴. Raising the surfactant concentration significantly enhanced the emulsion stability. To enhance the strength of adsorption in between the oil membrane phase and internal phase there is need to use higher surfactant concentration that can increase stability of formulation³⁸. In addition, higher surfactant concentration can also increase the viscosity of formulation.

DISCUSSION:

To protect labile drug, control drug release, increase drug solubility, increase bioavailability and reduce patient variability, there is need of surfactant and adjuvant co-surfactant with suitable concentration to formulate stable microemulsions. So to Identifying the suitable surfactant/cosurfactant is crucial to get efficient microemulsion formulation with optimum drug loading. Microbial surfactants have emerged as alternatives to their synthetic counterparts to solubilize natural oil in Microemulsions²⁴. Microemulsions with low surfactant levels can be formulated via the use of efficient surfactants, interfacially active cosurfactants, and proper formulation conditions. This approach has been used to reduce the cost and improve the performance of formulations. Similarly, interfacial chemical reactions can be facilitated using microemulsion-forming surfactants, in combination with indigenous cosurfactants. In the present review offers the interesting properties of surfactants and cosurfactant such as solubilisation, micellar, emulsifier, wetting ability as well as detergency. Here we suggesting that further profound study by identifying the suitable surfactant and co-surfactant that may be beneficial for formulation of microemulsion and to enhance the stability, bioavaibility of active drug moieties, which may have some incompatibility in other types of formulation.

CONFLICT OF INTEREST:

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest

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Received on 04.06.2021 Modified on 15.10.2021

Research J. Pharm. and Tech 2022; 15(10):4829-4834.

DOI: 10.52711/0974-360X.2022.00811

Classifications of Surfactants:

Advantages of Surfactants in Microemulsion23