Presented By Mahvash Ansari, M-Phil Pharmaceutics, 2008-2010, Roll # 02
Multiple EmulsionsPresented to Prof. Dr. Nazar Mohammad Ranjha
Multiple EmulsionMultiple emulsion systems are novel developments in the field of emulsion technology and are more complex type of dispersed system. Multiple emulsions are the emulsion system in which the dispersed phase contain smaller droplets that have the same composition as the external phase. This is made possible by double emulsification hence the systems are also called as double emulsion. Like simple emulsions, the multiple emulsions are also considered to be of two types: Oil-in-Water-in-Oil (O/W/O) emulsion system Water-in-Oil-in-Water (W/O/W) emulsion system In O/W/O systems an aqueous phase (hydrophilic) separates internal and external oil phase. In other words, O/W/O is a system in which water droplets may be surrounded in oil phase, which in true encloses one or more oil droplets. In W/O/W systems, an organic phase (hydrophobic) separates internal and external aqueous phases. In other words, W/O/W is a system in which oil droplets may be surrounded by an aqueous phase, which in turn encloses one or several water droplets. These systems are the most studied among the multiple emulsions. The immiscible oil phase, which separates two miscible aqueous phases is known as liquid membrane and acts as a different barrier and semi-permeable membrane for the drugs or moieties entrapped in the internal aqueous phase.2
Schematic Diagram of W/O/W & O/W/O Emulsions
Pre-Formulation of Double EmulsionThe formulate a double emulsion, it is necessary to choose, at least, an oil and two surfactants, one low in HLB and one high in HLB. In the example mentioned here, we have been working with span surfactants (HLB10) and with a vegetable oil (caprylic/ capric triglyceride). The first stage involves making state diagrams that provide the means for pre-selecting formulas. Those selected for in-depth study are the very white emulsions that cream slowly and/or moderately. HBL (blend)=f +HBL (A) + (1-f) *HBL (B)
Methods of PreparationMultiple emulsions are best prepared by re-emulsification of primary emulsion. The following are the method of multiple emulsions:
Two Steps Emulsification (Double Emulsification) Phase Inversion Technique (One Step Technique)
Two Steps Emulsification (Double Emulsification)Two steps emulsification methods involve re-emulsification of primary W/O or O/W emulsion using a suitable emulsifier agent. The first step involves, obtaining an ordinary W/O or O/W primary emulsion wherein an appropriate emulsifier system is utilized. In the second step, the freshly prepared W/O or O/W primary emulsion is re-emulsified with an excess of aqueous phase or oil phase. The finally prepared emulsion could be W/O /W or O/W/O respectively.
Two Steps Emulsification
Modified Two Steps Emulsification
Phase Inversion Technique (One Step Technique)An increase in volume concentration of dispersed phase may cause an increase in the phase volume ratio, which subsequently leads the formation of multiple emulsions. The method typically involves the addition of an aqueous phase contains the hydrophilic emulsifier [ Tween 80/sodium dodecylsulphate (SDS) or Cetyl trimethyl ammonium salt CTAB)] to an oil phase consisted of liquid paraffin and containg lipophilic emulsifier (Span 80). A well-defined volume of oil phase is placed in a vessei of pin mixer. An aqueous solution of emulsifier is then introduced successively to the oil phase in the vessel at a rate of 5 ml/min, while the pin mixer rotates steadily at 88 rpm at room temperature. When volume fraction of the aqueous solution of hydrophilic emulsifier exceeds 0.7, the continuous oil phase is substituted by the aqueous phase containing a number of the vesicular globules among the simple oil droplets, leading to phase inversion and formation of W/O/W multiple emulsion.
One Step Technique
Membrane Emulsification TechniqueIn this method, a W/O emulsion (a dispersed phase) is extruded into an external aqueous phase (a continuous phase) with a constant pressure through a Porous Glass Membrane, which should have controlled and homogenous pores. The particle size of the resulting emulsion can be controlled with proper selection of Porous Glass Membrane as the droplet size depends upon the pore size of the droplet size depends upon the pore size of the membrane. The relation between membrane pore size and particle size of W/O/W emulsion exhibits good correlation as described by the following equation : Y= 5.03 X + 0.19 Where X is the pore size and Y is the mean particle size of the multiple prepared using membrane emulsifier technique.
Average Globule Size and Size DistributionThe optical microscopy method using calibrated ocular and stage micrometer can be utilized for globule size determinations of both multiple emulsion droplets as well as droplets of internal dispersed phase.
Brightfield micrographs equipped with differential interference contrast optics have been used to characterize the internal droplet of multiple emulsions. Various other techniques used to characterize colloidal carriers like Coulter counter, freeze-fracture electron microscopy and scanning electron microscopy are also used to determine average globule size and size distribution of multiple emulsions. Recently, NMR self-diffusion methods are adapted to multiple emulsion characterization.
Area of InterfacesThe average globule diameter determined can be used in the calculation of the total area of interface using the formula S = 6/d S = Total area of interface (sa, cm) d = Diameter of globules (cm)
Number of GlobulesNumber of globules per cubic meter can be measured using the haemecytometer cell. The globules in five groups of 16 small squares (total 80 small squares) are counted and the total number of globules in per cubic mm are calculated using the formula (Chatterjee, 1985) :
Rheological EvaluationThe rheological of multiple emulsions is an important parameter as it relates to emulsion stability and clinical performance. The viscosity and interfacial elasticity are two major parameters, which relate to product rheology. The viscosity of the multiple emulsions can be measured by Brookfield rotational Viscometer.
Interfacial rheology (i.e., interfacial elasticity at the oil-aqueous interface) can be investigated at the mineral oil/water interface using an Oscillatory Surface rheometer.
Zeta PotentialThe zeta potential measurements are pivotal in the designing of surface modified or ligand anchored multiple emulsion systems. The zeta potential and surface charge can be calculated using Smoluchowskis equation from the mobility and electrophoretic velocity of dispersed globules using the Zeta-potentiometer. Zeta potential was calculated using following formula:
Percent Drug EntrapmentPercent equipment of drug or active moiety in the multiple emulsion is generally determined using dialysis, centrifugation, filtration and conductivity measurements. The % Entrapment can be calculated using the following equation :
In Vitro Drug Release
The drug released from the aqueous inner phase of a W/O/W emulsion can be estimated using the conventional dialysis technique. Aliquots were withdrawn at different time intervals and estimated using standard procedure and the data were used to calculate cumulative drug release profile.
In Vitro Stability StudiesEmulsion stability is determined by phase separation on storage of W/O/W emulsions. Freshly prepared multiple emulsion allowed to stand for one week at room temperature and the volume of aqueous phase separated (Vsep) is measured at suitable time intervais and percent phase separation is calculated using following formula:
Stability of Multiple EmulsionsEmulsion stability is a phenomenon, which depends upon the equilibrium between water, oil and surfactant. Unfortunately multiple emulsions are thermodynamically unstable. The possible indications of instability includes:
Leakage of the contents from the inner aqueous phase. Expulsion of internal droplets in external phase. Constriction or distension of the internal droplets due to osmotic gradient across the oil membrane. Flocculation of internal aqueous phase and multiple emulsion droplets. Disruption of oil layer on the surface of internal droplets. Phase separation.
Methods to Stabilize Multiple EmulsionsThe followings are some of the attempt or studies made to restore or strengthen the stability of multiple emulsions :
Liquid crystal stabilized multiple emulsion Stabilization in presence of electrolytes Stabilization by forming polymeric gel Stabilization by interfacial complexation between non-ionic surfactant and macromolecules Steric stabilization Phase-inversion stabilization of W/O/W emulsion
Drug Release Mechanisms & ModelsSome of the mechanisms includes :
Diffusion of unionized drug (hydrophobic species) through the oil layer Carrier medicated transport Micellar transport Rupture of oil membrane Thinning of oil membrane
Applications in Therapeutics & CosmeticsMultiple emulsion systems are finding unlimited uses because of their vesicular structure with innermost phase closely similar to that of liposomal vesicles and the selective permeability characteristic of liquid membrane.
Biomedical & Pharmaceutical Applications of Multiple EmulsionsApplications Enhanced oral bioavailability Masking action Drug over dosage treatment Vaccine adjuvant Se