Pharmaceutical emulsion p III

 HLB number

PART - III

Selection of suitable emulsifier according to HLB number. :-

To stabilize the emulsion, various types of compound are used such as amphilic polymers,  proteins, particles,  which have good oil water interface.

On industrial basis,  the emulsifier is expressed by hydrophile lipophile number (HLB).

The HLB number is an index of affinity of surfactant towards a solvent.

High HLB number indicates that water affinity is more as compare to oil affinity.

A perfect surfactant is used for a emulsion according to the determined number.

According to Bancroft rule, a oil soluble surfactant can make more preferably water in oil emulsion and water soluble surfactant can make more accurate oil in water emulsion.

The HLB number also reveals the ideal properties of solution like critical miceller concentration (CMC).

 

Griffin HLB scale equation

The equation proposed by Griffin is still commonly used. This equation expresses the structural balance between hydrophilic and lipophilic groups in surfactant molecule as numeric index from 0 to 20 on the basis of empirical Emulsification with POE emulsifier.

Specifically  the HLB number of POE alkyl ether are given by the following equation, in which the HLB number equals the weight fraction of the POE moiety in the molecule.

 

HLB = POE (wt%)/5

 

The Griffin HLB number was established empirical from Emulsification testing,  and thus this equation can be utilized as the convenient tool for industrial application.

On other hand,  Griffin also obtained other equation to calculate the HLB number for fatty acids esters chemically bonded with polyols as follows :

 

HLB = 20 X (1-S/A)

 

In which S and A are the saponification and acid numbers of esters respectively.

These equation can also be expressed by the following general equation, in which M is the molecular weight of the surfactant and Mw is the molecular weight of hydrophilic group.

 

HLB = 20 X Mw/M

 

o   Davies equation :-

 

Davies method,  which is one of the traditional method used to calculate the HLB number, is based on the unit value given to the each functional group as either a hydrophilic or hydrophobic elements, and HLB number of the molecule can be calculated as the summation of all whole unit values:

 

HLB = 7+ Σ (unit value of hydrophilic group) + Σ (unit value of lipophilic groups)

 

This versatile equation is able to calculate the HLB number even for ionic surfactant, although the resulting HLB value cannot be absolutely predict the properties of the surfactant solution.

Kawakami’s Equation :-

The calculation method reported by Kawakami, which is obtained from similar hypothesis by Davies equation defines the HLB number to be seven when hydrophilicity and lipophilicity of surfactant are equivalent, and can be given from following equation using two molecular weight by hydrophilic and lipophilic groups.

HLB = 7+ 11.7 X log Mw/Mo

Organic conceptual diagram :-

Another important equation used to calculate the HLB number is based on organic conceptual diagram . Initially the primary objective of this method was to determine the complex properties of organic compounds via intermolecular interaction.

The conceptual diagram consist of mainly organic value( OV) and inorganic value (IV) , which respectively relate to intermolecular Forces generated by van der waal Forces and electrostatic interaction and their corresponding values represent HLB number.

HLB = ΣIV/ΣOV X 10

The IV and IO values are to be assumed to have relation with respectively lipophilicity and hydrophilicity of the surfactant and the ratio.

·        Required HLB number for oil :-

 

For making a suitable emulsions a proper surfactant Must be choosen,  but due to some factors like oil polarity, electrolytes, polyols concentration in water, water in oil ratio and temperature.

In fact,  more polar the oil phase, the more hydrophilic the emulsifier should be.

For this a combination of an emulsifier should be choosen like one should be hydrophilic and another should be hydrophobic, which gives a better packing at the interface.

The average HLB number can be expressed by an equation –

 

Average HLB = Σn i=1 (HLBi. Xi)

 

Where, 

             HLBi = HLB number of emulsifier i

             xi = weight fraction of emulsifier i

On other hand, the effect of water soluble additives is not systemized.

For non ionic surfactant, one additives causes continuous decrease in cloud point that indicates low HLB number.

The effect of salt can be explained on the basis of Holfmeister series.

The HLB number of surfactant decreases upon addition of small amount of ionic water soluble additives because of electrical interaction between surfactant head groups and electric permittivity.

Recently Shinoda and Kunieda introduced new index ‘HLB temperature ‘ or phase inversion temperature (PIT).

When the temperature is raised the degree of hydration of POE surfactant decreases and the surfactant becomes less hydrophilic.

Consequently the oil in water may be inverted into water in oil emulsion with increasing PIT temperature.

PIT is Defined as specific temperature at which hydrophilic and hydrophobic nature  surfactant are balanced,  and it is intrinsic to the molecule but variable depending on the component in the system .

      The PIT temperature is a point where              surfactant are generally water soluble             becomes oil soluble I. t. it migrates from        external phase to internal phase.