Emulsion and HLB value

Emulsion is a mixture of two immiscible liquids that is formed by using an emulsifying agent. Emulsions are usually of the following two types,

·         Oil in water: Oil is dispersed in water.
Water in oil: Water droplets, dispersed in oil.

Emulsifying agents are the agents that crate compatibility between immiscible liquids. As they contain an affinity for both the lipophilic (oil loving) and hydrophilic (water loving) mediums, they decrease the interfacial tention and disperse one phase in the other.

HLB system assigns the emulsifying agents with a unique numerical value, which range from 0 to 20. This value provide us an idea about the nature of the emulsifying agents.

Emulsifying agents, whose HLB value is,

less than 9, are more lipophilic

between 9 – 11, are intermediate

above 11, are more hydrophilic

For the development of emulsions, the choice of the suitable emulsifying agent is very important.

If we use a blend of more than one emulsifying agents, we can simply add their HLB values to find the net HLB value.

Example,

Find the HLB value of the blend of Tween 80 and Span 80, if their concentrations are 60% and 30% respectively.

Tween 80                            66% x 15 = 9.9                   (HLB of Tween 80 = 15)

Span 80                              30% x 4.3 = 1.29                (HLB of Span 80 = 4.3)

                                            Net HLB = 11.19

In some cases, the HLB value is calculated on the basis of the oil phase. For some oils, the HLB value that is required for their emulsification is known. In such cases, the HLB value is calculated as follow.

Example,

Calculate the required HLB value to emulsify a blend of Castor oil and Corn oil, if their quantities are 20% and 30% respectively, in the formulation. The HLB values that are required for the emulsification of Castor oil and Corn oil are 14 and 10 respectively.

Castor oil             20% x 14 = 2.8

Corn oil                 30% x 10 = 3

                Required HLB = 5.8

(This means an emulsifying agent or a blend of emulsifying agents that have an HLB value = 3, can emulsify this oil mixture)

During formulation development, we usually encounter such ingredients that have unknown required HLB value. For such materials, experiments are designed for the evaluation of the required HLB value. In these experiments, different formulations are prepared with different blends of Tween and Span grades. In this way, the blend of emulsifying agents that produce the most stable emulsion, is selected, and its HLB value is calculated.

Sometimes a need arise to blend two emulsifying agents to produce a desired HLB value. In such cases, the following formulas are utilized.

% of Emulsifier A = 100 (Required HLB – HLB of Emulsifier B)

                               HLB of Emulsifier A – HLB of Emulsifier B

% of Emulsifier B = 100 - % of Emulsifier A

Where,

HLB of Emulsifier A > HLB of Emulsifier B

Example,

Achieve the HLB value of 5.7, by using Glyceryl monostearate and PEG 400 monostearate, if their HLB values are 3.8 and 11.6 respectively.   

By using the above mentioned formula, we get,

% PEG 40 = 100 ( 5.7 – 3.8) = 24.36%

                          11.6 – 3.8

% Glyceryl monostearate = 100 – 24.36% = 75.64%