Hydrophilic - Lipophilic Balance (HLB) scale
The hydrophilic–lipophilic balance (HLB) scale is a measure of balance between hydrophobic and hydrophilic parts of a surfactant.
For selecting a surfactant for stabilization of emulsion it is crucial that there is a degree of hydrophilicity of surfactant to confer the enthalpic stabilising force and a degree of hydrophobicity to secure adsorption at the oil/water interface.
The HLB of a surfactant is expressed using arbitrary scale known as HLB scale. For non-ionic surfactants the value ranges from 0 to 20.
1.At the top end of the scale, the surfactants are hydrophilic(water soluble) and act as solubilising agents, detergents and oil/water emulsifiers.
2.Oil-soluble surfactants that possess a low HLB value act as water-in-oil emulsifiers.
3.HLB values can be calculated according to empirical but useful formulae:
• For simple alkyl ethers in which the hydrophile contains only of ethylene oxide,
• For simple alkyl ethers in which the hydrophile contains only of ethylene oxide,
HLB value = E/5
where E is the weight % of ethylene oxide groups.
• The HLB of polyhydric alcohol fatty acid esters like glyceryl monostearate is calculated from the following equation:
• The HLB of polyhydric alcohol fatty acid esters like glyceryl monostearate is calculated from the following equation:
HLB value = 20 (1-S/A)
Here ,
S is sapanification number of ester
A is acid number of fatty acid
•For the materials like beeswax and lanolin derivatives for which it is not possible to obtain the saponification numbers, the HLB value is obtained from:
•For the materials like beeswax and lanolin derivatives for which it is not possible to obtain the saponification numbers, the HLB value is obtained from:
HLB = (E + P)/5
Here P is the weight % of polyhydric alcohol groups (glycerol or sorbitol) in the molecule.
• HLB values can also be calculated from group contributions using:
• HLB values can also be calculated from group contributions using:
HLB = Ʃ(hydrophilicgroup numbers) – Ʃ (lipophilic group numbers) + 7
• For a mixture of two surfactants that contains fraction x of A and (l – x) of B it is assumed that the HLB value is an algebraic mean of the two HLB numbers:
HLB of mixture = xHLB of A +(1 – x)HLB of B
Drawbacks of HLB system
1.The effect of temperature or that of additives or the concentration of emulsifying agents is not considered during the calculation of HLB value.
2.The presence of agents which salt-in or salt-out surfactants will, respectively, increase and decrease the effective (as opposed to the calculated) HLB values. Salting-out the surfactant (e.g. with NaCl) will make the molecules more less hydrophilic (more hydrophobic).
Authored By
S.M.Imroz , M.pharm (Pharmaceutics)
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