HLB Values of Surface Active Agents

“HLB VALUES” of Surface Active Agents

Surface active agents have varying properties depending on the nature and ratio of hydrophilic and hydrophobic groups. For example, among the anionic surfactants C₈-C₁₂ alkyl hydrophobes tends to be predominantly wetting agents, whilst the C₁₂-C₁₈ analogues exhibit better detergency and emulsifying properties. Clearly as the hydrophobic character of the surfactant is increased, the aqueous solubility decreases and oil solubility increases. Thus, the balance between the hydrophobic and hydrophilic moieties of a surfactant is a critical factor in determining its major characteristics. To dictate the properties of surface active agents, Griffin developed a system called Hydrophilic-Lipophilic Balance or HLB.

In this system, each surface active agent is given a number, called HLB value that numerically represents the hydrophilic and hydrophobic tendencies of the material. Such scale covers a range of value from 0 (the Lipophilic or hydrophobic extreme) to a hydrophilic extreme of 20. In HLB system a theoretical compound that could be considered 100% hydrophilic is arbitrarily assigned an HLB value of 20. A value of 10 approximately represents the point at which the hydrophilic and hydrophobic portions are in balance. This scale is especially useful in describing the properties of the nonionic ethoxylates and is of particular value in describing the formation of emulsions. Each material to be emulsified is also rated as to the HLB value of the emulsifier required to form an emulsion.

All surface active agents are amphiphilic or amphipatic compounds, i.e. each molecule contains an oleophilic or hydrophobic and a hydrophilic moieties. The former is a hydrocarbon tail and consists of material readily soluble in oil or solvents of low polarity. The seconds moiety is chemically similar to compounds, which are hydrated and extensively soluble in water. It consists of a negatively or positively charged ionic head-group in case of anionic or cationic surfactants plus the counter-ions for electro-neutrality. The HLB value depends on the ratio of the hydrophilic and hydrophobic properties. Thus, in general, the molecules that are soluble or dispersible in oil have low HLB values, whereas those, which are dispersible or soluble in water, have high HLB values.

In effect, the HLB value is determined by the chemical composition and the degree of ionization or hydration of emulsifier molecule. Nonionic compounds like glyceryl monostearate are strongly lipophilic and poorly hydrophilic and have low HLB values. Polyoxyethylene esters and ethers with long polyoxyethylene chains are strongly hydrophilic and have high HLB values. In general, the emulsifiers with low HLB values will tends to promote the formation of water-in-oil emulsions and those with high HLB values will form oil-in-water emulsions. However, the exact HLB values required for a specific oil phase must be determined experimentally, since each type of oil will probably require a different HLb system.

As a rough guide emulsifiers with HLB value of 6 or less are used useful in forming water-in-oil (W/O) emulsions and those the HLB values ranging from 8 to about 18 are useful in forming oil-in-water (O/W) emulsions. For solubilisation of oils, surfactants with HLB values of over 15 should be considered. The relation between the HLB and the behavior of the surface active agents is listed in the following table:

HLB values

Calculation of HLB Value:

The HLB values of various types of nonionic compounds can be calculated as follows:

1. For polyoxyethylene-fatty alcohol condensates,


Where E is the percentage by weight of the hydrophilic portion of the molecule. In the case of a polyoxyethylene-fatty alcohol containing 75% (by weight) of ethylene oxide adduct the HLB value would be 75/5=15.

2. For polyol and polyoxyethylene estersbof fatty acids,


Where S is the saponification number of the ester and A is the acid number of the fatty acid portion of the ester. Thus a given sample of sorbitan monolaurate with saponification number of 164 and with an acid number of 290 would have an HLB value of 20*(1-164/290) =8.7.

These equations do not appear to give proper HLB values for nonionic compounds containing polyoxyethylene oxide or polybutylene oxide adducts or for compounds containing nitrogen or sulphur. The actual constitution of a surfactant rarely conforms to its nominal structure. Consequently, the theoretical method of calculating is of limited utility, the practical methods are more reliable. The HLB value may determine directly by analysis or by comparison, with a range of surfactants of known HLB values. The HLB value of a mixture of surfactants is calculated by adding values (B) of the individual of a mixture surfactants weighted by respective fractional quantities (F).

B (mixture)=F₁B₁+F₂B₂+F₃B₃+………….

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