Since the principal emulsifying agents are compounds containing both hydrophobic and hydrophilic groups, and since the phase in which the emulsifier is more soluble is generally the continuous phase, the type of emulsion produced (i.e. O/W or W/O) can be predicted on the basis of the relative hydrophilic-lipophilic properties of the emulsifier. According to the hydrophilic-lipophilic balance (HLB) concept, each of the surface-active agents can be assigned a numerical value representing its hydrophilic-lipophilic balance.

Experimental determination of the HLB number for a given emulsifier is a tedious process. However, this value may be calculated with satisfactory accuracy based on easily determined characteristics of the emulsifier. The following equation was suggested by Griffin for polyhydric alcohol, fatty acid esters:

HLB = 20(1 - S/A), where S is the saponification number of the ester and A is the acid number of the acid. In certain cases, where accurate determination of the saponification number is difficult, the relationship HLB = (E+P)/5 is used, where E is the weight percent of oxyethylene and P is the weight percent of polyhydric alcohol. When ethylene oxide is the only hydrophilic group present the equation is reduced to HLB = E/5. HLB numbers for some common emulsifiers are listed below.

The solubility of emulsifiers in water generally follows their HLB rank. As a rule, emulsifiers with HLB values in the range 3-6 promote W/O emulsions; values between 8 and 18 promote O/W emulsions.

It has also been suggested that HLB values are algebraically additive so that the HLB of a blend of two or more emulsifiers can be obtained by simple calculation and that the blend of emulsifiers needed to produce maximum emulsion stability can be easily obtained. This, however, is not always the case. Although the HLB concept is useful as a guide of comparing emulsing-forming or stabilizing properties, it suffers from a number of limitations. First, commercial emulsifiers usually consist of a group of compounds rather than a single component. This makes direct calculation based on chemical properties very difficult. Furthermore, the HLB method does not take into consideration such factors as emulsifier concentration, mesomorphic behaviour, temperature, ionisation of the emulsifier, interaction with other compounds present, or properties and relative concentrations of the oil and aqueous phases. Pure monoglycerides, for example, have an HLB value of approximately 3.8. Accordingly they would be expected to form only W/O emulsions. However, at emulsifier concentrations that permit the formation of protective mesomorphic layers around the fat globules, pure monoacylglycerols promote O/W emulsions. Moreover, it is well known that O/W emulsions prepared from a blend of emulsifiers are usually more satble than those prepared from a single agent having the same HLB.