Introduction to the HLB System

by Joseph Albanese

HLB is short for hydrophilic-lipophilic balance. Created by William C. Griffin in the late 1940s, the HLB System is one of the most successful strategies for developing stable emulsions.^1,2 Although it might not be used as much today as in the past, it still remains a valuable time-tested tool used by formulators to select the best surfactants from the many hundreds available to emulsify oils. Surfactants may be anionic, cationic, nonionic, or zwitterionic. However, the HLB System was invented specifically to help in the selection of nonionic surfactants. The hydrophilic strength of a nonionic surfactant is calculated on a molecular weight basis. It equals the percentage of the molecular weight that is hydrophilic divided by 5. The HLB values assigned to nonionic surfactants ranges between 0.5 (most lipophilic) and 19.5 (most hydrophilic). By their very nature all surfactants (surface active agents) are amphiphilic. This means that their molecular structure has both a polar, water-loving group and a non-polar moiety, such as a hydrocarbon alkyl chain, with greater affinity for oil. Thus, as their name suggests, surfactants can actively orientate themselves at the oil-water interface. This makes them great emulsifiers. Without this interaction at the oil-water interface, the inherent thermodynamic instability of the immiscible constituents of emulsions would become quickly apparent to the observer—simply because water and oil, by themselves, do not mix. This dual attraction of surfactants for both water and oil is usually not a 50:50 relationship. Some surfactants have a stronger attraction for water (high HLB) and others for oil (low HLB). Highly water-soluble surfactants are best for cleansing while surfactants with intermediate solubility in water are best for oil-in-water (o/w) emulsions. On the other hand, surfactants with a greater affinity for oils are the right choice when making water-in-oil (w/o) emulsions. High concentrations of high HLB surfactants are needed to break up dispersed oil droplets to create more thermodynamically stable microemulsions. Selecting the correct surfactant system depends upon the Required HLB of the oil(s) to be emulsified. The Required HLB of several oils can be found in Table 1.

If the Required HLB of an oil is not in the table, it can be determined empirically by making multiple blends of high and low HLB surfactants whose ratios cover a range of HLB values, and then use them to make a series of emulsions while observing their stability. For example, if we were to make an oil-in-water emulsion containing palm oil, we would start with our specified quantity of water, and then add the desired amount of palm oil (e.g., 10% (w/w)). For the emulsifiers, a good starting place would be to have a total concentration equal to that of the palm oil. We would then use blends of the emulsifiers at specified ratios (e.g., see Table 2) to prepare a series of formulations, ultimately allowing us to select the correct ratio of emulsifiers in the final system based on stability, physical appearance, and other factors. Since we already know the HLB values of the emulsifiers, we can determine the Required HLB of the oil phase (in this case palm oil). The Required HLB of the oil is equal to the calculated HLB value of the binary blend of emulsifiers that provides the most stable and aesthetically pleasing emulsion.

Table 2: HLB values for mixtures of two stearate ester emulsifiers. One is lipophilic (sorbitan stearate) and the other hydrophilic (polysorbate 20). Data from Reference 4.

Matching this Required HLB with a surfactant system containing the correct ratio of hydrophilic and lipophilic portions is critical to achieving a stable emulsion. Most often a blend of two or more surfactants, each with a unique HLB value, is utilized to match the Required HLB of the oil phase rather than a single surfactant. Typically, such an approach creates the most stable emulsion.

Example
As an example, the first step in formulating an acceptable emulsion using the principles of the HLB system requires the cosmetic chemist to first select the components of the oil phase and then determine its Required HLB either by calculating it from the values provided in data tables or experimentally (as already explained above) using a combination of high and low HLB surfactants in various ratios. The second step is to find a surfactant, or combination of surfactants, with an equal (or at least very close) HLB value. These two initial steps will narrow the field of potential emulsifiers significantly.

Often, a process of further trial and error is the only way to find the best ‘chemical-type’ of surfactant. In doing these additional experiments, it is important to keep in mind that above a certain concentration surfactants in aqueous solution form molecular aggregates called micelles. This critical micelle concentration (CMC) is unique for each surfactant. The length of the alkyl chain influences the CMC and HLB value and these become important considerations when selecting the right ‘chemical-type’ for your emulsion.

Step 1. Select your oil phase and determine its Required HLB from the table.4
Mineral Spirits … 30% X Required HLB 14 = 4.2
Cottonseed Oil … 50% X Required HLB 6 = 3.0
Chlorinated Paraffin … 20% X Required HLB 14 = 2.8
Estimated HLB for emulsifier system … 10.0

Step 2. Find one or more nonionic surfactants (i.e. emulsifiers) to give an HLB Value of 10.0 +/- 1.0.4
Sorbitan stearate… 48% X HLB Value 4.7 = 2.256
Polysorbate 20… 52% X HLB Value 14.9 = 7.748
Calculated HLB Value… 10.004

Of course, there are additional approaches to stabilizing emulsions, such as thickening the external phase with a rheology modifier like carbomer or a hydrophobically modified analog. In addition, one may increase the percent concentration of the dispersed internal phase to increase the viscosity. Furthermore, forming lamellar gel structures and considering Hansen Solubility Parameters are also viable steps helping to achieve a stable emulsion. A multipronged approach works best.

The HLB system is not without its limitations, yet it remains a good place to start. It can be a time saving approach because it eliminates many surfactants from further consideration as suitable emulsifiers for your product.

References
1. W.C. Griffin, Classification of surface active agents by HLB, J. Soc. Cosmet. Chem., 1, 311-326 (1949).
2. W.C. Griffin, Calculation of HLB values of nonionic surfactants, J. Soc. Cosmet. Chem., 5, 249-256 (1954).
3. J.E. Zajic and W. Seffens, “Approaches to hydrophilic-lipophilic balance evaluation of microbes” in Biotechnology for the Oils and Fats Industry, Eds. C. Ratledge, P. Dawson, and J. Rattray, American Oil Chemists’ Society: Urbana, IL (1984).
4. The HLB System: A Timesaving Guide to Emulsifier Selection, ICI Americas Inc., Wilmington, DE (1992).