The problem: Why don't oil and water mix?

Imagine an oil-in-water emulsion, such as an oily fragrance dispersed in an aqueous disinfectant. Oil and water, as we know, do not mix naturally. They are like two characters from completely different stories: without a mediator, they simply cannot coexist.

That's where the protagonist of our story comes in: the surfactant.

Think of it as a teacher in a classroom full of students who are all very different from one another. On one side, we have water, which loves everything that’s polar. On the other, we have oil, which prefers its own nonpolar world. The teacher’s role—the surfactant—is to help these two “students” get along in harmony.

But not all teachers use the same methodology. Some communicate better with water; others, with oil. It is precisely this characteristic that the HLB system seeks to describe.

What is HLB?

The Hydrophilic-Lipophilic Balance (HLB) is a numerical system that expresses the balance between two components of a surfactant:

• Hydrophilic portion: the part of the molecule that has an affinity for water;
• Lipophilic portion: the part that has an affinity for oil.

The scale ranges from 0 to 20. The higher the HLB value, the more the surfactant “speaks the language of water.” The lower the value, the closer it is to the “language of oil.” In summary:

• High HLB - greater affinity for water.
• Low HLB - greater affinity for oil.

This balance directly guides the selection of the right surfactant for each formulation.

The HLB Scale in Practice

The chart below summarizes the HLB ranges and their most common applications:

Why use surfactant blends?

In practice, a single surfactant rarely possesses exactly the characteristics needed to stabilize an emulsion. For this reason, it is common to combine two or more surfactants. This strategy offers advantages beyond simply adjusting the HLB:

• Greater physical stability of the emulsion;
• Improved formation and homogeneity;
• Reduction in the total amount of emulsifier required;
• Synergistic effect between the molecules.

When different surfactants are used simultaneously, their molecules can organize themselves into structures called mixed micelles: combinations that take advantage of the characteristics of each component, resulting in performance superior to that of any of the surfactants used alone.

Want to learn more about surfactants and micellar behavior? Click here to read the full article on our blog.

How to Calculate the HLB of a Mixture

Os valores de HLB dos tensoativos são calculados a partir da estrutura molecular da molécula. Existem dois métodos principais:

Griffin's method (1949): for nonionic surfactants

Developed by William C. Griffin, this is the most common method for fatty acid esters and ethylene oxide derivatives (such as Tweens and Spans). The formula is based on the mass fraction of the hydrophilic portion:

• Mh = molar mass of the hydrophilic portion (e.g., ethylene oxide chains or polyols).
• M = total molar mass of the molecule.
• The factor of 20 is an arbitrary scale: a 100% hydrophilic molecule would have an HLB of 20.

Davies Method (1957): for ionic and nonionic surfactants

The Davies method assigns fixed numerical values to each chemical segment of the molecule. It has the advantage of also working for ionic surfactants, such as sodium lauryl sulfate.

Examples of group values: 

Sodium Lauryl Sulfate (SLS), for example, has a sulfate group (–SO-Na+, value +38.7) and a long carbon chain of 12 carbon atoms (12 × –CH–, total value –5.7). Using the Davies method, its HLB would be approximately 7 + 38.7 - 5.7 = 40—which indicates an extremely hydrophilic molecule, consistent with its use as a detergent and foaming agent.

In simple terms, the HLB can be calculated easily using the weighted average of the concentrations:

Here is a practical example:

• Surfactant A: HLB = 16, used at 50% of the mixture-contributes 8.0
• Surfactant B: HLB = 6, used at 50% of the mixture-contributes 3.0
• HLB of the mixture = 8.0 + 3.0 = 11

In other words, by combining equal parts of each surfactant, we obtain a mixture with an HLB of 11, which is suitable, for example, for oil-in-water emulsions with oils of moderate affinity.

Sorbitan Esters: A Classic Example

Among the surfactants most commonly used in research and in the development of emulsions are sorbitan esters, known by the brand names Span® and Tween®. They clearly illustrate the logic behind mixtures:

• Sorbitan esters (Span®): Relatively low HLB—more lipophilic.
• Ethoxylated sorbitan esters (Tween®): High HLB—more hydrophilic.

Combining the two allows you to adjust the average HLB to any desired value simply by varying the proportion of each component.

Note:

Sorbitan esters are widely used in academic studies, but that does not mean they are the best or most suitable choice for all cases. For each emulsion system, it is necessary to evaluate which type of surfactant is most appropriate.

What is Required HLB (HLBr)?

So far, we have discussed HLB as a property of the surfactant. However, there is also the concept of Required HLB, or HLBr, which is a property of the oil phase of the emulsion.

The HLBr is the HLB value that the emulsifying system must have in order to stabilize a given oil in a given formulation. In other words, each oil “requires” a specific type of surfactant.

• Oil-in-water (O/W) emulsions require surfactants with a high HLB value.
• Water-in-oil (W/O) emulsions require surfactants with a low HLB value.
• The HLB value varies depending on the type of oil used.

Some examples for reference:

• Sunflower oil: HLBr between 6 and 8.
• Isopropyl myristate: HLBr between 11 and 12.
• Silicones and mineral oils: HLBr generally above 10.

When the HLB of the emulsifying system approaches the HLBr of the oil phase, the result tends to be more stable and homogeneous emulsions.

How is HLBr determined in practice?

HLBr is not a fixed, absolute value. It is determined experimentally because it is influenced by various factors in addition to the oil itself: the composition of the aqueous phase, temperature, the ionic strength of the system, and the presence of other ingredients.

In practice, the methodology works as follows:

• A series of emulsions is prepared using the same oil and the same surfactant concentration, varying only the HLB of the emulsifying system (using mixtures with different ratios).
• The emulsions are evaluated for physical stability: observation of phase separation, centrifugation, or thermal cycles. At this stage, a lower surfactant concentration is intentionally used so that the emulsions are not completely stable, which allows for a comparison of the separation rates among the samples.
• The HLB range that produced the most stable emulsions is progressively refined until the optimal point is identified.

Once the HLBr has been determined, the next step is to determine the minimum surfactant concentration required to stabilize the emulsion for the product’s shelf life, again through a series of tests at different concentrations.

HLBr tables available in the literature

There are reference tables listing the HLB values of various oils. These are useful for reducing the number of initial tests, but should be used only as a starting point; the specific conditions of each formulation can alter the system’s actual HLB value.

HLB is a fundamental concept for anyone working with emulsions in the chemical industry. Understanding the scale, the logic behind surfactant blends, and the concept of HLBr allows for the development of more stable, more efficient formulations with fewer trials and errors.

Despite its limitations—especially the fact that the HLB value is influenced by multiple factors within the system—the HLB remains an excellent starting point for selecting emulsifiers, accelerating product development.

Do you have any questions about the HLB value of your formulation? Contact Macler’s SmartLab. Our technical team can help you identify and develop the emulsifier system best suited to your needs.