How to blend two emulsifiers to hit a target HLB: worked examples

Blend HLB is the weight-average of the component HLB values, so the ratio needed to hit a target is straightforward algebra: the weight fraction of the high-HLB emulsifier equals the target minus the low HLB, divided by the difference between the two. Hitting HLB 10.5 with Span 80 (4.3) and T80 (15.0) takes 58 % T80 and 42 % Span 80.

The calculation, stated once

The weighted-average rule is the only equation involved, and it is worth writing out in full before the examples. For a two-component blend, blend HLB equals the weight fraction of component A multiplied by the HLB of A, plus the weight fraction of component B multiplied by the HLB of B, where the two fractions sum to 1. Rearranged to solve for the ratio, the weight fraction of the high-HLB component equals the target HLB minus the low HLB, all divided by the high HLB minus the low HLB.

Two cautions apply to that arithmetic. The fractions refer to the emulsifier blend alone, not to the whole formulation, so a blend calculated at 58:42 is still dosed at whatever total emulsifier level the formulation calls for — the ratio sets HLB, the level sets how much interface can be covered, and the two are independent decisions. And the rule ignores every interaction between the two molecules, chain-length compatibility, temperature effects and the contribution of any other surface-active ingredient in the system. It reliably gets a formulator to the right neighbourhood, which is its purpose; the last part of the journey is done by testing a ladder of ratios around the calculated point.

Reference HLB values

Selecting a bracketing pair is easier with the available HLB values in one place. The table below lists industrial-grade nonionic esters commonly used as blend components, with values per manufacturer TDS.

Grade	Chemical name	CAS	HLB	Typical role in a blend
Span 80 (VS-201A)	Sorbitan monooleate	1338-43-8	4.3	Low-HLB anchor, liquid
Span 60	Sorbitan monostearate	1338-41-6	4.7	Low-HLB anchor, waxy solid
PEG400DO	Polyethylene glycol 400 dioleate	9005-07-6	7–8	Mid-range component
PEG600DO	Polyethylene glycol 600 dioleate	9005-07-6	10–11	Mid-range component
PEG400DL	Polyethylene glycol 400 dilaurate	9005-02-1	10.5	Mid-range component
PEG400MO	Polyethylene glycol 400 monooleate	9004-96-0	11–12	High-HLB partner
PEG600MO	Polyethylene glycol 600 monooleate	9004-96-0	13–14	High-HLB partner
T80 (Tween 80)	Polyoxyethylene (20) sorbitan monooleate	9005-65-6	15.0	High-HLB partner

HLB values per manufacturer TDS. All grades listed are nonionic.

Worked example 1 — target HLB 10.5 with Span 80 and T80

Take an oil-in-water system whose oil phase has been assessed at a required HLB of 10.5, to be emulsified with the Span 80 and T80 pair. The high-HLB fraction is 10.5 minus 4.3, divided by 15.0 minus 4.3, which is 6.2 divided by 10.7, or 0.579. The blend is therefore 57.9 % T80 and 42.1 % Span 80, rounded in practice to 58:42. Back-substituting confirms the result: 0.579 times 15.0 gives 8.69, 0.421 times 4.3 gives 1.81, and the sum is 10.50.

Converting to charge weights takes one more step. If the formulation carries a total emulsifier level of 8 parts per 100 of concentrate, the charges are 4.63 parts T80 and 3.37 parts Span 80. For the test ladder, hold that 8-part total fixed and vary only the ratio: blends at roughly HLB 9.5, 10.5 and 11.5 correspond to 48.6:51.4, 57.9:42.1 and 67.3:32.7 T80 to Span 80. Because this pair shares the same sorbitan-oleate backbone, the two molecules pack together closely at the interface, which is why the pair covers a wide HLB range from a single pair of raw materials.

Worked example 2 — an HLB range, and what it does to the answer

Consider a target HLB of 8.0 built from Span 80 at 4.3 and PEG-400 monooleate, which is specified as HLB 11–12 rather than a single value. Calculating at the low end, 8.0 minus 4.3 over 11.0 minus 4.3 gives 3.7 divided by 6.7, or 0.552 — a 55:45 blend. Calculating at the high end, 3.7 divided by 7.7 gives 0.481, a 48:52 blend. The two answers differ by about seven percentage points, which is the practical cost of a specified range rather than a point value.

Handling that spread is a judgement call rather than a calculation. Where the system tolerates a band, take the midpoint — about 52 % PEG-400 monooleate — and let the test ladder resolve it. Where the system is HLB-sensitive, either choose a partner with a single-point HLB such as T80 at 15.0, or plan the ladder deliberately to span the whole band so that the adopted ratio is validated at both ends. A third example shows how narrow the arithmetic can get: hitting HLB 6.5 from Span 60 at 4.7 and T80 at 15.0 requires 1.8 divided by 10.3, or 17.5 % T80 — a low-HLB-dominated blend in which small weighing errors on the minor component shift the result noticeably, so the high-HLB partner is best added as a pre-diluted solution rather than weighed neat.

Where the arithmetic stops and testing begins

The blend HLB calculation is a starting-point generator, and treating its output as a final answer is the most common way formulators lose time with it. Required HLB values for oils are themselves approximate, they shift with the polarity of additives already in the phase, and the optimum for a real formulation is frequently half an HLB unit or more away from the calculated figure. The calculation earns its value by replacing a broad search with a narrow one: instead of scanning the whole HLB range, the formulator prepares three to five samples around a calculated centre.

Run that ladder with a single variable — same oil, same total emulsifier level, same mixing order, same shear, same temperature — and assess it the way the product will actually be judged: initial droplet appearance, creaming or free oil at 24 hours, 7 days and 30 days, and behaviour at the temperature extremes of storage and use. Record the full recipe of each variant, because the ladder is also the evidence base for the next reformulation, and a documented HLB response curve for a given oil phase saves the whole exercise being repeated when a raw material changes.