There is a mistaken belief that HLD is "only for microemulsions". In fact it is crucial for optimising macroemulsions. This page gives a brief summary and a more complex app is available for those with a deeper interest.
This is a simple introduction to emulsion stability viewed through HLD-NAC. For a more detailed view, and a powerful predictive tool, go to the NAC Emulsion Stability section.
50:50 o:w emulsions
If you take a 50:50 o:w mix and shake it up with a small amount of surfactant, an emulsion will form which has a certain stability. A different surfactant might give a more or less stable emulsion. Each time that the stability of emulsions has been studied with a methodical variation of the Cc of the surfactant (though in many such studies they didn't know this is what they were doing) a clear pattern of stability emerges. Far away from HLD=0 the stability declines. This is not surprising - the surfactant will be much too soluble in the water (if Cc is very low) or in the oil (if Cc is very high) so will not be at the interface doing something useful. The Creaming and Ostwald ripening section in gives some further ideas for why the emulsion will be less stable. So as you get closer to the HLD=0 point the emulsion stability rises to a maximum because the surfactant molecules spend more and more time at the interface - but falls sharply to a minimum when HLD=0. Why the sudden fall? We have the answer from the section on Stability - when the interfacial energy is very low, the oil drops can fuse very rapidly so will cream very quickly. Far from HLD=0 we have creaming via Ostwald ripening, close to HLD=0 we have creaming via coalescence (though the rate of coalescence depends on other factors such as DLVO charge [ionic polymers] or steric [long-chains] stabilisation.
In general, people don't make 50:50 o:w emulsions. The point of this section is that there is a clear link (with plenty of experimental validation) between the theory of working with microemulsions and the theory of working with macroemulsions. In both cases, knowing where the HLD=0 point is in your system is vital. For macroemulsions, that point is the worst possible place to be. But it is a clear navigation marker in the complicated surfactant landscape. We'll now discuss how to use it to make real macroemulsions.
In principle it's very easy to use the basic HLD-NAC approach to formulate the perfect o/w macroemulsion. Set the values of the parameters you know are fixed (e.g. T, S, EACN), select the variable parameter (in this case, Cc) and look for the value where the solubility is maximum - i.e. the Type III point. Your perfect emulsion will be obtained with a Cc slightly lower.
How "slight"? At present there are no rules, but if the focus is on Cc then from the plot something like 0.5 lower would seem reasonable. This depends strongly on the tail lenth, L; the shorter the length, the sharper the solubility curve so the closer you need to be to the Type III spot. For 50:50 emulsions the technique is perfect. For other ratios we need the fishtail plot, as discussed in the next section on PIFs.
How do you get the right Cc value? Use the Cc calculator in Ch 1 to find whichever ratio of your two favourite surfactants will give you the right answer. If there is no such mixture (and often there isn't because many commercial surfactants are at useless extremes of Cc) you have to find two new favourites. In the php part have:
Simply go to the other side of the Type III point, say +0.5 Cc to get the most efficient w/o macroemulsion, with the caution about the need to understand PIF for O:W ratios different from 50:50.