Practical Surfactants & Foams

The graphics below sum up this site. They are used throughout the site as navigation aids so you know where you are in the overall story.

Surfactant Science Overview

Practical Surfactants started off as a guide to HLD-NAC theory as a counter to all the bad science out there invoking HLB, CMC or CPP as being keys to good emulsion formulation (which they are not). Over time it has become a more general repository of app-based tools for thinking through a whole range of surfactant issues. It is now changing once more because I am writing a free eBook called Surfactant Science for Formulators which will link to the apps on this site. One big change was that Practical Foams became integrated within Practical Surfactants as so many of the key ideas are shared.

Because HLD-NAC is key to so much of surfactant science, it still (rightly) dominates the site. But the Emulsions section is becoming steadily bigger and Other Issues will continue to grow. The Phase Diagram section emerged after a chance meeting with Seth Lindberg of Proctor & Gamble and is a major resource in its own right because we all find phase diagrams difficult and confusing.

I've greatly enjoyed the feedback from visitors to the site, many of whom are far more expert than I am. Please feel free to tell me that something is wrong, or could be better. I make no pretence to be a surfactants expert - my job is to bring the best available science to the attention of the formulation community in a format that makes it usable.


It is astonishing that a practical formulation tool for surfactants, HLD, which was created by Salager in the 1970s is still very little known and used. Instead people pretend to use HLB. In private everyone within surfactant companies admit that HLB is useless for everything other than ethoxylates, but if you ask most practical formulators how they should formulate they say "HLB", even though their own experience will tell them that it truly is useless. After HLB, some have heard of Critical Packing Parameter (CPP) a truly interesting concept which is valuable for understanding liquid crystal phases in concentrated surfactants but useless for most practical formulation. One can find RSN (Relative Solubility Number) as another simple scheme. This is in principle readily measured but the test (when does turbidity appear in a water/benzene/dioxane/surfactant mix) is not standardised (the benzene/dioxane ratio is not standardised, they are both toxic so a toluene/EGDE mix has been proposed instead) and it's not really clear with what formulation properties it actually correlates. Those who are interested can read Jiangying Wu et al, Development of a method for measurement of relative solubility of nonionic surfactants, Colloids and Surfaces A: Physicochem. Eng. Aspects 232 (2004) 229–237 for more details of the issues and of their attempts to correlate with HLB, for non-ionics only. After those three largely unhelpful concepts there may be some folk memories of other ideas for formulation, but nothing that can be put into practice.

But it turns out that HLD (Hydrophilic Lipophilic Difference) is a truly useful, practical tool for surfactant optimisiation. It was developed first by Salager and extended over the years by himself and key colleagues such as Aubrey, Sabatini and, most recently with the powerful NAC variation, Acosta.

Emulsion, Microemulsions, Macroemulsions

It is sometimes said that HLD is a fine theory for microemulsions but of no use for "real" emulsions. If that were true, there would be no point in writing this guide. Microemulsions are interesting in their own right and have some great applications, but they are only a small subset of formulations needs. HLD is very much applicable to "real" emulsions. Indeed, the fact that many such emulsions are made via the Phase Inversion Technique (PIT) is something that is best explained via HLD!

For the purposes of this guide we'll use the word "microemulsion" to mean a thermodynamically stable o/w or w/o phases which can, in principle, be made merely by adding the oil, water and surfactant together with a simple stir. Instead of saying "real" emulsion we will just use the word "macroemulsion", to describe phases that are only kinetically stable and which generally need substantial amounts of energy (or some clever tricks) to create and which over time will separate out by some combination of creaming, Ostwald ripening etc.

It is an unfortunate historical accident that microemulsions (which are nanosized) weren't called nanoemulsions. What are now called nanoemulsions are classic emulsions that happen to have nanosized droplets. In this unfortunate muddle, all microemulsions are nanosized (and none are micron-sized) and macroemulsions are micron-down-to-nano-sized. A lot of the time we will use the term "emulsion" in a neutral sense because the essentials of HLD apply to both microemulsions and macroemulsions.

Whilst on nomenclature, the terms Type I (o/w), Type II (w/o) and Type III (biphasic region between I and II) will be used for all emulsions. Some would object that the terms should be applied only to microemulsions, but by using them across emulsion space in general it is much easier to bring clarity to a lot of confusion.

Easy steps

This Guide takes you through the basics of HLD, always having an "app" right there to help you visualise what is going on. HLD is very simple and can take you a long way. As we start to get into the "NAC" part of the Guide the theory gets slightly more complicated but the power increases tremendously. Again, the apps will be right there to help you understand what is going on so you should never get lost.

The aim of all this is to give you practical formulation tools that you can use every day. You will find that to use HLD you need some key data about surfactants and oils. I've done my best to supply an up-to-date list of parameters. You can also gather this data yourself (it's not too hard) or demand it from your suppliers that they provide it. Once everyone provides the data on their surfactants and oils, formulation will get a lot easier for everyone!

Throughout the Guide if you click on buttons such as these some instant information appears, without leaving the current page.

Now start with the Surfactant Basics from the menu at the top of this page. The menu is always there to guide you.