Rational formulation of (micro)emulsions

HLD-NAC is a completely free software, dataset and theory package for understanding (micro)emulsions that I have written in collaboration with the theory's main developer, Prof Edgar Acosta of U. Toronto.

Its aim is to allow those who aren't necessarily great experts on surfactants to be able to design (micro)emulsions quickly and efficiently. Unlike HLB, CPP etc. HLD-NAC actually works for a wide range of oils and surfactants.

Background

For non-experts, rationally designing microemulsions is very hard. Although there are many excellent papers on the theory of microemulsions it is hard to know exactly what would happen if you used this surfactant at that concentration at that salinity at that temperature with that oil. HLB, CPP and many other tools just aren't useful for such purposes.

The work over many years by (in alphabetical order) Prof Aubry, Prof Sabatini and Prof Salager has created the highly useful concept of HLD - Hydrophilic Lipophilic Difference. With a few key parameters it is easy to find the conditions (salinity, temperature, oil) which would produce the minimum interfacial tension and the highest solubilisation of water in oil or oil in water. Building on HLD theory, Prof Edgar Acosta at U. Toronto (with his co-workers) has developed the NAC, Net Average Curvature extension to HLD theory so that many more details of a microemulsion can be modelled.

Why HLD-NAC?

The bottom line for practical surfactant formulators is that if you know the EACN (Effective Alkane Carbon Number) of your oil and the Cc (Characteristic curvature) of your surfactant then you already know how close you are to an optimal surfactant (maximum surfactancy for minimum surfactant). This is the HLD part. If you know the extended chain length, head area and "chi parameter" of your surfactant then the NAC part tells you how efficient your surfactant will be - getting the best emulsion for the least surfactant. If you have the key parameters for a range of surfactants then you can make a rational choice of surfactant (or surfactant blend) to get you started in your formulation optimization process.

What about "real" emulsion?

The theory is designed for microemulsions, the thermodynamically stable, clear emulsions that are of increasing interest in perfumes, cosmetics, and pharmaceuticals as well as in the field of extended oil recovery for which much of this theory was developed. However, there is strong evidence linking the understanding of microemulsions to that of real emulsions. The most basic fact is that emulsions are less stable when they are far from the best locations for o/w and w/o microemulsions, and are highly unstable at the optimal surfactant sweet spot. So the curves within the HLD-NAC software are exact predictors of general emulsion stability. There is much more to be said on this subject and a document will be posted here sometime in the near future.

Why a software package?

Although HLD and HLD-NAC are relatively simple theories, they are hard for non-experts to grasp in full. I (a non-expert) therefore wrote, at first for my own use, a user-friendly software implementation of HLD-NAC theory. Using it I was able to formulate right-first-time w/o and o/w microemulsions with difficult cosmetic oils. Since that early crude version I have worked with Prof Acosta on improving the software and also extending the database of key surfactant parameters including some of the latest extended surfactants. The software has now been used on several microemulsion formulation projects and has proven to be very useful in the hands of non-experts. As word has spread through the surfactant community there have been more requests for access to the HLD-NAC Optimal Surfactant software.

So now the package of software, data and explanation is offered, free of charge, to the surfactant community. It has, of course, many issues and limitations. In particular it provides parameters for only a tiny fraction of available surfactants. One of the hopes/expectations of providing the package is that users will each contribute the parameters of one or two of their favourite surfactants so that the database can grow rapidly. "Crowd sourcing" is a very powerful methodology in circumstances where neither academia nor industry can do the job for us. I am also proud to have been the project leader for the 30 Day Challenge sponsored by Intelligent Formulation that attempted to measure the Cc values of 22 "green" surfactants to aid the process of rational substitution of non-green surfactants. Click to download the Green Surfactant 30 Day Summary produced by the team from Syntopix and VLCI with the support of Prof Acosta and Dr Ian Callaghan. We managed to identify 13 Cc values and for the other 9 surfactants we defined the Cc range to make it easier for future studies to find the correct values.

Access to the package

To explore HLD-NAC the minimum requirement is to download Optimal_Surfactant.zip which you simply unzip into a convenient folder to be up and running - provided that the Microsoft DotNet Framework 4.x (freely available from Microsoft) is installed on your PC.

v1.1.11 has added the temperature dependence of each surfactant as a user-controlled variable. This is because more classes of surfactants are being included and automated ways to work out the dependence are no-longer feasible. It is now possible to over-ride the Cc and MWt values for both surfactants. This makes it easier to play with values to get an appropriate fit when you are using new surfactants. The Mole % of the two surfactants are also displayed as it is the Mole % which governs the Cc of a mixture.

More information about installation and PC requirements, plus legal disclaimers etc. is available in HLD-NAC_Software_Installation_and_use.doc

The Help file provided with the package contains a simple explanation of HLD-NAC theory and can be downloaded as Optimal_Surfactant_Help.rtf

A short document describing three different examples of the software in action (using parameter files provided with the software) can be downloaded from HLD-NAC_Working_Examples.doc

The core set of HLD-NAC papers for those who want to get into the theory is available as Core_Optimal_Surfactant_Papers.zip

A very simple Powerpoint description of HLD-NAC can be downloaded as Simple_HLD-NAC.ppt.

My personal view of HLD-NAC theory in comparison with other theories is available as a Powerpoint Presentation, HLD-NAC.ppt. I would positively welcome criticism and suggestions for improvements to this document.

Problems and possibilities

HLD-NAC is an imperfect theory. Even I can point out some of its faults. I have read many elegant papers on surfactant theory that do a better job in predicting the performance of one, specific, type of surfactant with one, specific, type of oil for one, specific, type of application. If those approaches give you what you want for your own purposes, please use those. But I have not found any framework that contains even a fraction of the power of HLD-NAC for practical formulation over a range of problems. If a superior alternative framework exists and I've missed it, I'd be happy to abandon HLD-NAC. I'm only interested in being able to formulate practical emulsions. Till then my hope is that surfactant providers and formulators will help in the task of providing the key datasets for the thousands of surfactants already out there and design surfactants (such as extended surfactants) that will allow users to formulate superior (micro)emulsions with less effort and frustration.

Acknowledgements

I would like to thank Profs Aubry (U. Lille), Sabatini (U. Oklahoma) and Salager (U. de Los Andes) who were most generous in their answers to my queries about their work on HLD.

I would especially like to acknowledge the wise and helpful input from Prof Acosta     U. Toronto and thank him for his encouragement and support for the HLD-NAC project.

Please send all feedback/comments on the HLD-NAC package to me at steven@stevenabbott.co.uk