Emulsions in Retrospect and Prospect

Presidential Address

Dr J.L. Moilliet

1 Introduction

First of all, may I explain the background to the title of this talk: “Emulsions in Retrospect and Prospect”? You see, most people who have the temerity to come out of retirement–even a fairly recent retirement–to address a meeting like this, soon make two discoveries: firstly, how quickly they have become out of date, and secondly, how much more clearly they remember things from the relatively distant past, compared with the more recent past. There are, of course, exceptions to this, such as Sir Eric Rideal or the late Professor N. K. Adam. But these remarks apply only too clearly to me, and so for the first of these “ageing phenomena”–viz. the fact that I am now a back number-I offer you my apologies. But I feel less inclined to apologize for the fact that I seem to have retained a clearer picture of emulsion science and technology up to about 1960 than in the past 10–15 years. Now let me try to justify that unrepentant attitude.

With the present-day exponential rate of growth of scientific and technical literature, there is an increasing tendency for us to adopt a sort of statute of limitations on scientific and technical knowledge, i.e. we tend to reduce our volume of reading to manageable proportions by setting a sort of “date-line limit” to our literature searches, rather than searching further back, on a narrower front. This tendency, which is probably unavoidable, seems to me to be equally noticeable among industrial workers and among academicals, and it is by no means unknown among patent examiners. For example, towards the end of my career in industry, I used to get quite a lot of satisfaction (or, in plain English, Schadenfreude !) from pointing out to younger friends that the use of cyclohexanol for solubilizing soaps in aqueous media was patented by a gentleman named Friesenhahn in 1922, or that the formation of interfacial emulsifying complexes was suggested by Schulman and Cockbain in 1940, or that the preparation of self-emulsifiable mineral oils by adding mixtures of soaps and phenolic compounds to the oil was known in 1887! I must not turn this into a lecture on industrial archaeology, but I hope that a few of my backward looks at emulsion systems may recall some worth-while targets and ideas for future research, and that retrospect may in fact lead to prospective ideas in this field.

2 Some thoughts on the classification of emulsions

We usually classify emulsions into general phase-relationship types such as oil-in-water and water-in-oil, with multiple emulsions such as water-in-oil-in-water thrown in for good measure, and then we go on to consider separately, under these headings, such topics as the preparation, stabilization, and destruction of emulsions, their practical uses, the selection of emulsifying agents, and so on. This classification is of course useful, and probably indispensable. But we must not allow it to lead to artificial distinctions or (to use a popular word) to a dichotomy in emulsion theory. For the same basic principles apply to the formation and behaviour of oil-in-water and water-in-oil emulsions. This becomes increasingly clear as our collection of water-in-oil emulsifying agents begins to catch up with our collection of oil-in-water agents, and as we see more and more clearly the overwhelming importance of steric stabilization, as distinct from electrostatic stabilization, in both types of emulsion. Let us consider for a moment a hobby-horse which I shall be riding again in a few minutes, viz. the preparation of “self-emulsifiable” compositions. This is of course a practical rather than an absolute concept: if a liquid (A) contains suitable additives, so that it will disperse in another liquid (B), which is immiscible with (A), either apparently spontaneously, or as the result of pouring (A) into (B) with very gentle agitation, we can describe (A) as being a self-emulsifiable composition. I think that 30–40 years ago, most of us would have said that (A) must be an oil, and (B) an aqueous medium, full stop. But now we have powerful agents (e.g. some of the specialized polyalkanoxy compounds) which will give us aqueous compositions which are self-emulsifiable when they are poured into water-immiscible liquids.

This brings us to an alternative method, not so much of classifying as of arranging emulsion systems in a sort of continuous spectrum, in terms of their degree of thermodynamical instability, or, if you prefer, in terms of the degree of steric and/or electrostatic stabilization which is needed if they are to survive. There is of course nothing new in all this: on the one side of the spectrum we shade off into miscellar solutions, via emulsion systems which are formed by self-emulsification, characterised by interfacial tensions which are too low to be properly measured, at present. At the other end of the spectrum, we have highly metastable emulsions, with boundary tensions running up into the tens of dynes per centimetre, prepared in ultra-high-pressure valve homogenizers or other special equipment, and stabilized by massive steric barriers, often formed by the adsorption of macromolecules. But, thanks to the progress in recent decades, our spectrum of degree of metastability becomes more and more obviously continuous, and cuts more and more clearly across the historical classification as oil-in-water or water-in-oil.

3 “Self-emulsifiable” compositions

Let us now examine the low-metastability end of our spectrum, what I am now calling “self-emulsifiable” compositions. I realize that this expression is, strictly speaking, self-contradictory, and it may be preferable to speak of “emulsifiable concentrates,” as does Ford in his paper to our Symposium. To me, “self-emulsifiable” is hallowed by usage, and emphasizes the fact that the user generally adds no further emulsifying agent–but I must emphasize that I use it in quotation marks.

The real distinction between a “self-emulsifiable” system and a micellar solution is that the state of the latter is one of genuine equilibrium and independent of its immediate history. In practice, this distinction may be difficult or even impossible to perceive, because it may take so long for equilibrium to be established. We have all had the experience, I expect, of adding more and more oil-soluble emulsifying agent to an oil, and then mixing with water, and eventually finding that we get very fine emulsions–even optically empty systems–apparently independently of how we mix the oil and the water. “Ah,” we say to ourselves, “these are thermodynamically stable systems.” But sometimes when we store these “emulsions” for a few months, we find that some of them remain stable, while others show some separation, or even collapse. And the onset of separation may not always correlate very well with the amount of emulsifying agent added. It seems to me in fact that methods for deciding whether we have an emulsion or a micellar solution–and in reasonable time-are an important target for future research.

As I indicated in my introductory remarks, “self-emulsifiable” oils are by no means new. They probably evolved, nearly a hundred years ago, from disinfectant emulsions, which were mixtures of water, water-insoluble phenolic compounds, and soaps. Some unknown person (unknown to me, at least) made the discovery that, if he mixed a mineral oil, phenolic bodies, and soap (including rosin soaps) in suitable proportions, he obtained a clear solution, which gave a very fine emulsion when it was poured into water. By the 1920’s mixtures of this type were available (often called “miscible oils”), which were used for making lubricating emulsions for jute-spinning and for metal-working. With the advent of much more effective agents (notably the polyalkanoxy compounds, of increasing complexity and elegance), and the stimulus afforded by the theory of interfacial complexes, “self-emulsifiable” compositions have become very important indeed–as will be clear from Ford’s authoritative review of pesticidal emulsions, which we are to hear tomorrow.

There appear to be two general approaches to explaining the behaviour of “self-emulsifiable” compositions. I think it was March who, in 1928, first examined the idea that, if oil–water interfacial tensions were small enough, emulsions might be thermodynamically stable, up to certain concentrations, because of the free energy absorbed by the Brownian motion of the droplets. (Or, to use a rather crude kinetic picture, the emulsion would be formed-and maintained–by disruption of the very fragile oil–water interface by the thermal motion of the liquid molecules.) I have tried very hard to persuade myself that this “Brownian free energy” approach is sound, for it has its great attractions, but is seems to me that it founders on the whole question of what we mean by an emulsion. An emulsion is surely by definition a two-phase system; if we now add one or more substances which cause the formation of a disperse or solubilized system which is thermodynamically stable, but which cannot be explained in terms of molecular solubilities, then it seems to me that we must think in terms of micellar solutions not emulsions. But...