Emulsions: setting the record straight

Bill_TogeBill_Toge Member, Professional Chemist
During my time in the industry, I have noticed that the finer details of emulsion formation are often poorly understood, even by people who are otherwise very knowledgeable.

Given that it is a subject that is not generally taught in schools or universities, most people are not exposed to it in any detail prior to entering the industry, and explanatory texts that are geared towards industrial rather than academic chemists are few and far between, this is not entirely surprising.

As a result, I have found there are two particular mistaken beliefs, or canards, which are frequently cited as fact (even on this forum), despite being impossible to reconcile with orthodox science or actual evidence. With that in mind, I would like to take this opportunity to examine them in detail and set the record straight. 

Canard 1: O/W emulsions will invert to W/O if the oil phase comprises over 50% of the formula, and vice versa.  (Whether this means 50% w/w, 50% w/v, 50% v/v or 50 mole % is never clarified.)

The HLB of any emulsifier at a given temperature is constant; it is an intrinsic property of the emulsifier, and it does not depend on the relative proportions of the oil and water phases. 

Therefore, at a constant temperature (hence, constant HLB) it is physically impossible for an emulsion to invert once a given ratio of phases has been exceeded, because there is no way for the emulsifiers to ‘know’ how much of each phase is present.

If the HLB did somehow depend on the ratio of the two phases, the process of determining the HLB would be much more laborious than it is in practise, and the requisite calculations would be considerably more complicated. 

Plus, the system would be acting in a manner grossly inconsistent with the normal laws of physics.

Phase inversions are possible, but whether or not they occur depends on how the emulsifiers’ HLB varies with temperature, and the range of temperatures encountered during the emulsion’s formation.  

Canard 2: O/W emulsions are formed if the oil phase is added to the water phase, and conversely, W/O emulsions are formed if the water phase is added to the oil phase.

Before examining this statement in detail, it is important to bear two fundamental physical principles in mind.

In any given system, wherever possible, nature will act to a) maximise entropy (as per the second law of thermodynamics), and b) minimise the amount of free energy available for thermodynamic work. (The latter is known as the Gibbs free energy.)

Entropy in the thermodynamic sense is defined as the degrees of freedom available to a given system, in other words the range of movement available to its constituent atoms and molecules. 

A homogeneous mixture has more entropy than two separate phases, because in the latter case, the molecules at the interface of the two phases have restricted movement.

Because of this, nature will favour homogeneous mixtures unless they are physically impossible to form.

For an emulsion which contains high HLB emulsifiers, it does not matter which phase is added to which.  If high HLB emulsifiers are present, the most stable emulsion is the one in which the water phase is continuous, i.e. O/W, regardless of which phase is initially continuous

This is due to the fact that water has much stronger intermolecular bonds than oil, and so a continuous water phase has less energy available for thermodynamic work than a continuous oil phase, making it more thermodynamically favoured. 

It is also because water droplets have a greater tendency to aggregate than oil, again due to the strong intermolecular bonds. 

These statements also apply to mixtures of high and low HLB emulsifiers.  These could in principle form either O/W or W/O, but in practise they are always O/W because a continuous water phase is thermodynamically favoured.

If the emulsion only contains low HLB emulsifiers, it is not physically possible for O/W emulsions to be formed, simply because water does not mix with either oil or low HLB emulsifiers.

The only way to form a homogeneous mixture in such a system is through formation of a W/O emulsion, and this must be done by adding the water phase to the oil phase, so that the water phase remains dispersed and cannot aggregate.

In summary:

For systems containing high HLB emulsifiers, whether on their own or mixed with low HLB emulsifiers:

·         Oil into water forms O/W

·         Water into oil forms O/W

For systems containing low HLB emulsifiers only:

·         Oil into water separates

·         Water into oil forms W/O


This can be summarised even further:

The nature of an emulsion is determined solely by the nature of its emulsifiers.

UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries

Comments

  • Chemist77Chemist77 Member, PCF student
    What a gem @Bill_Toge, glad I clicked on it. 
  • Thank you so much @Bill_Toge .

  • Bill_Toge , not agree with Canard 1: Phase inversion can occur not only with tºc (with ethoxylated surfactants), but also by change in the composition by the so-called phase inversion composition (PIC) technique. It takes advantage of the change in surfactant curvature along dilution pathway. To be able to carry out a phase inversion by composition you must be aware of the phase behavior of your system (ternary phase diagram) You can find a lot of literature of this method (profesor Solans, Kunieda and Salager are reference in this field). Furthermore it is a very useful method since you can produce small and homogeneous droplet size without adding high energy to the system.

  • Bill_TogeBill_Toge Member, Professional Chemist
    @jeremien I don't dispute this phenomenon - in fact, I've often replicated it both at the bench and on the plant - but I do question certain aspects of how it's interpreted, namely the characterisation of the initial stage as a W/O emulsion.

    When carrying it out, I've noted two key observations:

    * In reported studies, and at the bench, the water mixes readily with the oil - even without any mechanical mixing. 

    (In sharp contrast, W/O macroemulsions do not form spontaneously, and generally need a lot of shear just to achieve a complete and uniform mixture.)

    * When water is added the oil, it forms a milky white dispersion, which fades due to the dilution effect when it's mixed into the oil - the oil phase slowly becomes more opaque as more water is added. 

    (In contrast, when the dispersed phase is emulsified of either type, the continuous phase almost immediately turns opaque at low concentrations and the opacity does not vary with subsequent additions; the dilution effect is not observed.)

    Because of these inconsistencies, and my earlier points regarding thermodynamic stability, my view is that the alleged W/O phase is actually a dispersion of an O/W emulsion in oil; the oil phase remains continuous at low concentrations because the droplets of the emulsion are too finely dispersed and distant from each other to form a continuous phase. 

    (N.B. This is not a double emulsion, as the O/W phase is not emulsified into the oil.)

    The inversion occurs when the dispersed emulsion reaches a high enough concentration for the individual droplets to coalesce, form a continuous phase and absorb the remaining oil.  This generally results in a sudden increase in viscosity, which generates enough shear force to produce very fine, homogenous droplets.

    So while in practise, the continuous phases do invert at a well-defined ratio, the fundamental structure of the emulsion is the same at all phase ratios - the variable element is how it is dispersed.

    Which leads me back to my original point: in systems where this behaviour can occur, the inversion point depends on the nature of system, and is not necessarily the point where the phases are present in a 1:1 ratio of any unit.
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • I've never made a o/w emulsion. I have a question:
    I have no intention of ever making a sunscreen, is there any reason I should have any interest in o/w emulsions?
    Cosmetic Brand Creation. Concept to name to IMPI search to logo and brand registration. In-house graphic design inc. Pantone specs. Cosmetic label and box design & graphics.
  • AmeenAmeen Member
    Bill_Toge , thank you so much,  You are far much better than a god, You are human.
  • @Belassi you inverted your letters, didn't you?

    I guess a good reason for making W/O emulsions would be producing very effective skin-barrier creams, for instance.
  • Sorry yes, I did. I meant w/o emulsions.
    Cosmetic Brand Creation. Concept to name to IMPI search to logo and brand registration. In-house graphic design inc. Pantone specs. Cosmetic label and box design & graphics.
  •   :)  
  • Bill_TogeBill_Toge Member, Professional Chemist
    @Belassi Unless you're planning to expand into barrier creams or colour cosmetics, probably not.
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • Bill_Toge  I agree, that in the industrial report, this in not consistent with a phase inversion, but look like a uncompleted emulsification process.  If the system contain only a high HLB surfactants you will never form a W/O emulsion. To invert the system by PIC you need equilibrate HLB surfactant system (low and high HLB in the mixture to get an average between 8 and 12) to form lamellar liquid crystal or bicontinuous microemulsion in the dilution pathway. 

  • johnbjohnb Member
    edited March 2017
    A Gooooooogle search using the keywords "emulsion technology" will give a number of hits, not all necessarily agreeing with the views expressed here.

    A simple step by step slide show (93 slides!) is given in https://www.slideshare.net/MarciaRebelo1/emulsion-technology and there are more in the R.H. panel.

  • Bill_Toge 

    Thank you so much for sharing this. I am beginning to tread the waters of making emulsions and I have made these mistakes you have showcased. From a thermodynamic perspective, it makes so much sense! But, when presented with all of the technical information out there, it can be a bit daunting. So, thank you again for helping me to understand this concept even further, you have my eternal gratitude.

    johnb 

    Thank you for posting that link! You have most definitely shown me the importance of keywords--I know, silly. That presentation is extremely informative and helps a great deal with the ambiguities regarding emulsions. 

    Bill_Togejohnb 

    You guys are rockstars! I can not thank you enough for offering this information to nubile cosmetic chemists like myself.
  • Very interesting and very helpful!
  • Bill_TogeBill_Toge Member, Professional Chemist
    @Chemist77 @JulietK @Ameen thank you all!

    @SVOrtega glad to help, but I think the word you were after was "newbie" ;)
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • @Bill_Toge
    Yes, that is what I meant... How embarrassing.. lol
  • Thanks so much some of it is still over my head but then so are the trees. Thank  you
  • @Bill_Toge Thank you for a great thread, but @SVOrtega better to be nubile!
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