Dear, Titus, being physical chemist, with some theoretical & practical experience in the field, I guess that the kinetic energy needed to divide the dispersed inner phase is more efficiently transffered through a low viscosity (e g water), continuous phase than through a more viscous one (e g oil). Therefor I would expect sonication of an o/w emulsion to result in smaller droplets than for a w/o emulsion. / Johan.

O/W vs W/O is controlled by surfactant solubility in the two phases. Bancroft's rule states that the phase containing the surfactant will become the external phase. Check the HLB of the oil. It should have two HLB values, one as the internal phase (higher HLB), one as the external phase (lower HLB). Oil external requires more oil soluble surfactants.

No

the emulsion stability depends on the shear rate and viscosity of continuous phase, the W/O requires high shear rate while o/W require low shear and sonication same like shear.

@Ayman: I'd like to see your evidence for that point of view.

There is a series of papers on emulsion inversion as a function of many parameters by a team led by Lionel Choplin. Some of them with Tyrode on the author list are available on Researchgate. They show complex how it is to predict which form (O/W or W/O) will occur. I still maintain that sonication does not favor one form over the other. The result depends on the full set of formulation parameters.

I agree with Alan parker, sonication works both ways and factors such as those described by Jeffrey Harwell are more significant.

Thank you Alan to directing to informative articles and Eric Tyrode for providing them.

As written I started the discussion for a defined system with given emulsifier and oil.

Then for a given system what influences what emulsion type will be formed?

Searching the 'classical' literature in between I found several interesting information.

http://journal.scconline.org//pdf/cc1958/cc009n03/p00141-p00148.pdf

THE EFFECT OF THE NATURE OF THE EMULSIFYING AGENT ON EMULSION INVERSION

Paul Becher

For his procedure of emulsification (good stirring, continuous addition of water or oil) he found that for 1 % Span 80 mineral oil water system only above 90 % oil w/o is formed and below 20 %. In between o/w or w/o (guess it means both are present).

http://journal.scconline.org//pdf/cc1961/cc012n04/p00193-p00207.pdf

WHAT DETERMINES EMULSION TYPE?

J. T. Davies

in a colloid mill type machine 'the emulsion type may vary with

(a) the phase-volume of oil, o, in the system,

(b) the chemical type (H.L.B.) of the emulsifying additive,

(c) the viscosity of the oil, and

(d) the wettability of the surfaces of the rotor and stator plate'

For glass and Span 80 he found an inversion point at about 50 % oil, but on oil wet Perspex plates at about 15 % oil.

This leads me to the assumption that the polarity of the sonic tip (metal) may have an important role in preferring o/w. Shaking only in a glass tube 1/1 mixture oil/water with 1 % Span 80 dissolved in oil we get a layering of water with few large oil droplets, o/w layer, w/o layer and oil on top after staying over night.

With emulsification by sonication for 10 minutes we get a pure o/w emulsion, which conserves this nature even at temperatures of 60°C. We see only phase inversion at this temperature when concentrating the oil droplets by centrifugation for prolonged time. However, at lower temperature no inversion occurs even after concentration in the cream layer after prolonged centrifugation.

I find this amazing.

Any further input is highly welcomed.

Titus, to my experience there is no straight answer to your question. I worked with sonifier for three years and tried to produce w/o nano-emulsion which all failed even at low concentration.

Regarding the effect of oil viscosity, as proposed by Johan and Biplap, I have used a wide range of silicon oils with different viscosity; lower and higher than water. All of them produced o/w emulsion (AICHE 56(10):2751 - 2755).

In addition, sonication results in a high coalescence rate. for my formulation (consisted of C12/SDS/water) I could only produce emulsion with 50% oil phase ratio at 1% SDS while increasing the SDS concentration to 10% I could produce emulsion with 70% oil phase ratio. I think that coalescence has a great role in formation of reverse emulsion.

Cavity can occur in the oil phase which disrupts the oil phase but at the end it is the surfactant which rule the system and the immediate curvature of the interface which I think is the most important factor. For my system with SDS I did not end with reverse emulsion but a top layer of oil at the end of the emulsification ! So, I conclude that surfactant and its packing parameter (determining the spontaneous curvature at the interface) is an important factor and I stick to the Bancroft's rule.

To me, the µ-emulsion is the first choice. They are nice thermodynamically stable one-phase liquids, containing at least water, oil and surfactant. Since they are thermodynamically stable, there´s no need for sonication or other mechanical dispersion. If you just find the right composition, they will form spontaneously.

@Johan

maybe for you microemulsions are the first choice. Certainly they have an important place also in practice, however, 'real' emulsions have a much larger importance. One main reason for this is the need to use high surfactant concentrations.

@Elijah

For sure curvature determined by packing parameter is of great importance, however, as shown surface properties of the device and container can have a large impact. Maybe by promoting preferential coalescence of o/w or w/o, respectively?

During work for a master thesis I supervise, it could be seen that commercial anionic surfactant LABS-Na (Marlon A) produced only emulsions with low coalescence stability. I assume that in your case changing from SDS to a nonionic surfactant might increase coalescence stability considerably. Regarding phase inversion we were surprised that with Marlon A it seems that for nonane-water emulsions there is a phase inversion from o/w to w/o upon concentrating the cream layer by centrifugation, which again reverses to o/w upon release of centrifugal pressure. (not seen for paraffin oil-water). Maybe a microemulsion phase is involved here.

Dear Titus, as you reduce the oil content in the w/o emulsion, by creaming separation, the emulsion could more easily revert to o/w. I agree with you that the surface of the container significantly could have an influence, depending on its hydrophilic/hydrophobic nature and its specific area. And finally you know as well as I, that µ-emulsions are more easily formed, using ionic surfactants. It's mostly in pharma- and cosmetics that nonionic surfactants are preferred in µ-emulsions.

Titus, you described a very interesting phenomenon. I haven't worked with Marlon A and nonane and not familiar with their properties. I think a change in the packing parameter of surfactant at the interface due to thermodynamic variation (i.e. change in pressure during centrifuge) might be the reason for this phase inversion. Do you think that this might have any effect in your system?

Increasing the oil water ratio one usually has to expect at some point phase inversion. This is done by concentrating the cream layer by centrifugation. However, with anionic surfactant I did not expected it because in other system silicon oil-in-water emulsion stabilized by nonionic surfactants I experienced that addition of anionic surfactant, e.g. Marlon A, totally prevented the phase inversion.

On the other hand from a solubilization experiment I did long ago in nonane- (technical) LABS-Na system it seemed that nonane is solubilized in a lamellar system, i.e. molar amount of nonane half of that of the surfactant.

You are right pressure, especially in nonane system, might play an additional role, though pressure should be relatively low at about 2000 times earth gravity.