Does it really matter which phase is initially added to the other?

Now that I have been experimenting with formulations for about eight years, I am beginning to question some basic things that I was taught at the outset.  I will try to keep my inquiry brief, and keep in mind that this applies to all oil-in-water emulsions and water-in-oil emulsions, as this is an issue of chemistry and physics.

Here is  my question:  I am wonder how much (if at all) does it really matter whether you pour the oil phase into the water phase, or the water into the oil phase, while creating an emulsion.

In this case, the following can be assumed:
1) Both phases were measured separately and heated up in their own separate vessels, up to the necessary temperature (i.e. - highest melting point of the ingredient with the highest temperature requirement for liquifying).
2) After pouring one phase into the other, the high-shear mixing begins within seconds of this action.

NOTE:  On this issue, I have received contradictory responses from many other sources (other formulators and lab technicians).  I am always reluctant to conclude someone is wrong on any issue (unless it's a clearly objective fact, like a mathematical calculation that doesn't add up).  I have found in most cases the differences are a mix of facts and opinions, and they are simply talking about different things from different perspectives, and may have not realized it.

More specifically, my question involves the following concepts:

1)  The emulsifying agents are really what determines whether you have an oil-in-water, or water-in-oil emulsion . . . correct?  If your emulsifiers have an HLB value that is below 6/7, then you are pulling water in the lipids (the continuous phase), right?  And if the emulsifiers have an HLB value that is higher than 9/10, you are pulling lipids into the water phase (continuous phase), right?  This is assuming that you have an adequate amount of emulifying agents to get the job, and create a stable emulsion.

2)  So whether you begin by adding one phase or the alternative into the other vessel, they both end up in the same place, and as long as the heat is higher than needed to keep everything melted, then it's really the high-shear mixing (the heat and energy) that form the emulsion . . . correct?

3)  Without getting to complicated, some people have commented that this may create an issue with developing an emulsion that is actually closer to a "WATER-IN-OIL-IN-WATER" (W/O/W) which is uncommon.  Don't know if anyone here has ever tried to create an w/o/w emulsion, but that may be a separate discussion if it's not relevant to this specific topic.

Hopefully someone will have an articulate scientific explanation for this inquiry.

Comments

  • PharmaPharma Member, Pharmacist
    Now that I have been experimenting with formulations for about eight years, I am beginning to question some basic things that I was taught at the outset.  I will try to keep my inquiry brief, and keep in mind that this applies to all oil-in-water emulsions and water-in-oil emulsions, as this is an issue of chemistry and physics. Not chemistry and not schoolbook physics but, well, it's complicated and not even todays supercomputers are able to calculate these phenomena. At best, mathematics and physics can be used to explain an observed phenomenon.

    Here is  my question:  I am wonder how much (if at all) does it really matter whether you pour the oil phase into the water phase, or the water into the oil phase, while creating an emulsion. Sometimes it does, sometimes it doesn't and sometimes pre-mixing with low sheer is required prior to homogenisation.

    In this case, the following can be assumed:
    1) Both phases were measured separately and heated up in their own separate vessels, up to the necessary temperature (i.e. - highest melting point of the ingredient with the highest temperature requirement for liquifying).
    2) After pouring one phase into the other, the high-shear mixing begins within seconds of this action.

    NOTE:  On this issue, I have received contradictory responses from many other sources (other formulators and lab technicians).  I am always reluctant to conclude someone is wrong on any issue (unless it's a clearly objective fact, like a mathematical calculation that doesn't add up).  I have found in most cases the differences are a mix of facts and opinions, and they are simply talking about different things from different perspectives, and may have not realized it.

    More specifically, my question involves the following concepts:

    1)  The emulsifying agents are really what determines whether you have an oil-in-water, or water-in-oil emulsion . . . correct? Sometimes right, sometimes wrong If your emulsifiers have an HLB value Stop thinking in HLB unless you use PEG-based emulsifiers that is below 6/7, then you are pulling water in the lipids (the continuous phase), right?  And if the emulsifiers have an HLB value that is higher than 9/10, you are pulling lipids into the water phase (continuous phase), right? Wrong. Lipids remain in the oil phase and water remains in the aqueous phase whilst emulsifiers are supposed to be in the interphase (depending on the emulsifier/co-emulsifier, the bulk may form a depot in either phase) This is assuming that you have an adequate amount of emulifying agents to get the job, and create a stable emulsion.

    2)  So whether you begin by adding one phase or the alternative into the other vessel, they both end up in the same place No, they don't, it's not that simple. Emulsions are semi- or even unstable systems, not equilibrium states, and as long as the heat is higher than needed to keep everything melted, then it's really the high-shear mixing (the heat and energy) that form the emulsion . . . correct? Wrong. If you think in HLB, than you use PEG-based emulsifiers which have a phase inversion temperature (higher or lower than melting point possible) above which the o/w system is inversed = a o/w system which phase inverses when dropping below the PIT resulting in a fine o/w emulsion. This isn't true for other systems. Because an emulsion is a system which carries energy, it depends on how you obtain it and there are different emulsions which can be obtained.

    3)  Without getting to complicated, some people have commented that this may create an issue with developing an emulsion that is actually closer to a "WATER-IN-OIL-IN-WATER" (W/O/W) which is uncommon.  Don't know if anyone here has ever tried to create an w/o/w emulsion, but that may be a separate discussion if it's not relevant to this specific topic. That's yet another thing not necessarily related to this topic.

    Hopefully someone will have an articulate scientific explanation for this inquiry.

    See comments in italic.
    Read THIS if you want to understand more.
  • Thanks for putting your answers in italics throughout the text so I can see what you are specifically responding to for each part of the inquiry.  Yes, I was only referring to formulas use non-ionic emulsifiers, specifically the PEG or other ethoxylated emulifiers.

    I am thinking of moving towards other emulsification systems because at least if they work or don't work, you don't have to spend so much time obsessing over the issues related to peg-based non-ionic emulsifiers.

    I know there are pros and cons to both, which keeps it interesting, but challenging.

    Thanks for your feedback.

  • PharmaPharma Member, Pharmacist
    Uppss... Typo-alert: just realised that the second 'o/w' of the three should be w/o (i.e. the hot PEG emulsion).
  • Thanks, because I was a little confused; now that makes sense.
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