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Thread: The chemistry of this emulsion?

  1. #1 The chemistry of this emulsion? 
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    This isn't homework. It's just me trying to understand what has happened.

    I have some houseplants with mites and it appeared that there are several mineral oil based preparations available to treat them. Being the handy guy that I am, I found a formula for something online and made it. It went like this:

    185g tap water
    28 drops household ammonia
    2g gum arabic powder
    400g mineral oil from the drug store

    I put the water in a blender then added the ammonia. I started the blender then added the powdered gum arabic slowly and let it blend for a couple of minutes while I weighed the mineral oil. I added the oil and let the blender run for another minute or two. I got a sort of thick, white emulsion. I sprayed a little on just a few leaves and I'll watch and see what happens over the next few days. If it's all good, I'll spray the plants all over and start killing these critters.

    So, just for my own education,
    1.)What is the underlying chemistry of the emulsion process I followed?
    2.)What happened when I added ammonia to water? What was created?
    3.)What happened when the gum arabic powder was added to the blending water/ammonia? What was created?
    4.)What was happening in the blender as the mineral oil was added to the spinning contents of the blender?

    The weird ingredient volumes? I scaled down a formula that made 7 gallons. Seven gallons was a bit more than I needed for my 3 pepper plants.


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  3. #2  
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    Quote Originally Posted by bodangus View Post
    This isn't homework. It's just me trying to understand what has happened.

    I have some houseplants with mites and it appeared that there are several mineral oil based preparations available to treat them. Being the handy guy that I am, I found a formula for something online and made it. It went like this:

    185g tap water
    28 drops household ammonia
    2g gum arabic powder
    400g mineral oil from the drug store

    I put the water in a blender then added the ammonia. I started the blender then added the powdered gum arabic slowly and let it blend for a couple of minutes while I weighed the mineral oil. I added the oil and let the blender run for another minute or two. I got a sort of thick, white emulsion. I sprayed a little on just a few leaves and I'll watch and see what happens over the next few days. If it's all good, I'll spray the plants all over and start killing these critters.

    So, just for my own education,
    1.)What is the underlying chemistry of the emulsion process I followed?
    2.)What happened when I added ammonia to water? What was created?
    3.)What happened when the gum arabic powder was added to the blending water/ammonia? What was created?
    4.)What was happening in the blender as the mineral oil was added to the spinning contents of the blender?

    The weird ingredient volumes? I scaled down a formula that made 7 gallons. Seven gallons was a bit more than I needed for my 3 pepper plants.
    Interesting challenge!

    Possibly the key maybe the gum arabic. I looked this up on Wiki: Gum arabic - Wikipedia, the free encyclopedia

    and noted it can act as a thickener and also reduces surface tension. The latter suggests it would help emulsify immiscible liquids, such as oil and water. From the proportions of the mixture, which include more oil than water, and from your description of its high viscosity, it looks to me as though you have prepared an invert emulsion, i.e. one in which the oil is the continuous phase, with droplets of water dispersed in it, like mayonnaise, rather than the other way round (as with milk for example).

    I am not sure what reactions, if any, to expect between gum arabic and ammonia. It may be that making the water phase a bit alkaline somehow helps the emulsion stability, but I'm speculating. It could equally be that the ammonia helps kill the mites.

    I do recall from my years in the oil industry that we had products called "banana spray oils" that were often emulsifiable and seemed to work just by coating everything with a thin film of mineral oil - presumably stopping the pests from breathing, or from getting a foothold, or something.

    Sorry I can't be more definitive - maybe others can add something.


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  4. #3  
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    Thanks for the replies. Ammonia can be applied to plants as a source of nitrogen. I don't think it's used here in any way to kill bugs. There are many oil based things out there to kill mites and such and I think they all work by suffocating them. The oils have to be renewed a couple of times to get the hatched eggs. And I don't know if I have a reverse emulsion or not. I don't think that's what I was after, but I'm not sure.
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  5. #4  
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    Quote Originally Posted by bodangus View Post
    Thanks for the replies. Ammonia can be applied to plants as a source of nitrogen. I don't think it's used here in any way to kill bugs. There are many oil based things out there to kill mites and such and I think they all work by suffocating them. The oils have to be renewed a couple of times to get the hatched eggs. And I don't know if I have a reverse emulsion or not. I don't think that's what I was after, but I'm not sure.
    Yes, my recollection is dim but I rather thought with the banana spray oils the idea was have a low viscosity emulsion (oil in water) so that it could be easily sprayed. Then the water would evaporate and the oil droplets would be left behind, to form a film, which would suffocate the insect pests, as you say. It was the oil:water ratio you used, plus your comment about it being a thick emulsion, that made me think perhaps you had got an invert emulsion.

    P.S. I regularly try to make invert emulsions when I prepare salad dressing. When it doesn't work I get a normal one instead, which is much runnier - and tastes far more acid. I read somewhere this is the reason why commercial, bottled "salad cream" tastes so horrible (i.e. of bad vinegar), whereas mayonnaise is rather delicious. But I digress.
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  6. #5  
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    Gum arabic ( acacia gum) is a well known emulsion stabiliser and viscosity modifier. The form of the emulsion , that is water in oil or oil in water, is to some extent determined by the order of addition of the components, adding water to the oil phase will tend to give a water in oil emulsion for example. The stability of the resultant emulsion will be determined by the components, specifically which phase the emulsifier preferentially wets.

    Mayonnaise is delicious but salad cream, which is an entirely different concoction still has a place in my pantry.
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