An emulsion is a dispersion of a liquid in another liquid with which it is not miscible. The liquids exist as seperate phases, one of which is in small droplets (the dispersed phase) throughout the other (the continuous phase). If the continuous phase is hydrophilic and the dispersed phase hydrophobic (aka lipophilic) then the emulsion is an oil in water emulsion, and will be miscible with hydrophilic liquids. If the continuous phase is lipophilic, and the dispersed phase lipophobic (aka hydrophilic) then the emulsion is water in oil and will be miscible with lipophilic liquids. This writeup will look at the preparation of an oil in water emulsion containing liquid paraffin as the oily phase, water as the aqueous phase, and acacia as the emulsifying agent. Two methods will be discussed. This is not a definitive guide to preparing emulsions, but it'll give you a start.

The emulsion
One hundred millilitres of the emulsion of the moment contains 50 mL of liquid paraffin. After the addition of active ingredients, preservatives, the emulsifying agent, flavours (emulsions are not generally for internal use, since the texture is generally considered unappealing in the mouth, so flavouring is not always necessary), and whatever else you might want to include, the volume is made up to 100 millilitres with water (volumes are arbitrary, but the ratio between one and another should be preserved. The metric system is used because frankly, the imperial system is a mess, and I won't use it).

The primary emulsion
The first step in the preparation is to make a primary emulsion. The primary emulsion contains the liquid paraffin, some of the water, and the emulsifying agent. Four parts of oil to two parts of water to one part of Gum is used. This equates to 50 mL of liquid paraffin, 25 mL of water, and 12.5 g of acacia (to produce 100mL of product).

The Wet Gum method of primary emulsion formation:
First, the acacia is weighed and placed in a large earthenware mortar. 25 mL of water is added to the mortar and the mixture is triturated to form a mucilage (using the pestle). Your triturating arm may hurt slightly after this step.
Second, the liquid paraffin is incorporated, with continuous trituration, in small quantities (about 5-10mL), with each quantity being emulsified prior to the addition of the next. Once the oil has all been added, the measure that held it is held so as to drain the remaining paraffin into the mortar. This is important to get the quantities right, unless you measure by difference, which is risky, or don't care about exact quantities (pharmacy is not rocket science, as my friend Bones put it, but don't be surprised if you're sloppy and it all goes pear-shaped). Your triturating arm is probably feeling something by now.
Thirdly, trituration continues. A good primary emulsion has no lumps and will allow the pestle to remain standing when it is placed upright in the mortar. It will also make a clicking sound, which reminds me of nothing so much as the sound a young man I used to know used to make by opening his mouth, gripping his cheek and rapidly moving it in and out. The sound he produced was supposed to signify masturbation, which made hearing it coming from all angles in a pharmaceutics lab kind of disturbing. A good indication that the primary emulsion is ready is intense pain in your triturating arm.

The Dry Gum method of primary emulsion formation:
The acacia is placed into a large mortar, as in the wet gum method. It is important that the mortar be very dry. And lumps are removed by trituration, then all of the liquid paraffin is added at once and triturated. Approximately ten seconds after the paraffin is added, the 25 mL of water is added, also all at once, and this is triturated. Until the emulsion forms, this mixture is pretty disgusting. Continue to mix until the emulsion clicks and the pestle stands in it. This method takes less time than the wet gum method, and so your arm hurts less afterwards. If you don't add the water quickly enough then the emulsion will not work. Make sure you measure the water out and have it handy before adding the paraffin.

The rest of the method:
With the primary emulsion formed, the rest of the method is the same in either case. Any remaining ingredients, such as the active ingredient and preservatives (a note on preservatives: there should be one that will dissolve into each phase. If this means you need two preservatives, then you'd best use two preservatives) are added. If there is still some leeway before you reach volume, then adding some of the water will make the emulsion easier to pour. The emulsion is strained (emulsions will not pass through a filter intact) into a calibrated bottle and made to volume. The bottle is shaken after sealing.

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