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In chemistry, emulsification is defined as the "an additive which helps two liquids mix" (Source: Wikipedia), "a mixture of two or more liquids that are normally immiscible owing to liquid-liquid phase separation"(Google's Knowledge Graph), and a substance that stabilizes an emulsion (see https://rockedu.rockefeller.edu/component/emulsifiers-and-surfactants/). In biology, however, bile is defined as an emulsifying molecule that aids in the digestion, or breakdown, of fats (see https://www.hopkinsmedicine.org/health/conditions-and-diseases/biliary-system-anatomy-and-functions and other similar sources). So, does an emulsifier stabilize, or keep together fats and hydrophilic molecules or does it break down the fat by surrounding it and dispersing it? It's almost as if there are two opposite definition of emulsifier in chemistry and biology.

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    $\begingroup$ Emulsification is a process, an emulsifier a substance that helps to form and stabilize an emulsion - a dispersion system liquid in liquid. $\endgroup$
    – Poutnik
    Commented Dec 4, 2023 at 4:50
  • $\begingroup$ An emulsifier stabilizes an emulsion which is a process of phase change. Molecules are not being broken down, they are just being rearranged between phases. $\endgroup$
    – Buck Thorn
    Commented Dec 4, 2023 at 5:00
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    $\begingroup$ Opposite? It's the same meaning, just perspective change. $\endgroup$
    – Mithoron
    Commented Dec 4, 2023 at 13:31

2 Answers 2

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It seems you have confused the related word forms.

  • Emulsification is a process of forming an emulsion.
  • An emulsifier is a substance that helps to form and stabilize an emulsion.
  • Emulsion is a heterogeneous dispersion system of two non-miscible liquids, with droplet size typically in range $\pu{10 nm} - \pu{100 \mu m}$.
  • To emulsify is performing emulsification to form an emulsion.

IUPAC quote from Wikipedia:

Note 3: The diameters of the droplets constituting the dispersed phase usually range from approximately 10 nm to 100 μm; i.e., the droplets may exceed the usual size limits for colloidal particles.

Emulsifiers have two major, related effects:

  • They act as surfactants, decreasing surface tension at phase boundaries. This cause decreasing the required work to divide droplets to smaller ones.
  • They act as stabilizers, adsorbing on phase boundaries, acting against merging two droplets at the contact.

These effects make the formation of emulsions easier and cause higher thermodynamic and kinetic emulsion stability.

The mentioned eventual break of fats by bile has two aspects:

  • Forming fat emulsions from oils and liquefied solid fats, breaking them macroscopically, mechanically.
  • Aside of emulsifiers, the bile contains also lipase to enzymatically process emulsified fats.

Wikipedia: Bile - function:

Bile or gall acts to some extent as a surfactant, helping to emulsify the lipids in food. Bile salt anions are hydrophilic on one side and hydrophobic on the other side; consequently, they tend to aggregate around droplets of lipids (triglycerides and phospholipids) to form micelles, with the hydrophobic sides towards the fat and hydrophilic sides facing outwards. The hydrophilic sides are negatively charged, and this charge prevents fat droplets coated with bile from re-aggregating into larger fat particles. Ordinarily, the micelles in the duodenum have a diameter around 1–50 μm in humans.

The dispersion of food fat into micelles provides a greatly increased surface area for the action of the enzyme pancreatic lipase, which digests the triglycerides, and is able to reach the fatty core through gaps between the bile salts. A triglyceride is broken down into two fatty acids and a monoglyceride, which are absorbed by the villi on the intestine walls. After being transferred across the intestinal membrane, the fatty acids reform into triglycerides (re-esterified), before being absorbed into the lymphatic system through lacteals. Without bile salts, most of the lipids in food would be excreted in feces, undigested.

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An emulsion is just a colloidal system of liquid dispersed in a liquid phase just like smoke is a colloidal system of solids dispersed in air. In emulsions, one of the liquid phase is of typical colloidal dimensions (nano to micrometer size). Thus emulsify does not imply chemical breakdown, it simply means that one of the liquid phase has been "physically" broken down into nano or microdroplets in another immiscible liquid. This is why vigorous mechanical forces are needed to emulsify or make an emulsion. Look up the recipes for making mayonaisse at home.

Bile acids emulsify fats means that they help in making fat and oils smaller and smaller (no chemical decomposition) in size during the digestion process so that the body can absorb them easily.

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  • $\begingroup$ Droplets of stable emulsions need not to have colloidal size. $\endgroup$
    – Poutnik
    Commented Dec 4, 2023 at 8:10
  • $\begingroup$ IUPAC quote from Wikipedia: Note 3: The diameters of the droplets constituting the dispersed phase usually range from approximately 10 nm to 100 μm; i.e., the droplets may exceed the usual size limits for colloidal particles. $\endgroup$
    – Poutnik
    Commented Dec 4, 2023 at 8:13

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