Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

May, 2005

Emulsion and Latex Paint

Q. I would like to know alil bit more about latex acrylic and latex emulsions and the main difference between the two kinds in detail.

A. Here are some definitions of emulsion. I'll give an explanation as it relates to paints later in this reply.


Liquid droplets dispersed in another immiscible liquid. The dispersed phase droplet size ranges from 0.1 - 10 µ m. Important oil-in-water food emulsions, ones in which oil or fat is the dispersed phase and water is the continuous phase, include milk, cream, ice cream, salad dressings, cake batters, flavour emulsions, meat emulsions, and cream liquers. Examples of food water-in-oil emulsions are butter or margarine. Emulsions are inherently unstable because free energy is associated with the interface between the two phases. As the interfacial area increases, either through a decrease in particle size or the addition of more dispersed phase material, i.e. higher fat, more energy is needed to keep the emulsion from coalescing. Some molecules act as surface active agents (called surfactants or emulsifiers) and can reduce this energy needed to keep these phases apart.


A suspension of small drops of 1 liquid in a 2nd with which the 1st will not mix. Emulsions can be formed either by mechanical agitation, or by chemical processes. Unstable emulsions will separate with time or temperature. Stable emulsions will not separate.


A mixture in which two immiscible substances, like oil and water, stay mixed together thanks to a third substance called an emulsifier. The emulsifier is usually something like a soap, whose molecules have a water-soluble end and an organic-soluble end. The soap molecules form little balls called micelles, in which the water-soluble ends point out into the water, and the organic-soluble ends point into the inside of the ball. The oil is stabilized in the water by hiding in the center of the micelle. Thus the water and oil stay mixed.

An emulsion is an unstable mixture of two incompatible liquids, such as oil and water. In the case of mayonaise, we add oil to egg yolk VERY slowly so that the "emulsion" won't break. In the case of a latex paint we add water to an organic resin, such as an acrylic in such a manner that the emulsion won't break. Since the resin is an oil-based product (ultimately derived from crude oil) we add a small amount of a coalescing agent that enables us to mix these two unlikely components. In the case of a latex paint the coalescing agent is compatible with the resin and is also compatible with water. For instance, household soap is compatible with grease and water. After we have mixed the latext paint, (which is an emulsion), we apply the paint and ideally allow the water to evaporate off before the coalescing agent can evaporate. Eventially the co-solvent (coalescing agent in a latex paint) evaporates and a uniform paint film is formed.

I hope this helps

Best wishes,

Ron Joseph

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