Although egg whites separate from yolks better at refrigerator temperature, they whip to their maximum volume at room temperature. One reason is that the surface tension of the albumen is lower at room than at refrigerator temperature. Thus, small air pockets can be more easily incorporated into the albumen to form a foam.
A foam is a superstructure of bubbles. Each bubble is made up of a pocket of gas trapped inside a spherical film of liquid. Typically, the gas is air or carbon dioxide. The liquid cannot be pure water because its surface tension is so great that it's difficult to disperse pockets of gas within the water. Not so with albumen. Although it consists chiefly of water, it has enough other substances dissolved into it that its surface tension is sufficiently low to allow foaming.
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