A hydrophilic colloid, or hydrocolloid, is defined as a colloid system in which the colloid particles are hydrophilic polymers dispersed in water. Hydrocolloids can be either irreversible (single-state) or reversible. For example, agar is a reversible hydrocolloid of seaweed extract.
It can exist in a gel or liquid state and can alternate between states with either heating or cooling.
Many hydrocolloids are derived from natural sources. For example, gelatin is produced by hydrolysis of proteins from cows and fish, and pectin is extracted from citrus peel and apple pomace. Hydrocolloid-based medical dressings are used for skin and wound treatment. Biological molecules such as proteins contain both hydrophobic and hydrophilic parts. To allow the proteins to be suspended in the water in a cell, the protein folds such that the hydrophobic parts are in the middle and the hydrophilic parts are exposed on the surface. This keeps the protein in the solution.
A hydrophobic colloid, or emulsion, is defined as a colloid system where the colloid particles are hydrophobic polymers. Because the colloid does not interact with the hydrophilic aqueous solvent, hydrophobic colloids are inherently unstable and generally do not form spontaneously. Energy input -- through shaking, stirring, or homogenizing -- is needed to form the emulsion. Over time, emulsions tend to revert to the stable separated state of the substances comprising the emulsion. An example of this is seen in the separation of the oil and vinegar components of vinaigrette, an unstable emulsion that will quickly separate unless shaken almost continuously.
There are three types of instability in emulsions: flocculation, creaming, and coalescence. In flocculation, the dispersed phase comes out of suspension in the form of flakes. In creaming, one of the substances migrates to the top (or the bottom, depending on the relative densities of the two phases) of the emulsion. In coalescence, small droplets of colloid bump into each other and combine to form progressively larger droplets. In order for the emulsion to stay stable, additional substances are needed to stabilize the colloid.
An emulsifier is a substance that stabilizes the colloid so that it does not change significantly with time. One class of emulsifiers is known as "surface active substances," or surfactants. Soy and egg yolk lecithin are examples of surfactants. Surfactants are often used in food, such as mayonnaise and salad dressing, to keep the emulsions mixed over time. Detergents are another class of surfactants and will physically interact with both oil and water, thus stabilizing the interface between the oil and water droplets in suspension. This principle is also exploited in soap, to remove grease for the purpose of cleaning.