Hydrocolloids are hydrophilic polymers derived from plant, animal, microbial, or synthetic sources. When added to water, hydrocolloids disperse evenly as microscopic particles. At sufficiently high concentrations, the polymers become entangled with each other, forming loose networks that change the flow and spread properties of solutions. Many hydrocolloids, such as gelatin and pectin, can form gels by hydrogen bonding within and between polymers. The structure, charge, and concentration of a hydrocolloid and its interactions with other ingredients determine the rheology of the solution.
In addition to acting as rheology modifiers, hydrocolloids serve other functions in personal care products. By surrounding oil droplets in oil-in-water emulsions, hydrocolloids can prevent the droplets from coalescing. This droplet stabilization helps prevent products from separating, settling, or clinging to their container and improves their heat resistance. Some amphiphilic hydrocolloids, such as acrylate copolymers, can actually penetrate the oil-water interface of a droplet. By reducing the interfacial tension, amphiphilic hydrocolloids can make it easier to produce small oil droplets in an aqueous solution.
Another important function of hydrocolloids is to reduce the tendency of a fluid to thin upon agitation—the so-called thixotropic behavior.