A gel bridges the gap between a conventional liquid and a solid. Although it appears and behaves much like a solid, a gel is mostly liquid by mass. This unique semi-solid nature is achieved through the formation of an internal, stabilizing network within a fluid medium. This structure prevents the fluid from flowing freely, allowing the material to maintain a defined shape.
The Defining Molecular Structure of a Gel
The defining feature of any gel is its biphasic composition, consisting of a continuous solid phase and a dispersed liquid phase. The continuous phase forms a three-dimensional, interconnected scaffolding, often composed of long polymer chains. This microscopic network acts like a molecular sponge, physically trapping the liquid solvent within its pores and preventing its movement.
The stability of this solid framework is achieved through cross-linking, which connects the individual polymer strands. These connections can be chemical, involving strong, permanent covalent bonds that create an irreversible structure. They can also be physical, relying on weaker, reversible forces like hydrogen bonds, ionic interactions, or chain entanglements.
Unique Physical and Chemical Properties
The internal network structure grants gels measurable behaviors unlike those of simple solids or liquids. This includes viscoelasticity, meaning the material exhibits both viscous (liquid-like) flow and elastic (solid-like) shape recovery when subjected to stress. The polymer network contributes elasticity, allowing the gel to spring back after minor deformation. The movement of the trapped liquid provides the viscous component, allowing the gel to maintain its shape but deform significantly without breaking.
Gels also exhibit significant interactions with their surrounding liquid environment, demonstrated by the phenomena of swelling and syneresis. Swelling occurs when the dry polymer network absorbs a compatible solvent, expanding its volume as the solvent molecules are drawn into the network’s pores. Conversely, syneresis describes the process where a gel spontaneously shrinks over time, expelling some of the trapped liquid as the polymer network contracts. This is often an attempt by the network to reach a more stable, less strained configuration.
A third remarkable behavior is thixotropy, a time-dependent change in viscosity unique to certain gels. A thixotropic gel acts as a solid or highly viscous material at rest but temporarily liquefies, or shear-thins, when agitated or stressed. This transition occurs because the mechanical force temporarily breaks the weak physical bonds in the network. Once the stress is removed, the network slowly rebuilds its structure, and the material reverts to its more viscous, solid-like state over time.
Categorizing Gels by Composition
Gels are broadly classified based on the chemical nature of the solvent trapped within the network. The most common category is the hydrogel, defined by having water as the primary liquid medium. Hydrogels are formed from hydrophilic polymer chains, allowing them to absorb and retain vast quantities of water, often exceeding 90% of their total mass.
In contrast, organogels utilize an organic liquid, such as oil or alcohol, as the dispersed phase instead of water. These gels are formed by polymer networks or self-assembling small molecules that are compatible with the organic liquid. Organogels are valued for applications requiring water resistance or the delivery of non-polar active ingredients.
Two special classes of gels are distinguished by the removal of their liquid phase. A xerogel is the solid residue left behind when the solvent is removed through slow, conventional drying. This process typically leads to significant shrinkage and collapse of the original network structure.
Aerogels and Xerogels
An aerogel is created when the liquid is removed under supercritical conditions, which avoids the surface tension forces that cause network collapse. This specialized process results in a material with extremely high porosity and very low density, closely resembling the original expanded gel network.
Everyday Uses of Chemical Gels
The customizable structure and properties of chemical gels make them indispensable across numerous industries. In food science, gels like gelatin, pectin, and starch are used as gelling agents to create desserts, jams, and to provide texture and stability in many processed foods.
The medical and pharmaceutical fields utilize highly specialized hydrogels extensively due to their biocompatibility and high water content, which mimics natural body tissues. Applications include soft silicone contact lenses and advanced wound dressings that keep a wound moist to promote healing. Gels are also formulated as sophisticated drug delivery systems, where the polymer network is designed to release a therapeutic agent slowly and controllably over time.
Consumer products rely on gels for their unique texture and flow properties, particularly thixotropy. Products like toothpaste and hair gel are formulated to maintain a solid-like shape in the container but become fluid and spreadable when pressure is applied during use. Similarly, certain paints and cosmetics are designed as thixotropic gels to prevent dripping and ensure easy application before quickly setting again.