Here are some factors that contribute towards emulsion stability:

Interfacial Tension - As indicated above, most emulsifying agents are amphiphilic compounds. They will concentrate at the oil-water interface, producing a significant reduction of the interfacial tension and will need less energy to form emulsions. Despite a lowering of interfacial tension when surface-active agents are added, the free energy of the interface remains positive, leaving a persisting state of thermodynamic instability.

Repulsion by Electric Charge - Emulsion stability is often explained by the presence of repulsive electrical charges on the surfaces of emulsion droplets. According to the DLVO theory, the dispersed particles are subject to two independent forces: the van der Waals force of attraction and the electrostatic force of repulsion arising from the presence of electrical double-layers at the particle surfaces. The net interaction between the particles is obtained by summing these two terms. If the repulsion potential exceeds the attraction potential, an energy barrier opposing collision results. If the magnitude of this energy barrier exceeds the kinetic energy of the particles, the suspension is stable. The van der Waals negative potential becomes significant only when the distance between the particles is quite small.

At intermediate distances, the repulsive potential is larger than the attractive potential. Attention should be taken on application of the DLVO theory, which was originally developed for inorganic sols (in which the dispersed phase consists of submicroscopic spherical solid particles), to emulsions (where the dispersed phase consists of oil droplets stabilised by adsorbed emulsifying agents). For example, in emulsions, coalescence involves disruption of an adsorbed film around the droplets, and calculations of the potential energy barrier opposing the collision of oil globules must take into account such factors as the distortion or flattening of the oil droplets upon close approach. However, the DLVO theory still provides a good approximation of the electrostatic contribution to emulsion stability.

Ionic surfactants contribute significantly to the stability of O/W emulsions by contributing to the establishment of electric double layers in the aqueous phase adjacent to each oil droplet. Reversibly, this mechanism is of little importance in the stabilisation of W/O emulsions, since the oil phase does not generally supply counterions in sufficient amounts to establish a strong potential gradient.

Stabilisation by Finely Divided Solids - Solid particles of very small size, as compared with the size of the dispersed oil droplet, can stabilise an emulsion by adsorbing at the interface to form a physical barrier around the droplets. In addition, energy is required to dislodge solid particles from the interface, since the oil/water interface must be increased to do so. Powdered silica, various clays, basic salts and plant cell fragments are examples of such agents.

The emulsion type produced and its stability depend largely on the relative abilities of the two phases to wet the solid particles. The phase that preferentially wets the solid particle tends to become the continuous phase. If the interfacial tension between solid and oil (g _SO) is greater than that between solid and water (g _SW), the contact angle (q) of the solid with the aqueous phase is less than 90º, and the major portion of the solid particle resides in the water phase, thus favouring an O/W emulsion. The converse takes place if g_SW > g_SO. Nevertheless, if solid particles remain exclusively in either phase, they have no stabilising effect. On the other hand, the most stable emulsion is formed when the angle of contact between the two liquids and the solid surface is close to 90º. The surface of the solid and its contact angle may be modified by adjusting pH and by adsorbing various amphiphilic compounds to its surface. Concentration and chain length of the amphiphile’s hydrophobic group are important in this regard.

Based on these considerations, it has been recommended that, for the preparation of emulsions stabilised by solid particles, a surface-active substance should be added that is soluble in the least- wetting (discontinuous) phase, and that the concentration of the surface-active agent should be adjusted to give a contact angle in the vicinity of 90º between the powder and the two liquids.
Several methods have been introduced to aid in the selection of an appropriate emulsifier, or blend of emulsifiers, for a given purpose. The most prominent of these is the one based on the relative importance of the hydrophobic and hydrophilic properties of the molecules (HLB system).