Surface active agents or surfactants are substances that bind to the surface substances or between the contacts of two substances and decrease surface tension. Surfactants are adsorbed on different types of contact between substances (solid - liquid state, liquid - liquid state, liquid - gas state). They are amphiphilic, meaning they contain both hydrophilic heads and hydrophobic tails. Surfactants are divided by the charge of the polar head in anionic, cationic, nonionic and zwitterionic. Surfactants can also form micelles, which are aggregates of several surfactant unimers. Micelles begin to form when critical micelle concentration (CMC) is reached. Surfactants can be combined into spherical micelles, worm-like micelles, inverted micelles, lamellar phase, continuous structure and vesicles.
Rheology is the study of the deformation of the material and deals with the response of a substance to deformation. We know elastic and plastic deformation. Newtonian fluids (water, oil, honey, polymer solutions…) are ideal liquids, independent of the strength, direction and time of the force acting on the system. For these liquids, the shear stress is proportional to the shear rate. Non-newtonian fluids are real fluids. Viscosity depends on the shear stress. Viscoelasticity can be described by the distribution of energy during loading. Part of the energy is converted into heat due to the viscosity friction; the other part of the energy is used to return the substance back to its original form.
The rheological behaviour of viscoelastic solutions is manifested in time-dependent functions, such as transient viscosity or relaxation model. The rheological properties of viscoelastic substances are shown by a mechanical model, which assumes that, by the linear relationship between stress and velocity, the elastic properties are given by Hooke's law and viscous properties by Newton's law. Such a model is Maxwell's substance, which describes the viscosity of surfactant solutions. Non-newtonian behaviour is reflected in solutions where viscosity decreases and the shear increases. For example, a solution of cetylpyridinium chloride with sodium salicylate or a mixture of sodium lauryl ether sulphate and cocamidopropyl betaine with added octanoic acid.