How To Control The Stability Of Cosmetic Formulations

Emulsion is a thermodynamically unstable system, and its life span is limited. It indicates that the stability of the emulsion is an important issue for cosmetic formulators. Commercial cosmetics must have a shelf life of 2-3 years. The shelf life of an emulsion can be defined as the length of time it takes for the emulsion to deteriorate to an unacceptable level for consumers. It takes a long period of time for commercial cosmetics to be distributed and stored to consumers. Generally, the shelf life of cosmetics is 2 to 3 years. Some countries require over-the-counter (OTC) emulsions containing sunscreen agents to have a stability of longer than 5 years. Since real-time shelf life measurement is time-consuming, generally cosmetic companies rarely conduct real-time shelf life research, so it is important to design tests that accurately predict the shelf life of emulsions. Although there are many such programs at present, there is still a certain distance from the reality, indicating that the stability of the emulsion is not a simple matter.

　　 (1) The mechanism of emulsion instability

　　 When the emulsion is aged, it undergoes physical changes to make the emulsion unstable. Including the effects of gravity stratification, flocculation and disproportionation.

The possible result of gravity separation is flocculation. Flocculation is a phenomenon of sedimentation or stratification. During flocculation, it is still emulsified, but the rich dispersed phase layer is concentrated in the upper layer (in the O/W emulsion), or in the lower layer (in the O/W emulsion). W/O emulsion). The changes caused by these two kinds of gravity are reversible, and shaking can re-disperse the aggregated materials. These conditions obey Stokes' law, that is, the separation speed is proportional to the density difference between the oil phase and the water phase, and the viscosity of the continuous phase is proportional to the particle size of the dispersed phase. Therefore, increasing the viscosity of the continuous phase, or reducing the particle size using colloid mills or homogenization can increase the stability of the emulsion.

　　 When the dispersed phase droplets are brought together and combined to form larger droplets, coalescence occurs. Eventually phase separation will occur. If the amount of emulsifier is not enough to keep the droplets small, but enough to prevent flocculation, or further coalesce into an emulsion with larger particle distribution, this situation is called limited coalescence.

　　The disproportionation effect is caused by the internal pressure of the droplet being higher than the external pressure of the droplet. This driving force will cause the chemical components to diffuse from small droplets to larger droplets, or possibly to the continuous phase. Since different components of the surfactant system may diffuse from small droplets to large droplets at different rates, as a result, small droplets become smaller and large droplets become larger.

　　(2) Emulsion stability test

　　 Many experimental methods that predict emulsion stability have been reported in the literature. It mainly includes accelerated test, real-time test and rheological measurement.

　　 1. Accelerated test

　　Accelerated test is a method based on applying load to the system, such as increasing temperature or centrifugal action. The inherent danger of any accelerated test is to subject the emulsion to conditions far beyond what is actually experienced. Therefore, the accelerated test results should be used with care, and the conclusions should be compared and confirmed by the results under normal storage conditions.

　　The temperature-based accelerated test uses constant high temperature,

(For example, 40°C or 48°C) or freeze-melt cycle (-15~5°C to room temperature). The theoretical basis for using high-temperature accelerated tests is the Arrhenius equation describing the relationship between the chemical reaction rate constant and temperature, that is, if the temperature increases by 10°C, the speed of most chemical reactions doubles. Therefore, 3 years at 20°C should be equivalent to 4.5 months at 50°C. However, as the temperature increases, the viscosity of the emulsion decreases and the balance of surfactant solubility changes. Other changes can also occur, such as the hydration of colloidal solids and polymers, the partitioning of molecules between the two phases, and the melting of waxes or other substances. Some of the main factors that make emulsions stable will suddenly disappear at high temperatures, resulting in emulsions becoming unstable.

　　 Accelerated testing sometimes produces some misleading information. Some emulsions that are also separated at high temperatures may have different shelf life at room temperature. Nonionic emulsifiers rely on the hydration of their polyoxyethylene groups to stabilize emulsions, and their selection is based on the interface characteristics at room temperature. At high temperatures, the hydration of emulsifiers becomes smaller, and their HLB values are completely different. They can be considered as molecules of different properties. In some cases, the emulsion is more stable at high temperature than at low temperature. In this case, if the conclusion is made only based on the high-temperature accelerated test, it is not only incorrect, but also misleading.

　　 The freezing-melting cycle also increases the load of the emulsion, and there are some inherent problems. At freezing temperatures, the formation of ice crystals in O/W emulsions may elongate and flatten oil particles. In addition, the lipophilic part of the emulsifier loses its fluidity, and the hydrophilic part spontaneously "dehydrates" due to the freezing and precipitation of water. If the emulsion "cures" before coalescence occurs, the emulsion will withstand the test. If the re-dissolution rate of the components is slow, the emulsion is unstable, which is different from the process that occurs at room temperature.

　　 Centrifugal action is another method that uses accelerated gravity to apply a load to the emulsion. The radius is 10cm, and the speed of 3750r/min for 5h is equivalent to 1 year for gravity. Therefore, this method can be applied to processes where the speed is decisive for flocculation and stratification. Some people use the ultracentrifugation method to quantitatively determine the stability of the emulsion. The time required is shorter than that of the traditional method, but more work is needed to study the correlation between the results and real-time tests, and use the correlation to predict the stability of the emulsion. The speed of ultracentrifugation is up to 25000r/min. Under such high-speed conditions, there is a difference between the stability of the emulsion and the stability under normal conditions; under such high-speed centrifugation, only coalescence is the decisive stage of speed, so droplets or particles They pile up quickly, causing the water medium to separate quickly. Under normal storage conditions, reversible flocculation and irreversible flocculation occur, both of which may be the decisive stage of the process speed. In addition, applying such a strong force will cause the leakage of the liquid film of the aqueous phase liquid between the droplets and cause the destruction of the emulsifier film.

　　Only by further studying the correlation between the results of the accelerated test and the results under general storage conditions, can the accelerated test be used correctly to predict the stability of the emulsion.