Transparent cosmetic systems are of great interest not only for the detergent market (micellar systems) but also for skin care treatments (micro and nano-emulsions).
Their characterization is very important to evaluate some properties relevant for use.
In fact, during the research and development phases of a cosmetic product, the formulator must ensure quality standards and an adequate shelf-life. Nano-emulsions, for example, despite being transparent, are thermodynamically unstable systems. Monitoring the average diameter of the formulation’s particles is therefore useful for predicting any product instability and preventing particle aggregation phenomena.
Moreover, in the case of detergents, the volume of the micelles formed by the surfactants that compose it can influence the skin compatibility of the product, as the greater the diameter of the micelles, the less likely their ability to remove the barrier lipids of the stratum corneum will be.
For this reason, dimensional analysis can prove to be a simple and fast approach that is very useful for the characterization of nano-dispersed systems and for ensuring the chemical-physical stability of the product over time and compliance with its specifications.
In our laboratories, dimensional analysis is carried out using Dynamic Light Scattering (DLS), a non-invasive instrumental technique for measuring the hydrodynamic diameter of typically sub-micronic nanoparticles or colloids, dispersed or solubilized in a liquid. The sample is illuminated by a laser beam and the variations in intensity of the scattered light from the sample, generated by the Brownian movement of the particles, are measured as a function of time. At the same temperature and viscosity, the smaller particles move very quickly, generating rapid variations in the scattering intensity, while the larger particles move more slowly, generating delayed intensity variations. The signal is sent to an auto-correlator, which calculates the diffusion coefficient of the particles, which is then converted into a hydrodynamic diameter.
The use of this technique represents an innovative approach, which can efficiently integrate the accelerated stability assessments commonly performed during the development of a cosmetic product.
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