Consistency in the stability/structure characteristics of an emulsion is a necessary prerequisite to achieve the desired final product quality. Two reliable and well-established parameters that are intimately related to the physical stability and structure of an emulsion are particle (droplet) size and zeta potential (surface charge). Topical delivery systems (TDS) are many and varied; they include creams and lotions based on O/W and W/O emulsions. TDS are complex, multicomponent, heterogeneous compositions that require much iteration to attain the final desired formulation; many factors Cosmetic Delivery Systems Acoustic Attenuation Spectroscopy 24 play a role in determining the outcome. Additionally, cosmetic and pharmaceutical emulsions cannot be diluted without consequence to the droplet size and, hence stability. Additionally, product performance behavior such as delivery of actives, efficacy (e.g. SPF) and aesthetic acceptability (e.g. spreading) is directly affected by the droplet size (distribution). Unfortunately, current non-imaging instrumentation based on light scattering methods is severely limited in its application to such systems because of the requirement to dilute the system under investigation.
We report the results of a preliminary study of a variety of “real-world” cosmetic formulations, both O/W- and W/O-based emulsions, from sunscreens (with and without inorganic particulate actives) to moisturizers measured without dilution using a technique based on acoustic attenuation spectroscopy (AAS). A major advantage of the technique is the ability to study emulsion systems under flow conditions, allowing measurements under different shear conditions. The advantages and limitations of the method will be briefly reviewed. Data will be presented illustrating its relevance as a formulation aid to “fingerprint” emulsion composition. Further, a unique feature is the ability to non-destructively probe in situ structural characteristics of emulsions. This important new development will be discussed in light of its correlation with classical “shear” rheological measurements. AAS has deep common roots with rheology; ultrasound applies an extensional stress to the system on a very short time-scale. The output of AAS can be presented as “extensional” viscoelastic properties of the system at the frequencies within the megahertz region. These “extensional” viscoelastic properties can be used as a system fingerprint similarly to the classic “shear” viscoelastic properties.