RHEOLOGY OF COSMETIC PRODUCTS 489 hydrophobically modifi ed starch added to the different mixtures successfully reduced the drainage and the Ostwald ripening of the foams. Also, systematic model experiments showed that the surface tension and dilatational rheology were also strongly dependent on the presence of the additives. These effects were also strongly temperature dependent because drainage slowed down with the decrease in the temperature from 25 to 15°C. Most signifi cant drainage retardation was observed at 15°C in the presence of additives, where the surface tension was lowest and the surface viscoelasticity was highest. Drainage of thin horizontal fi lms was also dependent on the presence of additives and the tempera- ture. This was observed to be signifi cantly slower when the surface viscoelasticity in- creased. The obtained results demonstrated that additives leading to increased surface viscoelasticity of surfactant solutions could be used successfully for tuning the foam prop- erties of complex surfactant mixtures, of three and more surfactants even at alkaline pH, thus increasing the application range of the foams and ingredients to harsh conditions and requirements. Some mate rials are of interest for additional performance control of foams, and these include proteins. Proteins are excellent foaming agents because they strongly adsorb to the gas– water interface, and they tend to give good steric stabilization and some electrostatic Figure 2. Schematic d iagram showing interconnection between foam drainage, foam coarsening, and sub s e- quent rheology of foam.
JOURNAL OF COSMETIC SCIENCE 490 stabilization, and also the adsorbed fi lm tends to have a reasonably high surface rheologi- cal modulus which is because of interactions between the adsorbed molecules. Proteins adsorb well at the water–air interphase as a result of the hydrophobic patches they possess as a result of the hydrophobic amino acid deposits. Any factor that increases the exposure of the hydrophobic patches to the solvent will increase protein surface activity. Sánchez and Patino (48) show that there is a good correlation between the foaming of caseinate solutions and the rate of diffusion to the interface, which depends on the protein concen- tration and the state of aggregation of the caseinate. Foam as del ivery systems. Foams as de livery systems for actives have gained popularity in both cosmetics and pharmaceutical products because they possess advantages over tradi- tional methods of active or drug delivery such as gels, creams, and ointments. Foams, to a relatively high extent, eliminate negative sensorial attributes such as greasiness or tacki- ness. Although there is no direct connection between good foaming properties and good cleansing, it is important to note that consumers desire good foam generation and uncon- sciously link good cleansing to adequate foaming and any cleansing agent that does not generate adequate foam is termed an ineffective cleanser (49). There exist multiple applications of the foam technology to cosmetic products which include hard fi ne-pore shaving foam which helps uphold the hair during the shaving process. Hair mousse is able to give hair desirable shape and volume (50). For shampoos, good foamability of a shampoo formulation even under a strong fat contamination is of tremendous importance (51). There are also foam compositions for application to the skin as a barrier to skin irri- tants in the prevention of contact dermatitis caused, for example, by sodium lauryl sul- fate. Fowler (52) reported that a protective foam containing dimethicone and glycerine was shown to improve chronic hand dermatitis in individuals with previously uncon- trolled dermatitis, despite continuing their regular occupation. Cosmetic foaming com- positions can also contain keratolytics, lubricating agents, and germicide agents such as triclosan or sunscreens (53). EMULSION RH EOLOGY Emulsions a re heavily used in the cosmetic industry in formulating several kinds of cos- metic products. The composition of an emulsion plays a critical role in its rheological behavior, not only by infl uencing the long-term stability of droplets but also by affecting the kinds of interfacial interactions that exist between droplets through the emulsions’ overall interfacial structure (54). An interest ing aspect of emulsions is that although being composed entirely of viscous liq- uids or solutions, they can be made into soft solids that have tunable rheological properties which depend upon their compositions and fl ow histories. These solid-like properties emerge through crowding of deformable droplets as the droplet volume fraction φ is raised, which can lead to glassy behavior and, ultimately, jamming and deformation of droplets. The positional and interfacial structures of droplets in a concentrated emulsion, which are inherently linked to droplet size distribution through prior emulsifi cation and to be applied fl ow history, are crucial aspects that govern an emulsion rheological properties. Kim and Mason’s (55) review focuses on the important fundamental aspects governing the rheologi- cal properties of concentrated emulsions near and above the jamming transition. Developing a nd establishing a link between emulsion rheology and texture analysis could be really helpful to improve raw material choice for cosmetic formulations (56).
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