The Formation of Stable W/O, O/W, W/O/W Cosmetic Emulsions in an Ultrasonic Field

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This paper presents basic research results for production of W/O, O/W and W/O/W cosmetic emulsions under ultrasonic irradiation. A technology for the continuous and batch treatment of fluid mixtures with ultrasound was characterized using cosmetic emulsions as model systems. If cavitation is the dominant mechanism of droplet disruption, these results have to be taken into account for an optimal design of the emulsification apparatus geometry. The theoretical model supports experimental work. The experimental findings prove that final drop size essentially depends on specific power density. Drop size decreases with increasing residence time in the ultrasonic field until a system specific, minimum drop size is obtained. Ultrasonic irradiation leads to turbulent flow conditions on a macroscopic and a microscopic scale. The model based on the theory of turbulent drop bread-up predicts drop size and can be used to design equipment for ultrasonic emulsification. Emulsion used were stabilized using a combination of hydrophilic and hydrophobic surfactants. The ratio of this surfactants is important in achieving stable cosmetic emulsions.

It was shown that emulsifier concentration plays great role in controlling the emulsification processes as well as the structured features of the droplets.

The rheological behavior and morphological evolution during the ultrasonic emulsification were characterized systematically. The rheological properties were used to asses emulsion stability.

Introduction

A wide variety of cosmetic emulsions are used as bases for skincare products for healthy and diseased skin. These products can range in consistency from a cream to a lotion or milk and even a fluid. Since today's trends are increasingly short-lived, a manufacturer of cosmetic products needs to respond quickly to new market demands. This means that development times must be drastically reduced while maintaining the same high product quality. To be able to develop cosmetic products quickly and effectively, developers need reliable methods to obtain product stability without the need of long time testing. Here rheology has gained in importance in recent years (Kita et al., 1977, Pal, 1993, Princen, 1985).

In the past few years micro- and nanoemulsions have been the subject of extensive research work, because of the vast possibilities they offer. Their properties such as improved drug solubilization and bioavailability make them very attractive for application in personal care and cosmetics as well as in health care, because, due to small size of droplets, they can penetrate through the body membranes which enables transport of active substances. Small droplet size enhances emulsion stability since the Brownian motion may be sufficient for overcoming gravity; it means that creaming or sedimentation should not occur even on prolonged storage, also the small size prevents flocculation of the droplets as well as their coalescence. The long-term physical stability of fine emulsions (with no apparent flocculation or coalescence) makes them unique and they are sometimes referred to as ‘approaching thermodynamic stability’.

The recent surge in the cost of energy has led to an increased interest in emulsification processes. Intensive research on methods of carrying out this kind of processes ensures optimal utilization of resources, as well as externally applied energy. The application of ultrasound as a final step of emulsification, produces an emulsion with very small droplets, which has the effect of increasing the area of interfacial contact, and, hence, the efficiency of emulsification. The theory of emulsification by ultrasonic waves has not been fully elaborated as yet, owing to the complexity of the mechanisms involved. Cavitation is likely to control droplet disruption in sonicated liquid–liquid systems. The current view is that, apart from cavitation, it is the interfacial capillary waves that are responsible for the process of dispersion in a liquid-liquidsystem in an ultrasonic field (Li and Fogler, 1978a, Li and Fogler, 1978b, Mason, 1991, Tal-Figiel, 1990, Tal-Figiel, 2005). These waves spread out across the interface without penetrating into the bulk phase. An increase of ultrasonic intensity and, hence, of the amplitude of the capillary waves, eventually results in droplets of liquid breaking away from the crests of the wave, and their ejection into the dispersing phase.

The research results presented hereunder demonstrate the principles of formation of cosmetic fine emulsions by mixing and ultrasound. The objective of this work is to asses the impact of various physico-chemical properties of liquid, its flow rate, the irradiation time, amplitude or mean specific power density <ɛ> of ultrasound and apparatus geometry on the droplet size. This knowledge may prove helpful in composition and quality control.

Section snippets

Mechanism of Emulsification

To prepare an emulsion, oil, water, surfactant and energy are needed. The method of mixing those components is very important. The formation of large droplets, as is the case of macroemulsions, is fairly easy and hence high speed stirrers are sufficient to produce the emulsion. The formation of small drops is difficult and requires a large amount of surfactant and/or energy. To obtain emulsion with mean droplet diameter in the range 100–500 nm, ultrasound or high pressure homogenization should

Model of Drop Break in Ultrasonic Field

There are several possible mechanisms of droplet formation and disruption under the influence of ultrasound. One is the formation of droplets as a consequence of unstable oscillation of a liquid–liquid interface. These capillary waves may occur and contribute to dispersion, only if the diameter of droplets to be disrupted is larger than the wavelength of the capillary waves. A mechanism related to that of capillary waves is the oscillation and subsequent disruption of whole droplets due to the

Methods and Materials

An experimental investigation of cosmetic type emulsions: O/W, W/O and W/O/W obtained by two step method—preliminary mechanical mixing and final ultrasonification, has been carried out. Table 1 shows all components of emulsions investigated in this work.

The viscosity of the continuous phase was varied by addition of glycerol to the water phase in case of O/W systems and by using different oils for the W/O systems. Interfacial tension was lowered by adding mixture of commercially available

Experiments

The objective of this study was to evaluate the stability of W/O, O/W and W/O/W emulsions using droplet size analysis, creaming volume and rheological properties. On the basis of droplet size distribution, their mean diameter d32 was calculated. Experiments using ultrasound at various disperse phase content ϕ values at constant initial mass showed that the resulting droplet diameter increased and the dependence on energy consumption became weaker. A comparison of the average droplet diameter

Discussion and Conclusions

In the present study a technology for the continuous and batch treatment of fluid mixtures with ultrasound was characterized using cosmetic type O/W, W/O and W/O/W emulsions as model systems. This method is well suited for aseptic processing, an important issue in cosmetic and pharmaceutical development and production.

Ultrasound emulsification is an efficient method of obtaining finely dispersed emulsions. If cavitation is the dominant mechanism of droplet disruption these results have to be

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