Investigation of rheological and particle characterisation techniques for the optimization of accelerated stability testing of sunscreen emulsion

Abstract

Skin cancer has become an increasing health concern as a result of exposure to ultraviolet (UV) radiation. This has motivated the development of sunscreen emulsions which shield humans from potentially harmful radiation. Sunscreen emulsions are composed of active ingredients (UV filters), stabilizers (emulsifiers, thickening agents, solvents and preservatives) and sensory enhancers such as moisturizers, fragrances and emollients. They are available in different forms namely, emulsions (creams and lotions), gels, aerosols, solid sticks, oils, mousses and ointments. Sunscreens are categorized based on their appeal to customers, varying ingredients and how they are formulated. Sunscreen emulsions are thermodynamically unstable and tend to separate into oil and aqueous phases as a result of various mechanisms such as creaming, sedimentation, flocculation, coalescence and Ostwald ripening. Surfactants or emulsifiers, such as cetearyl alcohol, sodium dodecyl sulphate, and sorbitan oleate, play a crucial role in stabilizing emulsions to prevent them from separating. Before sunscreen emulsions are released to the market, they are subjected to a variety of tests to ensure their quality, safety and adequate shelf life. This study aimed to investigate whether alternative analytical techniques can predict the stability of sunscreen emulsions in a short period as opposed to the currently used accelerated tests that require about three to six months.

The focus of this study was to test various analytical techniques to characterize sunscreen emulsions, as an important local example of cosmetic emulsions, and thereby predict their shelf lives. The analyses were performed in comparison to the accelerated stability tests that are frequently practiced in cosmetic industries. Accelerated stability tests were conducted by exposing the formulations to different temperature conditions (5, 25, 40 and 50 oC) and monitoring the physiochemical properties of sunscreen emulsions for a period of up to six months. The physiochemical properties included organoleptic properties (colour, odour and appearance), pH, viscosity and density. A freeze-thaw test was also performed to monitor the thermal stability of the formulation when exposed to various temperatures for five days.

The selected analytical methods, namely, rheological tests (steady flow behaviour, amplitude sweeps, frequency sweep and freeze-thaw), particle size and zeta potential, were used to evaluate the flowability, structural strength, viscoelastic behaviour, the response of the emulsion to thermal stress and particle characteristics, such as particle size and surface charge, of the dispersed particles. In this case it is noted that the particles are droplets dispersed in the emulsion. A variety of samples ranging from physical and chemical sunscreens which included creams and lotions were tested. Samples containing different emulsifier concentrations were prepared to enable the preparation of emulsions of varying stability.

Accelerated stability tests showed that formulations with higher emulsifier concentrations did not show any phase separation which indicated stability, while the opposite was true for samples with lower emulsifier concentrations. The pH variation of each sample throughout the entire study period was less than 1 unit. The viscosity change was wider, as some samples showed more than 50% change, but the sample did not show any visible alterations that consumers could perceive. The concentration of the emulsifier did not affect the specific gravity, as there was no significant change observed during storage. All samples were thermally stable over the temperature range studied, as there was no visual alteration observed after the freeze-thaw test.

In terms of analytical techniques, the stability of one formulation was predicted within a week. All samples showed a shear thinning behaviour which is a typically expected behaviour for cosmetics. The concentration of the emulsifier was directly proportional to the zero-shear viscosity, with stable samples having a higher zero-shear viscosity which slows down the time required for phase separation to occur. Stable samples had a high consistency index compared to unstable samples that had lower values. The tested viscoelastic behaviour showed that all samples behaved as viscoelastic solids which were denoted by a storage modulus (G’) that was greater than the loss modulus (G”). Stable samples also showed a structural development (gel structure formation) which was characterized by a higher linear viscoelastic range, yield stress and flow point whereas the opposite was observed for the unstable emulsions. Frequency sweep results showed a crossover point for some of the unstable samples indicating the possibility of phase separation. However, some unstable samples did not show the crossover point within the tested frequency range. The formulations with the highest G’ at low frequency indicated stability. The particle sizes were inversely proportional to the emulsifier concentration resulting in stable samples showing smaller particle sizes and monodisperse distribution. There was no correlation between the zeta potential and the concentration of the emulsifier. All the samples were also thermally stable, characterized by a small structural change when exposed to varying temperatures.

Most of the proposed analytical methods generated results that corresponded to the accelerated stability results. They also showed a potential to reduce the time and cost required for stability testing of sunscreens and will assist formulators to launch their sunscreens to the market in a shorter period. In order to make the proposed methods accessible to smaller cosmetic houses, which may have limited financial and scientific resources, it is suggested that they be implemented at central, commercial analytical facilities to which samples may be submitted for analysis. Additionally, these methods will assist manufacturers in choosing the adequate concentration of emulsifiers and to analyse the quality of the sunscreen emulsion when raw materials are substituted. The methods may also find application with respect to other emulsion-based products, aside from sunscreens. Further research needs to be done for the frequency sweep of physical sunscreen creams to enable the method to differentiate between stable and unstable sunscreen creams.