Evaporation of Particle-Stabilized Emulsion Sunscreen Films

Abstract

We recently showed (Binks et al., ACS Appl. Mater. Interfaces, 2016, DOI: 10.1021/acsami.6b02696) how evaporation of sunscreen films consisting of solutions of molecular UV filters leads to loss of UV light absorption and derived sun protection factor (SPF). In the present work, we investigate evaporation-induced effects for sunscreen films consisting of particle-stabilized emulsions containing a dissolved UV filter. The emulsions contained either droplets of propylene glycol (PG) in squalane (SQ), droplets of SQ in PG or droplets of decane in PG. In these different emulsion types, the SQ is involatile and shows no evaporation, the PG is volatile and evaporates relatively slowly, whereas the decane is relatively very volatile and evaporates quickly. We have measured the film mass and area, optical micrographs of the film structure, and the UV absorbance spectra during evaporation. For emulsion films containing the involatile SQ, evaporation of the PG causes collapse of the emulsion structure with some loss of specular UV absorbance due to light scattering. However, for these emulsions with droplets much larger than the wavelength of light, the light is scattered only at small forward angles so does not contribute to the diffuse absorbance and the film SPF. The UV filter remains soluble throughout the evaporation and thus the UV absorption by the filter and the SPF remain approximately constant. Both PG-in-SQ and SQ-in-PG films behave similarly and do not show area shrinkage by dewetting. In contrast, the decane-in-PG film shows rapid evaporative loss of the decane, followed by slower loss of the PG resulting in precipitation of the UV filter and film area shrinkage by dewetting which cause the UV absorbance and derived SPF to decrease. Measured UV spectra during evaporation are in reasonable agreement with spectra calculated using models discussed here.

Recent developments in Pickering emulsions for biomedical applications

“…Other recent work has used chitosan based Pickering emulsions as a method to enhance wound healing, where the synergistic effect of the cargo therapeutic and other components of the Pickering emulsion demonstrated improved functioning compared to the drug alone. [16] Another innovative topical application of Pickering emulsions is for sunscreens, [25,26] where the emulsifying particles can be physical UV filters in combination with an encapsulated active agent, such as melatonin. [27] Such formulations can present a stable and effective sunscreen with the added benefit of protection against oxidative stress, thanks to melatonin's free radical scavenger and antioxidant activity.…”

“…[27] Silica particle-stabilised emulsions containing a dissolved UV filter have also recently been demonstrated to produce effective sunscreen films due to evaporation-induced effects. [26] Their unique combination of volatile and involatile components allows the UV filter to remain soluble throughout evaporation, maintaining excellent sun protection. These emulsion films offer advantages over solution sunscreen films, thanks to reduced film shrinking, which circumvents potential loss of sun protection.…”

Evaporative mass loss measurement as a quality control tool for quality assurance in the manufacture of inks suitable for high speed (≥60 m min−1) printing

“…Solvent evaporation from the liquid | air interface that is created by the "flat" circular "solvent disc", of radius r 0 , can be considered as a diffusional process [23][24][25][26][27][28][29]. Although the experiments were not thermostatted, ignoring temperature effects, if the solvent contained in the PTFE chamber is an infinite supply that is well-mixed, and evaporates to yield a perfect gas, the interfacial boundary condition on the gas side is the saturation concentration (c s ) of the solvent vapour.…”

“…Although the experiments were not thermostatted, ignoring temperature effects, if the solvent contained in the PTFE chamber is an infinite supply that is well-mixed, and evaporates to yield a perfect gas, the interfacial boundary condition on the gas side is the saturation concentration (c s ) of the solvent vapour. Assuming the solvent to behave ideally, and that there is no evaporative cooling of the liquid surface that leads to a reduction of the vapour pressure [23][24][25][26][27][28][29][30][31][32][33], nor any surface heterogeneity present that gives rise to an increase in the vapour pressure as a result of the Kelvin equation [34] or through specific adsorption at the liquid | air interface, this is given by equation (1).…”

Spectrophotometry of Thin Films of Light-Absorbing Particles

“…Dispersions and/or emulsions containing non-uniformly distributed light absorbing species are widely used in coatings, paints, inks, sunscreens and other applications in which light transmittance properties of thin films are important. For example, sunscreen films are generally applied at film thicknesses in the range 10-20 µm and contain a mixture of UVabsorbing dissolved molecules and small semiconductor metal oxide solid particles contained within either a solution, dispersion or emulsion [1][2][3] . We note here that, although specular transmittance is important in the range of application areas listed above, many additional factors (e.g.…”