Comprehensive evaluation of non-conventional lanthanum phosphate nanospheres inside water-in-oil microemulsion scaffolds and their utilization in the assessment of surfactant-free TiO2-based Pickering emulsion formulations

Abstract

Herein, non-conventional lanthanum phosphate (LaPO₄) nanospheres were synthesised in water-in-oil (W/O) microemulsion (Tween 20/1-butanol/water/toluene) scaffolds and employed in the assessment of surfactant-free TiO₂-based Pickering emulsion formulations. TEM and FESEM microstructural analyses were carried out to examine the nanospheres inside the core of the reverse micelles. Both types of electron microscopy identified two distinct phases in the growth of the nanospheres: a nucleation phase (5–15 min) in which only nanospheres of smaller dimensions were seen and a growth phase (15–60 min) wherein larger dimensions nanospheres were observed. LaPO₄ nanospheres are extremely hydrophilic and alone would fail to provide stability to the oil-in-water (O/W) Pickering emulsions and consequently require in situ hydrophobization. The determined zeta potentials suggested that the LaPO₄ nanospheres have a positive surface charge (+57 mV) and eventually, oppositely charged solid TiO₂ nanoparticles were therefore introduced onto the surface of the synthesised LaPO₄. The adsorption of TiO₂ ensured the evaluation of the amphiphilic nature of the nanospheres through the formation of LaPO₄@TiO₂ nanocomposites. Additionally, TEM analysis supported that no morphological alterations occurred upon the adsorption. The as-prepared nanocomposites thereafter started to provide extra stability (due to the lower interdroplet coalescence) to the O/W Pickering emulsions, which could be sustained for up to 4 weeks and even more. The stability of the emulsion droplets was supported by the optical micrography analysis. Moreover, such an attainment of stability therefore, successfully opens a new idea for obtaining surfactant-free stable Pickering emulsion formations.