Introduction: Self-assembling systems are those which get their supramolecular structure spontaneously, allowing them to be used as carriers for active substances in the formulation of cosmetics, foods, and medicines. The aim of this work was to study the ability of citronellol homopolymers (low and high viscosity) in forming self-assembling nanostructured lipid carriers using regular emulsification processes with non-ionic surfactants.
Method: Four 2³ DoE (Design of Experiments) were used, where percentages of surfactants 1:1 (Polysorbate 80 added of Ethoxylated oleic alcohol (EOA) or Poloxamer 188 (P188) - 4 and 8% w/w), propylene glycol (5 and 15% w/w), and content of citronellol homopolymer (low or high viscosity, CL or CH, respectively) in the oily phase (13 and 87% w/w) were factors and levels studied. Besides, two intermediate levels were included with regard to the content of citronellol homopolymer in the oily phase (25 and 75% w/w), resulting in 16 formulations for each DOE (DoE1: EOA-CL; DoE2: EOA-CH; DoE3: EOA-CL; DoE4: EOA-CH). All the formulations were produced by emulsification of oily and aqueous phases under heating (70 – 80 °C) and mixing for 10 minutes in a magnetic stirrer. Afterward, formulations were maintained under stirring as they cooled to room temperature, were visually inspected, pictured, and analyzed by laser diffraction (range of particle size analysis 40 nm – 2.5 mm) using purified water as the diluent. The Mie method was used by software for the calculation of sizes and size distribution, allowing the recording of d10, d50, d90, mean size (for volume and number passing), and the percentage of particles for a specific size range. These data made possible the calculation of span (size dispersion) and uniformity ratio, as parameters of uniformity of size distribution. The data were assessed in statistical software for Response Surface Regression (RSR) applying ANOVA with significance ≤ 0.05 and Principal Component Analysis (PCA) presented as Pareto charts for each parameter analyzed. The results were used to run optimization studies and estimate the desirability index for a target formulation.
Results and Discussion: The visual inspection of the produced formulations suggested that different emulsified systems were formed, since microemulsions (transparent formulations with nanometric dimensions - DoE1-F14) until nanoemulsions (translucid and nanometric - DoE3-F15), and ordinary emulsions, opalescent formulations containing particles with tens of micrometers, with (DoE3-F4) or without (DoE3 – F8) phase separation. Most of the tested formulations were unable to form a nanostructured system with good uniformity, although nanostructures were present in several of them. However, some formulations presented adequate mean size, lower than 300 nm (DoE1-F14, F15, F16; DoE3-F15, F16), and uniformity, with a span lower than 2.0 e uniformity ratio lower than 5.0. Concerning that, PCA analysis presented in a Pareto chart revealed that the proportion of citronellol homopolymer in the oily phase was the principal factor of variation for most of the parameter analyzed - be it alone, associated with another factor, or quadratic - including those related to uniformity (span and uniformity ratio). It was also observed that ethoxylated oleic alcohol was the surfactant that together with polysorbate 80 made possible to obtain at least two formulations for the DoE study whereas poloxamer 188 did not provide nanostructured systems. The ANOVA applied for RSR considering the studied parameters showed most of the p-values lower than 0.05 for the models. The same was observed for p-values for each factor, either isolated or in association, showing that factors studied are important for the size properties in tested formulations. Two optimization studies were done for each DoE, only considering parameters that had presented significance for the models in RSR. The desirability obtained was higher for those studies where size parameters were just minimized, D = 0.7630 for DoE3, whereas the indication of targets for size resulted in a lower desirability index, D = 0.7434 for DoE3, as an example.
Conclusion: It was possible to obtain self-assembling formulations using citronellol homopolymer as part of the oily phase in lipid nanoparticles and ordinary surfactants as polysorbate 80 and EOA. The citronellol homopolymer has shown an interesting feature as a solvent for lipophilic actives what might be interesting to carrier them in cosmetic and pharmaceutical formulations in a sustainable and ecological way.