Development of functional finishes based on natural products for leather applications
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Nanoemulsions are biphasic systems characterized by having droplets with nanometer size range, which confers them several advantages, such as high hydration capacity and increased bio-efficacy of the functionalities incorporated therein. In this context, the objective of this work was to develop a water-in-oil (W/O) nanoemulsion system based on natural compounds, like sweet almond oil and Cinnamomum zeylanicum aqueous extract, in order to produce a moisturizing and antimicrobial spray for leather application. Firstly, a systematic study was performed by testing different base formulations to establish the procedure for nanoemulsion preparation using a high-pressure homogenizer (HPH). A total of 16 base formulations were prepared by varying the W/O ratio and emulsifier composition. These samples were then analyzed by microscopic and visual analysis in order to evaluate the droplets size and morphology together with stability along the time. Based on these results, the best formulation revealed to be the system S4, corresponding to a W/O ratio of 40/60 (v/v), and Span 80/Tween 80 mixture (54/46 ratio, v/v) as emulsifier. Using this formulation, nanoemulsions functionalized with cinnamon at contents of 1.25%; 2.5%; 3.75%; 5% (w/v; water base) were prepared. Afterwards, these products, together with the base system, were characterized by Confocal Laser Scanning Microscopic (CLSM), color parameters, evaluated concerning the antimicrobial and antioxidant bioactivities, and stability. As results, CLSM confirmed the nature of the water-in-oil system, revealing also a change in the nanoemulsions microstructure due to the increase of cinnamon extract concentration. The color measurement evidenced a luminosity diminishing and a color variation to yellow and red as the extract content was increased. In addition, the formulations containing the cinnamon extract evidenced antimicrobial and antioxidant activity. The absence of bioactivity for the base nanoemulsion corroborated the effect of the cinnamon extract in the detected antimicrobial and antioxidant. Furthermore, for the antimicrobial activity, the formulations containing the extract presented a prolonged effect against E. coli and S. aureus bacteria, being detected similar inhibition zones for all the tested extract concentrations. Regarding the antioxidant activity, the rise in the extract concentration resulted in a relevant increase of this property. Finally, and according to the performed accelerated stability tests, a high stability against centrifugation for the base nanoemulsion was detected, evidencing that the presence of the extract contributes to the formulation instability. Beyond this, all the formulations showed high stability against thermal stress. From a general perspective, the results obtained are of great importance for the product design and development having in view the application as leather preservative and moisturizer, since the formulations presented antioxidant and antimicrobial activity, parameters that contribute to the protection and preservation of the shoes. In addition, their stability points for a good shelf life of these systems, being suitable for storage under commercialization conditions.
