IMPACT OF EUTECTIC SOLVENTS ON EMULSIONS STRUCTURE
399
Presented by: Helena Ribeiro
INTRODUCTION: Solvent selection represents an important part of cosmetics development. The reduction of organic solvents use is one of the main solutions to improve sustainability, as well as selection of green alternatives. One interesting alternative is the use of natural deep eutectic solvents (NADES), seen as a promising, green alternative to synthetic organic solvents to produce plant extracts. Contrary to organic solvents, NADES can be designed to be incorporated into the final product, providing additional opportunities to fine tune the desired properties.
AIM: This study is focused on the impact of natural deep eutectic solvents (NADES) used on the extraction of phenolic compounds from the brown seaweed Sargassum muticum on an O/W emulsion.
MATERIALS AND METHODS: Three emulsions were prepared: a blank emulsion (BC), an emulsion containing 1% (w/w) of NADES and S. muticum extract (EC), and one containing 1% (w/w) of lactic acid:fructose (7:1) with 50% of water (v/v) (SC). To evaluate the structure and physical stability of emulsions, each formulation was subjected to centrifugal cycles (4000 rpm for 5 min in each cycle, Medifuge small benchtop centrifuge), pH determination (pH-Meter, SevenEasyTM), microscopy (optical microscope with polarized light, Nikon eclipse Ci,), and viscosity and oscillation frequency sweep tests with a controlled stress Kinexus Lab+ Rheometer (Malvern) using a cone and plate geometry (truncated cone angle 4° and radius 40 mm). Droplet size distribution was also determined using a Malvern Mastersizer (Hydro 2000).
RESULTS/DISCUSSION: Phase separation was verified with centrifugation tests confirming that NADES’ formulations are not stable, meaning that the selected NADES has impact in their stability. pH values of the formulations SC and EC were very low, around 2, due to the high content of lactic acid in the NADES used. Results of droplet size show a monomodal distribution in the BC emulsion, used as the control, while SC and EC emulsions presented bimodal populations. These results were confirmed by microscopy. BC, used as control, showed higher viscosities, although not considerably different from the other formulations, representing higher resistance to the structural breakdown than SC and EC. At a shear rate of 1 s-1, the apparent viscosity values were 29.95 Pa.s, 21.54 Pa.s and 16.26 Pa.s for BC, SC and EC, respectively. The apparent viscosity decreases simultaneously with the increase of shear rate, a common behavior of non-Newtonian shear-thinning fluids. Concerning the oscillatory tests, all formulations resulted in G’ > G’’, meaning the elastic module is superior to the viscous module. It also suggests the existence of a strong network dominated by cohesive forces that allows good spreadability, adhesion and tackiness of emulsions. Results further indicate that the formulations SC and EC have higher elastic and viscous modules than the control, BC, meaning these formulations are slightly more structured. SC and EC creams differ from the BC (control formulation) since they contain the NADES lactic acid:fructose (7:1, 50% v/v H2O) and the seaweed extract, respectively. Despite the fact that the solvent and extract only represent 1% (w/w) of the SC and EC formulations, this percentage is responsible for the rheological differences between both formulations and the control. Lactic acid is one of the compounds most present in the solvent and in the extract introduced in the formulations and is responsible for the lowering of pH values that influences the structural differences observed. NADES, along with the extracted phenolic compounds (mainly phlorotannins), have reduced the apparent viscosity, although maintaining the rheological behavior, increased both elastic and viscous modules and induced phase separation, compared to the control formulation, BC.
CONCLUSION: The physical-chemical characterization of the O/W emulsion showed acidic pH value and a shear thinning behavior suggesting its suitability for skin application. However, the extract emulsion prepared, EC, revealed stability issues, highlighting the impact of NADES in topical formulations for skin care.
Acknowledgments: Funded by the Fundação para a Ciência e Tecnologia, and UIDP/04138/2020 to iMed.ULisboa, CEECINST/00145/2018 to J.Marto.
AIM: This study is focused on the impact of natural deep eutectic solvents (NADES) used on the extraction of phenolic compounds from the brown seaweed Sargassum muticum on an O/W emulsion.
MATERIALS AND METHODS: Three emulsions were prepared: a blank emulsion (BC), an emulsion containing 1% (w/w) of NADES and S. muticum extract (EC), and one containing 1% (w/w) of lactic acid:fructose (7:1) with 50% of water (v/v) (SC). To evaluate the structure and physical stability of emulsions, each formulation was subjected to centrifugal cycles (4000 rpm for 5 min in each cycle, Medifuge small benchtop centrifuge), pH determination (pH-Meter, SevenEasyTM), microscopy (optical microscope with polarized light, Nikon eclipse Ci,), and viscosity and oscillation frequency sweep tests with a controlled stress Kinexus Lab+ Rheometer (Malvern) using a cone and plate geometry (truncated cone angle 4° and radius 40 mm). Droplet size distribution was also determined using a Malvern Mastersizer (Hydro 2000).
RESULTS/DISCUSSION: Phase separation was verified with centrifugation tests confirming that NADES’ formulations are not stable, meaning that the selected NADES has impact in their stability. pH values of the formulations SC and EC were very low, around 2, due to the high content of lactic acid in the NADES used. Results of droplet size show a monomodal distribution in the BC emulsion, used as the control, while SC and EC emulsions presented bimodal populations. These results were confirmed by microscopy. BC, used as control, showed higher viscosities, although not considerably different from the other formulations, representing higher resistance to the structural breakdown than SC and EC. At a shear rate of 1 s-1, the apparent viscosity values were 29.95 Pa.s, 21.54 Pa.s and 16.26 Pa.s for BC, SC and EC, respectively. The apparent viscosity decreases simultaneously with the increase of shear rate, a common behavior of non-Newtonian shear-thinning fluids. Concerning the oscillatory tests, all formulations resulted in G’ > G’’, meaning the elastic module is superior to the viscous module. It also suggests the existence of a strong network dominated by cohesive forces that allows good spreadability, adhesion and tackiness of emulsions. Results further indicate that the formulations SC and EC have higher elastic and viscous modules than the control, BC, meaning these formulations are slightly more structured. SC and EC creams differ from the BC (control formulation) since they contain the NADES lactic acid:fructose (7:1, 50% v/v H2O) and the seaweed extract, respectively. Despite the fact that the solvent and extract only represent 1% (w/w) of the SC and EC formulations, this percentage is responsible for the rheological differences between both formulations and the control. Lactic acid is one of the compounds most present in the solvent and in the extract introduced in the formulations and is responsible for the lowering of pH values that influences the structural differences observed. NADES, along with the extracted phenolic compounds (mainly phlorotannins), have reduced the apparent viscosity, although maintaining the rheological behavior, increased both elastic and viscous modules and induced phase separation, compared to the control formulation, BC.
CONCLUSION: The physical-chemical characterization of the O/W emulsion showed acidic pH value and a shear thinning behavior suggesting its suitability for skin application. However, the extract emulsion prepared, EC, revealed stability issues, highlighting the impact of NADES in topical formulations for skin care.
Acknowledgments: Funded by the Fundação para a Ciência e Tecnologia, and UIDP/04138/2020 to iMed.ULisboa, CEECINST/00145/2018 to J.Marto.