We study volatile aroma substances (linalool, citronellol, geraniol, menthol) with respect to their ability to adsorb on air/ water (A/W) interfaces — they are highly surface active, and exhibit rather fast initial adsorption, as demonstrated by surface tension measurements. The equilibrium adsorption isotherms are also discussed, together with the relevant physico-chemical parameters. The „surfactant – like” properties of these perfume ingredients are influenced by evaporation. We carry out model experiments with gradual saturation of aqueous pendant drops from vapors, followed by subsequent evaporation into open air, and also do measurements with evaporation from drops of solutions. The A/W surface tension is found to increase vs. time, during evaporation. The data are interpreted according to simple model considerations which include the impact of barriers for desorption towards the gas phase [as described in Colloids Surfaces A 625 (2021) 126931, Langmuir 38 (2022) 2793]. An important kinetic parameter is determined — the mass transfer coefficient, quantifying the rate of molecular exchange through the interface. During the course of evaporation, the surface elasticity is also measured, by means of small oscillations in the area/ volume of the drop. The observed trend is a gradual increase of the elasticity moduli, which can be rationalised in the context of the contribution of local fluxes of mass transfer of the volatile amphiphile (perfume) between the interface and the adjacent sub-surface, when the area is expanded or contracted. Our results can be useful when explanations are sought for the role of aroma substances in stabilizing disperse systems (e.g., droplets), under dynamic conditions when mass exchange (evaporation) takes place simultaneously with adsorption/ desorption.
Acknowledgements
The authors wish to thank for financial support provided by the European Union – NextGenerationEU, through the „National Recovery and Resilience Plan” of the Republic of Bulgaria, contract # 70-123-260/2024, project BG-RRP-2.004-0008 SUMMIT.
