Air/water interfaces in microdroplets control molecular de-aggregation Jenifer Shantha Kumar, Pallab Basuri, Keerthana Unni, Sujan Manna and Thalappil Pradeep Department of Chemistry, Indian Institute of Technology Madras, India
The behaviour of molecules in a confined volume is distinctly different from those in bulk liquids. 1 It has been understood that the acceleration of reactions in microdroplets is strongly associated with processes at their gas– liquid interfaces through microscopy. 2 But, several critical pieces of information on molecules' properties in small volumes need to be understood in detail. 3 Through our work, we have found that molecular de-aggregation occurs at the air/water interface of aqueous microdroplets. We probed this phenomenon using dyes such as Rhodamine 6G (R6G), Rhodamine B, acridine orange, and fluorescein, which show aggregation-induced shift in fluorescence. The fluorescence micrographs of microdroplets derived from the aqueous solutions of these dyes show that they are monomeric at the air/water interface, but highly aggregated at the core. We propose that rapid evaporation of the solvent influences the de-aggregation of molecules at the air–water interface of the microdroplets. This phenomenon was further explored under various parameters such as microdroplet spray distance, nebulization pressure and applied voltage. These parameters greatly influenced the behaviour of dye molecules in the microdroplets. De-aggregation increases while the droplets move longer distances in the airuntil all the molecules become monomeric.An increase in the pressure and voltage positively affected the de-aggregation of molecules inside the microdroplets.We believe that such de-aggregation dynamics at the air/liquid interface help overcome intermolecular interactions and clustering, favouring the chemical reactivity in microdroplets. Microbubbles formed inside the microdroplets also showed distinct properties wherein the air/water interface showed complete de- aggregation of molecules. Such molecular behaviour in droplets could be utilized in understanding prebiotic, cellular and cloud chemistry. References 1. J. Rebek Acc. Chem. Res. , 2009, 42 , 1660 —1668,https://doi.org/10.1021/ar9001203. 2. B. M. Marsh,K. IyerandR. G. Cooks, J. Am. Soc. Mass Spectrom. , 2019, 30 , 2022 -2030,https://doi.org/10.1007/s13361-019- 02264-w. 3. P. Basuri,A. Chakraborty,T. Ahuja,B. Mondal,J. Shantha KumarandT. Pradeep, Chem. Sci. , 2022,https://doi.org/10.1039/ D2SC04589C.
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