To quantify the viscosity within the two separate BBOA phases, we measured the diffusion coefficients of large dye molecules (Rhodamine-6G, hereon referred to as R6G) through the phases as a function of RH and converted the diffusion coefficients into viscosity using the Stokes-Einstein relation . The Stokes-Einstein relation has been shown to be a valid conversion between diffusion coefficients and viscosity for organic-water matrices when the hydrodynamic radius of the diffusing molecules under investigation (here the R6G dye molecules) is greater than or equal to the mean hydrodynamic radius of the surrounding matrix . For our experiments, the hydrodynamic radius of the diffusing dye molecules of 5.89 Å ( ) is greater than the predicted mean hydrodynamic radius of the surrounding BBOA matrix of 4.23 Å (24), based on previous predictions of the average molar mass of BBOA being 248 g mol -1 (49) and a density of 1.3 g cm -3 (65, 66) and an assumed spherical geometry of BBOA molecules.