ABSORPTION OF SPARINGLY SOLUBLE VAPOR IN MICRODROPLETS - A STUDY BASED ON LIGHT-SCATTERING

A technique, based on intensities of light scattered by small droplets in the planes parallel and perpendicular to the polarization plane of the incident light, has been developed for the determination small size and refractive index changes that occur due to the absorption of minute amounts of vapor. The technique has been applied to study absorption of nearly-immiscible water vapor in single dioctyl phthalate droplets suspended in an electrodynamic balance under controlled humidities. Wavelengths of transverse magnetic (TM) and transverse electric (TE) mode resonances in the scattered light were measured independently by scanning a ring dye laser and used to determine the size and refractive index of a droplet in dry air. Shifts in the TE and TM mode resonating wavelengths, after an exposure of the droplet to a humid environment, were interpreted to obtain minute size and refractive index changes due to the absorption of water vapor. The results show that the absolute size and size change can be detected with a resolution of one molecular layer. Moreover, the technique can be used to detect the formation of an adsorbed monolayer on the surface of a micrometer size spherical particle. The results on the absorption water vapor show that the concentration of water in a droplet increases as the water vapor saturation ratio increases up to the saturation limit. The data have been analyzed to obtain vapor-liquid equilibrium data over a wide range of compositions.