In Situ Observation of Ultra-fine Particles in an Aqueous Solution by Laser Induced Photoacoustic Spectroscopy

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Laser induced photoacoustic spectroscopy (LPAS) was successfully applied to a study on colloid formation processes and coagulation processes. The photoacoustic signal intensity was linearly correlated to a product of the number of particles in a unit volume and light absorption cross section of one particle. This relationship was quantitatively confirmed by using gold sols (2 - 20 nm), silver sols (5 - 20 nm), polystyrene particles (100 - 2800 nm), α-FeィイD22ィエD2OィイD23ィエD2 particles (80 - 130 nm) and Tc(IV) colloids (200 - 700 nm). The LPAS method employed here is applicable to examining the colloid formation and coagulation processes with a wide range of particle size from nanometer to micrometer. The present method was useful for the determination of a refractive index of particles suspended even in a liquid medium. Furthermore, the growth of Tc(IV) colloid particles was found to proceed through coagulation of small particles and it was successfully analyzed by applying a newly developed collision-coagulation model in which the coagulation took place between two small particles whose kinetic energy was larger than the potential barrier.