Effects of Angular Dependency of Particulate Light Scattering Intensity on Determination of Samples with Bimodal Size Distributions Using Dynamic Light Scattering Methods

The angular dependency of light scattering intensity from differently sized particles strongly influences the apparent particle size distribution, as determined by dynamic light scattering (DLS) methods. Manufactured nanomaterials have size distributions more or less; therefore, the effect of detecting the angular dependency of the apparent size distribution by DLS is crucial. Commercial DLS instruments typically have two different types of detector angular position. The first is a detector angled at 90°, and the other is a backscattering angle detector. We therefore investigated the coverage and angular dependency when determining the relative concentrations of nanoparticles in polystyrene latex samples with a bimodal size distribution, using DLS methods both experimentally and theoretically. We used five differently sized polystyrene latex particles (one was a 70-nm nanoparticle and the others were various submicron-sized particles) in a variety of mixtures (the ratio of the difference of particle sizes ranged from approximately 2 to 7) to investigate the coverage and angular dependency of the recognition of the relative concentration ratio. In the case of size difference of approximately a factor of 2 or 3 between the two mixed particles (one was fixed at 70 nm), for DLS measurements at light scattering detector angles ranging from 60° to 150°, the homodyne photon correlation functions were approximately straight lines for mixtures of two differently sized polystyrene latex particles. The straight homodyne photon correlation functions were caused by the relatively strong light scattering from larger submicron particles masking the weaker light scattering from the smaller nanoparticles. As a result, DLS analysis could not recognize the relative concentration of nanoparticles in the mixture. In contrast to these samples, for mixtures of two differently sized polystyrene latex particles (one was 70 nm in size) with a size difference of a factor of 5, the homodyne correlation functions displayed an obvious curve for angles larger than 120°. This curve reflected an appropriate relative concentration ratio for the two differently sized polystyrene latex particles. Furthermore, for a mixture of two differently sized particles (one was again 70 nm) with size differences of a factor of 7, the homodyne correlation functions showed a clearly curved shape for detector angles larger than 90°, and yielded appropriate relative concentration ratios for the two different sizes of polystyrene latex particles. These observations were supported by theoretical investigation using Mie theory and asymmetric flow field-flow fractionation measurements with a multi-angle light scattering detector. Our investigation is crucial for achieving some degree of concordance on the determination of the size distribution of particles using DLS methods in industrial and academic fields