Conformational changes of humic acids in different aqueous solutions were studied by means of two novelty methods: high resolution ultrasound spectrometry (HRUS) and micro-rheology (μR). Obtained results were compared with those of more conventional method of dynamic light scattering (DLS).The HRUS method is based on the measurement of parameters of ultrasonic waves propagating through the sample. The attenuation describes the decay of the amplitude of the ultrasonic wave with the distance travelled. The velocity is the speed of this wave and is related to the wavelength and the frequency of oscillation of the deformation. It is determined by the density and elasticity of the sample, which is strongly influenced by the molecular arrangement. The μR technics (in contrast to conventional rheology) relies on tracking the motions of the micro-sized tracer particles embedded in the sample. It can give information about a local inhomogeneity especially in the ultra-small scale materials.Our results showed that the organization of particles in diluted and concentrated solutions of humic acids is different. The conformational arrangement was strongly influence both the concentration and the character of surrounding medium. The decrease in compressibility indicated the formation of a more rigid structure, which could lead to the decrease in the binding ability of humic acids. Changes in sizes of particles detected using DLS resulted in their different mobility and obtained micro-rheological parameters. It was confirmed that both novelty methods employed were very sensitive and could be utilized as indicators of conformational changes in humic systems.Keywords: Humic substances, molecular organization, conformation, ultrasound spectrometry, micro-rheology
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