engleski

Low-Frequency Dielectric Spectroscopy of Aqueous Solutions

Electrical signaling and complex dynamics in soft-matter systems are of the fundamental interest in life-sciences and biotechnology. With the aim of investigating dielectric relaxation in charged systems, polyions and colloids, in aqueous environment of varying ionic strength and pH we employ the low-frequency dielectric spectroscopy (LFDS). LFDS is application specific and non-destructive technique allowing detection and quantification of polarization response of charged systems in polar and non-polar solvents. LFDS is also well established in the solid state, for investigations of the collective electronic response in the low-dimensional synthetic materials [1]. In order to apply LFDS we designed and built a chamber for measurements of complex conductivity of liquid (mostly aqueous) samples of small volume, 50-200 μ L, with conductivity in the range of 2- 2000 μ S/cm. Temperature stabilization is &plusmn ; 10 mK in the range of -20 to +60°C. Reproducibility is 1.5% and the measured sample stability is achieved on the time scale of 2 hours. Measurements are performed by the precision impedance analyzer Agilent 4294A (range: 40Hz-110 MHz). However, the frequency range is limited by the electrode polarization effects, as well as by the parasitic impedances of the measurement circuit. Background subtraction methods and extensive test measurements established that the practical frequency range for detection of the dielectric modes is 0.5kHz-50 MHz. The purpose of the present investigation was to carry out test LFDS measurements on monodispersed polystyrene latex particles suspended in low-ionic strength electrolytes. Colloidal stability of these particles is generated due to their high surface charge. This charge, along with the well-defined counterion atmosphere, leads to the well-determined polarization response, and furthermore, to the appearance of two characteristic dielectric modes, which are well-described by generalized Debye function. Accordingly, the low-frequency mode, in the kHz range, is related to the particle size, while the high-frequency one, in the MHz range, is related to the thickness of the double-layer and defined by the Debye-Hückel screening length of the electrolyte [2, 3]. [1] M. Pinterić, T. Vuletić, S. Tomić and J.U. von Schütz, Eur.Phys. J. B 22, 335 - 341 (2001). [2] S.S. Dukhin and V.N. Shilov Adv.Coll.Interface Sci. 13, 153 (1980) [3] R.W. O'Brian, J. Colloid Interface Sci. 113, 81 (1986).

dielectric spectroscopy; colloids; soft matter; life sciences

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