Electrolyte solutions, conductance fluctuations

The best prospect for this field appears to be the electrophoretic technique. Variants of the light-scattering method such as fluorescence and Raman and infrared intensity fluctuations are also promising, although only the fluorescence technique has been used so far to measure reaction rate constants. Feher (1973) has introduced a technique which directly measures fluctuations of electrical conductivity of dilute electrolyte solutions. These fluctuations may be related to diffusion coefficients and reaction rate constants by methods similar to those described in this chapter. 

In biological ionic systems, conductance noise is induced by membrane structures such as ion channels and fluctuations in electrolyte conductance. Noise that originates from the switching of ion channels between different conductance states has been reviewed extensively (3-9). Therefore, we limit ourselves to a discussion of conductance fluctuations in electrolyte solutions (45) and new noise sources that have been identified recently for currents through open ion channels (46, 47). 

An excellent review of the early history of noise studies of different ionic systems, such as single pores in thin dielectric films, microelectrodes, and synthetic membranes, is reference 3. The review by Weissman (48) describes several state-of-the-art fluctuation spectroscopy methods that include (1) determination of chemical kinetics from conductivity fluctuations in salt solutions, (2) observation of conductivity noise that arises from enthalpy fluctuations in the electrolyte with high temperature coefficient of resistivity, and (3) detection of large conductivity fluctuations in a binary mixture near its critical point. 

This ideal gas behavior" of conductance fluctuations appears to be rather unaccountable. First, electrolyte solutions are systems with pronounced interactions that are attributable to a slow decrease of Coulomb forces between ions this brings about substantial correlation between mutual positions of ions in space. Second, even in the hypothetical case of weakly interacting charge carriers, the conductance fluctuation level is expected to be equal to the value calculated from the total number of carriers only when the mobilities of different carriers are identical. Indeed, substantial difference in mobilities, say for a 1 1 electrolyte, forces lower mobility carriers to be electrically invisible and, thus, the conductance fluctuations must be normalized only to ion species with higher mobility that is, to the total number of dissolved molecules. Figure 3 shows that this conclusion contradicts the HCl electrolyte experiments in which the mobility of cations is almost five times as large as that of anions. Nevertheless, the level of... 

Figure 3. Large-scale conductance fluctuations in aqueous solutions of several strong electrolytes measured by the laminar flow method vs. electrolyte concentration (45). The upper solid line shows the inverse total number of dissolved electrolyte molecules in the sample (that is, in solution volume confined by a fused quartz capillary channel of 13-pm radius and 0.4-mm length). The middle and lower lines correspond to inverse total numbers of ions for 1 1 and 2 1 electrolytes. At small electrolyte concentrations the fluctuation level is within several percent of the inverse number of ions independent of...

Figure 4. Mixtures of NaCl and HCl electrolytes show higher conductance fluctuations than individual electrolytes (45). The sample composition changes from pure NaCl solution (Kh = 0.0J to pure HCl solution (KH = 1.0) the total number of ions is held constant. Experimental points for pure electrolytes are in good agreement with the inverse number of ions in the capillary.

Chapter 13 includes a short introduction to the theory of nonequilibrium thermodynamics. A discussion of frames of reference in the definition of transport coefficients is given and a systematic theory of diffusion is presented. Fluctuations in electrolyte solutions are analyzed, and the parameters measured in electrophoretic light-scattering experiments are related to conductance and to the transference numbers—quantities usually measured in conventional electrochemistry. 

ABSTRACT The principle of electrical spectrography and its measurement system is discussed. The phenomenon of noise in electrolytes and interfaces receives attention. Noise spectrography is found to have applications in some biomolecular systems, viz., DNA helix-to-coil transition, thermal transconformation, and salt-free premelting effects. Noise conductivity emission spectra of collagen solutions gave information on permanent dipole fluctuations and hydrodynamic properties of the system. 

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