R.W. O'Brien

The equations which govern the ion distributions and velocities, the electrostatic potential and the hydrodynamic flow field around a solid colloidal particle in an applied electric field are reexamined. By using the linearity of the equations which determine the electrophoretic mobility, we show that for a colloidal particle of any shape the mobility is independent of the dielectric properties of the particle and the electrostatic boundary conditions on the particle surface. The mobility depends only on the particle size and shape, the properties of the electrolyte solution in which it is suspended, and the charge inside, or electrostatic potential on, the hydrodynamic shear plane in the absence of an applied field or any macroscopic motion.New expressions for the forces acting in the particle are derived and a novel substitution is developed which leads to a significant decoupling of the governing equations. These analytic developments allow for the construction of a rapid, robust numerical scheme for the solution of the governing equations which we have applied to the case of a spherical colloidal particle in a general electrolyte solution. We describe a computer program for the conversion of mobility measurements to zeta potential for a spherical colloidal particle which is far more flexible than the Wiersema graphs which have traditionally been used for the interpretation of mobility data. Furthermore it is free of the high zeta potential convergence difficulties which limited Wiersema's calculations to moderate values of ζ. Some sample computations in typical 1:1 and 2:1 electrolytes are exhibited which illustrate the existence of a maximum in the mobility at high zeta potentials. The physical explanation of this effect is given. The importance of the mobility maximum in testing the validity of the governing equations of electrophoresis and its implications for the colloid chemist's picture of the Stern layer are briefly discussed.

Sound waves can be generated in a colloid by the application of an alternating electric field. In this paper we describe the method for calculating this and the related electro-acoustic phenomenon of electric fields generated by sound waves. As an illustration of the procedure, we obtain formulae for these two effects for a suspension of spherical particles with thin double layers, in a parallel plate geometry.

SUMMARY Clostridium acetobutylicum has been studied during batch cultivation at pH 7 and 35° in a glucose + casein hydrolysate + vitamins and salts medium kept (i) anaerobic (E h, −400 to −370 mV), (ii) aerated (E h, −50 to o mV; dissolved 02, < 1 μ m), and (iii) aerobic (E h, + 100 mV; dissolved O2, 40 to 50 μ m). Shortterm (4 to 6 h.) exposure to oxygen was not lethal, though at sufficiently high concentrations oxygen decreased the rate of glucose consumption, halted growth and prevented net synthesis of DNA, RNA and protein. Under these aerobic conditions the organism was drained of‘reducing power’ and starved of energy - as evidenced by cessation of butyrate formation (but not of acetate production), and by a marked fall in intracellular ATP. These consequences of oxygenation were swiftly reversed when anaerobic conditions were re-established; ‘ normal ’ growth and glucose metabolism then resumed. There was no evidence of H202 production, nor could the effects of oxygenation be attributed merely to its elevation of the culture E h. Thus oxygen (40 μ m) inhibited growth even in a medium poised with dithiothreitol at − 50 mV, while growth and glucose metabolism continued unchecked when the E h of an anaerobic culture was maintained at + 370 mV by addition of potassium ferricyanide.