Zeta Potential of Nanobubbles Generated by Ultrasonication in Aqueous Alkyl Polyglycoside Solutions.
A simple and convenient method to measure microelectrophoretic mobilities was proposed to determine the zeta potential of nanobubbles generated by ultrasonication. Bubbles in pure water solutions and in aqueous solutions of alkyl polyglycoside (AG) with different alkyl chain lengths and degrees of polymerization in the head group were sonicated with a palladium-coated electrode designed specially by the manufacturer. The zeta potentials of bubbles with ordinary cationic and ionic surfactants are consistent with others' previous results. The average size of the bubbles generated by sonication is in the range of 300 to 500 nm. The zeta potentials of bubbles in both pure water and AG solutions at all pH values are negative. As the chain length of AG increases, zeta potentials significantly decrease at high pH. For nonionic AG, a possible charging mechanism based on known mechanisms is suggested to explain the negative charge, known to be unusual. Even with a very high concentration of H(+) ions in solution the bubbles are charged negatively because the interface is covered with slightly acidic alcohol groups of AGs. At high pH, the less polar the surfactant, the more negative the charge, since nonpolar surfactant molecules induce the adsorption of OH(-) ions, rather than H(+) ions that prefer hydration by water molecules. Copyright 2000 Academic Press.
Dynamic forces between bubbles and surfaces and hydrodynamic boundary conditions.
A bubble attached to the end of an atomic force microscope cantilever and driven toward or away from a flat mica surface across an aqueous film is used to characterize the dynamic force that arises from hydrodynamic drainage and electrical double layer interactions across the nanometer thick intervening aqueous film. The hydrodynamic response of the air/water interface can range from a classical fully immobile, no-slip surface in the presence of added surfactants to a partially mobile interface in an electrolyte solution without added surfactants. A model that includes the convection and diffusion of trace surface contaminants can account for the observed behavior presented. This model predicts quantitatively different interfacial dynamics to the Navier slip model that can also be used to fit dynamic force data with a post hoc choice of a slip length.
Calculation of the electric double-layer force between unlike spheres
Abstract A comprehensive account is given of the electrical double-layer interaction forces between spheres with the condition of either constant surface potential or constant surface charge density. The spheres may be of unequal size and the surface potentials at infinite separation of unequal magnitude and opposite sign. The Derjaguin approximation is used at small separations and the superposition formula for spheres at larger ones. In the Derjaguin approximation the double-layer force between two spheres is proportional to the interaction free energy of parallel plates and a discussion of the various constant charge cases for the latter is given. Simple formulas involving quadratures are derived for the plate interaction free energy at both constant potential and constant charge, and the difference formula of Frens is generalized. Transformations to standard elliptic integrals are given for the various constant charge situations. A number of curves of the force between spheres against separation are presented and the influence of geometry and of varying the constant potential or the constant charge density on one sphere is investigated. It is shown that the behavior of the electrical forces between two dissimilar surfaces at small separations can be complex, and many unusual effects may arise depending on which boundary condition is applicable. The small potential approximation is considered and found to be less effective at constant charge density than at constant potential.