Effective interaction of a charged colloidal particle with an air-water interface

Abstract

Within the framework of linear and non-linear Poisson-Boltzmann theory, we study the effective interaction of a single charged colloidal sphere in an aqueous electrolytic solution with an air-water interface. The effects of varying the salt concentration and the colloidal surface charge density on the effective interaction are being investigated, with a view to understanding some physical phenomena, which include electrostatic adsorption and trapping at the air-water interface. Results show an electrostatic double-layer barrier to the colloid's approach to the interface which can be lowered considerably by increasing the salt concentration. For enough added salt, the charged colloid should be able to suddenly pop up at the air-water surface, an effect which has actually been observed in recent experiments. We discuss the relevance of our results to other experimental observations, and emphasize the close analogy between the problem considered here and the classical problem of the interaction of two colloids in a bulk suspension.

Electrophoresis of a spherical particle normal to an air–water interface

“…A mesh with 40 Â 40 nodal points, in (x, Z) domain is found to be sufficiently accurate, and is used to carry out the calculations in this study. At such a distance from the free surface, the presence of the surface is hardly ''felt'' by the colloid [11]. The system is essentially like a single particle suspended in an infinite medium.…”

“…Colloidal crystallization [10][11][12] is a typical example of motion of particles near an air-water interface. Aggregation of particles and adsorption of proteins at an air-water interface find wide applications in denaturation and stabilization of products in the biological and food industries [12].…”

“…In corresponding theoretical analyses, Terao and Nakayama [10] studied the suspension of colloids trapped at an air-water interface by Monte Carlo simulations. Mbamala and von Grunberg [11] explored further the electrostatic interaction between a charged particle and an air-water interface when the particle is approaching or trapped at the interface. According to their results, the electrical double layer around the particle forms a significant barrier when the particle moves to the air-water interface.…”

“…As a result, the corresponding physical boundary condition considering interfacial polarization, or the Maxwell-Wagner effect [25][26][27], yields Eq. (25).There is an extremely thin ion-free layer near the air-water interface at equilibrium state, as inferred by the ''image charge repulsion'' theory in electrostatics [11,[19][20][21]. Assuming the essential physical situation remains the same in electrophoresis, as the applied electric field df is much smaller than the characteristic equilibrium state value f e , we have:…”

Physical ageing of the contact line on colloidal particles at liquid interfaces

“…First, after the discontinuity, we cannot pull the particle down with our optical trap. Second, at low salt concentrations, where electrostatics should prevent the particle from breaching 19 , there is no discontinuity, and the particle remains at a plateau (Fig. 1b).…”