Abstract
With the industrialisation of nanoparticle manufacture, the pervasive incursion of nanoparticles into the environment, the need to characterise nano-scale pharmaceuticals and living systems in replicated in vivo conditions, the continuing development of new theories to describe the electro-kinetic behaviour of nano-particles in representative ionic strengths and numerous other applications, there is an urgent requirement to provide simple and effective experimental tools to validate these models and explore new systems. Micro-electrophoresis implemented with a diffusion barrier, which isolates the dispersed phase from the electrode surface, is demonstrated as enabling such measurements for the first time, preventing the catastrophic outgassing, precipitation and sample degradation observed when the dispersed phase is in close proximity to the electrode surface. Using a measurement of a few minute’s duration in a standard laboratory light scattering instrument we reproduce the theoretically predicted phenomena of asymptotic, non-zero electrophoretic mobility with increasing ionic strength, the cationic Hofmeister series dependency, charge inversion and a continuously decreasing variation in mobility with pH as molarity increases. Standard operating procedures are developed and included to encourage further work.
Highlights
With the industrialisation of nanoparticle manufacture, the pervasive incursion of nanoparticles into the environment, the need to characterise nano-scale pharmaceuticals and living systems in replicated in vivo conditions, the continuing development of new theories to describe the electro-kinetic behaviour of nano-particles in representative ionic strengths and numerous other applications, there is an urgent requirement to provide simple and effective experimental tools to validate these models and explore new systems
On immersion into aqueous media, charged ionic species gather at the particle surface, creating a complex layer of charges known as the electrical double layer (EDL), comprising of a stationary layer of species adsorbed, chemically bound, to the particle surface and around this inner layer, a diffuse layer attracted via Coulombic interactions, which acts to minimise the total energy in the system by screening the first layer
In order to detect the residual electrophoretic mobility in high molarities from which zeta potential is calculated and which, as we report in this work, tend to be rather low in magnitude, a potential difference across the cell must be applied for periods long enough that the images of the particles explore a region of the detector plane larger than the Rayleigh resolution of the imaging microscope
Summary
With the industrialisation of nanoparticle manufacture, the pervasive incursion of nanoparticles into the environment, the need to characterise nano-scale pharmaceuticals and living systems in replicated in vivo conditions, the continuing development of new theories to describe the electro-kinetic behaviour of nano-particles in representative ionic strengths and numerous other applications, there is an urgent requirement to provide simple and effective experimental tools to validate these models and explore new systems. Within the outer (secondary) well the particles are loosely agglomerated and may be re-dispersed, if the equilibrium state has sufficient kinetic energy the particles may reach the inner (primary) well, where the attractive Van der Waals forces overcome the Coulombic force at short distances. This process is irreversible and en masse the dispersion will tend to aggregate over time. The judicious alteration of the pH of the continuous phase can be used to change the exposed charge at the particle surface and, subsequently, the zeta potential can be engineered to be either high in magnitude if aggregation is undesirable, or low in magnitude, if it is to be encouraged. (1). steric hindrance between ions[34,35], (2). the solvation shell around an ion lowers the local dielectric permittivity causing an additional dielectrophoretic force in an applied field gradient[36,37], (3). changes in permittivity due to the ordering of water molecules in the interfacial layer[38], (4). the possibility of a zero charge, but non-zero EDL, due to the ion specific penetration of a structured water layer at the particle surface[12,38], (5). the Debye length becomes smaller than the Bjerrum length over which ion-ion correlations become significant[39], (6). ion-ion pair correlations and ion size resulting in over screening of charged surfaces and charge reversal[36,39], (7). the Debye length becomes comparable to the ion size: ions ‘collapse onto the particle surface’[38] (8). and the detailed structure of the EDL at high salt depends on the identity of the ions present[36,39]
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