Role of electrostatic interactions in the adsorption kinetics of nanoparticles at fluid–fluid interfaces

Abstract

The adsorption of particles to the fluid-fluid interface is a key factor for the stabilization of fluid-fluid interfaces such as those found in emulsions, foams and bijels. However, for the formation of stable particle-laden interfaces, the particles must migrate to the interface from the bulk. Recent studies show that the adsorption of particles to the interface formed during emulsification is influenced by the surface charge of the particles. To further investigate this phenomenon, we study the effect of the surface charge of the particle on the adsorption kinetics of particles to the oil-water interface. By suspending a drop of aqueous dispersion of charge stabilized nanoparticles in decane, the adsorption dynamics of particles to the decane-water interface is studied using the dynamic surface tension measurements. When the particles are highly charged (low salt), a negligible change in the interface tension is observed indicating that almost no particles are adsorbed. These results show that the charged particles experience an energy barrier when they approach the interface. But when the particle surface charge is screened by the addition of monovalent salt, a significant reduction in surface tension is observed indicating the migration and adsorption of particles to the decane-water interface. We estimate the effective diffusivity of particles to the interface by analyzing the initial decay in the measured surface tension by considering particle laden drops containing different amounts of salt using the modified Ward and Tordai theory. This effective diffusivity is used to calculate the energy barrier for the adsorption of particles to the interface. The energy barrier from the analysis of dynamic surface tension data agrees well with the concept of image charge repulsion which inhibits the adsorption of highly charged particles to the interface. By considering various types of relevant interactions, we derive an analytical expression that qualitatively captures the effect of the surface charge on the equilibrium surface coverage of particles at the drop surface.

Adsorption of charged anisotropic nanoparticles at oil–water interfaces

“…These adsorption kinetics are in good agreement with previous ndings on CNC adsorption at the A/W interface. 12 Nanoparticle adsorption comprises two underlying subprocesses: 19,20 (i) diffusion limited transport to the interface and (ii) kinetically limited particle adsorption. In the case of charged nanoparticles, the kinetic adsorption barrier may be decreased by salt-induced charge screening, as demonstrated in Fig.…”

Adsorption of Ellipsoidal Particles at Liquid–Liquid Interfaces

“…In adsorption experiments, the colloidal interactions between the interface (which may carry electrostatic charges) and the charges on the particle may also influence the efficiency of the adsorption 22 , and this was not considered here. In the present work we only consider dynamics once the particle breaches the interface, when capillary energy is expected to be- It can be hypothesised that the origin of the contact line pinning lies in small surface heterogeneities.…”

“…For spherical particles, the adsorption energies and kinetics have been inferred from measurements of the interfacial tension using a pendant drop apparatus. Effects of particle size 21 and surface charge 22 on the global adsorption kinetics have been investigated. Using holographic microscopy Kaz et al monitored the adsorption of single particles and observed that the fast initial adsorption was followed by an intriguing and unexpectedly slow relaxation 23 , which was been attributed to surface defects and contact line pinning 24,25 .…”

Steady-state droplet size in montmorillonite stabilised emulsions

“…The efficiency parameter a is included to represent an energetic barrier to platelet adsorption. 24,25,52 In practice, when fitting the model to experimental data, it appeared that a and k detach were correlated adjustable parameters. In what follows, a was set at 0.5 and the detachment rate constant used as an adjustable parameter.…”

“…In the present system and those discussed above this is not found to be the case, therefore there may be some as yet unconsidered factor preventing full surface coverage. Previously reported factors that may affect particle adsorption at the interface include electrostatic barriers, 24,25 mass transport of particles 6,26 and particle aggregation. 14,22 Upon consideration of experimental findings and the energies involved (see ESI, † for calculations) we find that in conditions pertaining to our experiments it is likely that particles may be forced both onto and off the interface during high shear mixing, despite desorption not occurring under quiescent conditions.…”