Attractive interactions betweren colloids at the oil-water interface

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Spring 2012 entry - Anna Wang

References

"Attractive interactions between colloids at the oil-water interface," BJ Park, EM Furst, Soft Matter (2011) 7, 7676-7682

Background

The interaction of colloidal particles in bulk has been extensively studied, allowing great control over properties of colloidal suspensions. Their interaction at fluid-fluid interfaces, however, present additional challenges in understanding. For instance, DLVO theory can not completely explain the behaviour of long range repulsion, and short range attraction that's exhibited by particles confined to an oil-water interface.

This paper investigates the effects of salts and surfactants on the interaction between colloidal particles at a decane-water interface. This has implications for the creation of Pickering emulsions, and techniques which use interfaces to scaffold self assembly.

Theory

For colloids much smaller than the capillary length, the effect of gravity (and hence the 'cheerios effect') is neglible. There are then two major forces at play:

Dipole-dipole repulsion

For particles straddling an oil-water interface, there is charge dissociation on the portion of the particle in the water, but not in the oil. The result is that each particle is a net dipole with the form

<math>F_{rep}=\frac{3akT}{r^4}</math>

where a is a parameter that describes the magnitude of the repulsion and r is the interparticle separation.

Capillary forces

It has been shown that an irregularly shaped contact line over a particle can be responsible for an attractive interaction between particles at interfaces, as they move together to minimise the deformation of the interface. The cause of the irregularlity can be due to pinning to chemical or topological defects, and even a 50nm contact line undulation for a 1 micron particle can result in an attractive potential of order 1000kT. The form of this 'capillary quadrupole' (drawing parallels with electromagnetism) is

<math>F_{cap} = -\frac{4bkT}{r^5}</math>

where b is related to the height of the undulating contact line.

Experimental observations

Figure 1.Two particles are help in optical traps - one is stationary, the other moves.
Figure 2.Force between the particles as a function of separation in the presence of a) 250mM NaCl in the aqueous phase b) 0.1mM SDS and 250mM NaCl in the aqueous phase and c) 25um SPAN in decane. The neat system is the dotted green line in a).


Two parallel sets of experiments are done -

1. 3.1um diameter polystyrene latex particles, zeta-potential ~-80mV

2. two 'merged' 3.9um spheres forming a doublet, zeta-potential ~-10mV

In both sets of experiments, the particles are introduced to the interface, then held in time-shared optical traps. One particle is then brought closer to another stationary particle, and the displacement of the particles from the centre of the optical trap is noted as a function of particle separation (Fig. 1). As the effective trap stiffness is known, the force imparted by one particle on the other can then be determined. The results are shown in Figure 2 and fit well to an effective force encompassing capillary attraction and dipole-dipole repulsion.

Conclusions

The experimental results agree well with the model and shows the importance of capillary forces, even for particles smaller than the capillary length. The effect of salt and surfactant on the interaction also agrees well with the model:

  • The long range repulsion is weakened with the addition of salt as salt screens the dipole-dipole interaction.
  • The addition of surfactants weakens the fluid-fluid surface tension, meaning that the partiicle will push further into the oil phase. This reduces the amount of charged surface area in the water phase, resulting in less repulsion.