Mesoscopic Ordering of Polyelectrolyte Chains in Foam Films:  Role of Electrostatic Forces

Abstract

Recent studies using a thin film balance of free-standing thin aqueous films (foam films) containing polyelectrolytes resulted in jump-like discontinuities in the film thickness with increasing outer pressure. These jumps in film thickness correspond to an oscillation of the disjoining pressure. The oscillation period of the disjoining pressure scales as c -0.5 with the polyelectrolyte concentration c. A mesoscopic ordering of the chains in the film similar to that found in aqueous semidilute polyelectrolyte solutions is assumed. In experiments presented in this work, the electrostatic effect is investigated by adding low molecular salt and varying the degree of charge of the polyelectrolyte chains. The studies reveal that the amplitudes of the disjoining pressure oscillations are reduced with increasing ionic strength and decreasing degree of charge. The results show that the jumps in film thickness and therefore the structuring are due to the electrostatic repulsions between the polyelectrolyte chains.

Effect of polyelectrolytes on (de)stability of liquid foam films

“…With increasing ionic strength, the number of visible oscillations and their amplitude decreases, until they disappear at high salt concentrations (around 10 À2 mol l À1 ). 27,65 This is well correlated with SANS experiments, where a characteristic peak disappears above 10 À2 mol l À1 salt. 36 The degree of polymer charge has also a strong impact on the stratication behavior.…”

Polyelectrolyte-mediated surface interactions

“…Since the organization of the polyelectrolyte solution is caused by the repulsion between polyelectrolyte chains any variation that will reduce this repulsion, e.g. decreasing the linear charge density of the polyelectrolyte or increasing the ionic strength, will lead to a reduction in the oscillating structural force, which is confirmed by both theory [9] and experiments [62,66]. Simulations have shown that linear polyelectrolytes are aligned preferentially parallel to the confining walls [9], and experiments have shown that the periodicity of the oscillations scale with the bulk concentration to the power of À1/2 [63,67,68], i.e.…”

Neutral and charged polymers at interfaces

“…Measurements of the disjoining pressure in thin liquid films of PE solutions as a function of film thickness demonstrated an oscillatory pressure [95,239,240,241] with a period of the oscillation of the order of the peak position in the bulk structure factor (which was discussed in Sec. 3.6).…”