Forces between Carboxylic Acid Surfaces in Divalent Electrolyte Solutions.

The behavior of self-assembled monolayers of thiohexadecanoic acid adsorbed onto gold interacting in asymmetric 2:1 electrolytes has been studied with direct force measurements. The effects of two divalent cations (Mg(2+) and Ca(2+)) were studied at concentrations ranging from 1 µM to 10 mM. As compared to interactions in the presence of Na(+), the divalent ions adsorb strongly to the surfaces, with the effect of lowering the surface potential and decreasing the double-layer repulsion. At concentrations above 10 µM, the Ca(2+) ions were found to adsorb stronger than Mg(2+). Ca(2+) ions cause charge reversal at high concentrations, and the net interactions at 10 mM were attractive over the measurable range. Copyright 2000 Academic Press.

The Effect of Salt Concentration on Adsorption of Low-Charge-Density Polyelectrolytes and Interactions between Polyelectrolyte-Coated Surfaces.

In this investigation surface force, X-ray photoelectron spectroscopy and ellipsometry techniques have been used to study the adsorption of a low-charge-density cationic polyelectrolyte on negatively charged surfaces. It is shown that the low cationicity of this polyelectrolyte induces an adsorption behavior which is limited by steric factors rather than by the substrate surface charge or potential. It is also established that an increase in ionic strength of the solution results in desorption of the polyelectrolyte accompanied by an increase in layer thickness. This phenomenon is typical of a screening-reduced adsorption regime where electrostatic interactions predominate in the adsorption process. An increase in layer thickness most often occurs as a result of an increased adsorbed amount. Here, however, the increase in layer thickness occurs despite a reduction in the adsorbed amount. This can be understood as resulting from a reduced polyelectrolyte-surface affinity and a swelling of the adsorbed layer. Finally, it is demonstrated that the employed techniques complement each other and reveal new information on the interaction forces and conformation of polyelectrolytes at the solid-liquid interface. Copyright 1998 Academic Press.

Interactions between Adsorbed Layers of a Low Charge Density Cationic Polyelectrolyte on Mica in the Absence and Presence of Anionic Surfactant

Interactions between two negatively charged mica surfaces across aqueous solutions containing various amounts of a 10% charged cationic polyelectrolyte have been studied. It is found that the mica surface charge is neutralized when the polyelectrolyte is adsorbed from a 10-50 ppm aqueous solution. Consequently no electrostatic double-layer force is observed. Instead an attractive force acts between the surfaces in the distance regime 250-100 A. We suggest that this attraction is caused by bridging. Additional adsorption takes place when the polyelectrolyte concentration is increased to 100 and 300 ppm, and a long-range repulsion develops. This repulsive force is both of electrostatic and steric origin. The polyelectrolyte layer adsorbed from a 50 ppm solution does not desorb when the polyelectrolyte solution is replaced with an aqueous polyelectrolyte-free solution. Injection of sodium dodecyl sulfate (SDS) into the measuring chamber to a concentration of about 0.01 CMC (8.3 x 10(-5) M ) does not affect the adsorbed layers or the interaction forces. However, when the SDS concentration is increased to 0.02 CMC (0.166 mM ) the adsorbed layer expands dramatically due to adsorption of SDS to the polyelectrolyte chains. The sudden swelling suggests a cooperative adsorption of SDS to the preadsorbed polyelectrolyte layer and that the critical aggregation concentration between the polyelectrolyte and SDS at the surface is about 0.02 CMC. The flocculation behavior of the polyelectrolyte in solution upon addition of SDS was also examined. It was found that 0.16-0.32 mol SDS/mol charged segments on the polyelectrolyte is enough to make the solution slightly turbid.

Alkyl Glucosides on Hydrophobic Surfaces Studied by Surface Force and Wetting Measurements

The wetting properties of hydrophobized surfaces in the presence of alkyl glucosides have been studied using a Wilhelmy balance technique, and the interparticle forces acting between such surfaces have been investigated using the interferometric surface force apparatus. The adsorption at concentrations well below the cmc is rather limited both for the anomerically pure octyl beta-glucoside and for the technical product based on the branched octyl glucoside, 2-ethylhexyl glucoside. However, the adsorption results in a decreasing surface hydrophobicity, and consequently the force needed to separate two such surfaces from each other is reduced. At the cmc for octyl beta-glucoside, a surfactant monolayer is adsorbed on each surface. The interaction between the glucose units gives rise to a short-range repulsive force. Beyond the range of this repulsive force a weak attraction is experienced. Addition of a small amount of octyl alpha-glucoside does not change the short-range interaction significantly. The technical 2-ethylhexyl glucoside surfactant behaves similarly to the anomerically pure surfactant at low concentrations. The solution with the technical glucoside phase-separates at high surfactant concentration. It is observed that under such conditions a thick layer is deposited on each surface.