Abstract
Forces between charged particles in aqueous solutions containing multivalent coions and monovalent counterions are studied by the colloidal probe technique. Here, the multivalent ions have the same charge as the particles, which must be contrasted to the frequently studied case where multivalent ions have the opposite sign as the substrate. In the present case, the forces remain repulsive and are dominated by the interactions of the double layers. The valence of the multivalent coion is found to have a profound influence on the shape of the force curve. While for monovalent coions the force profile is exponential down to separations of a few nanometers, the interaction is much softer and longer-ranged in the presence of multivalent coions. The force profiles in the presence of multivalent coions and in the mixtures of monovalent and multivalent coions can be accurately described by Poisson-Boltzmann theory. These results are accurate for different surfaces and even in the case of highly charged particles. This behavior can be explained by the fact that the force profile follows the near-field limit to much larger distances for multivalent coions than for monovalent ones. This limit corresponds to the conditions with no salt, where the coions are expelled between the two surfaces.
Highlights
The systems investigated mostly included simple monovalent electrolytes and surfaces of mica, latex, or various oxides.[5,9,10,11,12,13,14,15,16,17,18,19,20]. These studies reveal that at lower salt concentrations the force pro les in these systems are dominated by double layer forces, and they can be well described by the mean- eld Poisson– Boltzmann (PB) theory or at lower charge densities by the linearized Debye–Huckel (DH) theory.[2,21,22]
We show that multivalent coions may induce unusually so and long-ranged double layer forces, which can be accurately described by PB theory
We present direct force measurements between pairs of similar colloidal particles in aqueous solutions containing multivalent coions
Summary
Electric double layer forces between solid substrates across aqueous solutions are currently a topic of intense research.[1,2,3,4,5,6,7,8] Such forces are becoming routinely accessible by various methods, including the surface force apparatus, the colloidal probe technique, or total internal re ection microscopy.[1,2,3,4,5,6,9,10,11,12,13,14,15,16] Double layer forces originate from overlapping diffuse layers forming near charged water–solid interfaces. This approximation assumes a constant capacitance of the inner layer, and introduces only one additional parameter.[23,26,27] The PB theory fails at smaller distances or higher salt levels, where one must consider additional interactions, especially van der Waals or hydration forces.[11,14,15]
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