Structural and Double Layer Forces between Silica Surfaces in Suspensions of Negatively Charged Nanoparticles

Abstract

Direct force measurements between negatively charged silica microparticles are carried out in suspensions of like-charged nanoparticles with the atomic force microscope (AFM). In agreement with previous studies, oscillatory force profiles are observed at larger separation distances. At smaller distances, however, soft and strongly repulsive forces are present. These forces are caused by double layer repulsion between the like-charged surfaces and can be quantitatively interpreted with the Poisson-Boltzmann (PB) model. However, the PB model must be adapted to a strongly asymmetric electrolyte to capture the non-exponential nature of these forces. Thereby, the nanoparticles are modeled as highly charged co-ions, while the counter ions are monovalent. This model permits to extract the effective charge of the nanoparticles, which is well comparable to the one obtained from electrophoresis. The PB model also explains the presence of a particle-free layer close to the interface.This dependence originates from the double layer repulsion between the nanoparticles, which forces them into a liquid-like structure. Equation ( 1) follows from geometrical considerations by identifying the wavelength with the spacing between nearest neighbors in a close-packed structure. The same scaling dependence was observed with X-rays or neutrons, in particular, with small angle scattering through the position of the structural peak in bulk suspensions 12,18 or with reflectivity near an isolated interface. 19,20 Similar oscillatory forces were also observed in concentrated polyelectrolyte [21][22][23][24] and micellar solutions. 25,26 While this similarity is not surprising for spherical micelles, an analogous structuring also occurs in polyelectrolyte solutions. In the latter case, the wavelength follows the scaling law given in eq.(1) only at lower concentrations. A similar scaling law remains applicable at higher concentrations, albeit with a different exponent. 23,27,28 Less information is available on double layer forces acting in these systems. In the standard experiments the nanoparticles and the planar (or quasi-planar) surfaces are negatively charged. Therefore, the presence of strongly repulsive double layer forces is expected too. Additional exponential repulsive forces were found recently in such systems, but these forces seem relatively weak. 29 In solutions of polyelectrolytes, strongly repulsive double layer forces were recently reported by some of us. 28 In the latter case, these forces were non-exponential, but compatible with predictions of Poisson-Boltzmann (PB) theory.Structuring and forces in colloidal suspensions of charged particles have also attracted much attention in the theoretical community. [30][31][32][33] Thereby, a more rigorous description beyond the PB theory was pursued, typically with computer simulations or integral equation theories. Some researchers have considered the so-called primitive model, where the macroions and counter ions are treated on equal footing. Another approach is to only treat the na...

Thickness of the particle-free layer near charged interfaces in suspensions of like-charged nanoparticles

“…However, experimental evidence concerning this layer is relatively scarce. Its existence has been confirmed in the slit geometry by direct force measurements in nanoparticle suspensions 9,10 and polyelectrolyte solutions. 19,20 The presence of a particle-free layer was also suggested in nanoparticle suspensions in contact with an isolated interface based on X-ray 11 and neutron reflectivity.…”

“…Subsequently, we report on measurements of the thickness of this particle-free layer in different nanoparticle suspensions with three different experimental techniques. Besides the established direct force measurements 4,9,25 and QCM technique 21,26,27 , we demonstrate that the thickness of this layer can be also measured with optical reflectivity. 28,29 We further argue that the characteristic separation between two interfaces, as observed in direct force measurements, corresponds to twice the layer thickness for an isolated interface.…”

“…Silica (HS30) and sulfate latex nanoparticle suspensions were purchased from Sigma-Aldrich and Invitrogen, respectively. The used batch of silica particles is similar to the one used in our previous studies 9,12 , but is not identical to the latter. Latex A is the same as the one used in a previous study by Scarratt et al 9 Prior to experiments, impurities were removed from the suspensions by ultrafiltration or dialysis.…”

“…The validity of the PB description for interaction forces was demonstrated for multivalent coions some time ago 34 , and more recently for polyelectrolytes and nanoparticles. 9,19 Equation ( 9) is useful to obtain an analytical estimate of the thickness of the salt-free gap through the condition ( 12)…”

“…The interactions between interfaces were mostly probed with various techniques in the slit geometry, including interferometry 5,6 , optical tweezers 1 , or the colloidal probe technique based on atomic force microscopy (AFM). 2,4,[7][8][9][10] The interesting observation made in these studies was that the typical force profiles were oscillatory. The corresponding wavelength was found to decrease with increasing particle concentration, thereby reflecting a liquid-like structure of the nanoparticle suspension.…”