Manipulating microparticle interactions using highly charged nanoparticles

An experimental study was performed to investigate the potential use of highly charged, spherical nanoparticles to control the forces between spherical microparticles, with the aim of developing novel methods for reversibly controlling the stability of a colloidal dispersion. A specific focus of the project was to determine the degree to which the nanoparticles strongly adsorb to the microparticles, as such adsorption would severely limit reversibly controlling forces and stability. Colloidal probe atomic force microscopy (CP-AFM) was used to measure the force between a silica microparticle and a silica plate in solutions of highly charged polystyrene nanoparticles at varying pH, covering the range over which the silica surfaces were very weakly charged (e.g., pH ≤ 3) to highly charged (e.g., pH 6.0). It was found that except for the highest pH, where the silica zeta potential was approximately −60 mV, addition of relatively low concentrations of the nanoparticles (e.g., 0.1% vol.) led to an increased repulsion between the microparticle and plate. This increased repulsion was evident even at pH 4.0 when the fractional surface coverage of the nanoparticles was only 1%. At low nanoparticle concentrations, this force decayed exponentially with a decay length equal to the bulk Debye length, indicating that it was electrostatic in origin. At higher nanoparticle concentrations (1.0% vol.), long-range depletion forces became significant, causing a deviation from the exponential behavior. This increased repulsion did not disappear upon flushing the nanoparticles out of the system, indicating that the nanoparticles were held in relatively deep energy wells. These results suggest that strong adsorption of nanoparticles should be expected in all but the most highly repulsive systems and also illustrate the challenges associated with using charged nanoparticles as a tool for controlling stability. © 2012 Elsevier B.V. All rights reserved. . Introduction and background The use of nanometer-sized particles (nanoparticles) to alter he interaction force between larger colloidal particles (microparicles) has been investigated for well over 50 years and continues o be a topic of interest. In the early 1950s, Asakura and Oosawa erived a simple algebraic expression for the interaction between wo hard spherical particles in a solution of nonadsorbing hard pherical ‘macromolecules’ that was based on the increase in free olume available to the macromolecules upon close approach of he two particles [1,2]. (An identical expression can also be derived y calculating the osmotic pressure gradient due to exclusion of the anoparticles from the gap region [3].) Since the initial work of Asakura and Oosawa, other researchers ave investigated the effects of nanoparticle charge and shape n the resulting depletion force [3–15]. It is also recognized hat as the concentration of nanoparticles increases beyond the ilute limit, interactions between the nanoparticles can lead to ∗ Corresponding author. Tel.: +1 540 231 4213; fax: +1 540 231 5022. E-mail address: [email protected] (J.Y. Walz). 927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. oi:10.1016/j.colsurfa.2011.12.040 oscillations in the local density of nanoparticles next to the surface of a microparticle, and the overlap of these density oscillations leads to an oscillatory force profile between the microparticles. These structural forces have also been studied in great detail, both computationally and experimentally [16–27]. In the majority of the studies referenced above, the assumption has been that the nanoparticles and microparticles are mutually repulsive, such as hard sphere or electrostatic repulsions arising from surface charges. Such repulsion leads to negative adsorption of the nanoparticles onto the surface. Recently, however, several researchers have begun investigating systems in which the microparticle–nanoparticle interaction is much weaker, or even locally attractive. For example, McKee and Walz [28] used colloid probe atomic force microscopy to measure the force profile between a large glass sphere and a glass substrate in solutions of either polystyrene or zirconia nanoparticles. The systems were conducted at pH values near the isoelectric point of the glass such that the average zeta potential of these surfaces was quite low. In addition, the impurities present in the glass (e.g., potassium, aluminum, calcium, and magnesium) allowed significant deposition of the nanoparticles to occur. Unlike the results at high pH when all surfaces were highly negatively charged, McKee and Walz found