Article pubs.acs.org/Langmuir
Effects of Ionic Strength on the Colloidal Stability and Interfacial Assembly of Hydrophobic Ethyl Cellulose Nanoparticles Navid Bizmark and Marios A. Ioannidis* Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada
Downloaded by TEXAS A&M INTL UNIV on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 20, 2015 | doi: 10.1021/acs.langmuir.5b01857
S Supporting Information *
ABSTRACT: Nanoparticle attachment at a fluid interface is a process that often takes place concurrently with nanoparticle aggregation in the bulk of the suspension. Here we investigate systematically the coupling of these processes with reference to the adsorption of aqueous suspensions of ethyl cellulose (EC) nanoparticles at the air− water interface. The suspension stability is optimal at neutral pH and in the absence of salt, conditions under which the electrostatic repulsion among EC nanoparticles is maximized. Nonetheless, hydrophobic attraction dominates particle−interface interactions, resulting in the irreversible adsorption of EC nanoparticles at the air−water interface. The addition of salt weakens the particle−particle and particle−interface repulsive electrostatic forces. This leads to destabilization of the suspension at ionic strengths of 0.05 M or greater but does not affect nanoparticle adsorption. The energy of adsorption, the surface tension and interface coverage at steady state, and the particle contact angle at the interface all remain unchanged by the addition of salt. These findings contribute to the fundamental understanding of colloidal systems and inform the utilization of EC nanocolloids, in particular for the stabilization of foams and emulsions.
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INTRODUCTION
The formation of nanoparticle aggregates (flocs) depends on the relative magnitude of attractive and repulsive interparticle forces. When attractive forces prevail, an initially homogeneous colloidal suspension is reduced over time to a phase-separated system through a process of particle aggregation and settling.20 The quantitative basis for assessing colloidal stability is the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory.20 In its original version, DLVO theory considers only van der Waals and electrostatic (double-layer) forces. Whereas classical DLVO theory is employed widely to predict the kinetics of nanoparticle coagulation,21−24 the need to include additional (so-called non-DLVO) forces of steric, hydrophobic, or acid/ base origin is widely recognized.25−28 The hydrophobic attractive force is a non-DLVO force manifested when at least one side of the interaction domain is hydrophobic.29 Significant deviations from classical DLVO theory have been attributed to hydrophobic interactions,29−31 although to date the quantification of the hydrophobic force remains empirical and its origin is still the subject of considerable debate.32 For a system containing colloidal EC nanoparticles, it has been proposed33 that the competition between attractive van der Waals and hydrophobic forces on one hand and repulsive electrostatic forces on the other hand controls system behavior. Colloidal stability and interfacial assembly are governed by the balance between these forces, a balance that shifts with changes in pH, ionic strength, and temperature. Coagulation in the bulk and adsorption at the fluid interface take place concurrently
The potential of colloidal nanoparticles to effectively stabilize foams and emulsions has been actively investigated during the past two decades. The use of nanoparticles represents an evolution in the formulation of so-called Pickering emulsions, a term which was coined a century ago when emulsions were successfully stabilized by suspended solid particles.1 Particlestabilized foams and emulsions have been used in a variety of applications, including templates for the synthesis of novel materials,2−4 drug delivery and controlled drug release,5,6 and oil recovery from subsurface formations.7,8 Hydrophobic ethyl cellulose (EC) nanoparticles, in particular, are biodegradable, demonstrate remarkable foamability and foam stability,9 and have found applications in the encapsulation of pharmaceuticals and controlled release.10−12 A preference for nanoparticlestabilized foams and emulsions over surfactant-stabilized ones is rooted in the superior stability of the former, a feature which has been the focus of several studies.13−15 The remarkable stability of particle-stabilized foams and emulsions is linked, at least in part, to the large amount of energy (several orders of magnitude larger than thermal fluctuations) released when a particle is adsorbed at the fluid interface, which makes the adsorption irreversible. The stability of foams and emulsions is optimal at specific values of ionic strength, pH, and temperature.9,16−19 Specifically for EC nanoparticles, it has been reported9 that a more stable foam is generated when the pH is low (
