Abstract
The influence of particle adsorption on liquid/liquid interfacial tension is not well understood, and much previous research has suggested conflicting behaviors. In this paper we investigate the surface activity and adsorption kinetics of charge stabilized and pH-responsive polymer stabilized colloids at oil/water interfaces using two tensiometry techniques: (i) pendant drop and (ii) microtensiometer. We found, using both techniques, that charge stabilized particles had little or no influence on the (dynamic) interfacial tension, although dense silica particles affected the "apparent" measured tension in the pendent drop, due to gravity driven elongation of the droplet profile. Nevertheless, this apparent change additionally allowed the study of adsorption kinetics, which was related qualitatively between particle systems by estimated diffusion coefficients. Significant and real interfacial tension responses were measured using ∼53 nm core-shell latex particles with a pH-responsive polymer stabilizer of poly(methyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (pMMA-b-pDMAEMA) diblock copolymer. At pH 2, where the polymer is strongly charged, behavior was similar to that of the bare charge-stabilized particles, showing little change in the interfacial tension. At pH 10, where the polymer is discharged and poorly soluble in water, a significant decrease in the measured interfacial tension commensurate with strong adsorption at the oil-water interface was seen, which was similar in magnitude to the surface activity of the free polymer. These results were both confirmed through droplet profile and microtensiometry experiments. Dilational elasticity measurements were also performed by oscillation of the droplet; again, changes in interfacial tension with droplet oscillation were only seen with the responsive particles at pH 10. Frequency sweeps were performed to ascertain the dilational elasticity modulus, with measured values being significantly higher than previously reported for nanoparticle and surfactant systems, and similar in magnitude to protein stabilized droplets.
Highlights
The study of colloidal particle adsorption at liquid−liquid and liquid−air interfaces is paramount to understand the characteristics of particle-stabilized foams and emulsions[1,2] and to optimize development of novel functionalized materials such as colloidosomes.[3,4] Effectively irreversible adsorption of particles leads to the extreme stability of particle-stabilized emulsions and foams.[2,5−7] Once adsorbed at the interface, the particle diameter and equilibrium contact angle at the interface determine the strength of adsorption
There is significant literature in this area, most of which concerns the development of Pickering emulsions and foams, and their use as precursors for new functional materials and so forth.[8−14] while equilibrium stabilization mechanisms are relatively well understood, generally less is known about the influence of particle adsorption on the dynamic interfacial properties which are currently the subject of differing opinion.[15−18] For example, pendant drop tensiometry as a method to investigate dynamic interfacial tension has been extensively used for surfactant systems at liquid−liquid interfaces; there are few Received: December 15, 2015
This sedimentation will cause a deformation in the droplet shape due to the weight of particles collecting at the bottom of the drop; which will influence the drop shape and create an apparent change in the interfacial tension measurement when compared to a pure water droplet (Figure S2.1b in Supporting Information)
Summary
The study of colloidal particle adsorption at liquid−liquid and liquid−air interfaces is paramount to understand the characteristics of particle-stabilized foams and emulsions[1,2] and to optimize development of novel functionalized materials such as colloidosomes.[3,4] Effectively irreversible adsorption of particles leads to the extreme stability of particle-stabilized emulsions and foams.[2,5−7] Once adsorbed at the interface, the particle diameter and equilibrium contact angle at the interface determine the strength of adsorption. Vignati et al.[15] investigated particles comprising a fluorescent core (300 nm) with a pure silica surface shell (0.51 and 0.77 μm thick) produced using a modified Stöber process on iso-octane/water and octanol/water systems The adsorption of both hydrophilic and hydrophobic particles (by hexamethyldisilazane (HMS) treatment) was not seen to modify the interfacial tension at any particle concentrations studied. Hunter et al.[22] compared the adsorption of ∼260 nm hydrophobic silica particles with competing adsorption of surfactant at an air−water interface to determine the impact of foam stability They found that the particles alone had no influence on the measured dynamic surface tension, their displacement of surrounding surfactant did influence dilatational elasticity in mixed systems. Stocco et al.[23] studied 20 nm silica particles at the air/water interface
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