Stimulus-Responsive Au@(MeO₂MA_<i>x</i>-<i>co</i>-OEGMA_<i>y</i>) Nanoparticles Stabilized by Non-DLVO Interactions: Implications of Ionic Strength and Copolymer (<i>x</i>:<i>y</i>) Fraction on Aggregation Kinetics

Functionalized nanoparticles can assist in stabilizing fluid–fluid interfaces; however, developing and applying the appropriate surface modification presents a challenge because successful application of these nanomaterials for biotechnological, food processing, and environmental applications requires their long-term stability in elevated ionic strength media. This work studies stimulus responsive polymeric materials based on random copolymers of di­(ethylene glycol) methyl ether methacrylate (<i>x </i>= MeO₂MA) and oligo­(ethylene glycol) methyl ether methacrylate (<i>y</i>= OEGMA) which, when grafted to gold nanoparticles, show significant, tunable, colloidal stability. The nanoparticles Au@(MeO₂MA_<i>x</i>-<i>co</i>-OEGMA_<i>y</i>) display tunable, reversible aggregation that is highly dependent on the (<i>x</i>:<i>y</i>) ratio and ionic strength. Effects of these parameters on the initial rate constant of aggregation (<i>k</i>_<i>11</i>) are studied by time-resolved dynamic light scattering (TR-DLS) experiments. At the same nanoparticle concentration, a strong sensitivity to salt concentration is observed. Over less than 300 mM increase in NaCl concentration, we observed a two-order of magnitude increase in aggregation rate constants, 4.2 × 10^–20 < <i>k</i>_<i>11</i> < 1.8 × 10^–18 m³s^–1. Additionally, for the same gold nanoparticles, a higher fraction of OEGMA requires a higher salt concentration to induce aggregation. A linear relationship between the critical NaCl coagulation concentration (CCC) and the copolymer composition is observed. Analysis of the experimental data with an extended Derjaguin–Landau–Verwey–Overbeek (xDLVO) theory that includes hydration and osmotic forces is used to explain the stability of these systems. We find the hydration pressure, 2.4 < <i>P</i>_h,0 < 7.2 MPa, scales linearly both with the osmotic pressure and the OEGMA monomer concentration (5 <<i> y</i> < 20%). Specific knowledge of <i>P</i>_h,0(<i>y, C</i>_NaCl) enables design of both aggregation kinetics and stability as a function of the copolymer ratio and external stimuli

Role of Collector Alternating Charged Patches on Transport of <i>Cryptosporidium parvum</i> Oocysts in a Patchwise Charged Heterogeneous Micromodel

The role of collector surface charge heterogeneity on transport of <i>Cryptosporidium parvum</i> oocyst and carboxylate microsphere in 2-dimensional micromodels was studied. The cylindrical silica collectors within the micromodels were coated with 0, 10, 20, 50, and 100% Fe₂O₃ patches. The experimental values of average removal efficiencies (η) of the Fe₂O₃ patches and on the entire collectors were determined. In the presence of significant (>3500 kT) Derjaguin–Landau–Verwey–Overbeek (DLVO) energy barrier between the microspheres and the silica collectors at pH 5.8 and 8.1, η determined for Fe₂O₃ patches on the heterogeneous collectors were significantly less (<i>p</i> < 0.05, <i>t</i> test) than those obtained for collectors coated entirely with Fe₂O₃. However, η calculated for Fe₂O₃ patches for microspheres at pH 4.4 and for oocysts at pH 5.8 and 8.1, where the DLVO energy barrier was relatively small (ca. 200–360 kT), were significantly greater (<i>p</i> < 0.05, <i>t</i> test) than those for the collectors coated entirely with Fe₂O₃. The dependence of η for Fe₂O₃ patches on the DLVO energy barrier indicated the importance of periodic favorable and unfavorable electrostatic interactions between colloids and collectors with alternating Fe₂O₃ and silica patches. Differences between experimentally determined overall η for charged heterogeneous collectors and those predicted by a patchwise geochemical heterogeneous model were observed. These differences can be explained by the model’s lack of consideration for the spatial distribution of charge heterogeneity on the collector surface