A new mechanism for regulating the stability of colloidal particles has been discovered. Negligibly charged colloidal microspheres, which flocculate when suspended alone in aqueous solution, undergo a remarkable stabilizing transition upon the addition of a critical volume fraction of highly charged nanoparticle species. Zeta potential analysis revealed that these microspheres exhibited an effective charge buildup in the presence of such species. Scanning angle reflectometry measurements indicated, however, that these nanoparticle species did not adsorb on the microspheres under the experimental conditions of interest. It is therefore proposed that highly charged nanoparticles segregate to regions near negligibly charged microspheres because of their repulsive Coulombic interactions in solution. This type of nanoparticle haloing provides a previously unreported method for tailoring the behavior of complex fluids. Colloidal suspensions enjoy widespread use in applications ranging from advanced materials to drug delivery. By tailoring interactions between colloidal particles, one can design stable fluids, gels, or colloidal crystals needed for ceramics processing (1), coating (2), direct write (3), photonic (4-9), and pharmaceutical (10, 11) applications. Long range, attractive van der Waals forces are ubiquitous and must be balanced by Coulombic, steric, or other repulsive interactions to engineer the desired degree of colloidal stability.The self-organization of highly charged nanoparticles and their influence on the behavior of complex fluids in which they dwell has received scant attention. The traditional view is that small particles or other species (e.g., polyelectrolyte, polymer, or micelles) in solution can promote flocculation of stable colloidal suspensions via an entropic depletion interaction (12-15). The term ''depletion'' describes the exclusion of these smaller species from the gap region between colloidal particles that arises when their separation distance becomes less than the characteristic depletant size. The resulting concentration gradient between the gap region and bulk solution gives rise to an attractive force, whose magnitude scales with the volume fraction of smaller species, their charge, and the size ratio of large to small species (12,15,16). However, emerging theoretical work (17-19) suggests that charged species in solution may affect system stability through other self-organizing pathways. For example, charged nanoparticles have been predicted to segregate to regions surrounding large uncharged colloids, especially in systems with high size asymmetry and many more small to large spheres (18). This segregation is driven solely by a Coulombic repulsion between smaller species in solution and occurs simply because the larger particles represent a big volume without charge. The key question we wish to explore is whether this type of haloing process can provide a mechanism for stabilizing colloidal species.Here, we study the effects of highly charged nanoparticles on the behavior of ne...
Cetyltrimethylammonium bromide (CTAB) induces partially irreversible compaction of DNA-adsorbed layers on hydrophobic silica. Additionally, there is a synergistic increase in the adsorbed amount when both CTAB and DNA are present as compared to the surface excess concentration of either of the individual components. In this study of the DNA adsorption and DNA-CTAB coadsorption by ellipsometry, emphasis has been placed on the DNA molecular weight as well as its conformation (single and double stranded). The DNA molecular weight and conformation have a large effect on the surfactant-free DNA adsorption behavior but not on the mixed DNA-CTAB adsorption behavior. Comparison between interfacial and bulk complexation has been made where possible. The DNA-CTAB complexes at the interface are neutral despite the large excess of DNA in the bulk. The final structure of the adsorbed layer was found to be dependent on the history of complex formation and DNA size.
Interfacial properties can be tuned by exploiting polymer/surfactant interactions. We find that coadsorption of the anionic surfactant sodium dodecyl sulfate (SDS) and the amphiphilic triblock copolymer poly-(ethylene oxide-b-propylene oxide-b-ethylene oxide), Pluronic F108, to silica is extremely sensitive to SDS concentration and ionic strength. First, using a pyrene solubilization assay we identify the surfactant concentration regimes where different F108/SDS aggregates form in bulk solution at several ionic strengths. We then measure the total surface excess concentration of coadsorbing F108 and SDS using optical reflectometry. Above the critical aggregation concentration where F108/SDS aggregates form, the coadsorbed amount decreases with increasing surfactant concentration until an SDS concentration is reached at which adsorption is prevented entirely. Furthermore, although adsorbed layers containing only F108 are irreversibly adsorbed, F108/SDS layers are reversibly adsorbed. These results suggest that F108 is "shuttling" the normally nonadsorbing SDS to the silica surface. At high ionic strength, we find that sequential coadsorption followed by removal of SDS from the adsorbed layer results in an enhanced adsorbed amount of F108 (compared to direct adsorption of F108 in the absence of SDS). Thus, surfactant-free F108 layers can be "sculpted" into a different conformation by sequential processing with SDS. Finally, scaling of our coadsorption data with the bulk binding transitions (onset of cooperative binding and saturation of the polymer) indicates that changes in adsorbed amount occur at SDS concentrations both below where aggregates form and above the point where the polymer is saturated in the bulk.
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