# Hybrid Colloids Made with Polymers
## 7 Hybrids Made of Nanoparticles  
Covalent and noncovalent functionalization modes give rise to such hybrids. The noncovalent ones occur through physical adsorption of polymers, or surfactants, on _NPs_. The above two adsorbents share several points in common, and substantial differences. Surfactants easily adsorb, but tend to be released the same; therefore, polymers are by far preferred. Polymer adsorption occurs by _NP_ dipping, eventually with co-solvents. Such preparation procedures date back to old times. Consider that _kajl_, decorating the lashes and brows of [PERSON] [43], is a very fine powder obtained by mixing galena, malachite, and antimony, and dispersing it in acacia resin, a polysaccharide. Kajl is, thus, a mixture of three different _hybrids_, and is still in use for aesthetic and eye-disinfect purposes. Figure 4: The hydrodynamic radii of cat-anionic _DDAB-SDS_ vesicles, \(<\)_R\({}_{H^{2}}\)_ (in nm), interacting with _BSA_ (bovine serum albumin) at _pH_ = 6.8 and 25.0 \({}^{\circ}\)C. The term _DDAB_ indicates didodecyldimethyl-lammonium bromide, and _SDS_ is an acronym of sodium dodecylsulfate. The _DDAB-SDS_ mole ratio is 3.8/1, when the overall surfactant content is 10.0 mmol kg\({}^{-1}\). Similar trends were observed if _pH_ is above the protein iso-electric point (and the protein, thus, negatively charged). The region drawn in yellow is turbid and polyphasic; it indicates the coexistence of very large adducts (some microns) and a precipitate. Symbols are larger than experimental errors. The figure is redrawn from Ref. [42]. Surface coverage prevents _NP_ coalescence (Figure 5). Polymer-wrapped _NPs_, _PW_-_NPs_, are relatively stable and remain as such for a long time. They are obtained in different ways: information on the preparation is outside our purposes, and is given in selected articles and reviews [44; 45; 46; 47; 48]. Hybrids are formed by silica and titania nanoparticles, carbon-based entities, fullerenes, graphenes, carbon nanotubes, glass spheres, polymeric nanoparticles, such as _PMMA_ (polymethyl-methacrylate), inorganic oxides, metals (including gold and silver), and so on. Among the many possibilities inherent to the above particles, let us refer to silica, titania, and carbon nanotubes (_CNTs_). The substantial differences among them are related to their different physical properties. Silica and titania are formed by reaction of their alcovides with ammonia in ethanol. Bare _CNTs_, conversely, have very high aspect ratios (mostly if they are single-walled), are strongly hydrophobic, and form bundles in polar media. To reduce their aspect ratios, they are functionalized [49; 50]. On this purpose, _CNTs_ are reacted with H2 SO4-HNO3 mixtures, or with H2O2. As a consequence of drastic oxidation, the axial ratios reduce significantly and polar groups, usually carboxylates, are introduced on the outer _CNTs_ surface. Further functionalization modes are possible by adsorption of oppositely charged species or by covalent reaction with proper species. Physical wrapping is concomitant to polymer partition with the bulk; depletion may occur [51; 52; 53; 54]. The latter phenomenon is due to unbalanced osmotic effects and dispersant flow from regions between the adducts; it depends on the polymer volume fraction, \(\varphi\), and on that of _NPs_. Depletion was observed in solutions of homo-polymers, co-polymers, poly-electrolytes, polymer-surfactant systems, surfactants, and their mixtures. Wrapping is controlled by electrostatic, steric, hydrophobic, \(vdW\) forces, and many combinations thereof [55]. Wrapping gives rough surfaces, with solvent molecules located in the polymer layers. Some polymer portions surface-adsorb; others protrude to the bulk. The protrusion probability is high; cases are known where extended coronas self-assemble around _NPs_[56; 57]; the wrapped layer can be some nanometers thick and uniform. Figure 5: Sketch of interparticle interaction modes. The repulsive one (implying stabilization) is reported above.