Silica particles polymers adsorbed

Snowden, M.J., Clegg, S.M., Williams, P.A., Robb I.D. (1991). Flocculation of silica particles by adsorbing and non-adsorbing polymers. Journal of the Chemical Society, Faraday Transactions, 87, 2201-2207. 

The amounts oi adsorption of the polymer on latex and silica particles were measured as follows. Three milliliters of the polymer solution containing a known concentration was introduced into an adsorption tube(lO ml volume) which contained 2 ml of latex (C = l+.O wt %) and silica(C = 2.0 wt %) suspensions. After being rotated(l0 rpm) end-over-end for 1 hr in a water bath at a constant temperature, the colloid particles were separated from the solution by centrifugation(25000 G, 30 min.) under a controlled temperature. The polymer concentration that remained in the supernatant was measured colorimetrically, using sulfuric acid and phenol for the cellulose derivatives(12), and potassium iodide, iodine and boric acid for PVA(13). From these measurements, the number of milligrams of adsorbed polymer per square meter of the adsorbent surface was calculated using a calibration curve. 

Polymer beads have also been tagged by treating them after each new diversity-introducing reaction with dye-containing, colloidal silica particles, which can be irreversibly adsorbed on the surface of the beads with the aid of polyelectrolytes such as poly(diallyldimethylammonium chloride) and poly(acrylic acid) [42,43]. Larger portions of support can also be linked to a chip that enables electronic tagging with a radio emitter [44-46]. 

Fontana and Thomas2 determined p, 6, and T for poly (lauryl methacrylate) of molecular weights of 33 X 103 to 1190 X 103 adsorbed from n-dodecane and cis-decalin onto silica particles by observing IR shifts of the C = O bands in the polymer and the surface silanol group. They found that 6 increased whereas p remained nearly constant with rising T. 

Fig. 3 Example of the microdomains created by embedding luminescent indicator-loaded silica gel nanoparticles into poly(dimethylsiloxane) before cross-linking to fabricate films for oxygen optosensing. The largest circles represent the silica particles the black lines represent the PDMS polymer chains, while the smallest circles represent the indicator dye molecules (the black circles depict those located in the organic polymer-free silica regions and the gray circles stand for those adsorbed on silica regions in contact with the PDMS)...

In open-column liquid chromatography, the test sample is added to the top of a column packed with adsorbent material (e.g. alumina, silica gel, polymer gel or fine-particle substrate coated with an organic compound). Differential movement... 

In order to apply the above procedure to determine the conditions of phase separation, we have chosen the system of polyisobutene-stabilized silica particles with polystyrene as the free polymer dissolved in cyclohexane. The system temperature is chosen to be the 8 temperature for the polystyrene-cyclohexane system (34.5°C), corresponding to the experimental conditions of deHek and Vrij (1). The pertinent parameters required for the calculation of the contribution of the adsorbed layers to the total interaction potential are a = 48 nm, u, =0.18 nm3, 5 = 5 nm, Xi = 0.47(32), X2 = 0.10(32), v = 0.10, and up = 2.36 nm3. It can be seen from Fig. 2 that these forces are repulsive, with very large positive values for the potential energy at small distances of separation and falling off to zero at separation distances of the order of 25, where 6 is the thickness of the adsorbed layer. At the distance of separation 5, the expressions for the interpenetration domain and the interpenetration plus compression domain give the same value for the free energy, indicating a continuous transition from one domain to the other. 

Fig. 1. Interaction potential between two colloidal particles as a function of the reduced centre-to-centre separation R = r/2a, where a is the radius of the particles. Curve 1, steric repulsion due to the adsorbed layer (Vs) curve 2, attraction due to the free polymer (Vd) curve 3, van dcr Waals attraction (X7.,) curve 4, sum of the contributions given by curves 1—3. System polvisobutene-stabilized silica particles and polystyrene (free polymer) in cyclohexane at 308 K. Molecular weight of the free polymer = 82,000, volume fraction of polystyrene, 0 = 0.02, a = 48 nm, thickness of the adsorbed layer 6 = 5 nm, x = 0.5 for polystyrene—cyclohexane, x, = 0.47 and xs = 0.10 for polyisobutene— cyclohexane, AjkT 4.54 and v = 0.10.

Fig. 5. The dependence of the limiting volume fraction of the free polymer,

The other kind of systems largely studied, consists of polymethylmethacrylate (PMMA) or silica spherical particles, suspended in organic solvents [23,24]. In these solvents Q 0 and uy(r) 0. The particles are coated by a layer of polymer adsorbed on their surface. This layer of polymer, usually of the order of 10-50 A, provides an entropic bumper that keeps the particles far from the van der Waals minimum, and therefore, from aggregating. Thus, for practical purposes uw(r) can be ignored. In this case the systems are said to be sterically stabilized and they are properly considered as suspensions of colloidal particles with hard-sphere interaction [the pair potential is of the form given by Eq. (5)]. 

Figure 2. Scattering from silica particles bound with strongly cationic polymers ( ), compared with that from free silica particles (+). The particles are spheres of precipitated silica with a radius of 19 nm In water at pH near 7 they bear 0.3 negative charge per nm of surface, most of which Is compensated by adsorbed counterions (15). The polymers are AM-CH copolymers with a ratio of cationic to total monomers equal to 0.3 the total amount of polymer In the floe approximately compensates the chemical charge borne by the silica particles (9).

After cross-linking of the VFA and BVU on the silica particle surface the IEP shifts from pH 3 (bare silica) to pH 5 (PVFA-co-PBVU/silica). The shape of the function = (pH) remains unchanged. It can be assumed that the silanol groups on the silica surface are weakly shielded by the adsorbed sections of the cross-linked polymer because covalent attachment of the polymer does not take place [63, 73]. Strong effects can be seen after conversion of the formamide groups into amino groups. The IEP is shifted into the... 

The functionalization of spherical silica particles and silicon wafers with PVFA-co-PVAm has been carried out using various synthetic procedures with VFA as the key monomer. The simplest approach used to produce PVFA-co-PVAm/ silica hybrid materials is adsorption of PVFA-co-PVAm on the inorganic component from aqueous solution. Depending on the copolymer composition about 0.02 to 0.1 g PVFA-co-PVAm per gram silica is immobilized. The increase of the PVAm content of the co-polymer leads to a significant increase in the amount of adsorbed PVFA-co-PVAm. 

Steric stabilization is another well-established method of stabilizing colloidal suspensions of submicron to micron size [23]. The particles are coated with a layer of adsorbed or grafted polymer chains that provides a steric repulsion of entropie origin and helps disperse the particles by counterbalancing van der Waals attraction (Fig. la). The polymeric nature of the adsorbed or grafted layer softens the interparticle interactions and makes the particles intrinsically deformable. Many polymer chain/particle combinations have been synthesized and studied, and are described in the literature. Several popular colloidal systems consist of silica particles covered with various polymers such as polydimethylsiloxane [24], stearyl alcohol [25], alkyl chains [26], and polyethylene oxide [27]. Polymethylmethacrylate and polystyrene particles grafted with polymer chains have also been used extensively. For a review on the impressive literature on the subject we refer the interested reader to Vlassopoulos and Fytas [2]. 

Lipases are manufactured by fermentation of selected microorganisms followed by a purification process. The enzymatic interesterification catalysts are prepared by the addition of a solvent such as acetone, ethanol, or methanol to a slurry of an inorganic particulate material in buffered lipase solution. The precipitated enzyme coats the inorganic material, and the lipase-coated particles are recovered by filtration and dried. Various support materials have been used to immobilize lipases. Generally, porous particulate materials with high surface areas are preferred. Typical examples of the support materials are ion-exchange resins, silicas, macroporous polymers, clays, etcetera. Effective support functionality requirements include (i) the lipase must adsorb irreversibly with a suitable structure for functionality, (ii) pore sizes must not restrict reaction rates, (iii) the lipase must not contaminate the finished product, (iv) the lipase must be thermally stable, and (v) the lipase must be economical. The dried particles are almost inactive as interesterification catalyst until hydrated with up to 10% water prior to use. 

When the toner is loaded with pyrogenic silica particles, these appear as isolated protrusions at the sur ce (Fig. IB-D). Depending on the modification, the diameters of the protrusions range between 50 and 200 nm. When HMDS is used as the silylation reagent, a silane monolayer is formed. In this case the diameter of the particles is ca. 100 nm. Such particles are adsorbed mainly as isolated aggregates to the toner surface (Fig. IB). PDMS-modified particles seem to coat almost the entire toner particle (Fig. 1C). Their topographical diameter is doubled conqrared to that of the HMDS-coated particles. This can be explained by the formation of a polymer-like PDMS layer. HMDS/PDMS-coated silicas appear only as isolated protrusions with a diameter up to 250 run (Fig. ID). 

A noteworthy finding has been that all the materials show two distinct relaxation dynamics, a fast and a slow relaxation [60]. The fast mode corresponds to relaxation of bulky polymer molecules, while the slow mode is related to relaxation of the filler structure with much longer time scales. As silica particles are physically connected with adsorbed polymer molecules, the formed polymer-particle network is a temporary physical network. On a long time scale, relaxation of this network occurs when immobilized polymer molecules connecting silica particles become free, via dissociation from silica particles or disentanglement from other immobilized polymer molecules. 

Hydrophobized silica nanoparticles were obtained by adsorbing the cationic surfactant CTMA-Cl on the surface subsequently the silica particles could be incorporated in polymer nanoparticles (Fig. 13b). Depending on the reaction conditions... 

---