Hydrophobic silica particles

Although silicone oils by themselves or hydrophobic particles (e.g., specially treated silica) are effective antifoams, combinations of silicone oils with hydrophobic silica particles are most effective and commonly used. The mechanism of film destruction has been studied with the use of surface and interfacial tensions, measurements, contact angles, oil-spreading rates, and globule-entering characteristics for PDMS-based antifoams in a variety of surfactant solutions.490 A very recent study of the effect of surfactant composition and structure on foam-control performance has been reported.380 The science and technology of silicone antifoams have recently been reviewed.491... 

In a similar manner, OAV/O emulsions were prepared by first emulsifying oil into an aqueous dispersion of hydrophilic silica particles, and then gently reemulsifying the OAV emulsion so formed into an oil dispersion of hydrophobic silica particles. Figure 6.21 shows a typical microscopic image of a double emulsion with toluene as oil. 

In the case where foam instability is desirable, it is essential to choose surfactants that weaken the Gibbs-Marangoni effect. A more surface-active material such as a poly(alkyl) siloxane is added to destabilize the foam. The siloxane surfactant adsorbs preferentially at the air/liquid interface, thus displacing the original surfactant that stabilizes the foam. In many cases, the siloxane surfactant is produced as an emulsion which also contains hydrophobic silica particles. This combination produces a synergetic effect for foam breaking. 

The stability of pseudoemulsion film was studied directly in our laboratory, by forming such a film from a surfactant solution (0.06 M sodium dodecyl sulfate) on the tip of a capillary (Figure 33). The tip of the capillary, which was filled with antifoam oil, was covered with a small pseudoemulsion film from the surfactant solution. Then, the oil was slowly pushed out of the capillaiy. In this way, the area of the pseudoemulsion film and the capillary pressure was increased. The pseudoemulsion film between silicone oil containing no particles and air was relatively stable, and it did not rupture before the capillary pressure maximum, (i.e., hemispherical shape). However, when the oil contained hydrophobic silica particles, the film was much less stable. At high particle concentrations (1—6 wt%), the film ruptured shortly after it was formed, (i.e., at very low capillary pressures). 

The presence of fine hydrophobic particles dispersed into the oil phase helps to prevent the formation of very small oil drops and the spreading of the oil at the air—water interface. The particle—particle interactions inside the oil phase were estimated by measuring the settling volume of particles in oil (98). Particles settling under gravity formed a three-dimensional gel-type structure. With small particles, the sediment contained a very low amount of particles, and the final settling volume depended on the type of oil. For example, with 0.2-fim sized, hydrophobic silica particles, the sediment contained 1.2 vol% particles in decane and 1.45 vol% particles in DMPS-V silicone oil. These results show that... 

Reduction of surface unity All defoamers could be said to operate by disrupting the surface layer. Some defoamer chemistries are made of particles that migrate to the bubble surface and disrupt the regular packing of molecules at the water/ air interface. Some particles may absorb surfactant molecules onto their surface. This reduces the surface cohesion. Examples are hydrophobic silica particles, EBS particles, or silicone particles. 

Particulate dewatering An example of a particulate dewatering defoamer is one containing hydrophobic silica particles. The silica particles are coated with insoluble silicone films, resulting in particles that have such a low surface energy that they exert a dewatering action on the foam lamella. 

For the first time, siUca-filled poly(l-trimethylsilyl-l-propyne) (PTMSP) layers on top of UF membranes for the pervaporative separation of EtOH-water mixtures was reported by Claes et al. (2010). Reduction of the thickness of the separating PTMSP top layer and addition of hydrophobic silica particles resulted in a clear flux increase as compared with dense PTMSP membranes. The performances of the supported PTMSP-silica nanohybrid membranes were significantly better than the best conunercially available organophilic PV membranes. The developed composite PTMSP-silica nanohybrid membranes exhibited EtOH-water separation factors around 12 and fluxes up to 3.5 kg/m h, establishing a sevenfold to ninefold flux inCTcase as compared with dense PTMSP membranes. 

The addition of nanoparticles to an immiscible blend greatly influences the viscosity ratio of the dispersed to continuous phase which in turn changes the phase inversion composition. When hydrophobic silica particles were added to... 

Stable mixtures of hydrophilic and hydrophobic silicas with water at different ratio between the concentrations of these oxides and a constant total amount of oxides were also studied using electrophysical methods (Mironyuk et al. 1999). The conductivity (a) of the oxide blend at different frequencies of the applied electromagnetic field increases relative to that for hydrophilic silica. However, the a value at the constant electrical current decreases as the concentration of hydrophobic silica increases. An increase in the frequency from 0.1 to 10 kHz gives the exponential growth of the ratio between dielectric loss (e") and dielectric permittivity (e ) due to a strong decrease in the e value. These effects are caused by additional ordering in the system to reduce the contact area between hydrophobic silica particles and water that influences formation of micelles with a high polarizability. 

There are, at least, three reasons responsible for diminution of the hemolytic effect on interaction of RBCs with TMS/A-300 (i) strong aggregation of partially hydrophobic silica particles and diminution of the amounts of individual primary silica nanoparticles, which are maximum bioactive, that causes diminution of the total contact area between RBCs and silica (per gram of silica) and a decrease of local interaction of solid particles with membrane proteins, (ii) changes in interaction energy between modified silica surface and the membrane structures due to a decrease in... 

The mechanism by which bubbles in foam are broken by at least partly hydrophobic silica particles is discussed in Chapter 4. Finely divided silica aggregates, made suffi ciently hydrophobic to be suspendable in a hydrocarbon or silicone oil, probably should retain some hydrophilic areas to be held at the bubble surface. The silica is treated with a silicone oil and heated to 245 C to react it with the surface, then suspended in an oil (634). A surfactant in the oil may be included (635). 

Previously, it was found that the collapse pressure and the calculated contact angles of hydrophobic particles significantly depend on the number of spread particles [41]. The effect was attributed to a surface pressure gradient along the cohesive particulate layer. More moderated mass-dependence, as expected, was found for smallest hydrophobic silica particles [41]. In order to... 

More recent studies, based on fluorescence labelling and microscopy, by Wang et al. (14), were carried out with hydrophobic particles in mineral-based defoamers. In this work, it was clearly shown that the hydrophobic silica particles concentrate in the oil/water interface near the three-phase contact angle line. The sequence of... 

S. Emmett, S.D. Lubetkin, and B. Vincent The growth of ordered sediments of monodis-persed hydrophobic silica particles, CoUoids Surf., 42 (1989) 139-153... 

Some of these early hypotheses are essentially naive and are easily disposed of. Thus, Sinka and Lichtman [226] attributed the role of the particle to inhibition of solubilization of the oil. However, it is easily shown that the presence of concentrations of antifoam oil alone, well in excess of the (usually extremely low) amounts that are solubilized, may still produce a negligible antifoam effect with aqueous solutions of, say, anionic surfactants [43, 71]. Povich [209] showed that the increase in bulk shear viscosity of PDMS due to the presence of the silica is not responsible for the 7-fold increase in antifoam effectiveness accompanying the presence of that material in the oil. Moreover, Ross and Nishioka [210] showed that the presence of hydrophobed silica (at concentrations of the <5 wt.% normal in antifoams) has no effect on the surface shear viscosity of PDMS. Garrett et al. [43,71] have shown that although the presence of hydrophobed silica particles can facilitate dispersal of the antifoam oil, that is not the principal role of the particles. Thus, mineral oil dispersions of essentially the same size distribution as hydrophobed silica-liquid paraffin dispersions reveal a markedly different antifoam effectiveness. This is exemplified in Figure 4.76, where the relevant size distributions are compared with the respective antifoam effectiveness as measured by F for a solution of a commercial sodium alkylbenzene sulfonate. 

Bergeron et al. [70] have also studied the effect of hydrophobed silica on the stability of the pseudoanulsion films formed by PDMS oil (of molecular weight 10 ), but in aqueous micellar solutions of AOT. They showed that the relevant pseudoemulsion films drained to form indefinitely stable white films of thickness <100 nm when subjected to c illary pressures of 60 Pa. However, the presence of hydrophobed silica particles caused the films to rupture within 30 s, even at applied c iUary pressures <60 Pa. Unfortunately, the concentration of hydrophobed silica used in this smdy was not reported. 

FIGURE 4.79 Effect of spread PDMS oil layers on ease of rupture of air-water-oil pseudoemulsion film, even in presence of hydrophobed silica particles, as revealed by measurements of critical capillary pressure p. Effect is seen to be insensitive to changes in drop diameter. (Reprinted with permission from Denkov, N.D. Langmuir, 20, 9463. Copyright 2004 American Chemical Society.)... 

FIGURE 4.82 Effect of spherical hydrophobed silica particles on stability of pseudoemulsion film formed by heptanes in saline aqueous solutions of AOT. (a) Apparatus used. Contact angles were varied by changing AOT concentration, (b) Plot of heptane drop half-life against + Bow (see text). (Aveyard, R., Clint, J.H., JCS Faraday Trans., 91, 2681, 1995. Reproduced by permission of The Royal Society of Chemistry.)... 

These antifoams may be prepared by mixing preformed hydrophobed silica particles with PDMS oils. They may also be prepared by mixing untreated silica particles with these oils in the presence of a catalyst that facilitates an in situ reaction between PDMSs and the surface hydroxyl groups of silica. A useful brief summary of the... 

The mode of action of PDMS-hydrophobed silica antifoams in aqueous surfactant solutions has been extensively stndied by Denkov et al. [53] and reviewed in detail in Chapter 4. Essentially the hydrophobed silica particles rupture the so-called air-water-oil pseudoemulsion film, thereby enabling the oil to emerge into the air-water surface. It is known that once they emerge into the air-water surface, drops of PDMS oils usually initially spread over that snrface, exhibiting either complete wetting or pseudo-partial wetting behavior (see Section 3.6.2). This means that the oil spreads as either a thick duplex layer or spreads and breaks up into lenses in equilibrium with a thin oil layer. Since such behavior is ubiquitous with aqueous surfactant solntions, it is reasonable to expect similar behavior when PDMS oil drops are introduced into the gas-blood surface. It is not, however, known whether complete or pseudo-partial wetting behavior is to be expected. 

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