Hydrophilic silica particles

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. 

The volume restriction effect as discussed in this paper was proposed several years ago by Asakura and Oosawa (12,13). Their theory accounted for the instability observed in mixtures of colloidal particles and free polymer molecules. Such mixed systems have been investigated experimentally for decades (14-16). However, the work of Asakura and Oosawa did not receive much attention until recently (17,18). A few years ago, Vrij (19) treated the volume restriction effect independently, and also observed phase separation in a microemulsion with added polymer. Recently, DeHek and Vrij (20) have reported phase separation in non-aqueous systems containing hydrophilic silica particles and polymer molecules. The results have been treated quite well in terms of a "hard-sphere-cavity" model. Sperry (21) has also used a hard-sphere approximation in a quantitative model for the volume restriction flocculation of latex by water-soluble polymers. 

Figure 3. Reaction of silicone oil with hydrophilic silica particle.

The addition of A-380 of 0.5-2.5 wt.% reduces the adhesion (Table 37.15) and this may be caused by weak interaction between the hydrophilic silica particles and the hydrophobic CH groups of ethyl cellulose. This assumption is supported by the increase in adhesion after the addition of modified silica. When TS are used as an additive, the adhesion increases for C< 1 wt.%, but for... 

As one example, the force between a hydrophilic silica particle and an air bubble at different concentrations of dode-cyltrimethylammonium bromide (DTAB) is shown in Fig. 10. Without surfactant, the particle is repelled by the air bubble. At distances above 5 nm, the electrostatic repulsion dominates. The reason being the negative surface charges on the silica surface and at the water-air interface [187-190]. Even at close distance, a stable water remains on the particle surface and no three-phase contact is formed. Adding even small amounts of the cationic surfactant DTAB changes the interaction drastically. At concentrations between 0.1 mM and typically 5 mM DTAB (critical micellar concentration is >= 16 mM), no repulsion was observed. When the particle comes into contact with the air-water interface, it jumps into the bubble and a three-phase contact is formed. Such a behavior can be explained with the strong adsorption of long-chain alkyltrimethy-lammonium ions to silica [191]. At a concentration of 0.1 mM, DTA+ forms a monolayer on the silica surface. This... 

Fig. 10 Normalized force-versus-distance curve measured with a hydrophilic silica particle in aqueous electrolyte with no added DTAB, 0.1 mM DTAB, 5.4 mM DTAB, and 13.2 mM DTAB (from Ref [143]). The electrolyte contained 0.3 mM KCl, the pH was around 5.5. The insert at 5.4 mM shows the electrostatic repulsion before the jump-in. The particle radius Rwas 2.5 pm.

It must be noted that the panicle ratio. F, is theoielical. Given the particle size of the finished form, it is obvious that hydroi bic silica panicles do not surround only one hydrophilic panicle they dehnitety surround an oily droplet of dried emulsion. This droplet may contain several hydrophilic silica particles (Fig. IC). 

Data on catalytic activity of adsorbed proteins give the information of great importance about conformational state in adsorbed states. Proteolytic enzymes show the ability to autholysis, that can proceed in the solution and in adsorption layers. We observed inhibition of autholysis of protein molecules in adsorption layers [22]. Sometimes the addition of suspensions with hydrophibic or hydrophilic particles leads to change in the rate of autholysis [101]. For savinase the half-life of enzymatic activity in solution is 3.5 hours, and this period is strongly reduced in the presence of particles with hydrophobic surfaces. On the contrary, hydrophilic silica particles stabilize the adsorbed enzyme against autholysis. 

Also, highly hydrophilic silica particles can be obtained when ethylene glycol chains are attached to the surface [41]. These groups are important if the matrix consists of hydrophilic polymers, such as epoxy resins. The ethylene glycol... 

Figures 12.1-12.3 show the rheological investigations performed on emulsions stabilized by very hydrophobic, mildly hydrophilic and very hydrophilic silica particles, respectively. From Figure 12.1 it is clear that emulsions stabilized by means of very hydrophobic particles are very stable when no voltage is apphed. However, the stability of these emulsions towards electrostatic destabilization drops...

Figure 12.6 Viscosity ratio as a function of the amount of mildly hydrophilic silica particles (Aerosil 7200) for 50% water cut emulsions from crude A ( ), crude B (O) and crude A with increased asphaltenes content ( ).

Since the emulsions stabilities for this oil-phase only were not sufficiently high to achieve reliable results, asphaltenes precipitated from crude A were added to the model emulsions in the same amount as in the actual crude oU. Figures 12.7 and 12.8 show the viscosity ratio as a function of particle concentration for the model emulsions stabilized with very hydrophobic and mildly hydrophilic silica particles, respectively. Model oil and crude oil emulsions show many similarities. For all model emulsions the stability in the absence of particles is lower, because the absolute viscosity is much lower than the one encountered in crude oil emulsions. Consequently, the effect of the shear rate on the destabilization is more pronounced. However, as particles are added in larger amounts, crude oil and model oil emulsions behave in the same way. Nevertheless, in Figure 12.8 the viscosity ratios are shifted towards lower values since a strong flocculation phenomenon enhanced by the low viscosity of the samples brings coalescence. 

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