Adhesion: increasing with colloidal silica

Bowen et al. [39] measured directly the adhesion (interaction) of cellobiose and cellulose with two polymeric UF membranes of similar MWCO, but of different materials. As probes, they used silica spheres (diameter 5-8 im) the surfaces of which were modified by static adsorption of cellobiose. They also used pure cellulose probes. Membrane ES 404 was made of poly(ether sulfone) alone, and EM 006 was made of a poly(ether sulfone)-polyacrylate blend, chosen specifically to increase the hydrophihc properties and decrease the fouling properties of the membrane. Study of ES 404 and EM 006 had shown that the interaction of cellobiose or of colloidal cellulose with the membranes was such that ES 404 always had the greater adhesion and greater fouling tendency. However, if the membrane was first fouled with cellobiose, the colloidal cellulose adhesion force was increased significantly, and the differences between the membranes diminished. Bowen et al. suggested that in the future, it would be possible to use the techniques developed to allow prior assessment of the fouUng propensity of process streams with different types of membranes. 

The most extremum behavior of all the characteristics is observed at low content of cells in the suspension (Figure 7.11). A minimum of Ys and C and a maximum of CZ are at Ch o = 98.4 wt% or Cy=C<-eii+Cicw=6.1 wt%. Notice that there is the extreme dependence of the Ys value on the total concentration of water in the aqueous suspensions of nanooxides (see Section 1.1.6) at a minimum at Chjo 93 wt%. This boundary concentration corresponds to transition from diluted suspensions to concentrated ones characterized by different particle-particle interactions. In the diluted suspensions, the systems can separate into a gel-like layer and upper layer with bulk, almost pure water. In the concentrated suspensions, the systems represent a continuous gel-like structure without separation of bulk water. With increasing size of particles, the critical concentration (CJ should increase. Therefore, one could expect a larger Q value for yeast S. cerevisiae cells (5-10 pm) than for nanosilica (primary particles 10 nm). However, the C<. values for yeast cells and nanosilica are relatively close due to the formation of silica nanoparticles aggregates 0.5-l pm and agglomerates >1 pm, which have sizes close to sizes of cells. Therefore, at Cy< 10 wt% (Cycolloidal dispersion with relatively weak intercell interactions. At these Cy values, the adhesion... 

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