The electrokinetic and rheological behavior of phosphatidylcholine-treated TiO2 suspensions

The rheological and electrokinetic behaviors of TiO2 suspensions in NaCl solutions after coating the particles with the biosurfactant l-α-phosphatidylcholine has been investigated in this study. Shear stress of the titanium dioxide suspension as a function of shear rate was measured for 2%, 5% and 10% particle volume fractions, and various pHs. Non-Newtonian, pseudoplastic behaviors characterize these suspensions. The yield stress of the suspensions in the absence of biosurfactant changes with pH and volume fraction in a way indicating the balance between vicinity to the isoelectric point (favoring aggregation and hence increasing yield stress), and tendency to sedimentation (increased at high volume fractions and isoelectric conditions), which produces a decrease in yield stress. The isoelectric point of the investigated TiO2 suspensions determined from viscoelastic behavior compares well with that determined from the zeta potential measurements. This makes evident the relationship between the yield stress and the state of aggregation of the suspensions, but leaves little control on the stability and rheology by just pH control. experiments with the biosurfactant, 5% TiO2 suspensions were used, and the concentrations of l-α-phosphatidylcholine used correspond to those required to reach monolayer or bilayer coating of the particles. The phosphatidylcholine treatment leads to a shift of the isoelectric point from around pH 6.3 to 4.85. The effect of pH on the rheological properties is slightly visible for lower concentrations of phosphatidylcholine. In the case of phospholipid bilayer this effect is evident. The obtained results indicate that it is possible to control the flocculation state both by adjusting the pH of the suspensions and by titanium dioxide surface modification. It is shown that both applied techniques are required, and their sensible combination can extend the range of application of concentrated titania suspensions.