As depicted in Fig. 2, the foaming capacity shows a
moderate increase from 1.96 to 2.46 upon increasing the pH
value from 1 to 5. Obviously, when the pH value was below 5,
substantive ionized H+ in the acid suspension prevented the
ionization of the valeric acid anion, which degraded the
modification of amphiphilic molecules on PZT particles. When
the pH value approached the isoelectric point of PZT colloids
(pH 6.5–7) [18], the viscosity of the initial suspension
increased, the particles tended to agglomerate, and the foaming
capacity decreased to 1.76 consequently. Under mild alkaline
condition (pH 9), valeric acid anion was hard to ionize,
resulting in the lowest foaming capacity of 1.32. Meanwhile,
the bulk density of samples varied between 2.38 and 5.09 g/
cm3 with a changing trend opposite to that of foaming
capacity.