Considering the above discussion, following analyses can be made
for the histogram generated at different pH (Fig. 4a). At pH 3.1, the relative
number of particles above about 45 nm in diameter is seemingly
increased. This can be interpreted as that the number of the dominant
particle sizes (i.e. below about 45 nm in diameter) has decreased due
to the shift in the particle-cluster equilibrium towards agglomeration
for the smaller particles. Such a size effect is explained by the DLVO theory
as that the smaller particles are more prone to form clusters due to
the lower electrostatic repulsion than the larger ones [15]. In addition,
this can be confirmed by the histogram at pH 3.4,which shows a similar
size distribution as pH2.0 because the agglomeration equilibria of larger
particles have also shifted away from monomers. The gradual shift of
themode of the size distribution to the larger diameters as pH increase