the interaction between FHAp surfaces and Tween80, whose
structure, as shown in Fig. 5, has a hydrophilic head and a
hydrophobic tail. Previous studies suggested that the interaction
between the HAp crystal and organic molecules was attributable
to electrostatic attraction, formation of calcium complex, and
hydrogen bonds [21]. Since Tween80 is a non-ionic surfactant,
the possibility of electrostatic action may be eliminated. Although
one carboxylic-ester group exists in the middle of Tween80
molecule, the steric effect hinders the adsorption on the nanoparticle
surface via the carboxylic-ester group [22]. Therefore,
three end hydroxyl groups and abundant ether oxygen may
contribute to the adsorption. The hydrophobic alkyl tail was
exposed outside of the surfaces and hindered the diffusion of the
growth units onto the crystal surfaces. Such adsorption and steric
hindrance led to the enlarged difference between the growth rates
of crystal faces, so that the FHAp crystal showed anisotropic
growth