Surface modification of CaCO3 nanoparticles by alkylbenzene sulfonic acid surfactant

Surface modification of CaCO₃ nanoparticles by two kinds of anionic surfactants, such as linear alkylbenzene sulfonic acid (LABSA) and branched alkylbenzene sulfonic acid (BABSA), was investigated by a quartz crystal microbalance with dissipation monitoring (QCM-D) measurement and contact angle analysis. Basically, both surfactant systems showed the same trend in the measurements of QCM-D and contact angle. Contact angle measurement has shown that CaCO₃ surface becomes more hydrophobic as surfactant concentration increases. However, the use of excess surfactant has led to the reverse change in surface property of CaCO₃ particles from hydrophobic to hydrophilic. QCM-D measurement using a quartz crystal coated with CaCO₃ revealed that monolayer adsorption of surfactant molecules at the particle–water interface occurs in a low surfactant concentration region and the thickness of forming molecular layers was found to be in the order of the length of a single surfactant molecule. On the other hand, both mass of adsorbed surfactant molecules and the thickness of molecular layers indicated bilayer adsorption of surfactant molecules in a high surfactant concentration region, which evidently resulted in an increase in hydrophilicity of CaCO₃ surface. Visual floating test has shown a good agreement with measurements of contact angle and QCM-D.