Adsorption configuration of stearic acid onto calcium sulfate whisker

The adsorption configuration of stearic acid onto calcium sulfate whisker is studied both computationally and experimentally. It is found experimentally that 8 stearic acids could be accommodated on the calcium sulfate whisker at the given conditions based on BET and TG analysis, and chemical binding between stearate ions and CSW is verified by IR analysis. However, the existing model St shows no adsorption when the number of stearic acids reaches 4. Therefore, we compare modified adsorption models > CaSt (> Ca denotes surface Ca) and CaSt(2) with traditional St model by using simulation method. The simulation results show that > CaSt is the most stable adsorption model in terms of molecular structure and binding energy. As the addictive number rises up to 4, the St model and CaSt(2) model show no adsorption and the binding energy of two cases becomes negative. Meanwhile, the binding energy is positive as the number reaches 8 in > CaSt model, which is more consistent with real process. Electrostatic force is found to contribute the most to adsorption, and stearic acid in form of stearate ion (St (-)) shows higher water contact angels, which indicates stronger adsorption at higher pH and demonstrates the contribution of electrostatic interaction between St(-) and Ca.