OPTIMAL ALUMINUM/ZIRCONIUM—PROTEIN INTERACTIONS 109 repulsion prevents the adsorption of AP onto BSA. In zone 2, BSA particles are negatively charged while AP particles still carry positive charges the electrostatic attraction causes adsorption on to the BSA surface. In zone 3, both BSA and AP particles are negatively charged—leading to repulsion between these two particles. The zeta potential measure- ment technique provides an effective and effi cient way to evaluate effi cacy of metal salts to use as AP product or coagulants/fl occulants in water treatment. CONCLUSIONS The possibility of using ζ-potential measurements to demonstrate the optimal interac- tion between BSA across a wide range of commercial AP actives has been successfully investigated. ζ-potential measurement is not only effective, but can also be used as a simple indicator to evaluate the effi cacy of an AP active when it is combined with a solu- tion containing a representative biomolecule (e.g., BSA) at IEP. As a result of minimum repulsion, an insoluble AP–BSA precipitate was formed at the pH where the molar ratio of AP/BSA allows for electrostatic neutrality and the ζ-potential of solution is zero, also known as IEP. The disparity between the turbidity of AP salts alone and turbidity of the AP–BSA combination implicates the importance of biomolecules in the Plug Theory. The electrostatically driven mechanism of plug formation is similar to that which is ac- cepted as the mode of action of primary coagulants in water treatment. The techniques and results described here should allow for more quantitatively analysis of new AP ac- tives, as well as providing insight into the rational design of new active salts. ACKNOWLEDGMENTS The authors warmly thank Dr. Andrei Potain from Colgate-Palmolive Co. for helping and providing advice on the operation of Zetasizer. REFERENCES (1) J. Duan and J. Gregory, Coagulation by hydrolyzing metal salts, Adv. Colloid Interface Sci., 100–102, 475–502 (2003). (2) S. D. Faust and O. M. Aly, Chemistry of Water Treatment. 2nd Ed. CRC (Press, Boca Raton, 2010), pp. 217–268. (3) Z. Chen, B. Fan, X. Peng, Z. Zhang, J. Fan, and Z. Luan, Evaluation of Al30 polynuclear species in polyaluminum solutions as coagulant for water treatment, Chemosphere, 64, 912–918 (2006). (4) B. Shi, Q. Wei, D. Wang, Z. Zhu, and H. Tang, Coagulation of humic acid: The performance of pre- formed and non-preformed Al species, Colloids Surf., A, 296, 141–148 (2007). (5) C. Staaks, R. Fabris, T. Lowe, C. W. K. Chow, J. A. van Leeuwen, and M. Drikas, Coagulation assess- ment and optimisation with a photometric dispersion analyser and organic characterisation for natural organic matter removal performance, Chem. Eng. J., 168, 629–634 (2011). (6) Z. Chen, Z. Luan, Z. Jia, and X. Li, Study on the hydrolysis/precipitation behavior of Keggin Al13 and Al30 polymers in polyaluminum solutions, J. Environ. Manage., 90, 2831–2840 (2009). (7) K. Laden, “Antiperspirants and Deodorants: History of Major HBA Market,” in Antiperspirants and Deodorants. K. Laden. Ed. Cosmetic Science and Technology Series. 2nd Ed. (Marcel Dekker Inc, New York, 1999), pp. 1–14. (8) C. J. C. Edwards and A. K. Mills, “A Guide to Understand Antiperspirant Formulations,” in Antiperspi- rants and Deodorants. K. Laden. Ed. Cosmetic Science and Technology Series. 2nd Ed. (Marcel Dekker Inc, New York, 1999), pp. 233–257.

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