OPTIMAL ALUMINUM/ZIRCONIUM—PROTEIN INTERACTIONS 99 RESULT AND DISCUSSIONS ZETA POTENTIAL AND TURBIDITY PROPERTIES OF INDIVIDUAL BSA AND AP SOLUTIONS WITH PH CONTROL The ζ-potential and turbidity properties of pure BSA and AP solutions were studied in a pH range of 3 to 12, and the results are shown in Figure 1. Table I summarizes the im- portant parameters found in this study. The original pH of BSA, activated ACH, ZAG, Al13, and ZG were 7.04, 4.73, 4.14, 4.62, and 2.99, respectively. All solutions appeared transparent at their original pH due to the strong repulsion force between the highly charged particles. The IEP of BSA was measured to be 4.7, which is in agreement with the value previously reported (34). The onset of precipitation is taken as the point where the turbidity is 50 NTU. The formation of visual precipitate was observed at the IEPs for all samples with the exception of BSA. However, a relatively maximum turbidity was measured for BSA at its IEP compared with other pH. For AP solutions, all IEPs fell at more basic pH. At low pH, polycations are the main species in AP solutions (35–39), which leads to a positive ζ-potential and low turbidity. The turbidity of AP solutions increases as the pH is raised. The formation of precipitate is most likely due to the formation of aluminum hydroxide or zirconium hydroxide when pH is raised to 5 or above (35,36): l 3+ 3 Al 3OH Al(OH) + , l 4+ 4 Zr 4OH Zr(OH) + , As the pH continues to increase, ACH and 13 Al solutions once again become clear due to the formation of the water soluble 4 Al(OH) species (35). This transition is not observed in Zr-containing AP solutions (ZAG and ZG) because the insoluble 4 Zr(OH) species is predominant. EFFECT OF AP ADDED ON ZETA POTENTIAL AND TURBIDITY OF SOLUTIONS AP polycations are adsorbed onto the surface of BSA via electrostatic interaction with the negatively charged asparagine and glutamine side chains of BSA (34). The adsorption of AP polycations onto the BSA surface changes the ζ-potential of the AP–BSA mixture solution from negative to positive with increasing AP dosage (16). To ascertain the amount of AP needed to make AP–BSA solutions possess a zero ζ-potential value, the ζ-potential was monitored while BSA solution was titrated with solid AP salts. Figure 2 shows turbidity as a function of ζ-potential and ACH concentra- tion in ACH–BSA mixture. All AP salts show similar behavior (see Figure 3 for details). ζ-Potential increases with increasing AP concentration in solution. As the amount of AP solution increases, the mixture becomes cloudy due to the formation of charge-neutral AP–BSA complexes. As expected, the maximum turbidity is achieved at the pH where ζ-potential was zero (IEP), which indicates the optimal interaction between BSA and AP actives. At high levels of AP, the turbidity dissipates possibly resulting from charge re- versal of the AP–BSA complex.

JOURNAL OF COSMETIC SCIENCE 100 Figure 1. Zeta potential and turbidity trend of selected AP solutions and BSA solution in pH range from 3 to 12, (A) activated ACH: precipitate formed when pH 5 IEP was around 9, where turbidity of solution reached its maximum precipitate disappeared when pH 9. (B) ZAG: precipitate formed when pH 5 IEP was around 9.5, where turbidity of solution reached its maximum precipitate did not dissolve with further addition of base. (C) Al13-mer: precipitate formed when pH 6 IEP was around 10, where turbidity of solution reached its maximum precipitate disappeared when pH 10. (D) ZG precipitate formed when pH 4 IEP was around 7, where turbid- ity of solution reached its maximum precipitate did not dissolve with further addition of base. (E) BSA: IEP was around 4.7, where turbidity of solution reached its maximum no precipitate was observed during titration process. Table II summarizes the IEP values, molar ratios at IEP, maximum turbidity at IEP, as well as the dosage ranges for precipitation of four AP–BSA mixture solutions. The onset of pre- cipitation is taken as the point where the turbidity is 50 NTU. Elemental analysis (EA) was