The RSM design of AA-2G stability study is shown in Table 2. Based on preliminary studies, the central point was set at 60 °C and pH 6.5. Temperature and pH were the two independent factors involved in AA-2G stability study. The cosmetic lotion was stored at the designed temperature and pH for 10 h before running the HPLC analysis. The peak area of the HPLC diagram represented the response value of RSM. After running the regression analysis, the coefficients of the second-order regression model were deter- mined and substituted into Eq. 1