STABILIZATION OF 1-ASCORBIC ACID OH I HOH2CHCyOyO Mg2+Mg2�-- Mg2++H -o OH Mg 2 + Mg2+ OH I HOH 2 CHC y O y O Mg2+Mg2�-- Mg2++Na -o OH Mg2+ Mg 2 + 5 Scheme 2. Schematic representation of the ionic shielding process of L-ascorbic acid in aqueous solution. - � ( 1.0 �----.- � / ■ --- ::: / /0 / -------- 0 □ / 0 .4 / □�- pH 2, 2 week storage -•- pH 7, 2 week storage -□- pH 2, 4 week storage -o- pH 7, 4 week storage 0.2 o.o----.--r-----,.--"""T""-.--......... --r--""""T"--.----r---r---. 0.00 0.15 0.30 0.45 0.60 0.75 0.90 Concentration of MgSO 4 (g/dl) Figure 1. Stability of L-ascorbic acid solutions with the ionic strength at different pH and storage times after 2-week storage(-■-, pH 2 -•-, pH 7) and 4-week storage(-□-, pH 2 -0-, pH 7) at 40 ° C. In this observation, the concentration of L-ascorbic acid was 3 wt%.

6 JOURNAL OF COSMETIC SCIENCE control the characteristics of the enediol group, especially the hydroxy group attached to C3 in the 1-ascorbic acid molecule, we introduce the concept of ionic shielding, which is applied generally to the swelling behavior of hydrogels in an ionic medium condition (23-25). In Scheme 2, the ionic shielding process of 1-ascorbic acid is represented schematically. When the ionic strength of the solution is increased, the counterions have a tendency coward binding co the species charged oppositely, which is responsible for the ion-ion interaction. The counterions include charged ionic species such as, Na + , K+, Ca2 + , Mg2+ , Cu2+, and Al3 + . (In chis study, we selected Mg2+ as a counterion.) The ionic strength in the system can be controlled by varying the ionic species and their concen­ tration. It can then be said chat the oxidation of 1-ascorbic acid co dehydro-1-ascorbic acid is possibly hindered. In this study, the effectiveness of ionic shielding on the stability of 1-ascorbic acid was examined by considering the medium's pH. Figure 1 shows the long-term stability of simple L-ascorbic acid aqueous solutions with the ionic strength at a high temperature, Figure 2. OM photographs for w/o/w double emulsions: (a) w/o/w double emulsions not contammg L-ascorbic acid (b) w/o/w double emulsions containing 3 wt% L-ascorbic acid in the internal aqueous phase.