Hofmeister Effect in Coagels of Ascorbic Acid Based Surfactants

Ascorbyl-6-O-alkanoates (ascorbyl−O−CO−(CH2) n - 1H) are single-chained anionic surfactants. When dispersed in water at room temperature, they produce coagels. At higher temperatures, these hydrated crystalline phases form either micellar solutions or gel phases, depending on the length of the aliphatic tail. We report the effect of different anions (Hofmeister series) on the coagel-to-micelle and coagel-to-gel phase transitions in ASCn/water systems, with 8 ≤ n ≤ 16. ASCn coagels in water are not greatly affected by solution pH, in the pH range investigated. The T and ΔH values related to the coagel-to-micelle (for ASC8 and ASC10) and to the coagel-to-gel (for ASC12, ASC14, and ASC16) phase transitions are quite similar to those found in pure water, suggesting that the phase transition process is driven mainly by the hydrocarbon chain length, when the concentration of the active ionic species is lower than 10-2 M (HCl and phosphate buffer saline solutions). The presence of 0.5 M salt solutions (Hofmeister series) has a large impact on the coagel properties, due to the adsorption of the different anionic species at the surface of ascorbyl-alkanoate aggregates. The phase transition temperature changes significantly with the anions, increasing in the following sequence: SCN- > I- > NO3 - > Br- > Cl- > H2O > BF4 - > H2PO4 - > SO4 2- > HCOO- > F- > AcO-. Acetate and formate ions produce anomalous results that do not fit the pattern. This is due to the hydrolysis equilibria that they generate in the presence of the ascorbic acid esters. What emerges very clearly from this study is that anions and not just cations adsorb strongly to the negatively charged interface of the ascorbyl-6-O-alkanoate aggregates. The different phase behavior reflects the interplay between intramolecular interactions that involve the hydrophobic chains, those between the ionic headgroups, and interaggregate interactions. The last two are consistently modified by co-ion adsorption.