Hofmeister Effect in Coagels from Ascorbic Acid-based Surfactants

LO NOSTRO, PIERANDREA; BARRY NINHAM,; MOIRA AMBROSI,; LAURA FRATONI,; SANTIAGO PALMA,; DANIEL ALBERTO ALLEMANDI; RUBEN MANZO,; PIERO BAGLIONI,

LANGMUIR

ACS

Año: 2003 vol. 19 p. 9583 - 9591

0743-7463

Ascorbyl-6-O-alkanoates (ascorbyl-O-CO-(CH2)n-1H, in brief ASCn) are single chain 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. The different phase behavior reflects the interplay between interactions that involve the ionic headgroups and water, and those between the hydrophobic chains. Solute induced modification of water structure ¨C e.g. by adding sucrose or urea - induce significant changes in the coagel-to-gel or coagel-to-micelle phase transitions. In this paper we studied the effect of different anions on the coagel-to-micelle and coagel-to-gel phase transitions in ASCn/water systems, with 8¡Ün¡Ü16. The results indicate the presence of a Hofmeister effect, with a significant variation in the ∆H value, according to the specific ion¡¯s polarizability. Phase transition temperatures show a lower dependence on ion specificity. The effect of pH on the ionization of the polar heads has been investigated as well, at pH = 2 and 7.4