Alina Iovescu

Some critical points in the interaction between homogeneous non-ionic surfactants and poly(acrylic acid)

Abstract Single hexaethylene glycol mono(n-dodecyl) ether or octaethylene glycol mono(n-dodecyl) ether in aqueous solution and their mixtures with poly(acrylic acid) (PAA) were studied by surface tension, fluorescence, dye solubilization, viscosity and pH methods. The CMC and several critical surfactant concentrations for the interaction were determined. The starting point of micellar aggregation on the PAA chain (T1) and the point at which free micelles appeared in the solution were found and identified by the various methods. The concentration at which free surfactant molecules reappear in the system, was a transitional range rather than a critical point and was identified with certainty only by surface tension measurements. The data were discussed taking into account the interaction of both ethoxylated and hydrophobic moieties of the surfactant with PAA (i.e. hydrogen bonding and hydrophobic bonding respectively). Finally, the relationship between T1 and the ethoxylated and hydrophobic moieties of the surfactants was considered.

A combined binding mechanism of nonionic ethoxylated surfactants to bovine serum albumin revealed by fluorescence and circular dichroism.

The study systematically investigates aqueous mixtures of fixed bovine serum albumin (BSA) and various ethoxylated nonionic surfactants belonging to a homologous series or not. Mono-disperse tetra-(C12E4), hexa-(C12E6) and octa-ethyleneglycol mono-n-dodecyl ether (C12E8), and poly-disperse eicosa-ethyleneglycol mono-n-tetradecyl ether (C14EO20) are respectively employed. Fluorescence and circular dichroism measurements are performed at surfactant/protein molar ratios (rm)s lower and higher than one. We aim to get new insights into the binding mechanism of these species and to differentiate among the interaction abilities of these surfactants. The relative magnitude of the binding thermodynamic parameters by fluorescence, and the increase of α-helix prove that hydrogen bonding drives the interaction next to the hydrophobic attraction. C12En (n=4,6,8) develop more H bonds with the albumin than C14EO20 owing to a zigzag conformation of their short ethyleneoxide chains. Among the homologous surfactants, C12E6 has a slightly stronger interaction with BSA due to a maximal number of H bonds at a minimal hindering. Static fluorescence and dynamic fluorescence indicate an inter-conversion between the tryptophan (Trp) rotamers which happens around the surfactants critical micellar concentration. For C14EO20, the meander conformation of the polar group determines a less evident conversion of the Trp rotamers and smaller α-helix rise. Binding isotherms of the homologous surfactants and the fluorescence quenching mechanism by C12E6 are also provided.