Syntheses and properties of novel non-ionic fluorinated multichains “star-like” surfactants

Abstract

A series of star-like nonionic surfactants (with two hydrophobic and two hydrophilic chains) with different lengths of hydrophilic and hydrophobic arms were synthesised on the basis of pyromellitic acid dianhydride. The hydrophilic arms were formed by polyoxyethylene and hydrophobic ones either by perfluoro- or by alkyl chains. The adsorption monolayers (Gibbs monolayers) were studied by surface pressure ( π) measurements as a function of time for different surfactant concentrations. For the spread monolayers (Langmuir monolayers), the measurements of the surface pressure ( π) versus the molecular area ( A) as well as the relaxation measurements of the area ( A) as a function of time at constant surface pressure were performed. The comparison between the characteristic parameters of two types of monolayers was made in order to understand the effect of the preparation conditions on the structure of these monolayers. It was found that decreasing the fluoroalkyl chain length induced a systematical decrease in the stability of Langmuir monolayers, which is manifested as the Marangoni–Gibbs viscoelasticity of the monolayers. For the surfactants, which have a large number of oxyethylene groups, adsorption at the air/water interface from the bulk solution required extremely long times to reach equilibrium due to the diffusion from the solution and to the conformational rearrangements at the interface. The observation of a hysteresis in the compression/decompression curves for these compounds is explained by the presence of the residual organic solvent molecules absorbed by oxyethylenic chains. A novel model describing the kinetics of desorption or rearrangement of molecules during the lateral compression was suggested, allowing the estimation of both characteristic time of this process and areas per molecule at the equilibrium from the relaxation curves A( t).