A new procedure of determination of alcohol sulfates and alcohol ethoxysulfates in agricultural soils

Comprehensive ethoxymer characterization of complex alcohol ethoxy sulphate products by mixed-mode high-performance liquid chromatography coupled to charged aerosol detection

The present work describes a simultaneous mixed-mode high performance liquid chromatography (HPLC) method combined with a universal and non-selective-response detector for the complete ethoxymer profiling of alcohol ethoxy sulphate mixtures. The optimized HPLC methodology combines the dual hydrophilic (HILIC) and reversed-phase selectivity of a surfactant-type column in order to render a comprehensive and simultaneous separation of more than 50 endogenous ethoxymers in a single analysis. Furthermore, an accurate quantitation of every single analyte was achieved using a final universal charged aerosol detector (CAD) including specific mathematical processing tools. Results obtained helped describing a complete alkyl chain and ethoxymer distribution of the investigated AES samples. Method validation evidences provided reliability of the individual ethoxymer contributions determined with the proposed HPLC-CAD methodology. Regarding accuracy including independent nuclear magnetic resonance (NMR) experiments, an excellent correlation was found between the structural information provided by a COSY NMR spectrum and the CAD results regarding the mono/polyethoxylated and the non-ethoxylated/ethoxylated distribution. Additional calculations including the average molecular weight and the degree of ethoxylation for the reference AES sample showed minimum differences (relative error < 1 %) between the two considered techniques. An outstanding precision and linearity along the working concentration range (r²>0.999) was also observed. The individual limit of detection for the target sulphate ethoxymers was determined to be in the low ppm range. Further validated distribution profiles for a large number of AES samples demonstrated the applicability of the optimized HPLC-CAD methodology to routine surfactant screenings. Therefore, the hereby developed methodology provided extensive information regarding the detailed individual ethoxymer profile of AES formulations, which can be extremely useful for the surfactant industry in order to gain information on specific synthesis routes and/or detergency properties.

Sorption, degradation and transport phenomena of alcohol ethoxysulfates in agricultural soils. Laboratory studies

In the present work, laboratory studies were conducted in order to determine and model the sorption, degradation and transport processes of alcohol ethoxysulfates (AES), one of the most important groups of anionic surfactants. Adsorption/desorption isotherms were obtained for several structurally related AES ethoxymers (homologue AES-C₁₂E_n with n = 0-10 ethoxymer units and homologue AES-C₁₄E_n with n = 0-7 ethoxymer units) using a batch equilibrium method. Data were fitted to a linear and a Freundlich isotherm models. Additionally, experiments in continuous-flow soil columns were also carried out and the breakthrough curves observed for each compound were studied. Breakthrough curves were used to determine the fundamental parameters of the transport model (hydrodynamic dispersion coefficient, degradation rate constant and adsorption/desorption isotherm slope), that is the main phenomena that take place simultaneously when AES move through agricultural soil. When the results obtained for the AES ethoxymers are combined, they reveal a clear and consistent trend towards a sorption increase with the number of ethoxylated units and with the length of the alkyl chain that opens the possibility to estimate the values of the transport parameters for other structurally related ethoxymers.