Quantification of hydrophilic ethoxylates in polysorbate surfactants using diffusion H1 NMR spectroscopy

Polysorbate surfactants (commercially available as Tween) are widely used in pharmaceutical, cosmetic and food products. They are generally considered as esters of ethoxylated sorbitan with fatty acids. Diffusion H1 NMR spectroscopy on a solution of polysorbate 20 in D2O revealed that only one diffusion coefficient was found for the fatty acyl part. Using the Stokes–Einstein equation, it became obvious that this diffusion behavior was caused by micelles. On the other hand, two significantly different diffusion coefficients were found for the methylene groups of ethylene oxide (EO). This indicates the presence of two distinct EO containing species in solution. Since the slowest diffusing EO species has the same diffusion coefficient as the fatty acyl part, it corresponds to the micellar (i.e. fatty acyl bound) ethoxylates. The diffusion coefficient of the fastest diffusing EO species was a factor of four larger than that of the slowly diffusing species and was attributed to water-soluble non-esterified ethoxylates. A solution of polysorbate 20 in the presence of NaOD was prepared to investigate if hydrolysis of the sorbitan ester could be the reason for the occurence of these hydrophilic ethoxylates. It was found that alkaline hydrolysis does lead to an increasing fraction of non-esterified ethoxylates, but is not the cause of its presence in untreated polysorbate samples since these species were also found in solutions of polyethylene glycol oleyl ether (commercially available as Brij), which are not susceptible to hydrolysis. Fractionation of the EO species present in polysorbate 20 into an amphiphilic and a hydrophilic fraction was only partly obtained by activated carbon adsorption. On the other hand, sequential extraction of aqueous polysorbate solutions by ethyl acetate and chloroform enabled a nearly complete fractionation. H1 NMR spectroscopy proved to be very useful since it allows in situ determination of the global composition of a surfactant sample, as well as quantification of both the amphiphilic and hydrophilic ethoxylate fractions via diffusion measurements.