Determination of the Concentration of Commercial Cationic Surfactants in Aqueous Solutions by the Colloidal Titration Method

Abstract

Cationic Surfactant's quantification in aqueous solution is relevant in fundamental and applied research, as well as for quality control in processes in the chemical industry. Colloidal titration is a commonly used method for surface charge determination in colloidal systems such as polyelectrolytes, macromolecules and solid particles, but it can also be applied for the detection of ionic surfactants in aqueous solution. It is a simple technique, easy to implement in a laboratory and its principle is based on the stoichiometric neutralization between two species of opposite charge, potassium polyvinylsulfonate (standard PVSK) and a cationic polyelectrolyte or cationic surfactant. When the stoichiometric ratio is exceeded, the excess polyelectrolyte reacts with a cationic indicator (crystal violet) forming an electrically neutral pair that modifies the solutions' absorbance value which is determined by spectrophotometry at 587 nm. To assess the feasibility of this technique, the colloidal titration method was normalized by titration between two standard polyelectrolytes (PDADMAC and PVSK), also with analytical grade cationic surfactants (DDBAB, CTAB and HDBAC) and finally with some commercial ones (Praepagen WK, Dodigen 1828, Empygen BAC 50). High molecular weight surfactants showed stoichiometric proportionality between their concentration and the amount of tritant added for concentration between 10-100 mN. This proportionality was not observed for low molecular weight surfactants. Finally, it was also observed that for commercial surfactants the presence of 0.01 M sodium chloride does not affect the titration results.