Toluidine blue-sodium lauryl ether sulfate complexes: Influence of ethylene oxide length

Abstract

Sodium Lauryl Ether Sulfates (SLES) are an increasingly important and versatile type of surfactants. The complexation between ortho-Toluidine blue (TBO) and a homologous series of SLES, including Sodium Lauryl Sulfate (SDS) without Ethylene Oxide (EO), has been investigated using visible spectrophotometry. Upon increasing the molar surfactant-dye-ratio (S/D), three TBO organization regimes can be identified from the absorption spectra: (i) Ion-pair formation, (ii) Pre-micellar aggregation and (iii) Micellar aggregation. The limited solubility of SDS-TBO ion pairs, leading to precipitation of ion-pair clusters, causes a decrease of the TBO monomeric peak height when S/D is increased. The better solvability of TBO-SLES ion-pairs causes the onset of precipitation to occur at higher S/D when the EO-block length increases. In an intermediate S/D range, metachromasy is observed as a result of dye-stacking onto the SDS arranged in a pre-micellar state at surfactant concentrations well below the critical micelle concentration (CMC). For SDS, the S/D range showing pre-micelles depends on TBO concentration but seems independent of variation of ionic strength (phosphate buffer, pH = 7.0, Cbuffer = 0.1–10 mM), whereas the metachromatic peak height increases with decreasing ionic strength. The pre-micellar region for all SLES-TBO is located at the same S/D range but the maximum metachromatic peak height follows the trend: EO2 > EO4 > SDS > EO12 ≈ EO30. The onset of the micellar range, in which TBO is again present in monomeric form, is shown to coincide with the CMC. These findings could proof helpful in either formulation or analysis of SLES. Likewise, understanding the dyes' interactions may facilitate formulation of dyeing solutions for optimal performance, or alternatively, as part of the textile plant effluent treatment to remove, and possibly recycle, dyes.