Self-Desorption of Mixtures of Anionic and Nonionic Surfactants from a Silica/Water Interface

Abstract

The adsorption of a mixture of two surfactants, the anionic sodium dodecyl sulfate and the nonionic nonylethylene glycol n-dodecyl ether, from water to a hydrophilic silica dispersion has been studied at pH = 5.0 and 25 °C in a large concentration range at several surfactant compositions by a classical depletion method. The adsorption of the latter compound onto silica induces the incorporation of the former by the formation of mixed surfactant aggregates. However, at high total surfactant concentration, about 100 times the mixed critical micelle concentration, both surfactants are partially desorbed from the silica surface. This behavior is qualitatively interpreted as the consequence of two phenomena: on one hand, the strong interaction between the anionic and the nonionic surfactant in the bulk above the critical micelle concentration and on the other hand the repulsion which occurs between the negatively charged dissociated silanol groups and the anionic surfactant in the mixed aggregates at the silica/water interface. At high total surfactant concentration and above a specific anionic/nonionic surfactant ratio the former phenomenon is favored over the latter, hence the desorption of both surfactants.

Partitioning behavior of silica in the Triton X-100/dextran/water aqueous biphasic system

“…Denoyel et al [62] also found the same behavior; i.e., addition of sodium n-octylbenzene sulfonate resulted in the suppression of TX100 adsorption on silica. In other mixed nonionic/anionic surfactant systems (e.g., pentaethylene glycol monodecyl ether/SDS, nonylethylene glycol n-dodecyl ether/SDS, and octaethylene glycol mono-ndodecylether/SDS), the adsorption of the nonionic surfactant onto silica also decreased in the presence of the anionic surfactant [58][59][60][61]. The experimental results from the present work (Fig.…”

“…With increased total surfactant concentration, the anionic surfactant also appears at the solid/liquid interface and forms mixed aggregates with the anchored nonionic surfactant. It is believed that the anionic surfactant is attracted to the silica surface by the hydrophobic interaction between its alkyl chain and the hydrophobic tails of the nonionic surfactant [57][58][59][60][61][62]. At the same time repulsive interaction develops between the mixed surfactant aggregates and the silica surface, as illustrated in Fig.…”

“…On the other hand, the uptake of SDS from its pure solution is negligible because of electrostatic repulsion between the negatively charged surfactant and the SiO 2 surface, which is negatively charged above its pzc (about pH 2) [56][57][58][59]. In the case of TX100 mixtures with anionic surfactants, at very low total concentration, only individual nonionic surfactant molecules are found on the silicon dioxide surface [57][58][59][60][61][62]. With increased total surfactant concentration, the anionic surfactant also appears at the solid/liquid interface and forms mixed aggregates with the anchored nonionic surfactant.…”

A model for the cooperative adsorption of surfactant mixtures on solid surfaces

“…Hence the nature of adsorption of the mixture is cooperative. A similar behavior was observed with the mixtures of sodium dodecylsulfate (SDS) and nonylethylene glyco n-dodecyl ether (C 12 E 9 ) at pH 5, where the nonionic C 12 E 9 is the active component to promote the adsorption of the SDS [25]. At concentrations above the critical aggregation concentration (cac), a relatively moderate decrease in the mole fraction of the active NP (x a s ) and an increase in that of passive DM (x p s ), accompanied by an enormous growth of surface aggregates (surfactant in aggregate%), were observed up to a threshold value till the onset of the maximum adsorption (Γ max ).…”

“…Thus, an agreement between x a b and x a s suggests ideal mixing both in the solution and at the solid/solution interfaces with no interaction between the surfactant molecules. However, in most cases, the system is nonideal and a deviation of x a s from x a b is obtained [25,26]. In this study, three equal molar mixture cases were studied with the first one under the ideal situation while the other two under nonideal conditions.…”