Understanding the Different Steps of Surfactant Adsorption at the Oil–Water Interface with Second Harmonic Generation

Abstract

Probing the behavior of surfactants at oil–water interfaces is crucial to understand their functionality. In this work, we present detection of the adsorption of several common surfactants at the hexadecane–water interface with second harmonic generation (SHG) and zeta potential measurements. Water molecules were used as reliable indicators of the adsorption of ionic surfactants in SHG analysis. With the change of the interfacial potential monitored by both SHG and zeta potential measurements, unique information about the multiple steps involved in the adsorption of typical surfactants at the oil–water interface is provided. It was revealed that the adsorption of sodium dodecyl sulfate (SDS) and cetyltrimethyl­ammonium bromide (CTAB) at the hexadecane–water interface is initialled by a step dominated by the adsorption of the hydrophobic part of the surfactant, and a latter step involves comparable contributions from both the hydrophobic part and the counterion. The adsorption free energies involved in the initial step can be quantitatively analyzed. In addition, the adsorption of two oil-soluble amphiphiles at the hexadecane–water interface was also studied. Analysis of the ionic strength dependent SHG signal at the hexadecane–water interface also reveals that the origin of the SHG emission is mainly the water molecules at the interfacial layer. The preferential orientation of water molecules is with the hydrogen atoms pointing to the oil phase.

Order-Disorder transition of carboxyl terminated chains in polydiacetylenes vesicles probed by second harmonic generation and two-photon fluorescence

“…The geometric constrains [58−60] from the continuous order-disorder transition of the spatial conformation of the interfacial carboxyl chains sterically hinder the electrostatic attraction interaction between the positively charged D289 molecules and the negatively charged surface of PDAs vesicles due to the decrease in the adsorption density of D289 molecules at the surface of PDAs vesicles with the increased UV irradiation. Besides, the chain-chain interaction is known to significantly influence the molecular energetics at the interfaces [40,44]. The interaction between the D289 molecules and the carboxyl chains of PDAs may be weakened by the ragged distortion and interleaving of the carboxyl chains in chromatic transitions from monomer to blue-to-purple-to-red forms as shown in FIG.…”

Synthesis, characterization, physical and thermodynamic properties of a novel anionic surfactant derived fromSapindus laurifolius

“…The stabilizing effect of oil-in-water emulsions at concentrations near the CMC could be attributed to the ability of longchain alkanes in the oil to associate and form mixed monolayers with the adsorbed ionic surfactant at the oil-water interface. [45][46][47] Therefore, the adsorption tendency is favoured till the CMC concentration is reached, following which the micellization tendency in the bulk oil phase is favoured. The emulsions formed in the presence of 5% oil (heptane, decane and dodecane) were found to remain stable even aer 24 h visual observation, as evident from Fig.…”

Charge-driven interfacial gelation of cellulose nanofibrils across the water/oil interface

“…At first, the rheological properties of the separated components were investigated over time (TS1). When only OA was present, spontaneous adsorption of the amphiphilic OA at the W/O interface occured, 32,36 although values of the interfacial complex modulus (|G i *|) are below the limit of detectability of the instrument (Fig. 2a).…”