Revealing New Structural Insights from Surfactant Micelles through DLS, Microrheology and Raman Spectroscopy

Abstract

The correlation between molecular changes and microstructural evolution of rheological properties has been demonstrated for the first time in a mixed anionic/zwitterionic surfactant-based wormlike micellar system. Utilizing a novel combination of DLS-microrheology and Raman Spectroscopy, the effect of electrostatic screening on these properties of anionic (SLES) and zwitterionic (CapB) surfactant mixtures was studied by modulating the NaCl concentration. As Raman Spectroscopy delivers information about the molecular structure and DLS-microrheology characterizes viscoelastic properties, the combination of data delivered allows for a deeper understanding of the molecular changes underlying the viscoelastic ones. The high frequency viscoelastic response obtained through DLS-microrheology has shown the persistence of the Maxwell fluid response for low viscosity solutions at high NaCl concentrations. The intensity of the Raman band at 170 cm−1 exhibits very strong correlation with the viscosity variation. As this Raman band is assigned to hydrogen bonding, its variation with NaCl concentration additionally indicates differences in water structuring due to potential microstructural differences at low and high NaCl concentrations. The microstructural differences at low and high NaCl concentrations are further corroborated by persistence of a slow mode at the higher NaCl concentrations as seen through DLS measurements. The study illustrates the utility of the combined DLS, DLS-optical microrheology and Raman Spectroscopy in providing new molecular structural insights into the self-assembly process in complex fluids.

Microrheological study of ternary surfactant‐biosurfactant mixtures

“…This behaviour is commonly seen in wormlike micelle systems . The reduction in viscosity has been attributed to either branching or shortening of micelle .…”

“…The viscosity was seen to increase with initial addition of sodium chloride reaching a maximum value and then decreasing with subsequent further addition of sodium chloride. This behaviour is commonly seen in wormlike micelle systems [18,22,23]. The reduction in viscosity has been attributed to either branching or shortening of micelle [18,[22][23][24].…”

“…The viscoelastic response of the three systems correlated with their viscosity profiles. The SLES/CAPB system exhibits Maxwellian response typically seen in entangled wormlike micelle as seen through the semi-circular shape of the Cole-Cole plot [18,[22][23][24]. On addition of CCB, G'/G" cross-over shifts towards higher frequency indicating shorter relaxation time.…”

Rheology of mixed solutions of sulfonated methyl esters and betaine in relation to the growth of giant micelles and shampoo applications

“…Along the abscissa, the total NaCl concentration is plotted, which includes both the added salt and the salt present as an admixture in the surfactants; see Section 2.1. Each experimental dependence has a typical shape of curve with maximum, called the salt curve [37][38][39][40][41][42]. In Fig.…”

“…Second, in systems containing ionic surfactants the dependence of viscosity on the concentration of added salt (the so called salt curve) often exhibits a high peak [28,[37][38][39][40][41][42][43]. In this case, the peak could be explained with a transition from wormlike micelles to branched micelles [44][45][46][47][48].…”