Aggregation states of water in reversed AOT micelles: Raman evidence (original) (raw)
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Aggregation processes in micellar solutions: a Raman study
Journal of Raman Spectroscopy, 2012
Ionic surfactants such as sodium dodecyl sulfate (SDS) belong to the amphiphile family: they possess a long hydrophobic hydrocarbon chain and a polar hydrophilic headgroup. In a polar solvent and over the critical micellar concentration these molecules join to form micelles. The micellar solutions, in turn, if doped with various ligands tend to aggregate.
2016
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.
Journal of Molecular Structure, 2002
The temperature dependence of the water structure in aqueous solutions of a poly(oxyethylene) surfactant C 12 E 5 was examined at concentrations 0, 20, 30, 45, 70 and 90 wt% by Raman spectroscopy of the O±H stretching band. The ratio of the intensities of the component around 3200 cm 21 (the collective in-phase O±H stretching vibrations of bonded aggregates) to the component around 3400 cm 21 (the O±H stretches in which the phase relations are lost) was monitored in the temperature range from 0 to 39 8C. The results show that the speed of the thermal destruction of the H-bond network increases as the concentration increases from 0 to 45 wt%. This change is attributed to the existence of a substantial amount of water in the system that takes part in the hydrophobic hydration of the poly(oxyethylene) headgroups, despite their predominantly hydrophilic character. The conclusion that this kind of water, which is known to have restricted mobility, plays an important role in the surfactant±water systems is consistent with the high viscosity of the liquid crystalline phases. q
Modulation of Dynamics and Reactivity of Water in Reverse Micelles of Mixed Surfactants
Journal of Physical Chemistry B, 2008
In this contribution, we attempt to correlate the change in water dynamics in a reverse micellar (RM) core caused by the modification of the interface by mixing an anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and a nonionic surfactant, tetraethylene glycol monododecyl ether (Brij-30), at different proportions, and its consequent effect on the reactivity of water, measured by monitoring the solvolysis reaction of benzoyl chloride (BzCl). The dimension of the RM droplets at different mixing ratios of AOT and Brij-30 (X Brij-30) has been measured using dynamic light scattering (DLS) technique. The physical properties of the RM water have been determined using Fourier transform infrared spectroscopy (FTIR) and compressibility studies, which show that with increasing X Brij-30 , the water properties tend toward that of bulk-like water. The solvation dynamics, probed by coumarin 500 dye, gets faster with X Brij-30. The rotational anisotropy studies along with a wobbling-in-cone analysis show that the probe experiences less restriction at higher X Brij-30. The kinetics of the water-mediated solvolysis also gets faster with X Brij-30. The increased rate of solvolysis has been correlated with the accelerated solvation dynamics, which is another consequence of surfactant headgroupwater interaction.
The Journal of Physical Chemistry, 1976
The Raman spectra of a series of cationic, anionic, and nonionic surfactants have been examined as pure solids or liquids, and in aqueous micellar solutions. The spectra show that cationic surfactants such as hexadecyltrimethylammonium bromide (CTAB), dodecylammonium chloride (DAC), and decyltrimethylammonium bromide (DeTAB), alpng with anionic surfactants such as sodium laurate (NaL) and sodium lauryl sulfate (NaLS) exist in all-trans structures in the crystalline state. Aqueous micellar solution of these surfactants show additional Raman lines which correspond to a liquidlike nature of the micellar core. Raman spectral studies indicate that the ethylene oxide chains in nonionic surfactant such as Igepal CO-880 and Brij 35 assume dihedral helical structures. Nonionic liquid surfactants with shorter ethylene oxide chains such as Triton X-100 and Igepal CO-630 have a major part of the ethylene oxide chain in an open coil form. No significant differences are observed in Brij 35 in HzO. However, micellization of Igepal CO-630 shows spectra which indicate a partial ordering of the ethylene oxide chains. The phase changes associated with the electrolyte-induced sphere-rod transition in micellar systems have also been examined by Raman spectroscopy. These studies show a greater ordering of the hydrocarbon chain in the rod-shaped compared to the sphereshaped aggregates.
Water Confined in Lamellar Structures of AOT Surfactants: An Infrared Investigation
The Journal of Physical Chemistry B, 2002
Both X-ray diffraction and infrared spectroscopy on water confined between the lamellar bilayers of aerosol-OT (AOT) allowed us to explore the differences in connectivity between bulk and confined water when water layer thicknesses are allowed to vary from 19.0 to 1.5 nm. Compared with previous studies on AOT reverse micelles, the present report in the lamellar mesophase allows one to cancel the micelle curvature contribution, which disrupts the water connectivity, too. The influence of AOT surface on the water connectivity degree was thus quantified alone in the mid infrared region (3000-3800 cm -1 ) in the OH stretching mode. The OH stretching mode peak can be fitted by three Gaussian peaks that describe three main connecting populations, namely the "multimer" water at c.a. 3585 cm -1 , the "intermediary" water at c.a. 3465 cm -1 , and the "network" water at c.a. 3320 cm -1 . It is shown that interactions with the surfactant heads do not alter the water structure until the water layer shrinks below 4.0 nm and the hydration water molecules per surfactant are found to be equal to 2.6, in good agreement with equivalent studies on micellar systems.
Materials, 2015
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. OPEN ACCESS Materials 2015, 8 3755
Thermodynamics of micelle formation in water, hydrophobic processes and surfactant self-assemblies
Physical Chemistry Chemical Physics, 2008
The critical micelle concentration (c.m.c.) for four cationic surfactants, alkyl-trimethyl-ammonium bromides, was determined as a function of temperature by conductivity measurements. The values of the standard free energy of micellisation DG 3 mic at different temperatures were calculated by using a pseudo-phase transition model. Then, from the diagram (ÀDG 3 mic /T) = f(1/T), the thermodynamic functions DH app and DS app were calculated. From the plots DH app = f(T) and DS app = f(ln T) the slopes DC p = n w(H) C p,w and DC p = n w(S) C p,w were calculated, with the numbers n w(H) and n w(S) negative and equal and therefore defined simply as n w . The number n w o 0, indicating condensed water molecules, depends on the reduction of cavity that takes place as a consequence of the coalescence of the cavities previously surrounding the separate aliphatic or aromatic moieties. The analysis, based on a molecular model consisting of three forms of water, namely W I , W II , and W III , respectively, was extended to several other types of surfactants for which c.m.c. data had been published by other authors. The results of this analysis form a coherent scheme consistent with the proposed molecular model. The enthalpy for all the types of surfactant is described by DH app = À3.6 + 23.1x w À x w C p,w T and the entropy by DS app = +10.2 + 428x w À x w C p,w ln T where x w = |n w | represents the number of molecules W III involved in the reaction. The term Dh w = +23.1 kJ mol À1 x w À1 indicates an unfavourable endothermic contribution to enthalpy for reduction of the cavity whereas the term Ds w = +428 J K À1 mol À1 x w À1 represents a positive entropy contribution for reduction of the cavity, what is the driving force of hydrophobic association. The processes of non polar gas dissolution in water and of micelle formation were found to be strictly related: they are, however, exactly the opposite of one another. In micelle formation no intermolecular electronic short bond is formed. We propose, therefore, to substitute the term ''hydrophobic bond'' with that of ''hydrophobic association''. The structure of (a) pure water and (b) aqueous solution of a hydrophobic substance.