Nanoconfinement of Water Layers in Lamellar Structures Prepared in the Presence and Absence of Organic Solvent

Abstract

An attempt is made to draw a line of comparison between the extent of rigidity of the hydration layers bound to the interfacial region of lamellar structures of Aerosol OT (AOT, sodium bis(2-ethylhexyl) sulfosuccinate) in water, in the presence and absence of an organic solvent using POM, SAXS, cryo-TEM, and time-resolved fluorescence spectroscopy. These systems are ternary mixtures of AOT, water, and n-heptane containing lamellar structures in an aqueous layer at higher w(0) values (w(0) = 300 and 150) and a binary solution of 20 and 50% AOT in neat water (w/w). The solvation shells residing at the vicinity of these lamellar structures are monitored using two different coumarin probes (C153 and C500). It is intended to envisage a comparative solvation dynamics study of the restricted aqueous region confined in lamellar structures formed in ternary mixture and binary solution. Though steady state measurements show a similar microenvironment probed by the fluorophores in lamellar structures formed in the two different aqueous phases, temporal evolution of the solvent correlation function C(t) unveils the existence of lamellar structures with different degrees of confinement of water layers in these two systems. A slower relaxation of the restricted aqueous region in lamellar structures of binary solution signifies the presence of more rigid interfacially bound water layers at the lamellar interface than in the ternary mixture having a similar weight percentage of AOT in water. The present investigation concludes that the lamellar structures formed under two different conditions provide a similar hydrophobic environment with different extents of localized water populations at the lamellar interface as manifested by the solvent relaxation time in agreement with SAXS and cryo-TEM images.

Anisotropic dynamics of guest molecules in aerosol OT lamellar structures

“…This property enables the imaging of these structures using cross polarization optical microscopy (POM). 16,20,21,22,23 4 Among these studies, the two that are most closely related to the present one are by Verma et al 16 and ourselves. 16 Attempts have been made in recent times aiming the elucidation of these aspects and the multimodal relaxation dynamics of both the free and bound water molecules found at the interfaces of these lamellar aggregates using a variety of techniques like X-ray diffraction, 12 infrared spectroscopy, 12, 13, 17 NMR and differential calorimetry.…”

“…31 Inclusion of C153 in lamellar structures formed in the lower aqueous layer of ternary mixtures of w 0 = 300 and 150 is marked by a blue shift in absorption spectra (Figure 1), as has been observed earlier as well. 22 This assignment is based on the earlier report of C153 in 6 micelles. This is indicated by the structured absorption spectra of C153 in the same region in neat heptane ( Figure 1).…”

“…15 Even though the fundamental structural aspects of these lamellar assemblies are similar to regular micelles, changes in degree of hydration and connectivity of water pools have been observed due to loss of dimensionality. 22 In the study of Verma et al, solvation dynamics in the lamellar structures has been reported, using coumarin 500. 18,19 However, only five fluorescence spectroscopic studies have been performed on these lamellar structures, to the best of our knowledge.…”

“…18,19 However, only five fluorescence spectroscopic studies have been performed on these lamellar structures, to the best of our knowledge. 22 Two kinds of lamellar structures have been compared in this context: those prepared from aqueous solutions of AOT following the method of Verma et al 16 and the other prepared from the aqueous layer of a ternary mixture of nheptane, water and AOT. 22 In the study of Verma et al, solvation dynamics in the lamellar structures has been reported, using coumarin 500.…”

“…Then, these results have been compared to those obtained earlier, with an excitation wavelength of 440 nm, 22 in an attempt to unravel the dynamics of migration of the instantaneously created dipoles from nonpolar to polar microenvironments. Besides, the inherent anisotropy in the structure of the aggregates has been interrogated by using fluorescence correlation spectroscopy using light of 402 nm as the excitation source.…”