Polymerization of N,N-dimethylacrylamide in Aerosol OT–water mixtures: from lamellae to segregation

Anisotropic dynamics of guest molecules in aerosol OT lamellar structures

The present work is focused on developing a description of an anisotropic microheterogeneous medium, exploiting the dynamics of a guest molecule. The medium in question is the lamellar structures formed in the aqueous layer of ternary mixtures containing aerosol OT (AOT), water and n-heptane. The guest used in this study is the fluorescent probe, coumarin 153 (C153). The dynamics of this molecule, within the lamellar structure, have been studied using a combination of steady state and time resolved fluorescence, as well as fluorescence correlation spectroscopy (FCS). The fluorophore is strongly solvatochromatic and so, the wavelength of excitation can be tuned so as to selectively excite fluorescent molecules residing in different regions of the microheterogeneous media, even if the spatial separation between these regions is below the diffraction limit. The excitation wavelength in the present experiments is chosen so as to exclusively excite those C153 molecules that reside in the hydrophobic region of the lamellar structures. This triggers two different modes of diffusion, one along and the other perpendicular to the bilayers of the AOT. Thus, the dynamics of the fluorescent probe provide an elegant manifestation of the anisotropy of the host medium.

Unique effects of aerosol OT lamellar structures on the dynamics of guest molecules

The behavior of lamellar structures of Aerosol OT (AOT) as hosts, vis-à-vis the flexible normal micelles and rigid nanochannels of Nafion membranes, has been investigated with two different fluorophores, [2,2'-bipyridyl]-3,3'-diol (BP(OH)2) and coumarin 102 (C102). Surprisingly, for BP(OH)2, a rise time is observed at intermediate emission wavelengths and not in the red edge of the fluorescence spectrum. A shoulder at 525 nm is observed in time resolved emission spectra (TRES) at initial times of BP(OH)2 in AOT lamellar structures. This feature is the signature of the monoketo (MK) tautomer, observed for the first time in a microheterogeneous medium. Also, the usually ultrafast single proton transfer in BP(OH)2 is retarded to an considerable extent in lamellar structures. The potential of this medium in promoting unusual intermediates is thus highlighted. This property may be ascribed to the rigidity of lamellar structures, compared to hosts such as regular micelles. However, studies using another fluorophore, coumarin 102 (C102), brings out the fact that these structures are significantly different from the rigid host, Nafion, as well. The absence of excited state proton transfer (ESPT) in this molecule in AOT lamellar structures indicates that it is not protonated, unlike in Nafion. Thus, the interfacial pH of lamellar structures is found to be significantly greater than that of Nafion nanochannels. From the time dependent Stokes shift (TDSS) of the emission spectra of C102, the relaxation time (0.85 ns) of interfacial water in lamellar structures is found to be an order of magnitude faster than that observed in Nafion nanochannels, in which H3O(+) ions have been substituted by different cations. Hence, this study demonstrates that AOT lamellar structures are rather unique hosts and that they behave very differently from conventional rigid and flexible hosts such as normal micelles and Nafion, respectively.

Incorporation of substituted acrylamides to the lamellar mesophase of Aerosol OT

The structure and stability of the lamellar liquid crystal formed by the surfactant sodium bis-2ethylhexyl sulfosuccinate (AOT) in water is perturbed by small amounts of the substituted acrylamides N-isopropyl, N,N-diethyl, N-acryloylmorpholine, and N,N-dimethyl methacrylamide, as revealed by small angle X-ray scattering (SAXS), deuterium NMR, and microscopy. These molecules are water soluble and stay mostly in the water layers between lamellae, but a small fraction of them (5-19%) are incorporated into the AOT bilayers, thereby producing dramatic changes. Both, the degree of anisotropy in the water molecules hydrating AOT (quadrupolar splitting in (2)H NMR) and the long period spacing between lamellae (SAXS), decrease with addition of this molecules at low concentrations, which is attributed to the lower average headgroup density at the AOT/water interface when the acrylamide is incorporated. The strength of these perturbations depends on the acrylamide, and goes in parallel with the hydrophobic character of the alkyl side groups in its molecule, which suggests that the acrylamides incorporated to the bilayer enter into contact with the lipophilic tails of the AOT molecule. An interaction with the hydrated heads of AOT is also suggested in the particular case of N-isopropylacrylamide. On increasing the molecule concentration an incipient melting of the lamellar phase towards an isotropic solution takes place, first at the microscopic level, then macroscopic. Near this phase transition, the ordered domains lose the random orientation prevailing at lower acrylamide concentrations, and adopt a preferred orientation, perpendicular to the magnetic field.