On the origin of the anomalous ultraslow solvation dynamics in heterogeneous environments

Abstract

Many recent experimental studies have reported a surprising ultraslow component (even >10 ns) in the solvation dynamics of a polar probe in an organized assembly, the origin of which is not understood at present. Here we propose two molecular mechanisms in explanation. The first one involves the motion of the 'buried water' molecules (both translation and rotation), accompanied by cooperative relaxation ('local melting') of several surfactant chains. An estimate of the time is obtained by using an effective Rouse chain model of chain dynamics, coupled with a mean first passage time calculation. The second explanation invokes self-diffusion of the (di)polar probe itself from a less polar to a more polar region. This may also involve cooperative motion of the surfactant chains in the hydrophobic core, if the probe has a sizeable distribution inside the core prior to excitation, or escape of the probe to the bulk from the surface of the self-assembly. The second mechanism should result in the narrowing of the full width of the emission spectrum with time, which has indeed been observed in recent experiments. It is argued that both the mechanisms may give rise to an ultraslow time constant and may be applicable to different experimental situations. The effectiveness of solvation as a dynamical probe in such complex systems has been discussed.

Impact of Topology on the Characteristics of Water inside Cubic Lyotropic Liquid Crystalline Systems

“…The plausible reasons for the origin of the ultraslow solvation component put forward by Bagchi et al is the role of the macromolecular chain dynamics. 50 They conceptualize that following the creation of the probe dipole, which is buried in the lipid bilayer, the neighboring water molecules need to reorient to minimize the energy. This involves not only the rotation of the water molecules but also the "breathing" motion of the surfactant chain.…”

“…The observed intermediate (∼1 ns) dynamics in Ia 3 d and Pn 3 m phases is ascribed to “trapped/bound” water molecules buried in the lipid headgroup region due to lipid fluctuation. Another plausible cause for the slow hydration component is the slow self-diffusion of the probe from the less polar regions (near the lipid–water interface) to the more polar regions (at the central core) of the nanochannels. , The origin of the ultraslow hydration component observed for C-343 in the two phases poses a unique question as the time scale of the component is too slow to correspond solely to water dynamics inside the aqueous channel. In fact, this kind of ultraslow dynamics (>5 ns) has also been observed in other organized assemblies such as microemulsions, micelles, and proteins .…”

“…Although the bimodal dynamic exchange model , satisfactorily explains the slow component contribution (≥1 ns), it is inadequate to explain this ultraslow component (∼10 ns). The plausible reasons for the origin of the ultraslow solvation component put forward by Bagchi et al is the role of the macromolecular chain dynamics . They conceptualize that following the creation of the probe dipole, which is buried in the lipid bilayer, the neighboring water molecules need to reorient to minimize the energy.…”

Solvation Dynamics in Different Phases of the Lyotropic Liquid Crystalline System

“…The diffusion coefficient measurement by the NMR study also indicates the existence of both mobile water and hydration water at the GMO–water interface (hydrogen exchange between the GMO hydroxyls and water molecules) , and thereby supports our conjecture related to two different water dynamics based on time-dependent Stokes shift measurement. The second explanation for the slow solvation component invokes the self-diffusion of the probe from relatively less polar interfacial region toward the highly polar central region of the cylinder. , In fact, the decrement (∼32%) of full width half-maximum (fwhm) of TRES at longer time-scale also indicates the possibility of contributing self-diffusion of probe toward the slow solvation component. Thus, we believe that both self-diffusion of C-343 as well as dynamic exchange between “bound” and “free” water molecules contribute toward the slow solvation dynamics observed in H II –LLC phase.…”

“…The second explanation for the slow solvation component invokes the self-diffusion of the probe from relatively less polar interfacial region toward the highly polar central region of the cylinder. 21,65 In fact, the decrement (∼32%) of full width half-maximum (fwhm) of TRES at longer time-scale also indicates the possibility of contributing selfdiffusion of probe toward the slow solvation component. Thus, we believe that both self-diffusion of C-343 as well as dynamic exchange between "bound" and "free" water molecules contribute toward the slow solvation dynamics observed in H II −LLC phase.…”