New Insights into the Dynamics of Zwitterionic Micelles and Their Hydration Waters by Gigahertz-to-Terahertz Dielectric Spectroscopy

Abstract

Gigahertz-to-terahertz spectroscopy of macromolecules in aqueous environments provides an important approach for identifying their global and transient molecular structures, as well as directly assessing hydrogen-bonding. We report dielectric properties of zwitterionic dodecylphosphocholine (DPC) micelles in aqueous solutions over a wide frequency range, from 50 MHz to 1.12 THz. The dielectric relaxation spectra reveal different polarization mechanisms at the molecular level, reflecting the complexity of DPC micelle-water interactions. We have made a deconvolution of the spectra into different components and combined them with the effective-medium approximation to separate delicate processes of micelles in water. Our measurements demonstrate reorientational motion of the DPC surfactant head groups within the micelles, and two levels of hydration water shells, including tightly and loosely bound hydration water layers. From the dielectric strength of bulk water in DPC solutions, we found that the number of waters in hydration shells is approximately constant at 950 ± 45 water molecules per micelle in DPC concentrations up to 400 mM, and it decreases after that. At terahertz frequencies, employing the effective-medium approximation, we estimate that each DPC micelle is surrounded by a tightly bound layer of 310 ± 45 water molecules that behave as if they are an integral part of the micelle. Combined with molecular dynamics simulations, we determine that tightly bound waters are directly hydrogen-bonded to oxygens of DPC, while loosely bound waters reside within 4 Å of micellar atoms. The dielectric response of DPC micelles at terahertz frequencies yields, for the first time, experimental information regarding the largest scale, lowest frequency collective motions in micelles. DPC micelles are a relatively simple biologically relevant system, and this work paves the way for more insight into future studies of hydration and dynamics of biomolecular systems with gigahertz-to-terahertz spectroscopy.

Time Domain Dielectric Spectroscopic Studies of Potassium Oleate and Cetyl Pyridinium Chloride in Acetate Buffer Solution

“…It was argued that the experimental relaxation time reflects the average dynamics of water molecules in the micelles. These assignments are in consistent with the more recent results from experiments . It is presumed that changes observed in the relaxation time of hydration water for the two surfactants are due to the different level of hydration of micelles.…”

“…Lanzi et al attributed the relaxation time of 120 ps for SDS to the interfacial bound water. Deepu K. George et al observed the cooperative molecular dynamics in hydrated layers of DPC micelles. Baar et al assigned the relaxation time τ = 25 ps to the water molecules surrounding the hydrophobic chain.…”

Dielectric relaxation spectroscopy of aqueous micellar electrolyte solutions: A novel application to infer Dukhin number and zeta potential of a micelle

“…The dielectric relaxation of aqueous NaCl solutions without surfactant in a frequency range from 20 MHz to 40 GHz features only one relaxation peak . This is different for the samples containing surfactants but no NaCl where up to four different relaxation processes are reported in this frequency range .…”

“…That is, the tangential micellar and the water molecules hydrogen bond relaxations. As mentioned in the Section Introduction, there is another relaxation process for aqueous surfactant solutions with a relaxation time of about 100 ps () reported in the literature . However, all of our attempts to incorporate this relaxation process in our regression analysis result in a diminished degree of the coefficient of determination of the fitted parameters.…”

“…Although there is some literature concerned with the application of DRS to micellar systems, such as Ref. , we are not aware of any attempt to use DRS to study these micellar systems in the presence of an additional electrolyte. However, an additional electrolyte is usually the case for the majority of surfactant solutions.…”