C. M. C. Gambi

Dielectric Relaxation Spectroscopy of Lysozyme Aqueous Solutions: Analysis of the δ-Dispersion and the Contribution of the Hydration Water

The dielectric properties of lysozyme aqueous solutions have been investigated over a wide frequency range, from 1 MHz to 50 GHz, where different polarization mechanisms, at a molecular level, manifest. The dielectric relaxation spectra show a multimodal structure, reflecting the complexity of the protein-water interactions, made even more intricate with the increase of the protein concentration. The deconvolution of the spectra into their different components is not unambiguous and is generally a delicate process which requires caution. We have analyzed the whole relaxation region, on the basis of the sum of simple Debye-type relaxation functions, considering three main contributions. Particular attention has been payed to the δ-dispersion, intermediate between the β-dispersion (rotational dynamics of the protein) and the γ-dispersion (orientational polarization of the water molecules). This intermediate contribution to the dielectric spectrum is attributed to the orientational polarization of water molecules in the immediate vicinity of the protein surface (hydration water). Our measurements clearly demonstrate that, at least at high protein concentrations, the δ-dispersion has a bimodal structure associated with two kinds of hydration water, i.e., tightly bound and loosely bound hydration water. In the concentration range investigated, the existence of a three-modal δ-dispersion, as recently suggested, is not supported, on the basis of statistical tests, by the analysis of the dielectric relaxations we have performed and a bimodal dispersion is accurate enough to describe the experimental data. The amount of the hydration water has been evaluated both from the dielectric parameters associated with the δ-dispersion and from the decrement of the loss peak of the γ-dispersion. The relative weight of tightly bound and loosely bound hydration water is briefly discussed.

Differential and double-differential dielectric spectroscopy to measure complex permittivity in transmission lines

This article presents and compares two differential methods for measuring the complex permittivity of dielectric materials: In the first method, two measuring cells built as coaxial transmission lines of identical cross section and terminations but different lengths are filled with a sample of the dielectric material. The complex dielectric permittivity is determined from the scattering parameter measurements and the length difference between the two cells, neglecting the resistive losses due to the cells. The second method is a double-differential one: Repeating measurements on the same cells empty, no other knowledge or limiting assumption is required.

Thermally stimulated dielectric polarization release in water-in-oil microemulsions

Abstract The thermally stimulated depolarization (TSD) of a water-in-oil microemulsion was analyzed as a function of increasing water contents in the concentration interval 0.024 ⩽ c ⩽ 0.5 (c = mass fraction). Two main peaks were clearly detected in the depolarization vs temperature spectra. The first peak, with its maximum at 253°K, persists over the whole range of concentration and is ascribed to the polarization of the electrified water-oil interface; the second peak, centered around 288°K, was interpreted in terms of the orientation of some structured dipolar entities which develop in a restricted range of concentrations preceding the liquid-crystalline phase. The activation energy and the relaxation time of the orientation process associated with the first depolarization peak (253°K) was calculated following the Bucci and Fieschi method.

A new insight on the dynamics of sodium dodecyl sulfate aqueous micellar solutions by dielectric spectroscopy

Aqueous sodium dodecyl sulfate micellar solutions were investigated by a recently developed double-differential dielectric spectroscopy technique in the frequency range 100 MHz-3 GHz at 22 degrees C, in the surfactant concentration range 29.8-524 mM, explored for the first time above 104 mM. The micellar contribution to dielectric spectra was analyzed according to three models containing, respectively, a single Debye relaxation, a Cole-Cole relaxation and a double Debye relaxation. The single Debye model is not accurate enough. Both Cole-Cole and double Debye models fit well the experimental dielectric spectra. With the double Debye model, two characteristic relaxation times were identified: the slower one, in the range 400-900 ps, is due to the motion of counterions bound to the micellar surface (lateral motion); the faster one, in the range 100-130 ps, is due to interfacial bound water. Time constants and amplitudes of both processes are in fair agreement with Grosses theoretical model, except at the largest concentration values, where interactions between micelles increase. For each sample, the volume fraction of bulk water and the effect of bound water as well as the conductivity in the low frequency limit were computed. The bound water increases as the surfactant concentration increases, in quantitative agreement with the micellar properties. The number of water molecules per surfactant molecule was also computed. The conductivity values are in agreement with Kallays model over the whole surfactant concentration range.

SANS study of a fluorinated water-in-oil microemulsion

The experimental results of a SANS analysis of fluorinated water-in-oil microemulsions with perfluoropolyether oil and surfactant are shown at T = 20°C and T = 35°C for samples with a constant water-to-surfactant molar ratio WS = 11. Under the hypothesis that the system is composed of interacting droplets, by a Guinier analysis of the dilute samples, a droplet radius of 23 A is found for the two temperatures studied. Furthermore, all the samples studied follow the Porod law, as expected for dispersed particles with a sharp interface. For the two temperatures studied, a microemulsion interfacial area per surfactant molecule of 50 A2 is found in the dilute region and about 35 A2 in the concentrated region.

Dielectric permittivity measurements on highly conductive perfluoropolyether microemulsions at frequencies up to 100 MHz

The impedance analysis technique has been used to measure the complex permittivity and conductivity of highly conductive perfluoropolyether microemulsions. The cell construction details, calibration procedure and data analysis procedure are reported. Examples of dielectric analysis on perfluoropolyether microemulsions are given.

Complexation of counter-ions in ionic micellar solutions: a small-angle neutron scattering study

Abstract The properties of LDS and SDS micellar solutions upon the addition of [2.2.2]-cryptand (C222) are studied by small-angle neutron scattering (SANS). The interfacial thickness of the micelle for three ligand to surfactant molar ratios ( L S = 0.5, 1.0 and 1.5 ) increases from 5.5 A (interfacial thickness of the micelles without ligand) to 10 A, indicating that the ligand is located at the interface. The micellar net surface charge and the average aggregation number decrease as L S increases, producing a slight decrease of the micellar size and a more spherical shape. The ionization degree of the LDS micelles is almost constant as L S increases, whereas it decreases slightly for SDS micelles. This supports the higher binding ability of C222 for sodium ions. The liogand/surfactant molar ratio at the interface is at most 1 2 when the ligand is added to the surfactant solution (LDS and SDS) in equimolar amounts. This implies that only half of the total amount is partitioned at the interface, while the remaining ligand molecules are in the bulk solution. Further addition of the ligand increases its amount in the bulk solution.

SANS on fluorinated water-in-oil microemulsions

Abstract Preliminary SANS results on water-in-oil microemulsions composed of water, fluorinated surfactant and fluorinated oil, indicate that the system is composed of interacting droplets in a large range of compositions. Work is in progress to find the interaction potential.

Dielectric behavior of highly conducting perfluoropolyether water-in-oil microemulsions

The dielectric and conductive properties of a new ternary water-in-oil microemulsion consisting of perfluoropolyether (PFPE) oil, PFPE surfactant and water, were investigated by an impedance analysis technique in the frequency range of 5 Hz to 100 MHz. The trend of both the electrical conductivity and the static permittivity was analyzed as a function of the temperature in the interval from -10 to +40/spl deg/C. Maxwell-Wagner relaxation phenomena in the MHz range and a percolative transition, of dynamic type against temperature, were observed. >

Effect of lithium encapsulation by a macrocyclic aza cage in micellar solutions of lithium dodecyl sulfate

Abstract The small macrobicyclic cage (5,12,17-trimethyl-1,5,9,12,17-pentaazobicyclo[7.5.5]nonadecane, CESTO), which can selectively encapsulate lithium ions, has been studied in the presence of micellar solutions of 0.037 M lithium dodecyl sulfate (LDS). The CESTO cage behaves as a fairly strong base in the first protonation step (log K 1 =11.83 where K 1 is the equilibrium constant) and exhibits two different behaviors in LDS micellar solutions. Surface tension, electron spin echo modulation (ESEM), 7 Li NMR and small-angle neutron scattering (SANS) measurements show that the macrocyclic cage at about pH 10.2 behaves as a bulkier counterion, while at about pH 12.7 it complexes selectively lithium counterions at the micellar surface. Furthermore, ESEM studies show that CESTO encapsulates lithium ions at the micellar surface by reducing the amount of water at the surfactant polar head groups. SANS results show that, at about pH 12.5, LDS molecules form micelles of quasi-spherical shape with an average aggregation number of 90. The effective ionization of the micelles is about 20%. The addition of CESTO leads to lithium ion complexation with a drastic reduction of the micellar surface charge, and of the hydration number. This is accompanied by the growth of the aggregation number and the micelles become more elongated.