Marcello Carlà

Adsorption behaviour of n-hexylamine at the Hg/water interphase and its comparison with a molecular model accounting for local order

Abstract The adsorption of n-hexylamine on mercury from aqueous 0.1 M NaOH was studied by means of electrocapillary measurements. Adsorption at constant charge was found to satisfy a Frumkin isotherm with a charge-dependent attractive interaction factor. Analogously, adsorption at constant potential was found to be characterized by a potential-dependent Frumkin interaction factor. The lack of a sound physical basis for a general monolayer model. The particular significance of the interaction factor under conditions of maximum adsorption was stressed. The charge dependence of the standard Gibbs energy of adsorption and that of the Frumkin interaction factor as well as the coverage dependence of the potential drop across the inner layer at constant charge were interpreted on the basis of a monolayer model of H-bonded water molecules and solute dipoles in which local order is accounted for on the basis of the quasi-chemical approximation.

Development of an ultralow current amplifier for scanning tunneling microscopy

A transimpedance amplifier for ultralow current scanning tunneling microscopy has been developed. Conditions for maximum signal-to-noise ratio have been explored, showing that best results can be obtained with a simple circuital arrangement. The amplifier associates a very high amplification factor (0.5 V/pA) to a sufficiently wide bandwith (1.6 kHz) and very low noise current (49 fA). Those features enable microscopy studies on an almost insulating surface, such as a freshly cleaved mica surface.

Non-conventional normal pulse polarographic techniques with a static mercury electrode for applications at very low reactant concentrations

Abstract It is well known that the use of static mercury electrodes in normal pulse polarography (NPP) allows the capacitive current to be eliminated completely. Here it is shown that upon integrating the current following the polarizing pulse immediately after the capacitive contribution has faded out (which requires a time tmin = 3–5 ms in aqueous solutions) up to the end of the pulse, the signal to noise ratio is increased by 1–2 orders of magnitude with respect to conventional NPP at a static electrode. This depresses the detection limit of electroactive substances exchanging two electrons per molecule to 2 × 10−8M, and allows a straightforward kinetic study of sparingly soluble electroactive substances. In the case of irreversible electrode processes involving specifically adsorbed reactants and/or products, integration of the current from the very instant of the application of the polarizing pulse up to a time (tmin after the end of the pulse, so as to include the capacitive contribution due to the backward potential jump, yields NP polarograms free of maxima as well as of the capacitive current. These polarograms exhibit a diffusion controlled limiting current and a rising portion which can provide valuable mechanistic information on the basis of a straightforward kinetic analysis. An application to the kinetic study of progesterone electroreduction on Hg from neutral aqueous media over the concentration range from 10−7 to 10−5M is provided.

A comparative adsorption study of 1-octanol and 4-octanol at the mercury/water interface

Abstract The adsorption behavior of 1-octanol and that of 4-octanol on mercury from aqueous solutions of 0.1 M NaF are compared in order to establish the analogies and differences between the adsorption properties of primary and secondary aliphatic compounds. The general trend, according to which secondary and tertiary compounds exhibit a more positive shift, E N , in the potential of zero charge at full coverage, is confirmed. The more negative values for the charge of maximum adsorption and for the Frumkin interaction factor as well as the more positive E N value exhibited by 4-octanol with respect to 1-octanol are rationalized on the basis of a molecular model of the interphase.

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.

A comparison between potentiostatic circuits with grounded work or auxiliary electrode

Potentiostatic circuit configurations with work electrodes and auxiliary electrodes at ground potential have been reviewed and compared. Though the former is by far the best known and most used, the latter was more convenient and accurate in interfacial capacity measurements.

An automated apparatus for interfacial tension measurements by the sessile drop technique

A fully computer controlled apparatus for interfacial tension measurements at the mercury/solution or solution/solution interface is described. The apparatus is based on the drop‐shape technique and employs a solid state video camera to take the drop profile; measurements can be accomplished as a function of interface polarization potential, temperature and solution concentration. Overall data reproducibility is better than 0.06%, with an acquisition time of about 45 s for each experimental data point.

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.

Hybrid polar compounds produce a positive shift in the surface dipole potential of self-assembled phospholipid monolayers

Hybrid polar compounds (HPCs) are powerful inducers of terminal differentiation of various types of tumors, including Friend murine erythroleukemia cells (MELCs). They are known to act synergistically with an increase in the extracellular concentration of cations, which causes a positive shift in the negative value of the ionic surface potential. Two HPCs, hexamethylenebisacetamide (HMBA) and suberoylanilide hydroxamic acid (SAHA), were adsorbed on self-assembled phospholipid monolayers supported on a mercury drop and the shift in the surface dipole potential chi of the lipid film due to their adsorption was estimated from charge measurements. At their optimal concentrations for inducing MELC terminal differentiation (5 mM for HMBA and 2.6 microM for SAHA), these HPCs cause a chi shift of about 15-20 mV, positive toward the hydrocarbon tails, both on neutral phosphatidylcholine films and on negatively or positively charged phosphatidylserine films. This strongly suggests that the nonspecific effect of HPCs of different structure in inducing cancer cells to rescue their differentiation program is related to a positive chi shift on the extracellular side of the cell membrane.

Electronic linearization of piezoelectric actuators and noise budget in scanning probe microscopy

The maximum resolution achievable with a scanning probe microscope is limited by the probe size, by the mechanism of interaction with the sample, as is widely known, and by the electronic noise in the instrument. The evaluation of this noise for the three motion axes of a linearized high resolution scanning electrochemical microscope has been carried through and the intrinsic maximum resolution is discussed.