Interfacial rotation

Rotational dynamics of a fluorescent dye adsorbed at the interface provides useful information concerning the rigidity of the microenvironment of liquid-liquid interface in terms of the interfacial viscosity. The rotational relaxation time of the rhodamine B dye was studied by the time-resolved total internal reflection fluorescent anisotropy. In-plane rotational relaxation time of octadecylrhodamine B cation was evaluated under the presence or absence of a surfactant [26]. Table 2.8 shows that by adding a surfactant, the relaxation time and the interfadal viscosity increased. Anionic surfactants SDS and HDHP (hydrogen dihexadecylphosphate) were more effective in reducing the rotational motion, because of the electrostatic interaction. HDHP with double long chains hindered the interfacial rotation more [40]. 

Statistical Thermodynamics of Adsorbates. First, from a thermodynamic or statistical mechanical point of view, the internal energy and entropy of a molecule should be different in the adsorbed state from that in the gaseous state. This is quite apart from the energy of the adsorption bond itself or the entropy associated with confining a molecule to the interfacial region. It is clear, for example, that the adsorbed molecule may lose part or all of its freedom to rotate. 

At lower frequencies, orientational polarization may occur if the glass contains permanent ionic or molecular dipoles, such as H2O or an Si—OH group, that can rotate or oscillate in the presence of an appHed electric field. Another source of orientational polarization at even lower frequencies is the oscillatory movement of mobile ions such as Na". The higher the amount of alkaH oxide in the glass, the higher the dielectric constant. When the movement of mobile charge carriers is obstmcted by a barrier, the accumulation of carriers at the interface leads to interfacial polarization. Interfacial polarization can occur in phase-separated glasses if the phases have different dielectric constants. 

Stationary microwave electrochemical measurements can be performed like stationary photoelectrochemical measurements simultaneously with the dynamic plot of photocurrents as a function of the voltage. The reflected photoinduced microwave power is recorded. A simultaneous plot of both photocurrents and microwave conductivity makes sense because the technique allows, as we will see, the determination of interfacial rate constants, flatband potential measurements, and the determination of a variety of interfacial and solid-state parameters. The accuracy increases when the photocurrent and the microwave conductivity are simultaneously determined for the same system. As in ordinary photoelectrochemistry, many parameters (light intensity, concentration of redox systems, temperature, the rotation speed of an electrode, or the pretreatment of an electrode) may be changed to obtain additional information. 

Samec Z, Weber J (1972) Reduction of ferric ion on a rotating platinum electrode of the turbulent type in the presence and absence of adsorbed sulfur. J Electroanal Chem Interfacial Electrochem 38 115-126... 

Cu9ln4 and Cu2Se. They performed electrodeposition potentiostatically at room temperature on Ti or Ni rotating disk electrodes from acidic, citrate-buffered solutions. It was shown that the formation of crystalline definite compounds is correlated with a slow surface process, which induced a plateau on the polarization curves. The use of citrate ions was found to shift the copper deposition potential in the negative direction, lower the plateau current, and slow down the interfacial reactions. 

Laboratory reactors for studying gas-liquid processes can be classified as (1) reactors for which the hydrodynamics is well known or can easily be determined, i.e. reactors for which the interfacial area, a, and mass-transfer coefficients, ki and kc, are known (e.g. the laminar jet reactor, wetted wall-column, and rotating drum, see Fig. 5.4-21), and (2) those with a well-defined interfacial area and ill-determined hydrodynamics (e.g. the stirred-cell reactor, see Fig. 5.4-22). Reactors of these two types can be successfully used for studying intrinsic kinetics of gas-liquid processes. They can also be used for studying liquid-liquid and liquid-solid processes. 

A schematic of the apparatus developed is shown in Fig. 3. Stirrers mix and push the lighter and heavier phases in each compartment, with the maximum rotation speed governed by the need to maintain the interface steady. Flow deflectors ensure that the phases are circulated in each chamber and that flow near the interface is laminar. The interfacial plate (thickness 2 mm) is rectangular with a hole at its center. The distance from the interface to the flow deflectors is less than 6 mm. The two phases are analyzed by withdrawing small volumes via sampling holes. 

The CLM method is a new technique, developed by Nagatani and Watarai [61]. This method produces a stable, ultrathin two-phase liquid membrane by the centrifugal force due to the rotation of a cylindrical cell, using the arrangement shown in Fig. 11. The inner diameter and inner height of the cylindrical cell were 19 and 29 mm, respectively. The rotation speed was controlled in the range 6000-7500 rpm. The summation of the absorption spectra of both interfacial and bulk organic phase species was measured in the direction perpendicular to the rotation axis with a diode array spectrophotometer. 

A high specific interfacial area and a direct spectroscopic observation of the interface were attained by the centrifugal liquid membrane (CLM) method shown in Fig. 2. A two-phase system of about 100/rL in each volume is introduced into a cylindrical glass cell with a diameter of 19 mm. The cell is rotated at a speed of 5000-10,000 rpm. By this procedure, a two-phase liquid membrane with a thickness of 50-100 fim. is produced inside the cell wall which attains the specific interfacial area over 100 cm. UV/VIS spectrometry, spectro-fluorometry, and other spectroscopic methods can be used for the measurement of the interfacial species and its concentration as well as those in the thin bulk phases. This is an excellent method for determining interfacial reaction rates on the order of seconds. 

By the total internal reflection condition at the liquid-liquid interface, one can observe interfacial reaction in the evanescent layer, a very thin layer of a ca. 100 nm thickness. Fluorometry is an effective method for a sensitive detection of interfacial species and their dynamics [10]. Time-resolved laser spectrofluorometry is a powerful tool for the elucidation of rapid dynamic phenomena at the interface [11]. Time-resolved total reflection fluorometry can be used for the evaluation of rotational relaxation time and the viscosity of the interface [12]. Laser excitation can produce excited states of adsorbed compound. Thus, the triplet-triplet absorption of interfacial species was observed at the interface [13]. 

Rotational dynamics of a fluorescent dye adsorbed at the interface provides useful information concerning the rigidity of the microenvironment of liquid-liquid interfaee in terms of the interfacial viscosity. The rotational relaxation time of the rhodamine B dye was studied by time-resolved total internal reflection fluorescent anisotropy. In-plane... 

Surfactant Cone. (M) In-plane rotational relaxation time (ns) Interfacial viscosity (Pas)... 

Eisenthal and coworkers have also measured interfacial friction via the rotational dynamics of probe molecules at the interface. In their first study, Eisenthal and coworkers probed the rotational dynamics of rhodamine 6G (R6G, structure shown in Fig. 2) at the... 

Another transport property of interfacial water which can be studied by MO techniques is the dipole relaxation time. This property is computed from the dipole moment correlation function, which measures the rate at which dipole moment autocorrelation is lost due to rotational motions in time (63). Larger values for the dipole relaxation time indicate slower rotational motions of the dipole... 

Monte Carlo and Molecular Dynamics simulations of water near hydrophobic surfaces have yielded a wealth of information about the structure, thermodynamics and transport properties of interfacial water. In particular, they have demonstrated the presence of molecular layering and density oscillations which extend many Angstroms away from the surfaces. These oscillations have recently been verified experimentally. Ordered dipolar orientations and reduced dipole relaxation times are observed in most of the simulations, indicating that interfacial water is not a uniform dielectric continuum. Reduced dipole relaxation times near the surfaces indicate that interfacial water experiences hindered rotation. The majority of simulation results indicate that water near hydrophobic surfaces exhibits fewer hydrogen bonds than water near the midplane. 

The convective diffusion theory was developed by V.G. Levich to solve specific problems in electrochemistry encountered with the rotating disc electrode. Later, he applied the classical concept of the boundary layer to a variety of practical tasks and challenges, such as particle-liquid hydrodynamics and liquid-gas interfacial problems. The conceptual transfer of the hydrodynamic boundary layer is applicable to the hydrodynamics of dissolving particles if the Peclet number (Pe) is greater than unity (Pe > 1) (9). The dimensionless Peclet number describes the relationship between convection and diffusion-driven mass transfer ... 

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