Interface relaxation processes

Riemann Randolf, Cantow Hans-Joachim, and Friedrich Carsten. Rheological investigation of form relaxation and interface relaxation processes in polymer blends. Polym. Bull. 36 no. 5 (1996) 637-643. 

The effectiveness of a crude oil demulsifier is correlated with the lowering of the shear viscosity and the dynamic tension gradient of the oil-water interface. The interfacial tension relaxation occurs faster with an effective demulsifier [1714]. Short relaxation times imply that interfacial tension gradients at slow film thinning are suppressed. Electron spin resonance experiments with labeled demulsifiers indicate that the demulsifiers form reverse micellelike clusters in the bulk oil [1275]. The slow unclustering of the demulsifier at the interface appears to be the rate-determining step in the tension relaxation process. 

For effective demulsification of a water-in-oil emulsion, both shear viscosity as well as dynamic tension gradient of the water-oil interface have to be lowered. The interfacial dilational modulus data indicate that the interfacial relaxation process occurs faster with an effective demulsifier. The electron spin resonance with labeled demulsifiers suggests that demulsifiers form clusters in the bulk oil. The unclustering and rearrangement of the demulsifier at the interface may affect the interfacial relaxation process. 

Fig. 46 Schematic diagram of elemental process during transition from Hex cylinder to bcc sphere (i) undulation of interface (a, b), (ii) break-up of cylinders into ellipsoids (b, c), (iii) relaxation of domains from ellipsoids into spheres (c, d), and (iv) relaxation in junction distribution to attain uniform distribution (d, e). Pole where concentration of junction points is low may work as memory of grain conservation upon reverse transition from bcc sphere to Hex cylinder. Small arrows in part (b) indicate diffusion of chemical junctions along interface in process (ii). From [136], Copyright 2000 American Chemical Society...

In the right-hand part of Figure 10 are shown simulation results obtained by using the above kinetic equations and the rectangular cell model which divides the air/water interface into one hundred cells. In this simulation, the relative magnitudes of the rate of relaxation processes and the rate of compression were set up as follows. ... 

If the PEP-PEE triblock chains are arranged in microdomains separated by well-defined interfaces, then the additional fast relaxation process observed by NSE would result from the undulation motions of the interface. The thermal motions of the interface are driven by the surface tension and overdamped. With an effective viscosity r eff in the interfacial region, the relaxation rate if Q) ... 

The combination of careful chemical synthesis with NSE and SANS experiments sheds some light on the fast relaxation processes observed in the collective dynamics of block copolymers melts. The results reveal the existence of an important driving force acting on the junction points at and even well above the ODT. Modelling the surface forces by an expression for the surface tension, it was possible to describe the NSE spectra consistently. The experimental surface tension agrees reasonably well with the Helfand predictions, which are strictly valid only in the strong-segregation hmit. Beyond that, these data are a first example for NSE experiments on the interface dynamics in a bulk polymer system. 

In the presence of the spacer (Fig. 17.35a), an initially high photocurrent value ( 6 mA/cm2) is achieved, but, due to the larger spacing between the two electrodes, the diffusion of the electron mediator is not fast enough to supply new reduced mediator to the Ti02/dye interface from which, under irradiation, is constantly depleted. Thus, a steady photocurrent value, significantly lower than the initial spike, is attained after a few seconds. In Fig. 17.35b, the reduced diffusional path for the electron mediator allows for a more effective mass transport that accounts for the generation of a stable photocurrent without the observation of photoanodic relaxation processes. 

An intense femtosecond laser spectroscopy-based research focusing on the fast relaxation processes of excited electrons in nanoparticles has started in the past decade. The electron dynamics and non-linear optical properties of nanoparticles in colloidal solutions [1], thin films [2] and glasses [3] have been studied in the femto- and picosecond time scales. Most work has been done with noble metal nanoparticles Au, Ag and Cu, providing information about the electron-electron and electron-phonon coupling [4] or coherent phenomenon [5], A large surface-to-volume ratio of the particle gives a possibility to investigate the surface/interface processes. 

In conclusion, we observe that the crossing of crystal phase boundaries by matter means the transfer of SE s from the sublattices of one phase (a) into the sublattices of another phase (/ ). Since this process disturbs the equilibrium distribution of the SE s, at least near the interface, it therefore triggers local SE relaxation processes. In more elaborated kinetic models of non-equilibrium interfaces, these relaxations have to be analyzed in order to obtain the pertinent kinetic equations and transfer rates. This will be done in Chapter 10. 

AC/ is known as the overpotential in the electrode kinetics of electrochemistry. Let us summarize the essence of this modeling. If we know the applied driving forces, the mobilities of the SE s in the various sublattices, and the defect relaxation times, we can derive the fluxes of the building elements across the interfaces. We see that the interface resistivity Rb - AC//(F-y0) stems, in essence, from the relaxation processes of the SE s (point defects). Rb depends on the relaxation time rR of the (chemical) processes that occur when building elements are driven across the boundary. In accordance with Eqn. (10.33), the flux j0 can be understood as the integral of the relaxation (recombination, production) rate /)(/)), taken over the width fR. 

Figure 10-14. Relaxation processes at a moving interface, a) Scheme of boundary a/0 during transformation, b) coupling of transport and relaxation processes, c) detailed structure element steps in the relaxation zone.

To complete the set of kinetic equations we observe that ub = (A/ /Ac)b where Acb can be expressed in terms of <5 ,b. Finally, the requirement of mass conservation yields a further equation. Considering the inherent nonlinearities, this problem contains the possibility of oscillatory solutions as has been observed experimentally. Let us repeat the general conclusion. Reactions at moving boundaries are relaxation processes between regular and irregular SE s. Coupled with the transport in the untransformed and the transformed phases, the nonlinear problem may, in principle, lead to pulsating motions of the driven interfaces. 

At present it is well established that the existence of the phase border between a polymer and any solid leads to the appearance of different types of micro- and macroheterogeneities at the molecular, supermolecular, and chemical levels78. It is established that, due to adsorption interaction at the interface, an essential decrease in molecular mobility takes place as a result of which the glass temperature of such systems increases79. At the same time, due to retardation of the relaxation processes in the surface layers, some loosening of packing takes place, whereas in pure adsorption layers some increase in density is observed80. ... 

The adsorption and desorption kinetics of surfactants, such as food emulsifiers, can be measured by the stress relaxation method [4]. In this, a "clean" interface, devoid of surfactants, is first formed by rapidly expanding a new drop to the desired size and, then, this size is maintained and the capillary pressure is monitored. Figure 2 shows experimental relaxation data for a dodecane/ aq. Brij 58 surfactant solution interface, at a concentration below the CMC. An initial rapid relaxation process is followed by a slower relaxation prior to achieving the equilibrium IFT. Initially, the IFT is high, - close to the IFT between the pure solvents. Then, the tension decreases because surfactants diffuse to the interface and adsorb, eventually reaching the equilibrium value. The data provide key information about the diffusion and adsorption kinetics of the surfactants, such as emulsifiers or proteins. 

Abstract. Surface pressure/area isotherms of monolayers of micro- and nanoparticles at fluid/liquid interfaces can be used to obtain information about particle properties (dimensions, interfacial contact angles), the structure of interfacial particle layers, interparticle interactions as well as relaxation processes within layers. Such information is important for understanding the stabilisation/destabilisation effects of particles for emulsions and foams. For a correct description of II-A isotherms of nanoparticle monolayers, the significant differences in particle size and solvent molecule size should be taken into account. The corresponding equations are derived by using the thermodynamic model of a two-dimensional solution. The equations not only provide satisfactory agreement with experimental data for the surface pressure of monolayers in a wide range of particle sizes from 75 pm to 7.5 nm, but also predict the areas per particle and per solvent molecule close to the experimental values. Similar equations can also be applied to protein molecule monolayers at liquid interfaces. 

The first type of relaxation processes reflects characteristics inherent to the dynamics of single droplet components. The collective motions of the surfactant molecule head groups at the interface with the water phase can also contribute to relaxations of this type. This type can also be related to various components of the system containing active dipole groups, such as cosurfactant, bound, and free water. The bound water is located near the interface, while free water, located more than a few molecule diameters away from the interface, is hardly influenced by the polar or ion groups. For ionic microemulsions, the relaxation contributions of this type are expected to be related to the various processes associated with the movement of ions and/ or surfactant counterions relative to the droplets and their organized clusters and interfaces [113,146]. 

Further experiments to examine the ion exchange mechanism for the acceleration in the relaxation process of the chemical oscillation were also performed. We varied the kind of hydrophilic anions (Cl , Br and I ) in the oil phase and investigated the dependence of the desorption rate of DS ions on the standard free energy of transfer of those anions. Since the ion exchange should occur at the oil/water interface, we expected that the more the standard free energy of transfer of the anions from the interface to the water phase would decrease, the more efficiently the ion exchange would occur between the hydrophilic anions and DS ions, resulting in the acceleration of the desorption... 

Tensions of non-relaxed interfaces are sometimes known by the adjective dynamic dynamic surface tension or dynamic inteifacial tension. The term dynamic is not absolute. It depends on De (i.e. on the time scale of the measurement as compared with that of the relaxation process). Some interfacial processes have a long relaxation time (polymer adsorption-desorption), so that for certain purposes (say the measurement of y] they may be considered as being in a state of frozen equilibrium. This last notion was introduced at the end of sec. 1.2.3. Unless otherwise stated, we shall consider static tensions and interfaces which are so weakly curved that curvature energies, bending moments, etc. may be neglected. 

M. van den Tempel, E.H. Lucassen-Reynders, Relaxation Processes at Fluid Interfaces, Adu. Colloid Interface Set 18 (1983) 281. 

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