Interfacial relaxation method

Surface induced spinodal decomposition leads, for properly controlled surface fields, to a two layer structure characteristic for coexisting phases. Hence it may be used to determine the coexisting conditions in a more convenient way that with the interfacial relaxation method as the initial bilayer geometry may be avoided. In practical terms the overall composition of the whole thin film may be much better controlled in experiments involving spinodal decomposition. Therefore in experiments studying the equilibrium composition vs depth pro-... 

Interfacial relaxation methods are typically based on a perturbation of the equilibrium state of an interfacial layer (equilibrium within the interfacial layer and with the adjacent bulk phases) by small changes of the interfacial area. The small relative change in area is defined by... 

We want to give only two examples of interfacial relaxation methods. The whole field of interfacial relaxations and rheology is so broad and of strong practical relevance that this topic deserves a whole book. At first, two examples, a harmonic and a transient experiment will be shown as example for slow relaxation experiments, while as second we will present results of experiments performed under ground and microgravity conditions, respectively, based on the principle of oscillating bubbles. 

There are many experimental techniques for studying interfacial relaxations of soluble adsorption layers. Except for the wave damping techniques, these methods are developed and used only by individual research groups. Up to now, no commercial set-up exists and therefore, relaxation experiments are not so wide spread. New developments in this field will probably increase the number of investigators studying the dynamic and mechanical properties of adsorption layers, since instruments are easy to construct and data handling is relatively simple. In this section, wave damping and other harmonic methods as well as transient relaxation techniques will be described. 

As mentioned above relaxation techniques are additional methods suitable to get insight into the mechanism of adsorption processes. Moreover, these methods represent the experimental tools to determine the dilational rheology of interfacial layers. The general principle of relaxation methods is the small disturbance of the interfacial layer, which has reached the equilibrium state beforehand. Particular methods are suitable to detect characteristic times of relaxations processes as they work each in a specific frequency range. This paragraph discusses briefly the most frequently used and very recently developed methods. 

In recent years, several theoretical and experimental attempts have been performed to develop methods based on oscillations of supported drops or bubbles. For example, Tian et al. used quadrupole shape oscillations in order to estimate the equilibrium surface tension, Gibbs elasticity, and surface dilational viscosity [203]. Pratt and Thoraval [204] used a pulsed drop rheometer for measurements of the interfacial tension relaxation process of some oil soluble surfactants. The pulsed drop rheometer is based on an instantaneous expansion of a pendant water drop formed at the tip of a capillary in oil. After perturbation an interfacial relaxation sets in. The interfacial pressure decay is followed as a function of time. The oscillating bubble system uses oscillations of a bubble formed at the tip of a capillary. The amplitudes of the bubble area and pressure oscillations are measured to determine the dilational elasticity while the frequency dependence of the phase shift yields the exchange of matter mechanism at the bubble surface [205,206]. 

The present chapter gives also detailed introduction to a large number of experimental methods, suitable for studying dynamic interfacial tensions. The methods are discussed in terms of the available time window. There are methods which complement each other such that a time interval from less than 100 microseconds up to hours and days of adsorption time can be covered (about ten orders of magnitude). The relaxation methods, also suitable for detecting the adsorption mechanism of surfactant s adsorption provide in addition the dilational rheology of interfacial layers. It is discussed that in particular these dilational rheological studies are most informative in respect to adsorption mechanisms, as the data interpretation includes the thermodynamic model as well as the adsorption dynamics. 

The drop-shape oscillation technique as developed by Tian et al. (206, 207) is another technique suitable for closing the gap in the experimental methods for liquid/liquid interfaces. This method is based on the analysis of drop-shape oscillation modes and yields again the matter-ex-change mechanism and the dila-tional interfacial elasticity. The method is similar to the transient relaxation methods applicable only for comparatively low oscillation frequencies. 

We will show that the ILIT method eliminates some of the problems associated with an electrical perturbation and, not surprisingly, creates new, interesting, and challenging problems. Improved electronics developed at the time of the termination of this program, coupled with picosecond or subpicosecond laser pulses, should, in principle, allow the ILIT method to probe interfacial relaxations of the order of 1 ns or less (our published work has used a slower system with a response function of the order of 15 ns). Of course, really dramatic improvement in response time will be achieved only with a pump-probe approach. Nevertheless, even at its present stage of development, ILIT effects significant improvement in time resolution over methods using conventional electrochemical perturbations where the time resolution is limited by solution resistance and interfacial capacitance (see Ref. 41) ... 

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]. 

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

Bavykin, Dmitry V. is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998) Luminescent and photocatalytic properties of CdS nanocolloids . Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. 

Solution of the coupled mass-transport and reaction problem for arbitrary chemical kinetic rate laws is possible only by numerical methods. The problem is greatly simplified by decoupling the time dependence of mass-transport from that of chemical kinetics the mass-transport solutions rapidly relax to a pseudo steady state in view of the small dimensions of the system (19). The gas-phase diffusion problem may be solved parametrically in terms of the net flux into the drop. In the case of first-order or pseudo-first-order chemical kinetics an analytical solution to the problem of coupled aqueous-phase diffusion and reaction is available (19). These solutions, together with the interfacial boundary condition, specify the concentration profile of the reagent gas. In turn the extent of departure of the reaction rate from that corresponding to saturation may be determined. Finally criteria have been developed (17,19) by which it may be ascertained whether or not there is appreciable (e.g., 10%) limitation to the rate of reaction as a consequence of the finite rate of mass transport. These criteria are listed in Table 1. 

Here it may be appropriate to note that in the literature the term dynamic interfacial (or surface tension is used in two senses, viz. (i) as the tension obtained by a dynamic method such as scattering or (li) as that obtained under nonequilibrium conditions, l.e. tensions measured at non-zero De, for interfaces that relax during the observation. We shall reserve the term dynamic interfacial tension for the latter case, that is, for Interfaces that are not equilibrated (sec. 1.14). Interfacial tensions derived from scattering are essentially static, although... 

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