Solutes at Interfaces Dynamics

Molecules adsorbed at interfaces are in constant motion, both from the point of view of their orientation and of the dynamic exchange with the bulk solution. The distribution of orientation angle is characterized by the parameter D, see Eq. (13), or the angle of orientation Q. Complete randomness for the angle 0, the angle of rotation around the principal... 

The overall picture arising from a comprehensive view of the solvation dynamics studies at interfaces that have been done can be summarized interfacial dynamics differ from bulk solution and cannot simply be considered the same as the bulk. In most cases, the structure of the interface appears to impact the dynamics by slowing them down. However, in a few cases, the dynamics appear to speed up. 

The microscopic structure of water at the solution/metal interface has been the focus of a large body of literature, and excellent reviews have been published summarizing the extensive knowledge gained from experiments, statistical mechanical theories of varied sophistication, and Monte Carlo and molecular dynamics computer simulations. To keep this chapter to a reasonable size, we limit ourselves to a brief summary of the main results and to a sample of the type of information that can be gained from computer simulations. 

Much less attention has been paid to the dynamic properties of water at the solution/metal interface (or other interfaces). Typical dynamic properties that are of interest include the diffusion constant of water molecules and several types of time correlation functions. In general, the time correlation function for a dynamic variable of interest A(t) is defined as... 

An important aspect of the study of water under electrochemical conditions is that one is able to continuously modify the charge on the metal surface and thus apply a well-defined external electric field, which can have a dramatic effect on adsorption and on chemical reactions. Here we briefly discuss the effect of the external electric field on the properties of water at the solution/metal interface obtained from molecular dynamics computer simulations. A general discussion of the theoretical and experi-... 

The theoretical modeling of electron transfer reactions at the solution/metal interface is challenging because, in addition to the difficulties associated with the quantitative treatment of the water/metal surface and of the electric double layer discussed earlier, one now needs to consider the interactions of the electron with the metal surface and the solvated ions. Most theoretical treatments have focused on electron-metal coupling, while representing the solvent using the continuum dielectric media. In keeping with the scope of this review, we limit our discussion to subjects that have been adi essed in recent years using molecular dynamics computer simulations. 

The main shortcoming of the molecular dynamics approach discussed in the previous section is that it ignores the fact that an electron transfer at the solution/metal interface occurs between an ion in a well-defined electronic state and a continuum of electronic states in the metal. For example, depending on the ion s orbital energy, the reorganization free energy and the overpotential, the electron could be transferred from, or to, any level around the Fermi level of the metal. Therefore, a sum over all these possibilities must be performed. Analytical theories of electron transfer at the solution/metal interface recognized this issue very early on, and the reader is referred to many excellent expositions on this sub-... 

Dynamic electroanalytical measurements at a solid electrode involve heterogeneous electron transfer. Electrons are transferred across the solution electrode interface during the electrode reaction. In fact, the term electrode reaction implies that such an electron-transfer process occurs. 

Fruitful interplay between experiment and theory has led to an increasingly detailed understanding of equilibrium and dynamic solvation properties in bulk solution. However, applying these ideas to solvent-solute and surface-solute interactions at interfaces is not straightforward due to the inherent anisotropic, short-range forces found in these environments. Our research will examine how different solvents and substrates conspire to alter solution-phase surface chemistry from the bulk solution limit. In particular, we intend to determine systematically and quantitatively the origins of interfacial polarity at solid-liquid interfaces as well as identify how surface-induced polar ordering... 

Complementing the equilibrium measurements will be a series of time resolved studies. Dynamics experiments will measure solvent relaxation rates around chromophores adsorbed to different solid-liquid interfaces. Interfacial solvation dynamics will be compared to their bulk solution limits, and efforts to correlate the polar order found at liquid surfaces with interfacial mobility will be made. Experiments will test existing theories about surface solvation at hydrophobic and hydrophilic boundaries as well as recent models of dielectric friction at interfaces. Of particular interest is whether or not strong dipole-dipole forces at surfaces induce solid-like structure in an adjacent solvent. If so, then these interactions will have profound effects on interpretations of interfacial surface chemistry and relaxation. 

Alexandridis P, Hatton TA. Polyethylene oxide)-poly(propylene oxide)- poly(ethylene oxide) block copolymer surfactants in aqueous solutions and at interfaces thermodynamics, structure, dynamics, and modeling (review). Colloid Surf A Physicochem Eng Aspects 1995 96 1 16. 

Equation (46), one form of the Gibbs equation, is an important result because it supplies the connection between the surface excess of solute and the surface tension of an interface. For systems in which y can be determined, this measurement provides a method for evaluating the surface excess. It might be noted that the finite time required to establish equilibrium adsorption is why dynamic methods (e.g., drop detachment) are not favored for the determination of 7 for solutions. At solid interfaces, 7 is not directly measurable however, if the amount of adsorbed material can be determined, this may be related to the reduction of surface free energy through Equation (46). To understand and apply this equation, therefore, it is imperative that the significance of r2 be appreciated. 

Viscosity and density of the component phases can be measured with confidence by conventional methods, as can the interfacial tension between a pure liquid and a gas. The interfacial tension of a system involving a solution or micellar dispersion becomes less satisfactory, because the interfacial free energy depends on the concentration of solute at the interface. Dynamic methods and even some of the so-called static methods involve the creation of new surfaces. Since the establishment of equilibrium between this surface and the solute in the body of the solution requires a finite amount of time, the value measured will be in error if the measurement is made more rapidly than the solute can diffuse to the fresh surface. Eckenfelder and Barnhart (Am. Inst. Chem. Engrs., 42d national meeting, Repr. 30, Atlanta, 1960) found that measurements of the surface tension of sodium lauryl sulfate solutions by maximum bubble pressure were higher than those by DuNuoy tensiometer by 40 to 90 percent, the larger factor corresponding to a concentration of about 100 ppm, and the smaller to a concentration of 2500 ppm of sulfate. 

There are several ways to reduce or suppress the electroosmotic flow in capillaries. These methods involve either eliminating the zeta potential across the solution-solid interface or increasing the viscosity at this interface. One approach is to coat the capillary wall, physically, with a polymer such as methylcellulose or linear polyacrylamide. Because of the difficulty in deactivating the capillary surface reproducibly, however, alternative methods employing dynamic reduction of solute-capillary interactions have been developed. Dynamic reduction of these interactions include the addition of chemical reagents such as methylhydroxyethylcellulose, S-benzylthiouro-nium chloride, and Triton X-100. 

Because the flow of electric current always involves the transport of matter in solution and chemical transformations at the solution-electrode interface, local behavior can only be approached. It can be approximated, however, by a reference electrode whose potential is controlled by a well-defined electron-transfer process in which the essential solid phases are present in an adequate amount and the solution constituents are present at sufficiently high concentrations. The electron transfer is a dynamic process, occurring even when no net current flows and the larger the anodic and cathodic components of this exchange current, the more nearly reversible and nonpolarizable the reference electrode will be. A large exchange current increases the slope of the current-potential curve so that the potential of the electrode is more nearly independent of the current. The current-potential curves (polarization curves) are frequently used to characterize the reversibility of reference electrodes. 

Miller, R., Fainerman, V.B., Aksenenko, E.V., Leser, M.E., and Michel, M. (2004). Dynamic surface tension and adsorption kinetics of p-casein at the solution-air interface. Langmuir. 20, ii -in. 

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