Surface potential attractive

If an atom is in an excited state, the character of the atom-surface potential (attractive versus repulsive) is determined by the sum of the van der Waals interaction (Eq. (2.100)) and the classical shift (Eq. (2.101)). The first term may be either positive or negative. The second term also may have either sign if one of the transition frequencies is close to the frequency of a surface excitation determined by the equation n (cp) + 1 = 0. As a result, the interaction constant C may be either positive or negative. 

Electrically insulating materials can be analyzed in HF-plasma SNMS by applying a square-wave HF in the 100 kHz range to the sample (Fig. 3.34). Dielectric charge transfer at the start of a period shifts the surface potential to the amplitude Uhfm applied. Ar" ions are attracted from the plasma and sputter the surface until the end of At . The potential increase AU = 1-100 V caused by their charge is then converted to a positive absolute AU which is reduced to less than 1 V within <0.1 ps by the... 

Since the prewetting transition may occur only for weakly attractive surfaces [146], we must choose an appropriate value for the parameter This value has been set as follows e = 3 /3/2 j = 6. This corresponds to a relative strength of the fluid-fluid and fluid-surface potential minima close to that for Ar in contact with sohd carbon dioxide [147]. The second parameter in Eq. (144), zq, was set to 0.8[Pg.219]

Select benzene from the molecules on screen, and select Surfaces. Potential Map refers to an electrostatic potential map. Select Transparent to present it as a transparent (actually translucent) solid. This will allow you to see the molecular skeleton underneath. The surface is colored red in the n system (indicating negative potential and the fact that this region is attracted to a positive charge), and blue in the a system (indicating positive potential and the fact that this region is repelled by a positive charge). 

Another important feature of polymer adsorption is the influence exerted on it by the surface roughness. Ball et al. [22] proposed that if the surface potential is not attractive enough to bind the polymer when flat, then corrugation can aid binding as follows. For a sinusoidal corrugation, one might anticipate that some... 

The surface potential can play an important role in the behavior of liposomes in vivo and in vitro (e.g.. Senior, 1987). In general, charged liposomes ai e more stable against aggregation and fusion than uncharged vesicles. However, physically stable neutral liposomes have been described (e.g.. Van Dalen et al., 1988). They are sufficiently stabilized by repulsive hydration forces, which counteract the attractive van der Waals forces. 

The adsorption of CO on to gold surfaces has attracted a huge amount of interest due to the potential of catalysis and this topic has been reviewed by Meyer [8]. 

Hartley showed that micellar effects upon acid-base indicator equilibria could be related to the ability of anionic micelles to attract, and cationic micelles to repel, hydrogen ions. More recently attempts have been made to quantify these ideas in terms of the behavior of a micelle as a submicroscopic solvent, together with an effect due to its surface potential (Fernandez and Fromherz, 1977). 

The quantities that best represent a particular property can often be rationalized on the basis of physical intuition. For example, those that reflect interactions between like molecules, such as heats of sublimation and vaporization, can be expressed well in terms of molecular surface area and the product vofot. A large value for this product means that each molecule has both significantly positive and significantly negative surface potentials, which is needed to ensure strongly attractive inter-molecular interactions, with consequently higher energy requirements for the solid —> gas and liquid —> gas transitions. 

In a qualitative way, colloids are stable when they are electrically charged (we will not consider here the stability of hydrophilic colloids - gelatine, starch, proteins, macromolecules, biocolloids - where stability may be enhanced by steric arrangements and the affinity of organic functional groups to water). In a physical model of colloid stability particle repulsion due to electrostatic interaction is counteracted by attraction due to van der Waal interaction. The repulsion energy depends on the surface potential and its decrease in the diffuse part of the double layer the decay of the potential with distance is a function of the ionic strength (Fig. 3.2c and Fig. 

Unlike charges attract and like charges repel each other, so there is a high concentration of counterions attracted to the particle surface whilst co-ions (those with the same sign charge as that of the surface) are repelled. Thermal motion, i.e. diffusion, opposes this local concentration gradient so that the counterions are in a diffuse cloud around the particle. Of course particles which have a like charge will also repel each other but the interaction of the particle surfaces will be screened by the counterion clouds between the particles. The interaction potential is a function of the surface potential, i]/o, and the permittivity of the fluid phase, e = r80, where r is the relative permittivity.12,27... 

This change has two attractive features (1) It eliminates the separation of activity coefficients into short- and long-range interactions, which cannot be evaluated separately in practice, and (2) implicitly incorporates an expected effect of surface potential on solution activity through the activity coefficient relationship of Equation 22. Table II summarizes the relevant reaction and activity coefficient terms based on the above modifications of the TLM. 

It is noted that the molecular interaction parameter described by Eq. 52 of the improved model is a function of the surfactant concentration. Surprisingly, the dependence is rather significant (Eig. 9) and has been neglected in the conventional theories that use as a fitting parameter independent of the surfactant concentration. Obviously, the resultant force acting in the inner Helmholtz plane of the double layer is attractive and strongly influences the adsorption of the surfactants and binding of the counterions. Note that surface potential f s is the contribution due to the adsorption only, while the experimentally measured surface potential also includes the surface potential of the solvent (water). The effect of the electrical potential of the solvent on adsorption is included in the adsorption constants Ki and K2. 

---