Conformation solution interfaces

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

After polarization to more anodic potentials than E the subsequent polymeric oxidation is not yet controlled by the conformational relaxa-tion-nucleation, and a uniform and flat oxidation front, under diffusion control, advances from the polymer/solution interface to the polymer/metal interface by polarization at potentials more anodic than o-A polarization to any more cathodic potential than Es promotes a closing and compaction of the polymeric structure in such a magnitude that extra energy is now required to open the structure (AHe is the energy needed to relax 1 mol of segments), before the oxidation can be completed by penetration of counter-ions from the solution the electrochemical reaction starts under conformational relaxation control. So AHC is the energy required to compact 1 mol of the polymeric structure by cathodic polarization. Taking... 

When rfc = 0, the polymeric structure is considered to be open enough (i = 0) that any subsequent oxidation will not occur under conformational relaxation control, hence P = 1. Every polymeric chain at the poly-mer/solution interface acts as a nucleus a planar oxidation front is formed that advances from the solution interface toward the metal/polymer interface at the diffusion rate. 

Several experimental parameters have been used to describe the conformation of a polymer adsorbed at the solid-solution interface these include the thickness of the adsorbed layer (photon correlation spectroscopy(J ) (p.c.s.), small angle neutron scattering (2) (s.a.n.s.), ellipsometry (3) and force-distance measurements between adsorbed layers (A), and the surface bound fraction (e.s.r. (5), n.m.r. ( 6), calorimetry (7) and i.r. (8)). However, it is very difficult to describe the adsorbed layer with a single parameter and ideally the segment density profile of the adsorbed chain is required. Recently s.a.n.s. (9) has been used to obtain segment density profiles for polyethylene oxide (PEO) and partially hydrolysed polyvinyl alcohol adsorbed on polystyrene latex. For PEO, two types of system were examined one where the chains were terminally-anchored and the other where the polymer was physically adsorbed from solution. The profiles for these two... 

Many studies of proteins at air-solution interfaces have indirectly established that the adsorbed proteins undergo detectable conformational changes. Similar studies at solid-liquid interfaces are few. We review here only several key studies. 

Schick and Harvey (49) summarize an interesting investigation of the effect of the choice of solvent on the conformation of a polymer adsorbed at the solution interface with Spheron 6 carbon black. A noteworthy conclusion concerns the occurrence of extended and looped configurations of the adsorbed polymer molecules formed from good or poor solvents, respectively. 

Radeva, T. and Petkanchin, I., Electric properties and conformation of polyethylenimine at the hematite-aqueous solution interface, J. Colloid Intetf. Sci., 196, 87,1997. Ozaki, M., Kratohvil, S., and Matijevic, E., Eormation of monodispersed spindle-type hematite particles, 7. Colloid Interf. Sci., 102, 146, 1984. 

The importance of adsorbed polymer conformation at interfaces was first recognized by Jenkel and Rumbach in 1951. A model of adsorbed polymer conformation was proposed based on the observation that amount of polymer adsorbed per unit area of the surface corresponds to a layer more than ten molecules thick. In that model, not all the segments of a polymer are in contact with the surface. As schematically shown in Figure 7.27, those segments that are in direct contact with the surface are termed trains, those between and extending into solution are termed loops, the free ends of the polymer extending into solution are termed tails. Sato and Ruch classified the possible conformations for most situations into the six types shown in Figure 7.28. 

Radeva Ts, Petkanchin I. Electric properties and conformation of polyeth-ylenimine at the hematite-aqueous solution interface. J Colloid Interface Sci 1997 196 87-91. 

An elastic network can be formed both in bulk, induced by a heating cycle causing denaturation of the protein, and at an interface, induced by adsorption, which likely caused changes in the conformation. The network formed at the air-protein solution interface can be considered as a kind of two dimensional gel. At low pH, a gel is more readily formed in bulk (lower heating temperature required)30,31 as well as at the air-protein solution surface. Likely, both phenomena are related to a lower stability of the molecule against conformational changes caused by external factors. 

Figure 13.2.5 is a graph of the relative surface excesses for the components of a 0.1 M KF solution in contact with mercury. Note that at potentials positive of the surface excess of F is positive and that of is negative. This negative surface excess of simply implies that the concentration of K" in the vicinity of the mer-cury/solution interface is smaller than that in the bulk solution. The opposite condition holds for potentials negative of Ej. The behavior of KF solutions thus conforms to what we would expect based on simple electrostatics. We can contrast this behavior with that of a 0.1 M KBr solution shown in Figure 13.2.6. Note here that at potentials positive of Ej (i.e., for > 0), Fk+ is positive. The reason for this interesting behav-...

In a novel experiment, Koyama et al. (57) obtained a spectrum of carotenoid BLM by resonance Raman spectroscopy—a major advance in BLM spectroscopy. For efficient charge transfer, the orientation of chlorophyll molecules at the membrane-solution interface is important. Brasseur et al. (58) developed a procedure for conformation analysis to define the position of chlorophyll in BLM. They found that the porphyrin ring is orientated at an angle of 45 5° to the plane of the BLM, which is in excellent agreement with the value reported previously (44). 

Adsorption and desorption phenomena often actually take place at the electrode surface (Fig. 2). Generally, adsorption from solution is characterized by competition between solute and solvent molecules. The electrode-solution interface can thus be assumed to conform to the illustration in Fig. 3, where electroactive species are adsorbed through replacement of preadsorbed solvent molecules. 

This distinction can be checked experimentally if the measurements are performed for different surface-inactive electrolytes but for the same electrode and solvent. According to the general theory [42-44] discussed in Sect. 2.1.7, the distance of the closest approach, which should be noticeably different for these electrolytes, has got almost no effect in the compact-layer capacitance if the ions do not penetrate into the region of the reduced dielectric response near the surface. This theoretical prediction turns out to be in conformity with experimental data [35, 37, 45, 46] for three mercury-aqueous solution interfaces for which the PZ plot at the p.z.c. gives practically identical values for the compact-layer capacitance, Ch(0) = 29gFcm-2 (Fig. 6). 

As previously discussed, NAD+ in solution is partially in a folded configuration [14-16]. It then seems clear that the folded conformation is preferentially adsorbed within the faradaic region, and differences may remain in the specific orientation, either parallel or perpendicular at the electrode/solution interface, depending on the electrode material. 

S. Ye, S. Nihonyanagi, K. Fujishima, Conformational order of octadecanethiol mono-layer at gold/solution interface internal reflection SFG study in Studies in Surface Science and Catalysis 132 (Eds. Y. Iwasawa, N. Oyama, H. Kunieda), Elsevier, Tokyo,... 

Precision in Linear Sweep and Cyclic Voltammetry, Vernon D. Parker Conformational Change and Isomerization Associated with Electrode Reactions, Dennis H. Evans and Kathleen M. O Connell Square-Wave Voltammetry, Janet Osteryoung and John J. O Dea Infrared Vibrational Spectroscopy of the Electron-Solution Interface, John K. Foley, Carol Korzeniewski, John L. Dashbach, and Stanley Pons... 

Chain structure departs from tgt and changes to ttt when PEO is blended with poly(methyl methacrylate) (88). The structure of adsorbed PEO chains in the air-solution interface was found to be very different from the disordered structure expected for the solution state (89-91). In fact, the vibrational bands for these adsorbed chains have characteristics similar to those in the crystalline state (90). Various theoretical studies have led to quite different results for chain conformations of PEO (62,92-98). Because of differences in the local dielectric environment, different sets of rotational isomeric states need be considered. 

The changes in polymer conformation in constrained geometries is of fundamental importance and can be studied most conveniently at the air-solution interface. 

Therefore, an understanding of the conformational behaviour of proteins at solid-solution interfaces is desirable for a variety of reasons. For example, a detailed mapping of conformational changes is necessary for understanding the mechanism of protein adsorption and can help identify optimal conditions to preserve functionality following protein immobilization. Even though major scientific contributions have been made to our understanding of proteins on solid surfaces in recent years... 

The properties of polymers adsorbed at the solid-solution interface are relevant to a wide range of technical problems. For example, the addition of pol3mers to colloidal suspensions can significantly modify their stability. The formation of interparticle bridges by the adsorbed pol3mier can result in flocculation important in water purification, while the addition and adsorption of large amounts of polymer can stabilize a suspension and provide lubrication capacity. The application of polymer films for microencapsulation and polymer adhesives similarly depends on both the amount and conformation of adsorbed polymer. The importance of the conformation of the adsorbed polymer in these effects has been reviewed by a number of investigators. 

The mechanism of action of the hydrophilic PEG chains can be explained in terms of steric interaction that is well known in the theory of steric stabilization. Before considering the steric interaction one must know the polymer configuration at the par-ticle/solution interface. The hydrophilic PEG chains can adopt a random coil (mushroom) or an extended (brush) configuration. This depends on the graft density of the PEG chains as will be discussed below. The conformation of the PEG chains on the nanoparticle surface determines the magnitude of steric interaction. This configuration determines the interaction of the plasma proteins with the nanoparticles. 

Fig. 13.11. A schematic representation of the complex situation near a crystal-solution interface. Shows solute molecnles with variable conformation in the bulk solution, partial structuring of solute molecules near the interface, and solute, solvent and impurity particles adsorbed on the solid surface. The structure of the solid at the surface may be different from the bulk solid structure.

---