Surface ionic groups

Althongh van der Waals forces are present in every system, they dominate the disjoining pressnre in only a few simple cases, such as interactions of nonpolar and inert atoms and molecnles. It is common for surfaces to be charged, particularly when exposed to water or a liquid with a high dielectric constant, due to the dissociation of surface ionic groups or adsorption of ions from solution, hi these cases, repulsive double-layer forces originating from electrostatic and entropic interactions may dominate the disjoining pressure. These forces decay exponentially [5,6] ... 

A stationary phase capable of ion exchange, on the other hand, has electric charges on its surface. Ionic groups such as SOs , COO NH3 or NR3 are incorporated in the resin or gel. Charges are neutralized by mobile counter ions. The mobile phase contains ions, and ionic sample molecules compete with these for a place on the surface of the stationary phase. 

As part of a study aimed at the rational development of immunoassays, the influence of surface characteristics on the interaction of F(ab )2 fragments obtained by peptic digest of rabbit polyclonal IgG has been investigated [12]. The latex particles differed in their hydrophilic character and in the nature and density of their surface ionic groups. The most important force governing the adsorption of the protein onto the surface was shown to be hydrophobic interactions, although in low ionic strength buffers electrostatic interactions were also found to play a part. On anionic latices, maximum F values were obtained at pH... 

In another study of the same type of latex, Ottewill and Vincent have shown that even with a non-ionic adsorbate there may be interactions with the surface ionic groups. They studied ethanol, n-propanol and n-butanol and concluded that the initial adsorption, at low concentrations of alkanol, probably involved ion-dipole association of hydroxyl with surface carboxylate anions. This would result in the hydrophobic tails of the alkanols being oriented towards the aqueous phase and, more importantly, a desorption of counterions from the double layer, thus affecting the surface electrical potential, IPq [33]. 

A picture of the electrical lines of force is given in Fig. XV-12 [114] in the plane CD of the ionic groups, it will be a periodic field, whereas a little further into the solution the effect will be more that of a uniformly charged surface. The Donnan treatment is probably best justified if it is supposed that ions from solution penetrate into the region of CD itself and might in fact, lie between CD and AB. 

In water, the hydroxyls react with protons and OH" groups, giving the surface ionic character. The following equilibrium reactions occur ... 

A second surface modification has been reported by Yamamoto et al. These workers added stearic acid to their carbon paste mixture. This produced an electrode which was relatively insensitive to ascorbic acid and DOPAC relative to dopamine. It is theorized that this electrode works because of electrostatic repulsion of the anionic ascorbate and DOPAC by surface stearate groups. Ionic repulsion has also been employed by covering the surface of the working electrode with an anionic polymer membrane. Gerhardt et al. used Nafion, a hydrophobic sulfonated perfluoro-polymer, to make a dopamine selective electrode. This electrode exhibited selectivity coefficients as large as 250 1 for dopamine and norepinephrine over ascorbic acid, uric acid, and DOPAC. 

As already mentioned, a network can be obtained by linking polymer chains together, and this linkage may be either physical or chemical. Physical linking can be obtained by (i) absorption of chains onto the surface of finely divided particulate fillers, (ii) formation of small crystallites, (iii) coalescence of ionic groups, or (iv) coalescence of glassy sequences in block copolymers. 

Structure controlled dendritic polymers that have been studied using gel electrophoresis generally behave as mimics of either proteins or nucleic acids, and possess similar ionic groups such as -NH3 , -COO , or P04 functionality. Dendrimer structures may be widely modified as a function of their interior composition and as well as the nature of their surface groups. Depending on their structure, the influence of pH may vary dramatically for different dendrimers. 

Hydroxide and carbonate typically form insoluble precipitates with polyvalent cations in natural waters. The activity of both of these species increases with pH. The presence of surface functional groups that are capable of exchanging a proton creates pH dependent-charge, whereby the ionic character of the surface increases with pH [158,284,285]. 

While the nature of the material is important, the surface of the material is also often critical. The human body wants to wrap around or connect to bodies within its domain. In some cases, the desired situation is little or no buildup on the polymer. Here, surface slickness is needed. Siloxanes and fluorinated materials such as PTFE are generally slick materials, but other materials can be made slick through surface treatments that present to the body few voids and irregularities at the atomic level. In other cases, body buildup, and surfaces and materials that assist this growth are preferred. Surface hydrophobicity or hydrophilicity, presence or absence of ionic groups, and chemical and physical (solid or gel) surfaces are all important considerations as one designs a material for a specific application. 

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