Surface charge modulation

Fig. 8.13 (a) Potential dependence of the phenomenological rate constant k,r derived for mechanism I. Note that for a two step mechanism, this rate constant contains terms associated with surface recombination, so that it is not the true rate constant for electron transfer. The influence of the modulation of potential due to surface charging is shown, (b) Potential dependence of the phenomenological rate constant k,t for case /. The influence of the dynamic modulation of surface potential by accumulated reaction intermediates is... 

In Sect. 7.3, Eqs. (18) and (19) describe the Maxwell stress forces acting on a conductive tip when a combined d.c./a.c. voltage is applied. For the PFM set-up we have to complete the total interaction force by the additional effects of piezoelectricity, electrostriction and the spontaneous polarisation. Both electromechanical effects cause an electric field-induced thickness variation and modulate the tip position. The spontaneous polarisation causes surface charges and changes the Maxwell stress force. If the voltage U(t)=U[)c+UAc sin((Ot) is applied, the resulting total force Ftotai(z) consists of three components (see also Eq. 19) Fstatic, F(0 and F2m. Fstatic is the static cantilever deflection which is kept constant by the feedback loop. F2a contains additional information on electrostriction and Maxwell stress and will not be considered in detail here (for details see, e.g. [476]). The relevant component for PFM is F(0 [476, 477] ... 

Gan, Q., Wang,T., Cochrane, C., and McCarron, P. (2005), Modulation of surface charge, particle size and morphological properties of chitosan-TPP nanoparticles intended for gene delivery, Colloids Surf. B. Biointerfaces, 44(2-3), 65-73. 

Changes in pH modulate the lipid phase behavior as a consequence of protonation/deprotonation of the lipid headgroups, which results in a change of the surface charge of the membrane (77). They also modify the surface polarity and hydration. Typically, protonation decreases lipid hydration and increases the main transition temperature (53). The effects of pH titration on the chain-melting transition temperature of dimyristoyl phospholipids is illustrated in Fig. 3e, which shows that single protonation increases the melting transition temperature by about 5-15° C. 

The use of micellar surface charge to modulate photochemical reactivity is also illustrated by oxidative quenching of hydrophobic 9-methylanthracene fluorescence by the dibromide anion. The cationic CTAB micelle is about 10 times more efficient than the anionic SDS micelle in promoting quenching, an effect that was attributed to electrostatic adsorption of Br2 on the surface of the CTAB micelle [62]. 

The basic idea in Osseo-Asare s model is that when protons, hydroxyl ions, ligands, or water molecules bind to the surface sites, they cause a polarization of the underlying silicon-oxygen bonds. With a sufficient number of bound species, the Si-O-Si bonds become sufficiently weakened to permit detachment of the surface silicon ion. In the absence of ligands, the dissolution rate is determined by the surface concentration of protonated species (MOH 2> at pH < pzc, of water at pH pzc, and of deprotonated species (MO ) at pH > pzc. The surface charge is modulated by the protons in the solution immediately adjacent to the solid surface according to the reactions... 

Lipids are not covalently bound in membranes but rather interact dynamically to form transient arrangements with asymmetry both perpendicular and parallel to the plane of the lipid bilayer. The fluidity, supermolecular-phase propensity, lateral pressure and surface charge of the bilayer matrix is largely determined by the collective properties of the complex mixture of individual lipid species, some of which are shown in Fig. 8.1. Lipids also interact with and bind to proteins in stiochiometric amounts affecting protein structure and function. The broad range of lipid properties coupled with the dynamic organization of lipids in membranes multiplies their functional diversity in modulating the environment and therefore the function of membrane proteins. 

Yamamoto Y, Nagasaki Y, Kato Y, Sugiyama Y, Kataoka K. Long-circulating polyfethylene glycol)-poly(D,L-lactide) block copolymer micelles with modulated surface charge. J Contr Rel 2001 77 27-38. 

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