Cationic monolayers

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

The DNA-aligned thin film could also be obtained with the LB method, in which anionic DNA strands were transferred with cationic monolayers at the interface using vertical dipping method. DNA was aligned along the dipping directions. 

Alkylammonium salts are typical cationic surfactants. The adsorption and the properties of the monolayers formed by these salts at water-air and water-oil interfaces have frequently been investigated [11, 24, 36, 37, 46, 63]. Mostly the adsorption of alkylammonium cations from the aqueous phase was studied. Special attention was paid to the relation between the structure of alkylammonium cations, their surface-active properties and the monolayer structure. Less attention was given to the influence of the anion nature of alkylammonium salt adsorption from the non-aqueous phase and competitive adsorption of counterions in cationic monolayers at the water-oil interface, although these problems are vital for understanding the mechanism of liquid ion-exchange extraction. 

The most detailed study of the interaction between inorganic ions and insoluble cationic monolayers was conducted by Goddard et al. [69-72]. A specific interaction was observed between anions and the monolayer of alkylammonium cations. Its value depends on the nature of the anion with the exception of the ion of fluorine. Only in the case when F is taken as a counterion, the dependence of the surface pressure on the area per molecule in the monolayer obeys the Davis equation of state for the charged monolayers. As the counterion radius decreases, the surface pressure on a given area... 

Figure 12 The interphase of a soluble cationic surfactant at the air-water interface at low (a) and high (b) bulk concentration. It consists of a charged topmost cationic monolayer, a diffuse layer of counterions and at higher concentrations a compact layer of directly adsorbed counterions. The charge density of the topmost monolayer reduced by the charge of the inner Stern layer determines the ion distribution within the diffuse layer. The prevailing ion distribution is given by solution of the nonlinear Poisson-Boltzmann equation. The excess of ions can be readily translated in a corresponding refractive index profile. The profile determines the reflectivity properties. Ellipsometric data modeled within this framework allow an estimation of the extent to which ions enter the compact layer.

Fig. 18. (a) Electrical make-up of the membrane (sectional view, superstructural protein elements not shown in protein units U. L lipid chains P negative groups of phospholipids M, H20 water-cation monolayer divalent cation M+ monovalent cation A" anion G Gouy layer, (b) Equivalent parallel-plate condenser. 

In the present concept lipids participate in at least three additional functions. The first is the establishment of a proton-conductive, water-cation monolayer discussed in Section III, C, I. The second, closely related to the first, is the drawing of cations to the water-cation monolayer the influence of fixed positive-charges on cation movements has been discussed in Section IV, B, 2. Last, but not least, phospholipids provide the necessary ligands for divalent cation bridges between membrane subunits. This role is important, not only to membrane cohesion, but to all forms of transport with the possible exception of simple diffusion (Section IV, B, 2). 

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