Ionized monolayers layer

The above definition of the symmetric surface excess and the classical Guoy-Chapman model of the diffuse double layer are combined to show that the surface excess cannot be considered a surface concentration in the presence of an ionized monolayer on an impenetrable solid/liquid interface. 

It is postulated that one of the ions of the adsorbed 1 1 electrolyte is surface active and that it forms an ionized monolayer at the solid/liquid interface. All counterions are assumed located in the diffuse double layer (no specific adsorption). Similions are negatively adsorbed in the diffuse double layer. Since the surface-containing region must be electrically neutral, the total moles of electrolyte adsorbed, n2a, equals the total moles of counterions in the diffuse double layer which must be equal to the sum of the moles of similions in the diffuse double layer and the charged surface, A[Pg.158]

Matijevic, E. and Pethica, B.A. (1958) The properties of ionized monolayers. Part 1. Sodium dodecyl sulphate at the air/water interface. Part 2. The thermodynamics of the ionic double layer of sodium dodecyl sulphate. Trans. Faraday Soc., 54, 1382-99. 

In flg. 3. lb the preference of the hydrophobic tails of the (anionic) surfactant molecules for the oil phase gives rise to the double layer. Such double layers are for instance encountered in some emulsions. They may also occur at the air-water interface then the driving force for their formation is the expulsion of the hydrocarbon tails from the aqueous phase. We speeik of ionized monolayers and return to them in Volume III. 

Figure 3.1. Examples of double layers (a) around a solid particle (b) at an Ionized monolayer of anionic surfactants, adsorbed at the oil-water Interface (c) on a hexagonal clay mineral particle at low pH (only the charge on the particle is drawn) (d) double layer generated by the adsorption of anionic surfactants on a hydrophobic surface. The pictures are schematic.

Davies (16) applied both equations to a wide collection of data at fairly low A for both soluble and insoluble ionized species and achieved only limited agreement between theory and experiment for some mono-layers. Nevertheless, the Davies term is the basis of nearly every subsequent discussion on the isotherms of ionized monolayers. We discuss elsewhere (25, 26) the validity of Equations 7 and 8 for intermediate and high surface charge densities as well as other proposed equations of state (14, 27, 28, 29, 30, 31, 32). In this paper we establish whether these two equations are suitable limiting forms at high A where many of the assumptions used in their derivation should be more valid. In particular we are interested in the limit of UA at zero n for both interfaces. 

Cation selectivity data for monolayers (summarized in the introductory section) coincide with the predictions of the field strength theory. Fatty acids have high apparent pKa values, and completely ionized mono-layers (2,3, 4,5,6) select a strong field sequence (XI in Table II). Alkyl sulfates have low apparent pKa values, and completely ionized mono-... 

Tn recent years, the influence of counterions on the properties of A ionized monolayers has received much attention. Even though Davies (I) application of the Gouy-Chapman double layer theory to ionized monolayers represented a major advance in the understanding of the properties of these systems, it has been increasingly recognized that we must account for the different effects (i.e., specific counterion effects) that counterions of the same net charge may have on the charged mono-layer. Because of counterion sequence inversions which have been ob-... 

A Model of the Electric Double Layer at a Completely Ionized Monolayer with Discreteness-of-Charge Effect... 

Tphe discreteness-of-charge effect (discrete-ion effect) is a general char-acteristic of electric double layers in aqueous media (I) and therefore should manifest itself in ionized monolayers. In a number of papers (2,3,4,5), one of the authors and co-workers investigated the role of this... 

Figure 1. Model of electric double layer for a completely ionized monolayer...

The equation of state for ionized monolayers has been discussed by Hachisu [32]. This author has shown by independent derivations using three different approaches that the equation proposed by Davies [33] is applicable in the presence or absence of added electrolyte provided that the Gouy-Chapman electrical double-layer model applies. The Davies equation may be written... 

The ionized forms of polypeptides exhibit many characteristics in common therefore, we have studied them under various conditions. The most interesting observation is the transition of a polyelectrolyte brush found by changing the polyelectrolyte chain density. The brush layers have been prepared by means of the LB film deposition of an amphiphile, 2C18PLGA(48), at pH 10. Mixed monolayers of 2C18PLGA(48) and dioctadecylphos-phoric acid, DOP, were used in order to vary the 2C18PLGA(48) content in the monolayer. 

The evaluation of the apparent ionization constants (i) can indicate in partition experiments the extent to which a charged form of the drug partitions into the octanol or liposome bilayer domains, (ii) can indicate in solubility measurements, the presence of aggregates in saturated solutions and whether the aggregates are ionized or neutral and the extent to which salts of dmgs form, and (iii) can indicate in permeability measurements, whether the aqueous boundary layer adjacent to the membrane barrier, Umits the transport of drugs across artificial phospholipid membranes [parallel artificial membrane permeation assay (PAMPA)] or across monolayers of cultured cells [Caco-2, Madin-Darby canine kidney (MDCK), etc.]. 

Fig. 11 Oxygen-projected unoccupied DOS of an NiO(lOO) monolayer. Plain and dashed lines refer to the positively ionized and neutral layers, respectively (from Ref. 85).

One of the central electrostatic characteristics of a monolayer is the potential difference created across the layer as a result of assembly and ionization of surfactants. Figure 3.17 gave an illustration. Important as this parameter may be for modelling and for controlling deposition of monolayers on solids, there is no way of measuring it. 

Figure 1. Schematic (26) of the processes which result in monolayer contraction. 1, ionization 2, desorption from the surface into an unstirred subphase layer of thickness e 3a, diffusion through the unstirred layer 3b, escape from the unstirred layer.

Unstable contracting monolayers form when either the fatty acid or its anion desorbs from the surface into the subphase. The desorption rate is controlled by tt, ionic strength, and temperature. These parameters can be varied so that the unionized fatty acid desorbs slowly, if at all, while the fatty acid anion desorbs rapidly. In a previous study, we (23) suggested that the rate of contraction, R, of an unstable fatty acid mono-layer maintained at constant tt provided a means to estimate ionization in the monolayer. R was defined by Equation 1 ... 

Desorption kinetics provide additional information that contributes to our understanding of surface phenomena such as the specific effects of tris and bicarbonate buffers on monolayers. Stable condensed mono-layers were expanded on tris buffers, and ionization appeared enhanced (8). Unstable, expanded monolayers did not expand further on tris buffers, but K data (Table I) showed a consistent decrease in the apparent pKa when fatty acids were spread on tris buffers. 

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