Monolayer ionized

While the v-a plots for ionized monolayers often show no distinguishing features, it is entirely possible for such to be present and, in fact, for actual phase transitions to be observed. This was the case for films of poly(4-vinylpyri-dinium) bromide at the air-aqueous electrolyte interface [118]. In addition, electrostatic interactions play a large role in the stabilization of solid-supported lipid monolayers [119] as well as in the interactions between bilayers [120]. 

Specific Cation Effect. Adam and Miller (2) noted a specific expansion in the tt-A isotherms of behenic acid on 2N solutions of NaOH and KOH. They attributed the expansion of the ionized monolayer to cou-... 

Sears and Schulman (51) measured surface pressures and potentials vs. molecular area (20 to 110 sq. A. per molecule) for the alkali metal stearates over 0.5N solutions of LiOH, NaOH, and KOH. Like Adam and Miller, they detected a specific cation effect on the 7r-A and AV-A isotherms of the ionized monolayer at high pH the ir-A curves were expanded in the order of the crystalline sizes of the alkali metal cations K > Na > Li. The sequence is the reverse observed for the long-chain... 

The kinetics and the equilibrium of adsorption of anions PoCl62" by ionized monolayers of cationic soaps have been studied and both phenomena are analogous to those observed with the polymer ion exchangers... 

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]

The electrical potential, < 0, of an ionized monolayer is given as a function of the concentration and valency of the counterions as follows (I),... 

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.

The presence of an electrical charge on the molecules forming the film greatly augments the film pressure. The third curve in Fig. 4 shows that the pressure exerted by the film is very high compared with that of the uii-ionized monolayers, due to the repulsion between the neighboring long-chain ions held in the interface. 

In this subsection we consider the notions of charged monolayer, ionized monolayer and dissociated monolayer as synonymous, although the term ionized is preferred over charged because monolayers are not externally charged. 

E.D. Goddard, Ionizing Monolayers and pH Effects, Adu. Colloid Interface Sci. 4 (1974) 45-78. (Older review but not dated because it contains much basic information on surface pressures and Volta potentials for ionized monolayers.)... 

Monolayers are more expanded at the O/W interface than at the A/W interface this has been generally confirmed since the early work of Davies (19). The condensing effect on fully ionized monolayers from increasing the electrolyte concentration (19) is shown in Figures 3-5. The equimolar Ci8 suIfate-Ci8 TAB monolayers are more condensed than the single-component charged monolayers shown by Phillips and Rideal (17, 18) and Brooks and Pethica (14) although in the present... 

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. 

Tn 1922 Adam (I) published the third paper in his extraordinary series on surface film structure. He observed that fatty acid monolayers greatly expanded on alkaline subphases. He also suggested that fatty acid anions desorbed or dissolved from the monolayer into the alkaline subphase. In 1933 he and Miller (2) showed that the composition of the subphase buffer significantly affected the monolayer thus palmitic and stearic acid monolayers were more condensed on 2N sodium hydroxide than on 2N potassium hydroxide. The expansion, desorption, and cation selectivity of ionizing monolayers are the subjects of this investigation. 

The initial observation of Sears and Schulman (3) that Na+ > K+ selectivity on strongly alkaline subphases reversed on a weakly alkaline subphase was confirmed in several studies. Desorption experiments (Figure 11) showed that palmitic acid monolayers with essentially complete ionization—strong fields—were condensed more by Na+ than by K+. At lower pH, the partially ionized monolayers—weaker fields—were condensed the same amount by Na+ and K+. 

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... 

The theory of the AV-A behavior developed by L.M.B. (2) was based on earlier calculations by Mingins and Pethica (M.P.) (9) from their experimental work on monolayers of SODS at the A—W interface. Recently these authors (10) reported a numerical error in their earlier work their conclusions question the model of the ionized monolayers used by L.M.B. (2) to explain the A V-A curves. The so-called Esin-Markov coefficient for adsorbed ions at the charged mercury/aqueous electrolyte has received considerable attention (11, 12, 13) particularly since it clearly demonstrates the discrete-ion effect. Its counterpart at ionized monolayers may be defined by the differential expression... 

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

Currently no adequate quantitative theory of the discrete-ion potentials for adsorbed counterions at ionized monolayers exists although work on this problem is in progress. These potentials are more difficult to determine than those for the mercury/electrolyte interface because the non-aqueous phase is a dielectric medium and the distribution of counterions in the monolayer region is more complicated. However the physical nature of discrete-ion potentials for the adsorbed counterions can be described qualitatively. This paper investigates the experimental evidence for the discrete-ion effect at ionized monolayers by testing our model on the results of Mingins and Pethica (9, 10) for SODS. The simultaneous use of the Esin-Markov coefficient (Equation 3) and the surface potential AV as functions of A at the same electrolyte concentration c yields the specific adsorption potentials for both types of adsorbed Na+ ions—bound and mobile. Two parameters which need to be chosen are the density of sites available to the adsorbed mobile Na+ ions and the capacity per unit area of the monolayer region. The present work illustrates the value... 

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