Counterion molecular model

The Gibbs equation allows the amount of surfactant adsorbed at the interface to be calculated from the interfacial tension values measured with different concentrations of surfactant, but at constant counterion concentration. The amount adsorbed can be converted to the area of a surfactant molecule. The co-areas at the air-water interface are in the range of 4.4-5.9 nm2/molecule [56,57]. A comparison of these values with those from molecular models indicates that all four surfactants are oriented normally to the interface with the carbon chain outstretched and closely packed. The co-areas at the oil-water interface are greater (heptane-water, 4.9-6.6 nm2/molecule benzene-water, 5.9-7.5 nm2/molecule). This relatively small increase of about 10% for the heptane-water and about 30% for the benzene-water interface means that the orientation at the oil-water interface is the same as at the air-water interface, but the a-sulfo fatty acid ester films are more expanded [56]. 

Enzymatic enantioselectivity in organic solvents can be markedly enhanced by temporarily enlarging the substrate via salt formation (Ke, 1999). In addition to its size, the stereochemistry of the counterion can greatly affect the enantioselectivity enhancement (Shin, 2000). In the Pseudomonas cepacia lipase-catalyzed propanolysis of phenylalanine methyl ester (Phe-OMe) in anhydrous acetonitrile, the E value of 5.8 doubled when the Phe-OMe/(S)-mandelate salt was used as a substrate instead of the free ester, and rose sevenfold with (K)-maridelic acid as a Briansted-Lewis acid. Similar effects were observed with other bulky, but not with petite, counterions. The greatest enhancement was afforded by 10-camphorsulfonic acid the E value increased to 18 2 for a salt with its K-enanliomer and jumped to 53 4 for the S. These effects, also observed in other solvents, were explained by means of structure-based molecular modeling of the lipase-bound transition states of the substrate enantiomers and their diastereomeric salts. 

In order to describe what happens in the molecular organization at smaller areas and understand the process undergone by water molecules and counterions under these conditions, it was also developed molecular modeling of a P4VPCi4 mono-layer with full quaternization. Two different surface areas were taken into account. One would correspond to the limiting area A0 (34 A2) and another to a larger area (40 A2) than the first one. When the compression is beginning to reach 40 A2, the lateral chains could be considered almost vertical at the air-water interface. (See Fig. 3.15). The situation shown in Fig. 3.15 seems to favor hydrophobic interaction between them. 

An approach with consideration of the two states (i.e, free with concentration c, and bound with concentration a,) of the B and A counterions is consistent with the loose quasi-crystal molecular model [41] that states that mass transfer in the exchanger may not occur by transfer of the complete counterions mass, as suggested by the classical quasi-homogeneous exchanger model [1-5,7-12,16,17], but solely by the free counterions that are not bound to the fixed exchanger groups. 

Physical interpretation of the X species employed in ion-exchange half reactions is governed by the molecular model used to describe the ion-exchange process. If the ion exchanger is viewed as a porous gel, for example, in which counterions are distributed within the charged solid lattice, the ion concentrations in the exchanger phase may be related to the corresponding bulk solution concentrations by (32)... 

Molecular Model of Counterion Dissociation Equilibrium. The following molecular concept is supported, or suggested, by both these spectroscopic observations and past ultrasonic investigations of simple aqueous electrolytes. In particular, a four-state model reminiscent of the multistep ionic dissociation mechanism of Eigen et al., (22, 23) was adopted (24). With regard to Figure 3, tFie states are classified as 1) completely dissociated hydrated ion pairs, 2) ion pairs at the contact of undisturbed primary hydration shells, and 3) outer and 4) inner sphere complexes. The relative populations of these states, (P ... 

In order to better understand the origin of the stereoselectivity observed with Mel, we performed molecular modeling on several key stmctures (Figure 6). Calculations were performed at both the M06-2X and B3LYP levels using the 6-31G(d) basis set. In order to simplify the calculations, only sodium and potassium were used as the counterion for the malonate and Mel was replaced with MeCl during the transition state analysis. 

Initiator efficiency in terms of conversions and molecular weights were similar for model compounds and polymerizations. The influence of chlorine and bromine-containing counterions on polymerization was similar to that found in model study. 

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