Concentrated salt solution, proton

The Langmuir-Blodged (LB) technique allows one to form a monolayer at the water surface and to transfer it to the surface of supports. Formation of the BR monolayer at the air/water interface, however, is not a trivial task, for it exists in the form of membrane fragments. These fragments are rather hydrophilic and can easily penetrate the subphase volume. In order to decrease the solubility, the subphase usually contains a concentrated salt solution. The efficiency of the film deposition by this approach (Sukhorukov et al. 1992) was already shown. Nevertheless, it does not allow one to orient the membrane fragments. Because the hydrophilic properties of the membrane sides are practically the same, fragments are randomly oriented in opposite ways at the air/water interface. Such a film cannot be useful for this work, because the proton pumping in the transferred film will be automatically compensated i.e., the net proton flux from one side of the film to the other side is balanced by a statistically equal flux in the opposite direction. 

The most trivial explanation for the effect of electrolytes on rate of proton dissociation is to consider the effect of salts on the dielectric constant of the solution (see also Equation 1). In concentrated salt solutions, a considerable fraction of the water molecules are oriented in an hydration shell around the ions thus, their dielectric constant is smaller than in pure water (Hasted et al., 1948). A decreased dielectric constant will accelerate ion-pair recombination and slow down ion-pair separation. 

In concentrated salt solutions, the vapor pressure is lower than that of pure water, and hence it exhibits reduced water activity. This phenomenon is explained by the fact that a considerable fraction of the water molecules are associated with the hydration of the salt ions. The binding energy of these water molecules (which forms the first and the second hydration shells) to the center ion is larger than 10 kcal/mol therefore, they are less likely to participate in the hydration of the newly formed proton. To observe successful proton dissociation, the thermodynamic stable complex must be formed within the ion-pair lifetime. The depletion of the solution from water molecules available for this reaction will lower the probability of the successful dissociation. As demonstrated in Figure 9, this function decreases with the activity of the water in the solution. 

Since the tryptophan signals are relatively well resolved, the spin-lattice relaxation times of the various protons of this amino-acid residue could be determined (Table 1). Rather unexpectedly it was found that in 0.15 M NaCl or phosphate at neutral pH the values of all signals and particularly of that presumably arising from the proton in position 2, were markedly lower than the values obtained in pure D2O. In 0.15 M sulphate, or in concentrated salt solutions, or at high pH values increased considerably. 

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of organic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution ate embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible appHcation of PEI. 

The data clearly indicate that the surface pH of the bile salt micelle is higher than the surface pH of a lauryl taurate micelle for a given bulk pH—i.e., the difference between bulk and surface pH is less with the bile salt micelle. The bile salt micelle should have a lower charge density and therefore a lower concentration of protons at the surface of the micelle. Therefore, the observed bulk pH at which micellar fatty acid ionizes is closer to the bulk pKa of molecularly dispersed fatty acid (4.9) in bile salt solution than in lauryl taurate solution. 

In this account, we will focus on the transient analysis of these systems, which has strongly contributed to a deeper understanding of the diverse reaction modes (Patemo-Buchi, proton abstraction, cycloaddition). In general, aromatic ketones were selected as electron acceptors for reasons of suitable excitation and long wavelength absorption of the radical anion intermediates. Among them, fluorenone 3 is particularly well suited since the concentration, solvent, temperature, and cation radius dependence of the absorption spectra of pairs formed with metal cations are already known [29]. Hogen-Esch and Smid [30, 10] pointed out that a differentiation between CIP and SSIP is possible for fluorenone systems. On the other hand, FRI s and SSIP s cannot be differentiated simply by their UV/Vis absorption spectra, whereas for instance conductance measurements may be successful. However, the portion of free radical ions in fluorenyl salt solutions was shown to be less important [9, 31]... 

Anhydro bases resulting from the proton abstraction by a base at an activated a -methyl group of a quaternary salt (see Section 4.19.2.3.3(iv)(a)) are active C-nucleophiles. These attack the C-2 position of a thiazolium salt affording adducts whose further reaction may lead to thiacyanines. Scheme 28 summarizes the successive steps in the reaction resulting from the addition of sodium ethoxide to a fairly concentrated ethanolic solution of 2,3-dimethylbenzothiazolylium salt (45) (c =0.1 moll-1). The initially formed anhydro base (46) cannot be isolated, it reacts as a nucleophile with a second molecule of benzothiazoly-lium salt yielding an adduct (47) which is deprotonated by ethoxide anion affording the dimeric anhydro base (48) whose reactivity will be discussed later (see Section 4.19.2.3.3.i). Monocyclic thiazolylium salts react similarly. 

The chaotropic effect is dependent on the concentration of the free counteranion and not the concentration of the protons in solution at pH < basic analyte Ka. This suggests that change in retention of the protonated basic analyte may be observed with the increase in concentration of the counteranion by the addition of a salt at a constant pH as shown in Figure 4-47 for a pharmaceutical compound containing an aromatic amine with a pKa of 5. 

The correlation between the availability of water and the rate constant of proton dissociation was measured in two systems. In one system, the ratio water methanol of a mixed solution modulated the availability of water [38]. In the other system, made of concentrated electrolyte solutions, the activity of the water was modulated by the salt [39]. The dependence of the measured rate of dissociation [60, 67, 68], either from photoacid or ground state acids, on the activity of the solvent yielded a straight log-log correlation function with respect to the activity of the water... 

As noted before, solubility of covalent organic compounds in molten salts seems to require proton interaction between acidic groups of the solute and anions of the solvent. For un-ionized solutes this may well involve structures analogous to the solvent-bridged ion-pair complexes postulated for concentrated aqueous solutions ... 

We attribute the effect of the amine at C-6 as deriving from the extent of dissociation of the ammonium salt at C-6, and it is an equilibrium effect which depends on the relative concentrations of the protonated phenolate group and the dissociated salt form. Strongly basic amines remove the proton completely and form the ammonium salt. Weakly basic amines do not cause complete ionization of the C-6 phenol. This same effect has been observed for monomeric models in solution. (We can ignore free amine quenching because the concentration of the amine is very low.)... 

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