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Yizhak Marcus

Publications and source records attributed to Yizhak Marcus.

6 recordsLinked to original sources
PubMed2006

Ionic volumes in solution.

The volumetric properties of electrolytes in solutions indicate the interactions of the constituent ions with their environment: the solvent and other ions. The interactions with the solvent alone are manifested at infinite dilution by the standard partial molar volume, V(infinity)(salt), obtained from density measurements. To study the interactions, it is necessary to split V(infinity)(salt) into the additive ionic contributions, V(infinity)(ion), using an extra-thermodynamic assumption. Values of V(infinity)(ion) for small ions depend cardinally on the electrostriction of the solvent that can be obtained from an iterative shell-by-shell calculation from a continuum model of the solvent. The solvent shrinkage per mol of ions is DeltaV(el)(ion)<0. Also, the molar electrostriction of the solvent S, DeltaV(el)(S)<0, is calculable. The ratio DeltaV(el)(ion)/DeltaV(el)(S)=n(infinity) is the solvation number of the ion in S at infinite dilution. The calculated V(infinity)(ion)(calc) are compared with the experimental values, showing good agreement for many univalent ions in both single solvents and in some binary solvent mixtures, where no appreciable preferential solvation takes place. Ion pairing sets in under certain circumstances of ionic charge and solvent permittivity. The difference DeltaV(ip)=V(ip)(infinity)-[V(infinity)(+)+V(infinity)(-)]>0 is obtained experimentally from the pressure derivative of the association constant. The ratio Deltan(ip)=DeltaV(ip)/DeltaV(el)(S) represents the number of solvent molecules released to the bulk on ion pairing by the diminution of the electrostriction.

Electricity↗
PubMed2006

Ion pairing.

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Journal Article↗
PubMed2005

Electrostriction, ion solvation, and solvent release on ion pairing.

The theoretical mean molar electrostriction volume of electrolytic solvents, DeltaVel(solvent), was calculated from their properties: the relative pressure derivatives of the density (the compressibility) and permittivity and their second pressure derivatives. The molar electrostriction caused by ions at infinite dilution was taken as the differences of their standard partial molar volumes in the solution and their intrinsic volumes: DeltaVel(ion) = Vinfinity(ion) - Vin(ion). The ratio ninfinity = DeltaVel(ion)/DeltaVel(solvent) then represents the solvation number of the ion in the solvent at infinite dilution. Similarly, from the molar volume change on ion pair formation, DeltaVip, the ratio Deltanip = DeltaVip/DeltaVel(solvent) represents the number of solvent molecules released thereby. These values were tabulated for those solvents, ions, and ion pairs for which the relevant information could be found, the extension to nonaqueous solvents not having been attempted previously.

Journal Article↗
PubMed2003

The connection between in vitro water uptake and in vivo skin moisturization.

BACKGROUND/PURPOSE: Humectancy or hygroscopy is the water absorption tendency of a substance from the surroundings. Our interest, from the clinical point of view, consists of correlating this tendency in vitro and its effect in vivo for the development of drugs and formulations for the treatment of dry skin syndrome or diseases accompanied by dry skin. METHOD: In vitro, water absorption was measured using the comparative isopiestic method. This method is based on bringing the vapor of the water to isothermal equilibrium between a reference system and the material to be studied. The in vivo model on guinea-pigs for the dry skin syndrome tested the therapeutic ability of mono-, di- and tri-glycerols to provide moisture to dry skin leading to healing. The moisture content in the stratum corneum was measured with a Corneometer CM 825 PC that measures merely the presence of high dielectric material (humectant or water), whereas the Mexameter MX 16 measures a pathological parameter - the erythema. RESULTS: Adding hydroxyl groups to a consecutive set of polyhydroxyalkanes increases the humectancy of the polyols in vitro. This elevation was found to be linear at low relative humidities (Relative humidity=31.9% and 37 degrees C). In vivo, moisture was returned to normal within a week in all three groups. However, only glycerol managed to abolish the erythema within 7 days. CONCLUSION: A rise in water absorption ability in vitro, at a rate of about 0.25 mol water per hydroxyl group was revealed in a consecutive set of glycerols (mono-, di- and tri-glycerols). One would expect that the better humectant is a material, i.e. in which the higher its physical ability to hold water in vitro the more effective it will be in recovering skin dryness. We have found, however, that glycerol, which has the lowest humectant activity in vitro, from the set of glycerols, di- and tri-glycerol, has been proven to be the best for eliminating the signs of skin dryness. Accordingly, we propose to distinguish between the in vitro humectancy (i.e. the water uptake of a material), and its in vivo moisturizing effect, i.e. its ability to cure skin dryness and erythema. This finding supports our conclusion that the connection between in vitro humectancy and in vivo moisturization is not a simple correlation.

Animals↗