PubMed · 11636967
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U Pohl. 1998. [Not Available].. https://pubmed.ncbi.nlm.nih.gov/11636967/
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The formation and disappearance of liquid bridges between two surfaces can occur either through equilibrium or nonequilibrium processes. In the first instance, the bridge molecules are in thermodynamic equilibrium with the surrounding vapor medium. In the second, chemical potential gradients result in material transfer; mechanical instabilities, because of van der Waals force jumps on approach or a Rayleigh instability on rapid separation, may trigger irreversible film coalescence or bridge snapping. We have studied the growth and disappearance mechanisms of laterally microscopic liquid bridges of three hydrocarbon liquids in slit-like pores. At rapid slit-opening rates, the bridges rupture by means of a mechanical instability described by the Young-Laplace equation. Noncontinuum but apparently reversible behavior is observed when a bridge is held at nanoscopic surface separations H close to the thermodynamic equilibrium Kelvin length, 2r(K)costheta, where r(K) is the Kelvin radius and theta is the contact angle. During the course of slow evaporation (at H > 2r(K)costheta) and subsequent regrowth by capillary condensation (at H < 2r(K)costheta), the refractive index of the bridge may vary continuously and reversibly between that of the bulk liquid and vapor. The evaporation process becomes irreversible only at the very final stage of evaporation, when the refractive index of the fluid attains virtually that of the vapor. Measured refractive index profiles and the time-dependence of evaporating neck diameters also seem to differ from predictions based on a continuum picture of bridge evaporation far from the critical point. We discuss these findings in terms of the probable density profiles in evolving liquid bridges.
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Partitioning of 153 di- to hexapeptides in an aqueous dextran-PEG two-phase system containing 0.15 m NaCl in 0.01 m sodium phosphate buffer, pH 7.4 was examined. The relative hydrophobicity of the peptides was estimated and expressed in equivalent numbers of methylene units. Analysis of the data shows that the additivity principle applies for the relative hydrophobicity of up to hexapeptides. The relative hydrophobicities of Trp, Glu, and Asp residues in heterooligopeptides are noticeably different from those in corresponding homooligopeptides. The relative hydrophobicity of peptides can be calculated and used as a structural descriptor in quantitative structure-activity relationship analysis. The peptide bitterness threshold is shown to be quantitatively related to the peptide structure described as a combination of the relative hydrophobicity and lipophilicity (logD) of peptides.