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Related Concept Videos

States of Water01:23

States of Water

Water exists in any one of the three classical states: solid (ice), liquid (water), and gas (steam or water vapor). The state of water depends on i) the intermolecular forces that draw molecules together and ii) the kinetic energy that leads to movements that pull them apart.
Water freezes when the intermolecular forces are greater than the kinetic energy. Unlike most other substances, water is less dense in its solid state than in its liquid state. This is because each water molecule can form...
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Irrotational flow is characterized by fluid motion where particles do not rotate around their axes, resulting in zero vorticity. For a flow to be irrotational, the curl of the velocity field must be zero. This imposes specific conditions on velocity gradients. For instance, to maintain zero rotation about the z-axis, the gradient condition:
Solubility03:00

Solubility

Solution, Solubility, and Solubility Equilibrium
A solution is a homogeneous mixture composed of a solvent, the major component, and a solute, the minor component. The physical state of a solution—solid, liquid, or gas—is typically the same as that of the solvent. Solute concentrations are often described with qualitative terms such as dilute (of relatively low concentration) and concentrated (of relatively high concentration).
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Surface Tension, Capillary Action, and Viscosity02:57

Surface Tension, Capillary Action, and Viscosity

Surface Tension
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded by other molecules and are attracted equally in all directions by the cohesive forces within the liquid. However, the molecules on the surface of a liquid are attracted only by about one-half as many molecules. Because of the unbalanced molecular attractions on the surface molecules, liquids contract to form a shape that minimizes the number...
Colloids03:22

Colloids

Children at play often make suspensions such as mixtures of mud and water, flour and water, or a suspension of solid pigments in water known as tempera paint. These suspensions are heterogeneous mixtures composed of relatively large particles that are visible to the naked eye or can be seen with a magnifying glass. They are cloudy, and the suspended particles settle out after mixing. On the other hand, a solution is a homogeneous mixture in which no settling occurs and in which the dissolved...
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Entropy and Solvation

The process of surrounding a solute with solvent is called solvation. It involves evenly distributing the solute within the solvent. The rule of thumb for determining a solvent for a given compound is that like dissolves like. A good solvent has molecular characteristics similar to those of the compound to be dissolved. For example, polar solutions dissolve polar solutes, and apolar solvents dissolve apolar solutes. A polar solvent is a solvent that has a high dielectric constant (ϵ ≥ 15); an...

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Probing the Structure and Dynamics of Interfacial Water with Scanning Tunneling Microscopy and Spectroscopy
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Probing the Structure and Dynamics of Interfacial Water with Scanning Tunneling Microscopy and Spectroscopy

Published on: May 27, 2018

The unstirrable water layer is unstirrable because it does not exist.

Robert A Scherrer1

  • 1BIOpKa, 3416 Ebba Street, White Bear Lake, Minnesota 55110, USA. rascherrer@biopka.com

Chemistry & Biodiversity
|November 26, 2009
PubMed
Summary

The unstirred water layer (UWL) model for membrane permeation is challenged. A new hypothesis explains apparent pKa shifts without UWL, proposing a transition state for absorption within the membrane.

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Magnetically Induced Rotating Rayleigh-Taylor Instability
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Magnetically Induced Rotating Rayleigh-Taylor Instability

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Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars
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Magnetically Induced Rotating Rayleigh-Taylor Instability
06:42

Magnetically Induced Rotating Rayleigh-Taylor Instability

Published on: March 3, 2017

Area of Science:

  • Membrane transport phenomena
  • Physicochemical hydrodynamics
  • Drug delivery systems

Background:

  • Unstirred water layers (UWL) are often invoked in membrane permeation models, particularly in PAMPA assays.
  • Apparent shifts in pKa (pKa(flux)) are observed for lipophilic acids and bases, attributed to UWL acting as a diffusion barrier.
  • Existing models struggle to explain these pKa shifts under conditions favoring unionized species.

Purpose of the Study:

  • To propose an alternative explanation for the observed pKa(flux) phenomenon that does not rely on the unstirred water layer (UWL).
  • To introduce the concept of a transition state for absorption within the membrane to explain pKa shifts.
  • To provide a testable hypothesis for the pKa(flux) term in membrane permeation.

Main Methods:

  • Theoretical modeling of ionizable compound behavior during membrane passage.
  • Analysis of free energy changes during absorption into the membrane.
  • Conceptual framework development based on transition state theory.

Main Results:

  • The pKa(flux) can be explained by a change in the compound's pKa as it enters the membrane.
  • A transition state for absorption is proposed, occurring at maximum free energy within the membrane.
  • This transition state pKa directly corresponds to the observed pKa(flux), eliminating the need for UWL.

Conclusions:

  • The unstirred water layer (UWL) is not required to explain apparent pKa shifts in membrane permeation.
  • The pKa of an ionizable compound changes during membrane transit, with the transition state pKa defining the pKa(flux).
  • This new model offers a testable alternative for understanding absorption and permeation rate limitations.