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Aqueous Solutions and Heats of Hydration02:42

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Water and other polar molecules are attracted to ions. The electrostatic attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. These attractions play an important role in the dissolution of ionic compounds in water.
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Hydronium and hydroxide ions are present both in pure water and in all aqueous solutions, and their concentrations are inversely proportional as determined by the ion product of water (Kw). The concentrations of these ions in a solution are often critical determinants of the solution’s properties and the chemical behaviors of its other solutes. Two different solutions can differ in their hydronium or hydroxide ion concentrations by a million, billion, or even trillion times. A common means of...
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Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions (attractive or repulsive) between charged species (permanent, partial, or temporary charges) and exist with varying strengths between ions, polar, nonpolar, and neutral molecules. The different types of intermolecular forces are ion–dipole, dipole–dipole, hydrogen bonds, and dispersion; among these, dipole–dipole, hydrogen...
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The reaction between a Brønsted-Lowry acid and water is called acid ionization. For example, when hydrogen fluoride dissolves in water and ionizes, protons are transferred from hydrogen fluoride molecules to water molecules, yielding hydronium ions and fluoride ions:
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Hydronium Ions Are Less Excluded from Hydrophobic Polymer-Water Interfaces than Hydroxide Ions.

Ryan L Myers1,2, Aoi Taira3, Chuanyu Yan1

  • 1Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.

The Journal of Physical Chemistry. B
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The study investigated how pH affects the cloud point temperatures of poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene) oxide (PEO) solutions. Results show hydronium ions (H3O+) promote polymer swelling, while hydroxide ions (OH-) reduce it, influencing their position in the Hofmeister series.

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Area of Science:

  • Polymer Science and Engineering
  • Physical Chemistry
  • Materials Science

Background:

  • Polymer solutions exhibit temperature-dependent phase transitions, crucial for applications like drug delivery and sensing.
  • The influence of pH and ionic composition on these transitions is complex and requires detailed investigation.
  • Understanding ion-polymer interactions is key to tailoring polymer behavior in aqueous solutions.

Purpose of the Study:

  • To determine the effect of pH on the cloud point temperatures of poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene) oxide (PEO) solutions.
  • To elucidate the role of specific ions (H3O+, OH-, Na+, Cl-) in modulating polymer solubility and phase behavior.
  • To correlate thermodynamic, spectroscopic, and molecular dynamics simulation data for a comprehensive understanding of ion-polymer interactions.

Main Methods:

  • Cloud point temperature measurements of PNIPAM and PEO solutions across a wide pH range (1.0-13.0) at constant ionic strength (100 mM).
  • Nuclear Magnetic Resonance (NMR) spectroscopy to probe polymer conformation and ion interactions.
  • All-atom molecular dynamics (MD) simulations of PNIPAM monomers and oligomers interacting with ions.

Main Results:

  • Cloud point temperatures of PNIPAM and PEO were largely stable between pH 2.0 and 12.0.
  • Acidic conditions (100 mM HCl) increased cloud point temperature, indicating polymer swelling, while alkaline conditions (100 mM NaOH) decreased it.
  • NMR and MD simulations confirmed that H3O+ ions promote swelling by interacting with PNIPAM, whereas OH- ions reduce swelling.

Conclusions:

  • H3O+ ions are less depleted from hydrophobic polymer interfaces than common cations, suggesting a position on the cation Hofmeister series.
  • OH- ions are excluded from polymer interfaces, placing them within the anion Hofmeister series.
  • The study provides a detailed understanding of pH-dependent polymer behavior and ion-specific effects, valuable for designing advanced polymer-based materials.