The alternation effect in ionic liquid homologous series

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• A density odd-even alternation effect was observed in alkyltrioctylphosphonium chlorides.
• A similar trend was detected in 1-alkyl-3-methylimidazolium ionic liquids.

Purpose of the Study:

• To investigate the cause of the odd-even alternation effect in ionic liquids.
• To explore the role of charge distribution and side-chain conformation.
• To determine if this effect extends to other molecular compounds.

Main Methods:

• Ab initio calculations
• Molecular Dynamics (MD) simulations
• Corresponding States Principle (CSP)

Main Results:

• Charge distribution and side-chain conformation are not the direct cause of the odd-even effect.
• Ionic liquid side chains adopt transoid conformations and pack head-to-head in the liquid phase.
• This packing mechanism explains odd-even alternation in solid-phase properties and extends to liquid phases of alkanes and alkanols.

Conclusions:

• The head-to-head packing of transoid side chains is the origin of the odd-even alternation effect in ionic liquids.
• The odd-even effect is a general phenomenon applicable to various molecular compounds with linear alkyl chains.
• The corresponding states principle provides a simple method to observe these trends in different liquid phases.