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Ordering and Nonideality of Air-Ionic Liquid Interfaces in Surface Second Harmonic Generation.

Renata Costa¹, Carlos M Pereira¹, A Fernando Silva¹

¹Departamento de Quı̀mica e Bioquı̀mica, CIQUP-Physical Analytical Chemistry and Electrochemistry Group, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal.

The Journal of Physical Chemistry. B

|April 24, 2020

Summary

Ionic liquid interfaces become more ordered with longer alkyl chains. Mixed ionic liquids show nonideal behavior, with long chains dominating the interface even at low concentrations.

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Area of Science:

Physical Chemistry
Materials Science
Surface Science

Background:

Ionic liquids (ILs) are tunable solvents with unique interfacial properties.
Understanding the air-IL interface is crucial for applications in catalysis, electrochemistry, and lubrication.
Alkyl chain length significantly influences IL behavior.

Purpose of the Study:

To investigate the effect of alkyl chain length on air-ionic liquid interfaces.
To study the interfacial behavior of mixed ionic liquids.
To determine the ordering and nonideality at IL interfaces.

Main Methods:

Polarization-resolved second harmonic generation (SHG) spectroscopy was employed.
A series of ionic liquids with varying imidazolium cation alkyl chain lengths ([Cmim]) and a common [NTf₂] anion were studied.
Mixed ionic liquid interfaces ([C12mim][NTf2] and [C2mim][NTf2]) were analyzed.

Main Results:

Increasing alkyl chain length enhanced the ordering of the air-IL interface.
Interfacial ordering was quantified by the orientational parameter.
Mixed IL interfaces exhibited nonideal behavior, with long alkyl chains dominating.
Even 10% long-chain IL significantly perturbed the interface.

Conclusions:

Alkyl chain length is a key factor in controlling air-IL interfacial ordering.
Mixed ionic liquids display significant interfacial nonideality due to preferential adsorption of longer chains.
The interface composition deviates from bulk composition, especially at low concentrations of long-chain ILs.