Dynamic and structural evidence of mesoscopic aggregation in phosphonium ionic liquids

Area of Science:

• Materials Science
• Physical Chemistry
• Chemical Physics

Background:

• Mesoscopic aggregation in aprotic ionic liquids arises from microphase separation of polar and non-polar components.
• This aggregation is strongly linked to physicochemical properties and potential applications.
• Low-q pre-peaks in scattering profiles are experimental evidence of this aggregation, attributed to phase alternation.

Purpose of the Study:

• To investigate the relationship between molecular structure and mesoscale aggregation in phosphonium ionic liquids.
• To understand how varying alkyl chain lengths on phosphonium cations affect aggregation and properties.
• To correlate scattering data and dielectric relaxation with mesoscale aggregate structure.

Main Methods:

• Small and wide-angle x-ray scattering (SAXS/WAXS) to probe structure.
• Dynamic-mechanical spectroscopy (DMS) to assess viscoelastic properties.
• Broadband dielectric spectroscopy (BDS) to study molecular dynamics and relaxations.

Main Results:

• Mesoscale aggregates were observed in phosphonium ionic liquids with varying alkyl chain lengths.
• Increasing symmetry of the quaternary phosphonium cation led to a disruption of these mesoscale aggregates.
• The disruption correlated with changes in scattering profiles and the absence of slow sub-alpha dielectric relaxation.

Conclusions:

• Molecular structure, specifically cation symmetry, significantly influences mesoscale aggregation in ionic liquids.
• Disruption of mesoscale aggregates impacts the physicochemical properties and dynamics of ionic liquids.
• These findings enhance the understanding of structure-property relationships in ionic liquids for targeted applications.