Self assembly of pH-sensitive cationic lysine based surfactants

Area of Science:

• Supramolecular Chemistry
• Physical Chemistry
• Materials Science

Background:

• Cationic surfactants with protonated amine groups exhibit pH-dependent charge.
• Understanding their solution behavior and self-assembly is crucial for applications.

Purpose of the Study:

• To investigate the solution behavior, interfacial adsorption, and thermotropic properties of N(ε)-acyl lysine methyl ester hydrochlorides.
• To correlate molecular structure and cationic charge with observed physicochemical properties.

Main Methods:

• Physicochemical methods including conductivity, nuclear magnetic resonance (NMR) diffusion, and surface tension (Wilhelmy plate, pendant drop).
• Phase behavior analysis using optical polarizing microscopy and small-angle X-ray scattering (SAXS).

Main Results:

• Critical micellar concentration (CMC) values varied between bulk techniques (conductivity, NMR) and surface tension methods.
• SAXS revealed rich thermotropic liquid crystalline behavior in lysine-based surfactants at low hydration.
• Small structural modifications led to significant alterations in phase behavior.

Conclusions:

• The pH-dependent charge and pK(a) are critical for understanding the aqueous solution behavior of these surfactants.
• Lysine-based surfactants exhibit complex self-assembly and diverse thermotropic liquid crystalline phases.
• Molecular structure plays a pivotal role in dictating the macroscopic properties and phase behavior of these cationic surfactants.