Thermal behaviour and structural properties of surfactant—Picrate compounds: The effect of the ammonium headgroup number

• Physicochemical properties of n-dodecyldimethylammonium picrates were examined.
• Temperature-dependent infrared spectroscopy detected adiabatic phase transitions.
• Linear growth of melting and crystallization temperatures with the headgroup number.
• Higher degree of oligomerization leads to the increased solid phase disorder.
• The crystal structure of dimeric picrate is determined.

Novel quaternary n-dodecyldimethylammonium picrates with two to four chains connected by an ethylene spacer at the ammonium headgroup level were synthesized. The effect of the headgroup number and the impact of picrate anion to their physicochemical properties was examined. A crystal structure of dimeric dipicrate is significantly different from bromide analogue. The dominant feature of this crystal packing is a layered structure with aromatic rings of picrate molecules, stacked via strong π···π interactions and connected to the alkylammonium molecules with their nitro groups by C–H···O hydrogen bonds. Weak NO2···HC hydrogen bonds further stabilize the structure. Higher degree of oligomerization leads to the less-ordered solid phase and amorphous picrate structure. These amorphous structures result from widespread and disordered hydrogen bonding and rupture of π–π stacking in samples with more than two headgroups. Unlike corresponding bromides, picrate compounds show reversible phase transitions without thermal decomposition. Temperature-dependent infrared spectroscopy in the liquid phase detected adiabatic phase transitions.

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