Alkylbetainate chlorides: Synthesis and behavior of monolayers at the air–water interface

In this study, cationic alkylbetainate chloride surfactants (CnBC, n = 10–16) are synthesized using more environment-friendly reagents (glycine betaine, thionyl chloride and primary alcohols) than in previous works (chloroacetyl chloride, trimethylamine and primary alcohols). The interfacial behavior of the CnBC monolayers is studied onto aqueous subphases at 20 °C using the Langmuir through technique. Different experimental conditions including pH, temperature, sodium salts and Na2SO4 ionic strength are investigated.The results obtained have shown that the CnBC with a hydrocarbon chain length ≥ 14 are able to form insoluble monolayers. The C16BC monolayer stability checked by the compression–expansion cycles has shown a superimposition of isotherms in the whole liquid-condensed region and an irreversible rearrangement of the molecules at high compression. The C16BC monolayer is not influenced by the pH and the presence of monovalent anions but it is influenced by the temperature, the divalent anions and the ionic strength. It has exhibited a great stability whatever the experimental conditions.

► Synthesis of cationic alkylbetainate chlorides (CnBC), glycine betaine based esters surfactants, have been optimized using more environment-friendly reagents (glycine betaine and thionyl chloride) than in previous works.. ► The interfacial organization of the C16BC monolayer is not greatly influenced by the pH and the monovalent anions but is influenced by the temperature, the divalent anions and the ionic strength mainly in term of interfacial area occupation. ► The CnBC molecules interact mainly via the van der Waals forces at the air-water interface and their polar heads are deeply immersed into the water subphase.