Effect of aromatic ring on surface and self-assembly properties of synthesized heterocyclic-based surfactants

Heterocyclic-based cationic surfactants with dodecyl tail group bridged to the head group either via aromatic ether [3-(4-dodecyloxybenzyl)-1-methyl-imidazolium bromide (Ar-I) and 4-(4-dodecyloxybenzyl)-4-methyl-morpholinium bromide (Ar-M)] or via aliphatic ether [3-(2-dodecyloxyethyl)-1-methyl imidazolium bromide (Al-I) and 4-(2-dodecyloxyethyl)-4-methyl morpholinium bromide (Al-M)] were synthesized and characterized using different spectral techniques. For the synthesis of Ar-I and Ar-M, 4-hydroxybenzoic acid was esterified initially, followed by O-alkylation, reduction, bromination and finally quaternization either with 1-methylimidazole or with 4-methylmorpholine. For the synthesis of aliphatic analogues (Al-I and Al-M), ethylene glycol was initially alkylated, followed by tosylation, bromination and quaternization either with 1-methylimidazole or with 4-methylmorpholine. Synthesized surfactants were evaluated for surface and self-assembly properties. Impact of the presence or absence of phenyl ring bridging the polar head and nonpolar tail of the surfactant and also type of heterocyclic moiety in the head group region on micellization and surface properties such as wetting, foaming and emulsification was studied. Surfactants with the bridged aromatic ether moiety were observed to display lower critical micelle concentration than those with the bridged aliphatic ether moiety irrespective of type of heterocyclic molecule in the head group region. Foam volume and stability of the aromatic ether-bridged surfactant were observed to be superior to their corresponding aliphatic analogs. Similar trend was observed in stabilizing the emulsion. However, aromatic ether-bridged surfactant depicted poor wetting ability compared to their aliphatic analog. Aliphatic ether bridged surfactant showed higher aggregation number compared to their aromatic counterparts, but depicted similar micellar core polarity.