Synthesis And Properties Of Imidazolium-based Surfactants

Imidazolium-based surfactants are defined as the surfactants with imidazole-ring as thecationic hydropholic head group. Recently, their surface activities have been one of theattractive topics in the field of colloid and interface chemistry. In order to investigate thestructure-performance relationship of the imidazolium-based surfactants, model surfactants,four kinds of cationic imidazolium-based surfactants1-alkyl-3-dodecylimidazolium bromides([C₁₂C_ni_m]Br, n=1,2,3,4) and three carboxylic zwitterionic imidazolium-based surfactants1-carboxymethyl-3-alkylimidazolium inner salts （CnimCM, n=10,12,14）, four sulfoniczwitterionic imidazolium-based surfactants1-sulfopropyl-3-alkylimidazolium inner salts（CnimSP, n=10,12,14） and1-（2-hydroxy propanesulfo）-3-dodecylimidazol inner salt（HPSDim）, have been designed and synthesized. The molecular structures of the abovecompounds were confirmed by means of fourier transform infrared spectroscopy, nuclearmagnetic resonance and electrospray mass spectrometry, respectively. Based on the surfaceactive parameters of the synthesized surfactants, the structure-performance relationships ofthe imidazolium-based surfactants were deduced.（1） The effect of the side chain length on the surface properties of [C₁₂C_ni_m]Br,（n=1,2,3,4） in aqueous solutions has been studied systematically. It is found that along with the sidechain length increasing, the critical micelle concentration （CMC） decreases, however, theCMC decreasing extent of adding one-CH2-to the side chain is relative smaller than that oflong side chain; meanwhile, the maximum surface excess （Γm） decreases, the surface pressureat the CMC （ΠCMC） and the minimum molecular cross-sectional area （Amin） at air-solutioninterfaces increase, which indicates that the [C₁₂C_ni_m]Br pack more loosely at the air-solutioninterfaces; the mean aggregation number （Nm） and the critical micelle aggregation number（Nm,c） of [C₁₂C_ni_m]Br decrease as the side chain length increases, which are opposite to thoseof the long-chain, furthermore, the micellar microenvironment polarity decreases and themicellar shape predicted via packing parameter （Pc） transforms from cylindrical to spheroidal.（2） The Krafft points （TK）, CMC and some surface activities parameters of CnimCM,（n=10,12,14） and CnimSP,（n=10,12,14） have been determined. The results show that, due tothe elongation of hydrocarbon chain, the TKvalues of imidazolium-based surfactants increase,and are generally higher than that of structurally related traditional betaine surfactants, butwill markedly decrease with the addition of inorganic salt. From the point of the view ofCMCs, the surface activities of zwitterionic imidazolium-based surfactants are superior to thestructurally related traditional betaine surfactants. Along with the long-chain length increasing,the hydrophobicity, the tendency of escaping from aqueous phase, the efficiency and thecapacity of reducing surface tension increase. The correlation between the long-chain length （n） and lgCMC obeys the Stauff-Klevens rule, which means that adding two methylenegroups to a hydrocarbon chain decreases the CMC by approximately a factor of10.Comparing with C₁₂C_ni_mSP, HPSDim shows higher Krafft point （65.8±0.5oC）, whichindicates that adding-OH group to the head group of the sulfonic zwitterionicimidazolium-based surfactants will decrease the water solubility.