The Synthesis, Self-Assembled Structures, and Microbicidal Activity of Cationic Gemini Surfactants with Branched Tridecyl Chains.

Cationic gemini surfactants with polymethylene spacer and linear alkyl chains containing an even number of carbon atoms have been extensively studied in the recent past, with the emphasis put on the determination of their aggregation behaviour in aqueous solution and their biological properties. However, the information on the aggregation of branched gemini surfactants with an odd number of carbon atoms in their alkyl chains is only sparsely reported in the literature. To help cover this gap in the research of cationic gemini surfactants, a series of branched bisammonium cationic gemini surfactants with an odd number of carbon atoms in alkyl chains (tridecane-2-yl chains) and a polymethylene spacer with a variable length ranging from 3 to 12 carbon atoms have been synthesized and investigated. Critical micelle concentration, which was determined by three methods, was found to be in the order 10-4 mol/L. A comparison of the obtained data of the novel series of tridecyl chain geminis with those of gemini surfactants with dodecyl chains and an identical spacer structure revealed that structural differences between both series of gemini surfactants result in different aggregation and surface properties for surfactants with 6 and 8 methylene groups in the spacer (N,N'-bis(tridecane-2-yl)-N,N,N',N'-tetramethylhexane-1,6-diaminium dibromide and N,N'-bis(tridecane-2-yl)-N,N,N',N'-tetramethyloctane-1,8-diaminium dibromide) with the cmc values 8.2 × 10-4 mol/L and 6.5 × 10-4 mol/L, respectively, as determined by surface tension measurements. Particle size analysis showed the formation of small stable spherical micelles in the interval between 2.8 and 5 nm and with zeta potential around +50 mV, which are independent of surfactant concentration and increase with the increasing spacer length. Microbicidal activity of 13-s-13 gemini surfactants was found to be efficient against Gram-positive, Gram-negative bacteria and yeast.

Biodegradable gemini surfactants. Correlation of area per surfactant molecule with surfactant structure.

Values of the area per surfactant molecule of various single chain and gemini quaternary ammonium surfactants containing biodegradable amide and ester groups are obtained from the surface tension measurements and they are mutually compared. It was found that surfactant molecules with the ester group in their structure occupy smaller area at the air/water interface than the corresponding molecules with the amide group, mainly due to the higher conformational flexibility of ester groups. In decreasing the area per surfactant molecule value, hydrogen bonding (both inter- and intramolecular) plays a significant role when amide groups are present in the spacer of a gemini molecule. They must be separated by a polymethylene chain or a flexible group such as cyclohexane which is short enough to allow intramolecular hydrogen bonds. The flexible cyclohexane group with the amide group in single chain surfactants may lead to the formation of intermolecular hydrogen bonds among surfactant molecules which also results in the reduction of the area per surfactant molecule.