ORIGINAL ARTICLE Surface-Active Properties and Antimicrobial Study of Conventional Cationic and Synthesized Symmetrical Gemini Surfactants Ketan Kuperkar • Jigisha Modi • Keshav Patel Received: 9 July 2010 / Accepted: 23 March 2011 / Published online: 24 April 2011 Ó AOCS 2011 Abstract Symmetrical gemini surfactants of cationic series a,x-alkanediyl bis (dimethyl ammonium bromide) commonly referred as ‘‘m–s–m’’ have been synthesized. Spectral analysis was performed to confirm compound structures and purity. Conductivity and surface tension measurements provide better understanding of the micel- lization process. Their self-assembly behavior in aqueous solution is also discussed in detail. The antimicrobial efficacy was measured by bacterial and fungal growth inhibition expressed as minimal inhibitory concentration values against five strains of a representative group of microorganisms viz. Bacillus subtilis, Staphylococcus aureus, Klebsiella pneumonia, Salmonella paratyphi B and Aspergillus niger. All of the synthesized surfactants showed antimicrobial activity against them, but at different levels depending on their structures. The surfactants pos- sessing longer alkyl chains (more hydrophobic environ- ment) demonstrated better antimicrobial functionality. The antimicrobial potency was found to be dependent on the representative target microorganism (Gram-positive bac- teria [ fungi [ Gram-negative bacteria), as well as on the ionic nature of the surfactant (cationic), alkyl chain length (m = 12, 16) and spacer length (s = 2, 4, 6) of the syn- thesized compounds. Gemini surfactants such as 12-2-12 and 12-4-12 were found to be weakly active whereas 16-2- 16 and 16-4-16 compounds proved to be the most potent antimicrobial surface-active agents among the synthesized gemini homologues. Keywords Gemini surfactants  Surface tension  CMC  Antimicrobial activity Introduction Conventional quaternized ammonium surfactants (QAS) normally contain one hydrophilic group and one hydro- phobic chain. Due to their ability to self-assemble in aqueous solution they exhibit superior micellar morpholo- gies such as spherical micelles, vesicles, lamellar struc- tures, etc. and attract a keen research interest [1, 2]. Such aggregates once formed create sharp polarity gradients at the interface and define clear hydrophobic regions. In recent years, interest has emerged for a new class of amphiphilic molecules with novel structures from which new aggregates and physical properties may be expected. One of these amphiphilic classes is the ‘‘gemini’’ or ‘‘dimeric surfactant’’, which has two hydrophobic and hydrophilic groups per molecule (m), separated by a covalently bonded spacer (s). A Spacer can be of variable length—short (with 2 methylene groups) or long (with 12 methylene groups), rigid or flexible, polar or nonpolar [3, 4]. Gemini surfactants enhance the adsorption and aggre- gation properties by strengthening the intra- or intermo- lecular hydrophobic interactions and therefore appear to be better than the corresponding single-chain conventional surfactants. They tend to have much lower critical micellar concentrations (CMC) and surface tension values than conventional surfactants at the same molar or mass con- centrations [5–8]. In addition to these, an important feature of these surfactants is their ability to design and tune the physico-chemical properties, supramolecular structure, and biodegradability by altering the nature and size of the K. Kuperkar (&)  J. Modi  K. Patel Department of Chemistry, Veer Narmad South Gujarat University, Magdalla, Surat, Gujarat 395007, India e-mail: ketankuperkar@gmail.com 123 J Surfact Deterg (2012) 15:107–115 DOI 10.1007/s11743-011-1269-0