Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 83–88 Rheological behavior of gemini-type surfactant/alkanolamide/water systems Suraj Chandra Sharma, Rekha Goswami Shrestha, Dharmesh Varade, Kenji Aramaki ∗ Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Received 10 March 2007; received in revised form 18 April 2007; accepted 18 April 2007 Available online 24 April 2007 Abstract The formation of the viscoelastic wormlike micelles in aqueous solution of mixed surfactant system of anionic gemini-type surfactant without a spacer group, disodium 2,3-didodecyl-1,2,3,4-butanetetracarboxylate (GS), and dodecanoyl-N-methyl ethanolamide (NMEA-12) has been studied at 25 ◦ C. With addition of NMEA-12 to the GS–water binary system, a micellar (W m )–lamellar (L  ) phase transformation takes place at low GS concentration. Within the W m phase region of GS–NMEA-12–water system, there exists a high viscosity region consisting of a viscoelastic micellar solution of entangled wormlike micelles. On increasing the concentration of GS, micellar growth begins at a slightly lower mixing fraction of NMEA-12. The oscillatory-shear rheological behavior of the viscoelastic solutions can be described by Maxwell model at low -shear frequency region. The increase in viscosity can be simply explained by reducing the effective cross-sectional area per amphiphile upon addition of NMEA-12. © 2007 Elsevier B.V. All rights reserved. Keywords: Gemini surfactant; Alkanolamide; Phase behavior; Wormlike micelles; Rheological behavior 1. Introduction Surfactant molecules in aqueous solution self-assemble to form a variety of microstructures such as spherical micelles, wormlike micelles, vesicles and liquid crystals [1–4]. These different aggregation structures have characteristic rheologi- cal properties. Transition from spherical to wormlike micelles corresponds to a drastic increase of elasticity and viscosity of the fluid [5]. The viscoelastic wormlike micelles have attracted much interest in fundamental research and practical applications [5–7]. Formation of viscoelastic wormlike micelles is a conse- quence of unidimensional micellar growth. Above some critical concentration, called the overlapping concentration, wormlike micelles entangle with each other to form a transient net- work and exhibit viscoelastic properties [8], analogous to those observed in flexible polymer solutions, with an important dif- ference from the polymeric network that the micelles can break ∗ Corresponding author. Tel.: +81 45 339 4300; fax: +81 45 339 4300. E-mail address: aramakik@ynu.ac.jp (K. Aramaki). and recombine on a time scale characteristic of the system [9]. It is well known that cationic surfactants can self-assemble in aqueous solution into long flexible cylindrical micelles (wormlike micelles) upon the addition of salts [5,10–12]. The salt serves to reduce the electrostatic interactions between the cationic headgroups, thus reducing the effective area per head group and thereby promoting the growth of cylindrical aggre- gates at the expense of spherical ones. There are some recent reports on non-ionic mixed surfactant systems [13–17]. How- ever, not much is known about the formation and rheological behavior of the wormlike micelles in anionic systems except sodium dodecyl sulfate (SDS) [18–21]. Gemini surfactants have a very high potential for practical applications because of their excellent ability to reduce surface tension of water and low Krafft temperatures. Compared to their monomeric analogs, gemini surfactants have much lower critical micelle concentration. Due to their high molecular weight, skin penetration of gemini surfactant is expected to be low, which is one of the desirable properties of a surfactant to be used in body care products such as soaps, shampoos and cosmetics. However, the main factor that has prevented the use of gemini surfac- 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.044