International Journal of Chemistry; Vol. 7, No. 2; 2015 ISSN 1916-9698 E-ISSN 1916-9701 Published by Canadian Center of Science and Education 213 Sound Velocities and Micellar Behaviour Studies of Dodecyltrimethylammonium Bromide in Aqueous Solutions at 295.15, 302.15 and 309.15 K Md. Harun-Al-Rashid 1 , Tania Tofaz 1 , M. M. Islam 2 , T. K. Biswas 2 1 Department of Chemistry, Begum Rokeya University, Rangpur-5400, Bangladesh 2 Department of Chemistry, University of Rajshahi, Rajshahi-6205, Bangladesh Correspondence: Md. Harun-Al-Rashid, Department of Chemistry, Begum Rokeya University, Rangpur-5400, Bangladesh. Tel: 880-191-534-2452, E-mail: harun.brur.bd@gmail.com Received: October 21, 2015 Accepted: October 22, 2015 Online Published: October 30, 2015 doi:10.5539/ijc.v7n2p213 URL: http://dx.doi.org/10.5539/ijc.v7n2p213 Abstract Speed of sound, u, of dodecyltrimethylammonium bromide (DTAB) aqueous solutions has been determined at 295.15, 302.15 and 309.15 K. The speeds of sound, u, data have been used to estimate isentropic compressibility, K s , apparent molal isentropic compressibility, φ k(s), acoustic impedance, Z, molal sound of speed, R m , and relative association, R A , of DTAB in aqueous solution. These parameters have been interpreted in terms of solute-solute and solute-solvent interactions and micellar behavior of DTAB. Keywords: surfactant, critical micelle concentration, isentropic compressibility, solute-solute and solute-solvent interactions 1. Introduction Surfactants are amphiphilic molecules consisting of polar (hydrophilic) and non-polar (hydrophobic) parts. Due to their dual character (hydrophilic and hydrophobic) surfactants are employed in different industrial applications (Herzog et al., 1998). In recent years, there has been growing interest in the interactions between protein and surfactant due to their many applications in biosciences, foods and cosmetics, drug delivery, detergency and biotechnological processes. Using various numbers of tools and techniques, these interactions have been studied and published in the past few years (Goddard et al., 1993, Brash et al., 1995, Sonesson et al., 2008, Mehta et al., 2008, Verdes et al., 2008, Mehta et al., 2009). Cationic surfactants are widely used in industry and have a great bearing on our day-to-day life (Rosen, 1989). The dodecyltrimethylammonium bromide (DTAB) is bio-degradable, non-toxic and is widely available material at low costs. Survey of available literature reveals that no serious attempt has been made to study the micellization phenomenon of dodecyltrimethylammonium bromide (DTAB) surfactant in aqueous solvent. The aim of the present work is to analyze the influence of the presence of dodecyltrimethylammonium bromide (DTAB) aqueous solutions upon several physical properties that allow the characterization of this kind of solutions. Studies on sound velocities of surfactants solutions are of great use in characterizing the structure and properties of solutions. Also compressibility is a powerful thermodynamic parameter for elucidating the behaviour of a solute in a solvent. (Chalikian et al., 1994).Various types of interactions exist between the solutes in solutions, and these solute–solute and solute–solvent interactions are of current interest in all branches of chemistry. These interactions provide a better understanding of the nature of the solute and solvent, i.e., whether the solute modifies or distorts the structure of the solvent. In the present work, an attempt has been made to provide an interpretation of solute– solvent and solute–solute interactions prevailing in the studied cationic surfactant dodecyltrimethylammonium bromide (DTAB) aqueous solutions as well as findings the micellar behaviour at the temperatures T=(295.15 to 309.15) K. 2. Experimental The surfactant used in this study was dodecyltrimethyammonium bromide, CH 3 (CH 2 ) 11 N + (CH 3 ) 2 CH 3 Br (molar purity  98%, Fluka AG, Switzerland ). Supplied distilled water was redistilled and deionized by passing through two ion exchange columns. The deionized water was distilled again in alkaline KMnO4 medium and used for