Detection of bicontinuous phase in some reverse micellar systems by positron lifetime spectroscopy

Abstract

Positron lifetime measurements have been made in some reverse micellar systems of cetyl tri methyl ammonium bromide (CTAB)/hexanol and tetra-decyl-tri-methyl-ammonium-bromide (TTAB)/pentanol by adding increasing amount of water in each system. For both the systems, the dependence of positron lifetime parameters on water concentration indicates the existence of bicontinuous phase in so called single reverse micellar phase. The results of this investigation have been presented in this paper.1 Introduction Cationic surfactants CTAB and its lower homologue TTAB of general formula, RN(CH 3)3Br, where R is an alkyl chain, have interesting applications in pharmaceutical, industrial and technological fields. They are among the most widely used bactericides, wetting and antistatic agents. CTAB and TTAB when dissolved in different solvents give rise to several aggregated phases. Detailed phase diagrams of CTAB/hexanol/water and TTAB/pentanol/water as given by Ekwall et al.[1] and Choudhary et al. [2] respectively, depict such phases. They show the existence of four phases namely the normal micellar phase L 1, the reverse micellar phase L2, lamellar phase D and liquid crystalline hexagonal phase E. L1 phase of this system has been extensively studied by several techniques [3,4]. However, few results have been reported to elucidate the detailed microstructure of the L2 phase. Recently positron lifetime spectroscopy has been employed to investigate normal micellar and lamellar phase of these systems [5][6][7][8]. These investigations have demonstrated the sensitivity of positron lifetime parameters in detecting various microstructural changes taking place in surfactant solutions. Besides this, it revealed the existence of two kinds of lamellar structures referred to as D 1 and D 2 phases in the systems, which were otherwise considered to be a single D phase by other conventional techniques [9,10]. In the present work, positron annihilation technique has been used to investigate the less studied L 2 phase of CTAB/hexanol/water and TTAB/pentanol/water.

Packing and mobility of hydrocarbon chains in phospholipid lyotropic liquid crystalline lamellar phases and liposomes: characterisation by positron annihilation lifetime spectroscopy (PALS)

“…[13][14][15] In recent years PALS has also been demonstrated to have utility as an in situ molecular probe in soft condensed matter systems for which it is highly sensitive to conformational, structural and microenvironmental transformations. PALS has been used to determine the critical micelle concentration (CMC) in aqueous micellar systems; [16][17][18][19] to distinguish transformations in micellar geometry [20][21][22][23][24][25] as well as phase transitions in liquid crystalline systems [26][27][28][29][30] and lipid bilayers. [31][32][33][34][35][36][37] The basic principle of PALS is the thermalisation of a positron (e.g.…”

Application of positron annihilation lifetime spectroscopy (PALS) to study the nanostructure in amphiphile self-assembly materials: phytantriol cubosomes and hexosomes

“…PALS has been used to determine the critical micelle concentration (CMC) of surfactants; [23][24][25][26] to distinguish transformations in micellar geometry [27][28][29][30][31][32] as well as phase transitions in lyotropic liquid crystalline systems [33][34][35][36][37][38] and lipid bilayers. In recent years, positron annihilation lifetime spectroscopy (PALS) has demonstrated its utility as a molecular probe in soft condensed matter systems.…”

“…In recent years, positron annihilation lifetime spectroscopy (PALS) has demonstrated its utility as a molecular probe in soft condensed matter systems. PALS has been used to determine the critical micelle concentration (CMC) of surfactants; [23][24][25][26] to distinguish transformations in micellar geometry [27][28][29][30][31][32] as well as phase transitions in lyotropic liquid crystalline systems [33][34][35][36][37][38] and lipid bilayers. 37,[39][40][41][42][43][44][45] The decay of radioactive nuclei such as 22 Na leads to the generation of ortho-positroniums (oPs); where oPs lifetime (t oPs ) may be considered an active probe of both intrinsic packing and chain mobility.…”

Location of Phase Boundaries of Lyotropic Liquid Crystal Employing Positron Lifetime Spectroscopy and Electrical Conductivity Measurement

“…However, due to relative insensitivity and perturbation introduced by some of the techniques, the molecular level information is scarce. The sensitivity of positron as a nondestructive probe for investigating such type of the systems has been amply demonstrated [2,[5][6][7][8][9]. It is, therefore, interesting to investigate the response of positron lifetime parameters on various phase boundaries of lyotropic liquid crystalline phases.…”