Optical Activity of Triton X-100 Micelles in Aqueous Solutions
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Optical Activity of Triton X-100 Micelles in Aqueous Solutions O. A. Fedyaevaa,* and E. G. Poshelyuzhnayaa a Omsk
State Technical University, Omsk, 644050 Russia *e-mail: [email protected]
Received December 24, 2019; revised February 17, 2020; accepted March 10, 2020
Abstract—Polarimetry is used to study the optical activity of colloidal aqueous solutions of nonionic surfactant (NS) Triton X-100. It is found that these solutions rotate the plane of polarization to the right. It is shown that a change in the concentration of nonionic surfactants alters the structure of micelles. It is found that the transition to a continuous hexagonal mesophase is accompanied by a stepwise change in optical rotation. Keywords: Triton X-100, micelles, optical rotation, optical activity DOI: 10.1134/S0036024420090058
INTRODUCTION It is known that colloidal surfactants in monomeric form are achiral and have low molecular solubility. Upon an increase in the concentration of their molecules in solution, however, aggregative processes begin to occur, leading to the formation of premicellar complexes with chirality. The authors of [1, 2], who studied the optical rotation of aqueous solutions of colloidal surfactants of various types (anionic sodium dodecyl sulfate, cationic hexadecyltrimethylammonium bromide and nonionic polyoxyethylene (4) mono-n-dodecyl ether), thus found that in the concentration range near of the critical micelle concentration (CMC), the optical activity of these substances passes through a maximum and then falls to some almost constant level. In their opinion, the drop in optical activity corresponds to the completion of the formation of complete spherical micelles. At the same time, the authors noted that the nonionic surfactant they studied, dodecanoyl-N-methylglucamide, was optically inactive, and all optically active surfactants had positive (right) rotation of the plane of polarization. In this work, we studied the optical activity of micellar aqueous solutions of nonionic surfactant Triton X-100. Solutions for analysis were prepared from commercial Triton X-100 from Sigma-Aldrich via serial dilution of 5.12 × 10−2 mol/L of solution with deionized water obtained with an Vodoley unit. Critical micelle concentrations (CMC1 and CMC2) were determined conductometrically using a Multitest KSL device. At 22°C, they were 1.72 × 10−4 and 3.2 × 10−3 mol/L, respectively [3]. The angle of rotation of the plane of polarization was measured using an SM-3 circular polarimeter with an optical path of 1 dm and a light wavelength of λ = 589 nm. Polarime-
ter sensitivity was 0.04°, and the error in the range from 0° up to ±35° was no more ±0.04°. Figure 1 shows the dependence of the optical rotation of solutions on the concentration of Triton X-100. We can see that Biot’s law on the linear dependence of the angle of rotation of the plane of polarization is not obeyed for the given optically active substance. Like the authors of [1, 2], we note an extreme change in the optical activity of solutions in the CMC re
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