Studies of the Association of Dyes with Surfactants in Aqueous Solutions: Spectrophotometry and Computer Simulation

Abstract

The association of cationic dye (pinacyanol, PC) with anionic dyes (ethyleosin, EE, or rhodamine 200 C, R200C), and also the interaction “dye+surfactant” has been investigated in aqueous solutions by visible spectroscopy and computer simulation. Cetylpyridinium bromide (CPB), sodium dodecylsulfate (SDS) and triton X-100 (TX) were used as cationic, anionic and non-ionic surfactants respectively. The formation of “dye+surfactant” associates takes place at low dye concentrations (≥10‑6 M) and concentrations of surfactant, which are much smaller than the critical micelle concentrations (CMC). In contrast, similar concentrations (10‑6 – 10‑4 M) of small cations of tetramethylammonium or tetraethylammonium have no noticeable effect on the absorption spectra of the dye in the “dye+tetraalkylammonium salt” systems; it indicates the absence of formation of new compound between the dye and salt. The standard enthalpy of formation of “dye+dye” or “dye+surfactant” associates has been determined by AM1 and PM3 semiempirical methods. The computer simulation confirms that the formation of these associates is energetically beneficial.

Association of rhodamine cations with tetraphenylborate-anion in aqueous solution: structural properties of ions and stability of associates

“…Association as a result of the manifestation of intermolecular interactions is accompanied by an increase or decrease in light absorption and changes in spectral bands. For many years, the formation of stable stoichiometric associations between oppositely charged dye ions has been experimentally proven [1][2][3][4][5][6][7][8][9] even at low (1•10 -6 -1•10 -5 mol/L) concentrations. It is shown that "non-Coulomb" interactions are essential in the processes of cation-anion association of dyes or other polyatomic (organic) particles.…”

“…Absorption spectra in the system "RB + TFB". Concentrations, mol/L, of RB: 1.07•10 -5 (1…7); of TFB: 0 (1), 1.01•10 -6 (2), 2.02•10 -6 (3), 5.04•10 -6 (4), 6.55•10 -6 (5), 8.06•10 -6 (6), 1.41•10 -5(7). The comparison solution is water.…”