The interaction of surfactants with proteins, polymers, nucleic acid, hydrophobic sols, etc., has been studied to establish certain characteristics peculiar to the surfactants. Controversy exists as to the nature of surfactant~lye binding and the composition of the reaction product. According to Mukerjee and Mysels 1 these interactions involve the formation of an insoluble dye-surfactant simple salt while Hiskey and Downey 2 have interpreted the spectral data for these reactions in terms of an association reaction. Dye-surfactant systems in which non-stoichiometric binding exists have recently been reported 3'4. Such systems have now been investigated to determine the combining ratios of the various surfactants to the dye and to find the c.m.c, of both anionic and cationic surfactants.
EXPERIMENTAL
Reagents
Dodecane sulphonic acid (DS) in pure form was prepared by the method of Noller and Gordon 5. Cetyltrimethylammonium bromide (CTMAB) and cetylpyridinium bromide (CPB) were BDH products. These were used without further purification since preliminary results on c.m.c, and the congo red binding data gave the same results for the commercial products as for the surfactants crystallized from acetone. Cationic dyes, viz., rosaniline hydrochloride, malachite green, rhodamine 6G and an anionic dye (congo red) were all BDH products. Congo red was recrystallized twice from 50~ ethyl alcohol and dried at 110 ยฐ C in an oven. Stock solutions of surfactants and dyes were prepared in double distilled water.
Walpole (pH 2.0, 4.5), McIlvaine (pH 7.0), and borax (pH 9.12) buffers were prepared in the laboratory.
Apparatus
Absorption measurements were carried out using a Bausch and Lomb spectronic '20'. A Cambridge bench type pH meter was used to measure pH of the solutions. A Philips conductivity bridge was used for conductivity measurements.
Formula used
The concentration of unbound dye to surfactant was calculated by Klotz's