J . Chem. SOC., Faraday Trans. I , 1986, 82, 2377-2383 The Hydrophobic Behaviour of Orange IV in Water and in Aqueous Electrolyte Solutions Michel De Vijlder Rijksuniversiteit Gent, Laboratorium voor Fysische Scheikunde, Krijgslaan 281, B-9000 Gent, Belgium Unlike the stepwise manner of self-aggregation shown by homologues of the surface-active azo dye Methyl Orange in aqueous solution, cooperative self-aggregation is shown in electrolyte solutions of Orange IV ( C , H , - N H - - ~ N = = N ~ SO; Na+). Evidence of a critical micellar concentration (c.m.c.) appears from the breaks in the plots of absorbance and surface tension values against the dye concentration. The quantitative treatment of the effects of NaCl and SrC1, reveals that the equation log (c.m.c.) = - Kcsalt + constant, derived for zwitterionics by Mukerjee (J.Phys. Chem., 1965, 69, 4038) on the basis of the salting-out of their hydrocarbon groups, is only obeyed at lower salt concentrations. An equation log (c.m.c.) = - K log Csalt +constant fits our experimental results over the whole range of salt concentrations considered and remains valid when 1.0 mol dm-3 urea, a well known disturber of hydrophobic bonds, is introduced. It is known that amphiphilic azo dyes [especially Methyl Orange (I) and its homologues] do not behave in aqueous solution like other ionic surfactants; e.g. their self-aggregation may proceed in a stepwise manner without cooperativity and without evidence of a critical micellar concentration (c.m.c.). However, the addition of foreign ions to such dye solutions, favouring the self- aggregation processes,2 may affect their mechanisms or the nature of the aggregated species.Since we observed recently2 that the amounts of electrolyte required for this purpose are small in the case of Orange IV (R, = C,H,, R2 = H), thus permitting surface-tension measurements, we focused our attention on the behaviour of this dye, referring to analogies with Methyl Orange where possible. Experiment a1 Methyl orange was purchased from B.D.H. and Orange IV from Fluka. Both were purified by a twofold recrystallization from water. Further purification through the foaming method recommended by Giles3 did not introduce any changes in the results of the surface-tension measurements in low concentrated dye solutions and was abandoned because of hastening precipitation in more concentrated ones.We used a Varian Techtron spectrophotometer for the light absorption measurements and a Philips PW 9504/00 bridge for the conductance measurements (the cell was equipped with blacked platinum electrodes). The surface tension measurements occurred with the du Nouy ring method: the force 79 2377 F A R 12378 Hydrophobic Behauiour of Orange IV / 100 /'/ I // 5 10 15 C/ 1 0-4 rnol dm-3 e I P z f ._ + j o y 1- :- n 60 50 10-3 I I I I I I I I I I I I 1 Clrnol dm-3 Fig. 1. Plots of the absorbance ( A ) , the conductance ( K ) and the surface tension (7) of aqueouc solutions of Methyl Orange (0) and Orange IV (+) as a function of concentration, C, at 25 "C. Absorbances are measured at A, = 465 nm for Methyl Orange and 443 nm for Orange IV (water as reference, cell path length 0.2 cm).exerted on the immersed ring by gently lowering the sample holder was recorded automatically by a R-G Cahn electrobalance. Calibration tests undertaken on pure solvents reveal an accuracy in the region of ca. 0.25 mN m-I. Results Spectrophotometric measurements in solutions where no salt is added apparently reveal a deviation from ideal behaviour at the same concentration for both dyes (5.5 x mol dm-3). Conductance measurements, however, show a rather marked difference: 6.0 x mol dm-3 for Orange IV. As will be seen further, extrapolating c.m.c. values of these dyes in electrolyte solutions to an electrolyte concentration equal to zero will confirm the values obtained by conductance measurements.Such a discrepancy associated with the method of observation is not unusual, and has been observed by other workers on Pentyl and Hexyl Orange as well.4 The higher critical concentration of Methyl Orange could be due to a more important hydrati~n.~ We note in passing that the value for Orange IV is fairly higher than those of homologues carrying aliphatic groups on th.= amino nitrogen: 0.5 x for Butyl Orange,6 0.35 x for Pentyl Orange4 and 0.3 x lop4 for Hexyl O ~ a n g e . ~ Apparently the aromatic group confers on Or IV a smaller tendency to aggregate than aliphatics do. As expected, surface-tension measurements fail to show any indication of c.m.c. neither at the above concentrations nor at higher ones. So, if it is true that a c.m.c. may rnol dm-3 for Methyl Orange and 5.0 xM .De Vijlder 2379 A * I E z E * Q .. Fig. 2. Plots of the absorbance ( A ) and the surface tension (7) of aqueous solutions of Orange IV in the presence of increasing amounts of NaCl. Dye concentrations are 3 x lop4 (+), 4.5 x (0) and 7.5 x ([I]) mol drnb3. be determined from the break in the Beer’s law curves of solutes absorbing in the visible or U.V. region^,^ each such deviation does not necessarily give evidence of the existence of a c.m.c. Repeatable results are all plotted in fig. 1.t From a critical amount dependent on the dye concentration, added electrolytes cause a decrease in the absorbance of both dyes. Critical electrolyte concentrations have already been reported for several azo dyes.2 On comparing the course of the decrease in absorbance and in the surface tension of Orange IV as a function of electrolyte concentration, we found that the absorbance values start to decrease precisely at those electrolyte concentrations for which surface-tension values remain unchanged, thus giving evidence of a c.m.c.(fig. 2). This should probably be valid for Methyl Orange as t Note that the degree of absorbance decrease as reported in the figures is not absolute; it depends as well on the surface-volume ratio of the recipient in which the dye solution is kept. The larger the ratio the less dramatic the decrease. The values reported in this paper are those of solutions in equilibrium in conventional 50 x dm3 flasks. 79-22380 Hydrophobic Behaviour of Orange IV A 2 3 4 5 I 20 1 5 1 I 4 / 2 3 C(0range IV) Fig.3. Graphic determination of the breaks in the Beer's curves of Orange IV, associated with a c.m.c. of the dye in the presence of NaCl (left, in mol dm-3) and SrCl, (right, in mol dmp3). well, but attempts at determining surface-tension values remain unsuccessful : the results drifted because of the much higher salt concentrations required. The change in c.m.c. of Orange IV as a function of added NaCl and SrCl, has been determined further only from breaks in the absorbance curves. The results are plotted in fig. 3. In the case of zwitterionics (which Methyl Orange and its homologues indeed are) and non-ionic surfactants it is expected that the logarithm of the c.m.c. vary linearly with the salt concentration.8 Fig. 4 and 5 reveal that such a relationship is only applicable here at lower electrolyte concentrations, and that a better fit is obtained by plotting log c.m.c.us. log Csalt. The same relationship has been reported in the case of Methyl Orange.2 The former equation is derived because of the effect of salts on the monomer-micelle equilibrium interpreted in terms of the salting-out of the hydrocarbon groups. Some deviations from this theory have been discussed,sc but as far as we know, a log-log relationship has not yet been submitted. It may be tempting to associate our results with the equation which is usually applicable to ionic surfactants : log c.m.c. = - K log Ci + constant where Ci is now the total counterion concentration in mole dmp3 (= Cc.m.c. + Csalt for univalent ions).This application may hold in the case of added NaCl, where Csalt % CCarnaC., but is inappropriate in the case of SrCl,. As seen above, extrapolating the c.m.c. values in fig. 4 to zero salt concentration permits us to recover the value of the critical aggregation concentration (c.a.c.) obtained from conductivity measurements (log c.a.c. = - 3.30, c.a.c. = 5.0 xM . De Vijlder 238 1 C(NaCl)/mol dm-3 0.0 5 01 0 0.1 5 Q20 I I I - 3 2 5 3 1 2 3 4 5 6 I I I 1 C (SrC12)/ 1 0-4 mol dm-’ Fig. 4. Plots of the log c.m.c. from fig. 3 us. the electrolyte concentrations. Table 1. Effect of urea on the critical aggregation concentration of Orange IV (all concentrations in mol dm-3) without urea 1 mol dm-3 urea 4 mol dmP3 urea in H,O 5.0 x 10-4 5.0 x 10-4 5.0 x 10-4 0.12 mol dmP3 NaCl 1.0 x 10-4 2.1 x 10-4 > 4.2 x 10-4 0.20 mol dm-3 NaCl 0.9 x 10-4 1.7 x 10-4 > 4.2 x 10-4 0.06 mol dm-3 NaCl 2.0 x 10-4 3.0 x lop4 not determined 0.8 x lop4 mol dm-3 SrCl, 1.3 x 10-4 2.8 x not determined 1.6 x mol dm-3 SrC1, 0.9 x 10-4 2.3 x lop4 not determined2382 Hydrophobic Behaviour of Orange IV A - 3.5 - 3.75 - L.0 - 1.5 -1 0 log [C(NaCl)] 2.0 1.5 1.0 0.5 Fig.5. Plots of log c.m.c. from fig. 3 vs. the log of the electrolyte concentrations. - 2 3 4 5 C(0range IV)/ 1 0-4 mol dmd3 Fig. 6. Graphic determination of the breaks in Beer’s curves of Orange IV, in aqueous 1 mol d ~ n - ~ urea solutions in the presence of NaCl (left) and SrCl, (right).M . De Vijlder 2383 C(SrC12)/10-4 mol d ~ n - ~ (+) 1 2 3 4 1 2 3 4 C(NaCI)/ lo-' mol dm-3 (0) 1 I I -4.5 -4.0 -3.5 -1.2 -1D -0.8 -0.6 1 1 Fig.7. Plots of the log c.m.c. from fig. 6 us. the electrolyte concentrations (left) and the log of the electrolyte concentrations (right) (both in mol dmP3). It also seemed of interest to check the influence of urea on the behaviour of the dye solutions. It is well known that urea lowers the tendency to aggregate through hydrophobic interaction^.^ The results of these experiments confirm that the aggregation of Orange IV in water and in electrolyte solutions are two different processes : apparently, urea has no effect on the value of the critical aggregation concentration in the former case, but lowers markedly the tendency to aggregate in the latter (table 1). The presence of 1 mol dmA3 urea does not affect the validity of the log-log relationship we found for the effect of the electrolyte concentration on the assumed c.m.c. of Orange IV (fig. 6 and 7). References 1 R. Reeves and Sh. Harkaway, J . Colloid Interface Sci., 1978, 64, 342. 2 M. De Vijlder, J . Chem. SOC., Faraday Trans. I , 1985, 81, 1369. 3 C. H. Giles and A. H. Soutar, J . SOC. Dyers Colour., 1971, 87, 301. 4 T. Takagishi, S. Fujii and N. Kuroki, J. Colloid Interface Sci., 1983, 94, 114. 5 R. L. Reeves, R. S. Kaiser, M. S. Maggio, E. A. Sylvester and W. H. Lawton, Can. J. Chem., 1973,51, 6 R. Burkhard, B. E. Buergert and J. S. Levitt, J . Am. Chem. SOC., 1953, 75, 2977. 7 D. G. Duff and C. H. Giles, J . Colloid Interface Sci., 1972, 41, 407. 8 (a) K. Shinoda, T. Yamaguchi and R. Hori, Bull. Chem. Soc. Jpn, 1961, 34, 237; (b) P. Mukerjee, 9 M. Schick, J . Phys. Chem., 1964, 68, 3585. 628. J. Phys. Chem., 1965,69,4038; (c) A. Ray and G. NCmethy, J . Am. Chem. SOC., 1971,93, 6787. Paper 5 / 1548; Receiued 9th September, 1985