SOLUBILIZATION OF A WATER-INSOLUBLE DYE
Solubilization of a Water-Insoluble Dye.
3611
111
by Hans Schott2 Research Center, Lever Brothers Company, Edgewater, New Jersey
~
(Received M a y 8, 1967)
~~
A method for determining number-average micellar molecular weights (mmw) by dye solubilization previously described3 was based on the fact that the solubilization limit was one molecule of Orange OT per micelle. This had been established with sodium decanesulfonate (SDSO,), dodecyl sulfate (SDS), and two C12ethylene oxide (EO) condensates. This method was extended to micelles of nonionic detergents with larger hydrocarbon cores and to cationic detergents. Light scattering gave the same m w as dye solubilization for n o n y l p h e n ~ l ( E Oand ) ~ ~ dodecyltrimethylammonium bromide (DT.4B) , but tripled the mmw for C18(EO)ls and cetylpyridinium chloride (CPyC), indicating that the latter two detergents solubilized three dye molecules/micelle a t saturation. Dye solubilization is therefore not an absolute method for determining mmw. It can be used to show relative changes of mrnw as a function of solubilized, flexible-chain impurities or concentration of added electrolytes and provides the lowest possible rnmw. hlmw of ionic detergents determined by light scattering were 1.00 or 3.00 times the mmw determined by dye solubilization in the presence of swamping electrolyte. The mmw of the ionic detergents determined by light scattering apparently decreased with decreasing amounts of added electrolyte, becoming smaller, in the case of DTAB, than the mmw determined by dye solubilization. Since decreasing amounts of added electrolyte either had no effect on the mmw of ionic detergents determined by dye solubilization or caused a small increase, a reexamination of the interpretation of light scattering theory for highly charged micelles seems in order. The number of carbon atoms in the micellar hydrocarbon core associated with one solubilized dye molecule was found to decrease in the order: SDSO,, SDS, DTAB, CPyC, C12(EO)16,C12(EO)2s,C1s(EO)ls,nonylphenol(E0)so. This is also the order of increasing size of the polar head groups, indicating that the larger the latter, the more loosely packed are the micelles and the greater their solubilizing power.
Introduction The preceding paper3 described a method for determining number-average micellar molecular weights (mmw), based on work with one Clo and three C12 detergents. It resulted from the finding that, a t saturation, one molecule of the n-ater-insoluble, oilsoluble dye Orange OT was solubilized by each micelle. Critical micelle concentration (cmc) and mmw were calculated from the amount of solubilized dye, which was determined spectroscopically. The mrnw which can be obtained by this method for other detergents might be smaller than those obtained by light scattering or ultracentrifugation for two reasons. (a) I n case of a rather l d e distribution of mmw, the weight-average values are always larger than hhe number-average
values. (b) If each micelle could accommodate two, three, etc. dye molecules, the apparent mmw determined by dye solubilization would be one-half, onethird, etc. of the correct value. The dye solubilization method gave good agreement with light scattering and ultracentrifugation data in the case of two nonionic detergents3 and with data for sodium dodecyl sulfate (SDS) obtained in the presence of swamping e l e ~ t r o l y t e . ~This agreement between (1) Presented a t the 154th National Meeting of the American Chemical Society, Chicago, 111.9 SePt 10-15, 1967. (2) U. S. Forest Products Laboratory, Madison, Wis. 53705. (3) H. Schott, J. Phys. Chem., 7 0 , 2966 (1966). (4) E. w. Anacker, R. M, Rush, and J. s, Johnson, ibid., 68, 81 (1964).
Volume 71, Number 1 1
October 1967
HANSSCHOTT
3612
the weight-average and number-average values, as well as the single, sharp peaks in the equilibrium sedimentation schlieren patterns, indicates that the micelles were monodisperse. For the anionic detergent, the mmw values determined by light scattering were found to decrease as the concentration of added electrolyte d e c r e a ~ e d , ~ whereas the mrnw values determined by dye solubilization were constant at NaCl molarities of 0.1, 0.03, and 0. Since it is highly unlikely that, at saturation, there is less than one dye molecule solubilized by each micelle, the mrnw values obtained by solubilization are the lowest possible values. Rforeover, there are important unresolved questions in the interpretation of light-scattering data for mrnw determination in multicomponent systems in general5 and for highly charged particles in media of low ionic strength in particular.*J Thus, the results of dye solubilization supported Hutchinson's conclusions that reexamination of the light-scattering data indicated independence of the mmw of ionic micelles from concentration of added electrolyte. I t had been found that small amounts of impurities such as hydrocarbons and long-chain alcohols did not affect the solubilization of Orange OTe3 The purpose of the present work was (a) to extend the dye solubilization method to cationic detergents, choosing those stmudiedextensively by Tartars-" and by A n a ~ k e r and ~ ~ (b) ~ ~ to ~ 'examine ~ the applicability of the method to micelles with hydrophobic portions considerably greater than those studied previously.3 Assuming the micelles to be spherical, with a core consisting of a hydrocarbon droplet of the same density as the bulk density of the hydrocarbon moiety of the detergent, the folloxing radii are calculated for the hydrocarbon cores of the detergents of the preceding paper: 17.4 A for l-dodecanol-28 ethylene oxide, Cl2(E0)2s; 18.5 A for Clz(E0)16; 28.1 A for sodium decanesulfonate (SDS03); and 22.6 A for SDS. Since it seemed questionable whether a single dye molecule could preempt a hydrocarbon core much larger than the above, it was of interest to ascertain how large a hydrocarbon core could be filled up by a single dye molecule and how many dye molecules could be contained in the larger cores at saturation. The latter number would then represent the factor by which the mmm determined by dye solubilization would be too small.
Experimental Section Materials. Purification of Orange OT (l-o-tolylazo2-naphthol) has been described. l 4 Both nonionic detergents used were made by General Aniline & Film T h e Joztrnal of Physical Chemistry
Corp. One was the commercial product Igepal CO 880, a branched nonylphenol with 30 EO units, NPh(EO)30, which was purified by ultrafiltration. l4 The other was stearyl alcohol with 14 EO units. It gave cloudy solutions in water a t room temperature, presumably owing to unreacted stearyl alcohol, which cleared up reversibly on warming. The product was soluble in hot ligroin (bp 97-103') except for a small residue identified as sodium phosphate. On cooling, 28% of the detergent remained in solution while 72% precipitated. The former fraction contained 6.8 EO by nmr analysis of the hydroxyl proton. The major fraction, which contained 18.5 EO and gave clear solutions in cold water, Cts(EO)ls, was used for the present work. Removal of residual ligroin by heating under reduced pressure of nitrogen introduced no carbonyl groups that could be detected in the infrared spectrum of an 8% solution in tetrachloroethylene. Hexadecyltrimethylammonium bromide (CETAB), technical grade of blatheson Coleman and Bell, was twice recrystallized from acetone containing about 5% water and once treated with ethyl acetate. Cetylpyridinium chloride (CPyC) and dodecyltrimethylammonium bromide (DTAB) were made by Fine Organics, Inc. (Lodi, K, J.). Both were recrystallized twice from acetone containing some ethanol and once from benzene. Purified CPyC contained 10.27% C1 and 4.08% N (theory 10.43 and 4.12%, respectively), while DTAB analyzed 25.67% Br and 4.39% S (theory 25.92 and 4.54%) respectively). Methods. The procedures were t h e same as before3 with the following two exceptions. Solutions used for light scattering and for absorbancy measurements were filtered through Millipore filters GS (mean pore size 0.22 p ) instead of ultrafine sintered-glass filters and plugs of cotton. Comparison of the filter media showed that the Millipore filters were equivalent to the other two. lIoreover, they eliminated the need for cleaning glass filters and afforded much speedier filtration than cotton plugs. The filters sorbed some Orange OT, so that the initial portions of the filtrates were colorless ~~
________
~
(5) R. K. Bullough, Proc. Roy. SOC.(London), A275, 271 (1963). (6) E. F. Casassa and H. Eisenberg, Adzan. Protein Chem., 19, 287 (1964). (7) J. P. Kratohvil, L. E. Oppenheimer, and M. Kerker, J . P h y s . Chem., 70, 2834 (1966). (8) E.Hutchinson, J . Colloid Sci., 9, 191 (1954). (9) H.V. Tartar, J . P h y s . Chem., 59, 1195 (1955). (10) H. V. Tartar and A. L. M. Lelong, ibid.,59, 1185 (1955). (11) H.V. Tartar, J . Colloid Sei., 14, 115 (1959). (12) E.W.Anacker, J . Phya. Chem., 62,41 (1958). (13) E. W.Anacker and H. M.Ghose, ibid.. 67, 1713 (1963). (14) H.Schott, ibid., 68,3612 (1964).
SOLUBILIZATION OF A WATER-INSOLUBLE DYE
but became readily saturated with the dye. The practice of discarding the first SO ml of the filtered solutions prevented further loss of solubilized dye. The light-scattering photometer was recalibrated with sucrose solution^'^ using the technique of Princen.''j The Rayleigh ratio for benzene was found to be 48.2 X cm-' a t 436 mp, in good agreement with published ~ a 1 u e s . l ~Since the light scattering apparatus had no temperature control, the temperature of the solutions varied between 26 and 30" during turbidity measurements.
3613
CETAB
(0.0130M KBr)
3.0 A/L
crn-'
2.0
1.0
Results Molar Extinction Coeficient. The molar extinction coefficient e of Orange OT was determined by adding small, measured volumes of acetone solutions of the dye to 2.0% detergent solutions and measuring the ab= ELC, wherelo is the reference sorbancy A = log (lo/l) intensity, measured on the plateau at 650 mp, I is the intensity at the absorption maximum around 500 mp, L is the path length in centimeters, and c the molar concentration. The absorption spectrum in the visible range of Orange OT solubilized by cationic detergents resembles that of the dye solubilized by anionic detergents. The strongest absorption is at 499 my, Jvith a weaker maximum appearing as a shoulder centered apparently at 522 mp. Figure 1 is a Beer's law plot showing that dye solubilized by three cationic detergents had the same extinction coefficient. The mean value of e, calculated by the method of least squares as the slope of the straight line going through the origin, is 1.872 X lo4 l./mole cm, the standard deviation of the mean is 0.012 X lo4, and the range of values is 4=0.034 X lo4. Dye solubilized by the two nonionic detergents had the same e as reported for the other
/
t 1.00 A/L,
cm-'
-E
't ,
0 DTAB
A CPyC
0.0I
i
10-6
,
, ,
/ 1 # , [
IO-^
I
I
I
I , , I , t
I
I
I d-4 D Y E CONCENTRATION, mole/l.
,
1.0
0
2.0
3.0
4.0
5.0
7.0
6.0
DETERGENT CONCENTRATION, g/l.
Figure 2. Absorbancy of nonionic and cationic detergent solutions saturated with Orange OT. Solid symbols represent mixtures prepared by diluting more concentrated solutions saturated with dye.
M NaCl
15
10
05
-
0 A
DTAE CPYC
0 IO
0
20
30
DETERGENT
40
50
60
70
CONCENTRATION, g / l
Figure 3. Absorbancy of cationic detergent solutions saturated with Orange OT. Solid symbols represent mixtures prepared by diluting more concentrated solutions saturated with dye.
nonionic detergent^,^^'^ namely, 1.740 X lo4 l./mole cm. Critical Micelle Concentrations. The cmc values listed in Table I were determined by dye solubilization. The absorbancy vs. detergent concentration plots of solutions saturated with dye (Figures 2 and 3) were linear, and their extrapolated intercepts with the abscissa mark the cmc. The constants in the equation of these linear plots (eq 1)
CETAB
p'
0
I , , .
~~
12-3
Figure 1. Beer's law plot for Orange OT solubilized by cationic detergents, on a double logarithmic scale.
~
(15) 8. H. Maron and R. L. H. Lou, J . Phys. Chem., 59, 231 (1955). (16) L. H. Princen, Thesis, University of Ctrecht, 1959.
(17) B. A. Brice, 768 (1950).
M .Halwer, and R. Speiser, J . O p t . SOC.Am., 40,
Volume 7 1 , Number 11 October 1967
HANSSCHOTT
3614
Table I : Critical Micelle Concentrations and Micellar Molecular Weights -----standard
Cmc, g/l.
Nonionic detergents at 25' CdE0)18 (water) NPh(EO),o (water) CETAB at 34" 0.0130 M KBr
Micellar molecular weights, deviation,O and range,b determined by--Dye Light solubilizationC scattering
0.134 0.4259
36,975 f 205 ( f 7 4 3 ) 55,697 f 106 (1397)
110,660 f l 8 2 0 56,930 1770
0.4347
17,628 f.104 ( f 4 3 7 )
See text for lit. values
4.627 1.2886 0.5152
30,442 f 71 ( 2 ~ 2 0 2 ) 28,821 f 208 (h618) 25,578 f 106 (f288)
0.0797 0.0171
14,874 f 8 5 ( f 2 7 2 ) 13,762 f 17 (f50)
1
*
DTAB at 28' Water 0.1001 M NaBr 0,510 M NaBr CPyC at 30" 0.01754 M NaCl 0.4382 M NaCl
' Of the mean.
* In parentheses. c = cmc
Mmw calculated as bLe.
+ b(A/L)
(1)
were calculated by the method of least squares. The solubility of Orange OT in water was found to be less than 2 X mole/l. It was somewhat larger in some detergent solutions below the cmc but still negligible compared to the solubility above the cmc. The crnc values listed in Table I agree within 10% with those reported for NPh(E0)30,18for DTAB in water,4,10,19-22 in 0.10 M ?;aBr,4 and in 0.51 M NaBr,4J3 and for CPyC in 0.44 M NaCl.l2 The only significant discrepancy between published cmc values and those of Table I is in the case of CPyC in 0.0175 M NaCl, where the present value, 0.0797 g/l., is considerably larger than the reported 0.0408 g/l. figure of Anacker.12 Since the crnc value of CPyC in 0.44 M NaCl of Anacker12 and that of Table I agree within 5010, it is interesting to ascertain which of the two values in 0.0175 iM NaC1 is consistent with Hartley's cmc value for CPyC in water, namely, 0.306 g/l.23 Linear interpolation of log crnc V S . the logarithm of the total gegenion c ~ n c e n t r a t i o ngives ~ ~ a cmc value of 0.0748 g/l. in 0.0175 M NaC1, which agrees with the value of Table I within 6%. Micellar Molecular Weights from Amount of Solubilized Dye. The following two assumptions are made to calculate mmw from absorbancy of detergent solutions saturated with dye: (a) one dye molecule is solubilized per micelle; (b) the concentration of detergent not associated into micelles is constant and equal to the cmc. Then, mmw equals be, with b defined by eq 1. The second assumption is not strictly but this causes only very small errors in mmw. As discussed above, errors in the first assumption lead to T h e Journal of Physical Chemistry
decreases in the apparent mrnw by integral factors equal to the number of dye molecules solubilized per micelle. As is seen in Table I, the mmw of C18(E0)18determined by light scattering is greater than that determined by dye solubilization by a factor of 2.99, indicating that there are three dye molecules solubilized per micelle. The following alternate possibilities to explain this discrepancy were considered and discarded. (a) The dye solubilization experiments were made at 25.0 f 0.2" whereas turbidities were measured between 26 and 30°, and it is known that increasing temperatures cause the turbidity of nonionic detergent solutions to rise, especially near the cloud point. However, the turbidity of a 4.432-g/1. solution of Cla(E0)Ig was the same at 22.5 and 29". The absorbancy of this solution saturated with dye increased by only 7% in going from 25 to 30". The half-filled circle in Figure 2 represents the absorbancy of the solution saturated at 30". (18) L. Hsiao, H. N. Dunning, and P. B. Lorenz, J . Phys. Chem., 60, 657 (1956). (19) A. B. Scott and H. V. Tartar, J . Am. Chem. Soc., 65, 692 (1943). (20) H. B. Klevens, J. Phye. Colloid Chem., 52, 130 (1948). (21) P. Debye, Ann. N. Y . Acad. Sci., 51, 575 (1949). (22) H. J. L. Trap and J. J. Hermans, Koninkl. S e d . A k a d . Wetenschap., Proc., Ser. B , 58, 97 (1955). (23) G. S. Hartley, J . Chem. Soc., 1968 (1938). (24) K. Shinoda, T. Nakagawa, B. Tamamushi, and T . Isemura, "Colloidal Surfactants," Academic Press Inc., New York, N. T., 1963. (25) P. Elworthy and K. J. Mysels, J . Colloid Interface Sci., 21,331 (1966).
SOLUBILIZATION OF A WATER-INSOLUBLE DYE
(b) Nost of the detergent solutions used to measure turbidity were more concentrated than those used for dye solubilization measurements, and higher concentrations often lead to larger micelles, culminating in gel formation. However, the range of concentrations overlapped (1A35-6.94 g/l. for dye solubilization and 2.000-16.61 g/l. for light scattering.) Moreover, the absorbancy us. detergent concentration plot (Figure 1) and the Debye plot were linear, indicating that the mrnw wae constant and independent of detergent concentration in the range studied. (c) The difference between the cmc values determined by solubilization (0.134 g/l. at 25") and by light scattering (0.20 f 0.07 g/l. a t 26-29") is much too small to account for the discrepancy in mmw. (d) Filtration of the solutions through Millipore filters did not reduce the detergent concentration through micelle retention, since the solids content of the filtered solutions were routinely compared to the initial concentrations as weighed out, and the largest difference ever found was 1% of the concentration. The mmw of NPh(EO)30determined by light scattering and by dye solubilization are identical within the accuracy of the measurements, since the difference is smaller than its standard deviation. As noted prev i o ~ s l y the , ~ precision of the mrnw determination by dye solubilization is an order of magnitude better than that of the light-scattering method. The following aggregation numbers have been reported for CETAB by light scattering: Tartar," 79.6 (water, 30"); Debye,?I 169.3 (0.0130 -11 KBr, 30"); Trap and Hermans,22207.4 (0.0125 JP KBr, 30") and 270.0 (0.025 AP KBr, 30"). These values are all larger than the apparent aggregation number found by dye solubilization, 48.4 (0.0130 A1 KBr, 34"), indicating that each CETAB micelle holds more than one dye molecule a t saturation, probably four or five. However, at higher salt concentrations, dissymmetry measurementsZ6gave aggregation numbers of 2181 (0.178 1%' KBr, 34") and 5103 (0.233 M KBr, 34") while ultracentriguati~n~~ gave 3375 (0.2 M KBr, 30") and 6119 (0.4 M KBr, 30"), both on the assumption of rodshaped micelles. This may be considered incipient precipitation by salting out. The aggregation numbers found for DTAB by dye solubilization at 28" were (Table I): 98.7 (water), 93.5 (0.100 J1 SaBr), and 83.0 (0.510 AP NaBr). Published values, obtained by light scattering, include: Tartar and Lelong,'O 50.0 (water, 23"); Debye,2150.3 (water, 30") and 56.4 (0.0130 M KBr, 30"); Trap and Hermans,2262.3 (water, 30"), 65.4 (0.0125 M KBr, 30"), 77.2 (0.025 M KBr, 30"), and 108.3 (0.050 M KBr, 30"); hnacker and Ghose,l3 84 (0.500 JP NaBr, 31").
3615
Anacker, Rush, and Johnson4found: 61 and 52 (water, 25"); 74 and 71 (0.100 AP IYaBr, 25"); 90 and 86 (0.502 M KaBr, 25"); 86-89 and 79-51 (0.510 M NaBr, as"), where the first aggregation numbers of each pair of values were calculated for the best value of x (micellar charge) and the second numbers were calculatedforz = 0. The agreement between mrnw from dye solubilization and from light-scattering measurements is good a t higher concentrations of added salt, where the interpretation of the turbidity measurements is best under~tood.~JThis indicates that each DTAB micelle solubilizes one dye molecule at saturation. The aggregation numbers of CPyC found by dye solubilization at 30" were 43.7 in 0.01754 111 S a C l and 40.5 in 0.4382 M S a C l (Table I). hnacker obtained 95 and 92.5 (0.01754 M YaCl, 31"); 117 and 114 (0.05843 M SaCl, 31"); 135 and 130 (0.3382 &I SaCl, 31"); 137 and 131 (0.7304 2k.f h'aC1, 31") by light scattering.I2 The diffusion measurements of Hartley and Runnicles,28 made in CPyC solutions containing between 0.1 and 1.0 N IYaC1, showed independence of micellar size from KaCl concentration within i=5%; observed radii were in the 26-28-h range. Using the mean radius of 27.2 A gives an association number of 134; water of hydration was neglected on the assumption that steric hindrance of the nitrogen prevents extensive hydration. The mrnw determined by diffusion agrees with that determined by light scattering in solutions containing the highest salt concentrations and is three times larger than the mmw determined by dye solubilization. This indicates that the CPyC micelles solubilize three dye molecules at saturation. As in the case of DTAB, light-scattering measurements indicate a sizable increase in mmw with increasing concentration of added electrolyte. while dye solubilization indicates a small decrease. The diffusion measurements show no trend, which is not necessarily in conflict with the dye solubilization measurements since the precision of the diffusion measurements is probably too small to detect a 7-S% decrease: an uncertainty of i5% in the radius corresponds to an uncertainty of ,tl2Y0 in the mmw.
Discussion Solubilizing Power of Detergents towaiV? Orange OT. The extremely low solubility of Orange OT in water, considered together with its moderate solubility in sol(26) P. Debye and E. W. Anacker, J . Phys. Colloid Chem., 5 5 , 644 (1951). (27) K. Granath, Acta C h ~ m Scand., . 7, 297 (1953). (28) G. S. Hartley and D. F. Runnicles, Proc. Roy. Soc. (London), A168, 420 (1938).
v o h m e 71, Number 11
OCtObEr 1967
HANSSCHOTT
3616
ventas of medium and low polarity, indicates that the hydroxyl group of the naphthol ring is internally hydrogen bonded to the azo group. This is favored by the proximity of the two groups. The most likely location for the solubilized dye is in the hydrocarbon interior of the micelle^.^^,^^ Table I1 summarizes the observation that a small, discrete, integral number of dye molecules is solubilized by each micelle, the number being characteristic of the detergent. This is in contrast to most other solubilizates, many molecules of which can be solubilized by each micelle and which swell the micelles in the process. The explanation for the difference in behavior lies probably in structural dissimilarities. AIost of the ~~
Table 11: Limits of Solubilization and Solubilitv of Orange OT I
Detergent
Radius of micellar hydrocarbon core, A
SDSOi SDS CI~(EO 115 Ci?(EOh NPh(E0)aO Cis(E0)ia DTAB CETAB CPyC
28.1 22.6 18.
