ASHOKARAYAND GEORGEN~METHY
804: but not t o both these nitrogens on the same adenine molecule, Moreover, Ni2+ binding solely to N(9) or N(3) does not explain the low value of k ~ ’ . The possible existence of higher order complexes and linkage isoimers formed in addition to the mono complex would make the rclaxation expression derived for the adenine systtm a simplified, limiting case of a more complicated schenie, However, no information is available on the formation of such complexes in solution, and the limited amount of temperature-jump data does not permit a quantitative consideration of these
factors. This complication might. partly explain the relatively poor fit of observed to calculate times (cf. Table 11). Nevertheless, the relatively low kl’ value would not be appreciably changed if the mechanism were incomplete in this way. Higher order complexes and/or linkage isomers may be present in the nickel(I.1)-hypoxanhhine (IBb) system. If the unresolvable, nonexponential effects for this ligand are the result of a multistep process, then this process may conceivably involve formation of more than one type of complex.
eetrophotometric Method for the Determination iccelle Concentrations1
ALshoka Ray and George Nemethy* The Rockefeller University, New York, New York ~100ll (Received April 18, 1970)
Pddzcation costs assisted by The National Science Foundation
A difference spectrophotometricmethod has been developed for the determination of critical micelle concentrations (cmc’s). It is particularly useful for nonionic detergents. The method is based on observing the red shift, accompanying micelle formation, of the aromatic absorption band of a chromophore which is either part of the detergent molecule or added to the solution as an indicator. The cmc’s so determined are in fair agreement with the values obtained by other available techniques. For detergents having very low cmc’s, the more intense uv difference absorbance bands at still lower wavelengths are shown to be useful.
I~troduct~oi~ Critical mkelle concentrations (cmc’s) of nonionic detergents have most frequently been determined in the past by three different methods,2 viz., from the break in the sLatic surface tension vs. logarithm of concentration curve, from the break in the turbidity vs. concentration curve, and by the iodine-solubilization technique.3 Since cmc’s of nonionic detergents are generally much lower than those of ionic detergents of comparable hydrocarbon chain lengths, the time needed to reach equilibrium in each measurement of surface tension can be several haursS4 The surface tension method thus becomes not only very time consuming, but may involve uncertainties5 due to evaporation from the surface and stagnant layer formation.6 The experimenial difficulhies involved in the precision determination of turbiditiea are well known,l though not insurmountable. The iodine-solubilization technique, although convenient, presumably involves some chemical reactions leading to the formation of hydrogen ioThe Journal of Ph.qa/sicnZ Chemistry, Vol. 76,No. 6 , 1971
dides-l1 and therefore cannot be considered as too reliable. (1) (a) This work was supported by Grants No. GB-5493 and GB8410 of the National Science Foundation. (b) Presented in part before the Division of Colloid and Surface Chemistry at the 157th National Meeting of the American Chemical Society, Minneapolis, Minn., April 1969, and the I V Congress0 Nazionele dell’ Associazione Italiana di Chiinica Fisica, Florence, Dee 1969. (2) P. Becher in “Nonionic Surfactants,” M:. J. Schick, Ed., Marcel Dekker, New York, N. Y., 1967, p 478. (3) S. Ross and J. P. Olivier, J . Phys. Chem., 63, 1671 (1959). (4) P. €1. Elworthy and C. B. MacFarlane, J . Pharm. Pharmacal., 14, lO0T (1962). (5) P. Mukerjee, Advan. Colloid Interface Sci., P , 241 (1967). (6) J. T. Davies and E. K. Rideal, “Interfacial Phenomena,” Academic Press, New York, N. Y., 1961. (7) P. H. Elworthy and C. B. MacFarlane, J. Chem. Soc., 537 (1962). (8) N. A. Allawala and 9. Riegelman, J . Pharm. Sci., 42,396 (1953). (9) W. B. Hugo and J. M. Newton, J.Pharm. Pharmacol., 15, 731 (1963). (10) P. G. Bartlett and W. Schmidt, A p p l . Mierobiol., 5 , 355 (1957). (11) T. Nakagawa in “Nonionic Surfactants,” M. J. Schick, Ed., Marcel Dekker, New York, N. Y., 1967, p 588.
DETERMINATION OP CRITICAL
MICELLE
CONCENTRATIONS
This paper reports the use of a difference spectrophotometric technique for the determination of cmc’s of nonionk detergents. The method is rapid and has a precision of rt:1%, It is based on the observation that the uv abccorption of an aromatic chromophore, either built into t,he detergent molecules, such as in a~ky~phenoxy~p(1ly~t~i~xy)ethanols) or added as a third component of the solution, such as phenol used with alkyl polyethoxy ethmiols, undergoes a red shift upon micelle formation in aqueous solution. The former class of detergents form micelles that are self-indicating, and thus provide mitable systems for studying the effects of various additives such as nonaqueous solvents,1z*18 salts,14etc., on them. For detergeni s having no built-in aromatic chromophores, solubiliaed phenol was used as an indicator of micelle formation. In principle, the phenol-solubilization technique is: aimllar to the past uses of water-soluble dyes16 and Like the latter methods, therefore, it is subject to the criticism that the addition of a third componerc may alter the structure and the stability of the micrlles’ in an undeterminable way, thereby making such methods less reliable. However, one advantage, posriibly minor, is that the phenol molecule is much smaller than the dye molecules, and hence the effects may be smaller with phenol than with dyes. The same would be true for iodine, which is also small. owever, the possibilities of chemical reactions8-ll may render the USE of iodine more objectionable than that of phenol. The difierence spectrophotometric technique has been extended to the determination of the cmc’s of ionic detergents having built-in aromatic chromophores, such as a l k y l p ~ 7 ~ i ~ ihalides, ~ ~ u m and should be applicable to alkyl benzerresulfonates and other similar detergents also. Similarly, the phenol-solubilization technique should, in principle, br applicable to ionic detergents having no intrimic chromophores. The uv abixrption spectra of alkylphenoxy(po1yethoxy )ethanols have been used in a somewhat different manner by LuckP7and more recently by Gsatzer and Beavenls for determining CMC’S. Rehfeld‘lg has rel y determined tbe cmc of sodium dodecyl sulfate by n z e n e - s ~ l ~ ~ i l ~ ztechnique a ~ ~ o n which is somewhat similar to the phenol technique presently described.
A. Materials. Triton X-100 (OPE9-l0) and Triton -102 (OPEli-la) v7ere supplied by the Rohm and aas Go., a r d Igepal CO-630 (NPE9) and Igepal C8-880 (N13PE30) were products of the General Aniline and Film Go The samples were polydisperse with respect to t‘he ethoxy chain lengths. The chain lengths, indicated above, represent mean values, as stated by the manuf actwers The abbreviations OPE9-I0and OPE,,-,, represent p ~er~-octy~~he~t~xy(p~)lyethoxy)ethanols of mean ethoxy ~
805 chain lengths of 9-10 and 12-13, respectively. Similarly, NPES and NPE30 stand for p-tert-nonylphenoxy(po1yethoxy)ethanols having mcan polyethoxy chain lengths of 9 and 30, respectively. The three homogeneous nonionic detergents, noctyl-, n-decyl-, and n-dodecyl(hexaethoxy)ethanol (CsEs,CIOEG, and ClZEs), were highly pure samples obtained as kind gifts from Dr. J. 11.Corkill. Dodecylpyridinium bromide (DPB), a gift sample from Diversey (U.K.) Ltd., was quoted as homogeneous with respect to the alkyl chain length and was recrystallized from ether-acetone mixture before use. Cetylpyridinium chloride (CPC) was ol a practical grade obtained from Matheson Coleman and Bell. Unless otherwise indicated, all detergents were used as obtained without further purification. “Chromatoquality,” reagent grade, ethylene glycol (Matheson Coleman and Bell) was used as obtained, without further purification. B. Methods. 1. Uv Di$erence S~ec~ro~hotometry. (a) Formation of Self-Indicating Micelles. For the detergents having built-in aromatic chromophores, viz. alkylpheiioxy (po1yethoxy)ethanols and alkylpyridinium halides, the following experimental procedure was used. Two carefully matched “split-compartment9’ mixing cuvettes20 were placed in the reference and sample beams of a Cary 14 spectrophotometer (Figure 1). Each of these cuvettes consists of two compartments, A and B, with exactly equal path lengths, I = 0.44 cm. Appropriate solvent (1 ml) was placed in compartment A, and an exactly equal volume of a detergent solution of a known concentration, co, was placed in compartment B of each cell. The net difference absorbance i s zero with such an arrangement, giving the base line (Figure 2 ) . Next, the liquids in the two compartments of the reference cell were mixed t h o r o ~ g h ~by y sealing the top of the cell with parafilm and rocking the cell several times. For eo 2cmc, micelles are present in the sample cell in an effective path length I , as also in the reference cell (after mixing) in an effective path length 21. The concentration of micelles in the sample cell The Journal of Phyrical Ch.emiatrg, V0L 76, ~VQ.6, 1971
ASHOKA RAYAND GEORGE N~METHY
808 i s given, a,s before, by eq 2; that in the reference cell is
I
given by
e": = c0/2
- cmc
(4)
0.I 2
Thus
tc
'
I
AA = Ar(c,*E -- cmr(2l)>= Ae(cmc)Z = constant (5) AAO o 8 i
The difference absorbance should ideally reach a maximat value at eo = 2cmc and should stay constant with further increase in the detergent concentration. Figure 3%indicates that AA values tend to level off at eo > 2cmc, but just as the transition near the cmc occurs over a range of concentrations, so this leveling off is also gradual and not an abrupt one. I n the experh"s described in section 1 ( b ) , the difference absorbance. AA, results from a difference in the optical absorption of those molecules of the phenol which are incorporated into the micelles, after mixing in the sample cell, and that of an equivalent amount of phenol pret,erit in aqueous environment in the reference cell. As is evident from Figure 3b, very little phenol is removed frsm aqueous environment below the cmc. The subsequent rise in the observed AA shows that more and nior'e phenol is incorporated into micelles, as the detergent concentration, c0/2, is increased beyond the cmc. At very high detergent concentrations, a l e v e h g of Ai1 is observed, Presumably, this occurs when most of the phenol present in solution is contained in the micelles The erne's of several nonionic and ionic detergents were determined by one of the two techniques described above. They are summarized and compared with the data obtained by other techniques in Table I. Molecular weightEi of d>PE9-loand OPE12--13were assumed to be 624 and 756, respectively. The cmc dam for ihe three homogeneous pure nonionic detergents, C8EB,Cl0E6,and C I ~ Eobtained ~, by the phenol solubiljzatlion method agree fairly well with the cmc's obtained by the surface tension method.21 This seems to indicade that ineorporation of phenol into the micelles doe:; not alter strongly the cmc's of these detergents. At least for CPp,EOlit is unlikely that more than one phenol molecule is incorporated into each micelle. The difference absorbance at constant phenol concentration (5 >< 10-4 If) continues to increase (Figure 3b) even when the concentration of micelles exceeds that of phenol (assuming thak the aggregation number of these micelles in the presence of phenol is about the same as in water,22viz., 32). owever, the possibility that more than one phenol moiecule is incorporated into micelles
The JouvmE of I'hyaieal (7hemietry, Vol. 76, No. 5, 1971
004
000
~
L__I_---__L-----200
225
250
275
X(m,u)-
Figure 4. The difference absorption spectrum of NPEBin water at 2 5 O , in the wavelength region 196-294 mp" Concentration of NPEg: 1.08 X 10.-4M.
of C10E6and of CltEe cannot be ruled out definitely, due to the much lower cmc (Table I) and the higher aggregation numbersz3 For alkylphenoxy(po1yethoxy)ethanols having very low cmc's, the sensitivity of the present technique was considerably increased by using the difference absorbance bands at shorter wavelengths. Figure 4 illustrates the complete difference spectrum of NPE9 in water a t 25" in the wavelength region 196-294 mp. The cmc's determined for this detergent using the various absorption bands are found to be in 'excellent agreement with each other (Table I). The 220-240 mp difference absorption band was utilized for the determination of the cmc of OPEs-l, also. The cmc values obtained from AA measurements at 232 mp (Table I) agree very well with the corresponding values obtained from AA measurements at longer wavelengths. However, the use of the shorter wavelengths is limited to solvents having low absorbances in these regions. The shorter wavelength absorption bands can be useful in the phenol-solubilization method, too. This would allow the use of much lower phenol concentrations than used presently.
Acknowledgments. We wish to thank Miss Hilda Malodeczky for her competent technical assistance. We thank Dr. J. M. Corkill for his gift of highly pure detergents. (21) J. M. Corkill, 3. F. Goodman, and S. P. Rarrold, Trans. Faraday Soc., 60, 202 (1964). (22) J. M. Corkill, J. F. Goodman, and R. W . Ottewill, ibid., 57,
1627 (1961). (23) R. R. Balmbra, J. S. Clunie, J. M. Corkill, and J, F. Goodman, ibid., 58, 1681 (1982) ; 60, 979 (1964).
