1554
M. E. GINNAND J. C. HARRIS
Vol. 62
EFFECTS OF PRACTICAL SURFACTANT C,OMPOSITIONSUPON CRITICAL MICELLE CONCENTRATION MEASUREMENTS BY THE SPECTRAL DYE METHOD BY M. E. GINNAND J. C. HARRIS Monsanto Chemical Company, Research and Engineering Division, Research Department, Dayton, Ohio Received June 20, 1968
The spectral dye method is demonstrated as sensitive t o surfactant-dye complex age, and t o unsulfonated oil present in commercial dodecylbenzene sodium sulfonate. The solubilizing character of the oil-containing surfactant appears to account for anomalous results by the spectral dye method between commercial and purified surfactant, and compositions t o which foam stabilizing agents were added. Methods for measurement of critical micelle concentration not affected by these factors are to be preferred when commercial surfactants and their combinations with detergent adjuncts are to be investigated.
Introduction Methods for evaluation of surfactants have been largely empirical and frequently measure several values rather than an individual characteristic of the compound. Desirable would be measurement of a single specific surfactant characteristic indicative of surface activity. Such a measure appears t o be that of critical concentration (c.m.c.) since direction in improvement of surfactant structure and composition can be predicted from these values. Precise work requires that the surfactant under test be a single entity, completely free from impurities. It is likely that some of the apparent variation in c.m.c. values reported has as its source deviation in one or the other or both of these factors. Critical micelle concentration can be determined by a variety of measurements, the spectral dye method,l+ because of its apparent simplicity of operation, frequently being used. However, any measurement that requires visual interpretation of a gradual color change is subject t o personal error, which may account further for some of the variation reported. Commercial surfactants, and even research products prior t o considerable purification, contain major or minor amounts of electrolytes, and greater or lesser proportions of unreacted or unreactable materials. Fatty alcohol sulfates after the sulfonation step invariably contain unreacted fatty alcohol, and alkylbenzene sulfonates contain unreacted alkylbenzene and unsulfonatable oils. I n both materials, these unreacted constituents affect c.m.c. values and for precise estimation must be removed. Furthermore, commercial products most frequently are not single entities, but represent a product range; so-called dodecylbenzene sulfonate, for example, may contain a mixture of decyl- to tetradecyl homologs. All of these factors affect the sharpness of the change occurring a t the critical concentration. Because the literature has not been specifically concerned with commercial surfactants, this paper records our refinements of the spectral dye method, in the hope that subsequent investigators using the technique may minimize time spent in procedure (1) M. L. Corrin, H. B. Klevens and W. D. Harkins, J . Chem. Phye., 14, 480 (1946). (2) M.L.Corrin and W. D. Hrtrkina, J . Am. Oil Chem. Soc., 69,679 (1947). (3) M.L. Corrin and W. D. Harkins, ibid., 69, 683 (1947).
standardization, A further objective is t o show the effect of the free oil in commercial dodecylbenzene sodium sulfonate on c.m.c. determination, and t o indicate the effect of certain detergent additives in the presence and absence of the free oil. Materials and Equipment Used.-Spectronic 20, a diffraction grating type spectrophotometer by Bausch & Lomb, equipped with l/z and 1-inch test-tubes. (Precision specified to within 0.5% in transmission reading.) Fisher Illuminator, with fluorescent lamps for yisual comparison of pinacyanol surfactant solutions, from Fisher Scientific. Impedance Bridge, ESI-Model 250 DA, cell constant = 0.1 (approx.) from Electro-Measurement, Inc. Cenco-duNouy interfacial tensiometer, equipped with a 70-ml. cylindrical dish for housing the test solutions. Thermometers-Beckmann differential thermometers; and normal thermometer, graduated in 0.1' and calibrated from -0.7 to +100.7". Water-baths-one a t 24.85 f 0.0024' for the photometric spectral dye method and one at 25 =t0.006' for the conductivity measurements. Constant temperature room-at 25 f 0.5' for surface tension measurements. Water-Deionized to 2 X 108 ohms resistance for the spectral dye and conductivity methods. Double distilled water for surface tension measurements.
Variables Influencing Spectral Dye Method.This widely accepted method has been well charact e r i ~ e d , l - ~pinacyanol chloride (pure) being the dye used t o follow changes in aggregation of surfactant. The color changes from red t o blue in going from below to above the critical concentration; it is purple a t the c.m.c. Color changes are attributed1-3 t o dye solubilization, the degree of solubilization increasing in the micellar state t o the blue color characteristic of the dye in nonpolar solvents. Much work has been done in establishing the absorption intensities of surfactant-pinacyanol chloride complexes,1t4three band maxima being apparent. The a- and @-bands are characteristic of the dye in solvent (blue) and the y b a n d (red) in aqueous media. Slight variations in peaks in these bands will be found; for our work absorption was measured a t 615 mp. Color intensity can affect spectrophotometric measurements by reducing apparent sensitivity. We found that 0.5, 1.0 or 2.0 X 10-6 M pinacyanol yielded the same c.m.c. for sodium oleate. 1.0 x 10-6 M dye concentration was most satisfactorily transmittant and was accordingly adopted for the photometric measurements. A level of (4) S. H. Herzfeld, TIIIBJOURNAL,
66, 963 (1952).
4
&C.,
1958
EFFECTS OF PRACTICAL SURFACTANT COMPOSITIONS ON C . M . C .
MEASUREMENTS 1555
TABLE I ANALYSESOF SURFACTANTS Surfactants
Sodium oleate A s recd. Purified
(%)
Property
Alcohol insol. 0.0 NaCl 0.94 Ash content ... Carbon 68.68 10.77 Hydrogen Sulfur (total) none Unsulfated or unsul... fonated matter6 Estd. active content 97.20 Estimated values, b Neutral oil-indicated R-C=CH-R, I
alkylbenzenes of the type By difference. By two-phase titrations.
Sodium lauryl sulfate As recd. Purified
Sodium dodecylbenzene sulfonate As recd. Oil-free
4.9 0.20 26.59 49.09 8.70 10.08 6.P
1.22 0.38 22.16 59.33 8.21 8.94 2.08b
...
(1.0)" 23.76 66.92 10.41
... ...
...
0.28 24.56 50.75 9.03 10.18 4.8"
(1.24)a 0.54 22.65 58.82 8.03 9.65
...
96.3c 98.2c 88.1d 95.2d 99.00 by infrared and ultraviolet absorption analysis t o contain a mixture of CHa
where R contains
5 X M dye concentration was generally more satisfactory for the visual method. Agreement on the effect of aging upon optical densities of pinacyanol-anionic surfactant solutions seems lacking : One experimenter4 specified a three-minute aging period, another5 two hours, and still a third6 indicated no change up t o 24 hours when stored in the dark. That NaDDBS is relatively unaffected by aging time is shown in Fig. 1, in contrast with the sensitivity of sodium oleate or sodium lauryl sulfate: sodium pleatepinacyanol complex shows a fairly sharp maximum in color intensity after 30 minutes aging, while extreme variation is obtained with sodium lauryl sulfate in any aging period less 'than 30 minutes. Aging time seems inversely related t o molecular weight, the shorter chain-length surfactants requiring longer aging periods for maximum color development or solubilization. However, a 30minute aging period can be accepted for a t least the three surfactants shown, and was selected for subsequent work. Other surfactants have caused serious pinacyanol color fading, even in 10 minutes time, so that an aging period suited t o the surfactant must be chosen. The combination of a relatively broad range of components and neutral oil tends t o obscure the critical concentration, and apparently smooth curves result from the plots of the data. It was possible t o determine the critical concentration in these instances by application of a rate of change technique,' where change in optical density is calculated as a function of the change in concentration. The c.m.c. values in Fig. 1 were obtained in this manner. One uncertainty of the visual method is the endpoint, since the color change is continuous and gradual. Greater differentiation was found using a pinacyanol concentration of 5 X 10-6 M , and direct comparison with a purple obtained with the surfactant a t its c.m.c. (as determined by another, (5) P. Mukerjee and K. Mysels, J . Am. Chem. Soc., 77, 2937 (1955). (6) H. B. Klevens, THISJOURNAL,61, 1143 (1947). (7) I. M . Kolthoff and E. B. Sandell, "Textbook of Quantitative Inorganic Analysis," 3rd Edition, The Macmillan Co., New York, N. Y., 1952,p. 488.
-6-,I
aliphatic hydrocarbon and small quantities of sulfones.
bH8 1.6 1.4 1.2
."g 1.0 3 Y
-
4 0.8
."$
0
0" 0.6 0.4 0.2
0
0.02 0.04 0.06 0.08 0.10 0.12 0.14 % concn. (solids basis). Fig. 1.-Effect of time on the c.m.c. of sodium dodecylbenzene sulfonate.
more absolute method). Where the visual color change is not sharp, use of the photometric procedure is indicated. Experimental Data Comparison of Values.-Shown in Table I1 for comparison purposes, and to demonstrate the effect of impurities, are the c.m.c. values for sodium oleate, sodium lauryl sulfate and eodium dodecylbenzene sulfonate (NaDDBS) by spectral dye and conductivity methods. The values for the purified materials by the conductivity method compare fairly well with literature values but the spectral dye values do not. Removal of extraneous materials such as inorganic electrolytes and unsulfonated materials for maximum purity is obviously necessary for agreement between investigators. Also apparent is the considerable divergence between the methods, though removal of unsulfonated oil from NaDDBS improved agreement. Table I1 indicates a threefold increase in c.m.c.
AT. E. GINNAND J. C. HARRIS
1556
Vol. 62
TABLE I1 COMPARISON OF “c.m.c.” BY THE SPECTRAL DYEAND CONDUCTIVITY METHODS WITH LITERATURE VALUES
Sample
Condition
C.m.0.
Spectral dye Visual Photometric titration, % % concn. Concn.’ YO?
Conductivity
%
Conon.
...
io8
Lit. values
% Concn.
Af X 10:
Ratio of c.m.c.’s (cond./ sp. dye)
Sodium oleate As recd? 0.022 0.72 0.076 2.64 0.024-0.07 0.8-2.3 3.5 (Fisher Purified) Sodium lauryl As recd. ... .021 0.73 .162 5.63 ... ... 7.7 sulfate Pursed .047 1.63 .116 4.03 .13 -0.14 4.5-5.0 2.5 Sodium dodecylAs recd. 0.07 .041 1.18 .153 4.40 .03 -0.153 0.9-4.39 3.7 benzene sulOil-free 0.132 .110 3.17 .132 3.80 1.2 fonate Per cent. concentration by weight. All samples run on dry basis. 7.0 The percentage increase in c.m.c. on purification
...
II
is 6.0
Photometric spectral dye Visual titration Conductivity Surface tension
5.0 X h
J 2 d 2
4.0
3 3.0 8 0
”8, 2.0
rn a
0 - As Received
P
- Oil-Free
1.0
0
Fig.2.-Effect
0.12
0.24
% concn.
0.36
0.4
of neutral oil on the c.m.c. of sodium dodecylbenzene sulfonate by conductivity.
44 A
ea 40 d h
?,
.-
3
$35 % 2
3 rn
30 0.
0.10
0.20
0.30
0.40
% concn. Fig. 3.-Effect of neutral oil on the c.m.c. of sodium dodecylbenzene sulfonate by surface tension..
values for the spectral dye method when the cornpound is purified, whereas a slight reversal is Shown by the conductivity method. Surface tension measurements for c.m.c. give values of NaDDBS Of 0*123 and O*l0%, for the as received and oil-free product.
168% 89 14 - 19
-
Influence of Neutral Oil.-Comparison of the visual and photometric spectral procedures with increasing concentrations of neutral oil show only differences in magnitude and not in the trend in c.m.c. values for the NaDDBS reconstituted neutral oil compositions. According to spectral dye determinations, neutral oil reduces c.m.c., in contrast to its effect as determined conductometrically (Fig. 2,). Surface tension data substantiated the conductivity measurements, varying only slightly in magnitude. Surface tension minimas have been attributed to interaction between uiisulfated or unsulfonated materials and fully sulfonated surfactant (e.g., lauryl alcohol and sodium lauryl sulfate). The surface tension minimum (Fig. 3) for the oil-free NaDDBS cannot be ascribed t o unsulfonated residue, hence probably is caused by lower chain sodium alkylbenxene sulfonates once the critical concentration has been exceeded. The effect of the unsulfonated oil completely obscures the sharp minimum found in the oil-free product and helps partially to explain the effect of oil-containing surfactant on the spectral dye procedure. Of the methods for measurement tested, the spectral dye procedure is the only one strongly sensitive t o neutral oil, hence is a poor choice for‘ use with those surfactants normally contaminated with oily materials. Influence of Additives on C.m.c.-Detergent adjuncts, such as sodium tripolyphosphate (STP, technical) and lauric diethanolamide (LDEA, technical), have been shown by o t h e r ~ ~ generally .~J~ to reduce the c.m.c. of surfactants (specifically alkylbenzene sulfonates). However, in our work (Table 111) using the spectral dye method, STP had no effect and LDEA actually increased the c.m.c. of “as received” NaDDBS. Conductivity and surface tension measurements, on the other hand, gave the expected decreases in c.m.c. for both additives. The decrease in c.m.c. by LDEA was shown by ( 8 ) c. D. Miles, THls JonRNaL, c9, 71 (1945). (9) 8. Ross and T. H. Brarnfitt, ibid., 61, 1261 (1957). (10) M. J. Schick and F. M. Fowkes, ibid., 61, 1062 (1957).
II
c
Dec., 1958
EFFECTS O F PRACTICAI,
SURFACTANT
COMPOSITIONS ON C.M.C. MEASUREMENTS 1557
TABLE I11 EFFECTOF ADDITIVES O N THE %.m.c." OF SODIUM DODECYLBENZENE SULFONATE AS MEASURED BY VARIOUS METHODS % Concn. of
Composition
NaDDBS Alone 79y0 NaDDBS 21% Sodium tripolyphosphate 79y0 NaDDBS 21% Lauric Diethanolamide 0 Approximate value.
Spectral dye (photometric)
0.04
as recd. NaDDBS at the "c.m.c."
Conductivity
0.153
i
p-*-----
1.5 I
Surface tension
0.123
.04
...
.110
.052
0.067
.021'
the spectral dye method using oil-free NaDDBS (Fig. 4). This reversal in effect was again attributed to the peculiar influence which neutral oil has on the spectral dye method. Since a c.m.c. decrease denotes improved lather stability while an increase in c.m.c. suggests reduction in lather,gthe anomalous situation existed that I I lauric diethanolamide acted as a foam stabilizer for 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 the deoiled NaDDBS but as a foam inhibitor with yo concn. the unpurified surfactant. Past performance tests Fig. 4.-Effect of lauric diethanolamide on the c.m.c. of and other c.m.c. data have shown that this is with and without neutral oil; photometric specnot the case. Therefore, use of the spectral dye NaDDBS d e method: 0,as received NaDDBS. @, 79y0 as remethod t o predict the foam stabilizing activity of tral ceiveJNaDDBS-21% lauric diethanolamide (LDEA); m, additives for surfactants as was done recently by oil-free NaDDBS; m, SO% oil-free NaDDBS-20% LDEA. other workers1' is not reliable when impurities are That neutral oil affects the solubilization of other present. Schick and FowkeslO showed consistent decreases in c.m.c. for surfactants by the visual dyes was ascertained by measuring the solubility titration spectral dye method on introducing foam of dimethylaminoazobenzene (DMAB, m.p. 114stabilizers such as laurylethanolamide. However, 115") in 0.1% solutions of unpurified and purified the surfactants used apparently were highly puri- NaDDBS when a considerably greater quantity fied (e.g., sodium 2-n-dodecylbenzene sulfonate of DMAB was eolubilieed by the oil-containing and sodium dodecane 1-sulfonate), and as shown NaDDBS. At this point, one may speculate concerning above when impurities normally found in commercial surfactants are present, completely mis- solubilization in the spectral dye method. I n this method a dye-detergent complex is leading results can, be obtained. Since greatest practical interest lies in unpurified (as produced) formed below the c.m.c. which becomes insoluble surfactants, use of the spectral dye method t o a t higher concentrations. At the low level inscreen foam stabilizers may produce questionable volved the salt appears to be completely dissolved. However, other workers have shown6that the salt values. Anomalous spectral dye results cannot be at- is actually in a suspended state and that protributed wholly t o the influence of the dye since longed standing or centrifuging results in flocculathe neutral oil effect is independent of pinacyanol tion or precipitation. The suspended salt must concentration. Conductivity and surface tension only be solubilized slightly in order to give the methods are not distorted by the presence of neu- characteristic blue color of pinacyanol in organic tral oil (see Fig. 3), not because pinacyanol is solvents (or micellar solutions). The method's absent, but rather because these methods do not sensitivity t o minute amounts of solubilizing imoperate through a solubilization mechanism1~11~12 purities, i.e., neutral oil, is therefore understandable. as does the spectral dye method. Acknowledgment.-We are indebted t o our co(11) J. W. McBain and Sister A. A. Green, J . Ana. Chena. Soc., 68, workers, E. L. Brown, C. L. Church and F. B. 1731 (1946). (12) I. M.Kolthoff and W. Stricks, THISJOURNAL, 63,915 (1948). Kinney, for much of the data presented. I
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