May, 1956
CRITICAL MICELLE CONCENTRATIONS OF NON-IONIC DETERGENT%
657
CRITICAL MICELLE CONCENTRATIONS OF POLYOXYETHYLATED NON-IONIC DETERGENTS' BY LUNHSIAO,H. N. DUNNING AND P. B. LORENZ Surface Chemistry Laboratory, Petroleum Experiment Station, Bureau of Mines, U.S. Dept. of the Interior, Bartlesville, Okla. Received November 18, 1866
The critical micelle concentrations of non-ionic detergents composed of nonylphenol and various ethylene oxide chain lengths have been determined by the surface tension method. Critica.1micelle concentrations increase with ethylene oxide chain length and decrease with increasing electrolyte concentration. The effect of chain length on critical micelle concentration may be represented by the expression, In (CMC) = 0.056R k, where CMC is expressed in molarity R is the average number of ethylene oxide units in the chain, and k is a constant depending on the type and concentration of electrolyte. Comparison of the critical micelle concentration depressions with the lyotropic numbers of the anions indicates that these detergents form hydrophilic micelles that have associated weak positive charges.
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and interfacial tension." Non-ionic detergents afford an unusual opportunity to study micelle formation. The hydroMaterials and Methods philic portion of the non-ionic detergent molecule The detergents used were Igepals (condensate products is generally larger than the hydrophobic part, of alkyl phenols with ethylene oxide) supplied by the General while it is much smaller with ionic detergents. Aniline & Film Corp. The samples were mixtures of I n addition, the absence of a distinct electric different molecular species varying in their ethylene oxide lengths. The major part of the investigation was charge on the micelles formed by non-ionic deter- chain conducted with four such detergents containing nonylgents eliminates a factor that complicates the phenol, and with averages of 10.5, 15, 20 and 30 units of interpretation of data obtained with ionic micellar ethylene oxide. These will be designated by NR-10.5, systems.2 The critical micelle concentrations NR-15, NR-20 and NR-30, respectively. Three other detergents were used less extensively; two containing nonyl(CMC) of non-ionic detergent solutions may be phenol, with averages of 9.5 and 100 units of ethylene oxide expected to be lower than those of ionic de- (NR-9.5 and NR-100); and one containing oct Iphenol, tergents because there is little or no electrical with an average of 8.5 units of ethylene oxide OR-8.5). force resisting aggregation of the non-ionic mole- These detergents are commercially available, except for NR-100. They contain 99% active ingredient, with water, cules. a salt,12 and unreacted nonylphenol or ethylene oxide as Detergents commonly are used in fresh water possible impurities. Other chemicals used were analytical systems for household or industrial use. How- reagent grade. ever, recent work has indicated that non-ionic detergents may be valuable additives to water injected into petroleum productive formations to increase the efficiency of petroleum p r o d ~ c t i o n . ~ This injected water and the water indigenous to these formations commonly are strongly saline. 1-15 NK-LU Therefore, the effects of detergent composition and .PLI-.-o-. n NR-10.5 of added electrolytes on the surface tensions and 1 -" critical micelle concentrations of non-ionic deter20 I I I I I l l 1 1 I I I I I l l 1 gent solutions were investigated. 10 20 50 100 200 500 1000 Many methods have been used to measure Concentration, p molar. CMC6; in fact, if almost any physical property of aqueous detergent solutions is plotted against Fig. 1 .-Surface tension-concentration curves of the Igepals. Values of surface tension were determined by the du concentration, the slope of the curve will change abruptly (Fig. 1) near the CMC,6 with slight Nouy ring method a t 25" as described elsewhere.18 Identical containers and solution volumes were used, and the differences depending on the property measured. glass container wall areas were kept a t a minimum to reduce The reported determinations of CMC were based effects due to adsorption of detergents on the containers. on the measurement of surface tension. Although Measurements were made a t intervals of a few minutes the validity of the surface tension method has until the values agreed within 0.1 dyne/cm. Usually three measurements were required to obtain constant been questioned,' it is supported by theory* and values for the detergents in distilled water and the most has been verified experimentally by compari- dilute electrolyte solutions. In the presence of more concentrated electrolytes the second reading generally agreed son with methods based on color changes of -0
(1) Presented before the Division of Colloid Chemistry, 128th meeting of the American Chemical Society, Minneapolis, Minn.. Sept. 15, 1955. (2) L. M . Kushner and W. D. Hubbard, THIS JOURNAL, 68, 1163 (1954). (3) C. R. Bury and J. Browning, Trans. Faraday Soc., 49, 209 (1953). (4) H. N. Dunning, H. J. Gustafson and R. T.Johansen, I n d . Eno. Chem., 46, 591 (1954). (5) N. Sata and K. Tyuzyo, BulE. Chem. Sac. J a p a n , 2 6 , 177 (1953). (6) J. Grindley and C. R. Bury, J . Chem. Soc., 679 (1929). 67, 898 (7) L. M. Kushner and W. D . Hubbard. THISJOURNAL, (1953). ( 8 ) C. R. Bury, Phil. Mae.. 4 , 080 (1927).
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