ANALYTICAL CHEMISTRY, VOL. 51, NO. 7 , JUNE 1979
RECEIVED for review September 25,1978. Accepted December 28,1978. T h e financial assistance provided by Department
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of Energy Grant EF-77-G-01-2696 and the Commonwealth of Virginia is greatly appreciated.
Determination of Alkylbenzenesulfonates and Alkylsulfates as Anionic Surfactants by Carbon- 13 Nuclear Magnetic Resonance Spectrometry Yoshio Kosugi” and Yu Yoshida Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya, Japan 464
Tsugio Takeuchi National University of Technology and Science at Toyohashi, Tenpaku-cho, Toyohashi, Aichi Prefecture, Japan 440
I3C NMR has been used for analysis of anionic surfactants. Using spectral peaks at both aromatic and aliphatic regions, the biodegradablesofl type of alkylbenzenesuifonate is easily discernible from the hard type. The purlty of the soft type is also deducible by the relative peak intensities of the aromatic carbons (C, or C2 and C3). Qualitatlve and quantitative analyses of dodecyl, tetradecyi, and cetyl sulfates are posslble at resonance signals of their terminal methyl carbons whose chemical shifts are more than 2 Hz apart from each other. Standard deviations in quantitation are less than 1.6. Applications of the I3C NMR method tor commercially available detergents are exempllfied.
Sodium salts of alkylbenzenesulfonic acids (ABS) and alkylsulfates prepared from high alcohols are anionic surface active agents widely used today. Recently household uses of these compounds in dry powder or liquid detergents have brought about doubts in health benefits and problems of environmental pollution. A number of methods have been proposed for analyses of anionic surfactants (1-3). Titration with a standard solution of cationic surfactants such as benzylcetyldimethylammonium chloride in the presence of methylene blue, infrared spectroscopy, and acid catalyzed hydrolysis followed by extraction for gas chromatographic analyses (GLPC) are commonly used. T h e last method seems to be best for both qualitative and quantitative determinations of alkylsulfates. However, hydrolyses of the sulfates under reflux conditions for a few hours and extraction from the aqueous solution are unavoidable. Since almost nothing is required prior to measurements, NMR spectroscopy often facilitates analyses of compounds such as organic sulfates, sulfonic acids, and carboxylic acids ( 4 ) for which hydrolyses or esterifications are needed in GLPC analyses. Our basic research on alkylsulfonic acids by 13C NMR ( 5 ) has now been extended to the present compounds. Herein described are some useful applications of 13C NMR analyses of the synthetic detergents most used today.
EXPERIMENTAL Sodium dodecylbenzenesulfonate (“hard type”) of extra pure grade and dodecene-1-LAS or linear dodecylbenzenesulfonic acid sodium salt (“soft type”) of ultra pure grade were purchased from Tokyo Kasei Kogyo Co., Ltd. Persoft SP (donated by Nippon 0003-2700/79/0351-0951$01 .OO/O
Oil and Fats Co., Ltd.) composed of 26% effective anionic surfactants, 70% water, 1% each of sodium chloride and sodium sulfate, and Monogen (Daiichi Kogyo Seiyaku Co. Ltd.) known to be sodium high alkylsulfonates were industrial materials for detergents, fabric softeners, etc. The household laundry powder (Nagoya CO-OP) was indicated as a high alcohol which contained alkylsulfates (19%), phosphates (8% as P205),inorganic sulfates and silicates. Natural abundance proton noise decoupled 13C NMR spectra were obtained on a Varian XL-100-15 NMR spectrometer operating at 25.16 MHz with a Varian DATA 620i computer of 8K memory. Normally accumulations of 1000 transients using pulses of flip angle of ca. 45’ in repetitive intervals of 3 s were Fourier transformed for spectral widths of 5000 Hz (for ABS), 2500 Hz (for alkylsulfates) or 1000 Hz (for methyl peaks in Figure 3). A 500-mg sample was dissolved in 3 mL of deuterium oxide with a small amount of sodium 3-trimethylsilylpropionate-d4 (TSP) or sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) as an internal standard.
RESULTS AND DISCUSSION Sodium Alkylbenzenesulfonates ( A B S ) . ABS was one of the first synthetic detergents commercialized and is still a main surfactant. Shown in Figures 1A and 1B are 13CNMR spectra of the so-called “soft type” and “hard type”, respectively. It has been revealed that the figures are very good probes for identification of “soft type” ABS or LAS (linear alkylbenzenesulfonate) from “hard type” ABS which is not biodegradable in environments. Aromatic carbon (C4)attached to alkyl groups of straight or nearly straight chain gave peaks a t ca. 151.5 ppm, while branched chain alkyl group of “hard type” ABS lowered the C, resonance peak by ca. 4 ppm which are comparable with the difference of 4.6 ppm between ipso carbon of ethylbenzene (144.1ppm) and that of isopropylbenzene (148.7ppm) (6). The C1 peak of the aromatic ring is affected very much by the sulfonic group, but little by various of para-substituted alkyl groups. The single peak observed at 143.5ppm agrees with a chemical shift calculated (143.1 ppm) based on additivity ( 7 ) of chemical shifts of n-butylbenzene (8) and benzenesulfonic acid ( 5 ) . The two strong peaks at 130.1and 128.3 ppm in Figure 1A are assigned to C2 and C3, respectively. Judging from peak intensities in the spectrum, the purchased LAS still contained “hard type” ABS as much as 25.670 calculated at C4 peaks or 27.470a t C2 and C3 peak intensities, averaged to be 26.570. Contamination of branched alkylbenzenesulfonates in LAS C 1979 American Chemical Society
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2
A) .-\
L*.IJw* B)
I
I
140
160
I i
9
120
I
r
50
I
I
I
10 ( p m )
30
Figure 1. i3C NMR spectra of (A) soft type of LAS and (B) hard type of alkylbenzenesulfonates
I
I
1
I
1
1
1
70
60
50
40
30
20
10
I
O (ppm)
Figure 2. i3C NMR spectrum of dodecyl sulfate in deuterium oxide with TSP
is apparent not only in the aromatic region but also in the aliphatic region where the number of peaks counted are more than 12 or so. Besides clear distinction between similar structures, another advantage of 13C NMR analyses of ABS lies in the easiness of sample preparation because chemical shifts are independent of p H values of aqueous solutions (5). Sodium Alkylsulfates (ROS03Na). ABS has gradually been replaced with alkylsulfates in the production of soapless soap. T h e alkylsulfate most often used is dodecyl or lauryl sulfate (R = C12Hw);its 13C NMR spectrum is shown in Figure 2. Peak assignments were made based on dodecanol (9). It is interesting that the long chain alkylsulfates have the same chemical shifts (71.6 ppm) for the CY carbons or C1. Although resonance signals of methylene carbons are not identical, many peaks are overlapped in a mixture sample. On the other hand, the terminal methyls show enough dif-
Table I. Analysis of a Mixture Sample Composed of Three Kinds o f Sodium Alkylsulfates (ROS0,Na) Terminal methyl carbons (cf. Fig. 3)
R
peak number and h C , ppm
Cl2H25 C,,H*, C,,H,,
1 16.38 2 16.26 3 16.18
a
Five measurements.
rel. intensities
std. dev.O
57.9 31.5
1.54 0.51 1.10
10.6
hydrolysisextractionGLPC methodb 60
30 10
Relative errors of ca. i4.0%.
ferences from each other in chemical shifts for finger prints. The peak intensities of terminal methyl varied proportionally with the concentration changes of dodecyl sulfate between 50
ANALYTICAL CHEMISTRY, VOL. 51,
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of 60, 30, and 10, respectively, with relative errors of ea. f4% in hydrolysis extraction followed by GLPC analysis. Only the methyl peaks of the mixture sample are shown in Figure 3. Since these sulfates are of similar structure, the terminal methyl carbons are in the same situation to receive nuclear Overhauser effects (NOE) of similar degrees and to have the same relaxation times (TJ. Peak separations of more than 2 Hz are larger than both resolution (0.24 Hz) and peak widths of freely rotating methyl groups whose transverse relaxation times (T2) are longer to give small peak widths according to the Equation of 1/d',. Relative peak intensities of peaks 1-3 were obtained with standard deviations of 1.5% in five time measurements of the mixture sample. T h e results are summarized in Table I. Applications of this method for industrial materials of detergents were attempted. It was found that "Persoft SP" contained dodecyl sulfate exclusively. but "Monogen" was composed of dodecyl and tetradecyl sulfates in a ratio of 2 to 1. Another application tried was a commercially available household detergent indicated "high alcohol". In order to save time, builders in the laundry detergent were removed as a n insoluble portion in 75% ethanol. The solution was dried roughly in a rotary evaporator, then was used for NMR measurements. The spectrum was almost the same as in Figure 3 with extra small peaks which might be of long alkyl chain functionalized by the ethoxy group or transesterified into phosphates.
I' Figure 3. 13C NMR peaks of methyl carbons of a mixture sample of
sodium dodecyl (peak l), tetradecyl (peak a), and cetyl sulfates (peak 3) in ratios of 57.9, 31.5, and 10.6, respectively
mg/3 mL and 500 mg/3 mL. Expansion of the dynamic range, if necessary, will be possible by employing water instead of deuterium oxide for better solubilities or by increasing the number of transients to as many as required using block memories of the computer for very diluted sample solutions. T h e calibration line obtained with a correlation coefficient of 0.997 was used for a n analysis of a mixture of three kinds of sulfates which R's were CI2Hz, C14HP9,and C16H33in ratios
A C K N O WLEDGM'ENT
The authors thank S. Suyama (Japan Oil and Fats Co., Ltd.) for the donation of alkylsulfates of C12-C16. LITERATURE CITED (1) Japanese IndustrialStandards Committee "Testing Methods for Synthetic Detergent", JIS-K 3362, Japanese Standards Association: Tokyo, 1978. (2) Rosen, M. J.; Goldsmith, H. A. "Systematic Analysis of Surface-Active Agents", Interscience: New York, 1966. (3) Hummel, D. "Identification and Analysis of Surface-Active Agents", Interscience: New York, 1962. (4) Kosugi, Y.; Takeuchi, T. Bull. Chem. SOC.Jpn. 1978, 5 1 , 2008. (5) Kosugi, Y.; Takeuchi, T. Org. Magn. Reson. in press (6) Stothers. J. B. "Carbon-13 NMR Spectroscopy", Academic Press: New York, 1972; p 97. (7) Savitsky, G. S. J . f h y s . Chem. 1983, 67, 2723. (8) Johnson, L. F.; Jankowski, W. C. "Carbon-13 NMR Spectra", WileyInterscience: New York, 1972; p 380. (9) Unpublished results from this laboratory
RECEIVED for review December 19, 19'78. Accepted February 12, 1979. Financial support by the Ministry of Education of Japan is gratefully acknowledged.
