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Overview of simple test for determination of surfactants by adhesion method Article in Journal of Environmental Science, Computer Science and Engineering & Technology · January 2013
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Journal of Environmental Science and Engineering B 2 (2013) 672-677 Formerly part of Journal of Environmental Science and Engineering, ISSN 1934-8932
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Overview of Simple Test for Determination of Surfactants by Adhesion Method Minori Kamaya Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, Hachioji-City 192 0015, Japan Received: September 2, 2013 / Accepted: October 5, 2013 / Published: November 20, 2013. Abstract: A simple, sensitive, and rapid analytical method is reported for the determination of surfactants. This is based on the use of an oppositely charged dye as the ion pair to form an ionic associate with the surfactant in a vessel, thus affording ion-associated adhesion on the inner wall of the vessel. After the adhesion, the remaining solution in the vessel is removed, and the ionic associate is dissolved in a suitable solvent. The absorbance of the resulting solution is measured spectrophotometrically to determine the concentration of the surfactant. Further, the mechanism of adhesion is elucidated. Key words: Anionic surfactants, soap, nonionic surfactants, cationic surfactants, simple determination.
1. Introduction Surfactants have industrial and agricultural applications in addition to domestic uses such as laundry and dish washing. Besides creating unsightly bubbles that mar landscapes, surfactants flowing into waterway can cause aquatic toxicity. Among the four main types of surfactants, anionic, nonionic, cationic, and amphoteric, most commonly used are anionic and nonionic surfactants and thus are produced in large quantities. Many methods are available for the analysis of anionic [1], nonionic, and cationic surfactants [2]. In the spectrophotometric determination of these surfactants, synthetic anionic surfactants are determined by the solvent extraction method using methylene blue and ethyl violet as the ion pairs and chloroform and toluene as the extraction solvent, respectively. Nonionic surfactants are determined by the extraction-spectrophotometric method with cobalt tetrathiocyanate as the ion pair and benzene as the solvent [3]. However, these methods are cumbersome, Corresponding author: Minori Kamaya, Ph.D., associate professor, main research field: spectrophotometric method for determination of metals and anions. E-mail:
[email protected].
and the large amounts of solvents used in the laboratory and fieldwork for extraction significantly contribute to pollution. In analytical chemistry, high-performance and sophisticated analytical instruments are mostly used, however, there is a need for simple and inexpensive analytical tools that can be used at all levels of expertise. As reported by Kamaya [4], an anionic surfactant adhered to the inner vessel wall when the air floatation method was used for the determination of anionic surfactants. In this study, the same analysis was performed using a tube through vigorous shaking without the air floatation method. The ion associate was dissolved in a miscible solvent such as alcohols and determined spectrophotometrically. The adhesion method has been used for the simple determination of anionic surfactants [4-7] and soaps [8] as well as cationic [9-11] and nonionic surfactants [12, 13]. In this paper, the author reports an adhesion method for the determination of surfactants as an extension of the previous studies.
2. Adhesion Mechanism The mechanism by which an ion associate adheres to the inner vessel wall has not been clearly explained.
Overview of Simple Test for Determination of Surfactants by Adhesion Method
The author proposed the following mechanism for the adhesion phenomenon. In the first step, the accumulation of an ion associate near a hydrophobic bubble occurs by vigorous shaking. In the second step, the adhesion of the accumulated ion associate occurs on the inner wall of the vessel.
sulfonyl functional groups, respectively. In contrast, a small blank value was obtained for PTFE (polytetrafluoroethylene) and PP (polypropylene) vessels. When a PTFE vessel was used, only a few drops of water adhered to the inner wall of the vessel. Therefore, the removal of water was easily performed by tapping, and the adhesion of the ion associate with methylene blue and the anionic surfactant was strong. However, PP vessels are better suited for this method because PTFE vessels are expensive. When a small-sized vessel was used, the addition time of the ion associate on the inner wall was short. The aforementioned vessel characteristics were the same for the determination of anionic and cationic surfactants.
3. Standard Procedure A specific amount of the surfactant sample was transferred into a vessel, and the corresponding charged dye (ion pair) was added to the solution. The vessel was shaken mechanically or manually by vigorous hand shaking. After the adhesion, the solution was discarded from the vessel. The remaining solution was removed by tapping the vessel on a paper, and the ion associate adhering to the inner vessel wall was dissolved in an alcohol solution. The absorbance of the resulting solution was measured at the maximum wavelength of the ion associate in the solution using a spectrophotometer. The various measurement conditions are shown in Table 1.
4.2 Volume Size of Vessels and Sample Volume A low concentration of surfactants was determined using a large volume of the sample and a small volume of the solvent for the dissolution of the ion associate adhering to the inner vessel wall. The sample volume was investigated using an anionic surfactant with methylene blue as the ion pair. The blank value increased proportionally with the PP vessel size. The parameters selected to achieve a high enrichment factor and sensitivity were a 500 mL vessel containing 250 mL of sample dissolved in 5 mL of ethanol. Under these conditions, an enrichment factor 20 was obtained, and the blank value and repeatability (coefficient of variation) were 0.2% and 3.6%, respectively. This method involves the formation of bubbles by vigorous shaking, however, the formation of bubbles
4. Parameters of Surfactants Determination 4.1 Vessel Materials The adsorption of the ion associates of surfactants and the adhesion of ion pairs to the inner vessel walls were affected by the composition of the vessel materials. The anionic surfactants determined using various 30 mL vessels are shown in Table 2. A high blank value was obtained with methylene blue as the ion pair for glass, polycarbonate, and polysulfone vessels because of their hydrophilic characteristics resulting from the presence of silanol, carboxyl, and Table 1
Measurement conditions.
Surfactants
Ion-pair reagents
Sample volume
Solvent
Shaking time (min)
Literature
Anionic
Methylene blue
1
3.5
1
5
Soap
Methylene blue
1
2
1
8
＊
Cationic
TPPS
Nonionic
TBPE
＊＊
＊
20
5
5
11
1
1
2
13
5,10,15,20-Tetrap(p-sulfonatophenyl)porphyrin;
＊＊
Tetrabromophenolphtalein ethyl ester.
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Overview of Simple Test for Determination of Surfactants by Adhesion Method
674 Table 2
Vessel materials for the determination of anionic surfactants [6].
Materials Borosilicate glass Polycarbonate Polypropylene Polysulfone PTFE
Absorbance at 534 nm Sample 0.487 0.749 0.537 0.607 0.581
Blank 0.122 0.351 0.057 0.181 0.071
Difference 0.365 0.398 0.480 0.426 0.510
8.64 μg of SDS, ion pair is Rhodamine 6G.
was affected by the ratio of the amount of air to solution in the vessel. The required ratio was more than 30% [7].
needed to be increased from 1 min to 5 min. Thus, the shaking time was dependent on the vessel size.
5. Individual Surfactant Determination
4.3 Dissolution of Ion Associate The dissolution of the ion associate adhering to the inner vessel wall was investigated. In the early stages of this method [5], the ion associate with methylene blue and an anionic surfactant was dissolved in zephylamine, and the resulting bubbles disappeared with the addition of an antifoaming agent. However, this procedure was tedious; therefore, the dissolution of the ion associate was carried out using water and a miscible solvent such as methanol, acetone, ethanol, 2-propanol, or methyl cellosolve (ethylene glycol monomethyl ether) [6]. Among these solvents, ethanol, acetone, and methyl cellosolve exhibited a high dissolution rate. The highest dissolution rate was observed for methyl cellosolve. However, because this solvent is toxic, a mixture of ethanol and water was used; an ethanol content of more than 60% (v/v) was necessary for complete dissolution [5]. When only water was used for dissolution, a minimum temperature of 80 °C was necessary. 4.4 Shaking Time Vigorous shaking was necessary for the adhesion of ion associates on the inner vessel wall. Long shaking times were achieved by using a Taitec model SR-II recipro shaker, which enabled us to obtain repeatability data. In the case of the determination of anionic surfactants, if the volume of the vessel was changed from 30 mL to 500 mL, the shaking time also
5.1 Anionic Surfactant Although soaps and synthetic detergents are anionic surfactants, a separate procedure was necessary for the determination of these surfactants because synthetic detergents contain sulfonate functional groups and easily dissolve in water. Moreover, the solubility of anionic surfactants is not affected by pH conditions; however, the solubility of soaps depends on pH because they contain a carboxyl group. Moreover, soap precipitates in the presence of calcium and magnesium ions. For these reasons, the simultaneous determination of these surfactants was impossible without the pretreatment by adding calcium ions and a masking agent. Anionic detergents easily associate with cationic dyes [6], the comparison of cationic dyes to be used as ion associates are shown in Table 3. Evidently, Rhodamine 6G was the most sensitive cationic dye. However, for comparing to a standard color sheet, it is difficult to prepare a standard fluorescence color sheet for Rhodamine 6G. For this reason, methylene blue was selected as the ion-pair dye for the test. A test kit for the anionic surfactants is commercially available [14]. Soaps contain various long-chain fatty acids. The dissolution of soap depends on pH because the fatty acids contain a carboxyl groups as mentioned before. For this reason, the ion associate with methylene blue was affected by the pH condition of the solution; the
Overview of Simple Test for Determination of Surfactants by Adhesion Method
Table 3
675
Cationic dyes as ion pairs for anionic surfactants [6].
Cationic dyes
Wavelength (nm)
Crystal violet Ethyl violet Brilliant green Rhodamine B Rhodamine 6G Rhodamine S Methylene blue Meutral red NDDP＊
595 597 608 548 534 534 622 450 575
Absorbance Sample 0.426 0.451 0.040 0.222 0.568 0.327 0.240 0.077 0.036
Blank 0.080 0.086 0.012 0.036 0.056 0.090 0.022 0.016 0.008
Difference 0.346 0.365 0.028 0.186 0.512 0.237 0.218 0.061 0.028
＊
1-(4-nitrobenzyl)-4-(4-diethylaminophenylazo) pyridinium bromide.
Table 4
Molar absorptivity (ε) for high fatty acids.
Fatty acids Stearic acid Oleic acid Linolenic acid Paslmitic acid Myristic acid
Chemical formula C17H35COOH C17H33COOH C17H29COOH C15H31COOH C13H27COOH
optimum pH range for this condition was 10-13. The molar absorptivity coefficient of high fatty acids, as shown in Table 4, was determined by standard calibration procedures. Among these fatty acids, oleic acid was the most sensitive. However, the molar coefficient of soaps could not be determined accurately because sensitivity differs among high fatty acids. 5.2 Nonionic Surfactants Nonionic surfactants do not produce ions in aqueous solution; therefore, they are compatible with other types of surfactants and are excellent candidates to form complexes by adding of potassium. Nonionic surfactant form complexes with rubidium and potassium and ion associates with anionic dyes such as of TBPE (tetrabromophenolphthalein ethyl ester) [12]. Potassium was selected as the ion pair because it is readily available and inexpensive. The anionic dyes tested were TBPE, bromothymol blue, bromophenol blue, tetrabromophenol blue, bromocresol purple, erythrosine B, and brilliant blue G [13]. Among these dyes, only TBPE was associated with the complex of
Sensitivity (× 104) 3.63 5.30 3.45 3.73 2.78
the nonionic surfactant and potassium. The ion associating ability of TBPE can be attributed to the existence of TBPE as a monovalent ion. In contrast, the other dyes existed as divalent ions. However, because TBPE cannot be easily solubilized in water, it was dissolved in methyl cellosolve. The solvent ethanol decreased the adhesion of the ion associate. Moreover, the blank value increased at a lower pH of 6. The Beer’s law is not obeyed in the region of the low concentration. For the determination of nonionic surfactants, the negative effect of anionic detergents was compensated by treating with an anionic ion-exchange resin. This method is limited in its applicability for ethoxylated nonionic surfactants. 5.3 Cationic Surfactants The anionic dyes investigated as the ion pairs for cationic surfactants were the same as those used for nonionic surfactants. All the anionic dyes combined with the cationic dyes are adhered to the inner vessel walls. Among these dyes, TBPE was the most sensitive. However, the optimum conditions were a shaking time of 15 min followed by allowing the
676
Overview of Simple Test for Determination of Surfactants by Adhesion Method
solution to stand for 30 min. For these reasons, TBPE was replaced investigated to be TPPS (tetraphenylporphyrin tetrasulfonic acid). TPPS was a better reagent because the optimum shaking time decreased from 15 min to 1 min. Moreover, there was no need to allow the solution to stand. Moreover, the sensitivity increased by twofold. The molar absorptivity coefficient of TPPS is very high; however, the composition of the ion associate with the cationic surfactant and TPPS was 4:1. Therefore, the merits of TPPS sensitivity could not be utilized. In the case of the determination of cationic surfactants, the effect of anionic surfactant was compensated by adding β-cyclodextrin [15].
6. Conclusions In analytical chemistry, highly sensitive and expensive instruments are commonly used; however, a simple test was needed for fieldwork and environmental analysis by all researchers. In this study, a sensitive and rapid method for the determination of surfactants without solvent extraction was developed. The developed method depends on the adhesion of the ion associates of surfactants with dyes as the counter ions on the inner vessel walls after vigorous shaking. The ion associates were solubilized with nontoxic alcoholic solvents such as ethanol and the concentration of the surfactants was determined by absorbance measurements or comparison with a standard color sheet. The determination of soaps was performed at pH 11. The addition of calcium ions and EDTA was effective in the simultaneous determination of soap and anionic detergents. Without the addition of EDTA, only anionic detergents could be determined because soap forms precipitates with calcium ions. The addition of EDTA facilitated the simultaneous determination of soaps and synthetic anionic detergents. Nonionic surfactants formed complexes with potassium ions; these complexes were associated with anionic dyes only on addition of TBPE. For the
determination of nonionic surfactants, the negative effect of anionic surfactants was compensated by treating with an anionic ion-exchange resin. In the case of cationic surfactants associated with TBPE, TPPS was a better reagent because of its sensitivity and reduced shaking time. The effects of the anionic surfactants were compensated by adding β-cyclodextrin. The developed method can be useful for the determination of surfactants in agricultural and domestic applications, however, the method has a disadvantage in that if the shaking is accomplished by hand, the difference in personal shaking strength can affect the adhesion rate of ion associates. Therefore, the development of an inexpensive tool is necessary for achieving consistent shaking conditions.
References [1]
[2]
[3]
[4]
[5]
[6]
[7]
M. Kamaya, Analytical methods of non-mental components in water (Part 9) anionic surfactants, Industrial Water 517 (2001) 45-52. M. Kamaya, Analytical methods of non-mental components in water (Part 10) Non-ionic surfactants and cationic surfactants, Industrial Water 518 (2001) 46-53. Y. Miura, H. Suzuki, M. Hasei, T. Koh, Extraction/spectrophotometric determination of micro amounts of non-ionic surfactants using ammonium tetrathiocyanatocobaltate (Ⅱ), Bunseki Kagaku [Online], 38 (1989) 15-19, https://www.jstage.jst.go.jp/article/bunsekikagaku1952/3 8/2/38_2_T15/_pdf. M. Kamaya, K. Nagashima, H. Namiki, Spectrophotometric determination of trace amounts of anionic surfactants by air stripping-ethyl violet method, J. Environ. Chem. [Online], 7 (1997) 291-295, https://www.jstage.jst.go.jp/article/jec1991/7/2/7_2_291/ _pdf. M. Kamaya, T. Tuchiya, K. Okauchi, T. Kawarabayashi, Development of a simple determination method for anionic surfactants, Journal of Water and Waste 41 (1999) 224-228. M. Kamaya, K. Tomizawa, K. Nagashima, Spectrophotometric method for the determination of an anionic surfactant without liquid-liquid extraction, Anal. Chim. Acta. 362 (1998) 157-161. M. Kamaya, A. Tushima, Y. Sekiguchi, K. Nagashima, Simple determination of anionic surfactant and cationic
Overview of Simple Test for Determination of Surfactants by Adhesion Method surfactant by adsorption method, Industrial Water 505 (2000) 21-27. [8] M. Kamaya, Development of a simple test method for soap, Journal of Water and Waste 44 (2002) 201-206. [9] M. Kamaya, J. Takahashi, K. Nagashima, Development of a simple test for cationic surfactants, Journal of Water and Waste 45 (2003) 751-756. [10] M. Kamaya, Y. Kaneko, K. Nagashima, Simple method for spectrophotometric determination of cationic surfactants without liquid-liquid extraction, Anal. Chim. Acta 384 (1999) 215-218. [11] M. Kamaya, J. Takahashi, K. Nagashima, Rapid and simple determination of a cationic surfactant by adsorption induced by vigorous shaking, Mikrochim. Acta 144 (2004) 35-39.
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[12] K. Kamaya, A. Hashimoto, K. Nagashima, Rapid and simple determination of ether type non-ionic surfactants by adsorption on the inner wall, Journal of Water and Waste [Online], 47 (2005) 41-47, https://www.jstage.jst.go.jp/article/jswe/27/11/27_11_735 /_pdf. [13] M. Kamaya, A. Hashimoto, K. Nagashima, Development of simple test for ethoxylated nonionic surfactants, Journal of Japan Society on Water Environment 27 (2004) 735-740. [14] Anionic surfactants set, http://kyoritsu-lab.co.jp/seihin/list/instructions/wa-det.pdf. [15] M. Kamaya, T. Ohte, K. Nagashima, A simple method for determination of anionic surfactants with β-cyclodextrin and phenolphthalein, Industrial Water 541 (2003) 2-6.
