750 Analyst, August, 1979, Vol. 104, $9. 750-755 Continuous Solvent-extraction Method for the Spectrophotometric Determination of Cationic Su rfa cta n ts Jiro Kawase and Makoto Yamanaka Tochigi Research Laboratories, Kao Soap Co. Ltd., 2606 Akabane, Ichikai-machi, Tochigi, Japan A rapid and automated spectrophotometric method for the determination of cationic surfactants, using the AutoAnalyzer, has been developed. This method is based on the continuous solvent extraction of the ion-pair complex formed in the reaction of Orange I1 with a cationic surfactant. Good traces and identical molar responses were obtained with seven different types of surfactant using methanol in the Orange I1 reagent. Fatty amines in mixtures of the amines and quaternary ammonium surfactants were deter- mined by changing the pH of the aqueous phase.The proposed method was applied to the determination of cationic surfactants in several commercial products. The results agreed with those obtained by the two-phase titration procedure. The detection limit is 5 p~ and the capacity is 10-20 samples per hour, with a relative precision of better than 1.5%. Keywords : Automatic analysis ; cationic swfactant determination ; fatty amine determination ; spectrophotometry ; Orange II Recently, new kinds of commercial products containing cationic surfactants have been extensively developed, in addition to the well known hair rinse, fabric softener and sanitiser types. The determination of the cationic surfactants has hitherto been effected by two- phase titration pro~edures.l-~ which has hitherto been conducted only manually by spectrophotometry, can be automated successfully by means of the Auto- Analyzer.We report here that the Orange I1 Experimental Reagents Benzethonium chloride (A), hexadecylpyridinium chloride (B) , benzylhexadecyldimet hyl- ammonium chloride (C) and trimethyloctadecylammonium chloride (D) were purchased from Tokyo Kasei Co. Ltd. Hexadecyl(2-hydroxyethyl)methyloctadecylammonium chloride (E) and hexadecylbis(2-hydroxyethyl)octadecylammonium chloride (F) were purchased from ICI Australia. Hexadecyldimethyloctadecylammonium chloride (G) , hexadecylamine (H) , octadecylamine (I), dialkylamine (R, and R, = C14-Cl8, relative molecular mass 499.83) (J) and alkyldimethylamine (R = C8-Cl8, relative molecular mass 293.69) (K) were produced by Kao Soap Co.Ltd. These reagents were purified by recrystallisation from acetone or an acetone - ethanol mixture. Stock solutions (3 mM) of A-G were prepared by dissolving the purified compounds in distilled water and standardised according to the direct two-phase titration procedure3 just before use. Amine surfactant (H-K) solutions ( 7 0 ~ ~ ) were prepared by weighing purified compounds and acidifying the amines with an adequate amount of dilute sulphuric acid. Dissolve 50 mg of Orange 11, 3.78 g of potassium dihydrogen orthophosphate, 1.99 g of disodium hydrogen orthophosphate and 520 ml of methanol in 300 ml of distilled water, dilute the solution to 1 1 with distilled water and adjust the pH to 7.3 with 1 N sodium hydroxide solution.Dissolve 50 mg of Orange 11, 2.24 g of potassium chloride, 3 ml of concentrated hydrochloric acid and 520 ml of methanol in 300 ml of distilled water, dilute the solution to 1 1 with distilled water and adjust the pH to 1.6 with 1 N hydrochloric acid. All other reagents were of analytical-reagent grade. Orange I I reagent, pH 7.3. Orange 11 reagent, pH 1.6.KAWASE AND YAMANAKA 751 Methylene blue solutiolz. Dissolve 0.03 g of methylene blue, 50 g of sodium sulphate (anhydrous) and 12 g of concentrated sulphuric acid in 500 ml of distilled water and dilute the solution to 1 1 with distilled water. Standardisation of Cationic Surfactants3 To 10 ml of sample solution (approximately 3 mM), measured in a measuring cylinder, add 25ml of the methylene blue solution and 15ml of chloroform. Titrate the sample solution with 0.004~ sodium lauryl sulphate solution by the usual method.At the same time, test a blank by titrating 10 ml of distilled water in the same manner. The end-point is when the blue colour is completely discharged from the aqueous layer. ,Wethod of calculation c = (v, - ‘vb) x 0.004 x 100 where C mmol is the concentration of the cationic surfactants, Vs ml is the volume of 0.004 M sodium lauryl sulphate solution used in the titration of the sample and v b ml is the volume of 0.004 M sodium lauryl sulphate solution used in the titration of the blank. Apparatus and Procedure The apparatus is a Technicon AutoAnalyzer, Model 11, consisting of the following com- ponents : automatic sampler, peristaltic pump, manifold, spectrophotometer equipped with a flow cell (light path length 15 mm) and 485-nm filter and recorder [Fig.l ( a ) ] . When necessary, a dilution loop [Fig. l ( b ) ] was used for analyses of commercial products. Pumping rate/ t mI min-’ ler wash recevtacle * PTFE tubing f’ Polyethylene tubing 0 PTFE LC three-way joint Fig. 1. (a), Diagram for extraction of cationic surfactants with Orange 11. (b), Flow diagram of a dilution loop. Except for Acidflex pump tubing, 2-mm bore PTFE tubing, 0.8-mm bore polyethylene tubing, mixing coils made of 2-mm bore PTFE tubing wound round a polyethylene pipe (25-mm diameter) and PTFE liquid chromatography (LC) three-way joints (Nihon Seimitsu JTO2U) are used for lines that come into contact with chloroform. All connections, especially between the phase separator and the flow cell, should be as short as possible.Before beginning a run, the tubing should be washed out by pumping methanol through the lines used to introduce chloroform. To prevent the inflow of the aqueous phase into the flow cell, it was found necessary to have the chloroform phase present in the tubing prior to intro- ducing the aqueous phase. As complex formation with fatty amines depends on the pH of752 Analyst, VoZ. 204 the aqueous phase,5 Orange I1 reagent at pH 1.6 was used for the over-all determination of quaternary ammonium and amine surfactants, and that at pH 7.3 for the determination of quaternary ammonium surfactants alone. The determination is effected automatically by passing a sample solution through a dilution loop, reacting the surfactant with the anionic dyestuff to form the ion-pair complex, extracting it into the chloroform phase and measuring the absorbance at 485 nm after phase separation.The sampling rate is 10-20 samples per hour. All calibration graphs were calculated with a custom-built microprocessor (Tachibana Electronic Co. Ltd.) connected to the recorder. The signal output of the recorder was monitored continuously at 1-s intervals over the run time. The absorbance, the equation of the straight line calibration graph, the regression coefficient and the calculated concentration of the sample were obtained from the print-out. KAWASE AND YAMANAKA : SOLVENT-EXTRACTION METHOD FOR Results and Discussion Effect of Methanol The slow extractability of a cationic surfactant in the Orange I1 method is well known.6 Ion-pair extraction in the continuous-flow analysis differs from that in the manual method, because it proceeds in the limited interfacial region between alternative aqueous and chloro- form segments in a definite length of mixing coil, and is seldom allowed to reach equilibrium.Thus, in the continuous-flow analysis the relative molar extractability of the ion-pair complex would be influenced by the corresponding extraction rate, which differs slightly for each type of cationic surfactant. However, in practical analysis it is necessary to obtain identical molar responses that are independent of the type of surfactant. The role of the solvating agent and its affinity for the ion-pair complex have been considered by Higuchi et al.6 The addition of methanol to the water - chloroform extraction system in the continuous-flow analysis would be expected to enhance markedly the extraction process, through solvation of the ion pairs that are fornied, and to be a powerful means of varying the relative molar extractability in the analysis.Investigation has shown that the relative molar extractability of the ion-pair complexes formed by the reaction of Orange I1 with cationic surfactants A-G, in the continuous-flow analysis, differs slightly and varies with the methanol content of the Orange I1 reagent to different extents (Fig. 2). The rate of extraction of the complex is also different for each surfactant. It should be noted that the rate of extraction for the surfactants is much more rapid with a higher methanol concentration and that the molar extractability of the complex increases with increasing methanol concentration (up to 40% V/V).0.26 ’ I I 1 I 0 10 20 30 40 50 f Methanol concentration, % V/V 3 Fig. 2. Effect of methanol concentration on the absorbance measured at 485nm, of: 0, surfactant B; A, surfactant C; A, surfactant D; and 0, surfactant G. Each cationic surfactant concentration was 200 p ~ . The pH of the Orange I1 reagent was 7.30.A zigmt, 1979 THE SPECTROPHOTOMETRIC DETERMINATION OF CATIONIC SURFACTANTS 753 There is also a distinct variation in the peak shapes with a change in the methanol con- centration (Fig. 3). A longer period of steady state was attained with a higher concentration of methanol; the peak shape for each type of surfactant deteriorates as the hydrophilicity increases.A,G AIG B,C DIE F Original peak. (methanol not added) Most improved peak shapes (methanol content, 52% V/V) Fig. 3. Variation in the peak shapes with the methanol content and the type of Sampling cam: 10 samples per hour, 3 parts of sample to 1 part surfactant, A-G. wash. The effect of methanol can be explained in terms of both solvating and dissolution effects. With increase in the concentration of methanol in the Orange I1 reagent, the methanol content of the chloroform phase also increases and this contributes to both the extraction rate and the extractability of the complex by its solvating effect. With 40% V/V or more of methanol in Orange I1 reagent, the ion-pair dissolution effect of methanol in the aqueous phase may predominate over the solvating effect and result in a decrease in the absorbance.In this work, 52% V/V of methanol in the Orange I1 reagent was chosen as the best concentration to give good peak shapes and almost identical molar responses for the seven different types of surfactant (Table I). TABLE I RELATIVE MOLAR EXTRACTABILITIES OF CATIONIC SURFACTANTS All values are normalised relative to the molar extractability of surfactant G. pH of buffer I A \ soh tion* A B C D E F G 7.30 103.3 102.0 100.0 98.9 101.1 97.9 100.0 1.60 103.3 102.0 100.2 99.1 101.6 98.2 100.0 Cationic surfactantt * Methanol content of the Orange I1 reagent was 52% V/V. t Each surfactant concentration was 200 p ~ . Calibration Graph and Precision The proposed method is suitable for determining up to 200 p~ of cationic surfactant, and provided that the dilution loop [Fig.l ( b ) ] is used, a linear calibration graph covering the range up to 3 mM can be realised. The detection limit was found to be approximately 5 p ~ . The resulting calibration graphs for each cationic surfactant were linear at pH 7.30 and 1.60. The precision of the method was evaluated by determining 1.5mM of each cationic surfactant (A-G). The results proved to be within 1.5% of the given concentration with a coefficient of variation of o.3-1.5y0. Cationic surfactant D serves as a representative example (Fig. 4). The cationic surfactant most frequently used in commercial products seems to be a dialkyldimethylammonium type, and therefore cationic surfactant G was used as a standard material for the commercial products analyses.Interferents likely to be present in commercial products were examined. Inorganic salts, acids and non-ionic surfactants found in the usual formulations had only a small effect on the determination.754 KAWASE AND YAMANAKA : SOLVENT-EXTRACTION METHOD FOR Analyst, Vol. 104 '"fl 172.7 180.7 172.7 172.7 r- I 0 3 6 9 12 15 18 21 24 27 30 33 36 Time/m in I Fig. 4. Determination of surfactant D. Values on peaks Rate of sampling is 20 per represent the concentration in p ~ . hour. Application to the Determination of Fatty Amines and Commercial Products Analyses Solutions (70 PM) of the fatty amines (H-K) were analysed at pH 1.60 using the calibration graph for surfactant G.The results were satisfactory (Table 11). Several commercial products containing cationic surfactants were analysed by the proposed method, after adequate dilution of the aqueous solutions. Orange I1 reagent at pH 1.6 was used for the over-all determination of quaternary ammonium and amine surfactants and that at pH 7.3 for the determination of quaternary ammonium surfactants alone. The results were com- pared with those obtained by the direct two-phase titration procedure,3 which determines TABLE I1 RECOVERY OF FATTY AMINES Amine Presentlpf Found/pM Recovery,* yo H 70.0 67.2 96.0 I 70.0 69.2 98.9 70.0 69.9 99.9 70.0 71.4 102 * The pH of the Orange I1 reagent was 1.60. The dilution loop [Fig. l(b)] was not used. TABLE I11 COMMERCIAL PRODUCTS ANALYSES AutoAnalyzer method r Sample* 1 2 3 4 5 6 7 8 9 pH 7.30: found/mM 2.78 2.03 1.90 2.10 1.82 3.33 1 .oo 1.08 3.33 pH 1.60: found/mM 2.85 2.06 2.14 2.13 1.80 3.52 - ._ Two-phase titration procedure3: found/mM 2.87 2.08 2.24 2.19 1.84 3.59 1.00 1.05 3.29 * Samples 1-3 were fabric softeners and samples 4-9 were hair rinses.August, 1979 THE SPECTROPHOTOMETRIC DETERMINATION OF CATIONIC SURFACTANTS 755 over-all cationic surfactants (Table 111). Good agreement was obtained between the auto- mated and conventional determinations. References 1. 2. IS0 Standard, IS0 2871-1973. 3. 4. 5. 6. Epton, S. R., Trans. Faraday SOL, 1948, 44, 226. Japanese Industrial Standard, K 3362, 1970. Few, A. V., and Ottewill, R. H., J . Colloid Sci., 1956, 11, 34. Scott, G. V., Analyt. Chem., 1968, 40, 768. Higuchi, T., Michaelis, A., Tan, T., and Hurwitz, A., Analyt. Clzem., 1967, 39, 974. Received October l l t h , 1978 Accepted February 27th, 1979