2124 Anal. Chem. 1980, 52, 2124-2127

Automated Determination of Cationic Surfactants by Flow Injection Analysis Based on Ion-Pair Extraction

Jiro Kawase Tochigi Research Laboratories, Kao Soap Co., Ltd., 2606, Akabane, Ichikai-machi, Tochigi, Japan

An automated ion-pair extraction of cationic surfactants by flow injection analysis (FIA) is described. A more simplified and effective solvent extraction system has been constructed. Novel components include the pumping system, an effective segmentor for the immiscible phases, and a highly selective phase separator with a poiy(tetrafiuoroethy1ene) membrane. The effects of segmentor and separator, inner diameter and length of extraction coils, and methanol content in the aqueous reagent have been examined on the extractability and sample band broadening. The relative molar extractability of the surfactants was studled In terms of both the type and the alkyl group of each homologue. Equal molar response, and good reproducibility (within 1.5 % coefficient of variation auto- matically injected), and linear response were obtained with the six different types.

Cationic surfactants are used commercially as fabric sof- teners, as hair rinses, as germicides and so on. Their total content has been determined on the basis of ion-pair extraction followed by two-phase tritration (1-3) and by colorimetry (4 , 5 ) . These procedures have been effected manually.

In the previous paper (6), we reported the automated de- termination of the cationic surfactants followed by the Au- toAnalyzer technique, in which methanol was used as the ion-pair solvating agent both to enhance the extraction process and to vary the relative molar extractabilities of different types of surfactant. The capacity is typically less than 20 deter- minations/ h.

Flow injection analysis (FIA) developed by RuiiEka e t al. and reviewed by Betteridge is a very versatile technique (7, 8). We applied ion-pair extraction of anionic surfactants by FIA at a rate of 80 samples/h (9). The approach involved the design of the phase-separating system with poly(tetrafluoro- ethylene) (PTFE) porous membrane which is permeable to chloroform but impermeable to the aqueous solution. Tamura (10) had previously applied porous PTFE membranes for phase separation.

This report is concerned with the fully automated deter- mination of cationic si7rfactants by FIA. A more simplified and practical solvent extraction system by FIA is designed which, because of the high selectivity of the porous PTFE membrane, permits both good phase separation and low band broadening.

EXPERIMENTAL SECTION Reagents. Sodium dodecylsulfate, 1 mM, in distilled water

and methylene blue reagent were prepared as described previously (9).

Cationic Surfactants. Homologues of alkyl(Clo-C1& pyridinium chlorides (monohydrate), alkyl(Clz-C18)trimethyl- ammonium chlorides and alkyl(C12Xle) benzyldimethylammonium chlorides (dihydrate), and hexadecyldimethyloctdecylammonium chloride were prepared in our laboratories. Benzethonium chloride (monohydrate) and hexadecyl(2-hydroxyethyl)methyloctadec- ylammonium chloride were obtained from Tokyo Kasei Co., Ltd., Japan, and ICI, Australia, respectively. These standard materials

were purified by recrystallization from acetone, an acetone-ethanol mixture, or a chloroform-ether mixture.

Cationic surfactants, 3 mM, in distilled water were prepared by dissolving the purified compounds just before use.

Orange I1 Reagent. Dissolve 0.1 g of Orange 11, 2.24 g of potassium chloride, and 3 mL of concentrated hydrochloric acid in ca. 300 mL of distilled water. With cooling add 520 mL of methanol to this solution slowly. Dilute the solution with distilled water to 1 L and adjust the pH to 1.6 with 1 N hydrochloric acid.

All the other reagents were of analytical grade. Apparatus and Procedure. The experimental arrangement

is shown as a flow diagram in Figure 1. Chloroform and Orange I1 reagent were pumped individually

with single-plunger pumps (Nihon Seimitsu NMP-~u , Tokyo, Japan) for HPLC. The flow pulsations were eliminated by the damping devices comprising pulse-dampers (Nihon Seimitsu air-damper NAD-2), pressure gauges (Nihon Seimitsu NPG-50, Bourdon tube Hi0 kg/cm2), and suppreesor coils (5 m X 0.25 mm bore PTFE tubing used in a flow-through configuration).

It is necessary to pump solvents for about 20 min to establish equilibrium state before analyses can be started.

Absorbance was measured with a spectrophotometer (Hitachi Model 100-50) equipped with a flow cell (inner volume 8 pL, light path length 8 mm) in combination with a multirange recorder (Hitachi Model 056) and an integrator (Shimadzu Chromatopac ElA).

Except for the flow cell, PTFE tubing and fittings were used for all flows. Extraction coils were made of 0.3-mm, 0.5-mm, 1.0-mm, and 2.0-mm bore PTFE tubing wound around poly- (ethylene) pipe (25 mm 0.d.).

At the mixing point of the phases, a 45/45-W segmentor shown in Figure 2 was used to obtain a regular pattern of alternate aqueous and chloroform segments.

A phase separator was constructed as indicated in Figure 3, with a PTFE porous membrane described previously (9) which is pressed into the inside flat surface of PTFE body 3 so as to make intimate contact with joint 5. The stream of alternate aqueous and chloroform segments enters via joint 1. The chlo- roform is separated from the stream by the PTFE membrane and exists via joint 5 . The residual chloroform and aqueous solution exit via joint 2.

In the preliminary experiments, the determination was effected by injecting a 6-pL aqueous sample solution with a microsyringe into the stream of the Orange I1 reagent through a PTFE L.C. line sample injector (Nihon Seimitsu NSL-1). For the automated determination, a synchronized automatic sampling device (in- jection volume: 20-pL, Iatron, Japan) as shown in Figure 1 was employed in the system.

The surfactant was allowed to react with Orange I1 to form an ion-pair complex, which was extracted into the chloroform phase during passage toward the separator, and the absorbance was measured at 490 nm after phase separation.

All the calibration curves were prepared by linear regression analysis.

RESULTS AND DISCUSSION Design of a Solvent Extraction System. Pumping

System. Due to the slow extractability of the surfactants as described previously (9), the ion-pair extraction in the flow analysis requires a longer extraction coil to increase the ex- tractability. Furthermore, a narrow and long suppressor coil behind the flow cell is effective to suppress air bubble gen- eration of organic solvent in the flow cell. But in the narrower

0003-2700/80/0352-2124$01.00/0 0 1980 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980 2125

s

Reagent

1.30 ml/min W

1 2 3 4

I b l

Aauecus

Figure 1. Schematic diagram of sokent extraction for FIA: (a) solvent extraction system; (b) automatic sampling system; (1) reservoir; (2) single-plunger pump; (3) pulse damper; (4) pressure gauge; (5) suppressor coil (5 m X 0.25 mm bore PTFE tubing); (6) segmentor (Figure 2); (7) extraction coil (5 m X 0.5 mm bore PTFE tubing); (8) phase separator (Figure 3); (9) detector (490 nm); (10)/(11) flow restrictor coils (2.68 m X 0.5 mm bore PTFE tubing)/(2 m X 0.5 mm bore + 0.4 m X 0.25 mm bore PTFE tubing); (12) automatic sampler; (13) programmer; (14) peristaltic pump; (15) slide valve (sampling volume 20 pL). (1 2), ( 14), and (1 5) are synchronized by the programmer (13).

h u e o u s

Reagent

Aaueous

Reagent Chlorcform

I \ I E x t r a t t l o n

! ! E r t r a c t l o n E x t r o t t l o n

GO1 I CC11

(a) ( b ) ( C )

Flgure 2. Segmentors designed for two immiscible phases. (a) 90/9C-T; (b) 30130-Y; (c) 45145-W. Inside diameter of capillaries was 0.8 mm.

and longer tubing, the flow pattern is not smooth because the pressure that is needed is close to the limit of the peristaltic pump. To resolve the problem, the pumping system was constructed by using single-plunger pumps for HPLC. By virtue of the system in which the tubing is not expendable, the flow ratio of chloroform in the spectrophotometer to waste is easily controlled by changing the length ratio of the flow restrictor coils, and then the flow diagram and the operation become simple.

Segmentor and Phase Separator. The form of the seg- mented flow pattern in the extraction coil (Figure 4) seemed to be a significant parameter in a narrower tubing both to decrease in the band broadening and to enhance the ex- tractability of the ion-pair complex, as the extraction proceeds in the limited interfacial region between alternate aqueous and chloroform segments. The effects of the segmentor on the segmenting of miscible phases have been studied by two groups (11-14). The design of the segmentor is also important when the two immiscible phases are combined. Three types of segmentor have been designed (Figure 2) for the deter- mination of sodium dodecylsulfate developed previously (9)

1 2

a

- - 4

Flgure 3. Construction of a phase separator: (a) side view; (b) bottom view of PTFE body; (1)/(2) PTFE joints (inletloutlet); (3) PTFE body (axial pitched cylindrical cavlty); (4) PTFE porous membrane sheet (7 mm diameter); (5) PTFE joint (outlet to flow cell). Inside diameter of the capillaries of both joints and body is 0.8 mm. The inner volume of the separator is 12.5 pL.

PTFE tub1 ng

I

( b ) ')

Aaueous solution I

( C ) c > C I c 3 < ) ~ * O

Chloroform I

Flgure 4. Segmented flow pattern in the extraction coil: (a) 1 mm bore tubing; (b) 0.5 mm bore tubing, 90/90-T or 30130-T segmentor; (c) 0.5 mm bore tubing, 45/45-W segmentor.

Width at I . half -height

I \ JL -

1 min Flgure 5. Effects of segmentors on both the extractability and the sample band broadening: straight line, 45/45-W; dotted line, 90/90-T. Five-meter length X 0.5 mm i.d. extraction coil was used.

tested using the extraction system described in this report. When 1 mm i.d. tubing was used as an extraction coil, these

segmentors showed no difference in the flow pattern (as in Figure 4a). By use of 0.5 mm i.d. tubing, both 90/90-T and 3Q/3Q-Y showed the flow pattern as in Figure 4b and 45145-W the different pattern as in Figure 4c, respectively. The peak

2126 ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980

2 5 - - 5 20-

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o 15'

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9 10-

a

: 10.0% t : 5,O ' i

25 1

I I I I ~ I I I 7 1 2 3 4 5 6 7 i 9 1 0

LENGTH OF EXTRACTION C O I L S ( m )

1 0 1 J. 1

0 0.1 0.2 0.5 0.4 0.5 0.6 0.7 0.8 0.9

1 5.0

Table I. Relative Molar Extractabilitiesa of Cationic Surfactants

extraction coils 0.5 mm 0.5 mm

cationic surfactantsb x 2 m x 5 m

hexadecylpyridinium chloride 102.0 101.0 trimethyloctadecyl- 94.0 100.9

benzyloctadecyldimethyl- 90.1 99.0

hexadecyldimethyloctadecyl- 98.2 100.0

ammonium chloride

ammonium chloride

ammonium chloride

methyloctadecyl- ammonium chloride

hexadecyl( 2-hydroxyethy1)- 69.8 98.9

benzethonium chloride 100.0 100.0 ' Values are normalized relative to the molar extrac-

tability of benzethonium chloride. the reagent was 52% v/v. mM) was manually injected.

Table 11. Group of Each Homologue

Methanol content of Each surfactant solution (3

Relative Molar Extractabilities' for the Alkyl

type of compound

EXTRACTION T l V E CALCULATED ( m ln . )

Figure 6. Effects of coil inside diameter and coil length on both the extractability and the sample band broadening. Abscissa: (a) length of extraction coil; (b) extraction time calculated. Extraction coils: (0) (m) 0.3 mm i.d.; (0) (0) 0.5 mm i.d.; (A) (A) 1.0 mm i.d. Solid and open symbols represent the peak areas and peak heights, respectweiy. One millimolar sodium dodecylsulfate solution (6 pL) was injected manually into the methylene blue reagent (detectlon 660 nm).

height and the band broadening (width a t half-height) was also affected by them. The results showed no significant difference between 90/90-T and 30130-Y but slight im- provement when the 45/45-W was used (Figure 5). Throughout the experiment, we preferred the 45145-W in the system.

In this system, the phase-separating system described previously (9) was improved, and the more simplified and practical separator as shown in Figure 3 was constructed. The structure of a phase separator was affected by the flow pattern. The inner volume (12.5 pL) of a separator was overflowed with an aqueous segment when 2 mm i.d. tubing was used for the extraction coil. Therefore the stream of alternate aqueous and chloroform segments should be as finely segmented as possible to reduce the inner volume of the separator. The separator showed good phase separation for a wide range of methanol content (up to 55% v/v), when 0.3 mm and 0.5 mm i.d. tubing were used.

Extraction Coils. Extraction coils of 0.3 mm, 0.5 mm, 1.0 mm, and 2.0 mm i.d. and various lengths were made and evaluated for the effects on the band broadening and the extractability for the determination of sodium dodecylsulfate using the system in this report.

The coil of 2.0 mm i.d. was unacceptable as an aqueous segment overflows the inner volume of the separator. Suc- cessive increases in length of the 1.0 mm i.d. coil gave rise to increases both in the peak area (extractability) and in the width at half-height (band broadening), whereas the 0.5 mm i.d. coil showed no significant differences (Figure 6a). With the increase in length with all inside diameter coils, the coefficient of variation decreased. At the same extraction time (Figure 6b), the peaks became sharper and higher when 0.5 mm i.d. coils were used, compared with 1.0 mm i.d. coils. Peak area (extractability) of 1.0 mm i.d. coils was larger than that of 0.5-mm i.d. coils independent of coil length. This may be the contribution of a fourfold increase of the interfacial cross

alkyl- alkylbenzyl-

pyridinium ammonium ammonium alkyl- trimethyl- dimethyl-

compd chloride chloride chloride

c,o 83.2 ClZ 99.2 94.7 98.6 Cl4 101.0 101.5 100.6 c, 6 101.3 100.1 103.2 c, 6 100.0 100.0 100.0

a Values are normalized relative to the molar extrac- tability of C,, alkyl group of each homologue. Meth- anol content of the reagent was 52% v/v. used was 5 m length X 0.5 mm i.d. Each surfactant solution (3 mM) was manually injected.

section between alternate aqueous and chloroform segments, arising from doubling the coil diameter. When 0.3 mm i.d. tubing was used, the extractability and the band broadening were similar to results with 0.5-mm tubing.

From these data, it is obvious that smaller inside diameter coils contribute significantly to the decrease in the band broadening in the extraction system when the length of the coils increases.

Effects of Methanol a n d Relative Molar Ext rac ta - bility. As described previously (6, 9), the concentration of methanol in the reagent is the important parameter to obtain good recorder traces and equal molar response for different types of surfactant in the continuous flow analysis.

The results of the experiment varying the methanol con- centration up to 55% v/v in the Orange I1 reagent showed that (i) similarly to the results previously reported (6, 9), an increase of methanol content in the reagent changes the relative molar extractabilities of cationic surfactants, con- verging to identical molar responses that are independent of the type of surfactant and reducing the tailing of peaks, (ii) when 0.3-mm bore tubing is used for the extraction coil, the relative extraction time is so short that equal molar response for different types of surfactant cannot be obtained because of the kinetically slow extractabilities of the surfactants which differs from one another by the type, (iii) the optimum con- dition is given when the content of methanol was made to 52% v/v in the reagent using a 5 m X 0.5 mm i.d. extraction coil, and (iv) the relative molar extractability is significantly af- fected by coil length more than by methanol content, when the hydrophilicity of the surfactant such as hexadecyl(2-hy-

Extraction coil

ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980 2127

miscible phases, the coefficient of variation was increased, and the extractability decreased compared to the proposed me- thod.

In the case of using the automatic sampling device shown in Figure Ib, the effects of sampling intervals were observed on both the peak height and coefficient of variation. If the time is too short, the surfactants in the sample solution adsorb on the Tygon tubing during the passage toward valve 15 reducing both the peak height and the precision. As the results of the experiment show, more than 60 s is necessary to obtain the adsorption equilibrium in the system.

In the proposed method, the calibration curve for the surfactants gave completely linear response (concentration range: 0.3-3.0 mM manually injected; 0.10-1.0 mM auto- matically injected) and good reproducibility (within 2.0% coefficient of variation manually injected and within 1.5% automatically injected), independent of the type of surfactant.

The reproducibility of the proposed method for different concentrations is listed in Table 111.

Figure 7 shows the representative example of the automated analysis. Negative peaks are due to the irregularity of flow disturbed by the injection pulse of rotary valve 15.

Futhermore, this method has the simplicity of solvent ex- traction by FIA and the enhanced capacity of an analytical method of industrial use.

ACKNOWLEDGMENT The author thanks K. Hirai for her technical assistance, A.

Nakae, M. Yamanaka, and K. Tsuji for their helpful discussion on this subject, and S. Tamura (Tokyo University, Japan) for his private communication on the PTFE membrane.

LITERATURE CITED (1) Eptons, S. R. Trans. Faraday SOC. 1948, 44. 226-230. (2) International Organization for Standard 1973, IS0 2871. (3) Japanese Industrial Standard 1970, K3362. (4) Few, A. V.; Ottewell, R. H. J. Colloid Sci. 1956, 7 7 , 34-38. (5) Scott, G. V. Anal. Cbem. 1988, 40, 768-773. (6) Kawase, J.; Yarnanaka, M. Anawst (London) 1979; 704, 750-755. (7) RuiiEka. J.; Hansen, E. H. Anal. Cbim. Acta 1978, 99, 37-76. (8) Betteridge, D. Anal. Cbem. 1979, 50, 832A-846A. (9) Kawase, J.; Nakae, A,; Yamanaka, M. Anal. Cbem. 1979, 57,

1640-1643. (IO) Tamura, S. "Trace Analysis"; Kagakudojin (Kyoto) 1978. 177. (1 1) Frei, R. W.; Michel, L.; Santi, W. J. Cbromatogr. 1976. 726, 665-677. (12) Frei, R. W.; Michel, L.; Santi, W. J. Cbromatogr. 1977, 742, 261-270. (13) Frei, R. W. J. Cbromatogr. 1979, 165, 75-86. (14) Oeirich, E.; Theuerkauf, D. HRC CC J. High Resolut. Chromatogr.

Cbromatogr. Commun. 1979, 2 , 256-258.

~ ~~~~

Table 111. of Different Concentration

Repeatability of the Proposed Method

surfac- tanta

mode of concn, peak height, mm injection mM X n range CV,b %

manual 1.5 103.3 5 101.9-105.0 1.28 ( 6 p L ) 3.0 203.4 5 200.9-209.9 1.83

(2OpL) 0.80 178.0 5 177.3-179.1 0.432 automatic 0.40 87.2 9 87.0-88.0 0.946

a Benzethonium chloride was used as an example. b Coefficient of variation. -

-

Scon

Figure 7. Example of the automated determination of cationic sur- factants. Values on the peaks represent the concentration (mM). Sampling volume was 20 yL, and sampling rate was 60 samples/h. Unknown samples were products of our industry.

droxyethy1)methyloctadecylammonium chloride increases. Table I lists results for six different types of surfactants.

The results of alkyl chain lengths for each homologue are compared in Table 11.

Precision of Analysis. When sample solutions were in- jected at the point just beyond the mixing point of two im-

RECEIVED for review March 27, 1980. Accepted August 11, 1980.