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Anal. Chem. 1990, 62,1084-1090
nature of TOF analysis enables mass spectra to be generated in excess of 50 scan files per second. Summation of successive transients increases the signal to noise in the resulting scan file and permits an optimization between the rate of scan file generation (which affects representation of the chromatographic profile) and the quality of the resulting mass spectra. The sampling speed allows complete analysis of each individual ion source extraction and results in accurate fragmentation patterns not affected by changes in partial pressure of the analyte due to the chromatographic process. In the case of overlapping chromatographic peaks, this process yields linear sums of unskewed spectra requiring simpler algorithms for subsequent deconvolution. The combination of high scan file generation rates and simpler deconvolution procedures yields the capacity to reduce the overall analysis time by alteration of the chromatographic parameters. Indeed, it appears that time array detection may be the method of choice for applications of high-speed high-resolution GC/MS. LITERATURE C I T E D Holland, J. F.; Enke, C. G.; Allison, J.; Stults, J. T.; Pinkston, J. D.; Newcome, 8.; Watson, J. T. Anal. chem. 1983, 55, 997A-1012A. Allison, J.: Holland, J. F.; Enke, C. G.; Watson, J. T. Anal. Instrum. 1987, 16, 207-224. Holland, J. F.; Tecklenburg. R.; et ai., unpublished work. W h y , W. C.: McLaren, I. H. Rev. Sci. Instrum. 1955, 2 6 , 1150-1 157
Pinkston, J. D.; Rabb. M.; Watson, J. T.; Allison, J. Rev. Sci. Instrum. 1988, 57, 583-592. Schultz, G. A.; Tecklenburg. R.; Holland. J. F.; Watson, J. T.; Allison, J. Implementation and Assessment of Beam Deflection Time-of-Flight Mass Spectrometry for Time Array Detection in GC-MS Applications. Presented at the 37th ASMS Conference on Mass Spectrometry and Allied Topics, Miami Beach, Florida, 1989; p 1063. Yefchak, G. E.; Enke, C. G.; Holland, J. F. Int. J. Mass Spectrom. Ion Processes 1989, 87, 313-330. Erickson, E. D.; Yefchak. G. E.; Enke, C. G.; Holland, J. F. Int. J. Mass Spectrom. Ion Processes, in press. Studier. M. H. Rev. Sci. Instrum. 1963, 3 4 , 1367. Ingle, J. D.; Crouch, S. R. Spectrochemical Analysis ; Prentice-Hall: Englewood Cliffs, NJ, 1988. pp 159-161. Chesler, S. N.; Cram, S. P. Anal. Chem. 1971, 4 3 , 1922. Holzer, G.; Bertsch, W. Am. Lab. 1988, (Dec), 15-19. Lanning, L. A.; Sacks, R. D.; Mouradan, R. F.; Levine, S. P.; Foulke, J. A. Anal. Chem. 1988, 6 0 , 1994-1996. Biller, J. W.; Biemann, K. Anal. Lett. 1974, 7, 515-528. Sharaf, M. A.; Kowalski, B. R. Anal. Chem. 1982, 5 4 , 1291-1296. Ghosh. A.; Andregg. R. J. Anal. Chem. 1989, 67,73-77.
RECEIVED for review October 31, 1989. Accepted February 23, 1990. This work was funded through a Biomedical Research Technology Program grant (No. DRR-00480) from the National Institutes of Health. Funding also was provided in part by a grant from the Office of Naval Research (Contract No. N00014-81-K-0834) under the auspices of John Michalski. In addition, Eric Erickson wishes to acknowledge receipt of a long-term training fellowship from the U S . Naval Weapons Center, China Lake, CA.
Microemulsion Structure and Its Effect on Electrochemical Reactions Raymond A. Mackay
CRDEC, U S . Army, Aberdeen Proving Ground, Maryland 21010-5423 Stephanie A. Myers, Liakatali Bodalbhai, a n d A n n a Brajter-Toth*
Department of Chemistry, University of Florida, Gainesuille, Florida 32611
Changes In the microstructure of dMn-water mlcroemulslons were Identified electrochemkally by uslng ferrocene derlvatlves, methyl vlologen, and ferricyanide as the electroactive probes. Microdropiets as well as the blcontinuous microstructure were detected. Thls was accomplished by determlnlng dlffrtslon coefficients of the probes. Use of probes of dlfferent hydrophoblclty/hydrophillcity and charge made it possible to Investigate different microenvkonments of mlcroemulslons Including oil, water, and surfactant/cosurfactant interface. Electrochemkai reverslbiltty of the probes was affected by the structure and appeared to reflect the ease of mobility across interphases. Reaction potential (E,,*) of the probes depended on the compositlon of the microemulsion.
INTRODUCTION Microemulsions are of considerable current interest, as model membrane systems, in catalysis and have also been attracting interest in analysis ( I , 2). Microemulsions are three-component solutions that contain water, water immiscible hydrocarbon, and a surfactant. Frequently a cosurfactant, an alcohol, is also present. Mi-
* Author
t o w h o m correspondence should be addressed.
croemulsions are thermodynamically stable and macroscopically homogeneous; however, the structure is heterogeneous on a microscopic scale. Interest in microemulsions is due to their solution environment, which combines the properties of hydrocarbons with those of aqueous media and surfactants. The ordered microenvironment of some microemulsions is also of considerable interest. The microscopic structure of microemulsions depends on composition and is still a subject of debate ( 3 ) . Ordered structures such as microdroplets of oil-in-water (O/W) or water-in-oil (W/O) have been found in microemulsions of high water or high oil content, respectively. The microdroplet structure is similar to the structure of micelles, with the overall larger size of microemulsion droplets. Structures similar to those of normal micelles are present at high water content, and structures like those of reversed micelles are present a t low water content (4, 5 ) . So-called bicontinuous structures, with hydrocarbon and water regions stretching over large distances and having no clearly defined structure, have been identified in the intermediate regions (5). In this region, as well as in the microdroplets, surfactant and cosurfactant form the oil/water interface ( 5 ) . Mackay and co-workers have been first to suggest that electrochemical methods can be used to obtain information about the microstructure of microemulsions (6). Typically microstructures have been characterized by spectroscopic methods such as light scattering (7) and NMR (8). Diffusion
0003-2700/90/0362-1084$02.50/0 0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990
coefficients of a water-soluble probe, which have been subsequently measured by Mackay and co-workers in several systems using dc polarography, were in agreement with those determined from quasi-elastic light scattering (QLS)and were used as evidence for the microdroplet structure ( 4 ) . Electrochemically determined values of diffusion coefficients of amphophilic probes with different lengths of hydrocarbon chain were also used as evidence for the existence of microdroplets ( 4 ) . Other electrochemical measurements have pointed to the existence of the bicontinuous structure (9). From these and other results (8,10) it follows that, through systematic changes in composition, microemulsion microstructure can be controlled. More significantly, the results suggest that electrochemical methods can be used to detect the structural changes; in order to identify the changes, the region in which the probe resides must be known. In this paper, we present results in which electrochemical methods were used to identify structures of microemulsions of different composition. Compositions of microemulsions that were studied were changed in a systematic fashion to span the range of possible structures from microdroplets to bicontinuous structures. The system we chose to study was a microemulsion of hexadecyltrimethylammonium bromide (CTAB)-hexadecane-1-butanol-water. This system was chosen because its phase diagram is known ( 1 1 ) and the literature contains results from other measurements in this system that could be used for comparison with electrochemical data. In order to obtain information about the structure, diffusion coefficients of electroactive probes, which were chosen to have different properties in order to probe different structural features, were determined. Our additional goal was t o investigate the effect of the microemulsion environment on the kinetics and reaction potentials of the electrochemical probes. The results confirm that the structural information which is obtained is determined by the choice of the probe and illustrate the properties of microemulsions which control potentials of electrochemical reactions.
EXPERIMENTAL SECTION Materials. Hexadecyltrimethylammonium bromide (CTAB) was obtained from Sigma. 1-Butanol was obtained from Fisher Scientific. n-Octane, n-hexadecane, and sodium bromide were from Alfa. All chemicals were used as received. Water used was deionized and then distilled. Methyl viologen dichloride hydrate (MVz+) and 1,2-diferrocenylethane (FcFc) were obtained from Aldrich. Acetylferrocene (AcFc) was obtained from Sigma. Potassium ferricyanide was from Fisher, and ferrocene (Fc) was from Arapahoe Chemicals. Tetrahydrofuran (THF) was purchased from Fisher. All electrochemical probes and THF were used without further purification. Apparatus. For cyclic voltammetry and chronocoulometry a Bioanalytical Systems electrochemical analyzer, BAS-100, with a Bausch and Lomb DMP-40 series digital plotter was used. In the electrochemical measurements, which were conducted in a three-electrode configuration, the working electrode was glassy carbon (High Performance, Englewood, CA, or Electrosynthesis). The auxiliary electrode was platinum wire and the reference was a saturated calomel electrode (SCE). Before each measurement, the working electrode was polished with Gamal y-aluminalwater slurry (Fisher) on a microcloth using Ecomet 1 polishing wheel (Beuhler). After polishing, the electrode was ultrasonicated in distilled water for about 5 min immediately before use. Methods. The working electrode areas were determined by chronocoulometry in 3.1 mM solution of potassium ferricyanide in 0.5 M KC1. By use of the diffusion coefficient Do= 7.6 X lo4 cm2/s (IZ),the electrode areas were 0.071 & 0.009 cm2. In the measurements of electrode area, the pulse width was 250 ms and the potential was stepped from 400 to -100 mV. In cyclic voltammetric measurements in microemulsions,typical resistances were between 100 and 300 R before compensation and
1085
S
P!
01L
WEIGHT PERCENT
W=WATER, O = H E X A D E C A N E AND
S = S U R F A C T A N T ( 5 0 WT. '1. C T A B
+
50 WT.
*/e
1- B U T A N O L )
Figure 1. Phase diagram of cetylammonium bromide (CTAB)/l-butanollhexadecanelwater microemulsion ( 7 7). were compensated to less than 5 R using the BAS-100. Peak potentials and peak currents were measured after iR compensation, unless otherwise noted. The scan rate was 100 mV/s for all cyclic voltammetric measurements. The potential step window for chronocoulometry was chosen following cyclic voltammetric experiments. The pulse widths were 500 ms. Diffusion coefficients for the reduced, D i , or the oxidized D J , forms of the probes were calculated from the slopes of the plots of Q vs (t)0.5(13). All measurements were carried out at 25 f 2 "C. Preparation of Microemulsions. The test microemulsion contained hexadecane as oil, hexadecyltrimethylammonium bromide (CTAB) as a surfactant, and 1-butanolas a cosurfactant. The phase diagram of this microemulsion has been reported (11) and is shown in Figure 1. The microemulsion region is shown in Figure 1 as the region between the two solid lines in the phase diagram. The phase diagram in Figure 1 describes a pseudothree-component system representing water, oil (hexadecane),and emulsifier, which is a surfactant (CTAB)/cosurfactant (1-butanol) mixture, where the ratio of CTAB to 1-butanol in the emulsifier is 1:l. Most microemulsion solutions were prepared along the dashed line shown in the phase diagram. This range of compositions corresponds to a relatively low oil content (percent oil) and a wide range of water content. Additional microemulsion compositions were tested to explore effects other than dilution by water. All compositions are summarized in Table I. Because percent water was varied over a wide range, changes in percent water were used to represent changes in composition. All results are, therefore, reported as a function of percent water. The second microemulsion that was studied contained as oil octane instead of hexadecane, the same surfactant (CTAB) and cosurfactant (1-butanol), and, in addition, the electrolyte NaBr (Table I). This microemulsion has been extensively characterized by spectroscopic methods ( 1 4 ) and served as a reference system in this work. In preparation of the microemulsions each component was added by weight and the solution was mechanically stirred until clear and homogeneous. The microemulsions were stable for several months and could be frozen and thawed. Ultrasonication was used to aid in dissolving the electrochemical probes in microemulsions. The microemulsion solutions containing probes were used within 2 days and were stored at 10 "C if not used immediately. Due to the limited solubility of 1,2-diferrocenylethane, this probe was first dissolved in THF, then the microemulsion was added. Chronocoulometric results with acetylferrocene led to large negative intercepts of Q vs (t)0.5plots with poor reproducibility and correlation coefficientslower than those of other probes. Solutions of methyl viologen were deaerated for about 5 min by bubbling nitrogen through the solution before measurements.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990
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0
Table I. Microemulsion Compositionso 7 0
surf+
% cosurfe
% oild
70 watere
% ef
30.6 29.9 37.4 30.2 30.0 28.0 27.5 23.0 18.6 18.0 18.0 14.0 9.9 9.5 7.0 5.4 4.7 4.6
19.5 19.8 4.3 9.8 3.6 4.0 3.8 3.9 4.1 4.5 4.0 1.9 1.3 1.0 1.1 1.0 2.38 2.38
19.5 20.5 20.9 29.9 36.4 40.0 41.2 50.1 58.9 59.5 59.9 70.0 79.3 80.0 84.9 88.6 89.7 89.8
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.0 1.o
30.5 29.8 37.4 30.1 29.9 28.0 27.5 23.1 18.4 17.9 18.1 14.0 9.5 9.5 7.0 5.0 2.3 2.3
F e r r o c e n e (Fc) 5 x , 0 4 M IN
,SOL"BlL,T"
H201
M e t h y l Viologen
Ferricyanide
Figure 2. Electroactive probes.
..I
10.00
.=...
5mM Fc in CTAB ME 5mM Fc i reference ME 5mM Me$ jn CTAB ME ( 8 G 5rnM MeV*' in reference ME -* 5rnM ferricyanide in CTAB ME OOOBO
8.00
/
"11 microemulsions are referred to in the text and figures by their percent water. surf, hexadecyltrimethylammonium bromide. ccosurf, 1-butanol. doil, hexadecane. e water, deionized distilled. 'e, NaBr. #Oil is octane instead of hexadecane (reference
4
microemulsion).
"0
4.00
a
2.00
4
/
;1
Solutions of ferrocene derivatives were not deaerated, since it was found that deaeration did not affect the electrochemical response. In order to avoid changes in composition of the microemulsion due to evaporation, microemulsion solutions were not used for extended periods. All potentials are reported vs SCE.
0.000.00
RESULTS Behavior of Electrochemical Probes. Electrochemical probes of different hydrophobicity which were used in this study are illustrated in Figure 2. The order of hydrophobicity of the probes is 1,2-diferrocenylethane > ferrocene > acetylferrocene. Methyl viologen and ferricyanide are both hydrophilic. Ferricyanide. Because of the limited solubility of ferricyanide in the presence of CTAB, the range of microemulsion compositions that could be tested was limited to those with a relatively high content of 1-butanol. In the microemulsions which were studied and which were stabilized by positively charged surfactant (Table I) both redox forms of cyanoferrate, which are negatively charged, can be expected to partition between the membrane phase, formed by the surfactant/cosurfactant, and the bulk aqueous phase. As a result, both redox forms should probe similar microenvironments. As shown in Table 11, in all microemulsions tested values, where Ellz = (E, + E,)/2, and the values of diffusion coefficients of ferricyanide ( D i ) remained relatively constant with changes in composition. The results also show that, at the experimental scan rate, reversible behavior of the redox couple (measured by AE,,and ipa/iw) is independent of composition. Electrochemical results are summarized in Table 11, and the changes in D,' with composition are illustrated in Figure 3. Diffusion coefficient values in Table I1 are an order of magnitude lower than the Do value for ferricyanide in aqueous
F c - C - CH3 Acetylferrocene
FC
