Langmuir 1998, 14, 7147-7154
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New Hydrophobic π* Indicators. Solvatochromic Properties and Interactions in Micellar Solutions R. Helburn,* Y. Dijiba, G. Mansour, and J. Maxka Department of Chemistry, Northern Arizona University, Box 5698, Flagstaff, Arizona 86011-5698 Received June 30, 1998. In Final Form: October 13, 1998 Solvent-sensitive probes have wide application in the study and characterization of interfacial systems. This work presents some new, more hydrophobic versions of the solvatochromic π* indicators of solvent dipolarity-polarizability. The new dyes form a homologous series of N,N-dialkyl-p-nitroanilines. The indicators are designed to act as improved probes of micelles and lipid bilayers in polar and aqueous solutions. UV-vis studies of the N,N-dipentyl- and N,N-dihexyl-p-nitroanilines show the two indicators to be strongly solvatochromic; values of -s estimated from two-point correlation are 3.081and 3.110, respectively. UV-vis spectra of N,N-dipentyl- and N,N-dihexyl-p-nitroanilines obtained at variable indicator concentration in cyclohexane and dimethyl sulfoxide, within the 10-5 M concentration range, display no concentration-dependent spectral shifting and no evidence of indicator aggregation in accordance with Beer’s law. Studies of the indicator series in aqueous solutions of varying concentration of the anionic surfactant sodium dodecyl sulfate (SDS) suggest that N,N-dipropyl-p-nitroaniline probes micelles of SDS more deeply than the commonly used N,N-dimethyl- and N,N-diethyl-p-nitroanilines. Of the three indicators in the series, N,N-dimethyl-, N,N-diethyl-, and N,N-dipropyl-p-nitroaniline, the dipropyl species is most responsive to the critical micelle concentration of SDS. The longer chain dyes N,N-dibutyl-, N,N-dipentyl-, and N,N-dihexyl-p-nitroaniline are more effective probes of larger micelles of the nonionic surfactant Triton X-114. UV-vis studies of N,N-dihexyl-p-nitroaniline in micellar solutions of Triton X-114 and model systems suggest that the longest chain indicator probes the outer polyoxyethylene portion of the Triton X-114 micelle.
Introduction The study of noncovalent interactions is becoming increasingly important in the investigation of interfacial solute binding. Weak intermolecular forces such as induction, dispersion, and hydrogen bonding play an important role in solute-solvent interactions and in the ability of a solute to distribute between two phases via a partitioning or adsorption process. These processes are relevant to a number of research areas including: (1) the absorption of drugs and nutrients in biological systems,1-3 (2) molecular recognition,4-7 (3) the behavior of pollutants in the natural environment,8,9 and (4) the analysis and optimization of analytical separations.10-12 Polarity-sensitive indicators probe solvent-solute interactions and provide a valuable approach to characterizing these processes. Researchers have used polarity-sensitive indicators to probe binding environments in micelles,13-19 in lipid * Address correspondence to this author. (1) Westphal, U. F.; Knoefel, P. K. In Absorption, Distribution, Transformation and Excretion of Drugs; Knoefel, P. K., Ed.; Charles C. Thomas Publishers: Springfield, IL, 1972; pp 56-76. (2) Palm, K.; Stenberg, P.; Artursson, P. Pharm. Res. 1997, 14, 568. (3) Poelma, F. G. J.; Breas, R.; Tukker, J. Pharm. Res. 1990, 7, 392. (4) Vasker, I. A.; Aflalo, C. Proteins (3rd Ed.) 1994, 20, 320. (5) Naray-Szabo., G. J. Mol. Recognit. 1993, 6, 205. (6) Paleos, G. M.; Tsiourvas, D. Adv. Mater. (Weinheim, Ger) 1997, 9, 695. (7) Fan, E.; Van Arman, A. A.; Kincaid, S. J. Am. Chem. Soc. 1993, 115, 369. (8) Sandeep, P. N.; Sabatini, D. A.; Harwell, J. H. Environ. Sci. Technol. 1994, 28, 1874. (9) Chiou, C. T. Environ. Sci. Technol. 1985, 19, 57. (10) Dorsey, J. G.; Dill, K. A. Chem. Rev. 1989, 89, 331. (11) Khaledi, M. G.; Smith, S. C.; Strasters, J. K. Anal. Chem. 1991, 63, 1820. (12) Kord, A. S.; Khaledi, M. G. Anal. Chem. 1992, 64, 1894. (13) Vitha, M. F.; Weckwerth, J. D.; Odland, K.; Dema, V.; Carr, P. W. J. Phys. Chem. 1996, 100, 18823.
bilayers,17-21 and in reversed-phase chromatographic stationary phases.22,23 The results from these studies can be used to make predictions about solute retention and about the properties of a solute as it occurs in the associated form. However, many commonly used indicators are not always optimal for this task.24 For example, those dyes which are sufficiently hydrophobic (e.g., Reichardt’s dye)25,26 are too nonspecific. Indicators which are more specific in the information they yield (e.g., the π* indicators of solvent dipolarity-polarizability)27 tend to be weakly lipophilic.24,27 The ability of the probe to respond to solvent polarity directly, without complication due to probe structure and conformation, is also an important consideration. In this work we present a homologous series of solvatochromic π* indicators (Figure 1). The new dyes in the series (1c-f) are designed to have increased hydrophobic character; they are relatively simple in structure. We (14) Vitha, M. F.; Carr, P. W. J. Phys. Chem. 1998, 102, 1888. (15) Kido, J.; Endo, M. Hiyoshi, C.; Nagai, K. J. Colloid Interface Sci. 1991, 142, 326. (16) Kessler, M. A.; Wolfbeis, O. Chem. Phys. Lipids 1989, 50, 51. (17) Shin, D. M.; Schanze, K. S.; Whitten, D. G. J. Am. Chem. Soc. 1989, 111, 8494. (18) Shin, D. M.; Whitten, D. G. J. Phys. Chem. 1988, 92, 2945. (19) Shin, D. M.; Schanze, K. S.; Otruba, J. P.; Brown, P. E.; Whitten, D. G. Isr. J. Chem. 1987, 28, 37. (20) Tsukamoto, I.; Ogli, K. J. Colloid Interface Sci. 1994, 168, 323. (21) Zachariasse, K. A.; Van Phuc, N.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676. (22) Helburn, R. S.; Rutan, S. C.; Pompano, J.; Mitchem, D.; Patterson, W. T. Anal. Chem. 1994, 66, 610. (23) Lu, H.; Rutan, S. C. Anal. Chem. 1996, 68, 1387. (24) Helburn, R.; Ullah, N.; Mansour, G.; Maxka, J. J. Phys. Org. Chem. 1997, 10, 42. (25) Dimroth, K.; Reichardt, C. Z. Anal. Chem. 1966, 215, 344. (26) Reichardt, C.; Harbusch-Gornert, E.; Schafer, G. Leibigs Ann. Chem. 1988, 839. (27) Kamlet, M. J.; Abboud, J. L.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027.
10.1021/la980787v CCC: $15.00 © 1998 American Chemical Society Published on Web 11/19/1998
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Figure 1. Homologous series of π* indicators: (a) N,Ndimethyl-p-nitroaniline, (b) N,N-diethyl-p-nitroaniline, (c) N,Ndipropyl-p-nitroaniline, (d) N,N-dibutyl-p-nitroaniline, (e) N,Ndipentyl-p-nitroaniline, and (f) N,N-dihexyl-p-nitroaniline.
Figure 2. Schematic representation of solvent effects on the electronic-transition energy of dipolar solutes; illustration of positive solvatochromism.
present our findings on the behavior and properties of the new lipophilic forms and on their ability to probe dipolar and dispersion effects in micelles in aqueous solution.
Of the three solvent polarity parameters, π* (dipolaritypolarizability),27,28 R (hydrogen-bond-donor (HBD) acidity,30 and β (hydrogen-bond-acceptor (HBA) basicity,31 the π* parameter is of particular significance because it represents a measure of solvent polarity without contribution from hydrogen-bonding interactions. New π* Indicators. We have selected the di-n-alkylp-nitroanilines (DNAP) as a base for creating a homologous series of π* indicators with increasing lipophilic character (Figure 1). 1a and 1b have been previously characterized; they are known to possess relatively high values of -s (eq 1).27 1c and 1d are the first dyes in our suite of more hydrophobic forms. In a prior paper, we showed that values of -s for 1c,d are slightly lower than -s for 1a,b, but still high when compared to those of other π* indicators.24 In this work, we give a very brief description of the synthesis of 1c-f. We use the reference solvents (DMSO and cyclohexane) to examine (1) the solvatochromic properties of 1e-f, (2) the concentration dependence of vmax for the four solvent-dye systems, and (3) the adherence of these systems to Beer’s law. For the longer chain indicators to act as effective solvent-sensitive probes, they must not self-associate in solution. Examining their spectroscopic behavior on a concentration basis is one way to evaluate this phenomenon. Last, we provide a beginning description of the suite of indicators in micellar solutions. The behavior of 1a-c in solutions of the anionic surfactant sodium dodecyl sulfate (SDS) and 1d-f in solutions of the nonionic surfactant Triton X-114 are discussed. In the latter section, we provide evidence that the longer chain indicators 1d-f are more well suited for probing the larger micelles of the nonionic surfactant Triton X-114. Experimental Section
Theory and Background
In Equation 1, υmax is the frequency of maximum absorption (in cm-1) for the indicator in the solvent of interest; υo is the frequency of maximum absorption for the indicator in cyclohexane. Within the context of the π* scale as established by Kamlet, Abboud, and Taft,27 the slope s reflects the magnitude of spectral shift between two reference solvents, cyclohexane (π* ) 0.00) and dimethyl sulfoxide (DMSO) (π* ) 1.00).27 The indicators in Figure 1 exhibit positive solvatochromism, implying that the excited state of the probe is stabilized (via intramolecular charge transfer) relative to the ground state with increasing solvent dipolarity (Figure 2), resulting in a shift of λmax to longer wavelengths.29
Reagents. Triton X-114 and spectroscopic-grade cyclohexane were purchased from Aldrich. SDS (99% purity) and spectroscopicgrade DMSO were from Acros. p-Nitrophenol was obtained from J. T. Baker; diethyl ether was from EM Science. 2-Ethoxyethanol was obtained from MCB. The indicators N,N-dimethyl-p-nitroaniline (1a) and N,N-diethyl-p-nitroaniline (1b) were purchased from Pfaltz & Bauer and Frinton Laboratories, respectively. Synthesis. Indicator 1c was prepared via nitration of N,Ndipropylaniline.24 Indicators 1d-f were synthesized via nucleophilic aromatic substitution of 4-fluoronitrobenzene with the corresponding dialkylamine. Procedural details are given elsewhere.24,32 Aspects of the synthesis discussed here are those which may affect the solvatochromic properties of the product. Solutions. Aqueous solutions containing surfactant and indicator were prepared with distilled deionized water, without added buffer or ionic-strength adjustment. For nonquantitative data (i.e., data exhibiting spectral shifts only), the indicator was added at concentration where the absorbance was 0.5 or lower. Spectroscopic Methods. All UV-vis spectra were collected using a Varian Cary 3E scanning UV-vis spectrophotometer. The spectrophotometer was interfaced to an IBM-PC equipped with Cary 13E software. For each dye-solvent system, separate spectra were collected of the dye solution and for the background solvent. The spectral data were digitized and imported into a Quattro Pro spreadsheet. Background spectra were subtracted directly. An offset correction was applied where appropriate. For data illustrating spectral shifts only, absorbances were normalized to a single value. Values of λmax were determined by the method of Kamlet, Abboud, and Taft.27 In this approach, λmax is taken as the midpoint between the two positions where the absorbance is 90% of the maximum.27
(28) Laurence, C.; Nicolet, P.; Tawfik Dalati, M. J. Phys. Chem. 1994, 5807. (29) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, Germany, 1988.
(30) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886. (31) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 377. (32) Mansour, G. Masters Thesis, Northern Arizona University, Flagstaff, AZ, 1998.
π* Indicators. Solvatochromic dyes act as indicators of solvent polarity by exhibiting shifts in the positions of their UV-vis absorption bands. The π* indicators are a class of solvatochromic dyes that respond to the nonspecific portion of van der Waals forces (e.g., solvent induction, dispersion, and dipolar effects).27,28 Scales of solvent dipolarity-polarizability (π*) have been developed on the basis of several indicator solutes.27 The π* parameter is calculated from the relative positions of UV-vis absorption bands. Values of π* are related to the transition energy through a linear solvation energy relationship (LSER) of the following form.27
υmax ) υo + sπ*
(1)
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Table 1. Physical Properties of DNAP Indicators compd
phase
mp (°C)
ref
1a 1b
crystal crystal
163-165 73-74 77-78 62-63 59 52-54
33, 34 35 36 36 37 24 this work this work this work
1c
crystal
1d 1e 1f
oil oil oil
Table 2. Values of λmax (nm) for Absorption Spectra of 1e and 1f as a Function of Molar Concentration in Cyclohexane and DMSO 1e in cyclohexane λmax
M (105)
0.89 1.79 2.69 3.59 4.49 5.38
367.7 366.8 366.8 367.6 367.5 366.7
0.88 1.46 1.76 2.34 2.93 nda
M (105)
λmax
M (105)
λmax
0.62 1.25 1.88 2.51 3.14 3.77
413.5 413.7 413.5 413.4 414.2 413.5
0.52 1.04 1.56 2.08 3.13 4.17
414.2 414.0 413.7 414.0 413.7 413.7
1e in DMSO
a
Figure 3. (a) Reaction scheme for the synthesis of 1c. (b) Reaction scheme for the synthesis of 1d-f.
Results and Discussion Synthesis and Physical Properties. Schemes for the synthesis of 1c-f are illustrated in parts a and b of Figure 3. Some physical properties of the indicators are given in Table 1.24,33-37 It is of interest to note that, under ambient conditions, compounds 1a-c are crystalline and 1d-f are oils. This phase behavior is analogous to that of other alkyl-substituted aromatic species.38-41 Where the alkyl chains are short (n ) 1-3; Figure 1) the crystal packing is dominated by aromatic and polar group interactions. As the alkyl chains get longer (n ) 4-6; Figure 1), hydrophobic interactions among the alkyl groups become equal in magnitude to those among the more polar moieties, and the melting point(s) drop below room temperature. In the preparation of 1d-f (Figure 3b), one concern was the formation of the contaminant p-nitrophenol (PNP). PNP is a solvatochromic species with spectroscopic properties similar to those of the DNAP indicators.31 Trace quantities of PNP can form via hydrolysis of 4-fluoronitrobenzene or hydrolysis of the DNAP product under strong-acid conditions. We observed42 PNP in the reaction (33) Eastes, J. W.; Alderidge, M. H.; Minesinger, R. R.; Kamlet, M. J. J. Org. Chem. 1971, 36, 3847. (34) Suhr, H. Justus Liebigs Ann. Chem. 1965, 689, 109. (35) Matsumura, E.; Tohda, Y.; Ariga, M. Bull. Chem. Soc. Jpn. 1982, 55, 2174. (36) Ibata, T.; Isogami, Y.; Toyoda, J. Bull. Chem. Soc. Jpn. 1991, 64, 42. (37) Nagornow, N. J. Russ. Phys.-Chem. Soc. 1907, 39, 699. (38) Bryan, R. F.; Hartley, P. Mol. Cryst. Liq. Cryst. 1980, 62, 259. (39) Bryan, R. F.; Hartley, P.; Miller, R. W. Mol. Cryst. Liq. Cryst. 1980, 62, 281. (40) Bryan, R. F.; Hartley, P.; Miller, R. W. Mol. Cryst. Liq. Cryst. 1980, 62, 311. (41) Desiraju, G. R. Crystal Engineering; the Design of Organic Solids; Materials Science Monographs #54; Elsevier: Amsterdam, 1989; Chapter 4. (42) Observation of PNP was carried out via TLC and GC-MS using known standards in addition to the product mixture.
1f in cyclohexane
M (105)
λmax 367.0 367.6 366.5 367.4 366.5 nda
1f in DMSO
No data taken.
mixture during extraction and on the alumina column during the final separation step.32 In DMSO, PNP has an absorption band with a maximum at 323 nm. In cyclohexane, λmax for PNP is at 287 nm. Aqueous solutions of PNP have a faint-yellow color due to absorption by the p-nitrophenolate anion. However, the predominant absorption band for aqueous solutions is that of the phenol (λmax ) 318 nm). These spectral bands do not overlap the solvatochromic bands of the DNAP indicators. Moreover, PNP was removed from the oil-phase products which are the subject of spectral studies in this work (1d-f). UV-Vis Studies of 1e and 1f. UV-vis spectra of 1e and 1f in DMSO and cyclohexane are illustrated in parts a and b of Figure 4. The spectra are reported for a range of indicator concentrations (Table 2). A concern for these long-chain dyes is the possibility that they may aggregate in solution, resulting in concentration-dependent spectral shifts. The λmax values in Table 2 show that the range of λmax for 1e and 1f in the two solvents was not more than 1.1 nm (1f in cyclohexane) and as small as 0.5 nm (1f in DMSO).43 More importantly, there was no trend in λmax for these systems. The λmax values were randomly distributed over the range of concentrations employed (Table 2). In addition, the linearity in the plot of absorbance vs concentration for the four systems, for a range of λ near λmax, suggests a lack of solute aggregation in accordance with Beer’s law (Figure 5) (values of r2 are g0.99 for all sets of data). Linear plots of absorbance vs concentration for the four dye-solvent systems (1e and 1f in DMSO and cyclohexane) over a selected range of λmax (Figure 5) have slopes that are very similar, suggesting that values of molar absorptivity () for the four systems (at or near λmax) are similar. For 1e and 1f in cyclohexane, values of slope were computed as an average from three linear regressions; the average values lie within each other’s range of error (0.210 ( 0.003 for 1e; 0.215 ( 0.003 for 1f). Slopes for 1e and 1f in DMSO, calculated in the same manner, differ slightly and are outside the range of error for the two respective values (0.245 ( 0.002 for 1e; 0.231 ( 0.002 for 1f). These small differences could be explained in terms (43) On the basis of instrument settings, values of λ can be considered accurate to the nearest 0.5 nm. In applying the 90% method,27 two points are needed to locate a single value of λmax. Thus, the inherent error in locating λmax should be ( 1.0 nm. In determining λ at 90% Amax, some averaging was used on account of spectral noise.
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Figure 4. (a) UV-vis absorption spectra of 1e in cyclohexane and DMSO at variable dye concentration (Table 2). (b) UV-vis absorption spectra of 1f in cyclohexane and DMSO at variable dye concentration (Table 2).
Figure 5. Plots of absorbance vs concentration for 1e and 1f in cyclohexane (A367) and DMSO (A414); 1e in cyclohexane (O), 1f in cyclohexane (0), 1e in DMSO (]), 1f in DMSO (4).
of variable band shape,44 where small changes in alkylchain length affect bandwidth to a greater extent for the more dipolar solvent. (44) Nicolet; Laurence, C. J. Chem. Soc., Perkin Trans. 1986, 11, 1071.
Solvatochromic Properties of 1e and 1f. The values of λmax for 1e and 1f in DMSO and cyclohexane can be used to evaluate the solvatochromic properties of the new indicators and to make an estimate of -s (eq 1) for the two dyes. The parameter -s is a quantitative measure which can be used to compare the spectral-shifting capability of different π* indicators over a range of solvents. In this work, we show that values of -s for 1e and 1f (from twopoint correlation)45 are 3.081 and 3.110, respectively; the spectral shifts between absorption bands for DMSO and cyclohexane (∆λmax) are 46.7 and 47.2 nm, respectively. A rigorous multipoint estimation46 of -s for a complete series of DNAP indicators is currently in progress. A listing of the solvatochromic parameters (-s and vo) for DNAP indicators that have been examined thus far (1a-f) is provided in Table 3. (45) In a recent paper,24 we reported values of -s for 1c and 1d using two- and seven-point correlations; the values differed by 1.4 × 10-2 and 4.9 × 10-2 units, respectively. (46) A rigorous estimation of -s is based on the slope of a linear plot of π* vs vmax for 20-30 nonHBD solvents (Equation 1).27 Values of π* are average values (based on several indicators) taken from the literature.27 Values of vmax are taken from UV-vis spectra of the new indicator in the solvent(s) of interest.
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Table 3. Values of -s (nm) and νo (nm) for DNAP Indicators, from the Literature and from Two- and Seven-Point Correlation indicator
νo
-s
ref
1a
28.10 28.20 28.23 28.52 28.58 27.57 27.39 27.41 27.32 27.33 27.27 27.27
3.436 3.456 3.464 3.182 3.136 3.122 3.007 3.021 3.051 3.002 3.081 3.110
27 24a 24b 27 24a 24b 24a 24b 24a 24b a a
1b 1c 1d 1e 1f a
Table 4. Structures and Properties of Surfactants Used in This Work
*Average value.
Two-point correlation. b Seven-Point correlation.
The estimates of -s (Table 3) for 1e and 1f show that the new longer chain π* indicators retain the good spectralshifting capability of the previously characterized 1c and 1d. Good spectral-shifting capability (i.e., large ∆λmax leading to large -s) is an important property for π* indicators that are to be used to probe micelles.24 The UV-vis spectrum of an indicator that is strongly solvatochromic will display a more prominent peak shift as the dye partitions into the nonpolar micellar environment. Of the π* indicators reported by Kamlet et al.,27 values of -s range from 1.281 to 3.436 with 1a having the highest value.27 In preparing more hydrophobic π* indicators, one of our goals was to employ a structural theme whereby -s would be relatively large and reasonably constant over a series of homologues. Use of the DNAP structural theme (Figure 1) has proven successful in this respect (Table 3). We note that Shin and Whitten18 prepared a homologous series of solvatochromic R,ω-diphenylpolyenes in which the indicator trans-1-(N,N-dimethylamino)-4′-nitrostilbene was altered sequentially by lengthening the conjugated spacer from ethylene to butadiene (1-p-(N,Ndimethylamino)phenyl-4-p-nitrophenyl-1,3-butadiene) and to hexatriene (1-p-(N,N-dimethyl amino) phenyl-6-pnitrophenyl-1E,3E,5E-hexatriene). The diphenylpolyene probes have less spectral-shifting capability than the DNAP indicators (∆λmax) 30 to 38 nm for DMSO and cyclohexane or heptane); -s was seen to decrease slightly with increasing spacer length, giving way to results (in heterogeneous media) that were slightly “probe specific”.18 New π* Indicators in Micellar Solutions. In designing indicators that can effectively probe micelles or lipid bilayers from an adjacent aqueous solution, a number of factors must be considered. The molecular structure of both indicator and surfactant govern the extent and nature of the partitioning process.47 Within that constraint, the hydrophobic character of the dye plays a role. The size and shape of the solute relative to that of the micelle are also important.47 Table 4 lists the surfactants used in this work, their structures, their molecular weights, and their critical micelle concentrations (CMC) in pure water. The anionic surfactant, SDS, is molecularly homogeneous while the nonionic surfactant Triton X-114 is molecularly nonhomogeneous.48,49 Triton X-114 has an average repeat unit (CH2CH2-O) of eight.50 For the latter surfactant system, the monomer-to-micelle transition is less (47) Meyers, D. Surfactant Science and Technology, 2nd ed.; VCH Publishers Inc.: New York, 1992; Chapter 4. (48) Kile, D. E.; Chiou, C. T. Environ. Sci. Technol. 1989, 23, 832. (49) Schwartz, A. M.; Perry, J. W. Surface Active AgentssTheir Chemistry and Technology; Robert E. Krieger Publishing Co.: Huntington, N. Y.; 1978.
Figure 6. Plots of π* vs SDS concentration above and below the CMC for SDS, for three indicators: 1a (9), 1b (b) and 1c (1).
abrupt; the process of aggregate formation in the Triton X system is continuous, due to the differing solubilities of the monomers in solution. Some reported monomer aggregation numbers are 71 for SDS and 276 for C9H19C6H4(OC2H4)10OH (a compound very similar to Triton X-114).48,51 From the aggregation numbers, we can see that micelles of Triton X-114 are larger than those of SDS. Previous studies of the solubilizing capability of micelles of different surfactants have shown that for surfactants having the same nonpolar chain length, the order of solubilizing power by the inner portion of a micelle for organic solutes is nonionic > cationic > anionic.52-55 One rationale behind this observation is that micelles of nonionic surfactants are more loosely packed and have more space for the incorporation of solutes.47 1a-c in SDS Solutions. Figure 6 illustrates plots of π* vs molar concentration of SDS in pure water for three indicators in the homologous series 1a-c (note that 1c is the first new indicator in our suite of more hydrophobic forms). All π* values were calculated from eq 1 using values of -s and vo obtained in our laboratory via a seven-point correlation (Table 3). The π* values were normalized so that π* for pure water, for each of the three indicators, would be the same (π*water ) 1.31 for 1a-c; Figure 6). These measurements were taken to ensure that the data would be comparative. Laurence et al.28 have emphasized that values of π* calculated from spectra of different π* indicators can vary, even within families of indicators.28 The three sets of π* values (1a-c) in Figure 6 show a break in trend over the SDS concentration range of 7.00 (50) Gu, T.; Galera-Gomez, P. A. Colloids and Surfaces, A 1995, 104, 307. (51) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Sons: New York, 1978. (52) McBain, J. W.; Richards, P. H. Ind. Eng. Chem. 1946, 38, 642. (53) Tokiwa, F. J. Phys. Chem. 1968, 72, 1214. (54) Saito, S. J. Colloid Interface Sci. 1967, 24, 227. (55) Satake, I.; Matsumura, R. Bull. Chem. Soc. Jpn. 1963, 36, 813.
7152 Langmuir, Vol. 14, No. 25, 1998
× 10-3 to 7.50 × 10-3 M; a value for the CMC of SDS in pure water has been reported at 7.28 × 10-3 M.48 The π* values (Figure 6) decrease following the CMC, suggesting that as micelles begin to form and increase in concentration, the indicators establish a partition equilibrium between the more nonpolar micelles and the adjacent monomer solution. The trend in the magnitude of decrease is as one might predict for three dyes with increasing hydrophobic character. Indicator 1a displays the smallest break, followed by 1b. Of the three dyes, the new indicator 1c is most responsive to the CMC of SDS. It has been stated that the determination of enhanced solute solubility by micellar solutions is a more sensitive measure of the CMC of a surfactant than conventional methods such as surface-tension measurements.48,49 The latter respond to the behavior of monomers in solution, while the former detects the incipient formation and subsequent increase in concentration of aggregates.48,49 Parameters relating to the change in solvation environment of a partitioning solute should be equally sensitive. Kessler and Wolfbeis have shown that the transition energy (∆ET) calulated from the solvatochromic band of 2,6-dichloro-4-(2,4,6-triphenyl-N-pyridinio)phenolate (ET33) can be used to probe the CMC of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB).16 In this work, we find that specific measures of solvent dipolar and dispersion effects (π*) can be effective in probing the CMC of the anionic surfactant SDS. The π* values in Figure 6 are influenced by two factors. First, the spectral data from which the values were calculated reflect a mixture of dye (in some position) within the SDS micelles and dye in the adjacent solution. Note that in this work no attempt was made to mathematically resolve the spectra of the micellar-phase dye. Second, the position and orientation of the indicator in the micelle is important. Nonpolar solutes will reside in the interior of the micelle, whereas more polar indicators, should be solubilized in the outer palisades layer.47 For the DNAP indicators the polar nitro group on the aromatic ring (Figure 1) will limit the extent to which the dye can be pulled into the micelle; we imagine that the nitro group should associate with the sulfate moieties at the solvated perimeter of the SDS micelle. Vitha and Carr13 determined π* values for aqueous SDS micelles using a suite of short-chain π* indicators; they employed a curve-resolution method to remove contributions to the spectrum from absorbance by aqueous phase indicator. Using indicator 1b in conjunction with this method, they reported a value of π*SDS of 1.152.13 On the basis of the π* scale,27 values of π* equal to or greater than 1.00 imply a very polar solvation environment. Vitha et al.13 reported values of π*SDS that were all greater than 1.00 but less than π*water (1.339 for 1b);13 note that our value for pure water using 1b and a value for -s of 3.182 (used by Vitha et al.) is 1.328. These researchers concluded that their indicators were residing in the outer palisades layer of the SDS micelles.13 The π* values in Figure 6 cannot be directly compared to those of Vitha et al. However, from their comparative nature, it can be seen that 1c has the least polar solvation environment and, thus, resides more deeply in SDS micelles than 1b or 1a. Longer Chain Indicators in Triton X-114. While increasing the hydrocarbon chain length (on DNAP indicators) from n ) 1,2 (1a,b) to n ) 3 (1c) improves, systematically, the indicator’s effectiveness in probing SDS micelles, there is no evidence at this time that indicators with longer alkyl chains, e.g. n ) 4, 5 or 6 (1d-f), will do a proportionally better job for that system. We find that visible spectra of 1e and 1f in 0.0093 M aqueous SDS
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solutions (as well as higher concentration SDS solutions) are essentially overlapped with that of 1a in solutions of the same SDS concentration, suggesting that the three indicators have similar solvation environments. Yet the solvatochromic band of 1c in a 0.0093 M SDS solution exhibits a hypsochromic shift of 17 nm relative to that of 1a. Spectral shifts for 1d (relative to 1a) in 0.0093 M SDS do occur, but they are smaller than that for 1c. A possible explanation for the apparent lack of partitioning by the very long alkyl chain dyes (1e,f) is that the nitro group on the indicator is left hanging outside of the micelle. The entire dye molecule must be pulled into the micelle in order for a spectral shift to occur. Conversely, indicators 1d-f are very effective in probing the larger micelles of the nonionic surfactant Triton X-114. For a 3.43 × 10-4 M solution of Triton X-114 (CMC ) 2.05 × 10-4 M),48 the solvatochromic bands of 1e and 1f exhibit hypsochromic shifts of 28 nm relative to that of 1a. The absorption band of 1d in the Triton X solution exhibits a hypsochromic shift of 23 nm relative to 1a (Figure 7a; Table 5). Thus, for the Triton X-114 system, the longest chain indicator (1f) performs best. The enhanced spectral shifts exhibited by 1f in solutions of Triton X-114 imply greater solubilization of the longer chain dye by the larger micelle. Again, the positioning of the indicator in the micelle, especially the NO2 group (Figure 1), must be considered. In micellar solutions of Triton X-114, the polar NO2 group on 1f could find a stable environment near the OH group on the micelle surface or among the polyoxyethylene chains (see Table 4). The λmax of 1f in micellar solutions of Triton X-114 is very similar to λmax of 1f in solutions of 2-ethoxyethanol (CH3CH2OCH2CH2OH) and in mixtures of 2-ethoxyethanol and hydrated diethyl ether (CH3CH2OCH2CH3). We used the latter two solvent systems as a model of the outer portion of the Triton X micelle (see Tables 4 and 5). Theoretical and experimental studies of Triton X-114 micelles in aqueous solution point to the occurrence of spherical aggregates56 and the existence of two distinct micellar regions: (1) a hydrophobic core containing aromatic and alkyl-chain portions of the surfactant, with some intercalated polyoxyethylene chains, and (2) a more flexible outer portion consisting of polyoxyethylene chains only.57 NMR studies indicate that water does not penetrate the inner hydrophobic core to any significant degree.57 The outer, more flexible portion may be hydrated to varying extents.57 On the basis of these previous studies and the positions of the solvatochromic bands of 1f in Triton X-114 and in the model systems (Figure 7a; Table 5), we suggest that indicator 1f partitions into the outer flexible portion of the Triton X-114 micelle. The widths of the absorption bands for indicators 1a, 1c, 1d, and 1f in the Triton X-114 solution (Figure 7; Table 5) are in support of this contention. Of the spectra in parts a and b of Figures 7, the width at half-maximum is smallest for 1f, which is an indication that the longest chain probe (1f) experiences the most restricted environment (i.e., the least variation in solute-solvent interactions). The spectral width for 1f in the Triton X-114 solution is consistent with that of 1f in the model solvents (Table 5). On the basis of the spectral shifts and peak widths for four dyes (1a, 1c, 1d, and 1f) (Table 5; Figure 7a,b), we propose the following hypothesis for the partitioning of DNAP indicators into Triton X-114 micelles. Dye 1a (56) Zheliaskova, A.; Sagnowski, S.; Derzhanski, A. Mol. Cryst. Liq. Cryst. 1990, 193, 99. (57) Dennis, E. A.; Ribeiro, A.; Roberts, M. F.; Robson, R. J. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1.
New Hydrophobic π* Indicators
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Figure 7. (a) UV-vis absorption spectra of 1a, 1d, and 1f in micellar solutions (3.43 × 10-3 M) of Triton X-114. (b) UV-vis absorption spectra of 1a and 1c in micellar solutions (3.43 × 10-3 M) of Triton X-114. Table 5. Values of λmax (nm) and Widths at 1/2 λmax (nm) from Spectra of 1a,c,d,f, in Triton X-114 and Model Solvents and Solutions dye
system
λmax
width (1/2 λmax)
1a 1c 1d 1f 1f 1f
Triton X-114 Triton X-114 Triton X-114 Triton X-114 solvent mixa 2-ethoxyethanol
422 415 399 394 393 396
80 91 70 56 56 58
a
Mixture of hydrated diethyl ether and 2-ethoxyethanol.
experiences a heterogeneous environment, but exists largely in the adjacent monomer solution. The λmax for 1a is almost that of 1a in pure water (423 nm). The partition equilibrium for 1c lies farther to the right (i.e., into the micelles). 1c exists in both micellar and monomer solution environments and thus has the widest peak (Figure 7b). 1d and 1f reside more predominantly in the micelles; the partition equilibrium for 1f lies farthest to the right. The λmax and narrow peak width for 1f is consistent with a polyoxyethylene environment. Probe-Specific Effects: Future Directions. Studies of solvatochromic dyes in heterogeneous media can yield
information about solute binding that is both fundamental and practical. However, results presented for different probes (e.g., values of λmax, ∆E, or π*) can sometimes conflict. Caution must be exercised when interpreting spectral-shift data. Our preliminary findings for 1d-f suggest that the more hydrophobic indicators in the DNAP series (Figure 1) are not good probes of SDS micelles. This result was the reverse of what we had expected for these longer chain dyes (1d-f). However, these results are not unlike those obtained by Shin et al.17 for a series of azobenzene indicators.17 The latter probes are strongly solvatochromic, possessing a chromophore similar in structure to that of the DNAP indicators. The azobenzene indicators (e.g., trans-p-(N,N-diethyl)-p′-nitroazobenzene) exhibited similarly red-shifted absorption bands (for SDS micellar systems) suggesting that the probes were experiencing a polar solvation environment.17 These two sets of results suggest that the size and shape of a probe relative to that of the partition medium may be an important criteria. Conversely, a series of the nitro-substituted R,ωdiphenylpolyenes18 (e.g., stibene, diphenylbutadiene, and diphenylhexatriene) exhibited solvatochromic absorption bands (for aqueous SDS) that were consistent with a pure hydrocarbon (e.g., heptane) environment; the extent of
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blue shift (relative to the λmax for heptane) was seen to increase with increasing conjugated spacer length. In this latter situation, there may be effects due to probe- and solvent-dependent conformations of the R,ω phenyl groups which, in turn, may influence the π conjugated chromophore, thereby affecting the shape and position of the solvatochromic band.18 Future investigations may involve the use of spectroscopic methods other than UV-vis spectroscopy to more accurately define the positioning of the indicator in a micelle or bilayer.58 With respect to interfacial systems, solvent-sensitive probes may have to be tailored to the specific environment of interest. The size and shape of a probe relative to that of the partition medium should be considered. Considerable work remains to be done in
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characterizing probe solubilization sites in interfacial and microheterogeneous media and in optimizing solventsensitive probes for specific applications. Acknowledgment. This work was supported by the National Science Foundation (Grant No. CHE-9410766), the Petroleum Research Fund (ACS-PRF Grant No. 31890GB4), and Research Corporation (Cottrell College Science Award No. CC4338). LA980787V (58) The orientation of 2H labeled solutes associated with micelles and bilayers may be examined using 2H NMR spectroscopy. Information on probe positioning and orientation could be inferred from changes in the motional anisotropy of the partitioned solute. An example of a related study can be found in J. Am. Chem. Soc. 1989, 111, 3176.
