J. Phys. Chem. 1996, 100, 18823-18828
18823
Study of the Polarity and Hydrogen Bond Ability of Sodium Dodecyl Sulfate Micelles by the Kamlet-Taft Solvatochromic Comparison Method Mark F. Vitha, Jeff D. Weckwerth, Kris Odland, Valdemia Dema, and Peter W. Carr* Department of Chemistry, UniVersity of Minnesota, Kolthoff and Smith Halls, 207 Pleasant St. S.E., Minneapolis, Minnesota 55455 ReceiVed: July 16, 1996; In Final Form: September 24, 1996X
We have studied the dipolarity/polarizability (π*), hydrogen bond donor acidity (R), and hydrogen bond acceptor basicity (β) of sodium dodecyl sulfate (SDS) micelles using the Kamlet-Taft solvatochromic comparison method. The micellar environments of solubilized indicator molecules are quite polar (π* ) 1.06) and have strong hydrogen bond donating ability (R ) 0.87) and moderate hydrogen bond accepting ability (β ) 0.40). Both the high dipolarity and especially the strong hydrogen bond acidity support previous work that indicates water is present in the indicators’ micellar microenvironments. In contrast to many previous UV-vis spectroscopic studies of the polarity of SDS micelles, we have resolved the recorded micellar spectra so as to obtain the micellar spectrum of the indicators free from contributions of the indicator in the accompanying aqueous phase. This study was conducted to rationalize and complement linear solvation energy relationship (LSER) studies of partitioning in SDS micellar systems which have recently appeared in the literature. To our knowledge this is the first study in which curve resolution is applied to micellar spectra for the purpose of characterizing both the polarity and hydrogen bond ability of SDS micelles.
Introduction Surfactants, amphiphilic molecules which self-assemble to form micelles in solution, are used in all areas chemistry.1 They act as catalysts, solubility enhancers, and detergents.2,3 They are also used to alter the selectivity of separations in liquid chromatography and micellar electrokinetic capillary chromatography.4-7 The effectiveness of surfactant micelles in all of these applications relies at least in part on their chemical interactions with solubilized molecules. These interactions include London dispersion, dipole-dipole, dipole-induced dipole, and hydrogen bonding (HB) interactions. Clearly, quantitative measures of the ability of micelles to participate in such interactions are important to understanding, predicting, and using micelles in the above applications. UV-vis and fluorescence spectroscopy have been used extensively to characterize the polarity of micelles.8-14 In general, these studies provide a qualitative assessment of the chemical nature of micellar solutions. Furthermore, depending on the spectroscopic probe used and its concentration, the studies have yielded different conclusions, some reporting nonpolar micellar environments13,14 and other reporting very polar environments.8-12 In this paper, we present a quantitative assessment of the dipolarity/polarizability and HB ability of sodium dodecyl sulfate (SDS) micelles obtained using UVvis spectroscopy in combination with the Kamlet-Taft solvatochromic comparison method described below. The Kamlet-Taft Solvatochromic Comparison Method In the mid-1970s, Kamlet, Taft, and co-workers developed three essentially independent scales to quantify the dipolarity/ polarizability, HB donating ability (HB acidity), and HB accepting ability (HB basicity) of bulk solvents.15-17 These scales are called the π*, R, and β solvent strength scales, respectively. All are based on the phenomenon of solvatochromism, which refers to the effect of changes in a molecule’s * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)02129-6 CCC: $12.00
surrounding environment on its spectroscopic properties. The molecular properties can be changes in intensities of absorption but are most commonly changes in the energy of electromagnetic radiation which is absorbed by the solute molecules or “indicators” as they are often called.18 Kamlet and Taft utilized solvatochromic shifts to construct the π*, R, and β scales, whose values were established so as to generally range from 0 to 1, with 1 representing a strong ability of the solvent to interact through the chemical force being described by the relevant scale. A few examples of some common solvents and their parameters19 are cyclohexane (π* ) 0.00), dimethyformamide (π* ) 0.88), acetonitrile (R ) 0.29), methanol (R ) 0.93), anisole (β ) 0.25), and dimethyl sulfoxide (β ) 0.75). The Kamlet-Taft solvatochromic scales have been used to characterize and rank hundreds of different solvents in terms of their dipolarity/polarizability, HB acidity, and HB basicity19 and to correlate and explain hundreds of solvent-dependent spectral shifts, reaction rates, and equilibrium constants.20 The goal of our study is to apply the Kamlet-Taft methodology to characterize SDS micelles using solvatochromic indicators and UV-vis spectroscopy. Specifically, we will use shifts in the λmax values of the indicators to characterize the SDS micellar phase. The advantage of the Kamlet-Taft approach compared to previous studies based on solvatochromism is that we can assign quantitative chemical meaning to the spectroscopic shifts. To our knowledge, this is the first application of the KamletTaft solvatochromic comparison method to the study of SDS micelles. We note that Handa, Nakagaki, and Miyajima21 used Reichardt’s betaine and methylene blue as indicators to simultaneously determine the π* and R values of micelles made from a closely related surfactant, sodium tetradecyl sulfate. Relative to that study, ours is more comprehensive in that eight different indicators were used, including four which probe micellar polarity directly without contributions from the micellar acidity. Additionally, we have used two indicators which probe the hydrogen bond basicity (β) of the micelles, a parameter which was not explored in the study mentioned above. © 1996 American Chemical Society
18824 J. Phys. Chem., Vol. 100, No. 48, 1996 A problem arises in applying the Kamlet-Taft methodology to micellar solutions, however, because they are not bulk liquids as are all the solvents that have been characterized using the methodology. If micelles could be formed in the absence of water, we could measure the absorbance spectra of indicator molecules in micelles directly. In order to form conventional micelles, however, the presence of water is required. This introduces a complication in the data analysis which is absent when using the Kamlet-Taft methodology to characterize bulk liquids. Specifically, because the indicator molecules are distributed between the aqueous and micellar phases present in a micellar solution, any UV-vis spectrum of an indicator in the solution is a concentrated-weighted sum of the indicators’ spectra in the aqueous and micellar phases. The presence of the aqueous phase makes it physically impossible to directly measure the spectrum of the indicator in the micellar phase free from contributions of the indicator in the aqueous phase unless the indicator prefers the micellar phase so strongly that it is completely removed from the aqueous phase. The system can be pushed in the direction of complete partitioning by working at high micellar concentrations or by using indicators with very large molar-based partition coefficients (K ) [indicator]micelle/[indicator]aqueous). For instance, it can be estimated that 99.4% of the indicator molecules will be in the micellar phase when using an indicator with a partition coefficient of 10 000 and an SDS concentration of 70 mM. In our studies, however, we do not reach the necessary conditions for the assumptions of complete partitioning into the micellar phase to be valid. First, we estimate that most of our indicators have partition coefficients which are below 1000. This arises from the fact that the solvatochromic indicators are rather polar (most are nitrobenzene derivatives) and therefore interact well with water. Unfortunately, UV-vis spectra of nonpolar molecules that would partition to a much larger extent than polar molecules tend to be insensitive to the polarity and HB ability of their environment and therefore do not exhibit sizable solvatochromic shifts. The second reason we do not achieve complete partitioning of the solute into the micellar phase is that we have restricted our work to concentrations below 70 mM SDS. We have done so to avoid the complication that, above approximately 70 mM SDS, the micelles undergo a shape change from spherical to rodlike micelles.22,23 This shape transition potentially introduces a new, chemically distinct indicator environment which is difficult to take into account in our data analysis method. Thus, for the spectra we collected, the complication remains that the total absorbance spectrum observed is a combination of significant contributions from the indicators in the aqueous and micellar phases. Therefore, some form of curve resolution is necessary to obtain the indicator spectra in the micellar phase. We note that knowledge of the water-to-micelle partition coefficients of the indicators would allow for an easy algebraic solution to the problem and eliminate the need for complicated curve resolution methods. However, accurate partition coefficients of typical solvatochromic indicators are not available. Alternatively, they can be estimated using the linear solvation energy relationships (LSERs) which have recently appeared in the literature,24-27 but the standard deviations of the LSERs are such that they lead to ranges of partition coefficients so large as to render them unreliable and of little use. (For example, the estimated partition coefficient for p-ethylnitrobenzene ranges from 475 to 1220 when the standard deviations of the fits are considered.) Given the chemical constraints of the system (that is, the SDS concentration range and small indicator partition coefficients) and the lack of published partition coefficients and applicable
Vitha et al. methods for measuring them, we could not make any reasonable simplifying assumptions or accurate algebraic statements that would produce the desired output, namely, the spectra of the indicators in the micellar phase free from contributions of the indicators’ absorbance spectra in the aqueous phase. We have used a modification of the curve resolution method of Kubista28 et al. to obtain the desired SDS spectra. In our approach we take advantage of the fact that we know the spectrum of each indicator in pure water, whereas the method of Kubista et al. does not require such knowledge. On the basis of the soundness of the mathematical approach, the results we generated using artificial test data, and the chemically reasonable results obtained with experimental data, we believe our approach works quite well and does indeed yield the spectra of the indicators in the micellar phase free from contributions of the indicator in the aqueous phase. Since the Kamlet-Taft methodology15-17 and the curve resolution technique28 have been discussed and validated elsewhere, the main topic of this paper is the presentation and interpretation of the π*, R, and β values we obtained for SDS micelles in the concentration range from 0 to 70 mM. Experimental Section Chemicals. The following chemicals were purchased from Aldrich and used without further purification: o-nitroanisole (99+%), p-ethylnitrobenzene (99+%), p-nitroaniline (99+%), p-nitrophenol (99+%), and SDS (98+%). The p-nitroanisole (97%) used in these experiments was also from Aldrich and was purified using chromatography on silica gel with methylene chloride as the solvent, followed by recrystallization from 2-propanol. Sodium methyl sulfate (SMS) was from Aldrich and heated under vacuum at 70 °C for 3 days to remove methanol. N,N-Diethyl-p-nitroaniline was from Frinton Laboratories and used as received. The two R-indicators, 2,6dichloro-p-(2,4,6-triphenyl-N-pyridino)phenolate (ET(33)) and bis[R-(2-pyridyl)benzylidine-3,4-dimethylaniline]bis(cyano)iron(II), (Fe(LL)2(CN)2), were prepared and purified using procedures given in the literature.29,30 All water was deionized and passed through Barnstead ion exchange and organic free cartridges followed by a 0.45 µm filter. Spectroscopy. A stock solution of each indicator in water was prepared such that the maximum absorbance of the peak of interest was approximately 0.5 (unless solubility limited, in which case an excess of the indicator was stirred in water for approximately 2 days followed by gravity filtration to remove the undissolved material). Indicator solutions of varying SDS concentrations (0-500 mM) were made by preparing either a 500 or 100 mM SDS solution using the stock indicator solution as the diluent, followed by serial dilution using the stock aqueous indicator solution. Corresponding aqueous SDS solutions were prepared using pure water and used as reference solutions. UV-vis absorbance spectra were collected using a Varian DMS 200 scanning spectrophotometer, 1 cm quartz cells (Starna Cells, Inc.), a scan rate of 20 nm/min, and a slit width of 0.2 nm. A holmium oxide filter was used to calibrate the spectrophotometer. Solutions were allowed to remain in the thermostated sample compartment for a minimum of 10 min before the spectra were collected. The temperature was maintained at 25.0 ( 0.2 °C using a Fisher Scientific Isotemp constant temperature circulator (Model 800) and a Utile Products Neslab Instruments Inc. (Model U-Cool) cooling unit. Data Analysis. Digitized spectra were collected using DMSSCAN software from Varian and analyzed using a curve resolution program written in Matlab for Windows. The algorithm upon which the program is based is similar to the method described by Kubista et al.28 Briefly, the method relies
H Bond Ability of SDS Micelles on principal components analysis of a matrix composed of spectra in micellar solutions followed by linear regressions of abstract vectors against real spectra and concentrations. The model assumes that only two components contribute to the observed spectra, namely, the indicator in the aqueous and micellar phases. Inherent in this model is the treatment of the SDS micellar phase as a separate and chemically distinct bulk phase. This is consistent with the numerous treatments of micelles using a pseudophase model.31-34 We have also assumed that the volume of the micelle is 15.5 L/mol, obtained by multiplying the molar volume of micellized SDS monomers (0.25 L/mol)31,35-37 by the average aggregation number (62).38-40 As was demonstrated elsewhere,28 the curve resolution method yields three results: (1) the partition coefficients of the indicators transferring from pure water to SDS micelles, (2) the spectra of the indicators in water (which can be compared to the experimentally measured spectra), and (3) the spectra of the indicators in the SDS phase free from contributions of the indicators in the aqueous phase. The third result is the most important to us as it represents the spectra that would be measured if SDS micelles could be formed in the absence of water. In performing the curve resolution, we have varied the range of SDS concentrations in which data were taken. This reduces the possibility that the results are skewed by any one spectra. In all cases at least four spectra (the indicator in water and the indicator in three SDS solutions above the critical micelle concentration) are required by the algorithm. For most indicators, however, we found it necessary to include as many as six spectra taken at different SDS concentrations above the critical micelle concentration (cmc) before consistent results are achieved. The variations of the results obtained as a function of the SDS concentration range included in the curve resolution will be discussed below. Following curve resolution the spectra of the indicators in SDS generated by the algorithm were imported into TableCurve, gently smoothed with a 10% FFT method, and the “9/10” method of Kamlet and Taft was applied to find λmax.15 The 10% FFT smooth was found to cause very little perturbation in the overall peak shape but eliminated enough noise to provide a smooth continuous curve near the peak maxima. This greatly aids in using the “9/10” method since it relies on finding the maximum absorbance value. Results and Discussion The partition coefficients and solvatochromic parameters obtained for each indicator as a function of the SDS concentration range included in the curve resolution are shown in Table 1. We have analyzed the data in this way so as to test the robustness of the curve resolution method. We begin this section with an analysis of the partition coefficients since they lend confidence to the reliability of the method. Our true interest, however, lies in obtaining the indicators’ micellar spectra and the solvatochromic parameters derived therefrom. These results are discussed in detail following the analysis of the partition coefficients. Partition Coefficients. We observe that when the four spectra recorded at the lowest SDS concentrations are the only spectra included in the analysis, the partition coefficients are significantly different than when data at higher SDS concentrations are included. This likely results from the fact that the spectra taken at the lowest SDS concentrations also exhibit the smallest spectral shifts, making curve resolution difficult to perform. As spectra with higher SDS concentrations, and therefore larger spectral shifts, are added, the method is better able to discern a second component apart from the dominant water component. In fact, for most indicators we studied, the
J. Phys. Chem., Vol. 100, No. 48, 1996 18825 TABLE 1: Partition Coefficients and Solvatochromic Parameters Obtained with Each Indicator [SDS]a
nb
Kc
π*d
0-20 0-30 0-40 0-50 0-60 0-70
4 5 6 7 8 9
60 112 133 150 154 156
0.983 1.035 1.055 1.055 1.054 1.054
0-16.6 0-25 0-33.3 0-50 0-100
4 5 6 7 8
44 77 119 151 161
0.975 1.008 1.031 1.044 0.994
0-20 0-25 0-30 0-40 0-50 0-75
p-Ethylnitrobenzene 4 578 5 597 6 603 7 626 8 624 9 594
1.004 1.005 1.003 1.006 1.005 0.998
0-16 0-20 0-25 0-30 0-40 0-50
N,N-Diethyl-p-nitrobenzene 4 2509 5 2553 6 2562 7 2568 8 2551 9 2525
1.152 1.153 1.152 1.152 1.151 1.151
o-Nitroanisole
p-Nitroanisole
n
0-20 0-30 0-40 0-50 0-60 0-70
4 5 6 7 8 9
ET(33) 4.09 × 104 3.85 × 104 3.82 × 104 3.95 × 104 4.01 × 104 4.00 × 104
0.736 0.735 0.735 0.736 0.736 0.736
0-40 0-50 0-60 0-70 0-80
4 5 6 7 8
Fe(LL)2(CN)2 9.23 × 104 5.26 × 104 3.56 × 104 3.74 × 104 3.84 × 104
1.011 1.010 1.009 1.009 1.009
[SDS] 0-20 0-30 0-40 0-50 0-60 0-70
K
R
[SDS]
n 4 5 6 7 8 9
K p-Nitroaniline 481 448 414 396 366 350
β 0.398 0.401 0.402 0.404 0.405 0.406
a SDS concentration range spanned by the spectra input into the curve resolution algorithm. The first three spectra are the indicator in water, 10 mM SDS, and an SDS concentration between 10 mM and the fourth highest concentration (the first value listed under each indicator). b Number of spectra used in the curve resolution. c Molar partition coefficient ([indicator]micelle/[indicator]water) obtained from the curve resolution algorithm. d Solvatochromic parameters obtained for SDS using the resolved spectra obtained from the curve resolution algorithm.
measured partition coefficients and resulting micellar phase spectra do not vary appreciably when six or more spectra are included in the analysis. Thus, including more data at higher SDS concentrations leads to more reliable determinations of the partition coefficients and curve resolution. This explanation is supported by the fact that the indicators with the smallest coefficients (p- and o-nitroanisole) most clearly demonstrate this behavior. Given their small partition coefficients, the spectra of these indicators in solutions of low SDS concentrations are very similar to those of the indicators in water and are therefore most difficult to accurately resolve.
18826 J. Phys. Chem., Vol. 100, No. 48, 1996
Vitha et al.
TABLE 2: Partition Coefficients Predicted from SDS LSERsa solute
Carrb
Abrahamc
Quinad
p-ethylnitrobenzene o-nitroanisole p-nitroanisole N,N-diethylnitroaniline p-nitroaniline
655-475 150-110 150-110 2420-1750 30-22
1220-560 370-170 370-170 4110-1880 170-80
1130-620 250-140 250-140 4680-2570 80-45
a
Solute ref 50 or coefficient coefficient coefficient
parameters used in the LSER equations were taken from estimated from parameters therein. b Predicted partition ( one standard deviation; ref 27. c Predicted partition ( one standard deviation; ref 26. d Predicted partition ( one standard deviation; ref 25.
The partition coefficients obtained for Fe(LL)2(CN)2 show the greatest variations as a function of the SDS concentration range analyzed. We do not know the origin of the behavior of these partition coefficients. We do note, however, that the spectra obtained for each analysis lead to very consistent R values and thus do not seem affected by the large variation in the partition coefficient. The same is true for the results obtained for p-nitroaniline. Despite the variations we observe, several qualitative comments can be made regarding the partition coefficients. From published LSERs,24-27 it is known that, all else being equal, large (hydrophobic) solutes will partition to the greatest extent. This is borne out by the large partition coefficients we obtained for the two largest indicators, ET(33) and Fe(LL)2(CN)2. Additionally, we predict that p- and o-nitroanisole will partition to a smaller extent than p-ethylnitrobenzene since the methoxy moiety is better able to interact with water than is the ethyl moiety. Thus, the partition coefficients obtained for these compounds make chemical sense relative to one another. Finally, we note that for p- and o-nitroanisole, p-ethylnitrobenzene, and N,N-diethyl-p-nitroaniline the observed partition coefficients generally fall within the range predicted by the published LSERs24-27 (Table 2). The observed partition coefficient for p-nitroaniline, however, is much larger than predicted by the LSERs. The spectroscopic behavior of this indicator is also inconsistent with SDS LSERs and is discussed in more detail below. Solvatochromic Parameters. Although an analysis of the partition coefficients and how they vary with the SDS concentration range included in the analysis is instructive and lends confidence to the reliability of the curve resolution method, our real interest lies in the λmax values obtained for the indicators’ spectra in the micelles generated by the curve resolution technique. The variation in λmax will ultimately determine the variations in the solvatochromic parameters. If the variations in the solvatochromic parameters are large, it is difficult, if not impossible, to assert anything about the chemical properties of micelles. The variations in the parameters that we observe, however, are quite small (Table 1), leading to very consistent estimates of the solvatochromic parameters independent of the SDS concentration range analyzed for each indicator (again excepting p- and o-nitroanisole which require that spectra up to 30 mM SDS be input before stable results are obtained). The behavior of the partition coefficients of Fe(LL)2(CN)2 also varies considerably as a function of the SDS concentration range, but the solvatochromic parameters are not affected by these variations. Dipolarity (π*) of SDS. The π* values of pure SDS micelles obtained with each indicator are shown in Table 3 along with the π* for pure water. The equations used to calculate π* from the λmax values were those of Kamlet, Abboud, and Taft.15 To obtain π*(SDS) for each indicator, the π* values in the regions of stable λmax values were averaged for each indicator. An
TABLE 3: Solvatochromic Parameters for SDS and Water π* π* R R β β (SDS) (water) (SDS) (water) (SDS) (water) o-nitroanisole p-nitroanisole p-ethylnitrobenzene N,N-diethyl-pnitrobenzene ET(33) Fe(LL)2(CN)2 p-nitroaniline p-nitrophenol average
1.055 1.038 1.005 1.152
1.163 1.098 1.175 1.339 0.736 1.010
0.956 1.122 0.401
1.063
1.194
0.873
1.039
0.401
0.112 0.459 0.286
overall π*(SDS) of 1.06 was determined by averaging the π* values obtained by the four indicators. As Kamlet and Taft suggested, averaging over several indicators reduces the contributions arising from specific solvent effects or spectral anomalies from any one indicator.15 This approach has been criticized recently for obscuring physically meaningful differences between indicators.41,42 For this reason we present the solvatochromic parameters obtained with each indicator in addition to the overall averages. We note that the qualitative conclusions about the chemical nature of micelles drawn from an analysis of the averages are the same as would be reached from an analysis of the individual indicators. The average π*(SDS) of 1.06 clearly shows that the micellar environment of the solubilized indicators is quite polar considering that the π* of other polar aprotic solvents such as nitromethane and dimethyl sulfoxide are 0.85 and 1.00, respectively.19 Furthermore, this high polarity strongly suggests that the indicators are solubilized in the hydrated polar head-group region31,43,44 of the micelles and not in the nonpolar core. These results and conclusions are consistent with and further support the conclusions we reached via an analysis of partition coefficients of small molecules transferring from water to SDS micelles45 and numerous published spectroscopic studies of the micellar environment of solubilized molecules.8-12 We note that the indicators in this study are polar and may therefore selectively reside in polar environments within the micelle, perhaps even protruding far enough out of the micelle as to be in a primarily aqueous environment. It has been shown that nonpolar indicators such as benzene, especially when present at high concentrations, reside in the nonpolar regions of the micelle,13 leading to different conclusion about the chemical nature of micelles than those drawn above. However, other spectroscopic studies of nonpolar indicators such as naphthalene and anthracene, as well as thermodynamic studies of benzene at low concentrations, suggest that these indicators are located in polar microenvironments inside micelles.14,45 The average π* of 1.06 for SDS micelles is in very good agreement with the π* value of 1.10 determined by Handa et al. for sodium tetradecyl sulfate micelles using Reichardt’s betaine and methylene blue as indicators.21 The agreement in π* indicates that the chain length of the surfactant has little effect on the chemical environment of the indicator molecules, again because they are located in the polar head group region and not in the nonpolar core. It is also instructive to compare π*(SDS) to π*(water) obtained with each indicator in this study. It can be seen that for all the indicators π*(SDS) is lower than π*(water), leading to an overall average π*(SDS) of 1.06, which is less than the average π*(water) of 1.19. This result is qualitatively consistent with and further supports the LSER obtained in this laboratory (eq 1) and those appearing in the literature24-26 describing the transfer of solutes from water to SDS micelles.
H Bond Ability of SDS Micelles
J. Phys. Chem., Vol. 100, No. 48, 1996 18827
log Kmw ) 0.31 + 3.02(Vx/100) - 0.58πH2 -
0.37∑RH2 - 1.65∑βH2 (1)
The subscript “2” is used to denote a solute parameter in contrast to a bulk liquid parameter. Vx represents a solute’s volume determined using McGowan’s additivity scale.46 The negative coefficient of πH2 (solute dipolarity/polarizability) shows that, all else being equal, polar molecules favor being in the aqueous phase rather than the micellar phase, suggesting that more or stronger dipole-dipole and dipole-induced dipole interactions exist in the aqueous phase relative to the micellar phase, in qualitative agreement with our measured differences in the π* of water and the micellar environment. Hydrogen Bond Donor Strength (r) of SDS. The R values for SDS micelles obtained using ET(33) and Fe(LL)2(CN)2 are given in Table 3 along with the values obtained for pure water using the same indicators. The following equations were used to obtain R:47
ET(33) ) 28590/λmax ) 39.09 + 14.47(π*) + 14.41(R) (2) ET(Fe) ) 28590/λmax ) 39.71 + 3.31(π*) + 4.50(R) (3) Values of 1.19 and 1.06 were used for the π* of water and SDS, respectively. It is evident from the average R values in Table 3 that water (average R ) 1.04) is a better hydrogen bond donor than are SDS micelles (average R ) 0.87). Notice also that the SDS environment is similar in HB donor strength to some bulk liquids which are considered to be good HB donors (Rmethanol ) 0.93, Rethylene glycol ) 0.90).19 The high R value of SDS micelles demands some explanation since SDS monomers per se have no active hydrogen atoms. Given that SDS monomers are not hydrogen bond donors, the HB acidity must arise from water which is associated with the micelle. A number of studies have shown that water penetrates to at least the second carbon from the head group attached to the alkyl chain.31,43,44 Thus, it is reasonable for the indicator molecules to experience hydrogen bonding inside the micelle. In fact, this result is qualitatively predicted by comparing the SDS LSERs with the LSERs describing the transfer of solutes from water to bulk alkanes and 1-octanol given in eqs 4 and 5.48
water-to-alkane transfer log Kaw ) 0.05 + 4.85(Vx/100) - 1.10πH2 -
3.13∑RH2 - 5.78∑βH2 (4)
water to 1-octanol transfer log Kow ) -0.13 + 4.11(Vx/100) - 0.44πH2 +
0.27∑RH2 - 4.18∑βH2 (5)
The coefficient of the solute’s hydrogen bond basicity (∑βH2 ) reports the difference in the hydrogen bond acidity of the bulk phases under consideration. Typically, LSER coefficients range from +6 to -6. Given the magnitudes of the ∑βH2 coefficients in eqs 4 and 5, it is clear that water has a significantly stronger hydrogen bond acidity than either bulk alkanes or 1-octanol. In contrast, the magnitude of the ∑βH2 coefficient in the SDS LSER (eq 1) is significantly smaller, indicating that SDS micelles are more similar to water in terms of HB acidity than are either bulk alkanes or 1-octanol. In other words, SDS micelles have considerable HB donating ability relative to these two solvents.
If one accepts that water does not penetrate into the very core of the micelle, then in order for the indicators to experience the HB donating ability of the water that is associated with the micelles, they must reside in the hydrated region which includes the head group and at least the first two methylene units from the head groups. This is the same conclusion drawn from the analysis of LSERs, the comparison of methylene unit partition coefficients describing the transfer from water to bulk solvents and SDS micelles, and the π* values presented above. The R value of 0.74 obtained using ET(33) is in good agreement with the R value of 0.63 reported by Handa et al. for sodium tetradecyl sulfate micelles using Reichardt’s betaine (a compound structurally related to ET(33)) and methylene blue as indicators.21 The agreement is expected since the micellar environments of ET(33) and Reichardt’s betaine are predicted to be the same given the structural similarities of the indicators and surfactants under consideration. We note that the determination of the R values is based on the assumption that the R indicators are located in the same environment as are the π* indicators. Given the complexity of the micellar interior and the chemical differences in the indicators, this assumption may not be valid and therefore may introduce some error in the measured R values, giving rise to the difference in R values reported by the two indicators. Notice, however, that the two indicators give different results in pure bulk water. Thus, some of the spread in the R values in SDS arises simply because of the chemical differences between the indicators. Hydrogen Bond Basicity (β) of SDS. After curve resolution and application of the Kamlet-Taft equations to calculate β using the shift of p-nitroaniline relative to N,N-diethyl-pnitroaniline, we obtain an SDS β value of 0.40. This is considerably higher than the value of 0.11 we obtain for pure water with these indicators and suggests that the SDS phase is a stronger HB base than is bulk water. This result is more difficult to interpret than those for the π* and R indicators because the SDS LSER coefficient of ∑RH2 (eq 1) shows that water and SDS micelles should differ only very slightly in their HB basicity. Additionally, the LSER shows that water should be a slightly stronger HB base than the micellar phase. Possible explanations for these discrepancies are discussed below. Using the p-nitrophenol/p-nitroanisole couple suggested by Kamlet and Taft, a pure water β value of 0.46 is obtained. When analyzing the p-nitrophenol spectral data in micellar solutions, however, the shift in wavelength as a function of SDS was not large enough to produce reasonable and reliable partition coefficients or spectra in SDS micelles. Therefore, it was impossible to generate a β value for the SDS micellar phase using this indicator. One possible explanation for the high basicity of SDS relative to water is that it arises from specific interactions between p-nitroaniline and the sulfate head groups. To test this hypothesis, the basicities of eight aqueous sodium methyl sulfate (SMS) solutions ranging in concentration from 0 to 100 mM were measured. Additionally, a 2 M SMS solution was studied. The β values for these solutions increased slightly from 0.11 for pure water to 0.13 for 2 M SMS. Thus, even at a very high SMS concentration the overall solution basicity does not approach that found for the micellar phase using p-nitroaniline as the indicator. A similar study using the p-nitrophenol/pnitroanisole couple showed no increase in the measured β values as a function of SMS concentration from 0 to 20 mM. The concentration range was limited by the fact that the pH values of solutions above 20 mM SMS are high enough to fully deprotonate p-nitrophenol.
18828 J. Phys. Chem., Vol. 100, No. 48, 1996 These studies seem to indicate that the increased basicity of SDS micelles relative to water does not arise from specific interactions between the indicators and the sulfate head groups. This conclusion cannot be made definitively, however, because of the complication of concentration and orientation effects that arise when considering micellar phases. Specifically, the orientation and localization of p-nitroaniline inside the micelle may be such that its local environment has a considerably higher concentration of sulfate groupsshigher than can be explored in bulk aqueous solutions of SMS. Additionally, the timeaveraged orientation of p-nitroaniline inside the micelle could favor hydrogen bond donor-acceptor interactions, an effect which may be absent or considerably reduced in bulk aqueous solutions. Another possible cause of the discrepancy between the pure water and SDS β values using the aniline indicators arises from the fundamental electronic causes of shifts in λmax. It is known that the shifts of the β indicators we used in this study arise from a blend of electrostatic and charge transfer phenomena.49 Given that β values determined using p-nitroaniline have been said to arise largely from an electrostatic contribution49 and that SDS micelles contain charged head groups, it is reasonable to suggest that SDS micelles may appear to be better HB acceptors than does bulk water. The p-nitrophenol/p-nitroanisole couple was shown to have a much smaller electrostatic contribution.49 This may explain why p-nitrophenol does not respond to the SDS micellar phase in the same way as p-nitroaniline. Finally, we point out that in the SDS micelles we are dealing with iondipole interactions which were completely absent in the construction and definition of the π*, R, and β scales. Ultimately, the origin of the high β value of SDS micelles relative to pure water and the discrepancy between the LSER and spectroscopic results is still unclear and requires additional study. Therefore, we tentatively report 0.4 as the β value of SDS micelles but warn that this value is questioned and still under study. Conclusions We have determined the solvatochromic parameters of SDS based on a pseudophase model using a previously described curve resolution algorithm based on principal components analysis. The π*, R, and β values of SDS micelles, free from contributions of the indicator molecules in the aqueous phase, are 1.06, 0.87, and 0.4, respectively. The last value is in question and only tentatively reported here. These values are consistent with the LSERs describing the transfer of solutes from water to SDS micelles and indicate that the indicators are solubilized in a very polar, hydrated region of the micelle. This conclusion about the indicators’ micellar environment is consistent with and further supports the conclusions drawn from other independent thermodynamic and spectroscopic studies of SDS systems. The advantages of this study are that the complication of assumptions regarding the distribution of indicator molecules between the aqueous and micellar systems have been removed and that spectra of the indicators in the micelle phase free from aqueous phase contributions have been obtained. Acknowledgment. The authors acknowledge the contributions of Dr. David Whitman and Dr. Sarah Rutan regarding the resolution of the spectral data. This work was supported by grants from the National Science Foundation and the Graduate School of the University of Minnesota. References and Notes (1) McIntire, G. L. Crit. ReV. Anal. Chem. 1990, 21, 257-278. (2) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975.
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