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Pyrene absorption can be a convenient method for probing critical micellar concentration (cmc) and indexing micellar polarity
Journal of Colloid and Interface Science 294 (2006) 248–254 www.elsevier.com/locate/jcis
Pyrene absorption can be a convenient method for probing critical micellar concentration (cmc) and indexing micellar polarity Gargi Basu Ray, Indranil Chakraborty, Satya P. Moulik ∗ Centre for Surface Science, Department of Chemistry, Jadavpur University, Kolkata 700 032, India Received 3 May 2005; accepted 1 July 2005 Available online 19 August 2005
Abstract The critical micellar concentration (cmc) of both ionic and non-ionic surfactants can be conveniently determined from the measurements of UV absorption of pyrene in surfactant solution. The results on a number of surfactants have agreed with that realized from pyrene fluorescence measurements as well as that obtained following conductometric, tensiometric and calorimetric methods. The absorbance vs [surfactant] profiles for all the major UV spectral peaks of pyrene have been found to be sigmoidal in nature which were analyzed according to Sigmoidal–Boltzmann equation (SBE) to evaluate the cmcs of the studied surfactants. The difference between the initial and the final asymptotes (ai and af , respectively) of the sigmoidal profile, a = (af − ai ) and the slope of the sigmoid, Ssig have been observed to depend on the type of the surfactant. The a has shown a linear correlation with the ratio of the fluorescence intensities of the first and the third vibronic peaks, I1 /I3 of pyrene which is considered as a measure of the environmental polarity (herein micellar interior) of the probe (pyrene). Thus, a values have the prospect for use as another index for the estimation of polarity of micellar interior. 2005 Elsevier Inc. All rights reserved. Keywords: cmc; Pyrene absorption; Micellar polarity index; Sigmoidal–Boltzman equation; Fluorescence
1. Introduction It is known that surfactants can assemble in solution and critical micellar concentration (cmc) is an important solution property of surfactants [1,2]. There are several frequently used methods like tensiometry, conductometry, fluorimetry and calorimetry for its determination [2–4]. Spectral methods using dye and other compounds as probes, and self-absorption of amphiphiles in solution are also used for the evaluation of cmc [5,6]. In addition, fluorescence of probe molecules has been successfully employed for the above purpose [7,8]. Among the fluorescent probes, pyrene (benzo[def]phenanthrene) has been widely used whose emission characteristics (ratio of intensities of the first and third vibronic peaks; I1 /I3 ) are very often considered to estimate the polarity level of its environment (say * Corresponding author. Fax: +91 33 2414 6266.
E-mail address: [email protected] (S.P. Moulik). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.07.006
micelles) as well as the cmc of amphiphiles [9,11]. Pyrene also exhibits characteristic absorption spectra with strong and weak peaks in the UV-region [12,13]. The UV spectral manifestation of it in aqueous surfactant solution should in principle lead to the estimation of its cmc like the useful pyrene I1 /I3 method [9–11]. But experimental endorsement of such a possibility is rarely found in literature. In this article, we have elaborated the potential of pyrene absorption spectra for the determination of cmc of different types of surfactant in aqueous medium. An attempt has also been made to compare the absorption method with the fluorescence method and correlate the data with I1 /I3 of pyrene to propose an absorption related micellar polarity index.
2. Materials The alkyltrimethylammonium bromides C10 TAB and C14 TAB were AR grade products of Fluka (Switzerland); C12 TAB and C16 TAB were AR grade products of Aldrich
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(USA). The non-ionic surfactant MEGA-10 (N-decanoylN-methylglucamine), a gift sample from Prof. G. Sugihara, Fukuoka University, Japan, was doubly recrystallized. The anionic surfactant sodium dodecylsulphate (SDS) was a 99% pure sample purchased from Sisco Research Laboratories (India) and sodium dodecylbenzenesulfonate (SDBS) was purchased from Sigma (India). The tetradecyltriphenylphosphonium bromide (C14 TPB) used was a purified sample obtained from Caledon Laboratories Ltd., Canada (Distributors for Lancaster Synthesis of England). Pyrene (Aldrich) was a gift sample from Polymer Science Laboratory of IACS, Kolkata (India). It was purified by sublimation, followed by recrystallization from ethanol. All the surfactants were dessicated before use. Doubly distilled deionized water was used for all sample preparations and dilution. All measurements were taken in a temperature controlled thermostated water bath.
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6. Results and discussion 6.1. Absorption and emission spectra of pyrene The absorption and emission spectra of pyrene (2 µM) in water, methanol and hexane are illustrated in Figs. 1a and 1b. The absorption spectra have evidenced eight [13] peaks strong (s) and weak (w) at 232w , 242s , 252w , 260w , 272s , 308w , 320s and 336s nm as depicted in Fig. 1a. The spectra have evidenced solvent dependence. At 242 nm, the trend of absorption was n-hexane > methanol > water, which changed to n-hexane < water < methanol up to 315 nm; thereafter, the trend of absorbance became water < methanol < n-hexane. The solvent dependent intensities of the effective part of the emission spectra between 370 and 410 nm followed the order water > methanol > n-hexane i.e.; in order of decreasing solvent polarity. There were five emission peaks at 373, 379, 383, 389 and 393 nm for pyrene [9, 11]. The [pyrene] used was 2 µM, which was within its sol-
3. Preparation of pyrene solution A stock solution of pyrene was prepared by adding a known weight of the compound in 20 wt% ethanol in water. The mixture was sonicated to yield a clear solution. The experimental 2 µM solution of pyrene was prepared from it by dilution wherein the ethanol concentration was 0.5%. Such a small concentration of the ethanol was considered not to affect the spectral and self-aggregation behavior of amphiphiles.
4. Absorbance study Absorbance measurements were taken in a UV 1601 Shimadzu (Japan) spectrophotometer using 10 mm path length quartz cuvettes. The spectra were recorded in the 200– 400 nm wavelength range. The concentration of the surfactant solution was varied from below to above cmc by progressive addition of a concentrated solution into water with a Hamilton microsyringe.
(a)
5. Fluorescence study Fluorescence measurements, using pyrene as the fluorescent probe, were taken in a FluoroMax-3, JOBIN YVON, Horiba (Japan), using a 10 mm path length quartz cuvette. Excitation was done at 331.5 nm and emissions were recorded in the 340–450 nm wavelength range. The slit widths for both excitation and emission were fixed at 0.4 nm. A concentrated surfactant solution was stepwise added into a 2 µM aqueous pyrene solution with the help of a Hamilton microsyringe and the emission spectra were recorded after excitation. The scan time was fixed at 0.8 s per scan. All measurements were taken at 298 ± 0.1 K.
(b) Fig. 1. (a) Absorption spectra of pyrene (2.0 µM) in water, methanol and n-hexane at 298 K. (b) Fluorescence spectra of pyrene (2.0 µM) in water, methanol and n-hexane at 298 K. Intensities are normalized against I3 .
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(a)
(a)
(b) (b) Fig. 2. (a) Absorption spectra of pyrene (2.0 µM) in C16 TAB, MEGA-10 and SDS micellar solution at 298 K. (b) Fluorescence spectra of pyrene (2.0 µM) in C16 TAB, MEGA-10 and SDS micellar solution at 298 K. Intensities are normalized against I3 .
ubility limit of 2–3 µM [9]. At this concentration, excimer formation was expected to be absent [9]. In Fig. 1b, the fluorescence intensities normalized against the intensity of the third peak are plotted against the wavelengths. Excimer formation was not observed in the spectra. The absorption and emission spectra at cmc for SDS, C16 TAB and MEGA-10 are presented in Figs. 2a and 2b (a, for absorption and b, for emission). The absorption peaks remained the same in micellar medium but the intensities varied depending on the system. The spectrum of MEGA10 was intermediate between SDS and C16 TAB. MEGA-10 has evidenced absence of two absorption peaks at 242 and 232 nm but has shown the usual emission pattern of pyrene. In SDBS medium, the absorption peaks 232, 242 and 272 nm of pyrene were not observed. This was due to strong absorption of MEGA-10 and SDBS in the near UV region, which masked the peaks of pyrene in question.
Fig. 3. (a) AT vs [SDS] profile for pyrene (2.0 µM) at 298 K. Inset: Absorbance vs [SDS] profiles for pyrene (1.172 µM) at 298 K. Curves 1, 2, 3 and 4 correspond to 242, 272, 320 and 336 nm, respectively. (b) AT vs [MEGA-10] profile for pyrene (2.0 µM) at 298 K. Inset: Absorbance vs [MEGA-10] profiles for pyrene (2.0 µM) at 298 K. Curves 1, 2 and 3 correspond to 272, 320 and 336 nm, respectively.
6.2. Absorption in surfactant solution In the illustrations of Figs. 3a and 3b, plots of the sum of absorbances of all the major peaks (AT ) against [surfactant] for SDS and MEGA-10 are shown. The absorbances of all the major peaks of SDS and MEGA-10 are also plotted separately against [surfactant] in the insets. The profiles were all sigmoidal in nature. Fitting them to the Sigmoidal– Boltzmann equation (SBE) (used by us on isothermal titration calorimetric results to determine the cmc) [14] was herein employed for cmc evaluation. Thus, (ai − af ) + af , (1) 1 + exp[(x − x0 )/x] where x is the concentration of surfactant, ai and af are the initial and final asymptotes of the sigmoid respectively, x0 is AT =
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(a)
(b) Fig. 4. (a) AT vs [ATAB] profile for pyrene (2.0 µM) at 298 K. Symbols 1 and 2 represent C12 TAB and C14 TAB, respectively. (b) AT vs equimolar [C12 TAB/C16 TAB] profile for pyrene (2.0 µM) at 298 K. Insets: (A) AT vs [C12 /C16 ] profile for pyrene (2.0 µM) at 298 K showing cmc1 and cmc2 and (B) AT vs [C14 TPB] profile for pyrene (2.0 µM) at 298 K.
the center of the sigmoid and x is the interval of the independent variable x. Aguiar et al. [11] have very recently used the SBE on [surfactant] dependent I1 /I3 ratio of pyrene for the determination of cmc. They have proposed that the sigmoidal plot can produce two cmcs, one at x0 and the other at (x0 + 2x). Further, the ratio x0 /x can be a guide to decide upon for the choice of the right cmc between the two. The surfactant systems that yield (x0 /x) < 10 produce cmc = x0 ; those which yield x0 /x > 10 by the SBE procedure produce cmc = (x0 + 2x). Based on this rationale, we have evaluated the cmcs of different ionic and nonionic surfactants from AT vs x Sigmoidal–Boltzman plots. In addition to the depictions in Figs. 3a and 3b, illustrations for the sigmoidal AT vs [surfactant] plots are also shown for C14 TAB and C12 TAB (in Fig. 4a); and 1:1 (mole/mole) C16 TAB:C12 TAB mixture (in Fig. 4b), and
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Fig. 5. AT vs [C16 TAB] profiles for pyrene at different temperatures. Curves 1, 2 and 3 correspond to 298, 303 and 308 K, respectively.
C14 TPB (in the inset of Fig. 4b). Both the C16 TAB:C12 TAB mixture and C14 TPB systems have been found to produce two sigmoidal courses registering two cmcs (cmc1 and cmc2 ), which has corroborated with our recent findings on the systems from conductance measurements [15]. We have also observed formation of double cmcs for alkyl triphenyl phosphonium bromides (C10 TPB and C12 TPB) from both conductometric and isothermal titration calorimetric measurements [4]. The absorption data processed by Eq. (1) and analyzed according to the proposition of Aguiar et al. [11] have produced x0 /x greater than 10 (actual value was 13.19) only for C10 TAB. The rest of the surfactants have produced x0 /x much lower than 10. Therefore, the cmc for C10 TAB was equivalent to (x0 + 2x), while the rest cmcs were taken equivalent to x0 . The suitability of pyrene absorption method has been also tested with variation of temperature. The results obtained on C16 TAB at three different temperatures 298, 303 and 308 K are illustrated in Fig. 5. Excellent sigmoidal pattern of pyrene absorption on [C16 TAB] at different temperature supported suitability of SBE procedure for the evaluation of cmc. The x0 /x values at the studied temperatures were all lower than 10, so that the cmc values were taken equal to x0 as discussed above. The cmc values thus determined for different systems are presented in Table 1 along with results by other methods and from different sources. The agreements were good. 6.3. Emission in surfactant solution In Figs. 6a and 6b, the sum of the emission intensities at 373, 383 and 393 nm in surfactant solution relative to the intensities in the absence of surfactant designated as (I /Iw )T are plotted against [surfactant]. The plots for the individual peaks are presented in the insets. Although the depictions for SDS (Fig. 6a) were sigmoidal, those for C16 TAB were like potential energy diagrams. The concentration of C16 TAB at the minimum of the well corresponded to its cmc. Kalyansundaran and Thomas [9] have reported the usability of the
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Table 1 The cmc of surfactants obtained from different methods at 298 K System C10 TAB C12 TAB C14 TAB C16 TAB C12 –C16 c SDS SDBS MEGA-10d C14 TPBc
cmc (mM) Absorbance
Fluorescence
Conductometry
Microcalorimetry
Surface tension
69.3 15.3 4.58 0.95, 1.26a , 1.41b 1.37 (1) 15.1 (2) 7.09 1.34 6.68 0.81 (1) 2.52 (2)
– 14.9 3.66 0.87 1.38 (1) – 7.05 – 7.05 –
66.9 [15], 65.6 [17] 15.3 [17] 3.88 [15], 3.94 [17] 0.91 [15], 0.96 [17] 1.32 [15] (1) 12.3 [15] (2) 7.75 [22] 1.82 [15], 2.90 [6] – 0.77 [4] (1) 2.43 [4] (2)
– 15.2 [18] 4.11 [19] 0.97 [20], 1.29a [28] 1.69 [15] (1) 16.3 [15] (2) 7.78 [21] 2.80 [6] – 0.80 [4] (1)
– 14.6 [15] 4.13 [15], 3.74 [23] 0.85 [15], 0.82 [24] 1.74 [15] (1) – 8.31 [15], 8.00 [25] – 6.45 [15], 5.00 [26] 1.05 [6b]
a cmc at 303 K. b cmc at 308 K. c The cmc values 1 and 2 refer to two cmcs of the C TAB:C TAB = 1:1 (mol/mol) and C TPB systems. Microcalorimetry and tensiometry have 12 16 14
evidenced a single cmc. d cmc at 6.7 mM was obtained by light scattering method [27].
(a)
(b)
Fig. 6. (a) (I /IW )T and I1 /I3 vs [SDS] profile for pyrene (2.0 µM) at 298 K. Inset: I /IW vs [SDS] profile for pyrene (2.0 µM) at 298 K. Curves 1, 2 and 3 correspond to 373, 383 and 393 nm, respectively. (b) (I /IW )T and I1 /I3 vs [C16 TAB] profile for pyrene (2.0 µM) at 298 K. Inset: I /IW vs [C16 TAB] profile for pyrene (2.0 µM) at 298 K. Curves 1, 2 and 3 correspond to 293, 373 and 383 nm, respectively.
pyrene fluorescence method from the plots between the fluorescence lifetime as well as I3 /I1 and [surfactant]. The plots were sigmoidal in nature and sharp. We have shown I1 /I3 vs [surfactant] profiles in our documentation (Fig. 6) wherein the features were also sigmoidal. Similar profiles have also been reported in recent literature [11]. The fluorescence plots for C12 TAB:C16 TAB systems at 1:1 (mole/mole) composition and that for C12 TAB are illustrated in Fig. 7. Indications for the formation of two cmcs were not observed as revealed from the absorption spectral measurements (Fig. 4b). Thus, the absorption spectral method has an advantage over fluorescence method in the identification of more than one cmc for a surfactant, which is otherwise difficult to determine straightforwardly by methods other than conductance, and for some systems by isothermal titration calorimetry. The cmc values obtained from emission experiments are also in-
Fig. 7. (I /IW )T vs [surfactant] profiles for pyrene (2.0 µM) at 298 K. Curves 1 and 2 represent C12 TAB and 1:1 C12 TAB/C16 TAB.
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Table 2 The a, Ssig and I1 /I3 (at cmc) values for different surfactant systems System
a
I1 /I3 (at cmc)
Ssig
C10 TAB C12 TAB C14 TAB C16 TAB C12 –C16 a
0.230 0.186 0.140 0.130 0.105 (1) 0.204 (2) 0.105 0.069 0.101 0.019 (1) 0.024 (2)
1.36 1.27 1.1 1.06 1.05 (1) 1.06 (2) 0.968 0.873 0.995 0.717 (1) 0.733 (2)
0.008 0.010 0.022 0.010 0.068 (1) 0.013 (2) 0.040 0.016 0.018 0.031 (1) 0.020 (2)
SDS SDBS MEGA-10 C14 TPBa
a Two values correspond to two micellization processes.
cluded in Table 1. Herein also the rationale of Aguiar et al. has been considered for cmc estimation. It is considered that the hydrophobic pyrene molecules remain associated in aqueous medium. The anionic amphiphiles may deaggregate them and form complex with pyrene and enhancement of absorption may take place by the first process, which levels off after micelle formation. The process of complexation in the case of cationic amphiphiles imparts a radiationless energy transfer so as to reduce the effective emission intensity up to the cmc point with enhancement of emission and its leveling off afterwards. But interestingly the I1 /I3 ratio evidences a negative sigmoidal trend with [surfactant] irrespective of the type. 6.4. Analysis of results with reference to polarity index It is considered that the I1 /I3 ratio of pyrene fluorescence is an index of polarity of its environment. The values in hydrocarbon solvent vary in the range of 0.57–0.61 and in polar solvents the range of variation is 1.25–2.00. In micellar medium the range of variation observed is 1.1–1.5. Kalyansundaran and Thomas [9] have elaborately discussed these results and the increased range of I1 /I3 for micellar solution compared to hydrocarbons has been explained in terms of water penetration into micelles [16]. According to them, the polarity index depends on the type of surfactant head group and should not depend on the surfactant chain length, concentration, counter ion and added electrolyte. Larger head groups as in CTAB produce less interfacial compactness and allow water penetration which compact micelles like that of SDS does not. The I1 /I3 index for CTAB is thus greater than that for SDS. The I1 /I3 values for C10 -, C12 -, C14 and C16 TAB as well as for SDS are presented in Table 2. The emission peaks one and three of pyrene were masked by self-absorption of C14 TPB and SDBS so that their polarity indices could not be experimentally determined. We have observed that at the constant [pyrene] = 2 µM, af − ai or a and the slope of the sigmoid Ssig (determined from the linear part around x0 ) depended on the surfactant system essentially on its micellar polarity. The a has shown a nice linear dependence while the linear dependence for Ssig was
Fig. 8. a vs I1 /I3 profile for the studied surfactants at 298 K. Inset: a and I1 /I3 vs carbon number (Cn ) profiles for C10 -, C12 -, C14 - and C16 TAB at 298 K.
only moderate. The linear profile between a and I1 /I3 is documented in Fig. 8. An equation of the following form a = (−0.2113 ± 0.024) + (0.321 ± 0.210)(I1 /I3 )
(2)
with correlation coefficient of 0.9914 holds. In this correlation, the anionic surfactant SDS and the nonionic representative MEGA-10 fairly fitted in the general course for the ATABs. Thus a can be set to a marker for the micellar polarity. In the insets of Fig. 8, the fair linear dependence of both a and I1 /I3 on the carbon number (Cn ) in the alkyl chain of the ATABs has been illustrated. In this correlation, both SDBS and C14 TPB were out of trend. This was due to the differences in the contributions of their head groups from the head group of ATABs. 6.5. Alkyl chain length related micellar polarity of ATABs The I1 /I3 values of the ATABs have shown a decreasing trend with increasing alkyl chain length. Increased penetration of water in the micelles of lower homologues of the ATABs was envisaged. Thus probability of pyrene to experience polar environment was maximum in C10 TAB micelles and minimum in C16 TAB micelles. This was contrary to the propositions of Kalyansundaran and Thomas [9] that the polarity index of pyrene in micelle should depend on the head group and should be independent of the surfactant chain length. In this study, regularity in the I1 /I3 values with the alkyl chain length in the homologous series of ATABs has been observed. Recognizing water penetration up to four to five carbon atoms of the alkyl chain [16], internal hydrophilic region per unit volume of micelles of ATABs should decrease with increasing chain length. Pyrene has a size of 0.77 nm, and the sizes of ATAB micelles are 2.81, 3.32, 3.80 and 4.28 nm for C10 -, C12 -, C14 - and C16 TAB, respectively [17]. Considering water penetration up to the fifth carbon atom in the micelles, the effective size of the hydrophilic regions of the above ATABs become 2.05, 2.65, 3.15 and 3.65 nm, respectively. Thus, pyrene had enough
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rooms in the ATAB micelles. Taking one pyrene on the average resided in one micelle, the proximity of polar environment to pyrene should increase with decreasing micellar size of the ATAB series from C16 to C10 TAB to augment increasing polarity index (I1 /I3 ). [6]
7. Conclusions [7]
(1) UV absorption of pyrene in surfactant solution can be a convenient method for the determination of their cmc. (2) The absorption and emission results of pyrene in different surfactant solution have evidenced fair agreement between the derived results. (3) A spectral parameter a (difference between the asymptotes of the sigmoidal Boltzman profile) at a constant [pyrene] has produced a fairly linear correlation with I1 /I3 . (4) Both a and I1 /I3 are linearly related with the carbon number of the ATAB homologues.
Acknowledgments
[8] [9] [10] [11] [12]
[13] [14] [15] [16]
G.B.R. and I.C. thank CSIR for a Junior Fellowship and a Senior Fellowship, respectively. S.P.M. thanks INSA for a Senior Scientist position during the tenure of which this work was performed. The fluorescence spectral measurement facility rendered by Dr. S. Mukherjee and Ms. Doyel Das of IACS, Kolkata is acknowledged with appreciation.
[17] [18] [19] [20]
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Pyrene absorption can be a convenient method for probing critical micellar concentration (cmc) and indexing micellar polarity Gargi Basu Ray, Indranil Chakraborty, Satya P. Moulik ∗ Centre for Surface Science, Department of Chemistry, Jadavpur University, Kolkata 700 032, India Received 3 May 2005; accepted 1 July 2005 Available online 19 August 2005
Abstract The critical micellar concentration (cmc) of both ionic and non-ionic surfactants can be conveniently determined from the measurements of UV absorption of pyrene in surfactant solution. The results on a number of surfactants have agreed with that realized from pyrene fluorescence measurements as well as that obtained following conductometric, tensiometric and calorimetric methods. The absorbance vs [surfactant] profiles for all the major UV spectral peaks of pyrene have been found to be sigmoidal in nature which were analyzed according to Sigmoidal–Boltzmann equation (SBE) to evaluate the cmcs of the studied surfactants. The difference between the initial and the final asymptotes (ai and af , respectively) of the sigmoidal profile, a = (af − ai ) and the slope of the sigmoid, Ssig have been observed to depend on the type of the surfactant. The a has shown a linear correlation with the ratio of the fluorescence intensities of the first and the third vibronic peaks, I1 /I3 of pyrene which is considered as a measure of the environmental polarity (herein micellar interior) of the probe (pyrene). Thus, a values have the prospect for use as another index for the estimation of polarity of micellar interior. 2005 Elsevier Inc. All rights reserved. Keywords: cmc; Pyrene absorption; Micellar polarity index; Sigmoidal–Boltzman equation; Fluorescence
1. Introduction It is known that surfactants can assemble in solution and critical micellar concentration (cmc) is an important solution property of surfactants [1,2]. There are several frequently used methods like tensiometry, conductometry, fluorimetry and calorimetry for its determination [2–4]. Spectral methods using dye and other compounds as probes, and self-absorption of amphiphiles in solution are also used for the evaluation of cmc [5,6]. In addition, fluorescence of probe molecules has been successfully employed for the above purpose [7,8]. Among the fluorescent probes, pyrene (benzo[def]phenanthrene) has been widely used whose emission characteristics (ratio of intensities of the first and third vibronic peaks; I1 /I3 ) are very often considered to estimate the polarity level of its environment (say * Corresponding author. Fax: +91 33 2414 6266.
E-mail address: [email protected] (S.P. Moulik). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.07.006
micelles) as well as the cmc of amphiphiles [9,11]. Pyrene also exhibits characteristic absorption spectra with strong and weak peaks in the UV-region [12,13]. The UV spectral manifestation of it in aqueous surfactant solution should in principle lead to the estimation of its cmc like the useful pyrene I1 /I3 method [9–11]. But experimental endorsement of such a possibility is rarely found in literature. In this article, we have elaborated the potential of pyrene absorption spectra for the determination of cmc of different types of surfactant in aqueous medium. An attempt has also been made to compare the absorption method with the fluorescence method and correlate the data with I1 /I3 of pyrene to propose an absorption related micellar polarity index.
2. Materials The alkyltrimethylammonium bromides C10 TAB and C14 TAB were AR grade products of Fluka (Switzerland); C12 TAB and C16 TAB were AR grade products of Aldrich
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(USA). The non-ionic surfactant MEGA-10 (N-decanoylN-methylglucamine), a gift sample from Prof. G. Sugihara, Fukuoka University, Japan, was doubly recrystallized. The anionic surfactant sodium dodecylsulphate (SDS) was a 99% pure sample purchased from Sisco Research Laboratories (India) and sodium dodecylbenzenesulfonate (SDBS) was purchased from Sigma (India). The tetradecyltriphenylphosphonium bromide (C14 TPB) used was a purified sample obtained from Caledon Laboratories Ltd., Canada (Distributors for Lancaster Synthesis of England). Pyrene (Aldrich) was a gift sample from Polymer Science Laboratory of IACS, Kolkata (India). It was purified by sublimation, followed by recrystallization from ethanol. All the surfactants were dessicated before use. Doubly distilled deionized water was used for all sample preparations and dilution. All measurements were taken in a temperature controlled thermostated water bath.
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6. Results and discussion 6.1. Absorption and emission spectra of pyrene The absorption and emission spectra of pyrene (2 µM) in water, methanol and hexane are illustrated in Figs. 1a and 1b. The absorption spectra have evidenced eight [13] peaks strong (s) and weak (w) at 232w , 242s , 252w , 260w , 272s , 308w , 320s and 336s nm as depicted in Fig. 1a. The spectra have evidenced solvent dependence. At 242 nm, the trend of absorption was n-hexane > methanol > water, which changed to n-hexane < water < methanol up to 315 nm; thereafter, the trend of absorbance became water < methanol < n-hexane. The solvent dependent intensities of the effective part of the emission spectra between 370 and 410 nm followed the order water > methanol > n-hexane i.e.; in order of decreasing solvent polarity. There were five emission peaks at 373, 379, 383, 389 and 393 nm for pyrene [9, 11]. The [pyrene] used was 2 µM, which was within its sol-
3. Preparation of pyrene solution A stock solution of pyrene was prepared by adding a known weight of the compound in 20 wt% ethanol in water. The mixture was sonicated to yield a clear solution. The experimental 2 µM solution of pyrene was prepared from it by dilution wherein the ethanol concentration was 0.5%. Such a small concentration of the ethanol was considered not to affect the spectral and self-aggregation behavior of amphiphiles.
4. Absorbance study Absorbance measurements were taken in a UV 1601 Shimadzu (Japan) spectrophotometer using 10 mm path length quartz cuvettes. The spectra were recorded in the 200– 400 nm wavelength range. The concentration of the surfactant solution was varied from below to above cmc by progressive addition of a concentrated solution into water with a Hamilton microsyringe.
(a)
5. Fluorescence study Fluorescence measurements, using pyrene as the fluorescent probe, were taken in a FluoroMax-3, JOBIN YVON, Horiba (Japan), using a 10 mm path length quartz cuvette. Excitation was done at 331.5 nm and emissions were recorded in the 340–450 nm wavelength range. The slit widths for both excitation and emission were fixed at 0.4 nm. A concentrated surfactant solution was stepwise added into a 2 µM aqueous pyrene solution with the help of a Hamilton microsyringe and the emission spectra were recorded after excitation. The scan time was fixed at 0.8 s per scan. All measurements were taken at 298 ± 0.1 K.
(b) Fig. 1. (a) Absorption spectra of pyrene (2.0 µM) in water, methanol and n-hexane at 298 K. (b) Fluorescence spectra of pyrene (2.0 µM) in water, methanol and n-hexane at 298 K. Intensities are normalized against I3 .
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(a)
(a)
(b) (b) Fig. 2. (a) Absorption spectra of pyrene (2.0 µM) in C16 TAB, MEGA-10 and SDS micellar solution at 298 K. (b) Fluorescence spectra of pyrene (2.0 µM) in C16 TAB, MEGA-10 and SDS micellar solution at 298 K. Intensities are normalized against I3 .
ubility limit of 2–3 µM [9]. At this concentration, excimer formation was expected to be absent [9]. In Fig. 1b, the fluorescence intensities normalized against the intensity of the third peak are plotted against the wavelengths. Excimer formation was not observed in the spectra. The absorption and emission spectra at cmc for SDS, C16 TAB and MEGA-10 are presented in Figs. 2a and 2b (a, for absorption and b, for emission). The absorption peaks remained the same in micellar medium but the intensities varied depending on the system. The spectrum of MEGA10 was intermediate between SDS and C16 TAB. MEGA-10 has evidenced absence of two absorption peaks at 242 and 232 nm but has shown the usual emission pattern of pyrene. In SDBS medium, the absorption peaks 232, 242 and 272 nm of pyrene were not observed. This was due to strong absorption of MEGA-10 and SDBS in the near UV region, which masked the peaks of pyrene in question.
Fig. 3. (a) AT vs [SDS] profile for pyrene (2.0 µM) at 298 K. Inset: Absorbance vs [SDS] profiles for pyrene (1.172 µM) at 298 K. Curves 1, 2, 3 and 4 correspond to 242, 272, 320 and 336 nm, respectively. (b) AT vs [MEGA-10] profile for pyrene (2.0 µM) at 298 K. Inset: Absorbance vs [MEGA-10] profiles for pyrene (2.0 µM) at 298 K. Curves 1, 2 and 3 correspond to 272, 320 and 336 nm, respectively.
6.2. Absorption in surfactant solution In the illustrations of Figs. 3a and 3b, plots of the sum of absorbances of all the major peaks (AT ) against [surfactant] for SDS and MEGA-10 are shown. The absorbances of all the major peaks of SDS and MEGA-10 are also plotted separately against [surfactant] in the insets. The profiles were all sigmoidal in nature. Fitting them to the Sigmoidal– Boltzmann equation (SBE) (used by us on isothermal titration calorimetric results to determine the cmc) [14] was herein employed for cmc evaluation. Thus, (ai − af ) + af , (1) 1 + exp[(x − x0 )/x] where x is the concentration of surfactant, ai and af are the initial and final asymptotes of the sigmoid respectively, x0 is AT =
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(a)
(b) Fig. 4. (a) AT vs [ATAB] profile for pyrene (2.0 µM) at 298 K. Symbols 1 and 2 represent C12 TAB and C14 TAB, respectively. (b) AT vs equimolar [C12 TAB/C16 TAB] profile for pyrene (2.0 µM) at 298 K. Insets: (A) AT vs [C12 /C16 ] profile for pyrene (2.0 µM) at 298 K showing cmc1 and cmc2 and (B) AT vs [C14 TPB] profile for pyrene (2.0 µM) at 298 K.
the center of the sigmoid and x is the interval of the independent variable x. Aguiar et al. [11] have very recently used the SBE on [surfactant] dependent I1 /I3 ratio of pyrene for the determination of cmc. They have proposed that the sigmoidal plot can produce two cmcs, one at x0 and the other at (x0 + 2x). Further, the ratio x0 /x can be a guide to decide upon for the choice of the right cmc between the two. The surfactant systems that yield (x0 /x) < 10 produce cmc = x0 ; those which yield x0 /x > 10 by the SBE procedure produce cmc = (x0 + 2x). Based on this rationale, we have evaluated the cmcs of different ionic and nonionic surfactants from AT vs x Sigmoidal–Boltzman plots. In addition to the depictions in Figs. 3a and 3b, illustrations for the sigmoidal AT vs [surfactant] plots are also shown for C14 TAB and C12 TAB (in Fig. 4a); and 1:1 (mole/mole) C16 TAB:C12 TAB mixture (in Fig. 4b), and
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Fig. 5. AT vs [C16 TAB] profiles for pyrene at different temperatures. Curves 1, 2 and 3 correspond to 298, 303 and 308 K, respectively.
C14 TPB (in the inset of Fig. 4b). Both the C16 TAB:C12 TAB mixture and C14 TPB systems have been found to produce two sigmoidal courses registering two cmcs (cmc1 and cmc2 ), which has corroborated with our recent findings on the systems from conductance measurements [15]. We have also observed formation of double cmcs for alkyl triphenyl phosphonium bromides (C10 TPB and C12 TPB) from both conductometric and isothermal titration calorimetric measurements [4]. The absorption data processed by Eq. (1) and analyzed according to the proposition of Aguiar et al. [11] have produced x0 /x greater than 10 (actual value was 13.19) only for C10 TAB. The rest of the surfactants have produced x0 /x much lower than 10. Therefore, the cmc for C10 TAB was equivalent to (x0 + 2x), while the rest cmcs were taken equivalent to x0 . The suitability of pyrene absorption method has been also tested with variation of temperature. The results obtained on C16 TAB at three different temperatures 298, 303 and 308 K are illustrated in Fig. 5. Excellent sigmoidal pattern of pyrene absorption on [C16 TAB] at different temperature supported suitability of SBE procedure for the evaluation of cmc. The x0 /x values at the studied temperatures were all lower than 10, so that the cmc values were taken equal to x0 as discussed above. The cmc values thus determined for different systems are presented in Table 1 along with results by other methods and from different sources. The agreements were good. 6.3. Emission in surfactant solution In Figs. 6a and 6b, the sum of the emission intensities at 373, 383 and 393 nm in surfactant solution relative to the intensities in the absence of surfactant designated as (I /Iw )T are plotted against [surfactant]. The plots for the individual peaks are presented in the insets. Although the depictions for SDS (Fig. 6a) were sigmoidal, those for C16 TAB were like potential energy diagrams. The concentration of C16 TAB at the minimum of the well corresponded to its cmc. Kalyansundaran and Thomas [9] have reported the usability of the
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Table 1 The cmc of surfactants obtained from different methods at 298 K System C10 TAB C12 TAB C14 TAB C16 TAB C12 –C16 c SDS SDBS MEGA-10d C14 TPBc
cmc (mM) Absorbance
Fluorescence
Conductometry
Microcalorimetry
Surface tension
69.3 15.3 4.58 0.95, 1.26a , 1.41b 1.37 (1) 15.1 (2) 7.09 1.34 6.68 0.81 (1) 2.52 (2)
– 14.9 3.66 0.87 1.38 (1) – 7.05 – 7.05 –
66.9 [15], 65.6 [17] 15.3 [17] 3.88 [15], 3.94 [17] 0.91 [15], 0.96 [17] 1.32 [15] (1) 12.3 [15] (2) 7.75 [22] 1.82 [15], 2.90 [6] – 0.77 [4] (1) 2.43 [4] (2)
– 15.2 [18] 4.11 [19] 0.97 [20], 1.29a [28] 1.69 [15] (1) 16.3 [15] (2) 7.78 [21] 2.80 [6] – 0.80 [4] (1)
– 14.6 [15] 4.13 [15], 3.74 [23] 0.85 [15], 0.82 [24] 1.74 [15] (1) – 8.31 [15], 8.00 [25] – 6.45 [15], 5.00 [26] 1.05 [6b]
a cmc at 303 K. b cmc at 308 K. c The cmc values 1 and 2 refer to two cmcs of the C TAB:C TAB = 1:1 (mol/mol) and C TPB systems. Microcalorimetry and tensiometry have 12 16 14
evidenced a single cmc. d cmc at 6.7 mM was obtained by light scattering method [27].
(a)
(b)
Fig. 6. (a) (I /IW )T and I1 /I3 vs [SDS] profile for pyrene (2.0 µM) at 298 K. Inset: I /IW vs [SDS] profile for pyrene (2.0 µM) at 298 K. Curves 1, 2 and 3 correspond to 373, 383 and 393 nm, respectively. (b) (I /IW )T and I1 /I3 vs [C16 TAB] profile for pyrene (2.0 µM) at 298 K. Inset: I /IW vs [C16 TAB] profile for pyrene (2.0 µM) at 298 K. Curves 1, 2 and 3 correspond to 293, 373 and 383 nm, respectively.
pyrene fluorescence method from the plots between the fluorescence lifetime as well as I3 /I1 and [surfactant]. The plots were sigmoidal in nature and sharp. We have shown I1 /I3 vs [surfactant] profiles in our documentation (Fig. 6) wherein the features were also sigmoidal. Similar profiles have also been reported in recent literature [11]. The fluorescence plots for C12 TAB:C16 TAB systems at 1:1 (mole/mole) composition and that for C12 TAB are illustrated in Fig. 7. Indications for the formation of two cmcs were not observed as revealed from the absorption spectral measurements (Fig. 4b). Thus, the absorption spectral method has an advantage over fluorescence method in the identification of more than one cmc for a surfactant, which is otherwise difficult to determine straightforwardly by methods other than conductance, and for some systems by isothermal titration calorimetry. The cmc values obtained from emission experiments are also in-
Fig. 7. (I /IW )T vs [surfactant] profiles for pyrene (2.0 µM) at 298 K. Curves 1 and 2 represent C12 TAB and 1:1 C12 TAB/C16 TAB.
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Table 2 The a, Ssig and I1 /I3 (at cmc) values for different surfactant systems System
a
I1 /I3 (at cmc)
Ssig
C10 TAB C12 TAB C14 TAB C16 TAB C12 –C16 a
0.230 0.186 0.140 0.130 0.105 (1) 0.204 (2) 0.105 0.069 0.101 0.019 (1) 0.024 (2)
1.36 1.27 1.1 1.06 1.05 (1) 1.06 (2) 0.968 0.873 0.995 0.717 (1) 0.733 (2)
0.008 0.010 0.022 0.010 0.068 (1) 0.013 (2) 0.040 0.016 0.018 0.031 (1) 0.020 (2)
SDS SDBS MEGA-10 C14 TPBa
a Two values correspond to two micellization processes.
cluded in Table 1. Herein also the rationale of Aguiar et al. has been considered for cmc estimation. It is considered that the hydrophobic pyrene molecules remain associated in aqueous medium. The anionic amphiphiles may deaggregate them and form complex with pyrene and enhancement of absorption may take place by the first process, which levels off after micelle formation. The process of complexation in the case of cationic amphiphiles imparts a radiationless energy transfer so as to reduce the effective emission intensity up to the cmc point with enhancement of emission and its leveling off afterwards. But interestingly the I1 /I3 ratio evidences a negative sigmoidal trend with [surfactant] irrespective of the type. 6.4. Analysis of results with reference to polarity index It is considered that the I1 /I3 ratio of pyrene fluorescence is an index of polarity of its environment. The values in hydrocarbon solvent vary in the range of 0.57–0.61 and in polar solvents the range of variation is 1.25–2.00. In micellar medium the range of variation observed is 1.1–1.5. Kalyansundaran and Thomas [9] have elaborately discussed these results and the increased range of I1 /I3 for micellar solution compared to hydrocarbons has been explained in terms of water penetration into micelles [16]. According to them, the polarity index depends on the type of surfactant head group and should not depend on the surfactant chain length, concentration, counter ion and added electrolyte. Larger head groups as in CTAB produce less interfacial compactness and allow water penetration which compact micelles like that of SDS does not. The I1 /I3 index for CTAB is thus greater than that for SDS. The I1 /I3 values for C10 -, C12 -, C14 and C16 TAB as well as for SDS are presented in Table 2. The emission peaks one and three of pyrene were masked by self-absorption of C14 TPB and SDBS so that their polarity indices could not be experimentally determined. We have observed that at the constant [pyrene] = 2 µM, af − ai or a and the slope of the sigmoid Ssig (determined from the linear part around x0 ) depended on the surfactant system essentially on its micellar polarity. The a has shown a nice linear dependence while the linear dependence for Ssig was
Fig. 8. a vs I1 /I3 profile for the studied surfactants at 298 K. Inset: a and I1 /I3 vs carbon number (Cn ) profiles for C10 -, C12 -, C14 - and C16 TAB at 298 K.
only moderate. The linear profile between a and I1 /I3 is documented in Fig. 8. An equation of the following form a = (−0.2113 ± 0.024) + (0.321 ± 0.210)(I1 /I3 )
(2)
with correlation coefficient of 0.9914 holds. In this correlation, the anionic surfactant SDS and the nonionic representative MEGA-10 fairly fitted in the general course for the ATABs. Thus a can be set to a marker for the micellar polarity. In the insets of Fig. 8, the fair linear dependence of both a and I1 /I3 on the carbon number (Cn ) in the alkyl chain of the ATABs has been illustrated. In this correlation, both SDBS and C14 TPB were out of trend. This was due to the differences in the contributions of their head groups from the head group of ATABs. 6.5. Alkyl chain length related micellar polarity of ATABs The I1 /I3 values of the ATABs have shown a decreasing trend with increasing alkyl chain length. Increased penetration of water in the micelles of lower homologues of the ATABs was envisaged. Thus probability of pyrene to experience polar environment was maximum in C10 TAB micelles and minimum in C16 TAB micelles. This was contrary to the propositions of Kalyansundaran and Thomas [9] that the polarity index of pyrene in micelle should depend on the head group and should be independent of the surfactant chain length. In this study, regularity in the I1 /I3 values with the alkyl chain length in the homologous series of ATABs has been observed. Recognizing water penetration up to four to five carbon atoms of the alkyl chain [16], internal hydrophilic region per unit volume of micelles of ATABs should decrease with increasing chain length. Pyrene has a size of 0.77 nm, and the sizes of ATAB micelles are 2.81, 3.32, 3.80 and 4.28 nm for C10 -, C12 -, C14 - and C16 TAB, respectively [17]. Considering water penetration up to the fifth carbon atom in the micelles, the effective size of the hydrophilic regions of the above ATABs become 2.05, 2.65, 3.15 and 3.65 nm, respectively. Thus, pyrene had enough
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rooms in the ATAB micelles. Taking one pyrene on the average resided in one micelle, the proximity of polar environment to pyrene should increase with decreasing micellar size of the ATAB series from C16 to C10 TAB to augment increasing polarity index (I1 /I3 ). [6]
7. Conclusions [7]
(1) UV absorption of pyrene in surfactant solution can be a convenient method for the determination of their cmc. (2) The absorption and emission results of pyrene in different surfactant solution have evidenced fair agreement between the derived results. (3) A spectral parameter a (difference between the asymptotes of the sigmoidal Boltzman profile) at a constant [pyrene] has produced a fairly linear correlation with I1 /I3 . (4) Both a and I1 /I3 are linearly related with the carbon number of the ATAB homologues.
Acknowledgments
[8] [9] [10] [11] [12]
[13] [14] [15] [16]
G.B.R. and I.C. thank CSIR for a Junior Fellowship and a Senior Fellowship, respectively. S.P.M. thanks INSA for a Senior Scientist position during the tenure of which this work was performed. The fluorescence spectral measurement facility rendered by Dr. S. Mukherjee and Ms. Doyel Das of IACS, Kolkata is acknowledged with appreciation.
[17] [18] [19] [20]
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