Article pubs.acs.org/Langmuir
Wet Interface of Benzylhexadecyldimethylammonium Chloride Reverse Micelle Revealed by Excited State Proton Transfer of a Localized Probe Aparajita Phukon, Nabajeet Barman, and Kalyanasis Sahu* Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India S Supporting Information *
ABSTRACT: Excited state proton transfer (ESPT) of an anionic photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) has been studied inside a cationic reverse micelle (RM), water/benzylhexadecyldimethylammonium chloride (BHDC)/benzene, using steady-state and timeresolved fluorescence spectroscopy. The observed ESPT behavior is found to be remarkably different from the known ESPT trend of HPTS inside anionic AOT and cationic CTAB RMs; the ESPT dynamics approaches that of bulk water at higher w0 (≥10) inside AOT RM while no ESPT was observed for CTAB reverse micelle [Sedgwick et al. J. Am. Chem. Soc. 2012, 134, 11904−11907]. The ESPT dynamics inside BHDC RM is remarkably slower compared to that of water at all w0 (= [water]/[surfactant]) values and relatively much less sensitive to w0 variation compared to AOT RM. 2D NOESY and fluorescence anisotropy measurements reveal that the probe (HPTS) is embedded inside the positive interface of BHDC RM. Despite its trapped location, HPTS is able to undergo ESPT due to significant penetration of water molecules into the interface. Furthermore, facile ESPT at higher w0 is consistent with higher degree of interface hydration as predicted by a recent MD simulation [Agazzi et al. Langmuir 2014, 30, 9643−9653]. The study shows that ESPT dynamics inside RM varies not only with the interface charge but also on the nature of the headgroup and solvation.
1. INTRODUCTION The study of structure, properties, and dynamics of reverse micelle (RM) remains a subject of continuous investigation because of its widespread applications in nanoparticle synthesis1−3 to enhance chemical and enzymatic reactions4−7 and as model water−membrane interface.8,9 A variety of surfactants (e.g., neutral, cationic, anionic, zwitterionic) spontaneously form RMs as spherical or ellipsoidal aggregates in nonpolar solvent encapsulating a certain amount of water (or other polar solvents). A surfactant monolayer is oriented in such a manner that its tails are directed toward nonpolar solvent and the polar headgroups encase a nanometer-sized aqueous droplet (called water pool). The molecular probe 8-hydroxypyrene-1,3,6-trisulfonate (HPTS or pyranine, Scheme 1) has been extensively used to probe the nature of water environment inside micelles,10,11 reverse micelles,12−15 and living cell.16−18 HPTS has mild acidity in the ground state with a pKa of 7.2−7.7 and remains predominantly in the protonated form in neutral bulk water.19,20 However, upon electronic excitation pKa* of HPTS drops to 0.5−1.4, resulting in an abrupt enhancement in acidity (by ∼7 orders of magnitude) and thus promptly releases a proton.20,21 The absorption and the emission maxima of the protonated (ROH) and deprotonated (RO−) forms are quite distinct, and hence, the excited state proton transfer © 2015 American Chemical Society
Scheme 1. Chemical Structures of the Protonated Form of HPTS and BHDC Surfactant with Proton Labels Used for NMR Signal Assignment
Received: May 21, 2015 Revised: October 28, 2015 Published: November 5, 2015 12587
DOI: 10.1021/acs.langmuir.5b03632 Langmuir 2015, 31, 12587−12596
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Scheme 2. Possible Environment around HPTS in the Interfacial Regions of BHDC Reverse Micelle in Comparison to CTAB Reverse Micelle
difference in ESPT nature was inferred to different locations of the probe supported by 2D NOESY measurements. As the electrical charge of the probe is highly negative (−3 and −4 for the protonated and deprotonated forms, respectively), the probe is repelled from the negative interface and forced to stay at the center of the water pool inside the anionic reverse micelle. In contrast, for the cationic reverse micelle, the probe remains buried inside the positively charged interface. As the hydroxyl group of HPTS does not have access to water, the ESPT was blocked inside the cationic CTAB reverse micelle.22 Very recently, Lawler and Fayer reported greatly different ESPT dynamics of HPTS inside IGEPAL reverse micelle compared to the AOT reverse micelle.24 In the neutral (IGEPAL) reverse micelle, the probe is located close to the interface and the ESPT dynamics is much slower than bulk water at all w0 values. In summary, the ESPT nature of HPTS and its location vary drastically with the interfacial charge. However, one should note that the cationic CTAB reverse micelle was actually a quaternary reverse micelle with the composition cyclohexane/CTAB/n-octanol/water.22 CTAB being a single tail surfactant typically needs a medium chain alcohol (e.g., n-octanol) as cosurfactant to form reverse micelle. The cosurfactant is usually assumed to be inserted in-between the CTAB surfactants at the interface (Scheme 2). However, a cosurfactant may alter interfacial property35 and, hence, may affect ESPT dynamics or the probe location. In order to get a consolidated idea of nature of ESPT in differently charged reverse micelles, it is important to undertake ESPT dynamics inside a cationic reverse micelle devoid of any cosurfactant. In this work, we used a cationic surfactant benzylhexadecyldimethylammonium chloride (BHDC, Scheme 1) which can form reverse micelle in aromatic nonpolar solvents (e.g., benzene, toluene, etc.) in the presence of water without aid of any cosurfactant. This cosurfactant free cationic RM has been subject of many studies.36−43 Interestingly, for BHDC reverse micelle, the observed ESPT dynamics differs sharply from that of the CTAB quaternary reverse micelles. In fact, we found moderate w0-dependent ESPT signatures. We also employed fluorescence anisotropy and 2D NMR to locate the position of HPTS inside the reverse micelle. The probe is found to be located at the interface like that in a CTAB RM but is still able to undergo ESPT.
(ESPT) could be conveniently followed by monitoring either transient absorption12,22,23 or emission at different wavelengths.24,25 The dynamics of HPTS in water was initially investigated by Pines and Huppert26 and later more extensively in the ultrafast domain by Tran-Thi et al.27 and Leiderman et al.28 using femtosecond fluorescence and absorption spectroscopy. Pines and Huppert showed that the ESPT process occurs in ∼70 ps time scale followed by an extended tail representative of a reversible geminate recombination of the HPTS anion with the ejected proton.26 Tran-Thi and co-workers observed three times constants of 0.3, 2.5, and 87 ps associated with the dynamics of HPTS in neat water.26 The two ultrafast components were attributed to solvation dynamics and LE− CT transition, respectively, whereas the 87 ps component was assigned to proton transfer.27 Mohammed et al. reassigned the ultrafast components (0.3 ± 0.2 and 3 ± 1.5 ps) to H-bond rearrangement of water around HPTS prior to proton transfer.29 The dynamics of proton transfer is dramatically linked with the availability of the proton acceptors, mobility of the ejected proton, and the ability of the solvent to reorganize and stabilize the transient ion pair formed during the process. Thus, ESPT serves as a sensitive indicator of local water environment and proton transport in micelle,11 reverse micelles,22,24 and other nanoconfined assemblies.30,31 The water confined inside the RM can be categorized into at least two types: water in contact with the interface and water residing at the core of the water pool.32−34 The hydrogen bonding and dynamics of water at the interface are markedly different from those of the bulk, while the water at the central core gradually attains the properties of bulk water for large water pools. The distinction between the interface and the core waters becomes more prominent with increase of the size of a reverse micelle. Thus, the dynamics of an ESPT probe is expected to modulate strongly depending on its location inside a RM. Recently, Levinger and co-workers have employed an elegant combination of 2D NMR and transient absorption spectroscopy to find out the location of HPTS and the corresponding ESPT dynamics in two oppositely charged RMs.22 They observed sharply contrasting modulation of ESPT dynamics inside anionic AOT and cationic CTAB reverse micelles. In the anionic reverse micelle, the ESPT dynamics varies strongly with the water content, while inside the cationic reverse micelle, no sign of ESPT was traced. This drastic 12588
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2. EXPERIMENTAL SECTION 2.1. Materials Used. 8-Hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS, pyranine) and benzylhexadecyldimethylammonium chloride (BHDC) were purchased from Sigma-Aldrich Chemicals. Benzene (HPLC grade) was purchased from Spectrochem India. Benzene-d6 and D2O were purchased from Sigma-Aldrich Chemicals. Water (resistivity 18.2 MΩ cm) was obtained from a Millipore system. 2.2. Instruments and Detection Methods. The absorption and emission spectra were recorded in a PerkinElmer Lamda-750 spectrophotometer and Jobin-Yvon FluoroMax4 spectrofluorometer, respectively. The time-resolved fluorescence were measured by a timecorrelated single photon counting (TCSPC) setup LifeSpec2 (Edinburg Instruments) using a picosecond laser diode EPL-375 (Edinburgh Instruments) with an excitation wavelength of 375 nm. Typical fwhm of the set up was ∼100 ps. The fluorescence decays were fitted using FAST software. The fluorescence transients were performed at a magic angle of the analyzer with respect to the polarizer. To measure fluorescence anisotropy decays, the analyzer was rotated at regular intervals to obtain parallel (I∥) and perpendicular (I⊥) components of a fluorescence decay separately. The anisotropy function, r(t), was constructed using the expression r(t ) =
Figure 1. Emission spectra of HPTS inside BHDC reverse micelle at various w0 values (λex = 390 nm). Emission spectrum in water (dotted line) is also included for comparison.
I (t ) − GI⊥(t ) I (t ) + 2GI⊥(t )
ps) in water compared to its fluorescence lifetime (∼5.5 ns),28 and hence, the steady-state emission contribution from the protonated (ROH*) form is negligible. On the contrary, inside the BHDC reverse micelle, the emission pattern markedly changes; emission is rather dominated by the protonated form. At low w0 (= 2), the emission spectrum is almost a single band emission centered at 434 nm with a very minute hump at ∼525 nm (Figure 1). This indicates strongly retarded or insignificant ESPT inside BHDC at this low w0. On increasing water content, the emission intensity of the protonated emission decreases gradually with a concomitant development of the deprotonated emission band. To get more insight into the emission characteristics of the two forms, we decompose the total emission spectrum at respective w0 values into two emission bands. For this, we first fit each of the emission spectrum with a superposition of two log-normal peak functions and then generate the two emission spectra from the fit parameters. An example of the analysis is given for the emission spectrum at w0 = 10 (Figure S2). The fitted emission maxima of the protonated and deprotonated emission bands were at 436 and 525 nm, respectively. Importantly, considerable overlap between the emission spectra of the two forms extends over a significant wavelength range. As warned earlier by Lawler and Fayer, emission decays measured at a fixed wavelength to monitor the fluorescence transients of the protonated and deprotonated forms are often problematic because of mutual spectral crossover.24 Our spectral decomposition suggests that fluorescence transient measured at a wavelength close to the peak of ROH* form is almost free of the RO*− emission but the reverse is not true. The emission spectrum of the ROH* stretches out significantly into almost all the wavelength regions of the deprotonated form. Thus, it is almost impossible to find a wavelength for the deprotonated emission absolutely free from the protonated emission. Nevertheless, a wavelength (e.g., 570 nm) significantly red-shifted from emission maximum of the deprotonated form may be a practical one where the contribution of ROH emission is much less compared to RO*− emission. From the decomposed spectra (Figure S2), it is also clear that not only the intensity ratio of the two emissive forms but
(1)
The G value of the setup was determined using HPTS in methanol, and the G value at 440 nm emission wavelength was found to be 0.63. Fluorescence anisotropy decays were fitted using Igor-Pro software. Dynamic light scattering (DLS) measurements were recorded using Malvern Nano ZS90 instrument. The He−Ne laser (λ = 632.8 nm) was used as excitation source. The scattering was collected at a fixed angle of 90°. The solutions were filtered with PTFE syringe filters having 0.2 μm pore size prior to DLS measurements. All measurements were performed at 298 K. The 1H NMR spectra of BHDC reverse micelle solutions with incorporated HPTS fluorophore were recorded using a Bruker 600 MHz NMR spectrometer. All NMR measurements were carried out in deuterated benzene solvents (Sigma-Aldrich, 99.96 atom % D) solvent containing 0.03% (v/v) TMS as chemical shift reference. The 2DNOESY NMR spectra were recorded in the 600 MHz NMR spectrometer; the 90° pulse width was 6.5 μs; the spectral window was 12 kHz in both t1 and t2, the acquisition time was 13.09 s; digital signal processing and frequency shifted quadrature detection were used. 2.3. Preparation of Reverse Micelle Solutions. The reverse micelle solutions at various w0’s (2−20) were prepared by adding a requisite amount of water (using the relation w0 = [H2O]/ [surfactant]) into 0.3 M BHDC solution in benzene. To incorporate the probe into the reverse micelle (RM) solution, a required amount of a stock solution of HPTS in water (or in D2O) were injected into the reverse micelle solution. In all experiments, the concentration of the HPTS was kept at ∼12 μM.
3. RESULTS 3.1. Steady-State Absorption and Emission Spectroscopy. In neutral water (pH ∼ 6.5), HPTS dominantly remains in the protonated form and exhibits an absorption maximum at 403 nm. Similarly, inside BHDC reverse micelle, the probe also remains exclusively in the protonated form with a slightly redshifted absorption (maximum at 406 nm) in the ground state as no significant absorption band was observed at ∼450 nm at any w0 value (Figure S1). The emission spectrum of HPTS in water is largely dominated by a strong emission band centered at ∼512 nm and a very faint emission band at ∼440 nm (Figure 1). It is well-known that HPTS undergoes relatively fast ESPT (∼90 12589
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Figure 2. TRANES (left panel) and normalized TRES (protonated form only, right panel) of HPTS inside BHDC reverse micelle at w0 = 2 and 20, respectively. The ESPT process is obviously more prominent at higher w0.
to the lifetime of HPTS in methanol (3.4 ns).48 Thus, the microenvironment of HPTS inside BHDC RM is greatly different from that of water. The lifetime of HPTS inside AOT RM was found to be 5.0 ns by the same this method while a w0dependent distribution of lifetime (1.8−2.1 and 5.0−5.4 ns) was obtained for neutral IGEPAL RM.24 The normalized TRES of the protonated emission displays significant spectral relaxation in the shorter times mostly within ∼1 ns (Figure 2). This time-dependent Stokes shift (characteristics of solvation dynamics) may be due to solvation of the initially excited HPTS (protonated form) by the positive headgroups, counterions, and water prior to ESPT process. Thus, three different processes are evident from the TRES measurementssolvation dynamics accounting for spectral shift in the faster time scale followed by excited state proton transfer and relaxation back to the ground state (lifetime). In bulk water, HPTS undergoes ultrafast solvation dynamics (∼0.3 and ∼2 ps) proceeding PT (∼90 ps).27,29 However, solvation dynamics faster than proton transfer has never been reported for HPTS in any confined system. Probably, in most systems, the level of hydration is high enough to activate ESPT more efficiently compared to solvation dynamics. Recently, Douhal and co-workers observed that the ESPT dynamics of HPTS bound to a protein human serum albumin (HSA) are associated with both ultrafast (∼0.15−0.2 ps and 3−5 ps) and slow (130 ps and 1.2 ns) components.49 They ascribed the fastest component to the direct ESPT to carboxylate group of amino acid and intermediate component to ESPT propagated via water bridges whereas the slow components were due to slow biological water molecules involved in ESPT process.49 Here, in the BHDC reverse micelle, almost no emission from the deprotonated form is observed at early time TRES at all w0’s, implying that the ESPT process unlikely possess any component faster than the time resolution of our setup. The lack of ultrafast component may be due to the unavailability of free (fast or bulk-type) water molecules or any other proton acceptor in vicinity of HPTS residing in the interface region of
their peak positions are also changing with water content. The RO*− emission maxima shifts from 521 to 527 nm as the w0 values vary from 2 to 20 (see Figure S3). Interestingly, these values are much different from the emission maximum of the deprotonated form in water (512 nm) or other RM (AOT or IGEPAL24). Such red-shifted emission of RO*− was reported for HPTS absorbed to CTAB micelle11 or lysozyme−CTAB aggregate.31 Thus, the red-shifting of HPTS may be a general feature of HPTS residing in a positive interface and may arise from the efficient electrostatic interaction and solvation by opposite charged headgroup. Cation−π interaction between the quaternary ammonium headgroup of BHDC and the aromatic ring of HPTS or π−π interaction between the benzyl group of BHDC and HPTS may also contribute to the red-shifting. Huppert and co-workers reported a red-shifted emission for HPTS linked to alumina surface via a covalent linkage.44 3.2. Time-Resolved Emission Measurements. The fluorescence transients of HPTS were measured at 10 nm wavelength intervals across the steady-state emission spectrum, and subsequently, time-resolved emission spectra (TRES) and time-resolved area-normalized emission spectra (TRANES) were reconstructed (Figure 2 and Figure S4). This is often considered as a better method of probing ESPT dynamics over single wavelength approach especially under strong overlap between protonated and deprotonated emissions and timedependent spectral relaxation.45−47 Clearly, at short time TRES (also in TRANES), only the protonated emission dominates and with lapse of time, the deprotonated from appears at the expense of the protonated emission. Comparing TRANES of different w0’s, it can be stated that the ESPT process becomes more prominent with increase of w0. The decay of the integrated total emission intensity (integrated area of the entire TRES) shows an exponential decay with time constant of 3.2 ± 0.2 ns (Figure S5). This time constant denotes lifetime of HPTS inside the BHDC reverse micelle and does not vary significantly with w0. The lifetime is very different from the reported lifetime in water (5.4 ns)28 and more closely resemble 12590
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Figure 3. Fluorescence transients of the (a) protonated, ROH*(t), and (b) deprotonated, RO*−, forms of HPTS at different w0 values. The emission transients were measured at 425 and 570 nm, respectively, upon excitation at 375 nm. The initial portion of deprotonated transients are displayed in the inset to show clearly the systematic variation present in the rise of the emission at different w0’s.
Table 1. Fluorescence Decay Parameters of Protonated, ROH*(t), and Deprotonated, RO*−, Emissions of HPTS inside BHDC Reverse Micelle, Respectively Measured at 425 and 570 nm; Decay Parameters of the D2O Containing RM Are Also Added for Comparison deprotonated, RO*−(t)
protonated, ROH*(t) w0 2 2 4 4 6 6 10 10 20 20
(H2O) (D2O) (H2O) (D2O) (H2O) (D2O) (H2O) (D2O) (H2O) (D2O)
τ1 (ps) 330 660 340 400 290 370 245 320 250 440
(0.20) (0.14) (0.28) (0.24) (0.30) (0.24) (0.32) (0.24) (0.306) (0.30)
τ2 (ps) 1435 1565 1550 1725 1340 1540 1250 1490 1180 1730
τ3 (ps)
(0.25) (0.18) (0.28) (0.25) (0.30) (0.26) (0.33) (0.31) (0.38) (0.34)
3450 3415 3410 3510 3250 3410 3140 3360 3080 3350
1240 1275 1700 1770 1640 1810 1500 1826 1480 1820
(−0.90) (−0.45) (−1.80) (−1.37) (−2.20) (−2.16) (−2.78) (−2.61) (−2.30) (−3.07)
τf (ps) 4770 4570 5000 4990 5100 5150 5200 5270 5230 5365
(1.90) (1.45) (2.80) (2.37) (3.20) (3.16) (3.80) (3.61) (3.30) (4.07)
forms show distinct rise time matching with the ESPT component observed at 425 nm. 3.3. Kinetic Isotope Effect. One convenient way to test whether the photophysical process of HPTS inside the BHDC reverse micelle involves solvation dynamics or ESPT is checking deuterium isotope effect.50 Thus, we replicate the fluorescence measurements by replacing water with D2O (see Figure S6). The ratio of deprotonated emission intensity to the protonated emission is clearly smaller in D2O containing BHDC RM compared to water analogue at all w0 values (see Figure S7). A comparison of the emission spectra of HPTS inside BHDC reverse micelle containing water vs D2O at w0 = 10 is given in Figure S8a. The overall KIE is found to be 1.5−2 based on the steady-state fluorescence. Likewise, the fluorescence transients at both 425 and 570 nm differ significantly in D2O from that in water. Figure S8b shows the fluorescence transients in water and D2O at w0 = 10, and all other data are supplied in Figure S6. The first two decay components (due to solvation dynamics and ESPT, respectively) in the D2O containing RM are significantly higher than the decay components in water containing RM (Table 1). We observe that the amount of KIE increases with increase of w0 (see Figure S7). However, KIE is significantly lower compared to that of bulk water vs D2O (KIE ∼ 3).50,51 Thus, we may conclude that only a fraction of the HPTS is undergoing ESPT. The non-negligible KIE supports the assignment of the
BHDC RM. The slow ESPT may be due to involvement of slow water molecules trapped within the headgroups of BHDC reverse micelle. We also compared the fluorescence days for different w0 at two fixed wavelengths: 425 and 570 nm respectively for the protonated and deprotonated forms. From Figure 3, a moderate variation among emission transients of the protonated form at different w0 is visible. The decays of the protonated form become progressively faster with increase of the w0 values. However, the decay even at the highest w0 value is markedly slower than that of bulk water. The decays at 425 nm (ROH* emission) can be best fitted by a triexponential function (Table 1) [ROH*(t )] = a1e−t / τ1 + a 2e−t / τ2 + a3e−t / τ3
(0.55) (0.67) (0.44) (0.51) (0.41) (0.50) (0.35) (0.45) (0.32) (0.37)
rise (ps)
(2)
This is consistent with three processes extracted from the TRES measurement. Thus, we may assign the first component to solvation dynamics while the longest component represents the lifetime and the intermediate component may be due to ESPT process. Note that there is only very little variation among the components at various w0’s, but a clear variation of the amplitude of the components can be noticed. Most notably, contribution (i.e., a2) of the ESPT process increases with w0. All the fluorescence transients (at 570 nm) of the deprotonated 12591
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(WIC) model where the anisotropy decay is assumed to originate from wobbling motion in which one part is attached to the interface and other part is undergoing an angular motion restricted to a cone with a semicone angle of θ and lateral diffusion of the probe along the interface.47,53,54 Note that WIC model usually includes overall rotation of reverse micelle. But this time scale is much higher (especially at higher w0) compared to the lifetime of HPTS; thus, we have not considered this, and we may expect that the overall motion may be responsible for the observed residual anisotropy. If the time constants of these motions are τW and τD, respectively, the anisotropy decay should be represented as
solvation dynamics and ESPT origin of the components. Note that similar small KIE (∼1.7) was observed for HPTS absorbed onto the chitin surface in water vs D2O.51 3.4. Dynamic Light Scattering. The size (diameter) of the reverse micelle at various w0 values was measured using dynamic light scattering. The size of the reverse micelle gradually increases with increase of w0. The measured sizes are listed in the Supporting Information (Figure S10 and Table S1). The values are consistent with an earlier report.52 3.5. Fluorescence Anisotropy Decay. Fluorescence anisotropy decays were measured to gain knowledge of the local environment around the probe. The anisotropy decays at various w0 values are much slower compared to that in a fluid solvent (e.g., in methanol or water). The rotational relaxation time of HPTS in bulk methanol was found to be 240 ps. The anisotropy decays in BHDC reverse micelle were found to be much slower compared to that of methanol, indicating a very restricted rotation of the probe (Figure 4). The anisotropy
r(t ) = r0[S2 + (1 − S2)e−t / τW ]e−t / τD]
(4)
where S is an order parameter and is related to the semicone angle θ as S=
1 cos θ(1 + cos θ) 2
(5)
Equating eq 3 and eq 4, one obtains
decays maintain very small but finite residual fluorescence anisotropies that do not decay completely probably due to short fluorescence lifetime (∼3.2 ns). The rotational anisotropy decays can be fitted with a biexponential function having a constant residual anisotropy (r∞) (3)
where r0 is initial anisotropy; τs and τf are the slow and fast rotational time constants, respectively, and as is the amplitude of the slow component. The anisotropy decay parameters are provided below (Table 2). The anisotropy decay of interface-bound fluorescence probes in reverse micelles is often analyzed via wobbling-in-cone Table 2. Anisotropy Decay Parameters of HPTS in Water/ BHDC/Benzene Reverse Micellar System at Different w0 Values w0
r0
r∞
as
τs (ns)
τf (ns)
τW (ns)
θ (deg)
2 4 6 10 20
0.33 0.33 0.34 0.39 0.35
0.03 0.03 0.04 0.08 0.07
0.92 0.88 0.85 0.78 0.75
4.09 3.27 2.99 2.85 2.61
0.58 0.56 0.44 0.50 0.50
0.67 0.67 0.51 0.60 0.62
13.2 16.5 18.9 22.8 24.7
(6a)
1 1 = τs τD
(6b)
1 1 1 = + τf τW τD
(6c)
In summary, very restricted rotation is observed for HPTS inside BHDC reverse micelle at all w0’s. A small but systematic w0 variation is observed for the long component (lateral diffusion) and also for the amplitude of slow components (Table 2 and Figure S11). Note that the semicone angle is relatively small and increases with w0. This implies that probe may be tightly confined within the interface and experience strong electric field and with increase of w0 the wobbling motion becomes somewhat facile because of higher hydration. The w0 dependence of the anisotropy is much less dramatic compared to the HPTS analogue (MPTS) inside AOT reverse micelle or even for the neutral reverse micelle IGEPAL.24 Lawler and Fayer found that the rotational relaxation time of MPTS inside AOT reverse micelle remarkably reduces with increase of w0. For example, at w0 = 5, the rotational dynamics was described by two components0.65 ps (80%) and 4.7 ns (20%)while it becomes single exponential with a time constant of 0.32 ns at w0 = 10. Furthermore, at w0 = 25, the anisotropy decay virtually merges with the rotational dynamics of HPTS in bulk water. On the other hand, for IGEPAL reverse micelle, at w0 = 3, the biexponential anisotropy decay with components1.9 ns (38%) and 8.9 ns (62%)reduces to 0.60 ns (67%) and 4.1 ns (33%) at w0 = 20. The very different anisotropy trend of MPTS inside AOT and IGEPAL reverse micelles was ascribed to very different location (in the core and the interface, respectively) of the probe. Note that rotational times are usually very sensitive to the rigidity of the environment where the probe is located. If the probe is located in the core, the rigidity of the environment should change dramatically as the size of the water pool increases but much less effect should be observed if the probe is situated within the interface. Thus, it may logical to assume that the probe may reside in the interface rather than in the core.
Figure 4. Fluorescence anisotropy decays of HPTS in bulk methanol (red) and in BHDC reverse micelle at w0 values of 2 (blue) and 20 (green) measured at λex = 375 nm and λem = 440 nm. The black line indicates a biexponential fit according to eq 2. It is evident that the anisotropy decays inside the RM are much slower compared to that in methanol and very less sensitive to water content.
r(t ) = (r0 − r∞)[ase−t / τs + (1 − as)e−t / τf ] + r∞
as = S2
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Langmuir 3.6. NMR Spectroscopy. Employing 2D NOESY experiments, we intend to measure interaction of HPTS protons with BHDC surfactant protons and water molecules, if it exists. This may help us to locate the position of HPTS inside the RM. Such a method was previously applied by Levinger and coworkers for locating HPTS inside CTAB and AOT reverse micelles.22 We performed experiment at two different w0 values: 5 and 10. Figure 5a shows several cross-peaks in the 2D NOESY spectrum at w0 = 5. Usually an off-diagonal or crosspeak arises if the spatial separation between two protons is very small (typically 20) w0 values.22,24,25 Contrarily, inside the CTAB reverse micelle no ESPT was observed. We need to understand the possible reasons for this unique ESPT behavior inside BHDC RM. The water environment inside a reverse micelle is generally believed to be very heterogeneouswith more rigid water near the interface and comparatively more dynamic water at the water pool. Variation of w0 influences dynamics of water near the interface much less compared to that of the water present at the water pool. At very large w0 values the dynamics of water present at the water pool asymptotically reaches to bulk water. Thus, ESPT should depend on the probe location. In the case of AOT RM, the negative charge of the interface forces the negative probe HPTS to stay at the center of the water pool, while the probe remains buried inside the positive interface of CTAB and hence no ESPT is possible. This is markedly different from the case of BHDC RM studied here. Thus, either locations of probe or the nature of the environment is largely different in BHDC RM from that of CTAB or AOT RM. From the NOESY measurement at w0 = 5, multiple crosspeaks between HPTS and BHDC protons were observed, indicating proximity of HPTS to the surfactant headgroups. The very slow rotational dynamics of the probe revealed by fluorescence anisotropy measurement supports this. These results lead to a conclusion that the probe is probably inside the interface. Notably, however, at w0 = 10, no significant interaction between HPTS and BHDC was detected. It may be inferred that the probe remains well hydrated inside the interface which leads to cross-peak only with water molecules. Note that fluorescence anisotropy becomes relatively faster
Figure 5. 2D NOESY spectrum of HPTS incorporated BHDC reverse micelles at (a) w0 = 5 and (b) w0 = 10. Multiple cross-peaks are evident between HPTS protons and BHDC protons and also with water protons at w0 = 5 but only cross-peak with water is observed at w0 = 10. 12593
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can be fitted with a power law with an exponent of −3/2 as expected from Debye−Smoluchowski equation.59 Fayer and coworkers found that the power law nature of the long time diffusion-controlled recombination retains for HPTS inside AOT reverse micelle with a modified exponent (−0.55) but not for IGEPAL reverse micelle.24 In our case the ROH emission could be best model by a triexponential dynamics rather than a power law one. Diffusion-assisted regeneration may be minor in this case because the negatively charged deprotonated species is residing in a positive interface; thus, the proton once dislocated far away may find it difficult to revert back.
with increase of water content. This may indicate that the interfacial arrangement becomes somewhat relaxed at higher w0 due to higher level of hydration. Another possibility is that the probe location is changed at higher w0, and it moves toward the core away from the interface. The fluorescence anisotropy decay inside BHDC RM is comparatively faster at w0 = 20 than at w0 = 2 but much slower than in bulk water or inside AOT RM at same water content (or similar size of water pool). If the probe were in the core, this should lead to much faster anisotropy decay and ultrafast ESPT. Hence, HPTS is most probably located inside the interface at all w0 values while the degree of hydration increases with increase of water content. Moreover, as the total population decays with a single lifetime, a very heterogeneous distribution of probe location is unlikely. The nature of the ESPT in the BHDC reverse micelle qualitatively matches with that of the neutral IGEPAL reverse micelle rather than in the cationic CTAB reverse micelle. No ESPT was found in the case of CTAB quaternary reverse micelle. The main difference between the two positively charged surfactants is that a methyl group of CTAB is replaced by a benzyl group in BHDC. This replacement lowers the cmc (critical micellar concentration) of BHDC compared to CTAB. 56 BHDC can form reverse micelle without a cosurfactant in nonpolar aromatic solvent but CTAB needs help of a cosurfactant (e.g., octanol). We need to consider whether the alcohol has any role on the ESPT dynamics observed in CTAB reverse micelle. As octanol is almost insoluble in water, it is unlikely that it will partition into the water pool at all w0. Octanol will mostly distributed between the interface and the continuous nonpolar phase.35 The coexistence of octanol and CTAB surfactants in the interface may alter the electrical properties and water dynamics quite differently from that of BHDC reverse micelle. The presence of octanol may replace water molecules solvating the quaternary ammonium headgroup. The location of probe is dependent on the electrostatic interaction and hydration. The presence of the cosurfactant may have significant role in keeping the balance between the two. Note that HPTS cannot transfer a proton to an alcohol.28,57 Thus, the presence of octanol at the interface may inhibit the ESPT completely at CTAB interface. The interfacial surfactant layer of BHDC is much less packed than conventional double-chain surfactant like AOT; thus possibly water may penetrate from the core into the interface. A recent MD simulation by Agazzi et al. revealed high degree of water penetration into the interface, and these waters are dynamically much restrained than water at the core.37 The degree of hydration of the BHDC surfactant headgroups increases with increase of water content.37 Physical process inside an interface is usually much less sensitive with water loading compared to core. Levinger et al. investigated solvation dynamics inside CTAB reverse micelle using different alcohols as cosurfactant and an anionic probe C343.58 They found that the emission spectrum is hardly affected by water loading or alcohol chain length. They concluded that the probe remains at the interface and experience similar environment at all w0 values. The observed ESPT behavior inside BHDC RM is much different in nature compared to that in bulk water. In water, the time constant for the initial deprotonation dynamics is about 90 ps; however, the fluorescence decay of the protonated form extends over a very long time. This long tail arises from the diffusion controlled recombination of the proton with the deprotonated form within the excited state. The long time tail
5. CONCLUSIONS In summary, ESPT of a negatively charged fluorophore HPTS has been investigated within a cationic BHDC reverse micelle using fluorescence spectroscopy. The location of the probe inside the RM was assigned by a combination of fluorescence anisotropy and NOESY measurements. The probe was found to be located at the interfacial region, similar to that of CTAB quaternary (n-heptane/CTAB/octanol/water) reverse micelle reported previously. However, the examined ESPT nature is very different from that of the CTAB reverse micelle; it undergoes detectable ESPT in BHDC RM whereas no ESPT was found inside CTAB RM. The difference in the ESPT trend in these two cationic interfaces may arise from difference in the interfacial properties of BHDC and CTAB reverse micelles. The presence of cosurfactant (e.g., octanol) may remove water from the CTAB interface, and hence no ESPT could occur. But in BHDC, the interfacial region may be quite wet due to significant penetration of water from the core into the interface. The ESPT in BHDC may serve as an excellent model for interfacial proton transfer at positively charged surface.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03632. Absorption, fitted emission spectra and emission maxima plots for both protonated and deprotonated forms of HPTS inside BHDC reverse micelle; TRES data, deuterium isotopic study, 1H NMR spectrum of HPTS and BHDC; figure and table for DLS size distribution results, fluorescence anisotropy decay fitting parameters (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (K.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work is funded by the Indian Institute of Technology Guwahati and Department of Science and Technology (DST), India (EMR/2014/000011), and Council of Scientific and Industrial Research (CSIR), India (01(2828)/15/EMR-II). We thank the Department of Chemistry and Central Instrument Facility for the instrument facility. We thank Dr. Sunanda Chatterjee of our department for important discussion on NMR assignments. We sincerely thank an anonymous reviewer for insightful suggestion. 12594
DOI: 10.1021/acs.langmuir.5b03632 Langmuir 2015, 31, 12587−12596
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