3514
Langmuir 2006, 22, 3514-3520
Fluorometric and Isothermal Titration Calorimetric Studies on Binding Interaction of a Telechelic Polymer with Sodium Alkyl Sulfates of Varying Chain Length Basudeb Haldar, Alok Chakrabarty, Arabinda Mallick, Malay Chandan Mandal, Paramita Das, and Nitin Chattopadhyay* Department of Chemistry, JadaVpur UniVersity, Calcutta-700 032, India ReceiVed December 13, 2005. In Final Form: February 17, 2006 Steady-state fluorescence measurements and isothermal titration calorimetric experiments have been performed to study the interaction between a telechelic polymer, pyrene-end-capped poly(ethylene oxide) (PYPY), and sodium alkyl sulfate surfactants having decyl, dodecyl, and tetradecyl hydrocarbon tails. Fluorometric results suggest polymersurfactant interaction in the very low range of polymer concentrations. The relative variation in the excimer to monomer pyrene emission intensities with varying surfactant concentration reveals that initial addition of surfactant favors intramolecular preassociation until the surfactant molecules start binding with the ethylene oxide (EO) chain. With the growing number of surfactant aggregates along the EO chain, the association becomes hindered due to the polyelectrolyte effect. The results from microcalorimetric titrations in the low concentration range of PYPY solution (∼10-6 M) with alkyl sulfates suggest two kinds of surfactant-polymer interactions, one with the polymer hydrophobic end groups and the other with the ethylene oxide backbone. The overall polymer-surfactant interaction starts at a much lower surfactant concentration for the hydrophobically modified polymers compared to that in the case of unsubstituted poly(ethylene oxide) homopolymer. From the experiments critical aggregation concentration values and the second critical concentration where free micelles start forming have been determined. An endeavor has been made to unveil the mechanism underlying the corresponding associations of the surfactants with the polymer.
Introduction Fluorescence spectroscopy has long been in the forefront of techniques for the study of the aggregation phenomena of various fluorophore-labeled amphiphilic polymers and surfactants.1 Isothermal titration calorimetry (ITC) or microcalorimetric titration, where the differential enthalpy during the binding process is monitored, is another sensitive technique for examining the binding interactions between the surfactant and the polymer. Both the techniques have been advantageously utilized in the present study to monitor the association behavior of a homologous series of alkyl sulfate surfactants with a representative hydrophobically modified telechelic polymer. Water-soluble polymers, which have been chemically modified by attaching hydrophobic groups along the polymer backbone or at the two ends, are denoted hydrophobically modified polymers (HMPs). Telechelic polymers are a class of HMPs, which have hydrophobe attachments at the two ends of a long water-soluble polymer backbone. HMPs are self-organizing polymers, and their unique solution properties in water are due to the association of the hydrophobic groups. Among the various types of HMPs, telechelic polymers prepared from poly(ethylene oxide) (PEO) perhaps constitute the best models for fundamental studies, since they have a very low polydispersity and the degree of functionalization of the extremities can be perfectly controlled. The associations of HMPs become modified in varying degrees with increasing polymer concentration in aqueous solution. In the low concentration range, telechelic polymers remain in a looped structure and they are called primary aggregates. At a given critical concentration, the critical association concentration, the R,ω-functionalized PEO primary * To whom correspondence should be addressed. E-mail: pcnitin@ yahoo.com. (1) Tazuke, S.; Winnik, M. A. In Photophysical and Photochemical Tools in Polymer Science; Winnik, M. A., Ed.; D. Reidel Publishers: Dordrecht, Holland, 1986.
aggregates associate to form “flower-like” micellar aggregates or clusters with a core of hydrophobic groups and a corona of loops. Above a second critical concentration, which corresponds to the overlap of these flower-like micelles, the clusters become interconnected, resulting in a “network” structure.2,3 A literature survey reveals that most of the studies so far reported with the PEO homopolymer or its hydrophobically modified homologues have been made within the 0.05-1.0 wt % range, i.e., at the micellar cluster stage or even more concentrated network structure stage.3-9 We have aimed to look for the interaction behavior of the anionic surfactants with a pyrene-labeled hydrophobically modified polymer, pyrene-end-capped poly(ethylene oxide) (PYPY; Scheme 1), at a much lower polymer concentration than so far reported. For this purpose, in both the fluorometric and ITC experiments the polymer concentrations were kept at an extremely low range. At this low concentration range only the primary aggregates prevail, and the simpler situation helps in understanding the properties of hydrophobically modified polymers in molecular terms. The study of the interaction of water-soluble polymers, especially HMPs, and surfactants employing numerous sophisticated techniques is a fertile field because of versatile practical uses of such systems and their interesting inherent properties.4 In several industrial applications, water-soluble polymer(2) Liu, F.; Frere, Y.; Francois, J. Polymer 2001, 42, 2969. (3) Persson, K.; Wang, G.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1994, 90, 3555. (4) Kwak, J. C. T. Polymer-Surfactant Systems; Surfactant Science Series Vol. 77; Marcel Dekker: New York, 1998. (5) Wang, G.; Olofsson, G. Pure Appl. Chem. 1994, 66, 527. (6) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276. (7) Dai, S.; Tam, K. C. Langmuir 2004, 20, 2177. (8) Dai, S.; Tam, K. C. Wyn-Jones, E.; Jenkins, R. D. J. Phys. Chem. B 2004, 108, 4979. (9) Dai, S.; Tam, K. C. J. Phys. Chem. B 2001, 105, 10759.
10.1021/la053370f CCC: $33.50 © 2006 American Chemical Society Published on Web 03/17/2006
Interaction of a Telechelic Polymer with SnS Scheme 1. Pyrene-End-Capped Poly(ethylene oxide) (PYPY)
surfactant systems are commonly encountered, such as foodstuffs, cosmetics, inks, water-based coating fluids such as thickeners, paints, and pharmaceuticals and in mineral processing, enhanced oil recovery, and polymer synthesis.10-12 Beyond the critical micelle concentration (cmc), surfactant molecules can selfassemble into aggregates of different morphologies.13 The presence of an interacting polymer can significantly alter the micellization behavior by inducing the aggregation of surfactant micelles onto the polymer chains. Among the several factors influencing the polymer-surfactant interaction behavior, the most important ones are the characteristics of the polymer and surfactant and the temperature.14 In general, polymer-surfactant interactions can be divided into two broad categories: (1) charged polymers and oppositely charged surfactants and (2) uncharged polymers and all types of surfactants.15 The study of interactions between the charged polymers (also known as polyelectrolytes) and oppositely charged surfactants in solutions is often complicated by the occurrence of precipitation induced by strong electrostatic interaction. The presence of electrolytes alters the binding strength, and varying salt concentrations can control the binding isotherms. On the other hand, interactions between uncharged polymers and surfactants are simpler due to the absence of strong electrostatic forces; thus, significant progress has been made in this field over the past three decades.3-7,14,16-21 The systems of various neutral homopolymers or unmodified polymers with different ionic and nonionic surfactants have extensively been studied, and a lot of information has been gathered about neutral polymer-surfactant interaction under varying conditions. The binding isotherms and the resulting mechanisms for neutral polymer-surfactant systems are known to depend on the nature of the surfactant, molecular weight of the polymer, chemical structures of the polymer and the surfactant, hydrophobic content of the polymer, electrolyte, temperature, and solvent quality.4,22 It is also reported that the binding interaction between an anionic surfactant and an uncharged polymer is much stronger than between an uncharged polymer and nonionic or cationic surfactants.22 Mixtures of anionic surfactants and flexible uncharged homopolymers, particularly poly(ethylene oxide) (PEO)-sodium dodecyl sulfate, have been the most widely (10) Finch, C. A. Industrial Water Soluble Polymers; The Royal Society of Chemistry: Cambridge, U.K., 1996. (11) Glass, J. E. AssociatiVe Polymers in Aqueous Media; American Chemical Society: Washington, DC, 2000. (12) Maestro, A.; Gonza´lez, C.; Gutie´rrez, J. M. J. Colloid Interface Sci. 2005, 288, 597. (13) Evans, D. F.; Wennerstrom, H. The Colloid Domain Where Physics, Chemistry, Biology and Technology Meet, 2nd ed.; Wiley-VCH: New York, 1999. (14) Goddard, E. D.; Ananthapadmanaban, K. P. Interactions of Surfactants with Polymer and Proteins; CRC Press: Boca Raton, FL, 1993. (15) Karsa, D. R. Annual Surfactant ReView, Volume 3: Surface ActiVe BehaVior of Performance Surfactants; CRC Press: Boca Raton, FL, 2000. (16) Shirahama, K.; Ide, N. J. Colloid Interface Sci. 1976, 54, 450. (17) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (18) Dai, S.; Tam, K. C.; Li, L. Macromolecules 2001, 34, 7049. (19) Couderc, S.; Li, Y.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 4818. (20) Panmai, S.; Prud’homme, R. K.; Peiffer D. G.; Jockusch, S.; Turro, N. J. Langmuir 2002, 18, 3860. (21) La Mesa, C. J. Colloid Interface Sci. 2005, 286, 148. (22) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. ReV. 1993, 22, 85.
Langmuir, Vol. 22, No. 8, 2006 3515
studied systems. However, detailed knowledge about the interaction of hydrophobically modified polymers with ionic surfactants is still lacking. Many fundamental questions such as the influence of hydrophobically modified polymers on surfactant aggregation and the role of these polymers involved in aggregate formation are still to a large extent unknown. Interestingly, though a number of reports have dealt with homologous series of cationic surfactants of varying alkyl chain length for the study of various polymer-surfactant systems,23-25 the effect of the chain length of the anionic surfactants is still obscure. Therefore, we were interested to explore the nature of interaction of homologous anionic surfactants with telechelic polymer PYPY. For this purpose, sodium alkyl sulfate (SnS) surfactants having decyl (C10), dodecyl (C12), and tetradecyl (C14) hydrocarbon tails have been used. Two critical concentrations are known to describe the interaction between a polymer and surfactant molecules in the polymer-surfactant systems.8,26 The first critical concentration, often denoted the critical aggregation concentration (cac), indicates the onset of the cooperative surfactant binding with the polymer backbone. The second critical concentration, usually denoted as C2, is the saturation concentration which implies that the polymer chains are saturated by surfactant molecules, no additional interaction between the surfactant and polymer chain occurs, and free micelles of the surfactant start to form.5 It is well established that the cac is nearly independent of the polymer concentration. However, the value of C2 is dependent upon the polymer concentration and increases with increasing polymer concentration.3,9 Both fluorometric and ITC techniques employed in the present work provide the means to determine cac and C2 of the three sodium alkyl sulfate systems while interacting with PYPY. The ITC thermograms, from the dilution experiments, also provide the cmc values for the surfactants. In addition, they provide significant information about the association mechanism for the hydrophobically modified PEO polymer-surfactant interaction for the series of anionic surfactants varying in hydrophobic chain length. Experimental Section PYPY was a kind gift from Professor Mats Almgren (University of Uppsala, Sweden). It is relatively monodisperse (Mw/Mn e 1.10) and has a molecular weight of 9500 determined from dynamic light scattering. The polymer contains about 200 ethylene oxide (EO) repeat units per PEO chain. Sodium decyl and tetradecyl sulfates (99%) were Lancaster (England) products. Sodium dodecyl sulfate was obtained from Aldrich, and all the surfactants were used as received. Triply distilled water was used throughout the experiments. The micellar solutions were prepared freshly to avoid aging. The concentration of PYPY was kept at ca. 2.5 × 10-6 M (0.003 wt %) for all fluorescence measurements, and for all the microcalorimetric experiments the concentration of PYPY was kept at 8.5 × 10-6 M (0.008 wt %). We tried to perform the ITC experiments using a 0.003 wt % solution, but the observed enthalpy changes were very small. To have a perceptible and reliable result, we had to move to a higher concentration of the polymer. Absorption and Steady-State Fluorescence Study. Absorption studies were performed using a microprocessor-controlled Shimadzu MPS 2000 spectrophotometer, and steady-state fluorescence measurements were carried out on a Spex Fluorolog-2 spectrofluorometer equipped with DM3000F software. During fluorescence measure(23) Schille´n, K.; Anghel, D.; Miguel, M. G.; Lindman, B. Langmuir 2000, 16, 10528. (24) Wang, G.; Olofsson, G. J. Phys. Chem. B 1995, 99, 5588. (25) Bai, G.; Wang, Y.; Yan, H.; Thomas, R. K.; Kwak, J. C. T. J. Phys. Chem. B 2002, 106, 2153. (26) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909.
3516 Langmuir, Vol. 22, No. 8, 2006 ment, the surfactant concentrations were varied by adding small aliquots of aqueous surfactant solutions through a microsyringe in a 2.5 mL polymer solution taken in a quartz cuvette of 1 cm path length. For the emission studies the systems were excited at 330 nm. The pyrene excimer emission intensity to monomer emission intensity ratio (IE/IM) was calculated by dividing the intensity at 480 nm by that at 376 nm, the excimer and monomer fluorescence emission maxima, respectively. The excitation spectra were recorded within 250 and 370 nm, monitoring the emission at 376 and 480 nm for the monomer and the excimer, respectively. All the spectra were measured with air-equilibrated solutions at ambient temperature (29 ( 1 °C). Isothermal Titration Calorimetric Study. The microcalorimetric measurements were performed with an OMEGA isothermal titration calorimeter (Microcal Inc., Northampton, MA). In an experiment, 1.325 mL of aqueous polymer solution of 0.008 wt % concentration was taken in the calorimeter cell. The reference cell contained the same volume of triply distilled water. Under computer-controlled conditions, a preset (programmed) number of 3-30 µL aliquots of temperature-equilibrated concentrated surfactant solution were injected into the cell at 4 min intervals. The heat flow in or out of the cell per injection and the corresponding enthalpy change per mole of the added surfactant were obtained using Microcal Origin software. Control runs using water instead of the polymer solutions to follow the corresponding dilution of the surfactants were taken following the same procedure as described above. During all the microcalorimetric experiments the temperature was kept constant at 30° C using a thermostated bath (NESLAB, RTE100).
Results and Discussion Steady-State Absorption and Fluorescence Study. The absorption spectrum (not shown) of PYPY (concentration ∼2.5 × 10-6 M or ∼0.003 wt %) is qualitatively similar to that of pyrene. One important difference is that the position of the band maximum is red shifted in the case of the polymer. The band maximum for the PYPY system in aqueous solution is observed at 343 nm, and the bathochromic shift of this band maximum from that of pyrene itself (334 nm) is significant. It is known that substitution on pyrene does make a red shift in the absorption spectra. Nevertheless, experiments have revealed that there is a significant red shift, although small, due to the ground-state association of pyrene in labeled polymers.27 In her classic review,27 Winnik has mentioned that preassociation of pyrenyl groups could be clearly recognized from the broadening of the pyrene absorption bands accompanied by a red shift in the band maxima and by a decrease of the extinction coefficients compared to those of model systems where pyrene is molecularly dissolved. PYPY reflects the entire requisite conditions of preassociation in its absorption spectra and therefore suggests that some of the pyrene units in PYPY remain as associated pairs prior to the photoexcitation. We believe that the red shift of lowest energy absorption from 334 nm (for pyrene) to 343 nm (for PYPY) comprises both these effects. An aqueous solution of PYPY exhibits a structured and characteristic emission spectrum with a pyrene monomer band along with a pronounced broad unstructured excimer band.28-30 At very low PYPY concentration (2.5 × 10-6 M), the excimer originates from the intramolecular interaction (since intermolecular excimer emission is observed at a much higher concentration of pyrene, in the range of 10-3 M). Hence, the excimer emission has a principal contribution from the intramolecularly (27) Winnik, F. M. Chem. ReV. 1993, 93, 587. (28) Haldar, B.; Mallick, A.; Purkayastha, P.; Burrows, H. D.; Chattopadhyay, N. Indian J. Chem., A 2004, 43, 2265. (29) Haldar, B.; Mallick, A.; Chattopadhyay, N. J. Mol. Liq. 2004, 115, 113. (30) Haldar, B.; Mallick, A.; Chattopadhyay, N. J. Photochem. Photobiol., B: 2005, 80, 217.
Haldar et al.
Figure 1. Normalized excitation spectra of PYPY in aqueous solution monitored at 376 nm (monomer, solid line) and at 480 nm (excimer, dotted line).
preassociated pyrene species. For an analogous pyrene-endcapped PEO polymer, Char et al.31 and later on Duhamel et al.32 proposed that a certain fraction of the polymer remains intramolecularly preassociated in the ground state. Furthermore, Duhamel et al.32 have shown with a pyrene-labeled single-endcapped and double-end-capped PEO polymer similar to PYPY that only for the double-end-capped homologue was the pyrene excimer emission observed at a polymer concentration as low as ∼10-6 M. This suggests that the excimer originates from intramolecular association. In addition to the absorption spectrum, the excitation spectra monitoring the monomer and excimer species (at 376 and 480 nm, respectively) give evidence of intramolecular preassociation of pyrene moieties for the present polymer system. Although the excitation spectra of the monomer and the excimer are similar in nature, they are not superimposable. First, the excitation spectrum for the excimer emission is 1-2 nm red shifted, compared to the spectrum of the monomer. Second, the bands in the spectrum monitored for the excimer are broadened. Third, the peak-to-valley ratio for the 0-0 transition is different for the monomer and the excimer excitation spectra. These observations are consistent with the report of Winnik et al. for other telechelic polymeric systems, viz., pyrene-labeled (hydroxypropyl)cellulose of different chain lengths, where they demonstrated a similar red shift of 1-4 nm for the formation of the ground-state preassociation.27,33 The normalized excitation spectra corresponding to the monomer and the excimer emissions of PYPY are shown in Figure 1. The variations of the emission spectra of PYPY in three aqueous surfactant media are similar in nature. The variations in the emission spectra on addition of sodium dodecyl sulfate (S12S) to aqueous PYPY solution are shown, as a representative plot, in Figure 2. A clearer picture concerning the variation in emission intensity emerges from the plot of relative emission intensities of the two emission bands (IE/IM) as a function of surfactant concentration. The plot for the three different polymer-surfactant systems is depicted in Figure 3. For all three PYPY-SnS pair systems, the initial additions of surfactant aliquots into the polymer aqueous solution could not produce a perceptible change in the IE/IM ratios and the ratios remain nearly unaltered until a certain critical concentration is reached. Above the critical concentration value for each individual surfactant system, IE/IM starts to increase, and on further increase in the surfactant concentration the IE/IM value reaches a second plateau, passing through a maximum. The first critical concentration value corresponds to cac, where (31) Char, K.; Frank, C. W.; Gast, A. P. Macromolecules 1989, 22, 3177. (32) Duhamel, J.; Yekta, A.; Hu, Y. Z.; Winnik, M. A. Macromolecules 1992, 25, 7024. (33) Yamazaki, I.; Winnik, F. M.; Winnik, M. A.; Tazuke, S. J. Phys. Chem. 1987, 91, 4213.
Interaction of a Telechelic Polymer with SnS
Langmuir, Vol. 22, No. 8, 2006 3517 Table 1. Parameters Obtained from Fluorescence and Fluorescence Quenching Experiments cac (M) C2 (M) surfactant (err ) 5%) (err ) 5%) cmclit. (M) KSV (M-1) S10S S12S S14S
Figure 2. Fluorescence spectra of aqueous PYPY solution in varying S12S concentrations. (a) and (b) basically represent the same set of spectra. (b) is not to the surfactant concentration scale and has been presented for clarity of the variation in the lower concentration range. In (a) the concentrations of S12S are given in the inset, and in (b) the concentrations (×10-4 M) of S12S are 0, 1.2, 2.4, 4.4, 5.6, 6.8, 8.0, 9.2, 11.2, 13.2, 16.0, 20.0, 32.0, 44.0, 68.0, and 92.0, respectively, for n ) 1-16. The concentration of PYPY is ∼2.5 × 10-6 M or ∼0.003 wt %. The excitation wavelength is 330 nm.
Figure 3. Variation in IE/IM with increasing surfactant concentration for the three surfactants. The lines are not fitted to any mathematical equation; they are just to guide the eye.
the hydrophobically modified polymer-surfactant interaction starts cooperatively and formation of small surfactant aggregates takes place. The cac values have been determined from the linear extrapolation of the first plateau and the initial sharp increase of the IE/IM ratios. The initial increase in the IE/IM ratio suggests that the primary surfactant aggregates induce intramolecular association of the pyrene chain ends for the polymer molecules. This early association occurs largely through hydrophobic interaction between the alkyl tails of the surfactants and the
NA
3.7 × 10 3.9 × 10 3.3 × 10 1.47 × 10 62 ((3) 3.0 × 10-4 4.5 × 10-3 8.0 × 10-3 9.24 × 103 68 ((3) 2.0 × 10-4 2.5 × 10-3 2.1 × 10-3 3.36 × 104 80 ((3) -3
-2
-2
3
hydrophobic end groups of the HMP, leading to an enhancement in the relative yield of the excimer fluorescence. A close inspection of Figure 3 reveals that the maximum value of IE/IM gradually increases with increasing length of the alkyl chain, pointing toward greater hydrophobic interaction. As expected, the maximum IE/IM values follow the order S14S > S12S > S10S. With increasing surfactant concentration, larger surfactant aggregates are likely to be formed as established for the PEO homopolymer-S12S system.34,35 Since the PEO chain in PYPY molecules (∼200 EO units) is reasonably large, it is anticipated that more than one surfactant aggregate will form on each chain and will produce a bead-necklace-like structure. The negatively charged surfactant aggregates are expected to be embedded along a coiled PEO string. The neighboring aggregates will exert electrostatic repulsion on each other, and thus, the polymer will experience, to some extent, the polyelectrolyte effect. The electrostatic repulsion will result in stretching of the polymer backbone and ultimately destroy the intrapolymer association. The fluorescence results and thereby the IE/IM ratios for all three anionic surfactants studied bear the signature of this effect. Figure 3 also provides the saturation concentrations (C2) for the three surfactants. The cac and C2 values estimated from the fluorescence measurements have been tabulated in Table 1 along with some other results extracted from the fluorescence quenching experiment to be discussed later. Isothermal Titration Calorimetric Study. ITC experiments were carried out through measurements of differential enthalpy during addition of concentrated surfactant solutions to water and very dilute PYPY solution at 30 °C. During dilution of concentrated solution containing a surfactant in micellar form, three different situations arise. In the experiments where the final surfactant concentration is below the cmc, the micelles break up to give monomers in solution. The observed enthalpy change ∆Hobsd includes the contribution from dilution of the micelle in the concentrated titrant solution, demicellization, and interaction between monomers. In experiments with the final concentration in the micellization region, part of the injected micelles demicellize. When the final concentration is above the cmc, the added micelles are only diluted and ∆Hobsd is the enthalpy of dilution of the concentrated micellar solution. The cmc can be determined as the concentration at the crossing point of the extrapolated premicellar region and the linear rise/decrease in the demicellization region. The difference between the titration curves for the surfactant/polymer and surfactant/water systems should be attributed to polymer-surfactant interaction.9 Since the PEO-S12S surfactant system is the most elaborately studied system, for convenience of comparison, we first discuss the PYPY-S12S system in more detail and then the other two polymer-surfactant systems in the upcoming subsections. Interaction between S12S and PYPY. The results of microcalorimetric titration of 0.008 wt % PYPY with 0.05 M S12S are shown in Figure 4. The figure shows the difference curve of titration of S12S into PYPY solution and water. The inset of (34) van Stam, J.; Almgren, M.; Lindblad, C. Prog. Colloid Polym. Sci. 1991, 84, 13. (35) Zana, R.; Long, J.; Lianos, P. In Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum: New York, 1985; p 357.
3518 Langmuir, Vol. 22, No. 8, 2006
Figure 4. Difference curve of titration of S12S into PYPY solution and water. The solid line is not fitted to any mathematical equation. It is just to guide the eye. The inset shows the observed enthalpy changes ∆Hobsd for S12S in the absence (open circles) and in the presence (filled circles) of PYPY.
Figure 4 comprises the observed enthalpy changes ∆Hobsd for each injection of surfactant solution plotted against the total S12S concentration in the absence and in the presence of the polymer. For S12S, the ITC experiment did not allow us to determine its cmc (inset of Figure 4); the reason for this is not very obvious. Although the purity of the S12S sample has been checked from other experiments and was found to be quite good, a small amount of surface-active impurity might be present in the sample which might be responsible for the uncanny breaks observed at the initial stage in the surfactant dilution curve of the ITC experiment. The PYPY system shows an endothermic peak and two exothermic peaks. It is pertinent to mention here that Persson et al. reported similar peaks for an analogous C12EO200C12 telechelic polymer system.3 A qualitative description of the calorimetric curve of our PYPY system could be as follows. The micelles added in the first couple of injections break up to monomers, and the slightly negative dilution enthalpy indicates a very weak exothermic interaction. This weak exothermic peak may be due to feeble noncooperative binding of the surfactant monomer and the polymer hydrophobes. When the cac region is reached, cooperative interaction starts to act between small-sized surfactant aggregates and polymer segments. Beyond the cac, PYPYS12S mixed micellar aggregates appear in solution, where the hydrophobic substituents and some of the EO segments in the PEO backbone of the PYPY molecule are removed from the water phase, and solubilize into the hydrophobic core of the small micelles. Like the unmodified PEO- S12S interaction, the enthalpy change for the transfer of the EO groups and the hydrophobic substituents from water to the dehydrated core would be positive and thus responsible for the emergence of an endothermic peak. At this initial stage of solubilization of hydrophobic units and EO segments, hydrophobic interaction should predominate, the extent of interaction must be higher for an HMP compared to an unmodified one, and the cac value is expected to be lower for an HMP than a homopolymer. The concentration corresponding to the start of the endothermic peak is ascribed to the cac. As the total concentration of S12S increases, the aggregation number of S12S in the mixed micelle is likely to increase, and the PYPY-S12S aggregation complex reorganizes itself. The exothermic contribution to ∆Hobsd could arise from the rehydration of EO segments that are expelled from the core during this reorganization process of the mixed micelle. Hence, the ∆Hobsd passes through a maximum and then drops to give an exothermic peak. During the approach toward the saturation concentration, C2, the EO groups will be found in the
Haldar et al.
Figure 5. Difference curve of titration of S14S into PYPY solution and water. The solid line is not fitted to any mathematical equation. It is just to guide the eye. The inset shows the observed enthalpy changes ∆Hobsd for S14S in the absence (open circles) and in the presence (filled circles) of PYPY. Table 2. Parameters Obtained from Isothermal Titration Calorimetric Experiments surfactant
cac (M) (err ) 5%)
C2 (M) (err ) 5%)
cmcexptl (M) (err ) 5%)
S10S S12S S14S
3.4 × 10-3 2.9 × 10-4 1.7 × 10-4
4.7 × 10-2 6.9 × 10-3 4.1 × 10-3
3.3 × 10-2 2.0 × 10-3
cmclit. (M) 3.3 × 10-2 8.0 × 10-3 2.1 × 10-3
outer part of the headgroup region, where to a large extent they will remain hydrated. After C2 the difference curve becomes flat and indicates no further interaction among the micellar aggregates and the polymer segments, and beyond that concentration, free micelles start to form. The estimated cac values for the PYPY system from the ITC experiments are consistent with the fluorometric results. The C2 value from microcalorimetry is, however, higher than that obtained from fluorometry, and the deviation in C2 is ascribed to the higher polymer concentration used in the ITC experiment. Consistent with expectation, the cac value is well below the cac value for the unmodified PEO polymer. Persson et al.3 with their aforementioned polymer and S12S system also found a cac value which was much lower than the cac value obtained in the case of unmodified PEO. The same authors in another study with S12S and two ethyl(hydroxyethyl)cellulose polymers of different hydrophobicities showed that the cac was significantly lowered for the more hydrophobic homologue.24 It is important to point out here that the cac value obtained for the PYPY-S12S system by us is even lower than that obtained by Persson et al. for their system. Hence, it may plausibly be assumed that the difference is due to the greater hyrophobicity of the pyrene-end-capped PEO polymer where the hydrophobic groups have a higher number of carbon atoms (C18) compared to the polymer (having an alkyl chain of 12 carbon atoms) that was used by Persson et al. A detailed study with HM polymers having varying alkyl chain length and S12S could give a conclusive explanation, but this was beyond the scope of the present work. Interaction between S14S and PYPY. ITC thermograms for sodium tetradecyl sulfate (S14S) and PYPY are depicted in Figure 5. The inset of Figure 5 contains the ∆Hobsd values for the dilution of 0.025 M S14S solution in the absence and in the presence of the polymer. From the dilution curve in distilled water the cmc of S14S has been estimated, and the value is consistent with the literature value (Table 2). The difference curve in Figure 5 also shows endothermic and exothermic peaks as has been observed for S12S. The interesting feature to observe is the exothermic
Interaction of a Telechelic Polymer with SnS
Figure 6. Difference curve of titration of S10S into PYPY solution and water. The solid line is not fitted to any mathematical equation. It is just to guide the eye. The inset shows the observed enthalpy changes ∆Hobsd for S10S in the absence (open circles) and in the presence (filled circles) of PYPY.
peak, which is the most pronounced among those of the three surfactant systems studied. The exothermic peak intensity increased gradually with an increase in the alkyl chain length. Since it is established that the emergence of the exothermic peak is due to rehydration of the EO segments of the PEO chain, the results indicate that with increasing alkyl chain length the rehydration process of EO segments becomes somehow favored. The reason may be larger aggregates and thereby less compactness of the surfactant headgroups having a longer alkyl tail length. Interaction between S10S and PYPY. The dilution behavior of sodium decyl sulfate (S10S) micelles in water can be examined by titrating S10S solution (concentration > cmc) into distilled water. The inset of Figure 6 (open circles) represents the ITC curve for titrating 0.5 M S10S into water. The transition point of 3.3 × 10-2 M corresponds to the cmc of S10S, which agrees well with the literature value.36 The microcalorimetric titration curve of 0.5 M S10S into 0.008 wt % PYPY (inset of Figure 6, filled circles) is nearly identical to that of S10S into water. The small difference in the two curves can be attributed to the frail interaction between S10S and PYPY at such a low polymer concentration. The weak interaction between the two components is perceptible from the difference curve of titration of S10S into PYPY solution and water (Figure 6). Though the ∆Hobsd values are low enough, the variation pattern is significant. For PYPY-S12S and PYPY-S14S the thermograms show both endothermic and exothermic peaks. For PYPYS10S, on the contrary, the entire thermogram gives positive ∆H values. The thermogram, however, shows that the interaction process is exothermic relatiVe to the noninteraction stage. This exothermicity is appreciably small and is insufficient to overcome the solvation endothermicity. The mechanisms behind the interaction are likely to be similar since both the polymer and surfactant headgroups are the same and the alkyl tail is slightly smaller in length in the case of S10S compared to S12S. The shorter hydrophobic chain of the S10S surfactant is therefore responsible for the weaker hydrophobic interaction reflected in the initial part of the plot. The reason behind the feeble second exothermic peak seems to be the residence of the hydrophobic groups along with some EO segments in the relative interior of the S10S micelles; the headgroups of which would be more compact compared to those of the longer tailed surfactant S12S. Steady-State Fluorescence Quenching Experiments. The steady-state fluorescence quenching experiment using Cu2+ ion (36) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS, Vol. 36; NIST: Gaithersburg, MD, 1971.
Langmuir, Vol. 22, No. 8, 2006 3519
Figure 7. Stern-Volmer plots in polymer-surfactant systems with S10S, S12S, and S14S.
as the quencher is an established technique to assess the accessibility of a fluorophore toward the quencher molecule in a micellar environment.37-39 A relative measure of the quenching efficiency (KSV) reflects the accessibility of one to the other. Thus, the experiment can throw light on the approachability of the quencher to the fluorophore, i.e., the compactness of the micellar headgroups. The surfactant concentrations were maintained above C2 for all three surfactants studied, and they were 7.4 × 10-2, 1.2 × 10-2, and 4.7 × 10-3 M for S10S, S12S, and S14S, respectively. At this high surfactant concentration the excimer emission is only feeble. Hence, quenching of the pyrene monomer emission of the PYPY-SnS systems by the heavy metal ion (Cu2+) was monitored. The Stern-Volmer plots according to eq 1 in the three micellar environments are
F0/F ) 1 + KSV[Q]
(1)
presented in Figure 7. F0 and F signify the fluorescence intensity of the pyrene monomer in the absence and in the presence of different concentrations of the quencher. KSV and [Q] are the Stern-Volmer constant and quencher concentration, respectively. The KSV values obtained from the plots in the three micellar environments increase remarkably with increasing alkyl chain length of the surfactants. The results indicate that the accessibility of the fluorophore to the ionic quencher is in the order S14S > S12S > S10S. In other words, the extent of water penetration and thereby less compactness of the headgroups are in the order S14S > S12S > S10S. The results from the quenching experiments thus corroborate the results of the ITC experiments. The results might mislead one to think that the longer the surfactant tail the more water is accessible to the hydrophobic core. The fallacy may be resolved by considering the fact that PYPY does not penetrate into the core of the micelles; rather it is located at the micelle-water interface.29 Hence, with an increase in the surfactant chain length, the distance between the headgroups of the micellar unit increases, allowing a greater penetration of water into the interface region. Fluorescence quenching experiments also provide the aggregation number (NA) of the three surfactant micelles at the concentration where free micelles are present along with polymer(37) Mallick, A.; Haldar, B.; Maiti, S.; Chattopadhyay, N. J. Colloid Interface Sci. 2004, 278, 215. (38) Mallick, A.; Mandal, M. C.; Haldar, B.; Chakrabarty, A.; Das, P.; Chattopadhyay, N. J. Am. Chem. Soc. 2006, 128, 3126. (39) Mallick, A.; Haldar, B.; Maiti, S.; Bera, S. C.; Chattopadhyay, N. J. Phys. Chem. B 2005, 109, 14675.
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Haldar et al.
bound micelles. To determine the aggregation number, ln(F0/F) has been plotted against the Cu2+ ion concentration according to eq 2,40 where [Stotal] is the total surfactant concentration at
ln(F0/F) ) [Q]NA/([Stotal] - cac)
(2)
which the quenching experiment is performed. Other terms have already been defined. From the slope of the plot we get the NA values for the three surfactants. Stern-Volmer constants (KSV) and NA values are tabulated in Table 1.
Acknowledgment. Financial support from CSIR and DST, Government of India, is gratefully acknowledged. B.H. and P.D. thank CSIR for providing fellowships. M.C.M. thanks the UGC for awarding a research fellowship. We are indebted to Prof. S. P. Moulik and Mr. I. Chakrabarty of Jadavpur University for their invaluable cooperation and suggestions in the ITC measurements. LA053370F (40) Winnik, F. M.; Regismond, S. T. A. Colloids Surf., A 1996, 118, 1.
