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Langmuir 2006, 22, 1526-1533
Binding of Sodium Dodecyl Sulfate with Linear and Branched Polyethyleneimines in Aqueous Solution at Different pH Values Hao Wang, Yilin Wang,* and Haike Yan Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Science, Beijing 100080, People’s Republic of China
Jin Zhang and Robert K. Thomas Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OXI 3QZ, United Kingdom ReceiVed July 22, 2005. In Final Form: December 7, 2005 Isothermal titration microcalorimetry (ITC), conductivity, and turbidity measurements have been carried out to study the interaction of sodium dodecyl sulfate (SDS) with polyethyleneimines (PEI) including linear PEI and branched PEI at different pH values of 3, 7, and 10. In all cases, the polymers show a remarkable affinity toward SDS. At pH 3, the polymer PEI is a strong polycation, and the binding is dominated by electrostatic 1:1 charge neutralization with the anionic surfactant. At pH 7, the electrostatic attraction between SDS and PEI is weak, and the hydrophobic interaction becomes stronger. At the natural pH of 10, PEI is essentially nonionic and binds SDS in the form of polymer-bound surfactant aggregates. The charge neutralization concentration (C1) of SDS for the PEI-SDS complex can be derived from the curves of variation of the enthalpy, conductivity, and turbidity with SDS concentration. There is good agreement between the results from the three methods and all show a decrease with increasing pH. The total interaction enthalpies (∆Htotal) of PEI with SDS are obtained from the observed enthalpy curves and the difference enthalpy (∆H*) between the total enthalpy of branched PEI with SDS, and the total enthalpy of linear PEI with SDS can be derived from the obtained ∆Htotal. The difference ∆H* increases dramatically as pH increases, which indicates that the interactions are different for linear PEI and branched PEI at high pH values. A schematic map of the different states of aggregation is presented.
Introduction Research in the area of polymer-surfactant interactions has accelerated rapidly over the last few decades, driven by both applications and fundamental interest in intermolecular interactions and hydrophobic aggregation phenomena.1-3 Strong electrostatic attraction and hydrophobic interaction between oppositely charged polyelectrolytes and surfactants occur at surfactant concentrations several orders of magnitude lower than the critical micelle concentration of the surfactant. The association of a polyelectrolyte with an oppositely charged surfactant is generally accepted to be an ion-exchange process, in which the electrostatic attraction is reinforced by cooperative aggregation of the bound surfactant molecules. Once all of the charged groups on the polymer backbone are neutralized, hydrophobic interaction begins to control the binding, which induces the restructuring of polymer chains to produce necklace-like aggregates.4-8 Among the studies on the systems of polyelectrolytes and oppositely charged surfactants, linear polymers have been widely investigated, but much less attention has been paid to branched * To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn. (1) Evans, D. F.; Wennerstro¨m, H. The colloidal Domain where Physics, Chemistry, Biology, and Technology Meet; VCH Publisher: New York, 1994. (2) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons Ltd: West Sussex, U.K., 1998. (3) Iliopoulos, I. Curr. Opin. Colloid Interface Sci. 1998, 3, 493-498. (4) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866-3870. (5) Konop, A. J.; Colby, R. H. Langmuir 1999, 15, 58-65. (6) Matulis, D.; Rouzina, L.; Bloomfield, V. A. J. Mol. Biol. 2000, 296, 10531063. (7) Fundin, J.; Hansson, P.; Brown, W.; Lidegran, I. Macromolecules 1997, 30, 1118-1126. (8) Wang, C.; Tam, K. C. J. Phys. Chem. B 2004, 108, 8976-8982.
and hyperbranched polymers. As a branched polymer, branched polyethyleneimine (PEI) has aroused special interest because of its intensive usage as an ingredient in pharmaceutical formulations, personal care products, food products, and household and industrial detergents. PEI exists in either linear or branched form. At low pH, the amine groups in PEI are protonated making it a highly positively charged polyelectrolyte, but at high pH, it is essentially a neutral polymer. There have been many studies on the solution behavior of PEI/SDS systems. Winnik et al.9,10 investigated the complex formation between branched PEI and SDS and interpreted an unusual increase in the conductivity of SDS in the presence of hyperbranched PEI in terms of cooperative ion transport processes across the polyamine/surfactant complex. Their results identified both monomeric and micellar binding of SDS to branched PEI and provided evidence for both hydrophobic and electrostatic interaction. Yui et al.11,12 showed that the electrostatic interaction between SDS and linear PEI leads to an increase in the hydrophobicity of the linear PEI. Li et al.13 have found that SDS has a remarkably high affinity for PEI. The appearance of precipitation was explained by 1:1 charge neutralization, whereas the resolubilization at higher surfactant concentration was interpreted in terms of repulsive micellar interaction. Me´sza´ros et al.14 have studied the hyperbranched (9) Bystryak, S. M.; Winnik, M. A.; Siddiqui, J. Langmuir 1999, 15, 37483751. (10) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C.; Strashko, V.; Macdonald, P. M.; Siddiqui, J. Langmuir 2000, 16, 4495-4510. (11) Yui, T. S. T. I.; Pal’mer, V. G.; Musabekov, K. B. IzV. Akad. Nauk, Kaz. SSR Ser. Khim. 1984, 19. (12) Yui, T. S. T. I.; Abilov, Z. A.; Pal’mer, V. G.; Musabekov, K. B. Issled. RaVnoVesnych Sist. 1982, 78. (13) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 3093-3100.
10.1021/la051988j CCC: $33.50 © 2006 American Chemical Society Published on Web 01/13/2006
Binding of SDS with PEI
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Figure 1. Structures of linear and branched PEIs.
PEI-SDS interaction and found that the interaction can be divided into different characteristic SDS concentration ranges. At low SDS concentration, dodecyl sulfate ions bind in monomer form to the protonated amine groups of PEI. Beyond a critical amount of the bound surfactant molecules, the PEI/SDS complexes would collapse and precipitate. At higher SDS concentration, SDS adsorbs on the surface layer of the collapsed particles. These results suggest that the observed mechanism of the branched PEI-SDS interaction is very different from the general characteristics of the interaction of linear polyelectrolytes with oppositely charged surfactants. However, there is still a lack of direct comparison of the behavior of linear and branched PEI with surfactants. Recently, Penfold et al.15 have characterized the adsorption of linear or branched PEI with SDS at the airsolution interface. It was found that the SDS adsorption at the interface is strongest when the PEI is branched and, unexpectedly, when the pH is high. For the branched PEI, a transition from monolayer to multilayer adsorption is observed, whereas for the linear PEI, only monolayer adsorption is observed. The present work extends that work to bulk aqueous solution. We have investigated the interaction of the same linear and branched PEI as used by Penfold et al. with SDS in aqueous solution in parallel microcalorimetry, turbidity, and conductivity measurements. Isothermal titration microcalorimetry is expected to give detailed thermodynamic information about the nature of the polymersurfactant interaction in aqueous solution.16,17 Turbidity is used to examine the formation of insoluble complexes. Combined with conductivity measurements, the results aim to give a more comprehensive understanding on the role of the PEI architecture on the mechanism of PEI-SDS interactions in aqueous solution and their relation to the surface behavior. Experimental Section Materials. Branched polyethyleneimine (BPEI) with mean molecular weight Mw ) 25 000 was purchased from Aldrich and linear polyethyleneimine (LPEI) with Mw ) 25 000 was obtained from PolySciences. Their generic structures are shown in Figure 1. Anionic surfactant sodium dodecyl sulfate (SDS), with > 99% purity, was obtained from Aldrich. All of the measurements were conducted at 298.15 K. Since we could not observe significant changes in ITC measurements at 20 ppm PEI solution (the concentration used in ref 15), a PEI concentration of 0.1% was used. The structure and molecular weight are different for the LPEI monomer and the BPEI monomer, so there are different mole concentrations at 0.1 wt %, i.e., 23.26 mM for LPEI and 7.75 mM for BPEI. The SDS concentration range was 0-25 mM. The solution pH was adjusted at 3, 7, and 10 by addition of HCl or NaOH solution. Measurements of the pH before and after the isothermal titration indicated only small shifts in pH of not more than 0.3 pH units. Because pure water is used, the ionic strength is unknown and largely determined by SDS and PEI concentrations. (14) Me´sza´ros, R.; Thompson, L.; Bos, M.; Varga, I.; Gila´nyi, T. Langmuir 2003, 19, 609-615. (15) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Langmuir 2005, ASAP article. (16) Wadso¨, I. Chem. Soc. ReV. 1997, 26, 79-86. (17) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276-9283.
Figure 2. Variations of the observed enthalpies (∆Hobs) of dilution of SDS into water with the final SDS concentration at pH ) 3, 7, and 10 and at 298.15 K. Isothermal Titration Microcalorimetry (ITC). The calorimeter used was a TAM 2277-201 microcalorimeter (Thermometric AB, Ja¨rfa¨lla, Sweden) with a 1 mL stainless steel sample cell. The cell was initially loaded with 0.5 mL of water or PEI solution, and the concentrated SDS solution of 50 mM was injected into the stirred sample cell in portions of 10 µL using a 500-µL Hamilton syringe controlled by a Thermometric 612 Lund Pump. The system was stirred at 60 rpm with a gold propeller. The interval between two injections was 9 min, which was sufficiently long for the signal to return to the baseline. The accuracy of the calorimeter was periodically calibrated electrically and verified by measuring the dilution enthalpies of concentrated sucrose solution.18 All experiments were repeated twice, and the reproducibility was within (2%. Turbidimetric Titration. The turbidity of the PEI/SDS solutions, reported as 100 - %T, was measured at 450 nm using a Brinkman PC920 probe colorimeter equipped with a thermostated watercirculating bath. The concentrations of SDS and PEI solution were the same as those used for ITC. The final turbidity titration curves were only recorded after the values became stable (about 2-4 min) and were corrected by subtracting the turbidity curve from a polymerfree titration. Conductivity Measurements. Conductivities for pure SDS and a mixed solution of PEI with SDS at different pH values were measured with a model 4320 Conductivity Meter (Jenway, England). The frequency for readings above 325uS is 800 Hz, and the frequency below 275uS is 40 Hz. Between 275 and 325, it can be either depending a little on the cell constant and also on the calibration. All measurements were performed in a double-walled glass container with the temperature being controlled by circulation of water. The conductivity meter was calibrated with a standard solution of known conductivity. The conductivity measurements for the PEI/SDS systems were carried out by titrating a concentrated SDS solution into a 0.1% PEI aqueous solution. Reference conductivity curves for SDS at different pH values were also measured. For all experiments, the conductivity was recorded when its fluctuation was less than 1% in 3 min.
Results and Discussion Micellization of SDS in Aqueous Solution at Different pH Values. Microcalorimetric curves for the dilution of SDS into water at pH 3, 7, and 10 are shown in Figure 2. As shown, the SDS dilution curves are all approximately sigmoidal in shape and each curve can be subdivided into two concentration regions separated by a transition region associated with micelle formation, corresponding to the critical micelle concentration (CMC). When the final SDS concentration is below the CMC, the added micelles (18) Gucker, F. T.; Jr.; Pickard, H. B.; Planck, R. W. J. Am. Chem. Soc. 1939, 61, 459-470.
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Figure 3. Determination of CMC and ∆Hmic from the observed enthalpy curve of SDS into pure water at pH 7 and 298.15 K. Table 1. Values of CMC, r, and Thermodynamic Parameters for SDS at Different pH Values and 298.15 K by ITC and Electrical Conductivity Measurements pH
CMCa (mM)
R
∆Hmic (kJ/mol)
∆Gmic (kJ/mol)
T∆Smic (kJ/mol)
3 7 10
8.85 (7.33) 8.86 (7.48) 8.58 (8.49)
0.43 0.42 0.30
-0.77 -0.97 -0.90
-18.40 -18.63 -20.05
17.63 17.66 19.15
a The CMC values are from calorimetric measurements with values from conductivity measurements in parentheses.
dissociate into monomers and the monomers are further diluted. When the final SDS concentration is above the CMC, the added micelles are only diluted without dissociation. As illustrated in Figure 3, these observed enthalpy curves are differentiated with respect to the final SDS concentration, and the position of the extremum is taken as the CMC.19 The values of the enthalpy of micellization (∆Hmic) are determined from the enthalpy difference between the two linear segments of the differential enthalpy curves extrapolated to the CMC.20 The values of the derived CMC and ∆Hmic per mole of surfactant at 298.15 K are presented in Table 1. The Gibbs free energy of micellization (∆Gmic) can be calculated using the following expression:21
∆Gmic ) RT(1 + β) ln CMC
(1)
where β is the degree of counterion association to micelle and can be obtained from the degree of ionization of the micelles, R (β ) 1 - R), obtained from the conductivity measurements. The entropy of micellization (∆Smic) can then be derived from ∆Gmic ) ∆Hmic - T∆Smic. All the thermodynamic parameters obtained are listed in Table 1. The CMC and ∆Hmic values at different pH values are in good agreement with literature values.22 Figure 4 shows the electrical conductivity measurements of SDS at three pH values of 3, 7, and 10. Each plot of the electrical conductivity (k) against surfactant concentration furnishes two straight lines that intersect at the concentration corresponding to micelle formation, allowing identification of the CMC. The degree of ionization of the micelles, R, is taken to be the ratio of the values of dk/dC above and below the CMC.23,24 The CMC and R obtained by electrical conductivity measurements are listed in Table 1 and there is excellent agreement with CMC values (19) Kira´ly, Z.; Deka´ny, I. J. Colloid Interface Sci. 2001, 242, 214-219. (20) Johnson, I.; Olofsson, G.; Jo¨nsson, B. J. Chem. Soc., Faraday Trans. 1 1987, 83 (11), 3331-3344. (21) Zana, R. Langmuir 1996, 12, 1208-1211. (22) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588-5596.
Figure 4. Specific conductivity vs the final concentration of SDS at pH values of 3, 7, and 10 and at 298.15 K.
obtained from microcalorimetry. Although the pH values have no obvious influence on the CMC, R of SDS at pH 10 is much lower than those at other pH values. Effect of pH on SDS-PEI Interaction. The branched PEI is known to contain three different types of amino groups: secondary and tertiary amino groups in the main chain and secondary and primary amino groups in the side chain. The ratio of primary-to-secondary-to-tertiary amino groups is 1:2:1.10,25,26 However, the LPEI only contains secondary amino groups in the main chain. The degree of protonation R* of the PEI amino groups can be easily adjusted by varying the pH of the solutions, and can be calculated from the electroneutrality condition27
R* ) [(CH)added - (CH)free + (COH)free]/[EI]
(2)
Here, (CH)added is the concentration of added acid (HCl) and (CH)free and (COH)free are the concentrations of free protons and hydroxyl ions, which can be determined from pH measurements by equating concentrations and activities and using the relationship (CH)free(COH)free ) 1.0 × 10-14; [EI] is the polymer concentration expressed as monomeric equivalents of PEI. For BPEI, the values of R* were reported in the literatures27-30 as 0.71, 0.22, and 0.01 at pHs 3, 7, and 10, respectively. As for LPEI, the degrees of protonation at the equivalent point best fit both BPEI and LPEI under high and low pH conditions.31,32 A. Linear and Branched PEI at pH 3. The ITC and turbidity titration curves for SDS binding to 0.1 wt % LPEI and BPEI at pH 3 are shown in Figure 5, respectively. These curves have the following features. (i) Initially the process is highly exothermic in the ITC experiment (Figure 5, parts a and c) and the negative ∆Hobs remains approximately constant up to the charge neutralization concentration (C1). Correspondingly, the turbidity increases steeply with increasing SDS concentration and reaches a maximum value at 5.2 mM SDS, and then these semitransparent (23) Zana, R. J. Colloid Interface Sci. 1980, 78, 330-337. (24) Evans, H. C. J. Chem. Soc. 1956, 579-586. (25) Rivas, B. L.; Gecheler, K. E. AdV. Polym. Sci. 1992, 102, 171-188. (26) Dick, R. C.; Ham, G. E. J. Macromol. Sci. Part A 1970, 4 (6), 13011314. (27) Winnik, M. A.; Bystryak, S. M.; Liu, Z.; Siddiqui, J. Macromolecules 1998, 31, 6855-6864. (28) Lindquist, G. M.; Straton, R. A. J. Colloid Interface Sci. 1976, 55, 45-59. (29) Kokufuta, E. Macromolecules 1979, 12, 350-351. (30) Van Treslong, C. J. B.; Staverman, A. J. Recl. TraV. Chim. Pays-Bas 1974, 93, 171-178. (31) Kokufuta, E.; Suzuki, H.; Yoshida, R.; Yamada, K.; Hirata, M.; Kaneko, F. Langmuir 1998, 14, 788-795. (32) Kobayashi, S.; Hiroishi, K.; Tokunoh, M.; Saegusa, T. Macromolecules 1987, 20, 1496-1500.
Binding of SDS with PEI
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Figure 5. (a) ITC curve for titrating 50 mM SDS into pure water and LPEI, (b) turbidity curve for titrating 50 mM SDS into BPEI solution, (c) ITC curve for titrating 50 mM SDS into pure water and BPEI solution, and (d) turbidity curve for titrating 50 mM SDS into BPEI solution. Polymer concentration is 0.1 wt %, and all the solutions are at pH 3 and 298.15 K. Table 2. Values of C1, C2, and CM and Thermodynamic Parameters for Surfactant in the Presence of PEI microcalorimetry C1 (mM)
conductivity
C2 (mM)
CM (mM)
C1 (mM)
C2 (mM)
CM (mM)
turbidity C1 (mM)
∆Htotal (kJ/mol)
pH
La
B
L
B
L
B
L
B
L
B
L
B
L
B
L
B
∆H*
3 7 10
8.47 4.58 2.97
12.34 7.63 2.60
10.27 10.57 10.11
13.67 10.35 7.56
14.37 16.54 20.10
16.54 15.18 13.58
7.49 4.88 2.21
11.47 7.68 1.78
9.22 10.20 11.09
13.19 10.76 8.26
14.28
16.45 15.78 13.16
8.44 4.57 1.98
12.23 5.76 2.42
10.87 4.30 5.38
10.82 6.33 10.17
-0.05 2.03 4.79
a
L and B indicate LPEI and BPEI, respectively.
solutions with high turbidity are stable with time until the C1 is reached (Figure 5, panels b and d). (ii) At higher SDS concentrations, there is a sudden decrease in the large exothermicity to a very small value, the turbidity reaches its maximum value at the C1, and precipitation occurs. Before C1, the solution is already very cloudy, but no precipitate is observed. (iii) After the C1, ∆Hobs becomes progressively less exothermic until a concentration C2 is reached, where ∆Hobs tends to a limit. Redissolution of larger size precipitated particles is not observed at C2, although there is a decrease in turbidity at this point. (iv) Above the point CM (marked in the figures), the ∆Hobs curves are similar to the dilution curves of concentrated SDS in water, indicating that free micelles begin to form above CM. The same shape of ITC curve at low pH has already been observed for the systems of SDS/PDMAEMA33 and SDS/D40OCT30.34 C2 signals the saturation of the polymer with bound SDS micelles. CM is often taken to be the same concentration as C2 since they are not always easy to distinguish. For the SDS/PEI system, free SDS micelles are formed after the saturation condition; that is, CM is larger than C2. The determinations of C1, C2, and CM from Figure 5 are summarized in Table 2. In the experiments at pH 3, we were unable to find significant differences in binding behavior between linear and branched PEI. Highly exothermic titration enthalpies as observed at the early stages of SDS binding with PEI at pH 3 can be attributed to charge neutralization by oppositely charged ions,13 because around 70% of the nitrogen atoms in the polymer are positively charged (33) Couderc-Azouani, S.; Sidhu, J.; Georgiou, T. K.; Charalambous, D. C.; Vamvakaki, M.; Patrickios, C. S.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2004, 20, 6458-6469. (34) Bai, G.; Nichifor, M.; Lopes, A.; Bastos, M. J. Phys. Chem. B 2005, 109, 518-525.
at this pH. In the case of oppositely charged polymers and surfactants, electrostatic contribution may result in a sufficiently high driving force for monomer binding. It can be expected that the dodecyl sulfate ion binds to the protonated amine groups of the PEI (see also Winnik et al.9,10). To test this monomer binding at the early stages, the turbidity dilution of 5 mM SDS into LPEI at pH ) 3 has been studied. It is observed that the turbidity curves of 5 mM SDS into LPEI and 50 mM SDS into LPEI have almost the same slope and there is no obvious change at the early regions. The total interaction enthalpy (∆Htotal) of SDS with PEI then should include three main contributions: (1) the added micelles break up to give monomers, (2) the monomers are further diluted, and (3) monomers and micelles bind to the PEI backbone. ∆Htotal can be calculated from the difference between the two linear segments that occur before and after the steep break shown in Figure 5(a), similar to the method by Bai et al.35 The results listed in Table 2 show that, for both the LPEI/SDS and BPEI/ SDS systems, ∆Htotal is endothermic, suggesting that the interaction is entropy-driven. ∆Htotal-B and ∆Htotal-L are the total enthalpies for BPEI and LPEI, respectively. The difference between them, ∆H*, can be derived from the difference ∆Htotal-B - ∆Htotal-L and is also given in Table 2. Phase separation is a characteristic feature of mixtures of strong polyelectrolytes and ionic surfactants when the degree of ion pairing is high. Dubin36 and other authors37 have studied the PAA/CTAC/C12E8 system and SDS/Polymer JR 400 systems at low pH and have attributed phase separation to a 1:1 electrostatic charge neutralization. Similarly here, phase separation in solution (35) Bai, G.; Santos, L. M. N. B. F.; Nichifor, M.; Lopes, A.; Bastos, M. J. Phys. Chem. B 2004, 108, 405-413. (36) Yashida, K.; Dubin, P. L. Colloids Surf. A 1999, 147, 161-167. (37) Goddard, E. D.; Hannan, R. B. J. Colloid Interface Sci. 1976, 55, 73-79.
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Figure 6. (a) ITC curve for titrating 50 mM SDS into pure water and LPEI, (b) turbidity curve for titrating 50 mM SDS into BPEI solution, (c) ITC curve for titrating 50 mM SDS into pure water and BPEI solution, and (d) turbidity curve for titrating 50 mM SDS into BPEI solution. Polymer concentration is 0.1 wt %, and all the solutions are at pH 10 and 298.15 K.
is probably due to the formation of a PEIH+-DS- insoluble complex. Over the range of SDS concentration used here, the precipitate cannot be redissolved. One possible explanation of this is the high charge density on the surfactant headgroup. This may lead to tight binding of SDS molecules to PEI and an increase in the effective ionization equilibrium constant (pKa) of the positively charged groups on the PEI when they are bound to SDS.38,39 B. Linear and Branched PEI at pH 10. ITC and Turbidity measurements resulting from titration of SDS into LPEI and BPEI solutions as well as into water at pH 10 are compared in Figure 6. It can be seen that the LPEI solution remains cloudy31 and there is a high turbidity in the absence of SDS. The first additions of SDS into LPEI solution result in exothermic ∆Hobs values below the C1 and there is a slow increase in the turbidity to a maximum value. When more surfactant molecules are added, the interaction between SDS and PEI becomes more endothermic leading to a pronounced endothermic peak (CR) followed by a broad shallow exothermic one at C2. Correspondingly, the turbidity remains fairly constant up to 5.8 mM SDS (CR) and then steeply decreases with SDS concentration until it reaches C2. The CR therefore indicates the onset of a redissolution process. The redissolution is due to electrostatic repulsion between negatively charged micelles attached to the polymer. Above C2, the enthalpy curve merges with the dilution curve of SDS at about 20 mM (CM), whereas the turbidity remains constant and the solution becomes transparent again. From the binding enthalpy and turbidity profiles values of C1, CR, C2, and CM can be estimated, and there is good agreement between the values from the two methods. At high pH, PEI in aqueous solution has only a small fraction of charged amino groups, approximately 5 per 100 monomer units according to ref 13. These solutions should have some properties of a polyelectrolyte, and the ethylene groups of the polymer are also expected to give the polymer some hydrophobic character. The first aliquots of SDS added to the LPEI solutions leads to an exothermic effect. In this process, the headgroup of an anionic surfactant can bind to high-affinity sites of the LPEIs (38) Thongngam, M.; McClements, D. J. J. Agric. Food Chem. 2004, 52, 987-991. (39) Wang, Y.; Kimura, K.; Huang, Q.; Dubin, P. L.; Jaeger, W. Macromolecules 1999, 32, 7128-7134.
cationic groups through electrostatic attraction, whereas the hydrocarbon tail of the surfactant can bind to the PEIs uncharged hydrophobic segments through hydrophobic attraction.40 With increasing SDS concentration, the enthalpy of the interaction between SDS and the LPEI become less exothermic. This change is probably due to the contribution of endothermic ion-ion repulsive interactions between SDS molecules or aggregates bound to the LPEI. When the C1 is reached, aggregates start to form with SDS incorporating EI segments to generate mixed micelles. Above the C1, the features associated with the LPEI/ SDS interaction are similar to those for uncharged polymer/ ionic surfactant systems.17 The enthalpy change for the transfer of EI groups from water to the dehydrated core should be positive, so the observed enthalpy ∆Hobs becomes more positive as the C1 is passed. When C > CR, the exothermic contribution to ∆Hobs could arise from the rehydration of EI segments that are expelled from the core in the reorganization of the mixed micelles. As the saturation concentration C2 is approached, the EI groups will tend to be found in the outer part of the headgroup region where they can be in a largely hydrated form.17 The ITC binding enthalpy curve for SDS with BPEI shows similar trends to the curve for SDS with LPEI for the first few aliquots of SDS. Unlike the linear polymer, the enthalpy curve of BPEI does not have a pronounced endothermic peak around 6-7 mM SDS, and there is only a gradual decrease from large exothermicity followed by a plateau region. At SDS concentrations beyond the maximum of the exothermic curve, the titration curve passes through C2 and merges with the SDS dilution curve above CM. The absence of an endothermic peak suggests that there is no dehydration process for the BPEI backbone, which can be attributed to the hyperbranched structure of BPEI. Strong steric restrictions of the hyperbranched structure will hinder the transfer of EI groups from water to the dehydrated core. At the beginning of SDS titration the mixture is transparent (Figure 6d). On further SDS addition, the turbidity increases steeply because of the formation of aggregates. When the SDS concentration is close to the C1, precipitation occurs, but there is no redissolution of precipitation with more SDS. We conclude that there are fundamental differences in the different contributions to the interaction for LPEI and BPEI at pH 10. (40) Kelley, D.; McClements, D. J. Food Hydrocolloids 2003, 17, 73-85.
Binding of SDS with PEI
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Figure 7. (a) ITC curve for titrating 50 mM SDS into pure water and LPEI, (b) turbidity curve for titrating 50 mM SDS into BPEI solution, (c) ITC curve for titrating 50 mM SDS into pure water and BPEI solution, and (d) turbidity curve for titrating 50 mM SDS into BPEI solution. Polymer concentration is 0.1 wt %, and all the solutions are at pH 7 and 298.15 K.
C. Linear and Branched PEI at pH 7. At pH 7, PEI molecules are positively charged in the absence of SDS because about 20∼30% of the nitrogen atoms in the polymer are positively charged. It would then not be surprising if some of the features of PEI at pH 7 in ITC experiments were similar to the curve for pH 3. Figure 7 shows the results of ITC and turbidity measurements for PEI-SDS system at pH 7. In Figure 7, panels a and b, the initial additions of SDS into LPEI solution result in highly exothermic ∆Hobs values. At these relatively low SDS concentrations (
