15554
J. Phys. Chem. B 2008, 112, 15554–15561
Characterization of Aggregates formed by Hydrophobically Modified Cationic Dextran and Sodium Alkyl Sulfates in Salt-Free Aqueous Solutions Marieta Nichifor,*,†,‡ Margarida Bastos,§ Sonia Lopes,‡ and Antonio Lopes‡ “Petru Poni” Institute of Macromolecular Chemistry, 700487 Iasi, Romania, Instituto de Tecnologia Quimica e Biologica (ITQB/UNL), P-2781-901 Oeiras, Portugal, and CIQ (UP), Department of Chemistry, Faculty of Sciences, UniVersity of Porto, P-4169-007 Porto, Portugal ReceiVed: December 14, 2007; ReVised Manuscript ReceiVed: October 9, 2008
The interaction between polyelectrolytes based on dextran with pendant N-alkyl-N,N-dimethyl-N-(2hydroxypropyl) ammonium chloride groups, where n ) 2, 4, 8, 12, or 16, and sodium alkyl sulfates, SCnS, with n ) 8, 10, 12, 14, and 16, has been studied by conductometry and fluorescence techniques. Comparison of cumulative specific conductivities of the mixtures of polymer-surfactant over a large surfactant concentration range, with those of pure surfactant and NaCl, has clearly shown that the surfactants start to bind to polymer at very low concentrations (10-6 M), forming mixed aggregates. The steady-state emission fluorescence measured in the presence of pyrene, 1,3,6-diphenylhexatriene (DPH), and 1-pyrenylbutyric acid sodium salt demonstrated the existence of a critical surfactant concentration (CACS) at which the previously formed mixed aggregates are interconnected due to self-association of surfactant molecules included in different mixed polymer/surfactant aggregates. Above CACS, the mixed aggregates change dramatically their properties (hydrophobicity, size, DPH solubilization) which depend on both polymer and surfactant hydrophobicities and concentrations. The characterization of the new formed aggregates at different surfactant concentration ranges is derived mainly from their ability to solubilize hydrophobic compounds. The variety of fluorescence techniques used, combined with conductometric measurements and previous calorimetric information allowed us to provide here a comprehensive study and new interpretation of the solution behavior of these polymer-surfactant systems. Introduction Interactions of surfactants with oppositely charged polyelectrolytes have been extensively studied over last decades for their relevance to colloid science and colloid applications. Many biological events or processes involving biological material can be described by similar interactions, like nonspecific association of DNA with proteins, enzyme immobilization in polyelectrolyte complexes, or the effect of surfactants on DNA transfection.1-4 Therefore, a deeper understanding of the fundamentals of these interactions can provide invaluable clues into important biological processes. The same interactions are useful in water purification, or in application of these mixtures as additives for food, cosmetics, or pharmaceutics.5-7 The strength and type of interactions depend on the properties of both surfactant and polyelectrolyte. Regarding the surfactant, the structure of the hydrophobic part and headgroup are the important factors. Charge density,8-10 chain flexibility,11,12 and hydrophobicity13 of polyelectrolytes have the larger influence on the interaction. When dealing with charged surfactants and hydrophilic polyelectrolytes, we have mainly electrostatic attractions and therefore the surfactant forms only self-aggregates surrounded by polyelectrolyte segments.7 The aggregation starts at a critical aggregation concentrations (CAC) which is several orders of magnitude lower than the corresponding CMC, * Corresponding author. Mailing address: ”Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41 A, 700487 Iasi, Romania. Tel.: +40-232-260333. Fax: +40-232-211299. E-mail: nichifor@ icmpp.ro. † “Petru Poni” Institute of Macromolecular Chemistry. ‡ Instituto de Tecnologia Quimica e Biologica (ITQB/UNL). § University of Porto.
indicating that the polyelectrolyte strongly facilitates the surfactant aggregation.7,10,14,15 If the polymer itself contains hydrophobic moieties (amphiphilic polyelectrolyte), it can self-associate to form hydrophobic microdomains, which can include the alkyl tails of the surfactants, giving thus rise to mixed aggregates. The characteristics of these mixed aggregates (size, shape, polymer/ surfactant ratio) are very important for the understanding of the mechanisms of their formation and the properties of the obtained system, especially from the rheological point of view,16 since these systems can act as viscosity modifiers, having large application in secondary oil recovery, coatings and good care formulations.17 The occurrence of surfactant self-aggregates at a defined CAC is less pronounced here, and the process is less cooperative.18-20 Surfactant binding will affect the polymer selfassociation, either by enforcing or weakening it.21 The properties of these systems have been studied by various methods like fluorescence,22 microcalorimetry,23-26 potentiometry,27-29 viscosity,19,30,31 surface tension measurements,32,33 NMR,34,35 and electron paramagnetic resonance (EPR).36 Among these techniques, fluorescence has been widely used to determine CAC values, the aggregation number, polarity, and compactness of surfactant self-aggregates or polymer-surfactant mixed micelles.19,22,27,28,37 Theoretical models have also been proposed to describe these types of systems.30,38 The purpose of the present work is to investigate in detail: (i) the evolution of the aggregation process and (ii) the characteristics of the aggregates formed in aqueous systems containing cationic amphiphilic polyelectrolytes and anionic surfactants, as a function of each species hydrophobicity and concentration. We have used several cationic amphiphilic
10.1021/jp802543s CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
Dextran and Sodium Alkyl Sulfate Aggregates
Figure 1. Chemical structure of cationic amphiphilic polyelectrolytes with dextran backbone. R ) ethyl, butyl, octyl, dodecyl, and cetyl.
polyelectrolytes (P), all based on dextran with pendant N-alkylN,N-dimethyl-N-(2-hydroxypropyl) ammonium chloride groups (Figure 1), where the alkyl chain has 2, 4, 8, 12, or 16 carbon atoms. These polymers have the charged group and the hydrophobe located on the same side chains, which are statistically distributed along the polysaccharide backbone, in a very well defined chemical composition. Sodium alkyl sulfates, SCnS, with n ) 8, 10, 12, 14, and 16, were used as surfactants (S). These systems were previously extensively studied by microcalorimetry.24-26 In this report we used conductometry and several fluorescence techniques (steady-state emission fluorescence, fluorescence anisotropy, and fluorescence quenching) to further investigate the properties of these systems in aqueous salt free solutions. The use of a panoply of experimental methods allowed us to provide further insight into the aggregation process as well as aggregate characterization. All this information taken together should provide an important input for further applications of these very interesting systems. Experimental Section Materials. Dextran D40 (Mw 40 000, polydispersity index 1.12) was obtained from Sicomed S.A. (Bucharest, Romania). The reagents for synthesis were from Aldrich and those for analytic measurements were from Sigma. Pyrene (Py) was recrystallized before use. The other fluorescent probes, 1,3,6diphenylhexatriene (DPH) and 1-pyrenylbutyric acid (PyBu), were used as received. The sodium salt of PyBu (PyBuNa) has been obtained by addition of a methanol solution of NaOH to a methanol solution of the acid in an amount which ensures complete neutralization of carboxylic groups. Synthesis of Polymers. Cationic polymers with pendant quaternary ammonium groups were synthesized by chemical modification of dextran, as previously described.39 The content in amino groups (degree of substitution, DS) was determined from the nitrogen content (elemental analysis) and chloride ion content (potentiometric titration with AgNO3). The general code of cationic dextrans is D40-RX, where R is the abbreviation for the substituent at amino group, according to Figure 1, and X indicates the average DS ((2-3 mol %) with amino groups carrying the R substituent. Accordingly, D40-Oct30 is the code for the polymer based on D40 with 30 mol % amino groups containing octyl as the R substituent. Methods. Conductometric measurements were performed with a model InoLab 1 (WTW GmbH) conductometer using a TetraCon 325 conductivity cell, cell constant 0.1 cm-1. The surfactant solution was added to polymer solution in increments of 0.05 mL, using surfactant stock solutions of increasing concentrations, thus the final increase in the total mixture volume was about 5%. Steady-state fluorescence spectra were obtained with a SPEX Fluorolog 212 in L conformation or a PerkinElmer L55 spectrofluorimeter. Emission spectra were recorded with the excitation wavelength set at 337 nm for Py, and at
J. Phys. Chem. B, Vol. 112, No. 49, 2008 15555 340 nm for DPH and PyBu. All analyzed solutions contained 1 × 10-6 M fluorescent probe, unless another concentration is specified. In the case of DPH fluorescence anisotropy, the emission wavelength was set at 430 nm. Quenching experiments were performed by measuring the intensity of the first peak in pyrene emission spectra in the absence (I0) and the presence (I) of different amount (0.01-0.1 mM) of cetyl pyridinium bromide (CPB) as a quencher. The linear Stern-Volmer plots allow the calculation of the aggregate concentration, [M], and aggregation number Nagg with the formulas: ln(I/I0) ) [Q]/[M] and Nagg ) CH/[M], where [Q] is the quencher concentration, and CH is the hydrophobe concentration (all concentration are expressed in millimoles). All mixtures used for fluorescence studies were allowed to equilibrate for 24 h at room temperature prior measurements. Conductometric and fluorescence data have been obtained at 25 ( 0.2 °C, in triplicate. Results Qualitative Evaluation of Surfactant Binding to Cationic Polymers. The binding isotherms in systems containing cationic surfactants and hydrophilic anionic polyelectrolytes have been obtained by a potentiometric method using surfactant specific electrodes.40 This method has been less used for anionic surfactants due to poor electrode sensitivity at low and high surfactant concentrations41,42 or for amphiphilic polymers which have tendency to stick onto the electrodes.27,28,43 Conductometry has been also used for the evaluation of the surfactant binding to neutral polymers,44 hydrophilic,45-47 or amphiphilic polyelectrolytes.23,48 The specific conductivity κ of the system P-S is a sum of the conductivities of all ionic species in solution (free P or S, free counterions Na+ and Cl-, P-S complex); therefore, the variation of κ with surfactant concentration (CS) can give only a qualitative evaluation of the extent of S binding to P. We have followed the variation of κ of the system P-SDS with SDS concentration, in comparison to what is observed for free SDS and NaCl (Figure 2). Plotting the data on narrow SDS concentration ranges (Figure 2b-d) allowed the detection of concentration ranges over which the slope of κ vs CS remains about constant and also the identification of several break points, called here CSi, in the order of their increasing value. At very low SDS concentration (CS < CS1), almost no variation of κ is observed for D40-Oct30 and D40-Dod30 polymers, indicating that all S molecules are bound to P. In case of D40-Cet30, κ varies with CS in an almost constant way (constant slope of κ vs CS) until charge neutralization (neutralization point, np, is about 1.2 mM for all polymers, at a polymer concentration CP ) 0.1 g/dL). The observed slope is higher than that for pure SDS monomer and lower than that of NaCl. D40-Cet30 has the highest hydrophobicity among the three studied polymers, and therefore both electrostatic and hydrophobic interactions are anticipated to be strong. Thus, the only explanation for this behavior is the constant release of the counterions from the complex to the bulk solution, which must be less pronounced in case of the other polymers, as the amount of released counterions increases with the strength of P-S interaction.11,21,47 Between CS1 and np the slopes of κ vs CS increase but are still lower than that of pure SDS and are different for different polymers, as follows: D40-Oct30 < D40-Dod30 < D40-Cet30 (Figure 2c and d). Near np, all systems show clear break points (CS3), after which the variations of κ with CS approach the slope of pure surfactant (Figure 2d). These break points (CS3) were also found by microcalorimetry, being actually the first visible break points in the calorimetric titration curve.24,26 The final break points occur at a surfactant concentration somewhat higher
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Figure 2. Variation of specific conductivity with SDS concentration (marks), in the presence of cationic polymers D40-R30 (CP ) 0.1 g/dL). Conductivity of pure SDS (solid line) and NaCl (dotted line) are also shown. For a better comparison of the slopes, the plots in graphs b, c, and d are enlargements of the xx scale of plots of graph a, and have been shifted differently along yy axes.
TABLE 1: Critical Concentrations, Polarity (Pyrene I3/I1), and DPH Solubilization Ability (IDPH) for the Systems Containing SDS and Cationic Amphiphilic Polyelectrolytesa critical concentrations, in mM polymer
method
CS1
CS2
D40-Oct30
conductivity pyrene DPH PyBuNa ITC conductivity pyrene DPH PyBuNa conductivity pyrene DPH
0.02 0.008 (CACS) 0.01 0.01
0.3 3
D40-Dod30
D40-Cet30
no polymer
CS3 1
CS*
(CS* - CMC)/ [amino group],mol/molb
9.12
I3/I1c
IDPHc
0.816 0.75 416
1.0d 1
0.1 0.17 (CACS) 0.15 0.1
1 1
0.21 (CACS) 0.3
1 1
9.53
1.14 0.8 427
1
conductivity
a
9.65
1.24 0.82 485
8.10 (CMC)
0.85
34
b
CP was 0.1 g/dL. Some properties of pure SDS micelles are included for comparison. CMC, determined from conductivity curve for pure SDS, was 8.1 mM. c Values corresponding to the upper plateau of the plots of fluorophore property variation with surfactant concentration. d Value from ref 24.
than the surfactant CMC, and these values have been taken as C*S (apparent CMC of the surfactant in the presence of polymer). The values of all break points discussed above are included in Table 1. We included also the number of SDS molecules bound per amino group of the polymer, calculated as the difference between CS* and CMC related to the amino groups of the polymer (7th column). As expected this value increases with increasing polymer hydrophobicity. Critical Aggregation Concentration of Systems. Most of the studies performed with hydrophobically modified polyelectrolytes (HMPE) and oppositely charged surfactants showed only a small monotonous variation of system properties (for instance pyrene I3/I1) with the surfactant concentration,20,22,27,28 suggesting the absence of a critical aggregation concentration (CAC) for
the surfactant in the presence of the polymer. Nevertheless, at polymer concentrations lower than its self-aggregation concentration (CACP), a CAC lower than CMC of the surfactant was found.19 The self-aggregation of P above a critical polymer concentration (CACP) was previously shown by fluorescence measurements carried out in the presence of Py, NPN, and DPH.39 In order to extensively characterize the aggregation process in P-S systems, we have performed fluorescence measurements in the presence of Py, DPH, and PyBuNa as fluorophores. Py can give information about the aggregate hydrophobicity (through the ratio I3/I1).49 We found a clear break in the variation of I3/I1 with SDS concentration, irrespective of the aggregation state of the polymer itself at the used concentration, CP (CP )
Dextran and Sodium Alkyl Sulfate Aggregates
J. Phys. Chem. B, Vol. 112, No. 49, 2008 15557
Figure 4. Variation of DPH emission intensity with SDS concentration in aqueous solution containing 0.1 g/dL cationic polymers. (a) D40Oct30, D40-Dod15, and D40Cet15; (b) D40-Oct30, D40-Et30, and D40But30.
Figure 3. Variation of pyrene I3/I1 with concentration in the systems containing SDS alone, cationic polymer (P) and mixtures SDS-P (0.1 g/dL). Polymers: D40-Oct30 (a) and D40-Cet15 (b).
0.1 g/dL is below CACP for D40-Oct30 and above CACP for D40-Cet15) (Figure 3a and b). According to our conductometric data, at these concentrations the surfactant is already bound to the polymer. Therefore, the increase in the ratio I3/I1 above this break point (CS1) can be assigned either to an increase in hydrophobicity of existing mixed aggregates, or to the start of the self-association of bound surfactant, with the formation of surfactant self-micelles. The maximum I3/I1 values depend on HP and are higher than those of polymer self-aggregates but lower than that for pure SDS micelles (Table 1). DPH intensity is a selective tool for discrimination of aggregate ability to solubilize hydrophobic compounds.50 Figure 4 shows the variation of the DPH emission intensity with SDS concentration. Addition of SDS does not change DPH intensity until a certain CS, above which a sharp increase in DPH intensity is noticed, suggesting a significant increase of DPH solubilization in the hydrophobic microenvironment. DPH intensity levels off at a higher SDS concentration. Above the CMC value of SDS, an intensity decrease is observed down to the value obtained for pure SDS. This behavior can not be explained only by an increase in system’s hydrophobicity, since SDS micelles are more hydrophobic than the mixed aggregates, and yet have a much lower ability to solubilize DPH (Table 1, Figure 4a). Therefore, we assign this very significant increase in DPH solubilization inside the P-S mixed aggregates to an increase in their size, determined by the collapse of the polymer chain
after complexation with SDS. Similar measurements with a hydrophilic polymer like D40-Bu30 (Figure 4b) show also an increase in aggregate size, and only in the case of D40-Et30 did we observe a behavior that is close to that of free SDS micelles. PyBuNa can be perceived as a fluorescent anionic surfactant which can be bound to oppositely charged cationic amphiphilic polyelectrolytes both by electrostatic and hydrophobic interactions. Occurrence of excimers in the system P-PyBuNa could be assigned to the association of pyrene surfactant molecules already bound to the polymer. Pure PyBuNa has a poor ability for excimer formation, and its water solubility is lower than 10-4 mM. However, in the presence of the cationic polymers (0.1 g/dL), PyBuNa is completely solubilized until 10-2 mM, proving its binding to the polymer. As Figure 5 shows, the occurrence of excimer (significant increase in the ratio IE/IM) is observed at PyBuNa concentrations close to SDS concentration related to I3/I1 (Py) or IDPH increase, indicating that at these concentrations we have the onset of an interaction between the hydrophobic parts of the surfactant molecules already bound to the polymer. In conclusion, all fluorescence measurements performed with the systems P-S under study revealed a good agreement between the surfactant concentrations (CS) at which the fluorophore properties start to increase as compared to those observed in the presence of pure polymer (Table 1). Therefore, these CS values (CS1) are considered here as the critical surfactant concentrations for which a significant variation in the system properties (mixed aggregates P-S) is observed. The value of this concentration (CACS) was taken as CS1 determined in the
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Figure 5. Variation of excimer to monomer ratio (IE/IM) with PyBu concentration in water or water solutions containing SDS or cationic polymers.
presence of Py. The next break point, (CS2), refers to the surfactant concentration for which a leveling off of the fluorophore property (Py or DPH) was observed. It should be stressed that although the values of CS2 determined by fluorescence and the break point obtained by conductometry (CS3) are very similar for CP ) 0.1 g/dL, the latter value is related to the neutralization of polymer and its variation with CP is different from that obtained by fluorescence. Therefore, these concentrations obtained by conductometry are included separately in Table 1. We have also included in the Table the values of the maxima for I3/I1 and IDPH for each system, as a measure of mixed aggregate maximum hydrophobicity and hydrophobe solubilization ability, respectively. Variation of I3/I1 with CS is sigmoidal and the slope of variation depends on the length of the alkyl chains of both surfactant (HS) and polymer (HP) (Figure 6). The cooperativity of the process described by these curves can be roughly evaluated from their slopes. We have calculated the cooperativity as the slope of the I3/I1 variation at the middle point between the minimum and the maximum of the sigmoidal curve I3/I1 versus CS, and the obtained values are plotted in Figure 7. One can clearly see the dependence of the cooperativity on the pair HP-HS. The highest cooperativity occurs in the system where HP and HS have the highest difference in length (C8 and C16), irrespective of their position, the lowest in the systems with similar length of the hydrophobes. Only for the system C16-C16 almost no break in the variation of I3/I1 with CS was found (Figure 6), as was reported for other previously described systems HMP/S.20,22 Dependence of Critical Aggregation Concentration on Surfactant Alkyl Chain Length. Figure 8 presents the variation of CACS values, calculated from the curves like those presented in Figure 6, as a function of the surfactant alkyl tail length (HS). The surfactant CMC values53 are presented for comparison. The CACS values are lower than the corresponding CMC and decrease in the order D40Cet15 ≈ D40Dod30 > D40Oct30. This order might indicate that bound surfactant self-association is favored in the presence of the more hydrophilic polyelectrolyte. The slope of CACS variation with alkyl chain length is lower than in case of CMC, depends on HP, and decreases in the following order: Oct30 (0.22) > Dod30 (0.11) ≈ Cet15. The slope of log(CACS) variation was related to the difference in stability of the P-S aggregates as compared with S selfaggregates, meaning that P has a larger influence on the stability of the aggregates formed by SCnS with lower n.45 The lower slope found for the systems containing Dod and Cet polymers
Figure 6. Variation of pyrene I3/I1 with the surfactant concentration for systems containing cationic polymers and alkyl sulfates of different hydrophobicities.
Figure 7. Slope of the variation of I3/I1 with surfactant concentration as a function of the polymer and surfactant hydrophobicity.
could indicate that the binding/dissolution of S molecules inside the polymer hydrophobic microdomains is less dependent on HS than for the polymer with shorter alkyl substituent, D40Oct30. Dependence of the Critical Aggregation Concentration on Polyelectrolyte Concentration. Results reported for other P-S systems showed either no or little dependence27,47,54 or a linear increase of CAC with CP.10,11,55 The latter behavior was explained by the increase in ionic strength of the solution with CP.10 The major part of the experiments performed in this study was carried out at a polymer concentration of 0.1 g/dL. Some
Dextran and Sodium Alkyl Sulfate Aggregates
Figure 8. Variation of critical aggregation concentration of surfactant as a function of the length of the surfactant alkyl chain. CMC values of pure surfactants53 are also included.
Figure 9. Influence of the polymer concentration on the value of critical aggregation concentrations of the SDS in its mixtures with cationic amphiphilic polyelectrolytes. CACS and CS2 significance are given in Table 1 and in the text.
experiments were also performed at different CP, in order to asses the CP influence on P-S system behavior. The results for the systems containing SDS are presented in Figure 9. The slope of the plots log(CACS) versus log(CP) is taken as a measure of the influence of the polyelectrolyte on bound surfactant selfassociation. In our case, the slope increases in the following order: Oct30 < Bu30 < Dod30 ≈ Cet30 < Cet15. The most significant difference is noticed between D40-Oct30 and the more hydrophobic polymers. This difference could be explained either by the difference in the amount of polyelectrolyte counterions released during surfactant binding56 or by different polymer self-aggregation ability. The former explanation is supported
J. Phys. Chem. B, Vol. 112, No. 49, 2008 15559 by the conductometric results here reported, but only for the system with D40-Cet30, not for that with D40-Dod30. However, the amount of S molecules solubilized inside polymer hydrophobic microdomains, preventing thus their self-association, clearly depends on P self-aggregation ability. Oct polymer selfaggregates at CP > 0.3 g/dL, and the formed aggregates are rather loose, with a lower ability to solubilize the HS. Therefore, the variation of CACS with CP is less steep, close to that for the system D40-Bu30-SDS. Oppositely, polymers with Dod and Cet hydrophobes have a CACP ∼ 0.001 g/dL, and above this concentration they form nonpolar and compact hydrophobic domains, the number and size of which increase with increasing CP.39 These hydrophobic microdomains have the ability to solubilize surfactant, and the number of solubilized S molecules increases with increasing CP, inducing the observed steep increase of CACS with CP. Such an explanation is supported by the clear tendency of all the lines in Figure 9a to intersect at very low CP (about 0.0001 g/dL), when all polymers are in extended conformation. Interestingly, no break point in the variation at CP ) CACP was found for the polymer D40-Oct30, showing again that the hydrophobic domains formed in this case are not very compact. However, if we plot the CS2 values as a function of CP (Figure 9b), a clear break point at a CP close to CACP is observed for this polymer and the second part of the plot parallels the similar plot for D40-Dod30. Size and Number of the P-S Aggregates. We have used fluorescence quenching experiments to determine the concentration of aggregates formed in P-S systems. Experiments were performed for concentration ranges of 0.1-1 g/dL for polyelectrolytes and 0.5-30 mM for SDS. The lack of binding isotherms, and therefore of the amount of S actually bound to polymer, makes the calculation of Nagg,complex unreliable, but the concentration of mixed aggregates, [M]complex, can give information about the aggregation process. Figure 10 presents the data obtained from quenching experiments, both for P-S systems and P alone, together with CACS, as a function of CP. One can see that [M]complex is lower than the polymer self-aggregate concentration, [M]P, determined in the absence of S. This result differs from other reported data, where an increase in aggregate concentration after CAC was found.57 In our case, a good correlation is observed between CACS and the number of HP included in one polymer aggregate (Nagg,P). This correlation explains the variation of CACS with CP. Another interesting finding is the relationship between CACS and [M]P. In the case of D40-Oct40, CACS < [M]P, for D40-Dod30, CACS > [M]P, and the difference increases with CP. The first relationship can be assigned to looser polymer aggregates which can not efficiently “hide” surfactant molecules from interacting with each other. The hydrophobic microdomains formed by Dod polymer are large and compact and can accommodate more than one HS before these hydrophobes can self-associate. Discussion The experimental data gathered in this study allow us to describe in detail the evolution of the aggregation process in the systems under study, and this represents an important step forward in the knowledge of these P-S systems. Several surfactant concentration domains and surfactant critical concentrations (Table 1) were highlighted through the present work. Conductometric measurements indicated that the binding of the surfactants to amphiphilic polyelectrolytes starts at very low CS, much lower than any critical concentration determined by fluorescence. The strength of the binding increases with increasing polymer hydrophobicity, allegation that is supported by the
15560 J. Phys. Chem. B, Vol. 112, No. 49, 2008
Figure 10. Dependence of SDS-cationic polymer aggregate characteristics on cationic polymer (P) concentration. CACS is the critical aggregation of the P-SDS system, [M]P is the polymer self-aggregate concentration, [M]complex is the polymer-SDS mixed aggregate concentration, and Nagg,P is the number of polymer hydrophobes included in a polymer hydrophobic microdomain. P was (a) D40-Oct30 and (b) D40-Dod30.
higher amount of polymer counterions released in the solution after binding and by the higher amount of surfactant bound to polymer at apparent saturation (Surf/X in Table 1). After initial binding, the behavior of the systems depends also on the polymer hydrophobicity and its aggregation state. When CP < CACP, the binding of the surfactant is induced by the presence of the polymer hydrophobes which become wrapped by surfactant molecules, giving rise to few and very small mixed aggregates. If CP > CACp, many mixed aggregates rich in polymer hydrophobes are formed. For CS < CACS both mixed aggregates have properties (hydrophobicity, DPH binding, and compactness) similar to the genuine polymer aggregates. Above CACS, both types of mixed aggregates change dramatically their properties: hydrophobicity becomes higher than that of the polymer but is lower than that of the pure surfactant micelles; the ability for DPH solubilization becomes much higher than that of pure polymer or pure SDS micelles but compactness remains constant (data not shown). The experiments performed with PyBuNa indicate that at CACS the interaction between bound surfactants starts to take place, and also that the interaction is much more pronounced for shorter Hp. The dependence of CACS on CP suggests that this interaction occurs when the polymer hydrophobic microdomains are saturated with monomeric S molecules. The much higher ability for DPH solubilization together with the decrease in mixed aggregate concentration support the hypothesis of increase in size of P-S mixed aggregates aboVe CACS by interconnecting two or more mixed aggregates Via surfactant tail association. The new formed aggregates have high DPH solubilization
Nichifor et al. capacity irrespective of their Hp. It is worth mentioning that the ability to form intra- or intermolecular connection between P-S mixed micelles is preserved down to an HP as low as C4 (Figure 5b) and only for a R ) C2 a pure self-micellization of S molecules bounded to oppositely charged surfactant seems to occur (as described by the neck-lace model). Thus, in our case CACS is an important break point in the properties of the mixed aggregates present in these systems. We should stress that this break point was detected by all fluorescence method used in the study as well as by conductometry. Between CACS and CS2, the mixed aggregates increase in size and hydrophobicity, and the cooperativity of the aggregation process depends on the pair HP-HS. Between CS2 and C*S, these properties remain constant. Another critical concentration reported, CS3, is correlated with the neutralization of most of the polymer charges, and can be lower, equal or higher than CS2 with increasing CP. CS3 was detected by conductometry and ITC measurements.24,26 The fact that the enthalpy of SDS dilution in the presence of the polymer indicates a first thermodynamic significant event at CS324,26 leads as to suppose that at this point the occurrence of free SDS molecules starts, but S binding still continues. Above CS*, when pure S micelles are supposed to be formed together with the mixed aggregates which becomes richer in surfactant molecules, the overall polarity of the aggregates present in the systems tends to that of pure SDS micelles. The formation of pure S micelles and S-like mixed aggregates is further corroborated by the fact that DPH solubilization capacity decreases and reaches the level found for pure SDS micelles. It should be stressed, however, that CS* cannot be considered the end point of S binding to P, but a threshold concentration that separates a region where P-S interactions prevails to one of strong S-S interactions. As has been already shown for other systems,42 there is a difference between the C*S values determined by conductometry and microcalorimetry, as the first method indicates the CS at which free micelles are formed and the latter reveals the real saturation point for the system P-S. The previous ITC experiments showed that these saturation values increases as expected with polymer concentration and are higher than CS*.26 The present results show that pure SDS micelle properties are observed at a SDS concentration as high as 6 times its CMC. Conclusion In order to get new insights into interactions occurring in the system containing sodium alkyl sulfates and oppositely charged amphiphilic polyelectrolytes, we have used a series of polymers with a well defined and versatile chemical structure which allowed us to establish a clear relationship between the properties of the polymer and those of its mixtures with surfactants. The data obtained by various fluorescence techniques, combined with conductometric measurements and previous calorimetric information, helped us to provide a comprehensive study and interpretation of the solution behavior of the P-S systems, as a function of surfactant alkyl chain length, polyelectrolyte hydrophobicity, and both component concentration. The results show some features also found in other related systems, but revealed some new aspects. The most relevant for the studied systems is the presence of a surfactant concentration, CACS, which is critical for surfactant-polymer mixed aggregate properties and is lower than the corresponding surfactant CMC. Above CACS these mixed aggregates increase in size and decrease in number due to their interconnection through the
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