2314
J. Phys. Chem. B 2001, 105, 2314-2321
Interaction of Hydrophobically Modified Cationic Dextran Hydrogels with Biological Surfactants Marieta Nichifor,*,†,‡ X. X. Zhu,‡ Doina Cristea,† and Adrian Carpov† “Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41 A, 6600 Iasi, Romania, and UniVersite´ de Montre´ al, De´ partement de chimie, C. P. 6128, succursale Centre-Ville, Montre´ al, QC, H3C 3J7, Canada ReceiVed: July 24, 2000; In Final Form: NoVember 15, 2000
Cross-linked cationic dextran hydrogels with 20-25 mol % N-alkyl-N,N-dimethylammonium chloride pendant groups were prepared, and their interaction with sodium salts of some bile acids was investigated. The binding parameters, initial ionization constant K0, stability constant K, and cooperativity parameter u, were calculated according to the nearest-neighbor interaction model. The influence of the length of the alkyl substituent of the hydrogel, the ionic strength, and the bile acid hydrophobicity on the binding parameters was studied. The increase in the length of the alkyl substituent of the hydrogel strongly increases the binding constants K0 and K, but decreases the cooperativity parameter u. This was explained by the formation of mixed micelles between pendant groups of the gel and surfactant molecules. The increase in the ionic strength leads to a significant decrease of the binding constants K0 and K and an increase in the cooperativity parameter for the binding of the bile salts by hydrogels with alkyl substituent length eC4. However, ionic strength has a small influence on the parameters for bile acid binding by hydrogels with alkyl substituent as long as C8 or C12. We have also found that the binding constants are higher for more hydrophobic dihydroxylic bile acids than those for trihydroxylic ones, but u values are less influenced by the hydrophobicity of the surfactant. Finally, we have shown that the water uptake at equilibrium by the hydrogels decreases rather monotonically with the increase in the binding degree, and the decrease is more significant for the more hydrophobic hydrogels, both in pure water and in the presence of 10 mM NaCl.
Introduction Systems based on the interactions between polymers and surfactants have attracted considerable research interest due to their widespread applications. Mixtures of polyelectrolytes and oppositely charged surfactants are found in fluids for enhanced oil recovery, food preparations, cosmetics, or pharmaceuticals.1-3 In addition, the study of such systems can provide insight into the physical and chemical processes based on hydrophobic and electrostatic interactions among proteins and biologically significant surfactants.1,4 The properties of polyelectrolyte-surfactant systems depend strongly on the relative charges and hydrophobicity of the components. The polyelectrolyte can interact electrostatically with the polar head of an oppositely charged surfactant, and the hydrophobic interactions among nonpolar segments of the bound surfactant molecules promote their aggregation. It is well established that this aggregation is stabilized in the presence of polyelectrolyte and starts at a surfactant concentration (critical aggregation concentration, cac) much lower than its critical micellar concentration (cmc).5-10 When the polyelectrolyte has also hydrophobic moieties (amphiphilic polyelectrolyte), it can strongly bind an oppositely charged surfactant, by both electrostatic and hydrophobic forces, promoting the formation of mixed micelles.11-15 The interactions between hydrophobically modified polyelectrolytes and surfactant micelles have been * Corresponding author. E-mail:
[email protected]. † “Petru Poni” Institute of Macromolecular Chemistry. ‡ Universite ´ de Montre´al.
studied by monitoring the rheological properties and phase separations13,15 as well as the binding isotherms.12 More recently surfactant binding by polyelectrolyte networks has been investigated16-22 and theoretically analyzed.23-26 These systems have interesting applications as “responsive” gels, superabsorbents, or controlled delivery systems.20,22 Their general features are similar to the systems involving watersoluble polyelectrolytes, but they have also special properties mainly induced by the high osmotic pressure inside the network and the variation of this pressure upon binding with surfactant. The influence of the charge density, ionic strength, or crosslinking degree on the binding characteristics has been outlined,20,25 and the occurrence of highly ordered supramolecular structures of the surfactant inside the gel has been demonstrated.21,27 Much less studied is the interaction of hydrophobically modified polyelectrolyte networks with oppositely charged surfactants.16 For example, the binding of bile salts by polyamines designed as bile acid sequestrants has been investigated,28 but the correlation between the hydrophobicity of the networks and their binding performances is scarce.29 The aim of the present work is to study systematically the influence of the hydrophobic moieties of polycationic networks on their interaction with anionic surfactants. The properties of the polyelectrolyte-surfactant systems depend strongly on the interactions among the species of the system. There can be electrostatic and/or hydrophobic interactions (a) among the polyelectrolyte moieties (charged groups and/or hydrophobic substituents), (b) among the surfactant molecules in solution, and (c) between the surfactant and the polyelectrolyte. A key
10.1021/jp002621k CCC: $20.00 © 2001 American Chemical Society Published on Web 03/01/2001
Interaction of Dextran Hydrogels and Surfactants
J. Phys. Chem. B, Vol. 105, No. 12, 2001 2315 formation of mixed micelles with lipids.30 The study of the interactions of bile acids with tetra- or hexadecyltrimethylammonium bromides (CnTAB) has shown a strong tendency to mixed micelle formation.31,32 The present study may provide useful information for designing cationic hydrogels as anticholesterolemic agents. In this paper we report studies on the influence of the hydrophobicity, i.e., the length of the alkyl substituent, R (C2 - C12, Figure 1) of cationic polyelectrolyte hydrogels on the binding of several bile acid sodium salts. Moreover, the swelling of the hydrogels in pure water or in the presence of NaCl, as a function of the degree of surfactant binding, is investigated.
Figure 1. General chemical structure of cationic dextran hydrogels DMRA (I), where R can be ethyl (DMEtA), butyl (DMBuA), octyl (DMOctA), or dodecyl (DMDodA) groups.
Figure 2. Chemical structure of bile acids used as anionic surfactants.
factor in the actual interaction between the system components is the chemical structure of hydrophobically modified polyelectrolyte, namely the flexibility of the polymer backbone and the relative positions of the charged sites and the hydrophobic moieties on the polymer backbone. The polyelectrolyte hydrogels under study have been designed to avoid as much as possible the interactions of type a. According to the general chemical structure depicted in Figure 1, the hydrogels have the charged sites and hydrophobic moieties located on the same pendant groups randomly distributed along the backbone of a moderately cross-linked dextran. Actually, each pendant group can be considered as a “polar-end-attached” cationic surfactant. Several investigations of anionic surfactant interactions with water-soluble or cross-linked cellulose derivatives with similar chemical structure have been already reported.15,16 These derivatives (Quatrisoft LM200) have a low degree of substitution (about 5 mol % N-dodecyl-N,N-dimethylammonium groups) and a large heterogeneity of substitution pattern.16 Our polymers were obtained by homogeneous chemical modification of dextran, a more reliable and flexible natural polysaccharide than cellulose, and have a moderate density of N-alkyl-N,N-dimethylammonium groups (20-25 mol %) at which an equilibration between repulsive electrostatic forces and attractive hydrophobic interactions is expected to be established. Therefore, as a first approximation, we can neglect the interactions of type a. The interactions of type b in solutions can be minimized by working at surfactant concentrations below its cmc. Consequently, only interactions of type c will be taken into consideration. The anionic surfactants used here are sodium salts of carboxylic bile acids with the general structure presented in Figure 2. Bile acids are facially amphiphilic anionic surfactants involved in many biological processes, especially in the
Experimental Section Materials. Sodium glycocholate (NaGCA), sodium taurocholate (NaTCA), sodium cholate (NaCA), and sodium deoxycholate (NaDCA) were purchased from Aldrich, as were all the tertiary amines used for the polymeric sorbent syntheses. N-Phenyl-1-naphthylamine (NPN) was from Sigma and was recrystallized twice from acetone. Synthesis of Cationic Dextran Hydrogels. The dextran polymer cross-linked with epichlorohydrin (1 mol/glucosidic unity in cross-linking reaction mixture) was functionalized by introducing pendent quaternary ammonium groups according to a procedure described in detail elsewhere.33 Briefly, crosslinked dextran (spherical particles of 100-200 µm in diameter) was swollen in water and allowed to react with a mixture of epichlorohydrin and a tertiary amine for 6 h at 70 °C. After purification and drying, the content in amino groups of the sorbents (in chloride salt form) was determined by elemental analysis of the nitrogen content and by potentiometric measurement of the Cl- ions. The equilibrium swelling capacity, Rw0 (the amount of water retained at equilibrium, in grams of H2O per gram of dry hydrogel), was determined according to a method described by Pepper et al.34 Synthesis of water-soluble cationic dextrans was performed by the same functionalization procedure described above, except for linear dextran of Mw ) 30 000 employed as starting material. The polymers were purified by dialysis against Millipore water. Binding Measurements. Equilibrium binding assays were performed in Millipore water or in aqueous NaCl of different concentrations. Cationic hydrogel (about 10 mg) was suspended in the bile acid sodium salt solutions (10 mL) the initial concentration (Ci) of which ranged from 0.1 to 3.0 mM. The amount of cationic hydrogel used in the binding experiments was always taken to result in a concentration of 1 mM cationic groups. The obtained suspension was kept for 24 h in a shaking bath thermostated at 25 ( 0.5 °C. After filtration, the equilibrium concentration (Ceq) of the bile salt in the filtrate was determined by HPLC analysis using a method previously described.35 The HPLC system was equipped with a Waters 600 pump and controller, a Waters 715 Ultra WISP autosampler, a Nova Pak C18 column 3.9 × 150 mm, and a Waters 410 differential refractometer as a detector. Isocratic elution was performed with a mixture of 0.1 M aqueous acetic acid and methanol in volume ratios ranging from 20/80 to 30/70, depending on the hydrophobicity of the bile acids. The calculation of bile salt concentration was based on calibration curves established for each individual bile salt. The amount of the bile salt bound by the gel was calculated as the difference between Ci and Ceq. Each sorption experiment was performed in triplicate, and the standard deviation was estimated to be e5%. Water Swelling of the Ionic Complex Hydrogel-Cholic Acid. The dry hydrogel in chloride form (100 mg) was
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introduced into a stainless steel tubing (20 mm i.d. × 30 mm long) fitted with a nylon sieve support, and the device was suspended without complete immersion in a stirred solution of known concentration in sodium cholate (100 mL). The system was kept for 24 h at room temperature; then the tubing was removed from the solution and the gel inside was rinsed carefully with water and centrifuged for 10 min at 2000 rpm. The content of bile salt in the remaining sodium cholate solution was determined by UV measurements, after the treatment with aqueous H2SO4 solution, according to a method previously described by Fini et al.36 The water content into the sorbent in cholate form was calculated with the formula
Rwβ ) (Wsβ - Wd - WCA)/Wd
(1)
where Wd is the gel weight in dry form, Wsβ is the gel weight in the swollen state after bile acid sorption and centrifugation, and WCA is the weight of cholic acid retained in the gel, calculated as the difference between the initial and final concentrations of sodium cholate solution. All the individual experiments were performed in triplicate. Steady-State Fluorescence Measurements. Aqueous solutions containing the same concentration of water-soluble cationic dextran (1 mM quaternary ammonium groups) and NPN (1.0 × 10-6 M) were vortexed with different amounts of sodium cholate or deoxycholate and kept at room temperature for 24 h. The steady-state fluorescence spectra of the solutions were obtained using a SPEX Fluorolog 212 in L configuration. Excitation wavelength was fixed at 340 nm. The cac values for bile salts were determined as the inflection points of the curves obtained by plotting NPN fluorescence emission relative intensity versus bile salt concentration. Results and Discussion Binding Isotherms. There are several theoretical approaches developed for describing the interactions occurring in the systems formed by polyelectrolyte networks and surfactants of opposite charge. Gong and Osada24 have extended the Zimm-Bragg model for cooperative binding with nearest-neighbor interaction37-39 to polyelectrolyte networks interacting with oppositely charged surfactants. They have estimated the effect of cross-linking degree on the binding properties and suggested that the binding takes place with surfactant monomer and aggregation of surfactant molecules inside the gel occurs between already bound surfactant molecules, at cac much lower than for the systems involving linear polyelectrolytes. Recently, Hansson26 has introduced a new model to describe the interactions of a highly flexible, highly charged, and weakly cross-linked polymer with oppositely charged surfactants. Using experimental data and Monte Carlo simulation for systems based on linear polyelectrolytes, Hansson developed his model based on shortrange interactions between the flexible polyelectrolyte chain segments and already formed surfactant micelles. According to this model, the surfactant aggregation inside the gel starts at the same cac as for linear polyelectrolytes of the same concentration, and the cross-linking degree has only a little influence on it. Since the hydrogels used in the present study have a moderate charge density and cross-linking degree, and the interest of the study is the interaction between the hydrophobic moieties of both components, we have chosen the nearest-neighbor-interaction model for analyzing the binding experimental data. This model is used by many groups working in the field of
polyelectrolyte-surfactant systems12,17-19,28 because it gives good fits to experimentally obtained isotherms and is an excellent way to characterize the binding strength and the cooperativity of the interaction. According to this model, the first step is the electrostatic binding of a surfactant molecule to an isolated charged site on the polymer. This step is characterized by the initial ionization constant K0. In the following step, the hydrophobic interaction between one bound and one free ligand molecule promotes an acceleration of the electrostatic binding to a charged site adjacent to the one already occupied on the polymer. This step is characterized by the cooperativity parameter u, an equilibrium constant for the aggregation of two ligand molecules bound by the sorbent. The whole binding process is described by the overall stability constant for the binding, K. The very complex equation describing the binding process is difficult to solve. Actually, K is not a constant, but a function of β (the molar ratio of the bound surfactant to the total amino groups of the gel).24 Nevertheless, at half saturation of binding (β ) 0.5), the relationships for the binding parameters can be obtained as shown in the equations12,17-19
K ) K0u )
(
dβ d ln Ceq
)
( ) 1 Ceq
β)0.5
(2)
β)0.5
)
xu 4
(3)
According to eq 2, K is the reciprocal of the free bile salt concentration at half saturation. The value of u can be calculated from the slope of the binding isotherm at the half-binding point (eq 3). Even if the nearest-neighbor model is an oversimplification and the calculations involve rather large errors in the u values,5,12 it can provide a semiquantitative evaluation of the binding parameters. The isotherms for binding of NaCA by dimethylalkylammonium hydrogels (DMRA) are presented in Figure 3. The isotherms are plotted as the degree of neutralization, β, versus free bile acid concentration at equilibrium (Ceq). The presented isotherms (as well as those obtained for the binding of all bile acids under study) are mainly sigmoidal, both in water and in 10 mM NaCl solutions. The concentration at which the binding starts depends on many factors, but in all cases the isotherms level off near β ) 1.0. The sigmoidal form of the semilogarithmic plots indicates a cooperativity of the binding process. To evaluate the influence of the system component characteristics on the binding process, eqs 2 and 3 were used for the calculation of the binding parameters K, K0, and u. Influence of the Polyelectrolyte Hydrophobicity. The values of calculated parameters for the bile salt binding in water are presented in Figure 4 as a function of the R substituent chain length. These plots clearly show an influence of the substituent R chain length on the binding process. In all cases under study, the binding constants K and K0 increase with increasing R chain length. However, the cooperativity parameter u remains relatively constant (with a slight variation from 1.5 to 2.5) in water. The increase in binding constants with increasing hydrophobicity of the polymer has been reported for other systems of linear polyelectrolytes-oppositely charged surfactants. Thus, the constant for the binding of C12TAB to polystyrenesulfonate (PSS) was several times higher than in case of the more hydrophilic dextran sulfate (Dex-S).40 Benrrau et al.12 have found that in the systems containing poly(maleic anhydride-co-alkylvinyl ether) (MA-co-VAE) and cationic surfactants, the binding constants increase also with increasing alkyl
Interaction of Dextran Hydrogels and Surfactants
J. Phys. Chem. B, Vol. 105, No. 12, 2001 2317
Figure 3. Isotherms for the binding of sodium cholate (NaCA) on DMRA hydrogels from pure water solutions (a) and from aqueous 10 mM NaCl solutions (b). R: ethyl (squares), butyl (circles), octyl (up triangles), and dodecyl (down triangles). The dotted lines are the best nonlinear least squares fits for the experimental data.
chain length of vinyl ether comonomer. In both cases the increase in binding constants has been explained by the interactions between the hydrophobic parts of the polymer and surfactants. The chemical structure of these polyelectrolytes is very different from that of cationic hydrogels under study, and the strength and type of interaction could be different. PSS has a hydrophobic backbone, with hydrophobicity close to a butyl chain, by analogy with the surfactants where the hydrophobic contribution of a phenyl ring is approximately equivalent to that of 3.5 methylene groups.40,41 Poly(MA-co-VAE)s are alternating copolymers of a hydrophilic monomer and a hydrophobic monomer, with a large content in hydrophobic moieties.12,42 Nevertheless, we can also assign the increase in ionization and stability constants with increasing polyelectrolyte hydrophobicity to hydrophobic interactions between polyelectrolyte and surfactant. They have as a result the formation of mixed micelles between pendant groups, which can be perceived as cationic surfactants, and the bile acid anions. Mixed micelles formed between C14TMAB31 or C16TMAB32 and bile salts have cmc values (as determined by conductometric titration and surface tension measurements) much lower than the cmc values of individual surfactants. This was assigned to the ion pair formation at very low surfactant concentrations. The high rate of formation and strength of interaction of similar ion pairs might explain the high K0 and K for the interaction of bile salts with hydrophobically modified dextran hydrogels. It has been already reported that the variation of u values with polymer hydrophobicity depends strongly on the chemical structure of the polymer.12,38,40 The cooperativity of surfactant binding by cross-linked polyelectrolytes depends on the osmotic pressure of the system and the network flexibility. The theoreti-
Figure 4. Overall stability constants (a), ionization constants (b), and cooperativity parameters (c) for the binding of bile acid salts by DMRA hydrogels from water solutions as a function of the length of R substituent. NaGCA (squares), NaCA (circles), NaDCA (triangles). Dotted lines serve as a visual guide.
cal investigation of Khokhlov23 and Gong-Osada24 indicated that the binding of the surfactant reduces the osmotic pressure which in turn reduces further binding, consequently reducing the cooperativity. An elastic network will respond by contraction, which in turn reduces the elastic free energy and maintains the osmotic pressure, thus promoting the cooperativity of the binding (u > 1). In a system containing a completely rigid network, the binding process will be noncooperative (u ) 1) or even anticooperative (u < 1). The increase in polyelectrolyte hydrophobicity can also results in a dramatic decrease in cooperativity. This has been demonstrated in the case of MAco-VAE linear polymers-cationic surfactant systems,12 for which an anticooperative effect has been found when the alkyl chain length increased above C10. The large content in hydrophobic moieties or these polymers leads to the formation of microdomains in which the surfactant molecules are preferentially included.42 The cooperativity parameter for the binding of bile salts by DMRA hydrogels in water is low, but is always higher than 1 (Figure 4c) as clearly shown by the shape of the binding isotherms. The results we obtained for u values can support
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Nichifor et al.
Figure 5. Water swelling capacity of DMRA dextran hydrogels as a function of degree of substitution with N,N-dimethyl-N-R quaternary ammonium groups. R: ethyl (diamonds), butyl (squares), octyl (triangles), and dodecyl (circles).
our hypothesis of the lack of significant interactions among the statistically distributed pendant groups of the hydrogel with no significant hydrophobic microdomain formation. This is suggested also by the data presented in Figure 5 where, in the range of the substitution degree of 20-25 mol % no significant decrease in water uptake of the gels occurs. A hydrophobic interaction between the alkyl R chains should result in a decrease in polymer hydration and a lowering of Rw0 values. Consequently, we can assume that mainly electrostatic and hydrophobic interactions between surfactant and pendant groups occur in water, with mixed micelle formation, together with a very low cooperative aggregation of bound surfactants. The low u values can be assigned also to the intrinsic low aggregation numbers of bile acids in water (2-4 molecules per aggregate30) or to the structure of mixed micelles, formed by the interaction of very dissimilar hydrophobic moieties of components (alkyl and steroid). As has been already shown for the systems of C16TMAB and bile salts, the initial aggregation as ionic pairs takes place very quickly (and this might explain the high K and K0 values) but further aggregation occurs only at very high concentrations of both surfactants.32 Influence of the Ionic Strength. Figure 6a,b shows that the K and K0 values for binding of the bile acid from 10 mM NaCl solutions increase also with R. The semilogarithmic plots included as insets indicate a certain leveling off of these values for R g C8. However, the cooperativity parameter is no longer constant but decreases with R (Figure 6c). An enhanced cooperativity due to increased ionic strength for the binding of bile salts to hydrophilic cationic hydrogels could be associated with the decrease in the osmotic pressure of the gel.18 In case of the bile salt binding by the more hydrophobic hydrogels, especially by DMDodA, the u values are very low, but still g1. This means that the process does not become anticooperative. The influence of the concentration of the electrolyte NaCl is exemplified in Figure 7 for the NaDCA/DMRA systems. No significant variation of binding parameter occurred at salt concentrations higher than 20 mM (data not shown). Nevertheless, below this salt concentration, the change in binding parameters is visible, especially in the case of the more hydrophilic hydrogels, DMEtA and DMBuA. In the systems involving these hydrogels the decreases in K and K0 as well as the increase in u values are very sharp. Instead, the binding
Figure 6. Overall stability constants (a), ionization constants (b), and cooperativity parameters (c) for the binding of bile acid salts by DMRA hydrogels from aqueous 10 mM NaCl solutions as a function of the length of R substituent. NaGCA (squares), NaCA (circles), NaDCA (triangles). The insets show the same data on a logarithmic y-axis. Dotted lines serve as a visual guide.
constants are less influenced by ionic strength in the case of hydrophobic hydrogels, DMOctA and DMDodA, and the cooperativity parameter is low even at salt concentration of 20 mM or higher. It seems that the salt concentration has a greater effect on the self-aggregation of bile salts inside the hydrophilic gels than on the formation of mixed micelles between the surfactant and pendant groups of polymer. There has been no clear reported influence of the ionic strength on the cooperativity of the binding of surfactants by hydrophobic linear polymers. The u parameter was found independent of the ionic strength in the binding of cationic surfactants by PSS,40 but a decrease in u with increasing ionic strength has been found for the systems involving more hydrophobic poly(MA-co-VAE).12 From the present results we can conclude that in the systems containing hydrogels with alkyl substituents lower than C8 the interactions are predominantly electrostatic, and aggregation occurs mainly between bound surfactant molecules. When R is longer, the hydrophobic interactions between polyelectrolyte and surfactant prevail over the electrostatic ones, and aggregation occurs mainly via mixed micelle formation.
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J. Phys. Chem. B, Vol. 105, No. 12, 2001 2319
Figure 8. Variation of critical aggregation concentration (cac) of NaCA (triangles) and NaDCA (squares) with the R alkyl chain length of watersoluble cationic dextran (DMRA) with the same chemical structure depicted in Figure 1, except for the lack of cross-linking. Polymer concentration: 1 mM as cationic groups.
NaCA (Figure 8). Less obvious is the explanation for the very low cooperativity parameter calculated for NaDCA binding on DMOctA and DMDodA hydrogels, very close to the values found for the trihydroxylic acids, in both water and salt solutions. This is in contradiction with large aggregates formed by dihydroxylic bile salts with C14TMAB.31 It seems that the binding process in NaDCA/DMDodA system is governed by the strong hydrophobic interactions between R and steroid nucleus, with formation of single ionic pairs.
Figure 7. Overall stability constants (a), ionization constants (b), and cooperativity parameters (c) for the binding of sodium deoxycholate by DMRA hydrogels from aqueous solutions as a function of NaCl concentration. R: ethyl (squares), butyl (circles), octyl (up triangles), and dodecyl (down triangles). Dotted lines serve as a visual guide.
Influence of the Surfactant Hydrophobicity. The three bile acids used in this study have different hydrophobicity, which increases in the order NaGCA < NaCA < NaDCA. The cmc values reported for the used bile salts are not consistent,30,32 but they can be estimated to be about 10 mM (NaGCA), 9 mM (NaCA), and 2.75 mM (NaDCA), respectively. The most striking difference in the binding behavior is among the trihydroxylic acids (NaGCA and NaCA) and the dihydroxylic NaDCA, and this difference is supported by the corresponding binding parameters (Figures 4 and 6). The values obtained for binding constants are very close for NaGCA and NaCA, but those for NaDCA are much higher under all the conditions studied, and more influenced by the R length. This finding can also be explained by the strong hydrophobic interactions between hydrogels and surfactants occurring in these systems. The results are in agreement with the cac values determined by fluorescence measurements in the presence of water-soluble cationic dextrans with the same chemical structure as hydrogels I. These values decreased with increasing R length for both sodium cholate and sodium deoxycholate, but the values for NaDCA were always at least 2 times lower than those for
Swelling Behavior. The shrinkage of a network hydrogel has been explained by the neutralization of the charged groups of the gel with the oppositely charged surfactant anions, which leads to (a) the displacement of the small counterions by partially immobilized surfactant ions, which results in a decrease in the osmotic pressure difference between the interior of the gel and the surrounding medium,17,20 and (b) the aggregation of the surfactants inside the hydrogel, accompanied by a loss in hydration and eventually the collapse of the network.20 There are two approaches to explain the volume change of ionic networks in the presence of surfactants. Khokhlov23 and Gong and Osada24 restricted their treatment to less flexible polyelectrolytes where the interaction with surfactant micelles promotes the shrinkage of the whole network. Accordingly, a sharp volume change will occur when the cac of the surfactant inside the gel is reached. Hansson’s model has been based on the microscopic perturbation of the network in the vicinity of each micelle. This shortrange interaction would promote a linear volume change as a function of β.22,26 We have followed the swelling in water at equilibrium at different degrees of sodium cholate binding. As a reference state, we have chosen the water uptake of the gel in equilibrium with pure water (Rw0). The collapse of the gel was characterized by the ratio Rwβ/Rw0. As Figure 9 shows, the contraction of the DMBuA hydrogel as a function of charge neutralization with cholate anion is monotonic and does not exceed 50% of the initial water uptake at β ≈ 1, both in water and in 10 mM NaCl. The decrease in the water uptake with increasing β is almost linear, as predicted by Hanson’s model.22,26 Despite the clear difference in binding behavior in pure water and in the presence of NaCl, the variation of the water content inside the gel with β is identical, indicating
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Nichifor et al. strength has a marked influence on the binding process when R < C8, but the effect is much lower when R > C8. The values of binding parameters and swelling behavior suggest different binding processes for different R chain length. In the case of shorter R the binding is predominantly electrostatic, and the aggregation occurs mainly between surfactant molecules inside the gel being enhanced by increasing ionic strength. When R is longer than C4, the binding is governed by strong hydrophobic interactions between substituent R and surfactant molecules, and aggregation occurs via mixed micelle formation. The length of R for a transition from one behavior to the other depends also on the bile salt hydrophobicity, being about C8 for trihydroxylic acids and C4 for dihydroxylic ones.
Figure 9. Variation of swelling capacity of DMRA-NaCA complexes with degree of binding, β, from water solutions (solid symbols, solid lines) and 10 mM NaCl solutions (open symbols, dotted lines). R: butyl (squares), octyl (triangles), and dodecyl (circles).
TABLE 1: Characteristics of Dextran Hydrogels with Structure I b dextran hydrogel content of amino groups solvent uptake, Rw (g/g) a (DMRA) mequiv/g mol % water 10 mM NaCl
DMEtA DMBuA DMOctA DMDodA
0.9255 1.1631 1.0042 0.9941
20 24 22 23
3.861 3.692 3.330 3.850
3.746 3.726 2.987 1.900
a Moles of amino groups/100 glucopyranosidic units: 100n/(m + n + p). b Amount of water or aqueous 10 mM NaCl solution retained at equilibrium per gram of dry gel.
a similar structure of ionic complex formed inside this rather hydrophilic gel. DMOctA and DMDodA hydrogels shrink faster with increasing β, and display a nonlinear variation with β. The volume change is still rather monotonic and tends to level off at around 30% of the initial value. The hydrophobic hydrogelbile salt complexes shrink more in the presence of the salt than in water. It is worth mentioning that the swelling of the hydrogels in 10 mM NaCl solutions (Table 1) is very close to that obtained for the same hydrogels in pure water, except for DMDodA. In the case of DMDodA we can assume that the increase in ionic strength reduces the electrostatic repulsion between the ionic groups and enhances the hydrophobic interaction between C12 alkyl chains, having as a result the collapse of the gel to 60% of its original volume in water. Even in this case, the gel does not collapse completely at β ≈ 1. The difference in the swelling behavior of the hydrogels can account for the strength of the immobilization of the surfactant on a charged site of the gel. This strength increases with increase in R length. Therefore, the equilibrium swelling dependence on R matches the variation of the binding constant with the same parameter. Conclusion The binding of bile salts to functionalized dextran hydrogels bearing pendant N,N-dimethyl-N-R ammonium chloride groups (where R is a C2-C12 alkyl chain) is characterized by a strong influence of the substituent R length on the binding parameters. The rate of initial binding and strength of the binding (expressed as the binding constants K0 and K, respectively) increase and the cooperativity of the binding (expressed as parameter u) decreases with increase in the length of R substituent. The ionic
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