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Interactions in mixed cationic surfactants and dextran sulfate aqueous solutions
Journal of Colloid and Interface Science 285 (2005) 342–350 www.elsevier.com/locate/jcis
Interactions in mixed cationic surfactants and dextran sulfate aqueous solutions V. Tomaši´c a,∗ , A. Tomaši´c b , I. Šmit c , N. Filipovi´c-Vincekovi´c a a Department of Physical Chemistry, Ruder ¯ Boškovi´c Institute, P.O. Box 180, HR-10002 Zagreb, Croatia b Division of Materials Chemistry, Ruder ¯ Boškovi´c Institute, Zagreb, Croatia c Division of Molecular Biology, Ruder ¯ Boškovi´c Institute, Zagreb, Croatia
Received 20 May 2004; accepted 29 November 2004 Available online 29 January 2005
Abstract The interactions between a hydrophilic anionic polysaccharide, dextran sulfate, and oppositely charged surfactants, n-alkylammonium chlorides (the number of carbon atoms per chain being 10, 12, and 14), were investigated by optical microscopy, X-ray diffraction, microelectrophoretic mobility, conductivity, surface tension, and light-scattering measurements at 303 K. The increase of surfactant alkyl chain length shifts both the critical aggregation (cac) and the critical micelle concentrations (cmc) toward lower surfactant concentration. Light-scattering and microelectrophoretic data revealed the coexistence of differently structured complexes beyond the cac. The presence of giant vesicles indicates that at least one type of species is ordered in bilayers. X-ray analysis of dry n-alkylammonium dextran sulfates exhibited mesomorphous ordering and interplanar spacings typical for lamellar structures; i.e., n-alkylammonium molecules form more or less disordered bilayers interconnected with dextran sulfate chains, thus forming multilamellar stacks. The average basic lamellar thickness increased linearly with the increase of surfactant chain length, whereas the average number of lamellar bilayers in the stack of lamellae decreases. 2004 Elsevier Inc. All rights reserved. Keywords: n-Alkylammonium chlorides; n-Alkylammonium dextran sulfates; Dextran sulfate; Giant vesicles; Lamellar structures
1. Introduction A lot of knowledge has been accumulated about complex interactions in aqueous mixtures of various surfactants and polyelectrolytes, because of their fundamental and practical importance (pharmaceutical formulations, food products, detergents, etc.). The extent of interaction between a polyelectrolyte and a surfactant depends on many factors, which include characteristic features of the polyelectrolyte and the surfactant, concentration and molar ratio of components, ionic strength, nature of solvents, temperature, etc. [1–25]. It is convenient for these interactions to classified according
* Corresponding author.
E-mail address: [email protected] (V. Tomaši´c). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.11.063
to surfactant and polyelectrolyte charges and/or according to their concentrations. The interaction between an ionic surfactant and an oppositely charged polyelectrolyte is a highly cooperative process, which starts at extremely low concentrations of both components. The multistep mechanisms of interaction include successive electrostatic, hydrophobic, and intra- and interpolyelectrolyte interactions, resulting in a formation of variety of single and coexisting phases, from differently ordered polyelectrolyte–surfactant monomer complexes to the stoichiometric precipitate and gel phase [22]. At low and fixed polyelectrolyte concentrations and with increasing surfactant concentration, three ranges of interaction can be distinguished. The first range corresponds to low surfactant concentration, well below the surfactant critical micelle concentration (cmc) in water. In this range, the electrostatic binding of surfactant on the polyelectrolyte chain
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has been the prevailing process, which leads to formation of polyelectrolyte–surfactant complexes. The second range is characterized by prevailing hydrophobic interactions between surfactant alkyl chains and formation of micelle-like aggregates on the polyelectrolyte chains, whereas the third range extends from the cmc to higher surfactant concentrations, with prevailing interactions between micelles and a polyelectrolyte. Basically, two important concentrations of surfactant can be distinguished: (i) the concentration needed to measure significant hydrophobic interactions between surfactant alkyl chains, known as the critical aggregation concentration (cac), and (ii) the concentration at which the surfactant micelles are being formed, known as the critical micelle concentration (cmc). In mixtures of oppositely charged polyelectrolytes and surfactants as systems of strong interactions, the cac value may be several orders of magnitude smaller than the cmc value. The most important characteristics of surfactant affecting the interactions are the hydrophobicity, rigidity, and bulkiness of the hydrophobic part of the molecule [26] and the type and position of surfactant head group [27]. Investigations of the surfactant chain length dependence on surfactants binding to dextran sulfate revealed a highly cooperative character of binding due to a large effect of the hydrophobic interactions between bound surfactants [16]. It has been shown that the free energy of binding [16], the concentration region where the binding proceeded [8,18,28,29], and the structure of polylelectrolyte–surfactant complexes [30, 31] strongly depend on the length of the alkyl chain. The increase of carbon atoms in the surfactant chains enhances hydrophobic association, shifts the cac toward lower surfactant concentrations, and change the structure of complexes, both in the bulk solution and in the solid state [29, 31,32]. Nevertheless a large number of new methods and applied models and a better understanding of the microscopic structure of the aggregates and the role of hydrophobic interactions may be needed. There is a wide range of open questions: binding mechanism, structural and specific binding forces, size and morphology of the aggregates formed, incorporation of polymers into liquid crystalline phases, etc. In this work, we examine the effect of the chain length of cationic surfactants, n-alkylammonium chlorides (RNH3 Cl), on the nature and strength of interactions with an oppositely charged hydrophilic polyelectrolyte, sodium dextran sulfate (DxS). The first two parts of the paper have focused on interactions at extremely low DxS concentration and increasing surfactant concentration without concomitant phase separation. The third part has been concerned with structural properties of stoichiometric solid phase, which exhibited lamellar structure with different degree of ordering. Investigations performed in dilute solutions revealed formation of giant vesicles as an indication of lamellar ordering from low to high polyelectrolyte concentrations.
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2. Experimental section 2.1. Materials and methods The preparation and purification of n-alkylammonium chlorides, general formula RNH3 Cl, with R = Cn H2n + 1 and nC = 10, 12, and 14, i.e., decylammonium chloride (DACl), dodecylammonium chloride (DDACl), and tetradecylammonium chloride (TDACl), were described earlier [33]. Their purity was checked by elemental analysis and surface tension measurements. The sodium salt of dextran sulfate (Pharmacia LKB, Uppsala, Sweden; average molecular weight Mw ≈ 5 × 105 ) was used without further purification. The reported sulfur content in ester sulfate is 17%, which corresponds to an average of 1.8 sulfonate groups per glycoside unit (Mw ≈ 380); i.e., the degree of substitution is 2, and the polymerization number n ≈ 1315. The number of sulfate groups of the present sample was evaluated to be about 2440 per DxS molecule; therefore c(DxS)/mol dm−3 = 1 × 10−10 , 4 × 10−10 , 1 × 10−9 , 4 × 10−9 , and 1 × 10−8 are equivalent to c(OSO3 )/mol dm−3 = 2.44 × 10−7 , 9.76 × 10−7 , 2.44 × 10−6 , 9.76 × 10−6 , and 2.44 × 10−5 , respectively. Stock solutions were prepared by weight using redistilled and filtered water (through Millipore Type HA 0.10 µm, to exclude dust before use). Systems were prepared with a constant DxS concentration and increasing concentrations of RNH3 Cl. They were stirred for 5 min and left standing for equilibration at 303 K for 1 day. During a 7-day observation period the systems did not show significant changes in measured physicochemical properties. Detection and microstructural identification of different phases were performed by optical microscopy (Model DMLS equipped with a video camera, Leica), tensiometry (K8 interfacial tensiometer, Krüss), conductometry (MPC 227 conductivity meter, Mettler Toledo), microelectrophoretic and light-scattering measurements (Zetasizer Nano ZS, Malvern), and X-ray diffraction by a Philips diffractometer with monochromatized CuKα radiation. 2.2. Interpretation of data The cac values were determined by the usual procedure from the specific conductivity (κ) versus concentration (c) plots [22] and by microelectrophoretic measurements as a concentration where change of particle charge occurred. The cmc values were determined from both the surface tension and the specific conductivity measurements as the surfactant concentration at the intersection of the two linear sections of γ or κ versus c curves by the method of nonlinear least mean squares of up to fourth order with fitting correlation coefficients equal or better than 0.999. The apparent degree of counterion dissociation from the micelle/solution interface (α) was determined from the conductivity measurements. To a first approximation, α was
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taken as a ratio of the slope of κ vs c lines above and below the cmc [34]. The maximum surface excess of surfactant molecule (Γmax ) at the air/solution interface as maximum adsorption density at the cmc was determined from the premicellar slopes of the γ vs log c(RNH3 Cl) curves by the Gibbs adsorption equation [35]. The minimum area occupied by surfactant molecule at the air/solution interface (Amin ) represents the effective area per molecule at the cmc and can be estimated from Γmax [22]. The average basic lamellar thickness (d/Å) was calculated from the first peak on the diffractograms according to the Bragg equation and the mean crystallite size in the [001] direction, L001 , was calculated by the Scherrer equation [36].
3. Results and discussion 3.1. Air/solution interface properties Fig. 1 shows the surface tension isotherms for DACl in the absence and presence of different DxS concentrations. The surface tension isotherm for pure DACl solution displays the decrease of surface tension up to the cmc and the constant value in micellar region. The surface tension with DxS added begins to drop at a concentration much lower than that with a pure DACl solution, indicating that the interactions began at very low concentrations. A strong lowering of surface tension has been found for all DACl/DxS mixtures. Systems with DxS added revealed two types of curves; at c(DxS) 1 × 10−9 mol dm−3 , curves with a minimum below the cmc are obtained, whereas curves with two plateaus appear at higher c(DxS). These behaviors are undoubtedly concerned with the changing of the composition of the air/solution interface and in the bulk solution. Below the cac and at very low and fixed DxS concentration, the adsorption of surfactant at the air/solution interface is enhanced due to formation of highly surface active
Fig. 1. Changes of surface tension (γ ) with c(DACl) in mixtures with constant c(DxS)/mol dm−3 = (") 0, (×) 1 × 10−10 , (E) 4 × 10−10 , (!) 1 × 10−9 , (P) 4 × 10−9 , (e) 1 × 10−8 . Temperature is 303 K.
complexes, the thickness of which increases with increasing surfactant concentration [37]. In bulk solution, however, the surfactant molecules are present as monomers, although some association with DxS can occur due to ion exchange of sodium ions by surfactant cations. Beyond the cac, the situation in bulk solution and at the air/solution interface changes drastically. The formation of micelle-like aggregates on DxS chains leads to a continuous desorption of the surface polyelectrolyte–surfactant complexes at the surface. Because of the removal of the DxS from the air/solution interface, the equilibrium between surfactant monomers at the air/solution interface and monomers in solution is shifted toward the bulk solution [38]. This explains the increase in surface tension close to the values of pure DACl solution. The appearance of two plateaus at higher DxS concentration may be interpreted as follows: at higher DxS concentration, there are sufficient DxS chains to form surface and bulk polyelectrolyte–surfactant complexes. With increasing surfactant concentration, the bulk polyelectrolyte– surfactant complexes continuously become more populated with surfactant molecules, while the surface polyelectrolyte– surfactant complex remains at the air/solution interface. Because of this there is no change in surface tension value. As the polyelectrolyte is saturated with surfactant, the surfactant monomer concentration and the activity start to increase again and there is a lowering of surface tension until the monomer concentration reaches the cmc. Once micelles of surfactant appear in solution, the surface tension becomes essentially equal to that of a polyelectrolyte free surfactant solution. New information accessible through X-ray and neutron reflectivity measurements show that several polyelectrolyte– surfactant complexes are being formed at the air/solution interface at surfactant concentrations lower than that seen in the bulk phase [39–41]. The polyelectrolyte configurations in the bulk and at the air/solution interface are very different and no doubt the complexes formed in the bulk phase differ from those formed at the surface. There are several kinds of the polyelectrolyte–surfactant complexes: monolayer and layered polyelectrolyte–surfactant complexes at the air/solution interface and polyelectrolyte– surfactant complexes in the bulk solution. While highly surface active complexes form at the air/solution interface, there are at least two kinds of bulk complexes, the first below the cac and the second between the cac and the cmc. Fig. 2 shows the surface tension data of RNH3 Cl/DxS mixtures. With the increase of the alkyl chain length all surface tension isotherms are shifted toward lower values. The addition of DxS affected neither the cmc not the surface tension at the cmc. All premicellar slopes of surface tension isotherms display a gradual increase with the DxS concentration, indicating an enhancement of the maximum excess surface concentration and a decrease in the minimum area per surfactant molecule at the air/solution interface with a DxS addition (Figs. 3a and 3b).
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Fig. 2. Changes of surface tension (γ ) with c(RNH3 Cl) in mixtures at constant c(DxS)/mol dm−3 = 1 × 10−9 : (!) DACl, (P) DDACl, and (1) TDACl. Temperature is 303 K.
Fig. 4. Specific conductivity (κ) vs c(DACl) at constant c(DxS)/ mol dm−3 = 1 × 10−9 at (a) lower and (b) higher c(DACl), at 303 K.
itively charged surfactant head groups due to the neutralization via electrostatic interactions with –OSO− 3 groups of DxS [42]. 3.2. Bulk properties
Fig. 3. (a) Surface excess concentration (Γmax ) and (b) minimum area per surfactant molecule (Amin ) at the air/solution interface vs c(DxS): (!) DACl, (P) DDACl, (1) TDACl. Temperature is 303 K.
The increase in Γmax and decrease in Amin are caused by different packing of surfactants monomers at the air/solution interface in the presence of polyelectrolyte ion in the subphase and by lowered electrostatic repulsion between the surfactant head groups, as similarly obtained in the systems with ι-carrageenan [22]. The condensation of the cationic surfactant monolayer obtained upon the DxS addition can be attributed to a decrease in repulsive forces between pos-
Fig. 4 shows typical change of specific conductivity as a function of DACl concentration with constant c(DxS). All measured κ values were lower than those obtained in the absence of DxS, thus indicating that the interactions between the components start at levels below 1 × 10−5 mol dm−3 of surfactants. In pure RNH3 Cl solutions, the κ vs c curves show typical break at the cmc, whereas those with the addition of DxS show two breaks: the first break in the premicellar concentration range (corresponding to the cac), and the second one in the vicinity of the cmcRNH3 Cl . In single-surfactant solutions, the increase of the number of carbon atoms leads to the decrease of the cmc according to the equation log cmc = −0.30nC + 1.71. The addition of DxS has not significantly affected the cmc values of all surfactants; i.e., the cmc values were almost the same as in the single surfactant solutions (Table 1). The cac values were found to be independent of DxS concentration. This is in good accordance with the literature data, which have established that the cac depends on the polyelectrolyte concentration only weakly [43].
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Table 1 Critical aggregation concentration (cac/mol dm−3 ), critical micelle concentration (cmc/mol dm−3 ), cac/cmc ratio, and reduction in free energy of micelles when bound to DxS (G/kJ mol−1 ) for n-alkylammonium chloride/dextran sulfate mixtures (temperature is 303 K) System
cac × 104
cmc × 102
cac/cmc × 102
G
DACl + DxS DDACl + DxS TDACl + DxS
25.0 2.20 0.30
5.6 1.50 0.38
4.46 1.47 0.79
−7.81 −10.57 −12.19
As the cmc, the cac decreases with increasing the chain length; in other words; the longer the hydrocarbon chain of the surfactant, the lower the cac (Table 1). An increase in amphiphile hydrophobicity results in an increase in the cooperativity of the interaction and in a stronger decrease of the cac. The chain length dependence of the cac obeys the equation log cac = −0.50nC + 2.33.
(1)
A slope similar to those obtained for log cmc vs nC indicates that the free energies of the surfactant methylene group transfer from the aqueous phase to a free micelle or to a bound aggregate are almost equal. This behavior is similar to what was observed by Hayakawa and co-workers in the binding of ionic surfactants by polyions of opposite charge using surfactant selective electrodes [4,15–17]. They have found that the overall binding constant increases with increasing surfactant chain length; a linear relationship between overall binding constant and surfactant chain length has been observed in various systems of surfactant cations and polyanions. From the slope, the difference in the free energy of surfactant binding for each CH2 group added to surfactant chains is estimated to be from −1.10 to −1.32 kT. This free energy change is close to the value of the free energy transfer of one CH2 group from an aqueous phase to the micellar phase (−1.20 to −1.38 kT) [44]. Wallin and Linse related the cac/cmc ratio to the free energy of complexation between a micelle and a polyelectrolyte in dilute solution [45]. Data presented in Table 1 show that the cac/cmc ratio decreases in parallel with the growth of the surfactant chain length, thus indicating that the cac reduction is faster than that of the cmc at increasing surfactant tail length. Two important aspects of the difference between the cmc and cac values are as follows: (i) a strong reduction of the electrostatic energy of the system and (ii) a release of counterions of the micelle and the polyelectrolyte. With the increase in tail length, the decrease in the electrostatic energy dominates the decrease in the complexation energy and the release of counterions becomes more important by enhancing the increase in entropy. Sirivat and co-workers [46] showed that the reduction in the Gibbs free energy of micelles, if polyelectrolyte-bound, can be expressed as G = RT ln[cac/cmc].
Fig. 5. Variation of the apparent degree of counterion dissociation (α) from the micelle/solution interface as a function of alkyl chain carbon number (nC ) in pure surfactant solutions (!) and in RNH3 Cl/DxS mixtures with constant c(DxS)/mol dm−3 = (P) 1 × 10−9 , (e) 4 × 10−9 , and (E) 1 × 10−8 . Temperature is 303 K.
(2)
The reduction in the free energy of polyelectrolyte-bound micelles listed in Table 1 exhibits an increase that indicates that the growth of surfactant chain length increases the stability of polyelectrolyte–surfactant complexes. In a solution where different types of counterions coexist, the counterion binding to the micelle/solution interface depends on the mol fraction and the type of each single ion in the bulk. Priority of binding displays counterions with higher valency, so a micelle formed in a polyelectrolyte solution binds the polyanion to its micelle/solution interface. The conductivity measurements showed an increase of the apparent degree of counterion dissociation from RNH3 Cl micelles upon the addition of DxS. Thus, the replacement of chloride counterions at the micelle/solution interface with DxS has been indicated; in other words, the micelle formed in a polyelectrolyte solution bound the oppositely charged polyanion to its surface. Fig. 5 shows that the α-values decrease with nC ; i.e., the counterion binding increases and diminishes the effective surface charge density at the surface of micelles. For all mixtures, the variation of α with nC can be approximated by the linear relation α = −0.02nC + B,
(3)
where B is a constant, being 0.53, 0.54, 0.57, and 0.58 for c(DxS)/mol dm−3 = 0, 1 × 10−9 , 4 × 10−9 and 1 × 10−8 , respectively. It seems that the B constant showing its dependence on the DxS concentration in Eq. (8) may be ascribed mainly to the electrostatic contributions, whereas the rest involves contributions governed mainly by surfactant hydrophobicity. The charge changes of the polyelectrolyte caused by surfactant binding can be followed by measuring the microelectrophoretic mobility. The microelectrophoretic mobility of DxS–surfactant complexes is shown as a function of surfactant concentration (Fig. 6). The microelectrophoretic mobility decreases with growth of the surfactant concentration. The reduced microelectrophoretic mobility indicates
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Fig. 6. Changes of the microelectrophoretic mobility (u± ) of particles vs c(RNH3 Cl) at constant c(DxS)/mol dm−3 = 1 × 10−9 : (! and ") DACl, (P and Q) DDACl, and (1 and 2) TDACl. Temperature is 303 K.
the decrease of the overall particle charge due to the electrostatic binding of ammonium head groups of surfactants to the anionic sulfate sites along the polyelectrolyte chain. When surfactant concentration reaches a critical aggregation value, DxS–surfactant complexes crossed zero mobility and become positively charged. This concentration coincides with the cac determined by conductivity measurements. The longer the hydrocarbon chain of the surfactant, the lower the surfactant concentration required to reach the zero mobility. The adsorption of surfactants at concentrations lower than the cac is considered to be mainly due to the ionic attraction, whereas for concentrations above the cac the surfactant monomer starts to bind cooperatively to the DxS chain through hydrophobic interactions. This process brought excessive positive charges to bear on the particles. Two populations of positively charged complexes were detected beyond the cac; the first one showing pronounced positive mobility and significant increase of mobility with surfactant concentration (empty signs in Fig. 6), and the second one showing the population with much lower mobility (filled signs in Fig. 6), which increased slightly with surfactant concentration. This finding is in accordance with the results of Shirahama and co-workers [47], who have found two kinds of migrating species in the DxS–dodecylpyridinium bromide system. They explained these results by the formation of complexes differing in the amount of adsorbed surfactants. In contrast, only one kind of complex species found for DxS–alkyltrimethylammonium bromide systems has been explained by the difference in the length of DxS and/or in nature of ligands used [28]. The variation of the mean particle size (D) with the increasing surfactant concentrations is illustrated in Fig. 7. At the lowest surfactant concentration, the head groups of RNH3 Cl bind to sulfate groups of DxS to form surfactant/DxS complex, reducing the net charge, and to weaken the electrostatic repulsion on the polyelectrolyte chains. Slight reducing of polyelectrolyte chains length is compensated for by increasing the molar mass of complex due to the
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Fig. 7. Changes of the mean particle size (D) in mixtures with c(RNH3 Cl) at constant c(DxS)/mol dm−3 = 1 × 10−9 : (! and ") DACl, (P and Q) DDACl, and (1 and 2) TDACl. Temperature is 303 K.
bound surfactant monomers, leading to an increase in the mean particle size. The first particles, which appear upon surfactant addition, exhibit the mean particle size close of 16 nm, which is close to that of polyelectrolyte itself (the DxS molecules in water resemble a rod with the radii of the cylindrical macroion a ≈ 0.4 nm [6], with hydrodynamic radius Rh = 15.8 nm [48]). Therefore it is reasonable to attribute it to an intrapolymer complex. The mean particle size increases with surfactant concentration until it reaches the cac, at level of which two differently sized populations appear. Thus, the particle size distributions broaden and become bimodal beyond the concentration corresponding to the cac. Bimodal distribution of mean particle sizes is in accordance with the microelectrophoretic data. With an increase of the chain length, the mean particle sizes of both populations increased, but in different ways. The particles with larger D continuously increased in the size with an increase in surfactant concentration and exhibited rather broad size distributions (empty signs in Fig. 7), whereas the particles with lower D were almost monodisperse (filled signs in Fig. 7). All samples were inspected between crossed polarizers in order to detect optically anisotropic phases. Microphotographs of samples taken from characteristic concentration regions (between the cac and the cmc and beyond the cmc) reveal the formation of giant vesicles with dimensions of several micrometers. Appearance of spherical vesicles was confirmed by characteristic Maltese crosses under crossed polarizers. By light-scattering techniques alone, it has not been possible to determine neither the shape nor the structure of polyelectrolyte–surfactant complex. However, optical microscopy undoubtedly shows that at least one kind of aggregates resembles a bilayer structure. As suggested by microelectrophoretic and light-scattering measurements, the organization of DxS–surfactant complex structure beyond the cac proceeded in two directions. It seems that in investigated mixtures, both inter- and intrachain hydrophobic aggregation of alkyl chains should be taken into account. It may be deduced that complexes of
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higher size are interpolymeric complexes; i.e., hydrophobic interactions between surfactant alkyl chains occurred between two surfactant–polyelectrolyte complexes. Consequently, open bilayers of alkyl chains with attached DxS chains in the head group regions have been formed. Increasing surfactant concentration promotes growth of bilayers, enhances positive charge and above all transforms open bilayers to the closed ones, i.e., vesicles. On the contrary, less charged and smaller particles represent intrapolymeric complexes with only slight excess of bound surfactant (with respect to the charge).
characteristic of a disordered liquid-like conformation of liquid paraffins [57]. The position of this halo does not change, and the experimental half-maximum width, β, decreases with the carbon number of alkyl chains (β = 3.75◦ θ for (DACl)2 DxS, β = 2.99◦ θ for (DDACl)2 DxS, and β = 2.66◦ θ for (TDACl)2 DxS), indicating the increase of ordering with the alkyl chain length.
3.3. Solid state properties There is a significant number of publications showing that the addition of a charged surfactant to an oppositely charged polyelectrolyte leads to materials that exhibit rich phase morphology of the surfactant [49–56]. In this work we have studied the effect of the alkyl chain length on the structural parameters of the alkylammonium dextran sulfates, which precipitated at concentration of c(DxS)/mol dm−3 > 1 × 10−7 . The elemental analysis revealed formation of an almost stoichiometric solid phase (with respect to the charge). X-ray diffractograms of dried samples displayed two sharp reflections in the small-angle region and at least two diffuse diffraction maxima in a wider-angle region (Figs. 8a–8c, inserted figures). Such X-ray patterns and the 1:2 relationship of the reciprocal interplanar spacings of two small-angle reflections are typical for lamellar mesophases with disordered alkylammonium chains within the ordered layers of cationic surfactant headgroups with electrostatically bound sulfate groups of polyelectrolyte chains, filling the space between. These diffraction lines were indexed as 00l (actually as 001 and 002) reflections and the basic lamellar thickness d from their interplanar spacings was calculated. A list of interplanar spacings (d), corresponding relative intensities (Irel ), and Miller indices (00l) for n-alkylammonium dextran sulfates is presented in Table 2. Beside two crystalline peaks, all samples displayed several diffuse diffraction maxima corresponding to the amorphous phase (Figs. 8a–8c). Hypothetical Bragg spacing of the very weak first halo increases somewhat with carbon number in the alkyl chain (Table 2). The most intensive second halo around 2θ/◦ = 20, i.e., at ≈ 4.5 Å, is
Fig. 8. X-ray diffraction patterns of solid n-alkylammonium dextran sulfates, taken at 303 K. Inserts: X-ray diffraction patterns taken at low angles. (a) (DA)2 DxS, (b) (DDA)2 DxS, and (c) (TDA)2 DxS.
Table 2 Interplanar spacing (d), relative intensities (Irel ), and Miller indices (00l) for n-alkylammonium dextran sulfates: decylammonium dextran sulfate (DA)2 DxS, dodecylammonium dextran sulfate (DDA)2 DxS, and tetradecylammonium dextran sulfate (TDA) 2 DxS (DA)2 DxS
(DDA)2 DxS
(TDA)2 DxS
d (Å)
Irel a
00l a
d (Å)
Irel a
00l a
d (Å)
Irel a
00l a
30.40 15.22 9.12 4.44
100 <1 vw s
001 002 A1 A2
35.32 17.66 9.80 4.45
100 4 vw s
001 002 A1 A2
39.94 19.97 9.98 7.24 4.43
100 7 vw 7 s
001 002 A1
a s—strong, vw—very weak, A 1 and 2 —amorphous maxima.
A2
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Fig. 9. Changes of the basic lamellar thickness (d) vs the number of carbon number in the alkyl chain (nC ) of n-alkylammonium dextran sulfates.
The change of lamellar thickness (d) with the carbon number in alkyl chains of n-alkylammonium dextran sulfates is presented in Fig. 9. The variation of d with nC in single n-alkylammonium chains shows linear dependence and can be approximated by the linear equation d = 2.5nC + 4.7. The constant increment of ∼ 2.5 Å in the long spacing of successive even numbers of the homologous series of alkyl straight chains corresponds to the distance between Cn and Cn + 2 atoms, indicating the trans conformation of chains. This change compares well with the one observed for a series of n-alkylammonium alkyl sulfates [58]. Fully extended lengths of the hydrocarbon chain are about 14, 16.7, and 19 Å, for hydrocarbon chains with nC = 10, 12, and 14, respectively [44]. Considering the values of ionic radii of ammonium and sulfate groups (2.5 and 4 Å) [59] as well as the fact that disordered hydrocarbon chains are not fully extended, the values of interplanar spacing d001 correspond to bilayer thickness of basic lamellae. The first 001 reflections are sharp, indicating multilayered packaging of lamellae; i.e., the mesophase domains consist of multilamellar bilayers. The mean crystallite size in the [001] direction, L001 /Å, is 933, 706, and 470 for (DA)2 DxS, (DDA)2 DxS, and (TDA)2 DxS. Dividing of L001 with the lamellar thickness indicated that the average lamellar bilayer in the stack of lamellae (mesomorphite) decreases in the order (DA)2 DxS (31) > (DDA)2 DxS (20) > (TDA)2 DxS (12). It seems that DxS chains are connected by surfactant bilayers into lamellae and the ability of lamellar stacking decreases with alkyl chains length. 4. Conclusions In this study we investigated the interactions in DxS/ n-alkylammonium chloride mixtures at the air/solution interface, in the bulk solution, and in the solid state. On the basis of different techniques, the formation of various polyelectrolyte–surfactant complexes at the air/solution interface, in the bulk, and in the solid phase has been proposed.
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A strong synergistic lowering of the surface tension found for all DxS/surfactant mixtures has been ascribed to the formation of polyelectrolyte–surfactant complexes at the air/solution interface. The addition of DxS to surfactant solutions causes an enhancement of the maximum surface excess concentration and a decrease in the minimum area per surfactant molecule at the air/solution interface. The increase of the alkyl chain length decreases the effectiveness of adsorption. The larger values of Amin reveal that higher homologous are less closely packed at the air/solution interface. An increase in amphiphile hydrophobicity results in a decrease of cac and an increase in the cooperativity of the adsorption process. Above the cac at least two kinds of complexes coexist in the bulk solution, one of them resembles bilayer structures. With further increasing of surfactant concentration, size and charge of layered complexes increase and the transformation of open to closed bilayers (vesicles) occurs. The complexation between micelles and DxS chains is associated with a release of counterions from the micelle/solution interface. X-ray analysis of solid alkylammonium dextran sulfates revealed mesophases with stacked lamellar bilayers. The n-alkylammonium molecules are in the form of disordered bilayers interconnecting dextran sulfate chains. The average basic lamellar thickness increases linearly with the increase of surfactant chain length, whereas the average number of lamellar bilayer in stack of lamellae decreases. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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Interactions in mixed cationic surfactants and dextran sulfate aqueous solutions V. Tomaši´c a,∗ , A. Tomaši´c b , I. Šmit c , N. Filipovi´c-Vincekovi´c a a Department of Physical Chemistry, Ruder ¯ Boškovi´c Institute, P.O. Box 180, HR-10002 Zagreb, Croatia b Division of Materials Chemistry, Ruder ¯ Boškovi´c Institute, Zagreb, Croatia c Division of Molecular Biology, Ruder ¯ Boškovi´c Institute, Zagreb, Croatia
Received 20 May 2004; accepted 29 November 2004 Available online 29 January 2005
Abstract The interactions between a hydrophilic anionic polysaccharide, dextran sulfate, and oppositely charged surfactants, n-alkylammonium chlorides (the number of carbon atoms per chain being 10, 12, and 14), were investigated by optical microscopy, X-ray diffraction, microelectrophoretic mobility, conductivity, surface tension, and light-scattering measurements at 303 K. The increase of surfactant alkyl chain length shifts both the critical aggregation (cac) and the critical micelle concentrations (cmc) toward lower surfactant concentration. Light-scattering and microelectrophoretic data revealed the coexistence of differently structured complexes beyond the cac. The presence of giant vesicles indicates that at least one type of species is ordered in bilayers. X-ray analysis of dry n-alkylammonium dextran sulfates exhibited mesomorphous ordering and interplanar spacings typical for lamellar structures; i.e., n-alkylammonium molecules form more or less disordered bilayers interconnected with dextran sulfate chains, thus forming multilamellar stacks. The average basic lamellar thickness increased linearly with the increase of surfactant chain length, whereas the average number of lamellar bilayers in the stack of lamellae decreases. 2004 Elsevier Inc. All rights reserved. Keywords: n-Alkylammonium chlorides; n-Alkylammonium dextran sulfates; Dextran sulfate; Giant vesicles; Lamellar structures
1. Introduction A lot of knowledge has been accumulated about complex interactions in aqueous mixtures of various surfactants and polyelectrolytes, because of their fundamental and practical importance (pharmaceutical formulations, food products, detergents, etc.). The extent of interaction between a polyelectrolyte and a surfactant depends on many factors, which include characteristic features of the polyelectrolyte and the surfactant, concentration and molar ratio of components, ionic strength, nature of solvents, temperature, etc. [1–25]. It is convenient for these interactions to classified according
* Corresponding author.
E-mail address: [email protected] (V. Tomaši´c). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.11.063
to surfactant and polyelectrolyte charges and/or according to their concentrations. The interaction between an ionic surfactant and an oppositely charged polyelectrolyte is a highly cooperative process, which starts at extremely low concentrations of both components. The multistep mechanisms of interaction include successive electrostatic, hydrophobic, and intra- and interpolyelectrolyte interactions, resulting in a formation of variety of single and coexisting phases, from differently ordered polyelectrolyte–surfactant monomer complexes to the stoichiometric precipitate and gel phase [22]. At low and fixed polyelectrolyte concentrations and with increasing surfactant concentration, three ranges of interaction can be distinguished. The first range corresponds to low surfactant concentration, well below the surfactant critical micelle concentration (cmc) in water. In this range, the electrostatic binding of surfactant on the polyelectrolyte chain
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has been the prevailing process, which leads to formation of polyelectrolyte–surfactant complexes. The second range is characterized by prevailing hydrophobic interactions between surfactant alkyl chains and formation of micelle-like aggregates on the polyelectrolyte chains, whereas the third range extends from the cmc to higher surfactant concentrations, with prevailing interactions between micelles and a polyelectrolyte. Basically, two important concentrations of surfactant can be distinguished: (i) the concentration needed to measure significant hydrophobic interactions between surfactant alkyl chains, known as the critical aggregation concentration (cac), and (ii) the concentration at which the surfactant micelles are being formed, known as the critical micelle concentration (cmc). In mixtures of oppositely charged polyelectrolytes and surfactants as systems of strong interactions, the cac value may be several orders of magnitude smaller than the cmc value. The most important characteristics of surfactant affecting the interactions are the hydrophobicity, rigidity, and bulkiness of the hydrophobic part of the molecule [26] and the type and position of surfactant head group [27]. Investigations of the surfactant chain length dependence on surfactants binding to dextran sulfate revealed a highly cooperative character of binding due to a large effect of the hydrophobic interactions between bound surfactants [16]. It has been shown that the free energy of binding [16], the concentration region where the binding proceeded [8,18,28,29], and the structure of polylelectrolyte–surfactant complexes [30, 31] strongly depend on the length of the alkyl chain. The increase of carbon atoms in the surfactant chains enhances hydrophobic association, shifts the cac toward lower surfactant concentrations, and change the structure of complexes, both in the bulk solution and in the solid state [29, 31,32]. Nevertheless a large number of new methods and applied models and a better understanding of the microscopic structure of the aggregates and the role of hydrophobic interactions may be needed. There is a wide range of open questions: binding mechanism, structural and specific binding forces, size and morphology of the aggregates formed, incorporation of polymers into liquid crystalline phases, etc. In this work, we examine the effect of the chain length of cationic surfactants, n-alkylammonium chlorides (RNH3 Cl), on the nature and strength of interactions with an oppositely charged hydrophilic polyelectrolyte, sodium dextran sulfate (DxS). The first two parts of the paper have focused on interactions at extremely low DxS concentration and increasing surfactant concentration without concomitant phase separation. The third part has been concerned with structural properties of stoichiometric solid phase, which exhibited lamellar structure with different degree of ordering. Investigations performed in dilute solutions revealed formation of giant vesicles as an indication of lamellar ordering from low to high polyelectrolyte concentrations.
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2. Experimental section 2.1. Materials and methods The preparation and purification of n-alkylammonium chlorides, general formula RNH3 Cl, with R = Cn H2n + 1 and nC = 10, 12, and 14, i.e., decylammonium chloride (DACl), dodecylammonium chloride (DDACl), and tetradecylammonium chloride (TDACl), were described earlier [33]. Their purity was checked by elemental analysis and surface tension measurements. The sodium salt of dextran sulfate (Pharmacia LKB, Uppsala, Sweden; average molecular weight Mw ≈ 5 × 105 ) was used without further purification. The reported sulfur content in ester sulfate is 17%, which corresponds to an average of 1.8 sulfonate groups per glycoside unit (Mw ≈ 380); i.e., the degree of substitution is 2, and the polymerization number n ≈ 1315. The number of sulfate groups of the present sample was evaluated to be about 2440 per DxS molecule; therefore c(DxS)/mol dm−3 = 1 × 10−10 , 4 × 10−10 , 1 × 10−9 , 4 × 10−9 , and 1 × 10−8 are equivalent to c(OSO3 )/mol dm−3 = 2.44 × 10−7 , 9.76 × 10−7 , 2.44 × 10−6 , 9.76 × 10−6 , and 2.44 × 10−5 , respectively. Stock solutions were prepared by weight using redistilled and filtered water (through Millipore Type HA 0.10 µm, to exclude dust before use). Systems were prepared with a constant DxS concentration and increasing concentrations of RNH3 Cl. They were stirred for 5 min and left standing for equilibration at 303 K for 1 day. During a 7-day observation period the systems did not show significant changes in measured physicochemical properties. Detection and microstructural identification of different phases were performed by optical microscopy (Model DMLS equipped with a video camera, Leica), tensiometry (K8 interfacial tensiometer, Krüss), conductometry (MPC 227 conductivity meter, Mettler Toledo), microelectrophoretic and light-scattering measurements (Zetasizer Nano ZS, Malvern), and X-ray diffraction by a Philips diffractometer with monochromatized CuKα radiation. 2.2. Interpretation of data The cac values were determined by the usual procedure from the specific conductivity (κ) versus concentration (c) plots [22] and by microelectrophoretic measurements as a concentration where change of particle charge occurred. The cmc values were determined from both the surface tension and the specific conductivity measurements as the surfactant concentration at the intersection of the two linear sections of γ or κ versus c curves by the method of nonlinear least mean squares of up to fourth order with fitting correlation coefficients equal or better than 0.999. The apparent degree of counterion dissociation from the micelle/solution interface (α) was determined from the conductivity measurements. To a first approximation, α was
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taken as a ratio of the slope of κ vs c lines above and below the cmc [34]. The maximum surface excess of surfactant molecule (Γmax ) at the air/solution interface as maximum adsorption density at the cmc was determined from the premicellar slopes of the γ vs log c(RNH3 Cl) curves by the Gibbs adsorption equation [35]. The minimum area occupied by surfactant molecule at the air/solution interface (Amin ) represents the effective area per molecule at the cmc and can be estimated from Γmax [22]. The average basic lamellar thickness (d/Å) was calculated from the first peak on the diffractograms according to the Bragg equation and the mean crystallite size in the [001] direction, L001 , was calculated by the Scherrer equation [36].
3. Results and discussion 3.1. Air/solution interface properties Fig. 1 shows the surface tension isotherms for DACl in the absence and presence of different DxS concentrations. The surface tension isotherm for pure DACl solution displays the decrease of surface tension up to the cmc and the constant value in micellar region. The surface tension with DxS added begins to drop at a concentration much lower than that with a pure DACl solution, indicating that the interactions began at very low concentrations. A strong lowering of surface tension has been found for all DACl/DxS mixtures. Systems with DxS added revealed two types of curves; at c(DxS) 1 × 10−9 mol dm−3 , curves with a minimum below the cmc are obtained, whereas curves with two plateaus appear at higher c(DxS). These behaviors are undoubtedly concerned with the changing of the composition of the air/solution interface and in the bulk solution. Below the cac and at very low and fixed DxS concentration, the adsorption of surfactant at the air/solution interface is enhanced due to formation of highly surface active
Fig. 1. Changes of surface tension (γ ) with c(DACl) in mixtures with constant c(DxS)/mol dm−3 = (") 0, (×) 1 × 10−10 , (E) 4 × 10−10 , (!) 1 × 10−9 , (P) 4 × 10−9 , (e) 1 × 10−8 . Temperature is 303 K.
complexes, the thickness of which increases with increasing surfactant concentration [37]. In bulk solution, however, the surfactant molecules are present as monomers, although some association with DxS can occur due to ion exchange of sodium ions by surfactant cations. Beyond the cac, the situation in bulk solution and at the air/solution interface changes drastically. The formation of micelle-like aggregates on DxS chains leads to a continuous desorption of the surface polyelectrolyte–surfactant complexes at the surface. Because of the removal of the DxS from the air/solution interface, the equilibrium between surfactant monomers at the air/solution interface and monomers in solution is shifted toward the bulk solution [38]. This explains the increase in surface tension close to the values of pure DACl solution. The appearance of two plateaus at higher DxS concentration may be interpreted as follows: at higher DxS concentration, there are sufficient DxS chains to form surface and bulk polyelectrolyte–surfactant complexes. With increasing surfactant concentration, the bulk polyelectrolyte– surfactant complexes continuously become more populated with surfactant molecules, while the surface polyelectrolyte– surfactant complex remains at the air/solution interface. Because of this there is no change in surface tension value. As the polyelectrolyte is saturated with surfactant, the surfactant monomer concentration and the activity start to increase again and there is a lowering of surface tension until the monomer concentration reaches the cmc. Once micelles of surfactant appear in solution, the surface tension becomes essentially equal to that of a polyelectrolyte free surfactant solution. New information accessible through X-ray and neutron reflectivity measurements show that several polyelectrolyte– surfactant complexes are being formed at the air/solution interface at surfactant concentrations lower than that seen in the bulk phase [39–41]. The polyelectrolyte configurations in the bulk and at the air/solution interface are very different and no doubt the complexes formed in the bulk phase differ from those formed at the surface. There are several kinds of the polyelectrolyte–surfactant complexes: monolayer and layered polyelectrolyte–surfactant complexes at the air/solution interface and polyelectrolyte– surfactant complexes in the bulk solution. While highly surface active complexes form at the air/solution interface, there are at least two kinds of bulk complexes, the first below the cac and the second between the cac and the cmc. Fig. 2 shows the surface tension data of RNH3 Cl/DxS mixtures. With the increase of the alkyl chain length all surface tension isotherms are shifted toward lower values. The addition of DxS affected neither the cmc not the surface tension at the cmc. All premicellar slopes of surface tension isotherms display a gradual increase with the DxS concentration, indicating an enhancement of the maximum excess surface concentration and a decrease in the minimum area per surfactant molecule at the air/solution interface with a DxS addition (Figs. 3a and 3b).
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Fig. 2. Changes of surface tension (γ ) with c(RNH3 Cl) in mixtures at constant c(DxS)/mol dm−3 = 1 × 10−9 : (!) DACl, (P) DDACl, and (1) TDACl. Temperature is 303 K.
Fig. 4. Specific conductivity (κ) vs c(DACl) at constant c(DxS)/ mol dm−3 = 1 × 10−9 at (a) lower and (b) higher c(DACl), at 303 K.
itively charged surfactant head groups due to the neutralization via electrostatic interactions with –OSO− 3 groups of DxS [42]. 3.2. Bulk properties
Fig. 3. (a) Surface excess concentration (Γmax ) and (b) minimum area per surfactant molecule (Amin ) at the air/solution interface vs c(DxS): (!) DACl, (P) DDACl, (1) TDACl. Temperature is 303 K.
The increase in Γmax and decrease in Amin are caused by different packing of surfactants monomers at the air/solution interface in the presence of polyelectrolyte ion in the subphase and by lowered electrostatic repulsion between the surfactant head groups, as similarly obtained in the systems with ι-carrageenan [22]. The condensation of the cationic surfactant monolayer obtained upon the DxS addition can be attributed to a decrease in repulsive forces between pos-
Fig. 4 shows typical change of specific conductivity as a function of DACl concentration with constant c(DxS). All measured κ values were lower than those obtained in the absence of DxS, thus indicating that the interactions between the components start at levels below 1 × 10−5 mol dm−3 of surfactants. In pure RNH3 Cl solutions, the κ vs c curves show typical break at the cmc, whereas those with the addition of DxS show two breaks: the first break in the premicellar concentration range (corresponding to the cac), and the second one in the vicinity of the cmcRNH3 Cl . In single-surfactant solutions, the increase of the number of carbon atoms leads to the decrease of the cmc according to the equation log cmc = −0.30nC + 1.71. The addition of DxS has not significantly affected the cmc values of all surfactants; i.e., the cmc values were almost the same as in the single surfactant solutions (Table 1). The cac values were found to be independent of DxS concentration. This is in good accordance with the literature data, which have established that the cac depends on the polyelectrolyte concentration only weakly [43].
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Table 1 Critical aggregation concentration (cac/mol dm−3 ), critical micelle concentration (cmc/mol dm−3 ), cac/cmc ratio, and reduction in free energy of micelles when bound to DxS (G/kJ mol−1 ) for n-alkylammonium chloride/dextran sulfate mixtures (temperature is 303 K) System
cac × 104
cmc × 102
cac/cmc × 102
G
DACl + DxS DDACl + DxS TDACl + DxS
25.0 2.20 0.30
5.6 1.50 0.38
4.46 1.47 0.79
−7.81 −10.57 −12.19
As the cmc, the cac decreases with increasing the chain length; in other words; the longer the hydrocarbon chain of the surfactant, the lower the cac (Table 1). An increase in amphiphile hydrophobicity results in an increase in the cooperativity of the interaction and in a stronger decrease of the cac. The chain length dependence of the cac obeys the equation log cac = −0.50nC + 2.33.
(1)
A slope similar to those obtained for log cmc vs nC indicates that the free energies of the surfactant methylene group transfer from the aqueous phase to a free micelle or to a bound aggregate are almost equal. This behavior is similar to what was observed by Hayakawa and co-workers in the binding of ionic surfactants by polyions of opposite charge using surfactant selective electrodes [4,15–17]. They have found that the overall binding constant increases with increasing surfactant chain length; a linear relationship between overall binding constant and surfactant chain length has been observed in various systems of surfactant cations and polyanions. From the slope, the difference in the free energy of surfactant binding for each CH2 group added to surfactant chains is estimated to be from −1.10 to −1.32 kT. This free energy change is close to the value of the free energy transfer of one CH2 group from an aqueous phase to the micellar phase (−1.20 to −1.38 kT) [44]. Wallin and Linse related the cac/cmc ratio to the free energy of complexation between a micelle and a polyelectrolyte in dilute solution [45]. Data presented in Table 1 show that the cac/cmc ratio decreases in parallel with the growth of the surfactant chain length, thus indicating that the cac reduction is faster than that of the cmc at increasing surfactant tail length. Two important aspects of the difference between the cmc and cac values are as follows: (i) a strong reduction of the electrostatic energy of the system and (ii) a release of counterions of the micelle and the polyelectrolyte. With the increase in tail length, the decrease in the electrostatic energy dominates the decrease in the complexation energy and the release of counterions becomes more important by enhancing the increase in entropy. Sirivat and co-workers [46] showed that the reduction in the Gibbs free energy of micelles, if polyelectrolyte-bound, can be expressed as G = RT ln[cac/cmc].
Fig. 5. Variation of the apparent degree of counterion dissociation (α) from the micelle/solution interface as a function of alkyl chain carbon number (nC ) in pure surfactant solutions (!) and in RNH3 Cl/DxS mixtures with constant c(DxS)/mol dm−3 = (P) 1 × 10−9 , (e) 4 × 10−9 , and (E) 1 × 10−8 . Temperature is 303 K.
(2)
The reduction in the free energy of polyelectrolyte-bound micelles listed in Table 1 exhibits an increase that indicates that the growth of surfactant chain length increases the stability of polyelectrolyte–surfactant complexes. In a solution where different types of counterions coexist, the counterion binding to the micelle/solution interface depends on the mol fraction and the type of each single ion in the bulk. Priority of binding displays counterions with higher valency, so a micelle formed in a polyelectrolyte solution binds the polyanion to its micelle/solution interface. The conductivity measurements showed an increase of the apparent degree of counterion dissociation from RNH3 Cl micelles upon the addition of DxS. Thus, the replacement of chloride counterions at the micelle/solution interface with DxS has been indicated; in other words, the micelle formed in a polyelectrolyte solution bound the oppositely charged polyanion to its surface. Fig. 5 shows that the α-values decrease with nC ; i.e., the counterion binding increases and diminishes the effective surface charge density at the surface of micelles. For all mixtures, the variation of α with nC can be approximated by the linear relation α = −0.02nC + B,
(3)
where B is a constant, being 0.53, 0.54, 0.57, and 0.58 for c(DxS)/mol dm−3 = 0, 1 × 10−9 , 4 × 10−9 and 1 × 10−8 , respectively. It seems that the B constant showing its dependence on the DxS concentration in Eq. (8) may be ascribed mainly to the electrostatic contributions, whereas the rest involves contributions governed mainly by surfactant hydrophobicity. The charge changes of the polyelectrolyte caused by surfactant binding can be followed by measuring the microelectrophoretic mobility. The microelectrophoretic mobility of DxS–surfactant complexes is shown as a function of surfactant concentration (Fig. 6). The microelectrophoretic mobility decreases with growth of the surfactant concentration. The reduced microelectrophoretic mobility indicates
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Fig. 6. Changes of the microelectrophoretic mobility (u± ) of particles vs c(RNH3 Cl) at constant c(DxS)/mol dm−3 = 1 × 10−9 : (! and ") DACl, (P and Q) DDACl, and (1 and 2) TDACl. Temperature is 303 K.
the decrease of the overall particle charge due to the electrostatic binding of ammonium head groups of surfactants to the anionic sulfate sites along the polyelectrolyte chain. When surfactant concentration reaches a critical aggregation value, DxS–surfactant complexes crossed zero mobility and become positively charged. This concentration coincides with the cac determined by conductivity measurements. The longer the hydrocarbon chain of the surfactant, the lower the surfactant concentration required to reach the zero mobility. The adsorption of surfactants at concentrations lower than the cac is considered to be mainly due to the ionic attraction, whereas for concentrations above the cac the surfactant monomer starts to bind cooperatively to the DxS chain through hydrophobic interactions. This process brought excessive positive charges to bear on the particles. Two populations of positively charged complexes were detected beyond the cac; the first one showing pronounced positive mobility and significant increase of mobility with surfactant concentration (empty signs in Fig. 6), and the second one showing the population with much lower mobility (filled signs in Fig. 6), which increased slightly with surfactant concentration. This finding is in accordance with the results of Shirahama and co-workers [47], who have found two kinds of migrating species in the DxS–dodecylpyridinium bromide system. They explained these results by the formation of complexes differing in the amount of adsorbed surfactants. In contrast, only one kind of complex species found for DxS–alkyltrimethylammonium bromide systems has been explained by the difference in the length of DxS and/or in nature of ligands used [28]. The variation of the mean particle size (D) with the increasing surfactant concentrations is illustrated in Fig. 7. At the lowest surfactant concentration, the head groups of RNH3 Cl bind to sulfate groups of DxS to form surfactant/DxS complex, reducing the net charge, and to weaken the electrostatic repulsion on the polyelectrolyte chains. Slight reducing of polyelectrolyte chains length is compensated for by increasing the molar mass of complex due to the
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Fig. 7. Changes of the mean particle size (D) in mixtures with c(RNH3 Cl) at constant c(DxS)/mol dm−3 = 1 × 10−9 : (! and ") DACl, (P and Q) DDACl, and (1 and 2) TDACl. Temperature is 303 K.
bound surfactant monomers, leading to an increase in the mean particle size. The first particles, which appear upon surfactant addition, exhibit the mean particle size close of 16 nm, which is close to that of polyelectrolyte itself (the DxS molecules in water resemble a rod with the radii of the cylindrical macroion a ≈ 0.4 nm [6], with hydrodynamic radius Rh = 15.8 nm [48]). Therefore it is reasonable to attribute it to an intrapolymer complex. The mean particle size increases with surfactant concentration until it reaches the cac, at level of which two differently sized populations appear. Thus, the particle size distributions broaden and become bimodal beyond the concentration corresponding to the cac. Bimodal distribution of mean particle sizes is in accordance with the microelectrophoretic data. With an increase of the chain length, the mean particle sizes of both populations increased, but in different ways. The particles with larger D continuously increased in the size with an increase in surfactant concentration and exhibited rather broad size distributions (empty signs in Fig. 7), whereas the particles with lower D were almost monodisperse (filled signs in Fig. 7). All samples were inspected between crossed polarizers in order to detect optically anisotropic phases. Microphotographs of samples taken from characteristic concentration regions (between the cac and the cmc and beyond the cmc) reveal the formation of giant vesicles with dimensions of several micrometers. Appearance of spherical vesicles was confirmed by characteristic Maltese crosses under crossed polarizers. By light-scattering techniques alone, it has not been possible to determine neither the shape nor the structure of polyelectrolyte–surfactant complex. However, optical microscopy undoubtedly shows that at least one kind of aggregates resembles a bilayer structure. As suggested by microelectrophoretic and light-scattering measurements, the organization of DxS–surfactant complex structure beyond the cac proceeded in two directions. It seems that in investigated mixtures, both inter- and intrachain hydrophobic aggregation of alkyl chains should be taken into account. It may be deduced that complexes of
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higher size are interpolymeric complexes; i.e., hydrophobic interactions between surfactant alkyl chains occurred between two surfactant–polyelectrolyte complexes. Consequently, open bilayers of alkyl chains with attached DxS chains in the head group regions have been formed. Increasing surfactant concentration promotes growth of bilayers, enhances positive charge and above all transforms open bilayers to the closed ones, i.e., vesicles. On the contrary, less charged and smaller particles represent intrapolymeric complexes with only slight excess of bound surfactant (with respect to the charge).
characteristic of a disordered liquid-like conformation of liquid paraffins [57]. The position of this halo does not change, and the experimental half-maximum width, β, decreases with the carbon number of alkyl chains (β = 3.75◦ θ for (DACl)2 DxS, β = 2.99◦ θ for (DDACl)2 DxS, and β = 2.66◦ θ for (TDACl)2 DxS), indicating the increase of ordering with the alkyl chain length.
3.3. Solid state properties There is a significant number of publications showing that the addition of a charged surfactant to an oppositely charged polyelectrolyte leads to materials that exhibit rich phase morphology of the surfactant [49–56]. In this work we have studied the effect of the alkyl chain length on the structural parameters of the alkylammonium dextran sulfates, which precipitated at concentration of c(DxS)/mol dm−3 > 1 × 10−7 . The elemental analysis revealed formation of an almost stoichiometric solid phase (with respect to the charge). X-ray diffractograms of dried samples displayed two sharp reflections in the small-angle region and at least two diffuse diffraction maxima in a wider-angle region (Figs. 8a–8c, inserted figures). Such X-ray patterns and the 1:2 relationship of the reciprocal interplanar spacings of two small-angle reflections are typical for lamellar mesophases with disordered alkylammonium chains within the ordered layers of cationic surfactant headgroups with electrostatically bound sulfate groups of polyelectrolyte chains, filling the space between. These diffraction lines were indexed as 00l (actually as 001 and 002) reflections and the basic lamellar thickness d from their interplanar spacings was calculated. A list of interplanar spacings (d), corresponding relative intensities (Irel ), and Miller indices (00l) for n-alkylammonium dextran sulfates is presented in Table 2. Beside two crystalline peaks, all samples displayed several diffuse diffraction maxima corresponding to the amorphous phase (Figs. 8a–8c). Hypothetical Bragg spacing of the very weak first halo increases somewhat with carbon number in the alkyl chain (Table 2). The most intensive second halo around 2θ/◦ = 20, i.e., at ≈ 4.5 Å, is
Fig. 8. X-ray diffraction patterns of solid n-alkylammonium dextran sulfates, taken at 303 K. Inserts: X-ray diffraction patterns taken at low angles. (a) (DA)2 DxS, (b) (DDA)2 DxS, and (c) (TDA)2 DxS.
Table 2 Interplanar spacing (d), relative intensities (Irel ), and Miller indices (00l) for n-alkylammonium dextran sulfates: decylammonium dextran sulfate (DA)2 DxS, dodecylammonium dextran sulfate (DDA)2 DxS, and tetradecylammonium dextran sulfate (TDA) 2 DxS (DA)2 DxS
(DDA)2 DxS
(TDA)2 DxS
d (Å)
Irel a
00l a
d (Å)
Irel a
00l a
d (Å)
Irel a
00l a
30.40 15.22 9.12 4.44
100 <1 vw s
001 002 A1 A2
35.32 17.66 9.80 4.45
100 4 vw s
001 002 A1 A2
39.94 19.97 9.98 7.24 4.43
100 7 vw 7 s
001 002 A1
a s—strong, vw—very weak, A 1 and 2 —amorphous maxima.
A2
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Fig. 9. Changes of the basic lamellar thickness (d) vs the number of carbon number in the alkyl chain (nC ) of n-alkylammonium dextran sulfates.
The change of lamellar thickness (d) with the carbon number in alkyl chains of n-alkylammonium dextran sulfates is presented in Fig. 9. The variation of d with nC in single n-alkylammonium chains shows linear dependence and can be approximated by the linear equation d = 2.5nC + 4.7. The constant increment of ∼ 2.5 Å in the long spacing of successive even numbers of the homologous series of alkyl straight chains corresponds to the distance between Cn and Cn + 2 atoms, indicating the trans conformation of chains. This change compares well with the one observed for a series of n-alkylammonium alkyl sulfates [58]. Fully extended lengths of the hydrocarbon chain are about 14, 16.7, and 19 Å, for hydrocarbon chains with nC = 10, 12, and 14, respectively [44]. Considering the values of ionic radii of ammonium and sulfate groups (2.5 and 4 Å) [59] as well as the fact that disordered hydrocarbon chains are not fully extended, the values of interplanar spacing d001 correspond to bilayer thickness of basic lamellae. The first 001 reflections are sharp, indicating multilayered packaging of lamellae; i.e., the mesophase domains consist of multilamellar bilayers. The mean crystallite size in the [001] direction, L001 /Å, is 933, 706, and 470 for (DA)2 DxS, (DDA)2 DxS, and (TDA)2 DxS. Dividing of L001 with the lamellar thickness indicated that the average lamellar bilayer in the stack of lamellae (mesomorphite) decreases in the order (DA)2 DxS (31) > (DDA)2 DxS (20) > (TDA)2 DxS (12). It seems that DxS chains are connected by surfactant bilayers into lamellae and the ability of lamellar stacking decreases with alkyl chains length. 4. Conclusions In this study we investigated the interactions in DxS/ n-alkylammonium chloride mixtures at the air/solution interface, in the bulk solution, and in the solid state. On the basis of different techniques, the formation of various polyelectrolyte–surfactant complexes at the air/solution interface, in the bulk, and in the solid phase has been proposed.
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A strong synergistic lowering of the surface tension found for all DxS/surfactant mixtures has been ascribed to the formation of polyelectrolyte–surfactant complexes at the air/solution interface. The addition of DxS to surfactant solutions causes an enhancement of the maximum surface excess concentration and a decrease in the minimum area per surfactant molecule at the air/solution interface. The increase of the alkyl chain length decreases the effectiveness of adsorption. The larger values of Amin reveal that higher homologous are less closely packed at the air/solution interface. An increase in amphiphile hydrophobicity results in a decrease of cac and an increase in the cooperativity of the adsorption process. Above the cac at least two kinds of complexes coexist in the bulk solution, one of them resembles bilayer structures. With further increasing of surfactant concentration, size and charge of layered complexes increase and the transformation of open to closed bilayers (vesicles) occurs. The complexation between micelles and DxS chains is associated with a release of counterions from the micelle/solution interface. X-ray analysis of solid alkylammonium dextran sulfates revealed mesophases with stacked lamellar bilayers. The n-alkylammonium molecules are in the form of disordered bilayers interconnecting dextran sulfate chains. The average basic lamellar thickness increases linearly with the increase of surfactant chain length, whereas the average number of lamellar bilayer in stack of lamellae decreases. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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