From Swollen Lamellae to PolymerâSurfactant Complexes - American

Article pubs.acs.org/JPCB

In Situ X‑ray Polymerization: From Swollen Lamellae to Polymer− Surfactant Complexes Yahya Agzenai,†,‡ Björn Lindman,‡ Viveka Alfredsson,‡ Daniel Topgaard,‡ Carmen S. Renamayor,† and Isabel E. Pacios*,† †

Dpt. CC y TT Fisicoquímicas, Facultad de Ciencias, UNED, P° Senda del Rey 9, 28040 Madrid, Spain Physical Chemistry, Lund University, Box 124, SE-221 00 Lund, Sweden

‡

ABSTRACT: The influence of the monomer diallyldimethylammonium chloride (D) on the lamellar liquid crystal formed by the anionic surfactant aerosol OT (AOT) and water is investigated, determining the lamellar spacings by SAXS and the quadrupolar splittings by deuterium NMR, as a function of the D or AOT concentrations. The cationic monomer D induces a destabilization of the AOT lamellar structure such that, at a critical concentration higher than 5 wt %, macroscopic phase separation takes place. When the monomer, which is dissolved in the AOT lamellae, is polymerized in situ by X-ray initiation, a new collapsed lamellar phase appears, corresponding to the complexation of the surfactant with the resulting polymer. A theoretical model is employed to analyze the variation of the interactions between the AOT bilayers and the stability of the lamellar structure.

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INTRODUCTION Lyotropic liquid crystals are used in multiple applications such as enhanced oil recovery, medicine, or cosmetics. For these purposes, active molecules are usually incorporated; the addition of a third component generally leads to changes in the phase behavior of the lyotropic systems.1 Monomers in the presence of surfactants are very much used in emulsion and microemulsion polymerization,2 but the monomers are rarely studied with surfactants when these form long-range ordered mesophases.3,4 In these cases, it seems crucial to understand where and how the monomer is located before polymerization. In fact, it has been demonstrated that some monomers are located in the interfacial domain of the lyotropic mesophases adopting preferential orientations with the consequent increase of the polymerization rate.4 Sodium bis(2-etylhexyl)sulfosuccinate (AOT) is an anionic surfactant widely used in industrial applications.5 It is also extensively studied in fundamental investigations because the lyotropic AOT/water system forms lamellar mesophases over a wide range of compositions, which is particularly interesting due to the similarity between the lamellar mesophase and the biological membrane.6 It has been reported that the lamellar structure of the AOT/ water system is maintained, when small amounts of N-alkylsubstituted acrylamides are incorporated. Nevertheless, at a given monomer concentration, the system splits into two phases, one lamellar and one isotropic. This macroscopic phase instability is a consequence of the perturbations introduced by the third component in the structure of the lamellar phase, and the concentration at which the phase separation takes place depends on its hydrophobic character.7 © 2014 American Chemical Society

On the other hand, the incorporation of water-soluble nonionic polymers in a lamellar mesophase has been extensively studied and it has been observed that the behavior is governed by entropic factors, related to the size of the macromolecule relative to the lamellar spacing. Thus, when the macromolecular coils are smaller than the water layer thickness, the polymer stays in the water layer of the lamellae, but when the polymer coils are larger than the water layer thickness, the polymer segregates from the lamellae with part of the water, forming an isotropic polymer-rich phase, which partially deswells the lamellar structure.8 The situation described corresponds to a case of a net repulsion between the polymer and the self-assembled surfactant and is typical for mixtures of nonionic polymers and surfactants. Over extensive ranges of concentration, this leads to segregative phase separation, i.e., to polymer- and surfactant-rich phases.9 In contrast, the addition of charged polymers (polyelectrolytes) to lyotropic systems formed by oppositely charged surfactants usually leads to the formation of polymer− surfactant complexes10 (PSC); this constitutes a very attractive field of research, because of the existing and potential technological applications in inter alia medicine,11 cosmetics,12 and personal care industry.13,14 PSCs usually exhibit long-range order (lamellar,15,16 cubic,17 or hexagonal18) as a consequence of the electrostatic interactions between the charged surfactant heads and the charged units along the polymer chains, and, for Received: December 4, 2013 Revised: January 9, 2014 Published: January 10, 2014 1159

dx.doi.org/10.1021/jp411894e | J. Phys. Chem. B 2014, 118, 1159−1167

The Journal of Physical Chemistry B

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Sigma and was dissolved in water and freeze-dried before used. The monomer diallyldimethylammonium chloride (D) with 97% purity from Aldrich was used as received. Deionized water (Milli-Q) and deuterated water (99.8% purity) from Armar Chemicals were employed for the sample preparation. Sample Preparation. Two sets of samples were prepared with the monomer (D) and surfactant (S) denoted DS-# and SD-#. The set DS-# is formed by six samples having the same D composition (1.25 wt %) and a surfactant fraction (#) that varies (# = 20, 25, 30, 35, 40, and 45 wt %). The set SD-# is composed by 11 samples having the same surfactant composition (25 wt %) and a variable weight fraction of D (# = 0.5, 0.75, 1.25, 2, 3, 4, 5, 6, 7, 8, and 11 wt %). The final composition of each sample was reached by weighing the proper amounts of AOT, D, and water. Samples were homogenized using a centrifuge by mixing back and forth for several days, and after this process, they were allowed to equilibrate at 25 °C. Additionally, for 2H NMR experiments, two sets were prepared with deuterated water as solvent, denoted HDS-# and HSD-#. These samples have similar compositions as the previous ones, although for the set HSD#, the highest D concentration was 5 wt %, and their equilibration time was 1 year. Techniques. Microscopy. A Nikon Labophot-2 microscope, with a Nikon DS-5M camera and crossed polarizers, was employed to determine the anisotropy of the samples, which where placed between a glass slide and a coverslip. Small Angle X-ray Scattering (SAXS). The measurements were performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble on the beamline BM16 (Spanish CRG). Samples introduced in capillaries were measured in three different locations with a wavelength of λ = 0.979 Å. The sample−detector distance was 1381 mm. Two-dimensional images were recorded using a CCD detector (MARCCD 165) with a resolution of 1024 × 1024 pixels and a pixel size of 159 μm. Images were integrated to obtain the scattered intensity (I) as a function of the modulus of the scattering vector, q = (4π/ λ) sin θ, where 2θ is the scattering angle, and were normalized to compensate for the intensity fluctuations of the synchrotron source. In the experiments of polymerization induced by X-ray irradiation, the diffractograms were recorded consecutively, and each scan was performed with irradiation times of 10 or 12 s. The accumulated irradiation time was, as minimum, 1100 s. 2 H NMR. A Bruker Avance-II 200 spectrometer (Bruker, Karlsruhe) operating at a deuterium resonance frequency of 30.72 MHz was used for the 2H NMR experiments. The magnet was equipped with a broadband probe (BBO, Bruker) having a 10 mm saddle coil for radiofrequency transmission and detection. The 2H signal was recorded after a 90° excitation pulse with length 8.12 μs using a receiver dead time of 50 μs. The raw time-domain data was converted to frequency-domain spectra using the TopSpin software. The quadrupolar splittings, Δ (Hz), were measured as the distance between the two peaks in the powder pattern.39 The samples were kept in flame-sealed 8 mm test tubes, and the temperature was regulated to 25 ± 0.2 °C by a flux of air through the sample holder.

certain polymers, also by hydrophobic interactions between the polymer backbone and the alkyl tails of the surfactants.19 In particular, the incorporation of polyelectrolytes into lamellar lyotropic phases often leads to the formation of several coexisting phases, sometimes without macroscopic phase separation.20 One of them is a swollen phase, while the other is a collapsed structure with a lamellar spacing independent of the global water content, in which the polymer is adsorbed flat onto the bilayers forming the complex.21,22 In this latter structure, the lamellar spacing is hardly independent of the global water content. Recently, a model has been proposed that characterizes the composition of the two coexisting phases: the collapsed structure of the complex and the swollen lamellar phase.23 Another method to prepare this kind of systems is by in situ polymerization. Polymerization usually needs the addition of chemical initiators and catalysts, which could modify the structure of the lyotropic systems. Additionally, a nonhomogeneous distribution of the initiator in the different domains (hydrophobic or hydrophilic) could occur, leading to a modification of the expected characteristics of the polymer. It has been previously reported that X-rays act as an initiator in the polymerization of acrylamides,24 acrylates,25 or ethylene glycol.26 Besides, the high photon flux of synchrotron radiation provides a method to follow the polymerization process in situ, by the change in the scattering curves, from the initial mixture to the postpolymerized state.27 This kind of experiments was performed by our group, concerning the polymerization of dimethylacrylamide in the AOT/water system,28 and afterward, other systems have been studied.29 Here we extend the previous study by incorporating a charged monomer, N,N-diallyldimethylammonium chloride (D), which is a quaternary ammonium salt and readily soluble in water. The monomer D has potential industrial applications, e.g., as a coagulant in the recovery of latex rubber.30 The polymer, poly(diallyldimethylammonium) chloride (PD), is also extensively studied because of its applications in different fields such as paper making, petroleum, domestic chemicals, pharmaceuticals, water treatment, and so on,31,32 where it is generally used as a flocculant agent,33 and it is also employed for fundamental research, e.g., in modeling of polyelectrolyte− surfactant complexes.34,35 When PD is incorporated into the lamellar system formed by lecithin and sodium dodecyl sulfate (SDS), above a critical concentration of 3 wt %, a collapsed lamellar phase is formed.36 The same behavior has been observed when it is incorporated into the lamellar system SDS/decanol/water, but in this case, the critical concentration is 4 wt %; when the polymer concentration is lower than 1%, only a typical swollen lamellar phase is observed, and at intermediate polymer concentrations, 2−3 wt %, the swollen and collapsed lamellar phases do coexists.37,38 With the aim of unveiling the different behavior of the monomer and the polymer in the lamellar AOT/water system, this research was performed in two steps: (i) to determine the influence of the monomer on the structure of the lamellar phase and (ii) to follow the X-ray initiated polymerization by characterizing the structure of the polymer− surfactant complexes that are formed as the polymerization progresses.

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RESULTS AND DISCUSSION Sample Characterization. There is no macroscopic phase separation for the set DS-#, and neither for the samples corresponding to the set SD-# with a D concentration lower than 6 wt %. All of these samples are slightly turbid and viscous fluids. The optical microscopy does not show any evidence of

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EXPERIMENTAL SECTION Chemicals. The surfactant 1,4-bis(2-ethylhexyl)sodium sulfosuccinate (AOT) with 99% purity was purchased from 1160



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example of this effect can be seen as the central hill in the spectra of HSD-0.5 in Figure 2. In fact, the existence of small lamellar domains in insufficiently equilibrated samples of the system AOT/water was previously reported.43 The values of the quadrupole splitting Δ can be rationalized using a two-site model,44,45 in which the water molecules are divided into two fractions, “free” and “bound”, where the latter is located in the nanometer-scale vicinity of the surfactant bilayer and is partially oriented due to interactions with the surfactant head groups. The size of this fraction is related to the number of water molecules required for hydrating one surfactant polar headgroup (nSW). The free water is further than, say, 1 nm from the bilayer surface and lacks any kind of preferential orientation. Since the observational time-scale of a 2 H spectrum is on the order of milliseconds, the nanosecond time-scale diffusional exchange between the free and bound water results in the observed value of Δ being a populationweighted average of the contributions from the two fractions, which can be expressed as

phase separation, and with crossed polarizers, the typical marbled lamellar texture is observed. In the other samples, i.e., samples with monomer content higher than 5 wt %, there is a macroscopic phase separation, with the upper phase transparent and optically isotropic, while the bottom phase is turbid and exhibits a lamellar texture (Figure 1). The volume of the bottom phase diminishes with the D concentration; in fact, for the sample with 11 wt %, it almost disappears.

Figure 1. Polarized light microscopy images of selected samples of the set SD-# (numbers displayed on top of each image).

S Δ = νQ SnW

The structural order of the samples studied in this work has been determined from 2H NMR and SAXS measurements. The 2 H NMR spectra provide information about the small, but readily detectable, orientational ordering of the water molecules within anisotropic mesophases on account of interactions between the water and the polar head of the surfactants.40 As shown in Figure 2, the spectra of the samples studied here

XS XW

(1)

where S is the average order parameter of the bound water, XS and XW are the mole fractions of surfactant and water, respectively, and νQ is the effective quadrupolar coupling constant for deuterium in deuterated water. In the presence of a third component, the splitting can be modified. When the additional component is hydrophilic, the hydration layer of the surfactant is disturbed, decreasing the average number of bound water molecules. Therefore, the concentration and hydration capacity of the third component should be considered, and the following modification of the above expression has been proposed:7 Δ

XW S XS D = νQ SnW − νQ SnW XD XD

(2)

where XD is the mole fraction of the third component, D in this case, and nDW is the reduction in the average number of water molecules bound to the surfactant, per molecule of D. The representation of eq 2 is depicted in Figure 2, with the data obtained from the samples without macroscopic phase separation. The ratio between the intercept and the slope provided by the linear fit of the data is nDW/nSW = 0.88, and knowing that nSW = 2.6 for the binary AOT/water system,46 it is obtained that nDW = 0.34. The value obtained in this work for nDW is lower than the previously reported for N-alkyl-substituted acrylamides in the AOT/water system (0.6−2), for which the distortion of the hydrating layer and the strength of this perturbation depend on the hydrophobic character of the alkyl side groups.7 Here, the main difference is the ionic character of D; in fact, a competition between D and the AOT counterion (Na+) is expected, which leads to a modification of the hydration layer. SAXS was employed in order to obtain the structural parameters of the mesophase. The diffractograms corresponding to the set DS-# (see Figure 3) present two or three peaks with relative positions 1:2:3, characteristic of a lamellar structure, Lα , with the exception of the sample with the lowest AOT content, which only exhibits the first order diffraction peak. Additionally, two features characteristic of the binary AOT/water system can be observed: (a) The intensity of the first order diffraction peak varies with the AOT concentration:

Figure 2. Upper: 2H NMR spectra of deuterated water in the samples HDS-45 and HSD-0.5. Bottom: 2H quadrupolar splitting Δ, scaled by the molar ratio water−monomer, XW/XD, vs the molar ratio surfactant−monomer, XS/XD. Squares, experimental data for the samples without macroscopic phase separation; solid line, fit of eq 2.

exhibit the characteristic powder pattern, i.e., two peaks and two shoulders, resulting from an isotropic distribution of microcrystallite orientations.39 Some samples also present a central peak that can be associated with microcrystallites that are smaller than the micrometer-scale diffusional displacement of water during the acquisition of the 2H spectra.41,42 One 1161



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Figure 3. Left: SAXS diffractograms of samples belonging to the set DS-#. Right: inverse lamellar spacing, 1/d, as a function of the surfactant volume fraction, ΦS.

Figure 4. Left: SAXS diffractograms of samples belonging to the set SD-#. For the samples with macroscopic phase separation, only the diffractograms of the bottom phase are depicted. Right: inverse lamellar spacing, 1/d, as a function of the monomer volume fractions, ΦD. Filled symbols correspond to the bottom phase in two-phase samples.

at first, it decreases and at higher surfactant concentration it increases.21 (b) A broad hump47 for wave vectors, q, between 1 and 5 nm−1. From the position of the first order diffraction peak, the lamellar spacing can be obtained as d = 2π/q. Figure 3 shows that d decreases with the AOT content in the set DS-#, following the dilution law that relates d with the composition in lamellar systems:

1/d = ΦS /dS

lamellar systems, and it has been explained as a consequence of the weakening of the effective interlayer interaction.48 In the samples with macroscopic phase separation, the same pattern is found for the bottom phase, with the exception of the sample with 8 wt % D, for which the first order peak has vanished. On the other hand, the upper phase of these samples shows a broad and weak peak, with the ratio between the position of this peak and the first diffraction peak of the bottom lamellar phase being around 1.6−1.7. This could indicate the existence of a sponge L3 structure in the upper phase, according to previous results reported for the AOT/brine system.49,50 Unfortunately, it is not possible to perform a more precise study of the L3 phase, due to the overlapping of this peak with the signal corresponding to the primary beam; therefore, additional experiments, could be performed to settle this assignment, e.g., freeze fracture TEM.51 In the set SD-#, the concentration of AOT is almost constant and the lamellar spacing does not change in the monophasic samples, suggesting that the monomer does not interact with the bilayer. In fact, it has been reported that, when a solute interacts with the surfactant, it produces a variation of the

(3)

where dS is the bilayer thickness and ΦS is the volume fraction of surfactant. The linear fit of the experimental data (Figure 3) renders a value of dS = 1.96 nm, in agreement with that previously reported for the binary AOT/water system.47 The samples of the set SD-# with a D content lower than 3 wt % show the same pattern, with two peaks at relative positions 1:2, but for the other monophasic samples, only the first order peak is observed. The intensity of the first order diffraction peak strongly diminishes with the D content, which would explain the vanishing of the second order peak (Figure 4). A similar behavior has been found in other surfactant/brine 1162



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Figure 5. van der Waals, B̅ vdw, electrostatic, B̅ el, and undulation, B̅ und, components of the smectic compression moduli and total smectic compression modulus, B̅ total, as a function of the volume fraction of D (left) and AOT (right).

Figure 6. X-ray induced polymerization of the sample SD-5. Left: Selected SAXS diffractograms for irradiation times corresponding to 10, 250, 550, and 1100 s, starting from the bottom. Right: Evolution of the diffraction peak of the complex with irradiation time.

lamellar spacing.7 On the other hand, Figure 4 shows that, in the samples with macroscopic phase separation, the lamellar spacing of the bottom phase decreases with the D content, indicating that this phase is enriched in surfactant. Additionally, in most of the samples, two weak peaks, at 2.14 and 4.27 nm−1, were detected which, as discussed below, are due to the presence of PD in the samples. With the aim of determining the influence of D in the lamellar phase and to obtain the critical conditions for the phase separation, the following model, which considers the dominant interactions, has been employed. In the lamellar phase, the distance between the bilayers depends on long-range interactions comprising attractive and repulsive contributions. The most important ones are van der Waals, electrostatic, and undulation; the van der Waals interaction is attractive, while the others lead to repulsion. The total interaction is usually given as

the sum of the individual potentials,52 defined as the free energy per unit bilayer area (see Appendix I for a detailed explanation of the expression employed to calculate the potentials: Vvdw, Vel, Vund); thus, V = Vvdw + Vel + Vund

(4)

On the other hand, D can be considered a strong electrolyte and, therefore, its incorporation in the AOT/water system induces a shielding that strongly diminishes the electrostatic potential and the rigidity of the bilayers with the consequent modification of the undulation interaction. Additionally, the van der Waals interaction is also affected through the zerofrequency contribution of the Hamaker constant, which is essentially an electrostatic interaction.53 1163



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Figure 7. SAXS diffractograms of the polymerized samples SD-# with D concentration higher than 3 wt %, after 1100 s of cumulative irradiation time. Left: samples without macroscopic phase separation. Right: samples with macroscopic phase separation. The bottom phase is depicted as a full line, and the upper, as a dashed line. Lamellar and optically isotropic structures, before polymerization, are indicated by black and red, respectively.

by studying the polymerization in situ, with the help of X-rays, and no other component as a polymerization agent. X-ray Polymerization. The X-ray polymerization process was followed in situ for the samples with monomer content higher than 2 wt %. As an example, Figure 6 shows the evolution of the SAXS profile with the irradiation time for the sample with 5 wt % D, where the polymerization was followed during 1100 s. At first, the scattering curves show the broad band between 1.5 and 5 nm−1 characteristic of the AOT/water lamellar system, the first order diffraction peak of the swollen lamellar structure (0.753 nm−1) characteristic of the D/AOT/ water system, and, as was mentioned above, two weak peaks (at 2.14 and 4.27 nm−1). It was previously reported that PD forms lamellar complexes with AOT, C, which are due to the interactions between the anionic surfactant and the oppositely charged polyelectrolyte,23 giving rise to collapsed structures. In this kind of systems, it has been proposed that the polymer is adsorbed flat onto the surfactant head groups of the individual bilayers.56 Therefore, the weak peaks appearing in the first diffractogram at 2.14 and 4.27 nm−1 would indicate that there is a small amount of complex in the sample. This could be due to the existence of PD impurities in the sample before the irradiation, but as we will see in the following, PD could also be generated by X-ray induced polymerization, even at short irradiation times. Indeed, the SAXS pattern of the samples is modified as a consequence of the irradiation as follows: (i) the intensity of the peak corresponding to the swollen lamellar phase, Lα , diminishes and finally vanishes and, at the same time, a broad and weak band appears at lower q values, having similar characteristics to the peak found for the upper phase of the samples with macroscopic phase separation before polymerization. These changes suggest that the swollen lamellar phase is transformed into a L3 phase, which also vanishes as the polymerization progresses. (ii) The intensity of the peak of the complex, C, increases, indicating that more phase of the complex is formed. On the other hand, its position changes slightly, from 2.14 to

According to the method proposed by Ligoure et al., in the absence of guest polymer molecules, the layer compression modulus, B̅ i , can be calculated from the potential as52 Bi̅ = d

∂ 2Vi ∂dW 2

(5)

where i refers to the different interactions and dW is the water layer thickness. The total smectic compression modulus, B̅ total, can be calculated as the sum of the attractive and repulsive B̅ i contributions. For a stable lamellar system, B̅ total should be positive; therefore, when it vanishes, the system becomes unstable and a phase separation occurs. In this system, macroscopic phase separation is observed for the samples with a D concentration higher than 5 wt %. Therefore, the vanishing of B̅ total is expected for a D concentration between 5 and 6 wt %. Figure 5 depicts the B̅ i values calculated using eq 5. In the case of the undulation contribution, the value of the bilayer bending modulus previously reported49,54 for the AOT/water system (κ = 3kBT) does not allow the phase separation observed in the samples with concentration of D higher than 5 wt % to be explained. Instead, it is necessary to assume a higher value, κ ≈ 17kBT, which is closer to the ones proposed for phospholipids (25−35 in kBT units).55 This κ value also explains the results of the set DS-#, where there is no phase separation and, consequently, B̅ total calculated is always positive. In summary, the above results describe the influence of the D monomer on the AOT/water lamellar system, indicating that the swollen lamellar phase remains stable until a D concentration close to 5 wt %, and this range of stability can be well explained by the balance of interactions calculated with the model just proposed. On the other hand, it has previously been reported that, when the polymer (PD), instead of the monomer, is incorporated into the lamellar AOT/water system, it forms a polymer−surfactant complex which gives a new collapsed lamellar phase. Therefore, it seems interesting to follow the whole process, from dissolved monomer to polymer, 1164



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2.02 nm−1. In fact, for intermediate irradiation times, the coexistence of two peaks in this region is observed (Figure 6, right). Similar patterns were found in the other samples. Nevertheless, in the samples with macroscopic phase separation, there are small differences in the evolution of the peaks with the irradiation that could be related with the different compositions of the phases. The initial diffractograms corresponding to the upper phase do not exhibit the peaks of the complex, although these are formed after a few seconds of irradiation, indicating that the monomer is distributed over the two macroscopic phases. After 1100 s of cumulative irradiation time (see Figure 7), the position of the peak belonging to the complex is almost the same, 2.04 ± 0.03 nm−1, which corresponds to a lamellar spacing of 3.09 ± 0.05 nm. At the end of the experiment, only the collapsed lamellar phase of the complex is detected by SAXS, with the exception of SD-3 for which a L3 phase is also detected. Nevertheless, it is observed that the samples become highly turbid in the region of the capillary where the X-rays traverse the sample, suggesting that the polymerization induces a phase separation process. Thus, as the polymerization progresses, the polymer is excluded from the swollen lamellar phase, retaining a part of the AOT and a small part of the water, giving rise to the collapsed lamellar phase containing the complex.57,58 Therefore, the AOT concentration of the swollen phase decreases and finally this phase becomes isotropic. The unreacted monomer can be located in both phases. On the other hand, the lamellar complexes, obtained by simple addition of PD to the lamellar AOT/water system, have spacings that vary only slightly from 2.84 to 2.90 nm, as a function of the sample composition.23 Therefore, the complexes formed by in situ polymerization are slightly thicker, indicating that the structure of the complexes could also be influenced by how the polymer is introduced. Unfortunately, it is not possible to analyze the composition of the samples after the in situ polymerization because the reaction occurs in the region where the sample is irradiated, i.e., a few millimeters. Nevertheless, some arguments can be proposed to explain these differences: (i) a molecular weight rather different for the polymer obtained in situ, (ii) presence of unreacted monomer, and (iii) a different structure of the polymer chains, e.g., existence of branches and/or reticulations.

The formation of polymer induces a splitting of the original lamellar phase into two phases, one of them being a collapsed lamellar phase, corresponding to the complex formed between the surfactant and the polymer, and the other a swollen phase, mainly formed by part of the surfactant and most of the water. X-ray polymerization is a new method to incorporate polymers into complex systems and could therefore improve the efficiency of technological applications where the polymers act as a flocculant agent.

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APPENDIX I The van der Waals interactions between two surfactant bilayers separated by a water layer with a thickness dW can be determined with the following expression Vvdw = −

⎤ AH ⎡ 1 2 1 ⎢ 2 − ⎥ + 12π ⎣ dW (dW + dS)2 (2dS + dW )2 ⎦ (6)

where AH is the nonretarded Hamaker constant that can be evaluated from the following expression after considering the screening effect:53 AH =

⎛ ε − εW ⎞2 3 kBT ⎜ S ⎟ (2dW κ )e−2dWκ 4 ⎝ εS + εW ⎠ +

3hνe(nS2 − nW 2)2 16 2 (nS2 + nW 2)3/2

(7)

with εi being the dielectric constant and ni the refractive index, for i = W (water) or S (surfactant). κ is the Debye screening length, νe is the plasma frequency of free electron gas, and h is the Planck constant (Table 1 contains the constants employed Table 1. Values of the Constants Employed to Estimate the Different Contributions to the Potential

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CONCLUSIONS The monomer diallyldimethylammonium chloride perturbs the lamellar mesophase of the AOT/water system, and when the monomer concentration is higher than 5 wt %, the results suggest that the lamellar phase is partly converted into an L3 phase. In the lamellar phase, the monomer is located in the water layer and its cationic part competes with the sodium counterions of the surfactant, leading to a modification of the hydration layer. The monomer is a strong electrolyte that induces a shielding of the charged bilayers, diminishing the electrostatic potential and the rigidity of the bilayers and, consequently, modifying the undulation interactions. A model, which evaluates the different contributions to the layer compression modulus, can be employed to explain the experimental results, i.e., the macroscopic phase separation. The monomer is polymerized by X-ray irradiation in the absence of a chemical initiator, and the process is followed by the change in the SAXS profiles using synchrotron radiation.

constant

symbol

AOT dielectric constant H2O dielectric constant Bjerrum length in water at 25 °C AOT refractive index at 25 °C Planck constant main electronic absorption frequency in the UV permittivity of free space AOT valency electronic charge Boltzmann constant surface area per polar group of AOT surface charge density on AOT bilayer

εS εW lB

24 (extrapolated) 78.3 e2/4πε0εWkBT = 7.2 Å

value

ref 60 61

nS h νe

1.463 6.626 × 10−34 J s ∼3 × 1015 s−1

62 61 53

εo Z E kB Σ

8.854 × 10−12 C2 J−1 m−1 1 1.602 × 10−19 C 1.381 × 10−23 J K−1 65 Å2

61

σ

e/Σ = −0.246 C m−2

61 61 47

to estimate the different contributions to the potential). The first term of AH is the zero frequency contribution, that can be neglected for the salt concentrations employed in this work. Therefore, the Hamaker constant is determined by the second term, i.e., the dispersion contribution, rendering a value of 4.5 × 10−21 J, which is lower than the one obtained for the AOT/ water system without salt (5.9 × 10−21 J).59 The repulsive electrostatic interaction between two charged surfaces separated by an aqueous solution with large enough 1165



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added salt concentration52 can be evaluated with the following expression obtained from the Poisson−Boltzmann equation Vel =

4kBT 2 −dW / λD′ γ e πλD′ lB

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(8)

where ⎡1 ⎛ l λ ′ ⎞⎤ γ = tanh⎢ argsinh⎜2π B D ⎟⎥ ⎝ Σ ⎠⎦ ⎣2

(9)

and λD′ =

0.304 (nm) cS′

(10)

where the effective salinity, cS′ , is larger than the real one, cS, due to the Donnan effect, and can be calculated as63 cS cS′ = 4λ′ 1 − dD (11) W

In lamellar systems, the bilayers are not rigid, and consequently, the undulation movements induce a repulsive interaction related to the confinement produced when two bilayers approach each other, which is given by Vund =

3π 2(kBT )2 128κdW 2

(12)

where κ is the bilayer bending modulus.

■

AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +34913987390. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work received financial support from MICINN (Spain), under grant CTQ2010-16414, and from DGUI (Comunidad de Madrid), under R&D Program MODELICO-CM/S2009ESP1691. We are indebted for beam time in the line BM16 (Spanish CRG) of the ESRF (Grenoble). Prof Arturo Horta is gratefully acknowledged for helpful discussions and advice.

■

REFERENCES

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