Water-Responsive Internally Structured Polymer–Surfactant Films on

Article pubs.acs.org/Langmuir

Water-Responsive Internally Structured Polymer−Surfactant Films on Solid Surfaces Charlotte Gustavsson,† Joaquim Li,† Karen J. Edler,‡ and Lennart Piculell*,† †

Department of Physical Chemistry, Lund University, P. O. Box 114, SE-22100 Lund, Sweden Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7 AU, United Kingdom

‡

S Supporting Information *

ABSTRACT: Water-insoluble films of oppositely charged polyion− surfactant ion “complex salts” (CS) are readily cast on solid surfaces from ethanolic solutions. The methodology introduces new possibilities to study and utilize more or less hydrated CS. Direct SAXS measurements show that the surface films are water-responsive and change their liquid crystalline structure in response to changes in the water activity of the environment. In addition to the classical micellar cubic and hexagonal phases, a rectangular ribbon phase and a hexagonal close-packed structure have now been detected for CS composed of cationic alkyltrimethylammonium surfactants with polyacrylate counterions. Added cosurfactants, decanol or the nonionic surfactant C12E5, yield additional lamellar and bicontinuous cubic structures. Images of the surfaces by optical and atomic force microscopy show that the films cover the surfaces well but have a more or less irregular surface topology, including “craters” of sizes ranging from a few to hundreds of micrometers. The results indicate possibilities to create a wealth of water-responsive structured CS films on solid surfaces.

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use of such coatings to give low adhesion and low friction.26 Notably, however, the cited studies were concerned with dry films. To the best of our knowledge, no published study to date addresses the consequences of water uptake for the structures of CS films, which is the topic of this article. Specifically, we look at surface films of water-insoluble alkyltrimethylammonium polyacrylate (CnTAPAp, where n is the number of carbons in the surfactant tail and p is the degree of polymerization of polyacrylate). We demonstrate that such coatings are typically water-responsive, changing their structure when their hydration is changed, and that stable and structured coatings persist even when the surfaces are totally immersed in water. Films with a gradient in the water content can be generated by simple experimental setups, revealing new details of the CS phase behavior. By adding a cosurfactant to the film, additional structures can be obtained.

INTRODUCTION Bulk systems of “complex salts” (CS), that is, surfactant ions neutralized by polymeric counterions, display a rich variety of liquid crystalline structures both in the dry state1−3 and in mixtures with water.4 Though typically water-insoluble, CS generally swell in water, taking up of the order of 10−20 wt % of water in ambient air and 40−70 wt % water in contact with pure water.4,5 Changing the water content is thus a simple way to control the structure of CS, and additional structures form with added cosurfactants. The knowledge of structures of hydrated CS in the bulk, with or without added (co)surfactant, has increased significantly through recent studies on bulk phase behavior.5−14 However, little has been done to utilize the structural richness of hydrated CS for coatings on solid surfaces. Studies of layers adsorbed from dilute aqueous mixtures of polyelectrolytes and surfactants have been performed using a variety of protocols,15,16 but the layers thus produced typically thin out substantially by desorption if the surrounding solution is diluted, as in typical laundry or shampoo applications.17 The layer-by-layer strategy has also been used to produce thin CS surface films, but studies published to date are few and the protocols are laborious, typically involving cycles of dipping, rinsing, and drying steps.18−21 Important steps toward efficient CS coating processes were the discoveries that CS can be dissolved in certain organic solvents22,23 and that surface films, or coatings, can be cast on solid surfaces from such solutions.22,24−27 Thus, Thünemann et al. studied spin-cast films of fluorinated CS and suggested the © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials. Polyacrylic acids (Aldrich) of MW 1800 g/mol (PA25) or 450 000 g/mol (PA6000) were purified by dialysis against Millipore water (5 days, Spectrum Laboratories membranes, MW cutoffs of 500 or 10 000). Dodecyltrimethylammonium bromide (TCI Europe, purity >99%), cetyltrimethylammonium bromide (Merck, PA grade), pentaethylene glycol dodecyl ether (C12E5, Nikko chemicals, purity Received: October 11, 2013 Revised: September 26, 2014

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99%), octaethylene glycol dodecyl ether (C12E8, Nikko chemicals, purity 99%), and 1-decanol (Aldrich, purity >99%) were used as received. Each CS was prepared by titrating the chosen poly(acrylic acid) with the hydroxide form of the chosen surfactant, prepared by ion exchange, as described elsewhere.5 Film Preparation. CS (with, in a few experiments, added cosurfactant) was dissolved in ethanol at 10−30 wt %. Surface films were made at room temperature by spreading 50−1000 μL of solution with a pipet on a freshly plasma cleaned mica sheet, covering 8−11 cm2 of the sheet. By varying the size of the mica sheet, as well as the concentration and amount of the solution, films of different dry thicknesses could be produced. The ethanol of a freshly cast film was evaporated overnight before further experiments were performed, unless otherwise specified. The same protocol was used to cast films on hydrophilic silica, hydrophobized or positively charged silica (obtained by silanization with dimethyloctylchlorosilane and 3aminopropyltriethoxysilane, respectively,28,29), and PVC surfaces. Film Characterization. The dry film thickness was evaluated using two methods, a micrometer screw gauge and mass-balance calculations based on (1) the coated area of the mica sheet, (2) the concentration of the initial ethanolic solution, and (3) the assumption of a uniform thickness of the film with a volumetric mass of 1 g/mL for the CS.6 The results of the measured and calculated thicknesses were generally in good agreement (in most cases within ±20%, but with occasional larger deviations), and values reported below refer to calculated dry thicknesses. Measurements with the micrometer screw gauge revealed that the film was thicker, sometimes by as much as a factor of 2, at the edges of the mica. The edge effect was seen for all films, regardless of CS, solution viscosity, and film thickness. The surface topology of CS films on mica substrates was investigated at increasing spatial resolution using optical microscopy and atomic force microscopy (AFM). Microscopy images through crossed polarizers were taken with an Axioplan microscope with an AxioCam MRc camera, both from Zeiss. The illumination was from a mercury arc lamp, and collection was achieved using either a 10× or a 40× objective (NA 0.30 or 0.60, respectively) The AFM images were recorded in noncontact mode using a model XE 100 instrument from Park Systems. Small-Angle X-ray Scattering (SAXS). SAXS measurements were performed at the I911-4 beamline at MAX-lab (Lund). The CScoated X-ray transparent mica substrate was mounted normal (unless otherwise specified) to the X-ray beam with a water bath underneath on a movable platform, see Figure 1. The position of the film relative

Article

RESULTS AND DISCUSSION

Surface Coatings Are Readily Prepared from Ethanolic Solutions. As shown in recent studies of CS phase behavior, CnTAPAp complex salts are highly soluble in ethanol.10 In the present study, drops of 7−30 wt % solutions of CnTAPAp in ethanol were found to readily spread on mica, hydrophilic silica, hydrophobized or positively charged silica, and PVC surfaces, forming films that remained attached to the surface after drying and subsequent immersion in water. Of practical importance for the coating process is the high viscosity of solutions of CS with high values of p. All CS films were sticky to the touch with a transparent appearance in the dry state and turned more opaque when immersed in water. Detailed imaging experiments were performed on CS films without added cosurfactant spread on mica sheets. Microscopy images revealed that, in general, the dry films were not microscopically smooth but had a more irregular appearance, see Figure 2. This was true for all films; however, the film thickness influenced the topology, and a larger thickness resulted in a smoother film as evident from the images d, e, and f in Figure 2, where films of the same CS, but three different thicknesses, are compared. The irregularities in the films often appeared as more or less deep “craters”. The craters observed in the films varied greatly in size, ranging from a few to a few hundreds of micrometers in diameter, and were often gathered in “bunches” of varying size. Patches where the film was considerably thinner were common throughout; however, patches of totally bare mica seemed rare. AFM images gave further insight into the nature of the irregularities, since smaller areas of the film could be investigated in more detail, see Figure 3. The AFM images show craters as small as a few micrometers as well as larger “valleys” and cracks with more irregular shapes. With AFM it was also possible to measure the depth of the craters and the amplitudes of other irregularities. The crater depth varied greatly, from 30−40 nm up to 1−2 μm. Unless otherwise specified, all films studied by SAXS were prepared using the protocol described above. However, we also investigated the visual appearances and images of films subjected to some other treatments. All films studied here stayed intact, forming liquid crystalline structures, when immersed in water. When a dry film was exposed to water it swelled and turned opaque but still adhered to the mica sheet. On the other hand, if a film of a still wet solution in ethanol was gently covered with water, the film could detach from the mica. The CS would then float in opaque aggregates which, nevertheless, could adhere to the mica again when brought into contact. Coatings resulting from ethanol-wet films that were immersed in water before being completely dried were more uneven and patchy upon drying compared to a coating that was allowed to dry prior to water immersion, see Figure 4a. A clear trend is that the earlier the wet film was covered with water, the rougher the dry film became. A drying time of 10 min before immersion in water was sufficient to produce a film with the same visual appearance as a film dried overnight. As shown in Figure S1 in the Supporting Information, no further changes in the appearance of the film were visible by light microscopy after a few minutes of drying of a freshly spread ethanol solution of CS. Placing a water droplet on top of a dry film and watching what happened as it dried revealed interesting phenomena, see Figures 4 b and 4c. In some cases, the retracting water edge

Figure 1. Experimental setup of a “wick experiment” with the filmcoated mica sheet positioned normal to the X-ray beam and partially immersed in a water bath that can be raised or lowered, either alone or together with the mica sheet, thereby varying the vertical distance h between the water surface and the spot irradiated by the X-ray beam.

to the bath could be varied and, also, the distance h between the surface of the bath and the position where the X-ray beam hit the film. Unless otherwise specified, the bottom end of the film was immersed at some depth in the water bath. The intensity, I(q), was monitored at a wavelength of 0.91 Å. A fast read-out pixel detector (Pilatus 1M) was used, and all data were treated with the software Bli911-4. B

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Figure 2. Representative microscopy images through crossed polarizers of the four studied CS films on mica substrates. Films with a dry thickness of roughly 20 μm are shown for (a) C12TAPA25, (b) C16TAPA25, (c) C12TAPA6000, and (d) C16TAPA6000; scale bar = 2000 μm. Images (e) and (f) show thin (5 μm) and thick (80 μm) films, respectively, of C16TAPA6000; scale bar = 50 μm.

and the rest of the film extends vertically out of the water into the ambient air. The water meniscus on the film extends a few millimeters from the surface of the water bath, making the area of the film covered by the meniscus accessible to the X-ray beam. With time, a gradient of water in the film develops, where the film is fully hydrated under the water meniscus but becomes increasingly dry at positions increasingly far from the surface of the bath. Since previously recorded bulk phase diagrams show a structural variation with the water content of the CS, a sequence of different structures is expected to appear with increasing distance from the water surface at steady state, or with increasing time for an initially wet film section pulled out of the bath.5,6,9 Films of Pure Complex Salts. Figures 5 and 6 contain sample SAXS profiles showing all the various structures that were observed for films of pure CS in wick experiments, either in steady-state experiments at different distances from the water surface or at different times after changing the position of the bath. The liquid crystalline structures expected from previous studies of hydrated CS in the bulk, that is, the Pm3n micellar cubic phase and the P6mm 2D hexagonal phase, were indeed found also in the films for all CS studied here. The values of the lattice parameters in the films were in excellent agreement with those previously observed in the bulk.6,9 Moreover, experiments using films of different thicknesses (in the studied range of 20− 80 μm dry thickness) showed no effect of film thickness on the sequences of liquid crystalline structures that were detected for films of the various complex salts, at different water contents, by SAXS. Interestingly, however, the films studied here also displayed two additional structures, which have not been observed or identified in previous bulk studies of binary mixtures of CnTAPAp complex salts mixed with water. One of these was the rectangular C2mm structure (Figure 5c), which can be described as a distorted hexagonal structure where the angles

Figure 3. AFM image (45 μm × 45 μm) with a contour map along the red trace in the image showing small craters in a C12TAPA6000 film dried from ethanol solution in ambient air.

gave rise to an alignment of the liquid crystalline structure. At the position of the boundary between the water droplet and the surrounding film, a string of craters with considerably thinner film was observed after the film had dried again. Different Structures Can Be Produced in Hydrated CS Films. Figure 1 shows the setup in what we call a “wick experiment″: One end of the film is dipped in the water bath, C

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Figure 4. Images of three C16TAPA25 films exposed to water. (a) Water was added gently to the film 2 min after deposition of the ethanolic solution, whereafter the film was dried. (b) Image with a drying water droplet (dark area to the left) creating aligned liquid crystalline domains as it dries. (c) String of craters created at the edge of a drying water droplet. Scale bars = 2000 μm.

Figure 6. SAXS profiles and assigned structures for a drying sequence of a C12TAPA25 film of 30 μm dry thickness at decreasing hydration. The profiles were recorded at (a) 1 min, (b) 5 min, and (c) 30 min after a fully hydrated film was partially pulled out from the bath.

Figure 5. SAXS profiles and assigned structures and cell parameters obtained at decreasing hydration in a drying sequence for a C16TAPA25 film of 30 μm dry thickness, recorded 1 min (a), 10 min (b), and 15 min (c) after the fully swollen film was partially pulled out of the water.

concentrations in water.30 Of particular relevance here are previous observations of rectangular ribbon phases for the alkyltrimethylammonium surfactants C16TABr, C16TACl, and C12TACl.31−33 These have been found in more or less narrow concentration regions situated above 75 or 80 wt % (depending on the system) of the surfactant. At concentrations above 70%

of the hexagon are no longer equal. This structure was observed at low water contents for films of all studied CS, although it did not always appear for the C12 complex salts. We note that analogous so-called “ribbon phases” have been detected for a number of different simple ionic surfactants at high D

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it is difficult to obtain homogeneous samples of aqueous CS by conventional mixing techniques, and, therefore, such high concentrations have typically been omitted in studies of their bulk phase behavior.5,6,9 It is, therefore, possible that the rectangular phase exists also in bulk binary CS/water mixtures but has escaped notice until now. On the other hand, it should be noted that similar rectangular structures, found in very thin surface films mesoporous silica containing cationic surfactant, have been explained as being a result of a pinning of the structure by the substrate surface.34,35 Another structure that has previously not been established for the CS studied here appeared consistently at the highest degree of hydration for the C12 complex salts, see Figure 6a. Again, previous studies of more or less similar systems are helpful and allow us to identify this structure as a hexagonally close-packed (HCP) arrangement of spherical micelles. Chu and co-workers studied a range of slightly cross-linked anionic copolymer gels of poly(sodium methacrylate-co-N-isopropylacrylamide) immersed in solutions containing a charge excess of alkyltrimethylammonium bromide36,37 and found after equilibration that the resulting hydrated charge-stoichiometric CS could form either a Pm3n cubic or a HCP structure. A shorter surfactant alkyl chain length and/or a lower charge content of the copolyion favored the HCP structure. Very recently, Liu and Warr reported that alkyltrimethylammonium surfactants neutralized by phosphate, oxalate, or carbonate cations also form a HCP P63/mmc phase, situated between the Pm3n micellar cubic phase and the disordered micellar phase.38 We note that both the peak positions for the C12 complex salts (Figure 6a) and the location of the HCP phase at higher water contents than the Pm3n micellar cubic phase fit excellently with the findings of Liu and Warr for C12TA surfactants.38 Our results thus support their conclusion that a more extensive hydration of the surfactant micelles facilitates the formation of spherical micelles, required for the HCP structure. Interestingly, we find that the HCP SAXS pattern identified in the present study also agrees with the “anomalous” SAXS pattern that was obtained in early studies in our own laboratory for some two-phase samples of C12TAPA30 and C12TAPA6000 at high water contents.6 The latter observation fits well with the results of the present study, since the film experiments conducted here are carried out at a huge excess of water. The Gibbs phase rule states, however, that the compositions and structures of two phases in equilibrium in a binary system must be independent of the phase volumes; therefore, the previously observed dependence of the structure of the concentrated phase on the relative phase volumes requires an explanation. One possibility is that the effect of dissolved carbon dioxide from the ambient air cannot be neglected at high dilution; this third component, which is present in systems in contact with air, should give rise to a partial protonation of the carboxylate polyion and a concomitant decrease in charge density. This could, in turn, give rise to a change in structure. Indeed, control experiments showed that the pH of the Millipore water used in our experiments was ca. 5.5. However, further experiments, outside the scope of the present investigation, would be required to test the possible effect of carbon dioxide. Films Containing Added Cosurfactants. When long-chain n-alcohols are added to CnTAPAp, aggregates of lower curvature result.10 Using the protocol described above, we could produce water-insoluble surface films from mixed solutions in ethanol of CS and decanol, and Figure 7 shows the lamellar structure of an initially hydrated C16TAPA6000 /decanol film (weight ratio

Figure 7. SAXS pattern from a mixed film of 60:40 wt % C16TAPA6000/decanol (dry thickness ca. 20μm) in a wick experiment. Increasing h from 0.15 mm (black) to 1.5 mm (red) and further to 4.5 mm (green) causes shrinkage of the lamellar repeat distance d. The profiles are offset for clarity.

60:40, 1:1.5 molar ratio) at various distances above the water surface in a wick experiment. As expected, the lamellar repeat distance decreases with decreasing water content (that is, increasing distance from the water surface). It was also possible to make films containing the nonionic surfactant dodecylpenta(ethylene oxide), C12E5, in addition to the CS, from mixed solutions of nonionic surfactant and C16TAPA6000 in ethanol. Previous bulk studies have shown that hydrated mixtures of CS and nonionic surfactant give rise to bicontinuous cubic structures in certain composition regions.14 For the aqueous mixtures of C 16 TAPA 6000 and C 12 E 5 specifically, a composition region was found where the concentrated bicontinuous structure was stable in equilibrium with an excess dilute solution of C12E5. In the present study, a bicontinuous structure was indeed obtained close to the water bath surface in a wick experiment for a surface film containing a 40:60 wt % (1:1.3 molar ratio) C16TAPA6000/C12E5 mixture, see Figure 8. Notably, the structure in the film switched to lamellar or hexagonal at a lower water contents, further away from the water surface (Figure 8b). Since the recorded SAXS profile only contained the √1 and √4 reflections, the latter structure could not be unequivocally determined; however, previous bulk phase studies confirm that either a hexagonal or a lamellar structure is indeed expected at lower water contents at the studied weight proportions of C16TAPA6000 and C12E5.14 The q value of the dominating first peak gives little guidance, since similar values have previously been obtained both for hexagonal C16TAPA6000/C12E5/water samples and for lamellar C16TAPA6000/octanol/water samples.10,14 Anisotropic Phases Are Oriented with Respect to the Substrate Surface and Show Some History Dependence. When liquid films containing colloidal objects are spread and dried on surfaces, orientation effects are often observed. The flow of the wet liquid, the drying process, and, obviously, the presence of the surface itself can induce an ordering of, especially, elongated objects such as polymers and surfactant rods. To investigate the possible presence of orientation effects, we studied the full 2D scattering patterns from a number of films, recorded both in the standard setup with the beam normal to the surface and in a geometry where the film had been tilted ca. 45° backward, so that the beam hit the surface at the corresponding angle. Representative 2D images are shown in Figure 9 for the rectangular phase of the C16TAPA25 complex salt. The left image shows only marginal variations in the scattering intensity with a changing azimuth angle for each of E

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orientation relative to the surface. In the left image, the first peak is more intense, but this varies, depending on the azimuth angle, for the tilted film. The latter observation sheds light on another observation, made when studying a rectangular CS film subjected to different hydration protocols. Initially, the film had undergone the standard protocol of casting from ethanol solution and drying, before being placed in the humidity gradient above an open water bath with the bottom end positioned just above the water bath. A rectangular SAXS profile was recorded after equilibration. Subsequently, the film was totally immersed in water, dried, and studied again in a wick experiment, with the bottom end immersed in the water bath. The same rectangular phase appeared in both experiments, but the relative intensities of the two first peaks in the C2mm rectangular SAXS pattern had changed (patterns shown as Figure S2 in the Supporting Information). Evidently, annealing in water, in a process where the film passes through the micellar cubic state, allows the average orientations of the microcrystals to change relative to the surface normal.

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CONCLUSIONS AND OUTLOOK The findings in this study have implications both for fundamental studies of polyion−surfactant ion complex salts and for possible applications. The wealth of structures established for bulk phases of hydrated CS, with or without additives, suggest a similarly rich variation in surface films. The “wick” experiments introduced here rapidly produce phase sequences of CS films at varying water contents, which should be of considerable general interest. Phase sequences can be determined in detail, and in particular water-poor states difficult to obtain as homogeneous bulk samplesare easily created. Indeed, two structures that have not previously been established for hydrated CnTAPAp complex salts were demonstrated here. Kinetic studies are obviously of interest when investigating responses of the films to changes in the environment, and such studies are currently in progress at our laboratory. Another aspect of interest, also under current investigation in our laboratory, is the responsiveness of the coatings to additives such as surfactant or salt in a surrounding solution. While the liquid crystalline structure of the phase at a given water content seems independent of the film history, it is interesting to note that it is possible to vary the alignment of liquid crystalline domains and, also, the detailed surface topology of the surface film, by the process of making the film.

Figure 8. SAXS patterns from a roughly 30 μm mixed film of 40:60 wt % C16TAPA6000/C12E5 in a wick experiment. Increasing h from 0.2 mm (a) to 6 mm (b) above the water meniscus causes the structure to switch from bicontinuous cubic (a) to lamellar or hexagonal (b).

Figure 9. 2D SAXS patterns of a C16TAPA25 complex salt film displaying a C2mm rectangular structure irradiated with the incident beam normal to the film (left image) and with the film tilted 45°, see text (right image). The two bright inner circles correspond to the intense two peaks in the low q region of the angle-averaged SAXS pattern, Figure 5c. Film thickness is approximately 80 μm. Inserted arrows highlight areas with higher intensities in the tilted film, indicating orientation of the microcrystals.

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ASSOCIATED CONTENT

S Supporting Information *

Drying sequence for a CS film and SAXS patterns for C2mm with varying relative intensities. This material is available free of charge via the Internet at http://pubs.acs.org.

the two circles, which correspond to the intense first two peaks of the rectangular scattering pattern (see Figure 5c). This indicates a powder sample with crystalline domains oriented randomly in the plane parallel to the mica sheet. By contrast, the tilted film in the right-hand image shows strong angledependent intensity variations of the two peaks. This immediately shows that the crystalline domains are preferentially oriented with respect to the surface normal. Similar observations, of an isotropic pattern for the vertical film orientation and an anisotropic pattern for the 45° “tilt”, were made for films of all four investigated CS, with both long and short polyions. We note in Figure 9 that also the relative intensities of the two first peaks of the C2mm pattern change with the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Grants from the Swedish Foundation for Strategic research (SSF), the Swedish Research Council (VR), and Lund University (guest professorship for K.J.E.) are gratefully acknowledged. We thank Tomás Plivelic and Sylvio Haas for F

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help with the SAXS experiments and Bernard Cabane and James Holdaway for valuable discussions.

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