Article pubs.acs.org/Langmuir
Water-Insoluble Surface Coatings of Polyion−Surfactant Ion Complex Salts Respond to Additives in a Surrounding Aqueous Solution Charlotte Gustavsson, Marc Obiols-Rabasa, and Lennart Piculell* Division of Physical Chemistry, Lund University, P.O. Box 114, SE-22100 Lund, Sweden S Supporting Information *
ABSTRACT: Hydrated, but water-insoluble, “complex salts” (CS) composed of alkyltrimethylammonium surfactant ions with polyacrylate counterions are known to exhibit a rich phase behavior in bulk mixtures with water and have recently been shown to act as water-responsive surface coatings. Here it is shown, by SAXS measurements, that surface coatings of CS also respond to various added solutes in a surrounding aqueous solution, by altering their liquid crystalline structure. The obtained results provide new information on the phase behavior of CS in contact with water and aqueous solutions. Solutes such as acids, salts, excess ionic surfactant, or water-soluble polymers act on the CS by altering the polyion charge density, screening the electrostatic interaction, changing the curvature of the surfactant aggregate, or increasing the osmotic pressuring in the surrounding solution, all of which may result in a phase transition in the film. In water, all studied CS surface coatings had a micellar cubic structure, which could change to 2D hexagonal, HCP, or disordered micellar structure, depending on the identity of the CS and the identity and concentration of the added solute. For some systems, even dissolved CO2 from the ambient air was sufficient to induce a structural change in the film. Especially the films containing the long polyions remained intact even for large concentrations of solutes in the contacting solutions, and extensive washing in water resulted, in most cases, in films with the “original” structure found in water.
1. INTRODUCTION Over the years oppositely charged polyion−surfactant ion systems have attracted considerable interest due to their versatility and broad range of applications such as rheology modifiers in the food industry, surface deposition agents in personal care products, and drug delivery vehicles.1−6 More recently “complex salts” (CS), consisting of polyions and surfactant ions of opposite charge with no other ions present, have been systematically investigated, resulting in an extensive knowledge about their behavior, especially in mixtures with water.7,8 CS are generally insoluble in water but can take up water from the surrounding environment. In ambient air some CS take up as much as 20 wt % water and, in direct contact with water, as much as 80 wt %.7 Aqueous CS containing polyions with a high charge density typically form liquid crystalline phases, similar to those formed by conventional ionic surfactants at high concentrations.7 Changing the degree of hydration is thus a simple method to gain access to a number of liquid crystalline structures, and if molecules such as cosurfactants are added, additional structures can be obtained.7 CS as structured surface coatings is a comparatively unexplored area; in particular, little has been done to explore them as hydrated coating materials. This could be due to the fact that previously used protocols to generate complex salt coatings from aqueous solutions have some clear drawbacks. © 2015 American Chemical Society
The layer-by-layer technique, for example, is a labor-demanding process that includes several cycles of dipping and rinsing, only to produce a rather thin film in the end.9−12 Another protocol used to deposit films from mixed polymer−surfactant solutions is “coacervation by dilution”, commonly employed in hair care applications.2,13 The drawback with the latter technique is again that quite thin surface layers are typically produced, especially at high dilution of the solution.13 A new approach to producing CS films on solid substrates was introduced by Antonietti et al.14,15 and Thünemann et al.,16 who showed that it was possible to cast CS films from certain organic solvents. Notably, these early studies all concerned “dry” films. Utilizing the fact that CS are soluble in ethanol, our group showed in a very recent study that it is possible to produce CS surface coatings which are facile to manufacture, can coat a variety of surfaces, and are responsive toward changes in hydration.17 The cited study focused on CS consisting of alkyltrimethylammonium surfactants, with alkyl chains 12 or 16 carbons long (CnTA+), and polyacrylate, with a DP of 25 or 6000 (PAp−). The coatings were found to be robust and could be hydrated, dried, and rehydrated several Received: March 4, 2015 Revised: May 25, 2015 Published: May 27, 2015 6487
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salts were stored in a desiccator over silica gel to minimize the water uptake. Sample Preparation. All samples for small-angle X-ray scattering (SAXS) were measured in a specially designed “sandwich cell”, where the film and a desired aqueous solution are contained between two mica sheets (see Figure 1). An ethanolic solution of the CS was
times without being damaged. It was also possible to incorporate a cosurfactant, such as decanol or a nonionic ethoxylated surfactant, in the film by simply including it in the original ethanolic solution. The present article continues our work on responsive CS films by focusing on their response to a range of different solutes in surrounding bulk aqueous solutions. In order to fully understand the potential of CS films, it is important to see how they respond not only to changes in water content but also to other triggers like simple salts and surfactants, known to affect the bulk behavior of CS. Can a film be triggered to change its structure simply by adding a well chosen additive to the bulk solution? Will the various additives dissolve the films and, if so, at what concentrations? These are questions that need to be addressed in order to think of possible applications like controlled release or selective membranes. We will here demonstrate that indeed CS films are susceptible to ionic surfactants, polymers, and simple salts added to a surrounding bulk solution. The additives have been chosen to demonstrate that one can influence the CS films in different ways, by directly incorporating small molecules in the surfactant aggregates in the film, by changing the charge density of the polyion in the CS, by screening the electrostatic interactions, or by altering the osmotic pressure in the bulk water. We will also demonstrate that the observed effects can be understood with the help of equilibrium phase diagrams obtained in bulk studies. Finally, the reversibility of the solute-induced structural changes is studied by equilibrating the coatings against pure water after having initially equilibrated them against a solution.
Figure 1. Holder for sandwich experiment. Left image shows the actual cell and the right its contents in a schematic cross section. prepared with a concentration of 10 wt %, and a film was cast by spreading five portions, each with a volume of 10 μL, of this solution on one of the mica sheets. The film was allowed to dry after each deposit to ensure that no ethanol was present to influence the SAXS measurements. After depositing the film and allowing the ethanol to evaporate, a film with an approximate dry thickness of 25 μm containing 4 mg of CS was obtained. On top of this film 60 μL of the desired solution was added, confined by an O-ring, followed by the second mica sheet. After closing the cell, the sample was left to equilibrate for 2 days. Samples to check for reversibility of the soluteinduced film structures were prepared in the following way. First, a film was cast as described above, but instead of being placed in the holder it was placed on the bottom of a small container with 60 μL of the desired solution on top, after which the container was sealed and left to equilibrate for 2 days. Second, the solution was removed, and the film-coated mica sheet was placed in an unstirred larger reservoir containing approximately 20 mL of Milli-Q water. The water was changed 3−4 times a day for 3 days, whereupon the film was transferred to the holder, 60 μL of Milli-Q water was added, and the holder was sealed. Small-Angle X-ray Scattering. Small-angle X-ray scattering measurements were performed in order to reveal the structures of the CS films. The experiments were carried out using a Ganesha 300XL (SAXSLAB ApS, Skovlunde, Denmark), a pinhole-collimated system equipped with a 2D 300 K Pilatus detector (Dectris Ltd., Baden, Switzerland), a Rigaku 100XL microfocus sealed X-ray tube (Rigaku, The Woodlands, TX), and a Genix 3D X-ray source (Xenocs SA, Sassenage, France), generating X-rays at a wavelength of 1.54 Å. The scattering data were collected for 2.5 h at a sample-to-detector distance of approximately 350 mm, giving a q-range of 0.014−0.753 Å−1. The magnitude of the scattering vector is defined as q = (4π/λ) sin(θ/2), where λ corresponds to the X-ray wavelength and θ is the scattering angle. The temperature was kept at 22.5 ± 0.5 °C by means of an external recirculating water bath.
2. EXPERIMENTAL SECTION Chemicals. Poly(acrylic acid) (PAA) samples with nominal molar masses of 2000 and 450 000 g/mol containing 30 and 6000 repeating units, respectively, were bought from Sigma-Aldrich. Both poly(acrylic acids) were purified by dialysis against Millipore water using cellulose membranes from Spectrum Laboratories with a molecular cutoff of 500 and 10 000, respectively. The dialyses were carried out for 5 days with daily changes of water. Dodecyltrimethylammonium bromide (C12TABr) from TCI Europe (purity >99%) and cetyltrimethylammonium bromide (C16TABr) from Merck (purity >99%) were used without further purification. Poly(ethylene glycol) (PEG, 20 kDa) from Fluka Analytical, sodium chloride (purity >99%) from Merck, and sodium bromide (purity 99,9%) from VWR Promolab were all used as received without further purification. Water purified with a Milli-Q system (Millipore Corporation, Bedford, MA) was used throughout this study. Preparation of Complex Salts and Acetate Surfactants. All complex salts and acetate surfactants were synthesized by titrating the hydroxide form of the surfactant with the respective acid until the equivalence point was reached. In the first step of the synthesis a solution of initially ca. 10 wt % CnTABr was converted to CnTAOH by successive treatments with three batches of a hydroxide-loaded Dowex anion exchanger (each time at an excess capacity), giving a final CnTAOH concentration of approximately 0.05 M. After this, a solution of polyacid (0.25 M) or acetic acid (0.5 M) was added dropwise over a period of 2−3 h to the solution of CnTAOH under constant stirring. The titration was stopped at the previously determined equivalence point. A detailed description is given elsewhere.8 The pH values at the equivalence points for C12TAPA25, C12TAPA6000, C16TAPA25, C16TAPA6000, C12TAAc, and C16TAAc were determined individually to be 8.7, 8.9, 8,7, 8.3, 8.9, and 8.8, respectively. All complex salts were left in a refrigerator overnight, followed by freeze-drying. Complex salts containing excess PAAp or CnTAOH were produced in the same manner, the difference being that the addition of the polyacid was continued beyond, or terminated before, the equivalence point. All
3. RESULTS Films of Complex Salt in Water. As a point of departure the liquid crystalline structures for the four investigated CS, C12TAPA25, C12TAPA6000, C16TAPA25, and C16TAPA6000, equilibrated against 60 μL of water (see the Experimental Section) were determined. In agreement with several previous studies of CS in bulk and as surface coatings, the micellar cubic Pm3n structure was found for all four salts also in this setup.17−19 We note, however, that deviations from this uniform behavior for the investigated complex salts have been observed in previous studies: the 2D hexagonal phase, rather than the Pm3n cubic phase, has sometimes been found for C16TAPA6000 in equilibrium with excess water.18 Studies of 6488
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Figure 2. SAXS patterns from C12TAPA6000 film exposed to different amounts of water: (a) 60 μL was added onto a dry film or (b) the film was preexposed to 20 mL of water prior to the measurement in the sandwich cell.
amount of water used for equilibration was reproduced here for C12TAPA6000 films. Figure 2a shows that a micellar cubic structure (Pm3n) was obtained for a film equilibrated with a small volume (60 μL) of water in the cell. By contrast, Figure 2b shows that a film that had been soaked for 2 days in a large excess of water (20 mL) in an open vessel, prior to mounting in the cell, displayed an HCP structure. Effect of Excess Polyacid or CnTAOH. It is well established that the polyion charge density can influence the structure of hydrated CS.19−21 Poly(acrylic acid) is a weak acid, and to get a better understanding of how the charge density influences the structure of CS, films containing excess uncharged poly(acrylic acid) units were here prepared according to two different protocols. In the first protocol, a CS was prepared where the titration of CnTAOH with PAAp was deliberately continued past the equivalence point, to produce a complex salt that finally contained a mixture of 77 mol % of ionic CnTA acrylate units and 23 mol % of uncharged acid units. This “copolymer” complex salt was then used to prepare a film that was subsequently contacted with water. In the second protocol, the usual stoichiometric CS was used, but the film was then contacted with a hydrochloric acid solution at a molar ratio of 1:4 (HCl in the solution:acrylate repeat units in the film). Assuming a quantitative reaction of the strong acid with the polyacrylate base, the second protocol produced, in addition to a 25 mol % content of uncharged acrylic acid units in the complex salt, an equivalent amount of excess CnTACl in the film. In addition to these experiments with excess acid, two control experiments were performed under very alkaline conditions. In the first alkaline experiment, CS with a deliberate excess of C12TAOH was produced by stopping the titration after the addition of ca. 70% of the stoichiometric amount of
Table 1. Structure and Lattice Parameters for C12 CS under Conditions Where the Charge Density of the Polyion Was Reduced, Either by Treating the Film with Aqueous HCl or by Overtitrating the CS in the Synthesis Stepa CS + condition
structure
C12TAPA25 in water (60 μL) C12TAPA25 in aqueous HCl C12TAPA25 with excess PAA25 C12TAPA6000 in water (60 μL) C12TAPA6000 in aqueous HCl C12TAPA6000 with excess PAA6000 C12TAPA6000 in aqueous NaOH C12TAPA6000 with excess C12TAOH
Cub HCP HCP Cub HCP HCP HCP Cub
lattice parameter (Å) 81.6 45.3; 45.7; 82.9 45.7; 47.2; 47.9; 82.0
74.2 76.0 74.3 73.6 76.1
a
See Experimental Section for details. For reference, results from one experiment with an undertitrated CS and one experiment with a contacting NaOH solution are also included; see Experimental Section for details.
ternary mixtures have shown, moreover, that an excess of PA6000 favors the cubic phase,19 whereas an excess of conventional surfactant, such as C16TAAc, favors the hexagonal phase.18 Thus, the structure of hydrated C16TAPA6000 should be sensitive to small excesses of either PA6000 or C16TAOH that could result from the titration process. Turning to the C12 complex salts, these have previously been found to display different structures depending on the proportions of water to complex salt,18 in apparent violation of the Gibbs phase rule. Recently, a 3D hexagonal structure of close-packed spherical micelles (HCP) was indeed found for C12TAPA6000 and C12TAPA25 films equilibrated against a large excess of water in an open bath.17 The dependence of the structure on the
Table 2. Structures and Lattice Parameters of Complex Salt Films Equilibrated Against NaCl and NaBr Solutions at Various Concentrations CS simple salt water (60 μL) NaCl (10 mM) NaCl (50 mM) NaCl (100 mM) NaCl (150 mM) NaBr (10 mM) NaBr (50 mM) NaBr (100 mM) NaBr (150 mM)
C12TAPA25
C12TAPA6000
Cub (81.6 Å)
Cub (82.9 Å)
HCP (46.9; 74.3 Å)
HCP (47.5; 75.1 Å)
micellar solution
HCP(47.5; 72.3 Å)
micellar solution
HCP (48.4; 76.6 Å)
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C16TAPA25 Cub (102.7 Å) Cub (101.9 Å) Cub (102.3 Å) Cub (108.3 Å) Cub (109.4 Å) Cub (102.2 Å) Cub (102.1 Å) Hex (50.7 Å) Hex (52.6 Å)
C16TAPA6000 Cub Cub Cub Cub Cub Cub Cub
(101.0 (101.1 (101.2 (101.8 (102.8 (101.8 (101.9
Å) Å) Å) Å) Å) Å) Å)
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Figure 3. SAXS profiles for C12TAPA25 (left) and C12TAPA6000 (right) films contacted with 5 wt % (red) and 20 wt % (green) aqueous C12TAAc, respectively. The paths for mixing the CS with the surfactant solutions are indicated in the previously established21 equilibration phase diagrams.
Figure 4. SAXS profiles for C16TAPA25 (left) and C16TAPA6000 (right) films contacted with 5 wt % (red) and 20 wt % (green) aqueous C16TAAc, respectively. The paths for mixing the CS with the surfactant solutions are indicated in the previously established8,18 equilibration phase diagrams.
respond to simple salt in a contacting solution. Thalberg et al. showed, in their pioneering study of the phase behavior of alkyltrimethylammonium bromide surfactants mixed with sodium polyacrylate, that the corresponding complex salts were completely soluble in aqueous solutions of sodium bromide at sufficiently large concentrations.22 Later, Hansson and Almgren made studies of the effects of added NaCl or NaBr on stoichiometric aqueous mixtures of C12TA+ or C16TA+ and polyacrylate.23 They found that the C16 complex salt could withstand a higher salt concentration than the C12 complex salt before dissolving completely and that NaBr was more efficient than NaCl to dissolve both CSs. Hansson and Almgren also determined the surfactant concentrations in the separating phases at different levels of saltgenerally higher than those studied here, but still below what was required for complete dissolutionbut the structures of the concentrated phases were not determined.23 The latter is the primary concern of our
PAA 6000 . In the second experiment, a stoichiometric C12TAPA6000 film was exposed to a solution of NaOH at a global hydroxide:acrylate molar ratio of 1:4. The results for the C12 complex salts are shown in Table 1. Interestingly, the only films that gave the micellar cubic Pm3n structure were the stoichiometric films in contact with 60 μL of water and the C12TAPA6000 film that contained an excess of C12TAOH. All other experiments gave HCP structures in the film. Similar experiments with excess polyacid, using either films of overtitrated CS or contacting HCl solutions, were performed for the C16 complex salts (see Supporting Information). As expected from a previous bulk study of C16TAPA6000 with excess polyacid,19 all the latter samples showed a Pm3n cubic structure. Added Simple Salts. Given that electrostatic interactions are the main reason that complex salts do not swell indefinitely in water, it is of interest to investigate how the CS films 6490
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Langmuir study, and Table 2 summarizes the observed phase behavior, with the corresponding lattice parameters, for the four investigated CS films contacted with solutions of NaCl and NaBr at various concentrations. Starting with the C12 systems, one can see that adding 50 mM NaCl or NaBr to a contacting solution transforms the cubic structure of C12TAPA6000 in water to the HCP structure. The latter structure is obtained also at 150 mM NaCl. The C12TAPA25 film, containing the short polyion, is more sensitive to added salt and also reveals an ion specificity in the response: the HCP structure is obtained in 50 mM NaCl, whereas 50 mM of NaBr is sufficient to dissolve the film, producing a single micellar solution in the sandwich cell. At 150 mM, also NaCl dissolves the C12TAPA25 film. For the C16TAPA6000 films, 50 mM NaCl seems to have the same effect as the solution with excess NaOH (48 mM on a concentration basis) studied above (Table 1): both of these films displayed an HCP structure with similar lattice parameters. None of the C16 films dissolved at the investigated salt concentrations (≤150 mM of NaCl or NaBr), but an ion specificity was observed: films contacting the NaCl solutions remained cubic, but NaBr eventually induced a transition to the 2D hexagonal phase for both C16TAPA25 and C16TAPA6000. In NaCl, an increase of the lattice parameter of the cubic cell with increasing salt concentration was noted (see Table 2); the effect was weak for the long polyion but strong at the highest NaCl concentrations for the short polyion. Added Excess Ionic Surfactant. This part of the study concerns added water-soluble amphiphilic additives that can be taken up from a contacting solution and enter into the surfactant aggregates of the film. We focus on the ionic surfactants C12TAAc and C16TAAc, which contain the same surfactant ions as the corresponding complex salts but carry monovalent, rather than polymeric, counterions. Equilibrium phase diagrams of bulk mixtures were already established for all these mixtures and are reported elsewhere.8,18,21 Using the equilibrium phase diagrams for added ionic surfactants of the type CnTAAc as a point of departure for the design of our experiments, two surfactant concentrations were investigated, 5 and 20 wt %, respectively. In Figure 3 below pathways, corresponding to adding a hydrated CS (i.e., the film equilibrated in air) to the respective surfactant solution, are indicated as red (5 wt %) and green (20 wt %) arrows in the respective equilibrium phase diagrams for C12TAPAp. Note that the tip of the arrow in each case gives the total composition of the sample in the sandwich cell, considering the total masses of the initial surfactant solution and the film. The corresponding experiments for C16TAPAp are shown in Figure 4. Common to all eight experiments is that the phases obtained by SAXS from the sandwich experiments agree with the predictions from the equilibrium bulk phase diagrams. Reference SAXS profiles of the micellar surfactant solutions alone, without any film present, can be found in the Supporting Information. The latter micellar scattering profiles are responsible for the broad peaks seen in all profiles in Figures 3 and 4. Structures and, where applicable, lattice parameters for all experiments on films contacted with CnTAAc surfactant solutions are summarized in Table 3. From Figures 3 and 4 and Table 3, it is evident that the CSs based on the long polyion, CnTAPA6000, can tolerate even 20% of ionic surfactant in a surrounding solution without dissolving. However, the structure of hydrated C16TAPA6000 switched from cubic to 2D hexagonal on equilibration against C16TAAc
Table 3. Summary of Structures and Lattice Parameters for the Four CS after Equilibration Against Various Solutions of CnTAAc (Structures in Water Are Included for Reference) C12TAPA25 water (60 μL) 5 wt % CnTAAc 20 wt % CnTAAc
C12TAPA6000
C16TAPA25
C16TAPA6000
Cub 81.6 Å
Cub 82.9 Å
Cub 102.7 Å
Cub 101.0 Å
2 micellar solutions single micellar solution
Cub 82.0 Å
Cub 103.4 Å
Hex 46.5 Å
Cub 82 Å
single micellar solution
Hex 47.3 Å
solutions. There was a slight swelling of the latter phase with increasing surfactant concentration, as indicated by the lattice parameters in Table 3. For the CS with a short polyion, CnTAPA25, on the other hand, adding surfactant to the surrounding bulk solution eventually destroyed the ordered structure, resulting in a disordered micellar solution. Both C12TAPA25 and C16TAPA25 films dissolve completely in the corresponding 20% surfactant solutions. A 5 wt % solution of surfactant was enough to destroy the structure of the C12TAPA25 film but not enough to dissolve the film. After 2 days of equilibration, the cell with the latter system contained two disordered micellar solutions: one concentrated and one dilute. The C16TAPA25 film, on the other hand, retained the cubic structure, somewhat swollen as indicated by an increase in the lattice parameter (Table 3), after equilibration against a 5% surfactant solution. Added Nonionic Polymer. All solution additives previously discussed were chosen since they interact directly with the CS in the film, by changing the polyion charge density, by entering into the surfactant aggregates, or by screening the electrostatic interactions between the polyion and the surfactant aggregate. PEG, on the other hand, does not enter into the film; given that the molecular weight is high enough, it only lowers the water chemical potential or, equivalently, increases the osmotic pressure, in the contacting solution.24,25 The consequence for the CS film is that it dries out to some extent and takes on the liquid crystalline phase that is stable at the new, lower water content. Figure 5 illustrates how C16TAPA25 and C16TAPA6000 films respond to 25 and 40 wt % PEG in the bulk. The 40 wt % PEG solution was very viscous, almost paste-like, and no attempts were made to use PEG solutions at still higher concentrations. With 25 wt % PEG in the solution phase, the C16TAPA25 film had a Pm3n cubic structure, as in water, and there was little difference in the unit cell size, 102.4 and 102.7 Å, respectively. For C16TAPA6000, on the other hand, a dominating hexagonal structure of the film was obtained already at 25 wt % PEG, although one peak, greatly reduced in size, remained from the cubic structure. With 40 wt % PEG in the contacting solution phase, both C16TAPAp films showed very similar hexagonal structures, with rod-to-rod distances of 45.7 and 45.8 Å for C16TAPA6000 and C16TAPA25, respectively. The osmotic pressures for 35 kDa PEG solutions at various concentrations have previously been established by Rand,26 and from these values the relative humidities for a 25 and 40 wt % solution could be determined to be 99.2 and 97.5%, respectively. Data on the water content of CS at these water activities are not available. However, the results are in agreement with previous observations that a C16TAPAp film, partially immersed in an open water bath, displays a cubic phase inside the water meniscus extending along the film above the water surface, but a hexagonal phase forms already at sub-millimeter distances 6491
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Figure 5. SAXS profiles and lattice parameters for C16TAPA25 (left) and C16TAPA6000 (right) equilibrated against water (top) and PEG solutions of 25 or 40 wt %.
Table 4. Structures and Lattice Parameters Obtained in Re-equilibration Experimentsa C12TAPA6000 after 2 days of equilibration against 20 wt % CnTAAc 40 wt % PEG 150 mM NaBr after 2 days of equilibration followed by 3 days of washing 20 wt % CnTAAc 40 wt % PEG 150 mM NaBr reference in water
C16TAPA25
C16TAPA6000
Cub 82.0 Å
single micellar solution Hex 45.7 Å Hex 52.6 Å
Hex 47.3 Å Hex 45.8 Å Hex 50.7 Å
HCP 46.8; 74.3 Å
washed off Cub 102.6 Å Cub 103.0 Å Cub (60 μL) 102.7 Å
Hex 45.9 Å Cub 100.9 Å Cub 100.1 Å Cub (60 μL) 101.0 Å
HCP (exc H2O) 47.0; 74.9 Å
a
Top part of the table summarizes structures and lattice parameters observed after 2 days in contact with the indicated solutions (see the respective tables above) and the bottom half after an initial equilibration, followed by extensive washing in water. Reference values in water are included for comparison.
above the water/film/air three-phase line.17 The differences between the C16TAPA6000 and C16TAPA25 films at 25% PEG in the solution (Figure 5) are in line with the general observation that the cubic structure is more stable, relative to the hexagonal structure, for the CS with the shorter polyion. Re-equilibration of Films against Water. For a film that has changed its structure in response to a solute added to a contacting aqueous solution, it is of obvious interest to investigate whether or not the equilibrium structure in water is regained after extensive soaking of the film in water. Table 4 summarizes the re-equilibration experiments performed in the present study, where the film was first contacted with a solution of the additive for 2 days, followed by 3 days of washing against
Milli-Q water with 3−4 changes of water per day, according to the protocol detailed in the Experimental Section. For easy comparison, we reproduce in Table 4 data, already presented above, on structures and lattice parameters for films that were measured after equilibration in the sandwich cell against the respective solutions, together with data for the various CS equilibrated directly against water. For the C16TAPA25 and C16TAPA6000 films first exposed to PEG or NaBr solutions and for C12TAPA6000 exposed to 20 wt % CnTAAc, the 3-day washing procedure was found to be sufficient to generate the same structures as for the corresponding films equilibrated directly against large amounts of water, namely, a cubic Pm3n structure for the C16 complex 6492
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For the penetration of the more slowly diffusing species, we may assume that water equilibration has already occurred and that the film in to which these species enter is essentially a hydrated CS film with a water content of ca. 50% (cf. the phase diagrams in Figures 3 and 4), which, accordingly, has expanded to a thickness of ca. 5 × 10−5 m. Using the value for the acetate diffusion coefficient in a 50% cubic phase cited above, we obtain a time scale of 6 s for the average salt penetration a distance corresponding to the film thickness. By the same argument, the leaching out of salt from a salt-loaded film by soaking in water should also be rapid. Here and in the following, we do not take into account the effect of phase changes on the self-diffusion coefficient, since the difference in the obstructing effect between spheres (as in a cubic phase) and rods (as in a 2D hexagonal phase) is small.28 Clearly, equilibration of the surfactant ions between the film and a contacting surfactant solution is the slowest equilibration process in the systems considered here. As is evident from the arrows in Figures 3 and 4, depletion of the surfactant solution through transport of molecules in to the film is negligible. We can then regard the contacting surfactant solution as a reservoir with an approximately constant concentration of diffusing surfactant ions. From data presented in ref 30 we estimate the diffusion coefficient for C16TA+ in a 20 wt % contacting solution to ca. 5 × 10−12 m2/s, i.e., more than 2 orders of magnitude larger than in the hydrated film. Therefore, the ratedetermining step for equilibration against an external surfactant solution in our sandwich experiments must be the surfactant diffusion within the film. Of the systems studied here, the slowest equilibration should occur for C16TA+ ions penetrating into the concentrated C16TAPA6000 films (the latter films swell only marginally as a result of surfactant penetration, see Figure 4 and Table 3). With a diffusion coefficient of 10−14 m2/s, we obtain a time of 125 × 103 s = 35 h, approaching the employed experimental equilibration time of 2 days, for C16TA+ to diffuse an rms distance of 5 × 10−5 m across the film. The slow establishment of an equilibrium C16TA+ concentration in the films, coupled with strong indications that only a small surfactant excess is required for the 2D hexagonal phase to be stable, is also the likely explanation for our observation that a C16TAAc-loaded C16TAPA6000 film did not revert to a cubic structure even after an extensive soaking in water (see Table 3 and Figure 4); all C16TAAc was probably not removed from the film. For systems based on the C12TA+ ion, equilibration should be much more rapid, since the fraction of free unimers is higher. Bull and Lindman reported a self-diffusion coefficient of 5 × 10−13 m2/s for C12TA+ in the Pm3n cubic phase of C12TACl.29 Using this value in calculations as above, we obtain a time scale of 2500 s ≈ 40 min for the penetration of the C12 surfactant ion from an external solution. On the topic of film equilibration times, we note that for films of submicron thicknesses, or for dispersions of submicron complex salt particles,30 a complete equilibration of the molecular composition should be much faster than for the films studied here, since the diffusion time is proportional to the square of the film thickness. The relatively thick films of the present study were chosen partly for sensitivity reasons; the recording of each SAXS profile still took 2.5 h. Cubic Structure of Hydrated C12TAPAp Can Easily Be Switched to HCP. Under several different conditions explored in our study, C12TAPAp films equilibrated against liquid water, or various aqueous solutions, exhibited an HCP structure,
salts and an HCP structure for the C12 complex salt (see Figure 2 and the accompanying text). Moreover, the cell parameters were nearly identical to those obtained on direct equilibration of the film against a large volume of water. We should note, however, that for some of these systems (see Tables 2 and 3) the structure does not change, and the unit cell size changes only slightly, at the lowest level of added solute compared to that obtained in pure water. Thus, the SAXS results should in some cases be rather insensitive to a possible presence of remaining solute. By contrast, the C16TAPA6000 film that had originally been contacted with 20 wt % C16TAAc was still hexagonal even after the extensive 3-day washing procedure, indicating that some of the added solute still remained in the film. We defer a more thorough discussion on equilibration and molecular diffusion processes in the films to the Discussion section.
4. DISCUSSION Equilibration of Films against Liquid Water or Aqueous Solutions. The main purpose of this study is to demonstrate that structural transitions can be induced in CS films by contacting the films with aqueous solutions of different types. A detailed experimental investigation of the time scales required for full equilibration of the various molecular species between the films and their surrounding solutions is outside of the scope of our study. Nevertheless, some estimates of these time scales are clearly relevant for the interpretation of our results. To that end, we will in the following use available data on molecular diffusion coefficients in relevant CS and surfactant systems. An NMR study of the self-diffusion the various molecular species in ternary C16TAPA30/C16TAAc/water mixtures was performed by Svensson et al.27 A number of samples with water contents in the range 50−55 wt % were investigated in the Pm3n cubic phase, which extends across the phase diagram for all ratios of complex salt and surfactant (see the phase diagram, reproduced in Figure 4). The various self-diffusion coefficients were found to vary only very weakly with the C16TAPA30/ C16TAAc ratio, a fact that we will use in our analysis. The most rapid diffusion occurred for water, with a self-diffusion coefficient of ca. 5 × 10−10 m2/s. The acetate diffusion was slightly slower, ca. 2 × 10−10 m2/s, and the slowest diffusion was observed for the C16TA+ ion, with an effective self-diffusion coefficient of the order of 10−14 m2/s or less. The reason for the slow surfactant ion diffusion is that the surfactant aggregates in the cubic phase are effectively stationary, so that all surfactant ion transport has to occur via the very small fraction of dissociated surfactant unimers that move in the continuous aqueous domain while exchanging rapidly with the surfactant ions in the micelles.27 The above-cited self-diffusion coefficients immediately suggest that water is the molecular species that will equilibrate most rapidly between a CS film and a contacting reservoir of water or an aqueous solution. However, the initial diffusion of water into the film should be slower than the above cited value of 5 × 10−10 m2/s, since a CS initially in equilibrium with ambient air contains only ca. 10−20 wt % water. Assuming an initial diffusion coefficient of 10−10 m2/s, we find that the time for a water molecule to travel a rms distance in the film corresponding to the film thickness, 25 × 10−6 m, is 3 s. Thus, full equilibration of a pure CS film with water, or a solution of PEG that does not penetrate the film, should be very fast. 6493
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Langmuir rather than the Pm3n cubic structure found previously in bulk mixtures of C12TAPAp with excess water.18 Other authors have similarly observed HCP phases under certain conditions for alkyltrimethylammonium surfactant aggregates at high degrees of hydration. Chu and co-workers found, for CnTA+ surfactant ions of different alkyl chain lengths associated with copolymer gels of sodium methacrylate and N-isopropylacrylamide, that an increased proportion of the neutral comonomer eventually resulted in an HCP structure rather than the cubic Pm3n structure.31,32 Very recently, Loh and co-workers similarly found a HCP structure for the most hydrated liquid crystalline phase of a complex salt of C16TA+ and a copolymer of charged methacrylate and uncharged ethoxylated methacrylate units.33 In simpler surfactant systems, Liu and Warr very recently found the HCP structure for C12TA+, C14TA+, and C16TA+ surfactants with certain “strongly hydrated” counterions (such as oxalate), and they reported a sequence HCP → Pm3n cubic → 2D hexagonal with a decreasing water content.34 The same phase sequence with decreasing hydration was observed for C12TAPAp films in our recent study of hydrated CS films partially immersed in open water baths.17 The present systematic studies extend our knowledge on conditions that can induce the HCP structure in CS. In ternary mixtures, we have demonstrated that a switch from Pm3n to HCP in C12TAPAp films can be induced by adding excess PAA to the film or by equilibrating stoichiometric films against solutions of HCl. These experiments agree with the previous observations that an introduction of uncharged (here acrylic acid) comonomers in the polyion can result in a switch from Pm3n to HCP in complex salts.31−33 However, we here also found that added NaCl, NaBr, or NaOH all induce the HCP structure in maximally hydrated C12TAPAp. Since excess NaOH should have a negligible effect on the effectively fully charged polyion in the stoichiometric complex salt, the effect of NaOH (Table 1) is most probably a salt effect, analogous to the effect of NaCl and NaBr, rather than a pH effect. That a transition to HCP can result from added electrolyte has not, to our knowledge, been reported previously. Such an effect of salt on the C12TAPAp systems is quite unexpected, since added salt and especially NaBrnormally decreases, rather than increases, the curvature of a cationic surfactant aggregate. Indeed, for C16TAPA25, we observed (Table 2) that NaBr added to the contacting solution eventually causes a switch from Pm3n to a 2D hexagonal phase. The only common denominator that we find at present for additives that produce the phase change from Pm3n to HCP is that they are hydrophilic and contribute to the volume of the continuous aqueous domain in the micellar liquid crystalline structure. Turning to the structure in excess water, our study here supports previous findings from our laboratory that the volume of the contacting water phase can influence the structure of the stable phase of maximally hydrated C12TAPAp.17,18 For the interpretation of this effect, in apparent violation of the Gibbs phase rule, our findings of the effect of excess PAA provides an important clue. Unless particular precautions are taken, laboratory water rapidly becomes equilibrated with carbon dioxide taken up from the air, so that the water becomes a twocomponent solution. (Other dissolved gases should have a negligible effect on the CS.) A pH value of 5.5 was measured in the water kept in an open reservoir after 2 days of exposure to the surrounding air. In a system open to the surrounding air, this will be the equilibrium pH of the water phase, regardless of its volume. At this slightly acidic pH, the PA will become
partially protonated since PAA is a rather weak acid with a pKa of 4.2.35 We propose that it is the presence of these uncharged protonated units in the polyion that in analogy with the effect seen in Table 1 induces the HCP structure. If, on the other hand, the mixture of water and C12TAPAp is sealed from the outside air after mixing, only the amount of CO2 that was present in the water before sealing will be available to react with the polyacrylate. For a sufficiently small relative volume of wateras presumably, in our experiments with 60 mL of water and 4 mg of complex saltthe available amount of CO2 will not suffice to induce a phase change. Conversely, at sufficiently high water/CS mass ratios, even sealed mixtures may give the HCP structure, as has previously been observed.18 Cubic Structure of C16TAPA6000 Can Be Switched to 2D Hexagonal by Various Solutes. Here, and in our recent work on films dipped in open baths, the Pm3n cubic phase has consistently been observed at maximum hydration for C16TAPAp. However, we also found here that various additives in the contacting solution can induce the 2D hexagonal phase, but by different molecular mechanisms: C16TAAc by entering the surfactant aggregates, NaBr (at rather high concentrations) presumably by the specific binding of bromide to the cationic surfactant aggregates,36 and high concentrations of PEG by osmotically dehydrating the CS phase. Why added C16TAAc favors the 2D hexagonal structure is not obvious to us, but we note that the same trend in the curvature is seen for both C12 and C16 complex salts: excess polyacid favors the structure with the higher curvature (HCP and Pm3n for C12 and C16, respectively), whereas excess ionic surfactant (including the alkaline C12TAOH, see Table 1) favors the structure with the lower curvature (Pm3n and 2D hexagonal for C12 and C16, respectively). The sensitivity of C16TAPA6000 to a small excess of surfactant ions carrying monovalent counterions could explain the fact that both the 2D hexagonal and the Pm3n cubic structures have previously been observed for this CS in in contact with excess water.18,19 Effects of Polyion Length and Surfactant Alkyl Chain Length. A marked dependence of the CS phase behavior on the length of the polyion is apparent from our film experiments as well as from previous bulk phase studies.18 The differences in their sensitivity to added surfactant, seen in Figures 3 and 4, have been discussed in detail in ref 21, using a simple Flory− Huggins approach as a theoretical tool to identify the mechanisms involved. Briefly, it was concluded that for CSs with long polyions, the failure of added surfactant to dissolve the concentrated CS phase could be understood as follows. When an ionic surfactant is added to a biphasic water/CS mixture, it will partition roughly equally between the two phases, since this distribution is favored by the entropy of mixing of the numerous monovalent surfactant counterions. The latter ions will also dominate the osmotic pressure of both phases, and hence the partitioning of water between the two phases will not be strongly affected by an increasing content of the nearly uniformly distributed ionic surfactant. The polyions, finally, will remain in the concentrated phase if they are very long; since their translational entropy is small (on a mass basis), they will strongly prefer the concentrated phase where their strong electrostatic and bridging attractions to the surfactant aggregates are best satisfied.8,37 This molecular picture can thus explain our striking result that even a solution with a huge concentration of surfactant (20 wt %) fails to dissolve complex salt films containing long polyions (Figures 3 and 4). The translational entropy of short polyions, on the other hand, is 6494
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C12TAPAp in water appears to depend on the water/CS ratio: Most likely, the CO2 content in water equilibrated with air is sufficient to produce a transition from a cubic to an HCP structure. An interesting feature of most of the CnTAPAp complex salts studied here is that they undergo a phase transition close to their stoichiometric composition, so that small excesses produce a phase change. For the investigated C12TAPAp complex salts, a transition cub → HCP can be generated by a small excess of the respecive PAp. For C16TAPA6000, a transition cub → hex can be induced by a small excess of the C16TA+ surfactant ion, neutralized by a small monovalent counterion. This suggests that small amounts of acid or base could be used as structure-switching agents for these complex salts. Experiments, combined with estimates based on available self-diffusion data, suggest that equilibration in contact with a new solution environment will sometimes be very swift, while in other cases it takes very long. Transitions induced by small solutes, like simple salt, or by solutes that do not enter the film, such as PEG, should be fast and could quickly be reverted by washing the films excessively against water. However, equilibration of an amphiphilic substance, such as a surfactant, between the film and a contacting solution can take very long if the monomer solubility of the amphiphilic substance is very low. Indeed, a preliminary test in our laboratory showed that the nonionic surfactant pentaethylene glycol monododecyl ether, C12E5, which has a low cmc, did not equilibrate with a C16TAPA6000 film on the time scale used in the present study. This feature could be a practical advantage: a film using a watersoluble cosurfactant to generate a desired structure could in practice be kinetically very stable, even though it at equilibrium should lose all the cosurfactant to a surrounding aqueous reservoir. Obvious potential applications of CS coatings of the type studied here are to release or take up solutes from a surrounding aqueous solution. For the various specific applications, detailed kinetic studies would clearly be necessary. However, already the information provided in the present work provides important guiding principles for the design of such functional coatings: How the structure of the films can be tuned by various water-soluble agents and, also, how the rate of exchange of the solutes between the film and a surrounding solution is expected to vary with the nature of the solute.
non-negligible. Hence, the latter will partition to the dilute phase to a significant extent when the concentration of surfactant aggregates builds up in the latter phase, eventually leading to a critical point and a complete merging of the concentrated and the dilute phases. The entropy of mixing of the polyions should also explain the observation that it takes less salt (NaCl or NaBr) to dissolve C12TAPA25 films compared to films of C12TAPA6000 (Table 2). For the C16TAPAp systems, no dissolution occurred, but we observed a strong increase of the lattice parameter of the cubic cell at the highest salt concentrations for the short polyion and a much weaker effect for the long polyion. These differences again illustrate the importance of the polyion length for the translational entropy of the polyions. Turning to the effect of the surfactant alkyl chain length, Svensson et al. showed that the miscibility gap in a binary mixture of CS and water decreases with decreasing length of the surfactant alkyl tail.18 This can be explained by the fact that the concentration of surfactant unimers in the solution, and thus the osmotic pressure and resulting water uptake, decreases with increasing surfactant tail length. Furthermore, the micelles decrease in size and increase in number for the shorter surfactant, and consequently, the average interaction strength between a polyion and an individual micelle becomes weaker. The same arguments should explain why C12TAPA25 films are more easily dissolved than films of C16TAPA25, both by added salt (Table 2) and by added surfactant (Figures 3 and 4).
5. CONCLUSIONS AND OUTLOOK This work has demonstrated that complex salt coatings on mica in contact with aqueous solutions are quite robust and that they can respond to additives in contacting aqueous solution by changing their structure. A number of different additives have been chosen, representing different modes of interaction with the CS. The response of the film was found to depend not only on the additive but also on the nature of the CS. In total, three different liquid crystalline structures were obtained in films equilibrated against the various solutions: HCP, micellar cubic, and 2D-hexagonal. The responsiveness of the films to the various solutes varied, but in the majority of cases, the films were stable even in environments with high solute concentrations. Moreover, the solution-treated films reverted, in the majority of the cases, to the structures found in pure water after a standardized extensive washing in water. Cycles of soluteinduced transitions, followed by reverse transitions induced by washing, thus seem eminently possible. When available, previously established phase diagrams could predict the responses displayed by the CS films. The agreement was excellent, as would be expected for the comparatively thick films used hereorders of magnitude thicker than the dimensions of the unit cells of the liquid crystalline structures. Thus, the sandwich setup offers a good method for rapid screening of the effect on CS of other water-soluble components, for which the relevant phase diagrams have not been determined. For instance, our study has illustrated that added simple salt can induce structural changes in hydrated CS phases and, moreover, that the obtained structures may be sensitive to the anion of the added simple salt. It was found here that a contacting acidic aqueous solution may induce the HCP structure of C12TAPAp complex salts by decreasing the charge density of the polyion. This finding provides an explanation to the puzzling, but consistent, observation that the structure of maximally swollen
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ASSOCIATED CONTENT
S Supporting Information *
SAXS profiles, for C16TAPAp films overtitrated or exposed to HCl solutions, and CnTAAc surfactants without any CS film present. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b00831.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (L.P.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Swedish Foundation for Strategic research (SSF), grant RMA08-0056, is gratefully acknowledged. 6495
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The authors thank Olle Söderman for fruitful discussions and Joaquim Li for help regarding the PEG solutions.
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