Effects of Aggregate Charge and Subphase Ionic Strength on the

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Cite This: Langmuir 2018, 34, 2312−2323

Effects of Aggregate Charge and Subphase Ionic Strength on the Properties of Spread Polyelectrolyte/Surfactant Films at the Air/ Water Interface under Static and Dynamic Conditions Andrea Tummino,†,‡ Jutta Toscano,† Federica Sebastiani,†,§ Boris A. Noskov,∥ Imre Varga,*,‡,⊥ and Richard A. Campbell*,† †

Institut Laue-Langevin, 71 avenue des Martyrs, CS20156, Cedex 9 38042 Grenoble, France Institute of Chemistry, Eötvös Loránd University, 112, P.O. Box 32, Budapest H-1518, Hungary § Department of Biomedical Science, Faculty of Health and Science, Malmö University, 21432 Malmö, Sweden ∥ Institute of Chemistry, St. Petersburg State University, Universitetsky pr. 2, 198904 St. Petersburg, Russia ⊥ Department of Chemistry, University J. Selyeho, Komárno, Slovakia ‡

S Supporting Information *

ABSTRACT: We demonstrate the ability to tune the formation of extended structures in films of poly(sodium styrenesulfonate)/dodecyltrimethylammonium bromide at the air/ water interface through control over the charge/structure of aggregates as well as the ionic strength of the subphase. Our methodology to prepare loaded polyelectrolyte/surfactant films from self-assembled liquid crystalline aggregates exploits their fast dissociation and Marangoni spreading of material upon contact with an aqueous subphase. This process is proposed as a potential new route to prepare cheap biocompatible films for transfer applications. We show that films spread on water from swollen aggregates of low/negative charge have 1:1 charge binding and can be compressed only to a monolayer, beyond which material is lost to the bulk. For films spread on water from compact aggregates of positive charge, however, extended structures of the two components are created upon spreading or upon compression of the film beyond a monolayer. The application of ellipsometry, Brewster angle microscopy, and neutron reflectometry as well as measurements of surface pressure isotherms allow us to reason that formation of extended structures is activated by aggregates embedded in the film. The situation upon spreading on 0.1 M NaCl is different as there is a high concentration of small ions that stabilize loops of the polyelectrolyte upon film compression, yet extended structures of both components are only transient. Analogy of the controlled formation of extended structures in fluid monolayers is made to reservoir dynamics in lung surfactant. The work opens up the possibility to control such film dynamics in related systems through the rational design of particles in the future.

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INTRODUCTION Over the past century, the relation between the bulk and surface properties of oppositely charged polyelectrolyte/ surfactant (P/S) mixtures has attracted growing attention in colloid science.1,2 The increasing interest has a twofold motivation. The first is to develop a clear understanding of the structure and formation mechanism of mixed layers involving macromolecules at fluid interfaces. The second concerns the widespread applications of these materials in everyday life products,3 water treatment,4 and biocompatible materials.5 Moreover, since many biologically relevant macromolecules are polyelectrolytes themselves, there is scope to extend a comprehensive understanding of the behavior of P/S systems to fields including the delivery of functional molecules (e.g., DNA,6 protein/peptides,7 and drugs8) as well as the fabrication of organic photovoltaic devices.9 © 2018 American Chemical Society

The bulk phase behavior of aqueous P/S mixtures changes strongly with the composition. At low bulk surfactant concentrations, the system is in a single phase and can contain swollen equilibrium complexes formed by micelle-like surfactant aggregates wrapped by polyelectrolyte chains. These complexes are in equilibrium with individual surfactant molecules. As stoichiometric charge binding is approached, the diminishing charge on the complexes leads to their compaction, which in turn gives rise to reduced electrostatic repulsion. The complexes lose colloidal stability, and aggregates involving many polyelectrolyte chains are formed.10,11 With time, precipitation (sedimentation or creaming) occurs, and the Received: November 17, 2017 Revised: January 11, 2018 Published: January 11, 2018 2312

DOI: 10.1021/acs.langmuir.7b03960 Langmuir 2018, 34, 2312−2323

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Langmuir

spread material at the air/water interface has also been reported.36 These observations together prompted our recent study on the preparation of loaded films from aggregates of NaPSS and dodecyltrimethylammonium bromide (DTAB) at the air/water interface using dropwise spreading of a small aliquot of an aqueous mixture containing neutral aggregates and free surfactant.37 Film formation results from the dissociation of aggregates on the subsecond time scale upon contact with the surface of water, which results in the fast spreading of material by Marangoni flow. Equilibration is not reached on experimentally accessible time scales as the films are kinetically trapped in a metastable state; e.g., an activation barrier resulting from the entropy change of the counterion release into bulk phase prevents dissolution. The films were shown to have 1:1 charge binding and a much higher surface excess than equilibrium layers resulting from the complex adsorption in a prediluted system. Also, they behaved as insoluble membranes where the film at limiting monolayer coverage (i.e., a monolayer of surfactant molecules in contact with air as well as a layer of hydrated polyelectrolyte bound to the head groups) can be expanded and recompressed to the same surface area without observable loss of material. A benefit of this route for the production of films for transfer applications is that it is fully aqueous and thereby may have economic and environmental advantages due to reduced cost and toxicity.38 As a step in the development of this methodology into a viable technological route, it is necessary to understand factors that affect formation of the spread films as well as their physical properties exhibited under static and dynamic conditions. The main aim of the present study is to resolve in detail the behavior of films spread from NaPSS/DTAB aggregates with respect to their charge/structure and the ionic strength of the subphase. These variables have profound effects on bulk P/S interactions, but a systematic investigation of their effects on spread films is missing. Following a description of the bulk properties of the aqueous aggregate mixtures, we present data from a range of techniques (ellipsometry, neutron reflectometry, and Brewster angle microscopy) as well as different methodologies (adsorption, single spreading, successive spreading, and surface pressure isotherms). This work represents a necessary precursor for future studies on different synthetic and biocompatible systems including their transfer to solid substrates for a range of applications.

system evolves to an equilibrium two-phase state (associative phase separation).12,13 At even higher bulk surfactant concentrations, the free surfactant adsorbs on the surface of the compact aggregates, thus providing electrostatic stabilization.14−16 These mixtures are not in equilibrium but are instead electrostatically stabilized two-phase colloidal dispersions. Such nonequilibrium effects play an important role in determination of the bulk properties of aqueous P/S mixtures. Further, steady state bulk properties can be affected by the sample history, e.g., changing of the mixing protocol.14,17 In particular, kinetically trapped aggregates can be produced as a result of concentration gradients during mixing. The ionic strength influences strongly the bulk phase behavior. It has been shown that with increasing ionic strength the stability of P/S mixtures can decrease constantly until a critical coagulation concentration is reached.18 The phase separation region then extends to higher bulk surfactant concentrations as a result of the charge screening of aggregates.18,19 In addition, it was shown in work on mixtures of poly(sodium styrenesulfonate) (NaPSS) and hexadecyltrimethylammonium bromide (CTAB) that associative phase separation is suppressed fully when the ionic strength is high enough.19 Therefore, the addition of inert electrolyte can be used effectively to tune the equilibrium versus nonequilibrium nature of aqueous P/S mixtures. In spite of the established nonequilibrium features of P/S systems, their interfacial behavior has been typically described over the years in terms of equilibrium adsorption of the complexes.20−22 Over the past decade, however, there has been growing interest in the influence of nonequilibrium effects at the air/water interface. For example, it has been systematically demonstrated that depletion of material at the interface can occur as a result of slow bulk aggregation,23 while enhancement can result from direct aggregate interactions at the interface.24 Indeed, it has been outlined recently how steady-state interfacial properties frequently reported in the literature must have been far from equilibrium.25 Further, the addition of inert electrolyte can be used to convert kinetically trapped steady states at the interface to the equilibrium ones.26 More and more studies are emerging where data interpretations are set in the context of a nonequilibrium physical framework.27,28 Significant attention has been paid over the years to the mechanism of P/S film formation. The possibility to form compressible,29 gel-like,30 and heterogeneous31 molecular films that are not in equilibrium with the bulk has been well established. Films have also been produced by spreading complexes from an organic solvent onto the air/water interface.32 Recently, work has focused on film formation through the interaction of aggregates from bulk aqueous solution with the air/water interface. For example, two main steps of interfacial layer formation were resolved for refined aggregates of poly(ethylene imine) and sodium dodecyl sulfate (SDS): their adsorption followed by their dissociation and spreading.33 It was also discovered that on the continually expanding surface of an overflowing cylinder a majority of the material at the interface can originate from the dissociation of aggregates and spreading of material as opposed to complex adsorption.34 Further, it was shown that highly charged aggregates of poly(amidoimine) dendrimers and SDS remained embedded intact at the air/water interface, while those with lower charge were not detected.35 The interfacial dissociation of P/S aggregates triggered by particle−particle interactions to

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EXPERIMENTAL SECTION

Materials. 17 kDa NaPSS (analytical standard; Sigma-Aldrich), D2O (99.9% d-atoms; Sigma-Aldrich), NaCl (99.999%; Fluka Analytical), ethanol (≥99.8%; Honeywell), and chain-deuterated DTAB (d25-DTAB; 98.7% d-atoms; CDN Isotopes) were used as received. Acetone (>99.5%; Fisher Chemicals) was distilled once. DTAB (≥98%; Sigma-Aldrich) was purified by recrystallization in a 4:1 acetone/ethanol mixture followed by drying under vacuum first at room temperature and then at 50 °C. Purified H2O (18.2 Mohm·cm) was generated using a Milli-Q machine. Sample Preparation. Stock solutions of the individual components were made in purified H2O or ACMW (air contrast matched water; 8.1% v/v D2O in purified H2O; zero scattering length density). 200 ppm of NaPSS stock solutions involved overnight stirring to ensure full dissolution of the powder. 50 mM DTAB, cm-DTAB (contrast matched DTAB; 4.4% v/v d25-DTAB in DTAB; zero scattering length density), or d25-DTAB stock solutions were diluted to the required concentration for a given measurement prior to use. None of the stock solutions contained added NaCl. 2313


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Langmuir For the spread films, NaPSS/DTAB mixtures were produced by rapidly mixing, contacting in midair above a container, equal volumes (up to 300 μL) of a 200 ppm of NaPSS stock solution and a DTAB, cm-DTAB, or d25-DTAB solution at a bulk concentration equal to double the final required value. This method was used to minimize concentration gradients during mixing that enhance the production of kinetically trapped aggregates.15,16 The NaPSS/DTAB mixed solutions were used within 1 min of their preparation in order to limit aggregate growth prior to their use. Liquid was dispensed dropwise directly onto the surface of an aqueous subphase using a pipet at an angle of 45°. The drops were deposited by letting each one touch the surface, avoiding their falling onto the surface from a height. For the ellipsometry and BAM experiments at the static air/water interface, 100 μL of the mixed solution was dispensed onto 25 mL of an aqueous subphase contained in a 7 cm diameter Petri dish made of glass. For the stepwise addition experiments, five 100 μL aliquots were dispensed onto 25 mL of pure water at 5 min intervals. For the NR experiments, the quantities of materials were scaled up to 500 μL of the mixed solution dispensed onto 125 mL of an aqueous subphase contained in a Langmuir trough (Nima, Coventry, UK; surface area of 283 × 100 mm). The BAM experiment at the dynamic air/water interface was performed in a larger Langmuir trough (Nima, Coventry, UK; surface area of 900 × 100 mm), and the spreading aliquot was scaled by volume to 1800 μL. The aqueous subphase comprised purified H2O or 0.1 M NaCl in purified H2O for the laboratory-based experiments and ACMW or 0.1 M NaCl in ACMW for the NR experiments. For the adsorbed layers, the same quantities of all the materials were used yet the experimental pathway was changed to ensure that the sample remained in the equilibrium one-phase region throughout its history. In this case, each stock solution was prediluted with 12.5 mL of an aqueous subphase prior to their mixing, again contacting in midair above a container to minimize the production of concentration gradients. The total liquid volume was then poured into a 7 cm diameter Petri dish, and the surface of the liquid was cleaned by aspiration to remove any aggregates that had become trapped there during its creation.24 Note that equivalent surface excesses were achieved when samples in the absence of polyelectrolyte were prepared using these two methods, i.e., exchange of the NaPSS stock solution for pure water (see part 1 of the Supporting Information for details). Hence, we infer that any differences in the surface excess of the spread films and adsorbed layers for the NaPSS/DTAB mixtures result from the spreading of material due to the dissociation of aggregates. All the measurements were carried out at 23 ± 1 °C. UV−Vis Spectroscopy. The turbidity of NaPSS/DTAB solutions was measured using a PerkinElmer Lambda 2 UV−vis spectrophotometer with a semi-micro quartz cell of 1 cm path length. The optical density of samples was determined at 400 nm (O.D.400). Measurements were carried out both at 5 min (fresh) and 1 month after mixing. Since neither the polyelectrolyte nor the surfactant has an adsorption band above 350 nm, increasing O.D.400 values indicate the presence of aggregates suspended in the samples, while a reduction in the values with time is symptomatic of depletion from precipitation. Ellipsometry. Ellipsometry is a precise and sensitive tool that can be used to provide a measure of the surface excess at the air/water interface.39 The technique exploits the change in polarization of light upon reflection, described by

rp rs

= tan ΨeiΔ

and a data acquisition rate of 0.2 Hz. While the sensitivity of the technique is slightly higher at angles of incidence commensurately above than below the Brewster angle, the angle of incidence used was chosen to follow the approach adopted in our previous studies,24,40 which optimizes the tolerance of the measurements to effects of evaporation and misalignment. Calculation of the surface excess from dΔ can be performed using an optical matrix model for layers without strong anisotropy, but even for an ionic surfactant monolayer anisotropy can render such an approach to be inaccurate.41,42 For P/S mixtures, an alternative approach can be to use a calibration from neutron reflectometry.43 In the present work, however, data are presented with respect to the aggregate charge, ionic strength, surface age, and experimental methodology, and such a calibration would rely on equivalent dielectric functions across the interface in every case. Further, low area coverage of aggregates embedded in the films can have a small effect on the measured value, either positively or negatively depending on their size. As a result, therefore simply we present values of dΔ as approximate relative measures of the surface excess. Neutron Reflectometry (NR). NR is a technique that can be used to resolve the structure and composition of mixed layers at the air/ water interface.44 Measurements were performed on the FIGARO reflectometer at the Institut Laue-Langevin (ILL, Grenoble, France).45 The instrument was operated in a high flux configuration with dephased choppers to give neutron pulses in the wavelength range λ = 2−16 Å at 9−9.6% resolution in dλ/λ. The data were collected on the area detector of the instrument at an incident angle θ = 0.62°. Only data in the range λ = 4−12 Å, corresponding to Qz = 0.01−0.03 Å−1, were used, where Qz is the momentum transfer normal to the interface, given by

Qz =

4π sin θ λ

(2)

Analysis was performed by fitting data to a model involving a single mixed layer of polyelectrolyte and surfactant. As these samples were measured in ACMW at the thin film limit and analyzed only at low Qz values, NR is insensitive to details of the actual structure, and the scattering properties of the material simply account for the amounts of the components.37 Assuming an additive contribution to the scattering from different components, the fitted scattering excess, ρ × τ, is given by ρ × τ = NA∑ (Γi × bi) i

(3)

where ρ is the scattering length density used in the model fit, τ is the fitted thickness, bi is the scattering length of species i (i.e., NaPSS or DTAB), Γi is its surface excess, and NA is Avogadro’s number. For a binary mixture, two independent equations may be written and solved simultaneously to calculate the surface excess of each component following measurement of data involving NaPSS in ACMW with (1) cm-DTAB and (2) d25-DTAB. Data were generated in 2 min slices for NaPSS and 1 min slices for DTAB using the same approach as in our study on spread NaPSS/DTAB films.37 The surface excesses resolved are dominated by material in the surface monolayer, but a small contribution can result from a low area coverage of aggregates embedded at the interface. The data were analyzed using Motofit.46 The data were modeled using the approximation of full counterion release, and the scattering lengths used in the calculations were 47.3 fm for PSS−, 0 fm for cm-DTA+, and 242.2 fm for d25-DTA+. Examples of the reflectivity profiles, model fits, and scattering length density profiles can be found in part 2 of the Supporting Information. Brewster Angle Microscopy (BAM). BAM is an optical imaging technique that can be used to highlight lateral inhomogeneity on the micrometer scale in films at the air/water interface.47,48 Measurements were made using a Nanofilm EP3 machine equipped with an Nd:YAG laser and a 10× objective at the Brewster angle for the air/water interface of 53.1°. Only fast focusing correction was enabled due to the dynamic nature of the films studied. The angle of polarization of the incident beam and the analyzer were set to zero; i.e., p-polarized light

(1)

where rs and rp are the Fresnel reflectivity coefficients for s- and ppolarized light, and Ψ and Δ are the ellipsometric angles of the amplitude change and the phase shift, respectively. In the thin film limit at the air/water interface, Δ is much more sensitive than Ψ to changes in the surface excess. Therefore, we measured time-resolved values of dΔ = ΔNaPSS/DTAB − Δwater, where ΔNaPSS/DTAB and Δwater are the values for the NaPSS/DTAB system and a pure water, respectively. A Beaglehole picometer light ellipsometer was used for this purpose with a wavelength of λ = 632.8 nm, an angle of incidence of θ = 50°, 2314


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Langmuir was reflected at the interface. Since the signal amplification was kept constant for all of the measurements, a bare air/water interface was invisible (black) and a homogeneous monolayer was visible (gray), while extended structures had higher intensity (white).49 Background subtraction was not performed. Gamma correction was applied equally to all of the images during postprocessing to emphasize the features observed. Surface Pressure Isotherms. Surface pressure isotherms offer insight into the dynamic response of films to changes in the surface area.50 Measurements were made using the smaller of the two Langmuir troughs described above according to the Wilhelmy method using a plate made of filter paper. A fresh plate was used in each measurement. In order to study the spread films under nonequilibrium conditions of perturbations in the surface area, a compression/ expansion rate of 9 cm2/min was used.

O.D.400 with respect to the bulk surfactant concentration is reported at two sample ages (fresh and one month). Electrophoretic mobility measurements for this system have been previously reported,37 where it was shown that there is a continual change in the charge of the complexes and aggregates from negative through neutral to positive with increasing bulk surfactant concentration. The concentration corresponding to charge neutrality is marked in Figure 1 with a vertical dashed line. At bulk compositions close to charge neutrality, O.D.400 of the freshly mixed samples is high due to the formation of macroscopic aggregates due to lack of colloidal stability of the formed complexes. When these samples are aged, O.D.400 drops dramatically as precipitate settles (shaded area in yellow in the figure). As the bulk surfactant concentration deviates from the phase separation region, O.D.400 of the freshly mixed samples decreases. This occurs because the species formed during mixing are charged up by the excess of polyelectrolyte segments or surfactant molecules. As they gain colloidal stability, aggregation is inhibited so the turbidity decreases. On the basis of the above information and knowledge of the general physical picture of the behavior of these systems, we have chosen eight bulk compositions to use in the preparation of spread films at the air/water interface. This choice was made so that each sample represents a different physical state of the system. The bulk surfactant concentrations used as well as some general descriptive remarks on the characteristics of the aggregates present are listed in Table 1. Note that some of the remarks are based on the accepted physical picture for these systems, which was formed a result of work on mixtures of NaPSS/CTAB23 and related systems.14,22 The samples were used always within the first minute after mixing the components, so they contained a dispersion of submicrometer aggregates; the size of neutral aggregates is approximately 500 ± 50 nm,37 and it follows that charged aggregates are smaller because they aggregate slower. Further, while the internal liquid crystalline structure of the aggregates was not investigated in the present work, data have been published on the phase diagram of the PSS+DTA−/water system where coexisting hexagonal and micellar phases form.51 Effects of Aggregate Charge. We turn now to an examination of film formation by exploiting the dissociation and spreading of material from NaPSS/DTAB aggregates as a function of their charge/structure. Figure 2a shows timeresolved values of the ellipsometric phase shift for samples where an aliquot of aqueous aggregate dispersion is dropped onto the surface of pure water. For comparison, Figure 2b

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RESULTS AND DISCUSSION Bulk Behavior. In order to select a range of different types of NaPSS/DTAB aggregates to use in the preparation of spread films, first we need to understand key characteristics of the bulk phase behavior with respect to the extent of surfactant binding in the complexes and aggregates formed. Optical density and electrophoretic mobility measurements can be used to characterize the bulk interactions and phase behavior in oppositely charged P/S systems. In Figure 1, the variation of

Figure 1. Optical density measurements at 400 nm of 100 ppm, 17 kDa NaPSS/DTAB mixtures with respect to the bulk surfactant concentration and sample age for freshly mixed samples (blue circles) and samples aged for one month (green triangles). The equilibrium two-phase region is shaded in yellow. The vertical dashed line marks the bulk composition of charge neutrality. The letters A to H correspond to the bulk compositions of mixtures used to form spread films below.

Table 1. Summary of the Relative Characteristics of Eight Different Bulk Compositions with Respect to the Phase Behavior and Nature of Aggregates Present in the Samples Including Their Relative Quantity, Structure, Size, and Charge label

[DTAB] (mM)

A B C D

0.43 1.0 2.5 4.5

E F

6.0 10

G H

15 25

characteristics equilibrium one-phase region; negligible amount of aggregates; moderate in size; swollen in structure; high negative charge equilibrium one-phase region; small amount of aggregates; moderate in size; swollen in structure; high negative charge equilibrium one-phase region; moderate amount of aggregates; moderate in size; swollen in structure; high negative charge equilibrium two-phase region; large amount of aggregates that lack colloidal stability; becoming larger in size; becoming more compact in structure; low negative charge equilibrium two-phase region; large amount of aggregates that lack colloidal stability; largest in size; compact in structure; charge neutral equilibrium two-phase region; large amount of aggregates that lack colloidal stability; becoming smaller in size; compact in structure; low positive charge colloidal aggregate dispersion; large amount of stabilized aggregates; moderate in size; compact in structure; positive charge colloidal aggregate dispersion; large amount of stabilized aggregates; smallest in size; compact in structure; high positive charge 2315


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Figure 2. Ellipsometric phase shift of (a) spread films on pure water and (b) adsorbed layers in pure water prepared from NaPSS/DTAB mixtures. The colored labels refer to the bulk compositions specified in Figure 1 and Table 1, and the corresponding samples in the two panels have the same total concentrations but were prepared using different protocols (see Experimental Section). The inset to panel a contains a longer measurement of the spread film of sample H (gray) and the adsorbed layer of sample E (pale blue); the value corresponding to an adsorbed layer of a full sample of sample A is indicated for reference (dashed line). The inset to panel b contains the ratio of the initial phase shifts of the spread films to the corresponding adsorbed layers.

as the quantity of negatively charged aggregates trapped in the equilibrium one phase region decreases (from samples C to A). The behavior is different for the spread films prepared from compact, positively charged aggregates (samples F to H). In all three cases the surface excess exceeds that of a film at limiting monolayer coverage, the enhancement is more pronounced with increasing charge of the aggregates, and the material in excess of a film at limiting monolayer coverage decreases with time. To determine the final steady state of sample H, longer measurements were performed (gray curve in the inset to panel a), and over several hours the surface excess converged to a value exceeding that of the monolayer reference. Further, there are temporal fluctuations in the data for the films prepared from aggregates with the highest charge. These observations can be attributed to islands of local extended structures in the film (e.g., reservoir loops or multilayer stacks) and/or collections of intact aggregates embedded in the film that pass in and out of the area probed by the laser on the second time scale,24 and with time the extra material is lost to the bulk. This gradual loss of material has been confirmed with direct measurements of the surfactant surface excess by NR measurements, which are included in part 3 of the Supporting Information. To see if there is an underlying difference between the behavior of spread films prepared from aggregates of low/ negative and high positive charge, we performed experiments involving stepwise additions. Figure 3 shows time-resolved values of the ellipsometric phase shift for four samples where the addition of aggregates was performed five times consecutively over 5 min intervals. The stepwise addition of slightly undercharged (sample D) and overcharged (sample F) aggregates gives rise only to minor increases of the surface excess, and the limiting value is very similar to that of a film at limiting monolayer coverage. These data show that addition of more aggregates does not result significantly either in their retention in the film or their dissociation to spread more material at the interface. In contrast, the stepwise addition of compact aggregates with high positive surface charge (samples

shows with corresponding colors data for the adsorbed layers of samples with the same final concentrations of NaPSS and DTAB (prediluted separately prior to mixing). A substantially higher surface excess is observed using the spreading method than from bulk adsorption for all of the samples measured. For the spread film prepared from neutral aggregates (sample E), as characterized previously,37 the initial surface excess is about 5 times higher than that for bulk adsorption (inset to panel b). The value is stable and almost equivalent to that of an equilibrium adsorption layer in a concentrated sample in the equilibrium one-phase region containing 250 times the amount of polyelectrolyte (dashed line in the inset to panel a). These results demonstrate the high efficiency of the spreading process, which hints at its scope for exploitation in the production of films for technologically relevant transfer applications. An interesting question is if the spread films have a higher surface excess than a fully equilibrated adsorbed layer for a system with equivalent total concentrations of NaPSS and DTAB. As most of the data in Figure 2b have not reached equilibrium within 30 min, a long measurement of the corresponding adsorbed layer for sample E was recorded (light blue curve in the inset to panel a). The surface excess of the spread film (light blue curve in panel a) exceeds that of the equilibrated adsorbed layer by a factor of 2. This result demonstrates that our process to create spread films by exploiting aggregate dissociation does not simply accelerate the path to equilibrium through localization of the materials at the interface (circumventing the slow diffusion of complexes from a dilute bulk solution) but in fact results in films present in a kinetically trapped state. For the spread film prepared from swollen, negatively charged aggregates in the equilibrium two-phase region (sample D), a very similar surface excess is produced, which demonstrates that the dynamic film formation process occurs also for swollen, negatively charged aggregates and not only for compact, neutral ones. As expected, the surface excess decreases 2316


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from the location where droplets touch the subphase, and consecutive waves result in the observed irregular patterns due to the dropwise nature of the procedure. The bright regions are attributed to the presence of islands of extended structures beyond that of a surface monolayer, as has been discussed previously for monolayers of carboxymethyl cellulose with cationic surfactants.52 After 15 min (panel c) smaller wormlike domains appear, after 1 h (panel d) the wavy domains have almost disappeared, and after 2 h (panel e) only wormlike features are observed. This evolution can be interpreted in terms of a reduction in the amount of material present in extended structures as a result of a loss of material to the bulk, which is qualitatively consistent with the data from ellipsometry and NR. Only after several hours (panel f) is an optically homogeneous layer formed. Even so, it is worth noting that the intensity is still greater than that of the bare interface. This observation may be related to the presence of a homogeneous film, the presence of some aggregates embedded in the film, or the retention of extended structures at the interface with their size below the spatial resolution of the instrument of ∼2 μm. The results described above show that positively charged NaPSS/DTAB aggregates can dissociate upon contact with pure water to spread loaded, kinetically trapped films by Marangoni flow and that they can form extended structures. The origin of these structures in terms of reservoir loops or multilayer stacks has not been resolved from the data presented above, but it is clear that they are not stable with time as the extra material is slowly lost to the bulk. Further, the presence of embedded aggregates in the films to activate the formation of extended structures cannot be excluded from these data. The situation for films spread from aggregates of low/negative charge is different as the maximum surface excess achieved is that corresponding to a film at limiting monolayer coverage. An important question is if the formation of extended structures can be controlled through compression of spread films prepared from aggregates of different charge/structure. We performed experiments on a Langmuir trough where the surface area was compressed and then expanded five times at a constant rate. The low-Qz analysis method of NR44 was applied to resolve the interfacial composition in situ while measuring the surface pressure. Figure 5 shows data from two films prepared from aggregates of sample D (swollen and negatively charged; panels a and b) and sample F (compact and positively charged; panels c and d); data corresponding to the neutral aggregates of sample E have been reported previously.37 Note that the slightly charged aggregates of sample F were chosen in preference to more highly charged aggregates of samples G or H because the surface excess for the static system from ellipsometry is more stable with time, yet fluctuations in the data were still detected, hinting at an underlying similarity in the behavior. The maxima and minima in the interfacial composition (panels a and c) mark the points of full compression and expansion of the surface area, respectively. Trends in the surfactant surface excess and surface pressure during the first compression are indicated with green circles and are compared directly in panels b and d. Similar to the behavior of films spread from neutral aggregates,37 the surface pressure and surfactant surface excess reach plateau values corresponding to a full monolayer (29 mN/m and 3.2 μmol/m2, respectively) during the first compression of the film spread from swollen, negatively charged aggregates (sample D). Note that even though extended structures of trilayers were observed for NaPSS/

Figure 3. Ellipsometric phase shift of spread films on pure water prepared from NaPSS/DTAB mixtures; five additions of aliquots containing aqueous aggregate mixtures at 5 min intervals as indicated with arrows.

G and H) results in successive loading of material into the film, again with temporal fluctuations and a loss of material from the interface with time. Therefore, compact, positively charged aggregates can activate the formation of local extended structures in the film and/or become embedded in the film themselves, and these films are far from equilibrium as material is subsequently lost. It is not possible to distinguish between the two possibilities using these ellipsometry data alone, so other experimental techniques and methodologies were applied. BAM was next applied in order to see if extended structures in a loaded film could be visualized. Sample H prepared from compact aggregates with a high positive charge was chosen as the films exhibit the greatest extent of loading. Figure 4 shows a

Figure 4. BAM images of (a) a bare air water/interface and (b−f) a spread film prepared by spreading a 100 μL aliquot of a freshly mixed sample of 100 ppm, 17 kDa NaPSS with 25 mM DTAB on 25 mL of pure water. The sample ages for the film are (b) 5 min, (c) 15 min, (d) 1 h, (e) 2 h, and (f) 15 h. The scale bars are 100 μm.

reference measurement of pure water as well as five images of the NaPSS/DTAB film at different sample ages. The mean intensity of the film is initially very high and decreases with time but remains much higher than that of the bare interface (panel a). After 5 min (panel b), irregular wavy domains are visible. Their shapes can be attributed to the nature of the spreading process: material spreads radially by Marangoni flow 2317


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Figure 5. Surface excesses of PSS− (blue circles) and DTA+ (red squares) during five compression/expansion cycles of a film spread on pure ACMW from aggregates of (a) sample D and (c) sample F measured using NR. Corresponding surface pressure data are shown in the insets. Plots of the surfactant surface excess (red squares) and surface pressure (black line) during the first compression are shown for (b) sample D and (d) sample F, as highlighted with green circles.

DTAB layers in concentrated mixed systems,53 the limiting surface excess of these spread films is consistent with a monolayer of surfactant molecules with hydrated polyelectrolyte bound to the head groups. The lack of increase in either value during film compression is attributed to subsequent loss of material to the bulk; i.e., the film does not form extended structures. The film has a consistent P/S stoichiometry of 1:1, which is attributed to high energetic cost of the formation of polyelectrolyte loops as a result of released counterions to the bulk combined with its low ionic strength. During the first expansion of the film, and upon further compression/expansion cycles, the film acts as an insoluble membrane with minimal hysteresis in the surface pressure isotherms, as was observed for films prepared from neutral aggregates.37 The behavior is different for the film spread from compact, positively charged aggregates (sample F). Although the surface pressure exhibits a plateau at the same limiting value during the first compression, the surface excesses of both components continue to increase. Upon expansion, initially there is loss of both components from the interface that is accompanied by retention of the maximum surface pressure (see green arrow in the inset to panel c). This behavior is indicative of the fast resupply of material from the extended interfacial structure to the surface monolayer to maintain the high surface pressure. Over successive compression/expansion cycles, there is a small loss of material from the film, and this is accompanied by progression of the surface pressure data toward a limit cycle

that retains the hysteresis characteristic of the repeated formation of extended structures. We have also observed directly in real space the extended structures formed in films spread on pure water from compact, positively charged aggregates (sample F) during compression of the surface area using BAM. In this case, we used a larger Langmuir trough that could access higher area compression ratios than were possible during our NR experiments. Figure 6 shows images collected at six different surface areas. In this case, the appearance of the extended structures in the images is different to that observed after the spreading (cf. Figure 4). Although absent in the film spread initially, linear white bands orthogonal to the direction of compression appear with increasing intensity as the film is compressed. Comparisons of these interfacial morphologies may be made to those observed in other systems under compression, e.g., silica nanoparticles modified by cationic surfactants that undergo a transition from a monolayer to multilayers above a critical surface pressure,54 and extended structures of multilayers that are also formed upon compression in films of cyclolinear poly(organosiloxane)s.55 As the stoichiometry of films spread from aggregates of different charge/structure studied under dynamic conditions are similar, and the total bulk concentrations are highly dilute, we infer that the activation of extended structures only in the films spread from compact, positively charged aggregates may be attributed to their presence embedded in the films. This inference is qualitatively consistent with the long ellipsometry 2318


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upon expansion.56,57 Of course, the system studied in the present work has strong differences to lung surfactant: it is a model system that is not biocompatible and the surface tension values are too high. Nevertheless, our demonstration that the formation of extended structures can be switched on or off through tuning the charge/structure of aggregates embedded in a fluid monolayer may open the possibility in the future of the rational design of synthetic or artificial structures to activate reservoir formation in films involving biocompatible materials such as dioctadecyldimethylammonium bromide58 or hyaluronic acid.59 Further, the fact that we can also control the morphology of the extended structures orthogonal to the direction of compression may lead to the creation of patterned structures at fluid interfaces for use in transfer applications.60 Effects of Subphase Ionic Strength. The ionic strength is an important variable that can be used to alter the electrostatic interactions in P/S systems as well as tune their equilibrium versus nonequilibrium nature.18,19,26 Figure 7a shows the ellipsometric phase shift of films of selected samples prepared from NaPSS/DTAB aggregates spread on 0.1 M NaCl, and Figure 7b shows data for the corresponding adsorbed layers in prediluted systems with the same total concentrations of the components. The spread films on 0.1 M NaCl have rather similar surface excesses regardless of the quantity and nature of the aggregates used in their preparation. The film spread from compact, positively charged aggregates (sample H in panel a) is loaded only transiently and exhibits a rapid loss of material to limiting monolayer coverage (dashed line in inset to panel a). The increased ionic strength of the subphase therefore results in a much faster loss of additional material to the bulk: a few minutes instead of a few hours (see Figure 2a). The initial surface excesses are around 8−11 times higher than that of the adsorbed layers (see inset to panel b), yet in time the surface excess matches that of the equilibrium adsorbed layer (see inset to panel a). These results may be interpreted that the spreads films are not present in a kinetically trapped state when the subphase has high ionic strength, but instead they simply reach

Figure 6. BAM images of an NaPSS/DTAB film spread from aggregates of sample F with surface areas of (a) 700, (b) 400, (c) 250, (d) 180, (e) 100, and (f) 85 cm2. The onset of the plateau in the surface pressure, where the formation of extended structures begins, corresponds to a surface area of 580 cm2 (i.e., between images in panels a and b), and the image in panel c was taken at an area compression ratio close to the highest value achieved during the NR experiment.

data of sample H converging to a steady-state surface excess higher than that of a film at limiting monolayer coverage and the finite intensity in the steady state image from BAM. Further, there is a slight excess of surfactant in the films studied under dynamic conditions from NR, which may be attributed to the presence of a low area coverage of compact aggregates coated in excess surfactant. Since films spread from compact, neutral aggregates were shown not to form extended structures,37 we may infer that the critical factors in their formation may be attributed to the aggregate charge/structure rather than their size. Analogy of the formation of extended structures in films prepared from NaPSS/DTAB aggregates may be made to lung surfactant, in which protein activates the formation of reservoirs upon compression that resupply the interface with material

Figure 7. Ellipsometric phase shift of (a) spread films on 0.1 M NaCl and (b) adsorbed layers in 0.1 M NaCl prepared from NaPSS/DTAB mixtures. The colored labels refer to the bulk compositions specified in Figure 1 and Table 1, and the corresponding samples in the two panels have the same total concentrations but were prepared using different protocols (see Experimental Section). The inset to panel a contains a longer measurement of the spread film of sample H (gray) and the adsorbed layer of sample E (pale blue); the value corresponding to an adsorbed layer of a full sample of sample A but in 0.1 M NaCl is indicated for reference (dashed line). The inset to panel b contains the ratio of the initial phase shifts of the spread films to the corresponding adsorbed layers. 2319


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Figure 8. Surface excesses of PSS− (blue circles) and DTA+ (red squares) during five compression/expansion cycles of a film spread on 0.1 M NaCl in ACMW from aggregates of (a) sample D and (c) sample F measured using NR. Corresponding surface pressure data are shown in the insets. Plots of the surfactant surface excess (red squares) and surface pressure (black line) during the first compression are shown for (b) sample D and (d) sample F, as highlighted by green circles in panels a and c, respectively.

during compression and expansion shows that loss of material during compression and addition of material to the film during expansion are rapid compared with the period of the surface area cycles. Nevertheless, the surface pressure isotherms exhibit extreme hysteresis, which demonstrates the energy barrier involved with the exchange of material between the film and subsurface. Note that BAM was used to try to confirm the lack of extended structures on the micrometer scale in films spread on 0.1 M NaCl from compact, positively charged aggregates (sample H), and indeed no optical inhomogeneities on the micrometer scale were observed. High ionic strength of the subphase clearly switches off the formation of extended structures of both components for the films spread from compact, positively charged aggregates. We have not resolved if the underlying reason for this change is that there are no longer aggregates embedded in the film to activate the formation of the extended structures or that such structures can be activated by aggregates in the film yet are much less stable. This issue merits work in the future. A further point of interest is the extent to which polyelectrolyte loops are formed in films spread on solution with high ionic strength during compression. Future work could focus on tuning the extent of loop formation by varying the compression rate as well as the extent of formation of extended structures of both components by adjustment of the ionic strength in a range of systems.

equilibrium faster through localization of the components at the interface; i.e., the film formation method is an efficient way to circumvent slow diffusion of complexes as a result of their low bulk concentration. Also, it may be inferred that there is not a measurable quantity of embedded aggregates trapped in the spread films following their equilibration with the bulk. Direct measurements of the surfactant surface excess using NR confirmed the observations above and are included in part 3 of the Supporting Information. The dynamic properties of spread films on 0.1 M NaCl were examined by measuring the interfacial composition during five compression/expansion cycles on a Langmuir trough. Figure 8 shows the data for spread films prepared on 0.1 M NaCl from aggregates of samples D (panels a and b) and sample F (panels c and d); data corresponding to the neutral aggregates of sample E can be found in part 4 of the Supporting Information. Contrary to the behavior of films spread on pure water, those spread from both the swollen, negatively charged aggregates and the compact, positively charged NaPSS/DTAB aggregates on 0.1 M NaCl behave similarly. As the films are compressed, a full monolayer of material is quickly achieved, beyond which surfactant is lost to the bulk. This behavior demonstrates that extended structures of both components cannot be formed when the ionic strength of the subphase is high. Most of the polyelectrolyte is also lost to the bulk upon compression yet an excess remains. This observation can be rationalized in terms of loops of polyelectrolyte stabilized by excess small ions. The symmetric shape of the surface excesses of both components 2320


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CONCLUSIONS AND OUTLOOK

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ASSOCIATED CONTENT

S Supporting Information *

In the present work, we have elaborated our approach to prepare loaded P/S films out of equilibrium conditions by exploiting the dissociation and Marangoni spreading from liquid crystalline aggregates. The approach is a quick method of preparing films that have a much higher surface excess than that accessible by equilibrium complex adsorption. In addition to possible economic benefits of the preparation of films for transfer applications, the methodology is based entirely on aqueous materials and hence may also have benefits of reduced toxicity. The focus of this study was to examine effects of the charge/structure of the aggregates used in the spreading process and the ionic strength of the subphase under static and dynamic conditions using a combination of surface-sensitive techniques. For films spread on pure water, use of compact, positively charged aggregates activates the formation of extended structures, which can form either from spreading enough aggregates or by compression of a film beyond monolayer coverage. Different morphologies are created upon spreading and compression: disordered structures formed by successive Marangoni flows at the interface in the former case and linear structures orthogonal to the direction of compression in the latter case. The identity of the extended structures in terms of reservoirs loops or multilayer stacks has not yet been resolved, and this issue merits further work. Films prepared from swollen, negatively charged aggregates, by contrast, do not form the extended structures, showing that formation of such structures can be tuned by the charge/structure of the aggregates used in film preparation. We noted an analogy between the tunable formation of extended structures in spread NaPSS/DTAB films with the behavior of lung surfactant. While there are large differences between the systems, the possibility to activate reservoir formation at fluid interfaces by tuning the charge/ structure of embedded particles may be a promising direction for future work. For films spread on solution of high ionic strength, the formation of extended structures is suppressed, yet loops of polyelectrolyte stabilized by excess small ions form regardless of the charge/structure of the aggregates used in film formation. The exchange of material between the subsurface and the film during cycles of the surface area is faster than the period of the surface area cycles, but the energy barrier in the process results in an extreme hysteresis in the surface pressure isotherms. We noted how variation of the compression rate and fine-tuning of the ionic strength may be attempted in future work to control the extent of formation of polyelectrolyte loops versus extended structures of both components. This comprehensive study lays the foundation to develop the methodology of films spread from aggregate dissociation for various applications. In terms of the underlying physical chemistry, it will be interesting next to examine films involving polyelectrolyte with respect to its molecular weight, architecture, or degree of dissociation. Additionally, exciting directions can be work on biocompatible systems and encapsulation of drugs or other functional molecules for use in transfer applications.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03960. Ellipsometry measurements of adsorbed layers and spread films of DTAB solutions, examples of neutron reflectivity profiles in the low-Qz range, surfactant surface excess in NaPSS/DTAB films spread on pure water and 0.1 M NaCl, and interfacial composition of a film spread from neutral aggregates on 0.1 M NaCl (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (I.V.). *E-mail [email protected] (R.A.C.). ORCID

Federica Sebastiani: 0000-0002-7405-6125 Boris A. Noskov: 0000-0001-8117-1490 Richard A. Campbell: 0000-0002-6296-314X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Institut Laue-Langevin for allocations of neutron beam time on FIGARO (DOIs: 10.5291/ILL-DATA.9-10-1433 and 10.5291/ILL-DATA.9-12-461) as well as the Partnership for Soft Condensed Matter for access to the ellipsometer and Brewster angle microscope. This research has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/20072013/under REA grant agreement no. 290251 and from the Hungarian National Research, Development and Innovation Office (NKFIH K116629), which is gratefully acknowledged. This publication is the partial result of the Research & Development Operational Programme for the project “Modernization and Improvement of Technical Infrastructure for Research and Development of J. Selye University in the Fields of Nanotechnology and Intelligent Space”, ITMS 26210120042, cofunded by the European Regional Development Fund. B.A.N. is grateful to St. Petersburg State University for the grant no. 2.40.532.2017.

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REFERENCES

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Effects of Aggregate Charge and Subphase Ionic Strength on the

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