Effects of subphase pH, temperature and ionic strength on the aggregation behavior of PnBA-b-PAA at the air/water interface

Lage E, Casas M, Concheiro A, Alvarez-Lorenzo C. Langmuir monolayer studies of non-ionic surfactants and DOTMA for the design of ophthalmic niosomes

The worldwide increase in diabetes entails a rise in associated diseases, with diabetic retinopathy on the forefront of the ocular complications. To overcome the challenges posed by ocular barriers, self-assembled nanocarriers have gathered increasing attention in recent years, with niosomes revealing themselves to be suitable for the delivery of a variety of drugs. This study investigated the mechanical properties of Langmuir monolayers comprising cholesterol, Tween 60, and 1,2-di-O-octadecenyl-3-trimethylammonium propane (DOTMA), both individually and in binary and ternary systems. The cholesterol monolayer was characterized by an L-shaped isotherm, reflecting two surface aggregation states. Tween 60 exhibited expanded conformation and progressive aggregation, transitioning through a phase change. The addition of cholesterol to Tween 60 resulted in a subtle reduction in surface compressional modulus. The compression isotherms highlighted the stabilizing effect of cholesterol on the monolayer, affecting the film's resistance to compression. The introduction of DOTMA in Tween 60 monolayers revealed concentration-dependent effects, where the compression resistance of the film was proportional to DOTMA concentration. Ternary systems of cholesterol, DOTMA and Tween 60 exhibited unique behavior, with DOTMA enhancing film stability and cholesterol modulating this effect. Temperature and subphase ionic strength variations further exacerbated the effects of DOTMA concentration. Brewster Angle Microscopy confirmed the absence of microdomains in the compressed monolayer, supporting the hypothesis of a monolayer collapse. Overall, the research provided valuable insights into the intricate interactions and mechanical behavior of these surfactant systems and the feasibility of obtaining cationic niosome-based drug delivery.

On the Preparation of Thin Films of Stearyl Methacrylate Directly Photo-polymerized at the Air-Water Interface

Characterization of bidimensional polymeric films at the air-water interface in the Langmuir trough, despite being a recurrent topic, usually refers to films of already formed polymeric materials, with very scarce reports on direct polymerization at the air-water interface. In the present work, we studied the photo-polymerization of stearyl methacrylate directly at the air-water interface under a nitrogen atmosphere, with the radical initiator solubilized in the aqueous phase. Two-dimensional (2D) polymerization was monitored by measuring the pressure-area isotherm at different irradiation times. The polymerization leads to a film with an isotherm different from that observed for the monomer, where the surface pressure is directly related to the irradiation time. The shape of this isotherm confirms the presence of a compressed liquid phase, where a higher order can be attained as a consequence of stronger packing forces involving polymer chains. The presence of inter-chain interactions allows rearrangements on the surface of the subphase, and even before the collapse a dense 2D ordering (with a solid phase-like behavior) can be observed. We present a new one-step, solvent-free procedure to obtain a photo-polymeric film directly at the air-water interface, which can be transferred to a solid surface by the Langmuir-Blodgett method, allowing film preparation of controlled thickness. Films were characterized by measuring properties such as thickness, roughness, and hydrophobicity and comparing them with films obtained from a conventional polymer. We report the differences between the interfacial behavior of amphiphilic molecules and nanomaterials such as films obtained by photo-polymerization, PSMA, directly on the air-water interface.

Self-Assembly of Polystyrene-b-poly(2-vinylpyridine)/Chloroauric Acid at the Liquid/Liquid Interface

The self-assembly of polystyrene-block-poly(2-vinylpyridine) at the liquid/liquid interface has been systematically investigated to develop a series of primary morphologies of the aggregates. The block copolymers self-assembled into large areas of nanodot arrays, parallel nanostrands, layered films, parallel nanobelts, honeycomb monolayers, and foams by reacting with chloroauric acid, depending on the molecular structure of the block copolymers and the amount of chloroauric acid. The formation of the first four ordered structures resulted from interfacial adsorption and self-assembly, and nucleation and epitaxial growth. The latter two structures were attributed to the water hole templating effect and spontaneous interfacial emulsification, respectively. This work provides insight into the self-assembly behavior of block copolymers at the interface and provides a facile approach for fabricating functional structures.

Effects of Ionic Strength and Ion Specificity on the Interface Behavior of Poly(dimethylaminoethyl methacrylate)-Poly(lauryl methacrylate)

The ion specificity effect on the water solubility of poly(N-isopropylacrylamide)-containing copolymers complies with the Hofmeister series, which is applicable to other copolymers or not need to be explored. In this work, effects of ionic strength under acidic conditions and ion specificity under alkaline conditions on the air/water interface behavior of two amphiphilic diblock copolymers poly(dimethylaminoethyl methacrylate)-poly(lauryl methacrylate) (PDMAEMA-PLMA) were systematically studied. Under acidic conditions, the surface pressure-area isotherms of a predominantly hydrophilic copolymer are insensitive to ionic strength. In contrast, the isotherms of a predominantly hydrophobic copolymer successively shift to the large, small, and large molecular area with the increase of ionic strength. Under alkaline conditions, the interfacial stretch degrees of PDMAEMA chains of two copolymers change with salt species and concentrations, which do not comply with the Hofmeister series. All of the Langmuir-Blodgett films of the former copolymer exhibit separate circular micelles. Nevertheless, those of the latter copolymer obtained under alkaline conditions exhibit various distinctive morphologies such as separate circular micelles, large separate PLMA cores within large PDMAEMA domains, and large PLMA domains/aggregates surrounded by short PDMAEMA shells. It can be attributed to the high deformability of PLMA chains, the ion specificity effect on the stretch degree of PDMAEMA blocks, and their underwater solubility upon compression.

Analysis of the self-assembly process of Aspergillus oryzae hydrophobin RolA by Langmuir-Blodgett method

Hydrophobins are small, amphipathic proteins secreted by filamentous fungi. Hydrophobin RolA, which is produced by Aspergillus oryzae, attaches to solid surfaces, recruits the polyesterase CutL1, and consequently promotes hydrolysis of polyesters. Because this interaction requires the N-terminal, positively charged residue of RolA to be exposed on the solid surface, the orientation of RolA on the solid surface is important for recruitment. However, the process by which RolA forms the self-assembled structure at the interface remains unclear. Using the Langmuir-Blodgett technique, we analyzed the process by which RolA forms a self-assembled structure at the air-water interface and observed the structures on the hydrophobic or hydrophilic SiO₂ substrates via atomic force microscopy. We found that RolA formed self-assembled films in two steps during phase transitions. We observed different assembled structures of RolA on hydrophilic and hydrophobic SiO₂ substrates.

Interfacial self-assembly of amphiphilic conjugated block copolymer into 2D nanotapes

In the present work, the evaporation-induced interfacial self-assembly behavior of an amphiphilic conjugated polymer, poly(3-hexylthiophene)-b-poly(acrylic acid) (P3HT-b-PAA), at the oil-water interface is explored. Novel 2D nanotapes of P3HT-b-PAA are prepared via the interfacial self-assembly. It is inferred that P3HT segments adopt a special conformation at the oil-water interface, which facilitates the packing of alkyl side chains and π-π interaction. The UV-vis spectrum further confirms that the ordering degree of P3HT segments is increased while transmission IR and Raman spectroscopic studies suggest that the P3HT chains adopt a more planar conformation at the oil-water interface. It is proposed that the formation of the nanotapes is driven by the ordered packing of the P3HT chains at the oil-water interface. Finally, the packing model of the P3HT chains inside the nanotapes is roughly proposed.

Aggregation Behavior of the Blends of Homo-PS and PS-b-PEO-b-PS at the Air/Water Interface

Aggregation behaviors of the blended Langmuir monolayers of a homopolymer polystyrene (h-PS) and a triblock copolymer polystyrene-b-poly(ethylene oxide)-b-polystyrene (PS-b-PEO-b-PS) were studied by the Langmuir film balance technique, and the morphologies of their Langmuir-Blodgett (LB) films were studied by atomic force microscopy. The isotherms of the h-PS/PS-b-PEO-b-PS blends shift to small areas with the increase of h-PS content, and a pseudoplateau appears as h-PS content is below 60 wt %. It is worth noting that the blended isotherms appear at the left of their corresponding ideal ones, which means that the blended monolayers are a little more condensed due to attractive interactions between the two components. Hysteresis phenomena exist in all of the blended monolayers, and the higher the PS-b-PEO-b-PS content, the larger the hysteresis degree becomes because of the stronger looped-PEO entanglements. All the blended LB films of h-PS and PS-b-PEO-b-PS prepared under low pressure exhibit the mixed structures of small and large isolated circular aggregates. The small aggregates are the copolymer micelle cores and the large ones are attributed to coalescence of the local h-PS chains and some PS blocks. Upon further compression, the aggregates in the blended LB films become a little denser as h-PS content is below 60 wt %, whereas those become totally close-packed with decreased size as h-PS content is 80 wt %.

Synthesis and physicochemical properties of dual-responsive acrylic acid/butyl acrylate cross-linked nanogel systems

HYPOTHESIS

A cross-linked amphiphilic nanogel containing a high mole% of hydrophilic pH-responsive moiety can provide enhanced functionality regarding stimuli-responsiveness, water-dispersibility, hydrophobic substance loading, and structural stability under harsh environmental conditions. These nanogels could be synthesized using a one-pot procedure for large-scale applications. Moreover, the interplay of various interaction forces in these colloidal systems is being investigated.

EXPERIMENTS

Model nanogels consisting of acrylic acid-butyl acrylate-ethylene glycoldimethacrylate were synthesized using an emulsion copolymerization via a seeded semi-batch process under an acidic condition. The structures were assessed by Fourier transform infrared spectroscopy and potentiometric-conductometric titrations. Zeta potential, field-emission scanning electron microscopy, and transmission electron microscopy were used to evaluate the dispersion stability, size distribution, and structural distribution, respectively. Their stimuli-responsive behavior was studied by combining static and dynamic light scattering and titration analyses.

FINDINGS

Monodisperse nanospheres of approximately 150 nm were successfully prepared by implementing a one-pot practical pathway. These nanogels displayed a dual thermo- and pH-responsive behavior, reflecting the high efficiency of physical cross-linking make it ideal for drug delivery and oil industry applications. Moreover, a novel symmetric pH-activated morphology transformation behavior was revealed. Accordingly, a compositional distribution was proposed and assessed by exploring the polymerization process.

Anion Specificity Effects on the Interfacial Aggregation Behavior of Poly(lauryl acrylate)-block-poly(N-isopropylacrylamide)

Aggregation behavior of an amphiphilic diblock copolymer poly(lauryl acrylate)-block-poly(N-isopropylacrylamide) (PLA-b-PNIPAM) on neutral aqueous subphases with different salt species and salt concentrations, as well as the structures of its Langmuir-Blodgett (LB) films, were systematically studied. The presence of NaCl or Na₂SO₄ in subphases makes PNIPAM chains shrink on the water surface and reduce their solubility underwater. On the contrary, the presence of NaNO₃ or NaSCN makes PNIPAM chains more stretched on water and increase their solubility underwater, whose stretch degree and solubility both increase with the increase of salt concentration. Solubility of PNIPAM chains in the above subphase solutions is ranked as NaSCN ≫ NaNO₃ > pure H₂O > NaCl ≈ Na₂SO₄, which is almost consistent with the Hofmeister series except for the latter two close cases. All the initial LB films of PLA-b-PNIPAM exhibit tiny isolated circular micelles. Upon compression, the LB films in the case of pure H₂O exhibit the dense mixed structures of circular micelles and wormlike aggregates. The formation of wormlike aggregates is due to connection of some adjoining cores, which is less possible in other subphase cases because of the conformation difference of PNIPAM chains.

Effects of Spreading Conditions on the Aggregation Behavior of a Symmetric Diblock Copolymer Polystyrene- block-poly(methyl methacrylate) at the Air/Water Interface

Langmuir monolayers and Langmuir-Blodgett (LB) films of a symmetric diblock copolymer polystyrene- block-poly(methyl methacrylate) (PS- b-PMMA) were characterized by the film balance technique and tapping mode atomic force microscopy, respectively. Effects of both the spreading solution concentration and the surface concentration on the aggregation behavior of PS- b-PMMA at the air/water interface and the morphologies of its LB films were studied in detail. When the monolayers spread in different concentrations (≤0.50 mg/mL), all their initial morphologies exhibit tiny circular micelles because of the long hydrophilic PMMA block in the copolymer. The initial tiny circular micelles form spontaneously and then aggregate into small ones upon compression, which can further coalesce into rodlike aggregates or large micelles depending on the spreading concentrations. The LB films of PS- b-PMMA usually exhibit various mixed structures of rodlike aggregates and circular micelles, which can further transform into labyrinth patterns under some special spreading conditions. Besides spreading concentration and volume, we discover that the detailed spreading process should also be responsible for the initial and final morphologies of the LB films. Furthermore, the LB films prepared under different spreading conditions can be regarded as in the equilibrium or nonequilibrium structures because of the kinetic effect difference resulting from the different PS chain entanglement degrees.

Crossover from semi-dilute to densely packed thin polymer films at the air-water interface and structure formation at thin film breakup

A series of poly(n-butyl acrylate) (PnBA, 5 to 32 kg mol-1) homopolymers and diblock copolymers with poly(ethylene glycol) (PEG, constant molecular weight of 0.3 kg mol-1) is synthesized for the purpose of the investigation of quasi-2D polymer films at the air-water interface. The presented compression isotherms show a transition from θ solvent behavior for PnBA homopolymers to good solvent conditions when the volume fraction of the PEG in the block copolymers is increased by decreasing the molecular weight of PnBA. A transition from a semi-dilute regime to a densely packed layer is observed in the pressure isotherms for all the polymers. In the densely packed films we found first evidence for thin film breakup of a thin polymer film directly at the air-water interface. Combination of results from Brewster-Angle-Microscopy and Surface X-ray scattering provide a consistent picture of the film breakup. Our results suggest a preferred length scale of 2.5 μm. This scenario is analogous to a spinodal mechanism driven by thermal fluctuations of the film height.