Subphase pH Values Research Articles

Aggregation behavior of symmetric poly(n‐butyl acrylate)‐block‐poly(acrylic acid) on subphases of different ionic strengths

AbstractInfluences of subphase pH value and ionic strength (NaCl concentration) on the aggregation behavior of a symmetric diblock polyelectrolyte poly(n‐butyl acrylate)‐block‐poly(acrylic acid) (PnBA‐b‐PAA) at the air/water interface and its Langmuir–Blodgett (LB) films were studied. With the increase of subphase pH value, surface pressure–molecular area isotherms of PnBA‐b‐PAA monolayers shift toward smaller area due to the gradually enhanced ionization and underwater solubility of hydrophilic PAA chains. All the initial LB films transferred from different subphases with/without NaCl (before monolayer compression) exhibit the densely mixed structures of worm‐like aggregates and circular micelles, which indicates that the aggregate formation belongs to the spontaneous mechanism. The micelle core diameters in the LB films increase with the increase of subphase pH value, owing to the enhanced PAA chain solubility and PnBA block aggregation. Under acidic condition, micelle cores are slightly larger upon addition of NaCl due to the enhanced underwater solubility of the uncharged PAA chains triggered by their possible interaction with Cl−. Under neutral/alkaline conditions, micelle cores are also larger upon addition of NaCl, which is attributed to the electrostatic shielding effect of Na+. Upon compression, the LB films prepared under neutral/alkaline conditions exhibit the predominantly large circular micelles.

Transparent and Hydrophobic “Reduced Graphene Oxide–Titanium Dioxide” Nanocomposites for Nonwetting Device Applications

Graphene oxide–titanium dioxide (GO–TiO2) nanocomposite sheets were transferred onto Si and quartz substrates by Langmuir–Blodgett (LB) technique at different subphase TiO2 concentrations and pH values. The effects of subphase and heat treatment conditions on the composition, surface morphology, microstructure, and hydrophobic performance of the composite sheets were investigated by XPS, Raman, AFM, HR-TEM, and contact-angle measurements. The wetting behavior of composite sheets before and after vacuum heat treatment was analyzed in conjunction with the extent of TiO2 uptake, distribution of nanoparticles over GO sheets, and overall surface roughness. The degree of subphase ionization significantly affects the uptake and aggregation behavior of TiO2, which tends to be dominated by the interaction of TiO2 with carboxylic acid functional groups at the edges of the GO sheets. Wettability studies revealed improvement in the hydrophobicity of composite sheets compared to the GO sheets, which is attributed prim...

Capacity of cholesteryl hemisuccinate in ion pair/phospholipid complex to improve drug-loading, stability and antibacterial activity of clarithromycin intravenous lipid microsphere.

Current research was to evaluate the capacity and molecular interaction of ion pair/phospholipid complex during preparation of clarithromycin intravenous lipid microsphere (CLA-LM) for improving drug-loading, stability and antibacterial activity. The optimum pH range for the presence of ion pair formed by CLA and cholesteryl hemisuccinate (CHEMS) was found to be between 6.4 and 8.2. CLA-LM prepared by ion pair/phospholipid complex possessed improved drug-loading (5-10 mg/ml), plasma stability, storage stability (24 months at 4 ± 2℃) and better activity for multi-drug-resistant Mycobacterium tuberculosis (MIC = 0.058 μg/ml). The release profile in vitro was pH-sensitive and greater in more acidic condition, which was effective for pathological targeting. CLA-LM presented controlled release and low drug leakage in plasma. Langmuir monolayer showed that the incorporation of CHEMS obviously improved interfacial molecular interactions of the mixed monolayer. The ordering and condensing effect of CHEMS resulted in higher collapse surface pressure and smaller limiting area, as well as reduced compression modulus. Close aggregate network distribution with particular micro domains in atomic force microscopy images reflected the enhanced miscibility and thermodynamic stability. The change of the spatial location and protonation degree of CHEMS resulted from the aqueous subphase pH value from 6 to 8 was observed. More acidic environment allowed for better molecular interaction and interface behavior. These results indicated that CHEMS enhances the phospholipid packing to achieve stable CLA-encapsulation in phospholipid interface and identify the preference of ion pair/phospholipid complex as a valuable delivery strategy to develop CLA formulations for improving drug-loading, stability and antibacterial activity.

Dynamic Behaviors and Morphology Change of Anionic Phospholipid DPPG Monolayer Caused by Bovine Serum Albumin at Air-Water Interface

The interaction between bovine serum albumin (BSA) and the anionic 1.2-dipalmitoyl-sn-glycero-3-(phospho-rac-(1-glycerol)) (sodium salt) (DPPG) phospholipid at different sub-phase pH values was investigated at air-water interface through surface pressure measurements and atomic force microscopy (AFM) observation. By analyzing surface pressure-mean molecular area (π-A) isotherms, the limiting molecular area in the closed packing state-the concentration of BSA (Alim-[BSA]) curves, the compressibility coefficient-surface pressure (C−1S-π) curves and the difference value of mean molecular area-the concentration of BSA (ΔA-[BSA]) curves, we obtained that the mean molecular area of DPPG monolayer became much larger when the concentration of BSA in the subphase increased at pH=3 and 5. But the isotherms had no significant change at different amount of BSA at pH=10. In addition, the amount of BSA molecules adsorbed onto the lipid monolayer reached a threshold value when [BSA]>5×10−8 mol/L for all pHs. From the surface pressure-time (π-t) data, we obtained that desorption and adsorption processes occurred at pH=3, however, there was only desorption process occurring at pH=5 and 10. These results showed that the interaction mechanism between DPPG and BSA molecules was affected by the pH of subphase. BSA molecules were adsorbed onto the DPPG monolayers mainly through the hydrophobic interaction at pH=3 and 5, and the strength of hydrophobic interaction at pH=3 was stronger than the case of pH=5. At pH=10, a weaker hydrophobic interaction and a stronger electrostatic repulsion existed between DPPG and BSA molecules. AFM images revealed that the pH of subphase and [BSA] could affect the morphology features of the monolayers, which was consistent with these curves. The study provides an important experimental basis and theoretical support to understand the interaction between lipid and BSA at the air-water interface.

Langmuir monolayers and Langmuir-Blodgett films of pH-sensitive lipid

Colloidal membranous vesicles based on the functionalized liposomes are promising for development of novel efficient systems of controlled and stimuli-triggered drug delivery. In particular, the specific metabolism of cancer cells results in significant acidification in tumors, and thus the corresponding pH value changes can be used as a stimulus for drug release specifically in the tumor areas. pH-sensitive liposomes can be formed via incorporation of special pH-sensitive amphiphilic compounds into the liposomal membrane. Molecules of such compounds undergo a conformational transition caused by certain changes in the pH value of aqueous environment and resulting in corresponding membrane perturbations with rapid leakage of drug content from those liposomes into the areas with corresponding acidity. In the present work the pH-induced changes of Langmuir monolayer compression isotherm and of the structure of corresponding Langmuir-Blodgett films of pH-sensitive synthetic lipid trans-4,5,didodecyloxycarbonyl-trans-2-morpholinocyclohexanol have been observed and studied. The substantial expansion of Langmuir monolayer of that lipid caused by change of the aqueous subphase pH value from 7 to 5 was observed. Corresponding monolayer Langmuir-Blodgett films were deposited on mica substrate surface and studied using AFM technique. The substantial differences in the structure of Langmuir-Blodgett films deposited at pH values 7 and 5 were found. The observed effects directly demonstrate the pH-induced structural transformations in organized condensed planar molecular systems formed by pH-sensitive lipid molecules which underlie the pH-sensitivity of corresponding liposomal bilayer membranes containing such lipids.

Effect of ionization on the behavior of n-eicosanephosphonic acid monolayers at the air/water interface. Experimental determinations and molecular dynamics simulations.

Monolayers of n-eicosanephosphonic acid, EPA, were studied using a Langmuir balance and a Brewster angle microscope at different subphase pH values to change the charge of the polar headgroups (Zav) from 0 to -2. Molecular dynamics simulations (MDS) results for |Zav| = 0, 1, and 2 were compared with the experimental ones. EPA monolayers behave as mixtures of mutually miscible species (C20H41-PO3H2, C20H41-PO3H(-), and C20H41-PO3(2-), depending on the subphase pH). The order and compactness of the monolayers decrease when increasing |Zav|, while go from strongly interconnected by phosphonic-phosphonic hydrogen bonds (|Zav| = 0-0.03) through an equilibrium between the total cohesive energy and the electrostatic repulsion between the charged polar groups (0.03 < |Zav| < 1.6) to an entirely ionic monolayer (|Zav| ≈ 2). MDS reveal for |Zav| = 0 that the chains form spiralled nearly rounded structures induced by the hydrogen-bonded network. When |Zav| ≈ 1 fingering domains were identified. When Z ≈ 2, the headgroups are more disordered and distanced, not only in the xy plane but also in the z direction, forming a rough layer and responding to compression with a large plateau in the isotherm. The monolayers collapse behavior is consistent with the structures and domains founds in the different ionization states and their consequent in-plane rigidity: there is a transition from a solid-like response at low pH subphases to a fluid-like response at high pH subphases. The film area in the close-packed state increases relatively slow when the polar headgroups are able to form hydrogen bonds but increases to near twice that this value when |Zav| ≈ 2. Other nanoscopic properties of monolayers were also determined by MDS. The computational results confirm the experimental findings and offer a nanoscopic perspective on the structure and interactions in the phosphonate monolayers.

PH controlled condensation of polysiloxane networks at the water–air interface

Structural and mechanical properties of molecularly thick polysiloxane membranes were studied on different liquid subphases to investigate the impact of the subphase's pH value on the cross-linking process. The lateral structure of these films was studied in-situ by grazing incidence diffraction while torsions pendulum experiments reveal the response of the system to mechanical stress. The results show a hindered cross-linking on acidic subphases. At alkaline and neutral pH conditions the cross-linking process was not effected. The data revealed that the degree of polymerization can be tuned by regulating the subphase's pH value, which opens the opportunity to build complex polysiloxane membranes in a controlled manner.

Mössbauer study of C18N/Fe Langmuir-Blodgett layers

Langmuir-Blodgett (LB) films of octadecanoyl hydroxamic acid (C18N) complexed with Fe3 + ions have been prepared at various subphase pH values. The LB films consisting of different number of layers were investigated by 57Fe conversion electron Mossbauer spectroscopy (CEM) at room temperature. The CEM detector contained a piece of α-iron, enriched with 57Fe, using as an internal standard. The Mossbauer pattern of the C18N/Fe LB films is a doublet with parameters δ = 0.35 mm/s and Δ = 0.74 mm/s. A gradual increase of the relative occurrence of the doublet compared to the sextet of the internal standard was observed with the increasing number of layers, indicating the nearly uniform distribution of Fe among the LB layers.

Physical–Chemical Properties and Transfection Activity of Cationic Lipid/DNA Complexes

Investigation of DNA interactions with cationic lipids is of particular importance for the fabrication of biosensors and nanodevices. Furthermore, lipid/DNA complexes can be applied for direct delivery of DNA-based biopharmaceuticals to damaged cells as non-viral vectors. To obtain more effective and safer DNA vectors, the new cationic lipids 2-tetradecylhexadecanoic acid-{2-[(2-aminoethyl)amino]ethyl}amide (CI) and 2-tetradecylhexadecanoic acid-2-[bis(2-aminoethyl)amino]ethylamide (CII) were synthesized and characterized. The synthesis, physical-chemical properties and first transfection and toxicity experiments are reported. Special attention was focused on the capability of CI and CII to complex DNA at low and high subphase pH values. Langmuir monolayers at the air/water interface represent a well-defined model system to study the lipid/DNA complexes. Interactions and ordering of DNA under Langmuir monolayers of the new cationic lipids were studied using film balance measurements, grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR). The results obtained demonstrate the ability of these cationic lipids to couple with DNA at low as well as at high pH value. Moreover, the observed DNA structuring seems not to depend on subphase pH conditions. An influence of the chemical structure of the lipid head group on the DNA binding ability was clearly observed. Both compounds show good transfection efficacy and low toxicity in the in vitro experiments indicating that lipids with such structures are promising candidates for successful gene delivery systems.

Conformation of Lysozyme Langmuir Monolayer Studied by Infrared Reflection Absorption Spectroscopy

The surface chemistry and spectroscopy of the reduced lysozyme Langmuir monolayer were investigated at different pH values to compare with the native one. It was found that the limiting molecular area of the reduced lysozyme was not subphase pH dependent as the native lysozyme. To explain this result in terms of the conformation and orientation of the lysozyme Langmuir monolayer at various subphase pH values, we have used infrared reflection absorption spectroscopy. The interpretation of the results make plausible change of the conformation and orientation of the native lysozyme Langmuir monolayer with the subphase pH 3, 6, and 11.

Langmuir films of monohydroxamic acid on various ion-containing subphases

In order to get the most dense octadecanoyl monohydroxamate (C 18N) monolayer developed at the air–water interface, the influences of various ions (Ca 2+, Mg 2+, Sr 2+, Ba 2+, and Fe 3+ used at low concentrations) at various subphase pH values were studied by pressure–area ( π– A) isotherms and Brewster angle microscopy (BAM). The change in the mean molecular area as a function of time at constant surface pressure was monitored to examine the monolayer stability under various experimental conditions. Both the density and the stability of the C 18N Langmuir films are influenced by the presence of metal ions in the subphases. The change which depends on the type of ions added to the subphase as well as on the subphase pH value could be explained by the complex forming ability of cations with the C 18N in the monolayer characteristics. Due to the high stability of C 18N monolayers developed on ion-containing subphases, high-quality LB films onto solid substrates could be fabricated.

Monolayer behavior of a pyridyl head-group-containing amphiphile and its miscibility with poly( d, l-lactide-co-glycolide) on different pH subphase

In this paper, we investigated the Langmuir film and Langmuir–Blodgett (LB) monolayer film of a nonionic amphiphilic molecule, 4-(6- p-pyridyloxyl)hexyloxyl-4′-dodecyloxylazobenzene (C 12AzoC 6Py) and its mixture with poly( d, l-lactide-co-glycolide) (PLG) at different subphase pH values (2.0, 2.6, 3.3, 4.4, and 6.5, respectively) by surface pressure–area ( π – A ) isotherms, in situ interface Brewster angle microscopy (BAM), and ex situ atomic force microscopy (AFM). For pure C 12AzoC 6Py, its π – A isotherms display a plateau when the subphase pH value is lower than 3.0. The pressure of the plateau increases with the decrease of pH until 2.0. Over the plateau, the π – A isotherms become almost identical to the one under neutral conditions. The appearance of such a plateau can be explained as the coexistence of protonation and unprotonation of pyridyl head groups of the employed amphiphile. In contrast to the homogeneous surface morphology of pure C 12AzoC 6Py near the plateau by BAM observation, the surface in the case of its mixing with PLG exhibits a dendritic crystalline state under low surface pressure at subphase pH lower than 3.0. The crystalline state becomes soft and gradually melts into homogeneous aggregates with surface pressure increasing to a higher value than that of the plateau. Meanwhile, the hydrolysis of PLG in the mixture system at the interface has been affirmed to be restrained to a very large extent. And the PLG was believed to be compelled to the up layer of the LB film due to the phase separation, which is examined by AFM. Based on the experimental results, the corresponding discussion was also performed.

Cross-linking of polybutadiene at the air/water interface: Toward an easy access to two-dimensional polymeric materials

A novel approach to two-dimensionally crosslink polydienes at the air/water interface is proposed. The acid-catalyzed condensation of the triethoxysilane pendant groups of triethoxysilane-functionalized polybutadiene chains at the air/water interface successfully led to the formation of an insoluble crosslinked material which could be directly removed from the water surface. The efficiency of the cross-linking reaction was demonstrated through surface pressure measurements such as surface pressure–mean molecular area isotherms recorded at different reaction times and isobar experiments for different subphase pH values. The evolution of the monolayer topography during cross-linking was studied by atomic force microscopy imaging of the Langmuir–Blodgett films.

Polystyrene-b-Poly(tert-butyl acrylate) and Polystyrene-b-Poly(acrylic acid) Dendrimer-Like Copolymers: Two-Dimensional Self-Assembly at the Air−Water Interface

The two-dimensional self-assembly at the air/water (A/W) interface of two dendrimer-like copolymers based on polystyrene and poly(tert-butyl acrylate) (PS-b-PtBA) or poly(acrylic acid) (PS-b-PAA) was investigated through surface pressure measurements (isotherms, isochores, and compression-expansion hysteresis experiments) and atomic force microscopy (AFM) imaging. The two dendrimer-like block copolymers have an 8-arm PS core (Mn = 10 000 g/mol, approximately 12 styrene repeat units per arm) with a 16-arm PtBA (Mn = 230 000 g/mol, approximately 112 tert-butyl acrylate repeat units per arm) or PAA (Mn = 129 000 g/mol, approximately 112 acrylic acid repeat units per arm) corona. The PS-b-PtBA sample forms stable Langmuir monolayers and aggregates into circular surface micelles up to a plateau observed in the corresponding isotherm around 24 mN/m. Beyond this threshold, the monolayers collapse above the interface, resulting in the formation of large and irregular desorbed aggregates. The PS-b-PAA sample has ionizable carboxylic acid groups, and its A/W interfacial self-assembly was therefore investigated for various subphase pH values. Under basic conditions (pH = 11), the carboxylic acid groups are deprotonated, and the PS-b-PAA sample is therefore highly water-soluble and does not form stable monolayers, instead irreversibly dissolving in the aqueous subphase. Under acidic conditions (pH = 2.5), the PS-b-PAA sample is less water-soluble and becomes surface-active. The pseudoplateau observed in the isotherm around 5 mN/m corresponds to a pancake-to-brush transition with the PAA chains dissolving in the water subphase and stretching underneath the anchoring PS cores. AFM imaging revealed the presence of circular surface micelles for low surface pressures, whereas the biphasic nature of the pseudoplateau region was confirmed with the gradual aggregation of the micellar PS cores above the PAA chains. The aggregation numbers for both samples were estimated around 3-5 dendrimer-like copolymers per circular surface micelle. These rather low values confirmed the tremendous influence of molecular architecture on the two-dimensional self-assembly of block copolymers.

Organophosphorus Hydrolase at the Air−Water Interface: Secondary Structure and Interaction with Paraoxon

The secondary structure of organophosphorus hydrolase (OPH) at the air-water interface was studied using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The shape and position of the amide I and amide II bands were used to estimate the surface conformation and orientation of OPH. The PM-IRRAS results indicated that the enzyme did not unfold for the range of surface pressure used (0-30 mN/m). At low surface pressures, the signal of amide I was very weak and the intensity was almost the same as amide II. Upon further compression, the PM-IRRAS signal and the ratio of the intensity of amide I and amide II both increase, implying an increased interfacial concentration of the enzyme. From the amide I/amide II ratio and the band position, it was deduced that the enzyme adopts a conformation which gives a higher occupied surface at low surface pressure and rotates to a more vertical orientation at high surface pressures. The compression and decompression of the OPH monolayer indicated that the fingerprint of the secondary structure at the air-water interface was reversible. PM-IRRAS was also used to investigate the pH effect of the subphase on the secondary structure of OPH. The secondary structure of OPH at the air-water interface was well defined when the pH of the subphase was near its isoelectric point (IP, pH 7.6). However, it adopted a different orientation when the subphase pH values were higher or lower than the IP with formation of random coil structure. The hydrolysis of organophosphorus compound paraoxon by OPH was also studied at the air-water interface by PM-IRRAS. The pH effect and the interaction with paraoxon both seem to orientate the enzyme more in the plane of the interface and to produce random coil structure.

Special properties of monolayer formed by a novel Gemini amphiphile at air–water interface

A novel Gemini amphiphile, N, N′-bis-(4-decyloxy-2-hydroxybenzylidene)-ethane-1,2-diamine, was synthesized and characterized. This amphiphile is capable of forming stable monolayer at air–water interface. The limiting area has a minimum when the pH value of subphase is 12. The unique behavior is attributed to the formation of intramolecular hydrogen bond between hydroxyl groups. The existence of this hydrogen bond was supported by the FTIR results. With the addition of copper ion into subphase, the limiting area showed a decreasing value, which is quite different from that of the conventional neutral amphiphiles. XPS and UV–vis were also carried out to investigate the effect of copper ion on molecular area.

Ion-Sensitive Monolayers and Langmuir–Blodgett Films of Amphiphilic Cyclen: Selectivity and Regeneration

Complexation of dicetyl cyclen with transition metal ions in the monolayers on the surface of aqueous solutions of Cu(II), Ni(II), Zn(II), Ag(I) and their mixtures was studied. It was established that the selectivity of the interaction of the monolayer composed of this ligand with transition metal ions is determined by the subphase pH value. It is disclosed that, in the acidic region of subphase pH values, dicetyl cyclen in the monolayer bounds predominantly the Ni(II) ions from solutions containing Cu2+ and Ni2+ ions, although its complexes with Ni(II) in the bulk under these conditions are less stable than similar complexes with Cu(II). The effect of conformational and charge states of the ligand on the protonation of macrocycle and the stability of its complexes is discussed. The possibility of the reversible regeneration of the monolayers and the Langmuir–Blodgett films of the complexes of dicetyl cyclen and copper(II) ions is shown to occur with no changes in the structure and properties of this planar system. It is shown that the Langmuir–Blodgett films based on dicetyl cyclen can be used as a sensor element for the quantitative analysis of the content of Cu(II) ions in dilute solutions.

Langmuir–Blodgett Films of Fatty Acid Salts of Bi- and Trivalent Metals: Y, Ba, and Cu Stearates

Dependences of the composition and properties of the monolayers and Langmuir–Blodgett films of yttrium, barium, and copper stearates on the conditions of their formation were studied by FTIR spectroscopy, X-ray photoelectron spectroscopy, and quartz crystal microbalance technique. The identity of film compositions on the surfaces of liquid and solid substrates was established. It was shown that, in the case of copper and yttrium, the Langmuir–Blodgett films are formed from corresponding basic salts of stearic acid at high subphase pH values. The conditions of the preparation of high-quality Langmuir–Blodgett films from yttrium, barium, and copper stearates are determined.

Structure characterization and stability of mixed lipid/protein monolayer at the air/water interface

Abstract A mixed protein/lipid monolayer has been constructed by the protein adsorption from subphase into the spread phospholipid monolayer. A precisely controlled pump was used to exchange the protein solution with different pH values after the protein was ensured to reach the less condensed surface. The domains formed in the coexistence region of D-dipalmitoylphosphatidylcholine (D-DPPC) have been recorded by Brewster angle microscopy (BAM) combined with the film balance before and after the penetration of the protein, human serum albumin (HSA). The subphase was exchanged by gradually increasing or decreasing pH value of the solution. Three isotherms of the mixed D-DPPC/HSA monolayer with the subphase of pH=4.2, pH=7.0 and pH=9.1, respectively, were obtained. It indicated that the area per lipid molecule with protein increased as the subphase pH value was lowed. Simultaneously, morphological dynamic changes caused by the gradual changing on subphase pH were observed. These variations can be ascribed to the conformation change of protein under the fluctuation of pH value. The hydrophobic and electrostatic interactions between the phospholipid and HSA were as considered for the interpretation of domain change based on the current experimental results

Static and Dynamic Properties of Calixarene Monolayers at the Air/Water Interface. 2. Effects of Ionic Interactions with p-Dioctadecanoylcalix[4]arene

The static and dynamic properties of p-dioctadecanoylcalix[4]arene monolayers on the air/water interface are examined relative to the effects of inorganic and polymeric counterions (i.e., spermine, poly(ethylenimine), and poly(l-lysine)) in the subphase over a large range of pH. The methods adopted are the techniques of the Wilhelmy plate and surface light scattering (SLS). At moderate subphase pH values, 3.5−10, the polyions significantly expand the monolayer but do not appreciably change the monolayer surface viscoelasticity as detected by SLS. A scheme of complex formation and acid−base equilibrium is used to explain the surface pressure isotherms while strong lateral packing interactions of the hydrophobic chains and aromatic rings, which predominate over the electrostatic interactions of the phenoxide rings of calixarene, are invoked to explicate the viscoelastic properties. Simple univalent inorganic ions do not show any specific effects on either the static or dynamic properties of these monolayers.