Iso-branched semifluorinated alkanes in Langmuir monolayers

A series of semifluorinated n-alkanes (SFAs), of the general formula: (CF3)2CF(CF2)6(CH2)nH (in short iF9Hn), n = 11-20 have been synthesized and employed for Langmuir monolayer characterization. Surface pressure and electric surface potential measurements were performed in addition to Brewster angle microscopy results, which enabled both direct visualization of the monolayers structure and estimation of the monolayer thickness at different stages of compression. Our paper was aimed at investigating the influence of the iso-branching of the perfluorinated fragment of the SFA molecule on the surface behavior of these molecules at the air/water interface. It occurred that iF9 SFAs with the number of carbon atoms in the hydrogenated moiety from 11 to 20 are capable of Langmuir monolayer formation. Monolayers from iF9H11 to iF9H13 are instable, whereas those formed by iF9 SFAs with longer hydrogenated chains form stable films at the free surface of water. As compared to SFAs containing perfluorinated chain in a normal arrangement, iso-branched molecules have a greater tendency to aggregate. Lower stability of monolayers formed by iF9 SFAs as compared to F10 SFAs originated from the surface nucleation observed in BAM images, even at the very initial stages of compression. The dipole moment vector for iso-branched SFAs was found to be virtually aligned with the main axis of the molecule, contrary to F10 SFAs, where the dipole moment vector was calculated to be tilted with respect to the main molecular axis. Quantitative Brewster angle microscopy measurements (relative reflectivity experiments) enabled us to monitor the changes of monolayer thickness at different stages of monolayer compression.     

Interactions of a fluoroaryl surfactant with hydrogenated, partially fluorinated, and perfluorinated surfactants at the air/water interface

A novel surfactant containing pentafluorophenyl moiety attached at the terminal position of undecanol (11,11-difluoro-11-(pentafluorophenyl)undecan-1-ol, abbr. PBD) was synthesized and employed for the Langmuir monolayer characterization and miscibility studies with a semifluorinated alkane (perfluorodecyleicosane, abbr. F10H20) and four alcohols differing in the degree of fluorination in their hydrophobic chains: octadecanol (C18OH), perfluorooctyldecanol (F8H10OH), perfluoroisononyldecanol (iF9H10OH) and 1H,1H-perfluorooctadecanol (F18OH). Pure monolayers of all of the investigated surfactants as well as their mixtures were investigated with surface pressure-area isotherms complemented by Brewster angle microscopy (BAM) images. PBD was found to form stable Langmuir monolayers of liquid-expanded character. Characteristic dendritic structures were formed at the very early stage of compression and remained up to the vicinity of collapse, where 3D crystallites appeared. 2D miscibility studies revealed that PBD forms mixed monolayers with the investigated semifluorinated alkane (F10H20) as well as with perfluorinated alcohol (F18OH) within the whole composition range, do not mix with octadecanol to the fully hydrogenated alcohol, whereas it is partially miscible (up to a certain surface pressure value) with the studied semifluorinated alcohols. The analysis of the miscibility derived from the surface pressure-area isotherms (collapse pressure vs composition dependencies) agrees well with BAM images. Molecular interactions in the investigated systems have been quantified with interaction parameter, alpha.     

Semifluorinated alcohols in Langmuir monolayers--a comparative study

A series of semifluorinated alcohols differing in the proportion of the perfluorinated to hydrogenated chains length was synthesized and investigated in Langmuir monolayers using surface pressure and surface potential measurements. All the investigated semifluorinated alcohols were found to be capable of stable floating monolayer formation. The stability of monolayers was found to be higher upon increasing the length of the perfluorinated segment. A lower stability of the monolayers from alcohols having shorter perfluorinated fragment was attributed to the aggregation process, which was visualized with Brewster angle microscopy (BAM). Most condensed monolayers were formed by compounds with longer perfluorinated moiety, whereas monolayers composed by molecules with an iso-branched perfluorinated segment were found to be more expanded. The change of electric surface potential was negative along the whole compression. The maximum absolute values of DeltaV varies, depending on the number of CF(2) groups, from ca. -400 mV for F6H10OH to ca. -700 mV for F10H10OH. The dipole moments of free molecules were calculated with Hyperchem, and the obtained values were approximately the same (within the experimental error), i.e., 2.8D for all the investigated molecules, independently on the perfluorinated fragment length. The dipole moment vector was found to be virtually aligned to the main molecular axis for the studied compounds. Therefore, the observed differences in the measured values of DeltaV can result from a different dielectric permittivity of a particular monolayer.     

Two-dimensional miscibility studies of alamethicin and selected film-forming molecules

Alamethicin (ALM), a 20-amino acid antibiotic peptide (peptaibol) from fungal sources, was mixed in Langmuir monolayers with six different surfactants: semifluorinated (F6H18, F10H19, F8H10OH, F6H10SH) and hydrogenated (C18SH and DODAC), aimed at finding appropriate molecules for ALM incorporation for nanodevice construction. Alamethicin-containing mixed monolayers were investigated by means of surface manometry (pi-A isotherms) and Brewster angle microscopy (BAM). Our results show that only semifluorinated alkanes can serve as an appropriate material since they form miscible and homogeneous monolayers with ALM within the whole concentration range. All the remaining surfactants, possessing polar groups, were found to demix with ALM. This effect was explained as being due to the existence of strong polar interactions between vertically oriented surfactant molecules, which tend to separate from horizontally oriented alpha-helices of the peptide. On the contrary, semifluorinated alkanes, lacking any polar group in their structure and bearing a large dipole moment, interact with ALM, also possessing a huge cumulative dipole moment. These dipole-dipole interactions between ALM and SFAs are more attractive than those between SFA molecules in their pure monolayers, causing the large ALM molecule, situated parallel to the interface, to be surrounded by SFA molecules in perpendicular orientation, leading to the formation of a highly organized binary mixed monolayer. BAM images of the ALM monolayer indicate that this peptide collapses with the nucleation and growth mechanism, like the majority of surfactants, which contradicts the model of ALM collapse by desorption, previously published in the literature.     

Semifluorinated chains at the air/water interface: studies of the interaction of a semifluorinated alkane with fluorinated alcohols in mixed langmuir monolayers

Mixtures of a semifluorinated alkane 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-henicosafluorotriacontane (abbr. F10H20) and different alcohols were investigated at the air/water interface using surface pressure-area isotherms complemented with BAM images. In our studies, octadecanol and its fluorinated derivatives differing in the degree of fluorination were researched. To verify the influence of an iso branching of the fluorinated segment in an alcohol molecule, the properties of perfluorooctyldecanol and perfluoro-iso-nonyldecanol in mixtures with F10H20 were compared. From the isotherms datapoints, the excess of free energy of mixing (DeltaG(exc)) together with the interaction parameter (alpha) were calculated. On the basis of the additivity rule and BAM images, phase diagrams for all of the investigated systems were constructed. It occurs that F10H20 mixes with the fully hydrogenated alcohol, octadecanol, within the whole range of alcohol mole fractions, whereas it is completely immiscible with its perfluorinated analogue. Regarding the mixtures of F10H20 with semifluorinated alcohols, it turned out that these systems exhibit limited miscibility, i.e., are miscible at a low semifluorinated alcohol proportion, whereas upon increasing alcohol content, the systems start to demix. It may be concluded that the molecular packing in mixed monolayers is the key factor determining the miscibility of F10H20 of the investigated alcohols.     

Mixed Langmuir monolayers of gramicidin A and fluorinated alcohols

Mixed monolayers of gramicidin A (GA) and three alcohols, differing in the degree of fluorination, namely C18OH, F18OH, and F8H10OH have been investigated by means of: surface manometry (pi-A isotherms) and Brewster angle microscopy (BAM) aiming at finding appropriate molecules for incorporating gramicidin A for a biosensor design. Our results proved that only the semifluorinated alcohol is appropriate material for this purpose since it forms miscible and homogeneous monolayers with GA within the whole concentration range. The experimental results have been supported by the calculations of van der Waals energy profiles using the Insight II program. Both the hydrogenated and perfluorinated alcohols were found to aggregate at higher surface pressures, which exclude their application for gramicidin-based biosensor construction.     

Long-range nanometer-scale organization of semifluorinated alkane monolayers at the air/water interface

We have determined the structure formed at the air-water interface by semifluorinated alkanes (C(8)F(17)C(m)H(2m+1) diblocks, F8Hm for short) for different lengths of the molecule (m = 14, 16, 18, 20) by using surface pressure versus area per molecule isotherms, Brewster angle microscopy (BAM), and grazing incidence x-ray experiments (GISAXS and GIXD). The behavior of the monolayers of diblocks under compression is mainly characterized by a phase transition from a low-density phase to a condensed phase. The nonzero surface pressure phase is crystalline and exhibits two hexagonal lattices at two different scales: a long-range-order lattice of a few tens of nanometers lateral parameter and a molecular array of about 0.6 nm parameter. The extent of this organization is sufficiently large to impact larger scale behavior. Analysis of the various compressibilities evidences the presence of non organized molecules in the monolayer for all 2D pressures. At room temperature, the self-assembled structure appears generic for all the F8Hm investigated.     

Water-soluble gold nanoparticles protected by fluorinated amphiphilic thiolates

The preparation and the properties of gold nanoparticles (Au NPs) protected by perfluorinated amphiphiles are described. The thiols were devised to form a perfluorinated region close to the gold surface and to have a hydrophilic portion in contact with the bulk solvent to impart solubility in water. The monolayer protected clusters were prepared, in an homogeneous phase using sodium thiolates because of the low nucleophilicity of the alpha-perfluorinated thiols, and fully characterized with (1)H, (19)F NMR spectrometry, IR and UV-vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Au NPs with core diameters ranging from 1.6 to 2.9 nm, depending on the reaction conditions, were obtained. Water-soluble NPs (MPC-F8-PEGs) were obtained with the thiol HS-F8-PEG ending with a short poly(ethylene glycol) unit (PEG-OMe 550), whereas thiols with shorter PEG chains give rise to NPs insoluble in water. MPC-F8-PEGs undergo an exchange reaction with amphiphilic alkyl thiols. ESR investigations, using a hydrophobic radical probe, indicate that the MPC-F8-PEG monolayer shows a greater hydrophobicity compared to the analogous hydrogenated monolayer.     

Fluorinated vs hydrogenated surfactants in mixtures with valinomycin—The Langmuir monolayer study

Selected fluorinated and hydrogenated surfactants, namely a semifluorinated alkane (SFA): 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-henicosafluorononacosane (F10H19), two long chain alcohols: 18,18,18,17,17,16,16,15,15,14,14,13,13,12,12,11,11-heptadecafluorooctadecane-1-ol (F8H10OH) and octadecane-1-ol (C18OH) and with two long chain thiols of the analogous apolar part structure to the above-mentioned alcohols, i.e.: 18,18,18,17,17,16,16,15,15,14,14,13,13,12,12,11,11-heptadecafluorooctadecane-1-thiol (F8H10SH) and octadecane-1-thiol (C18SH) have been tested in mixtures with valinomycin as potential artificial matrixes for its immobilization. The thermodynamic analysis (ΔG^exc vs X_val plots) based on surface pressure–area isotherm registration for particular valinomycin/surfactant mixtures, complemented with BAM images of the films structure indicate that only fluorinated surfactants are suitable materials for valinomycin incorporation as they form homogeneous miscible monolayers at X_val below 0.5.     

Langmuir monolayers from perfluorobutyl-n-eicosane

Perfluorobutyl-n-eicosane (abbreviated as F4H20) was spread at the air/water as Langmuir monolayers and studied under different experimental conditions, such as spreading volume, subphase temperature and compression speed. The Langmuir monolayer experiments (π-A isotherms) have been complemented with Brewster angle microscopy results, which enabled direct visualization of the monolayers’ structure and estimation of the film thickness at different stages of compression. It has been found that the molecules are oriented almost vertically (with respect to the interface) in the vicinity of film collapse. The negative sign of the measured surface potential, ΔV, is evidence for the orientation of F4H20 molecules with their perfluorinated parts exposed towards the air. In the case of F4H20 a limited fluorination relative to perfluoroeicosane also results in monolayer formation, in contrast to eicosane itself, which forms lenses.     

Semifluorinated alkanes--primitive surfactants of fascinating properties

Semifluorinated alkanes (SFAs) are diblock molecules, in which two mutually immiscible moieties, namely the hydrocarbon segment and the perfluorinated segment are bound covalently. The presence of two opposing segments within one molecule makes semifluorinated alkanes a very interesting class of compounds, which show a particular behavior both in bulk and at interfaces. Their highly asymmetric structure, arising from the incompatibility of the both constituent parts, results in surface activity of these molecules (so-called primitive surfactants) when dissolved in organic solvents, and allows for the Langmuir monolayer formation if spread at the air/water interface, despite of the absence of any polar group. Since 1984 (when SFAs have been characterized for the first time by Rabolt et al. [Rabolt JF, Russell TP, Twieg RJ. Macromolecules 1984;17:2786]), semifluorinated alkanes have been subjected to many studies. The present article reviews the results obtained so far and covers the aspects of their synthesis, properties in bulk (solutions and solid state) and applications. Special emphasis is put on the Langmuir monolayer properties and self-organization of SFAs on solid substrates.     

Spectral properties and structure of the J-aggregates of pseudoisocyanine dye in layered silicate films

Monolayer behavior of an ion pair amphiphile (IPA), hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), with normal long-chain alcohols at the air/water interface was analyzed by the Langmuir trough technique with the Brewster angle microscope (BAM) observations, and the pronounced stability enhancement of a HTMA-DS monolayer with the presence of the alcohol additives was demonstrated. Two normal long-chain alcohols with alkyl chain lengths of C16 and C18, 1-hexadecanol (HD) and 1-octadecanol (OD), were chosen as the additives. The surface pressure-area and surface potential-area isotherms of the monolayers with BAM images of monolayer morphology implied that the addition of either HD or OD with a comparatively small head group in a double-chained HTMA-DS monolayer at the interface led to better molecular packing and attractive interaction between the molecules, showing a similar condensing effect as that observed in mixed phospholipid/cholesterol systems. Moreover, the monolayer hysteresis and relaxation curves indicated that the incorporation of the alcohols into a HTMA-DS monolayer was able to lessen the monolayer hysteresis and to enhance the monolayer stability. In comparison with OD, HD seemed more effective as an additive in stabilizing a HTMA-DS monolayer, most likely due to the relatively better molecular packing of HTMA-DS and HD molecules at the interface. It is inferred that the stability of a monolayer or vesicular bilayer structure composed of IPAs can be improved by adjusting the molecular packing/interaction with a suitable long-chain alcohol as the additive.     

The perfluorinated alcohols (F5C6)(F3C)2COH and (F5C6)(F10C5)COH: synthesis, theoretical and acidity studies, spectroscopy and structures in the solid state and the gas phase

The syntheses of the perfluorinated alcohols (F(5)C(6))(F(3)C)(2)COH (1) and (F(5)C(6))(C(5)F(10))COH (2) are described. Both compounds were prepared in reasonable yields (1: 65%, 2: 85%) by reacting the corresponding ketone with C(6)F(5)MgBr, followed by acidic work-up. The alcohols were characterized by NMR, vibrational spectroscopy, single-crystal X-ray diffraction, acidity measurements and gas-phase electron diffraction. A combination of appropriate 2D NMR experiments allowed the unambiguous assignment of all signals in the (19)F spin systems, of which that of 2 was especially complex. High acidity of the alcohols is indicated by acidity measurements as well as the calculated gas phase acidities. It is also supported by the crystal structure of 2, which exhibits only a single weak intermolecular hydrogen bridge with an O...O distance of 301 pm. This shows the low donor strength of the oxygen atom in the compound, which is partly compensated through formation of two intramolecular CF...H contacts of 220 and 232 pm length to the proton not involved in the hydrogen bridge. The pK(a) values in acetonitrile are 22.2 for 1 and 22.0 for 2; their calculated gas phase acidities are 1367 and 1343 kJ mol(-1) (MP2/TZVPP level).     

Nucleation and growth in the collapsed langmuir monolayers from semifluorinated alkanes

The 3D phase formation was monitored in relaxation experiments of the collapsed Langmuir monolayers of selected partially fluorinated tetracosanes, that is, F6H18, F8H16, and F10H14. To carry out these experiments, the classical method of surface manometry, such as pi-A isotherms registration and the molecular area-time dependencies, under quasi-static monitoring conditions were applied. The evolution of 3D structures at the water/air interface was observed with Brewster angle microscopy (BAM). The obtained data were interpreted according to the nucleation-growth-collision theory model. It occurred that, even though the investigated chemicals are not classical surfactants and do not possess any polar headgroup, their evolution from a 2D monolayer to 3D structures can be successfully modeled with the above-mentioned theory. The influence of the subphase temperature on the nucleation process is also discussed.     

Semifluorinated side group poly(oxyethylene) derivatives having extremely low surface energy: Synthesis, characterization, and surface properties

Poly[oxy[[2-(perfluorooctyl)ethyl]thiomethyl]ethylene]s (H2F8TP-Xs, where X is mole% of perfluorooctyl groups in the side chain) with different levels of conversion were synthesized using polymer analogous reactions from poly[oxy(chloromethyl)ethylene] and 2-perfluorooctyl ethane thioacetate. H2F8TP-20, 41, 64, and 85 were obtained by changing the poly[oxy(chloromethyl)ethylene] to 2-perfluorooctyl ethane thioacetate mole ratio in the reaction from 0.35 to 1.50. H2F8TP-85 (85% conversion) was found to have an extremely low surface energy of 6.2 mN/m at room temperature, which was attributed to the highly ordered perfluorinated alkyl groups on the surface as a result of phase separation between the perfluorinated side chain part and the hydrogenated flexible backbone. The films of the polymers were characterized by electron spectroscopy for chemical analysis (ESCA) and near edge X-ray absorption fine structure (NEXAFS).     

The role of nonsurface-active species at interfacial molecular recognition by melamine-type monolayers

The main characteristics of Langmuir monolayers are radically changed by molecular recognition of hydrogen bond nonsurface-active species. The change in the thermodynamic, phase, and structural features by molecular recognition of dissolved uracil or barbituric acid by 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2C11H23-melamine) monolayers is characterized by combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and Grazing incidence X-ray diffraction (GIXD) measurements. Phase behavior of the 2C11H23-melamine monolayer and morphology of the condensed phase domains are changed drastically, but in a specific way, by molecular recognition of uracil or barbituric acid. The main characteristics of the interfacial system can be essentially affected by the kinetics of the recognition process. Pure 2C11H23-melamine monolayers show only small compact, but nontextured domains. The monolayers of 2C11H23-melamine-uracil assemblies develop well-shaped circular condensed-phase domains having an inner texture with alkyl chains essentially oriented parallel to the periphery and having a striking tendency to two-dimensional (2D) Ostwald ripening. The 2C11H23-melamine-barbituric acid monolayers form large homogeneous areas of condensed phase that transfer at smaller areas per molecule to a homogeneous condensed monolayer. BAM imaging of corresponding assemblies with ((CH3(CH2)11O(CH2)3)2-melamine having modified alkyl chains demonstrates the specific effect of the monolayer component. GIXD results reveal that molecular recognition of pyrimidine derivatives gives rise only to quantitative changes in the two-dimensional lattice structure. The striking differences in the main characteristics between the supramolecular species are related to their different chemical structures. Quantum chemical calculations using the semiempirical PM3 method provide information about the different nature of the hydrogen-bonding-based supramolecular structures.     

Monolayers of mono- and bipolar palmitic acid derivatives

Monopolar and bipolar derivatives of hexadecanoic acid (HA), 16-hydroxyhexadecanoic acid (HHA), methyl hexadecanoate (MH) and methyl 16-hydroxyhexadecanoate (MHH) have been investigated on pure water and NaCl solutions with different ion concentrations (1, 2 and 3 mol l⁻¹). Surface pressure area isotherms show that HA forms a fully condensed monolayer on pure water at 20 °C [E. Teer, C.M. Knobler, S. Siegel, D. Vollhardt, G. Brezesinski, J. Phys. Chem., B104, 43, 2000, pp. 10053–10058] whereas in the case of the corresponding bipolar HHA the hydroxy group as a second polar moiety leads to a destabilization of the monolayer. The presence of two relatively strong hydrophilic polar groups at opposite ends of the chain prevents the formation of condensed films. The esterification of the carboxyl group (MH) changes the phase sequence from L₂–Ov–LS for HA to L₂–LS. Inserting a hydroxy group at the end of the chain (MHH) shifts the liquid expanded/liquid condensed (LE/LC) phase transition to higher surface pressures but does not change the phase sequence, however it increases the chain tilt. The pressure of the first-order phase transition LE/LC is strongly temperature dependent for MH, while the transition pressure of MHH is almost temperature independent. The phase behavior of MHH and MH on pure water was further studied by surface potential, Brewster angle microscopy (BAM), fluorescence microscopy and grazing incidence X-ray diffraction (GIXD) measurements. The LC domains of MHH on pure water are so small that no inner texture can be observed by BAM in contrast to the LC domains of MH. 3M NaCl in the subphase does not change the MH textures, while it increases the size of the LC domains of MHH. The influence of the hydroxy group on the monolayer behavior is discussed in terms of the formation of hydrogen bonds. The presence of NaCl in the subphase expands the monolayers. The results obtained are explained by changes in monolayer–monolayer and monolayer–subphase interactions.     

Adsorption of DNA to octadecylamine monolayers at the air-water interface

In this work, surface properties of octadecylamine (ODA) monolayers in the presence of different concentrations of calf thymus DNA in the aqueous subphase covering a range of 2-8μM have been investigated. The increase of DNA concentration is accompanied by a marked increment in the expansion of the corresponding isotherms. In addition, there is a change in the profile of the isotherms ranging from an abrupt liquid-solid transition for the lipid monolayer on pure water to a slow condensation of the monolayer in a liquid state when DNA is added to the subphase, demonstrating the effective adsorption of the polynucleotide to the long chain amine monolayer. Additional phase transitions appear in the isotherms upon addition of sufficient amount of DNA, revealing the existence of specific processes such as folding or squeezing out of the DNA. This system is, however, highly reversible during compression-expansion cycles due to the strong interaction between the two components. These results are also supported by Brewster Angle Microscopy (BAM) images showing significant changes in the morphology of the film. Integral reflectivity of the BAM microscope has been used to study both isotherms themselves and the kinetic process of DNA inclusion into the lipid-like ODA monolayer. This parameter has been proven to be very effective for quantification of the monolayer processes showing high consistency with the compressibility and kinetics results.     

Effect of the molecular structure on the hierarchical self-assembly of semifluorinated alkanes at the air/water interface

Semifluorinated alkanes (C(n)F(2n+1)C(m)H(2m+1)), short FnHm display local phase separation of mutually incompatible hydrocarbon and fluorocarbon chain moieties, which has been utilized as a structure-forming motif in supramolecular architectures. The packing of semifluorinated alkanes, nominally based on dodecyl subunits, such as perfluoro(dodecyl)dodecane (F12H12) and perfluoro(dodecyl)eicosane (F12H20), as well as a core extended analogue, 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene) (F11H1-core-H12), was studied at the air/water interface. Langmuir monolayers were investigated by means of neutron reflectivity directly at the air/water interface and scanning force microscopy after transfer to silicon wafers. Narrowly disperse surface micelles formed in all three cases; however, they were found to bear different morphologies with respect to molecular orientation and assembly dimensionality, which gives rise to different hierarchical aggregate topologies. For F12H12, micelles of ca. 30 nm in diameter, composed of several circular or "spherical cap" substructures, were observed and a monolayer model with the fluorocarbon block oriented toward air is proposed. F12H20 molecules formed larger (ca. 50 nm diameter) hexagonally shaped surface micelles that were hexagonally, densely packed, besides more elongated but tightly interlocked wormlike structures. Conversely, F11H1-core-H12 films organized into linear rows of elongated surface micelles with comparable width, but an average length of ca. 400 nm, apparently formed by antiparallel molecular packing.