Semifluorinated chains in 2D-(perfluorododecyl)-alkanes at the air/water interface (original) (raw)
Related papers
Langmuir, 2011
Semifluorinated alkanes (C n F 2n+1 C m H 2m+1 , FnHm for short) are molecules that associate two antagonist moieties in the same chain: a hydrocarbon block and a fluorocarbon block. 1À4 The former block exhibits a hydrophobic character and substantial conformational freedom because of the low-energy barrier of trans/gauche interchanges (typically 1.2 k B T). 5 The latter block adopts a helical and more rigid configuration with a higher energy barrier to the rotation of carbonÀcarbon bonds (typically 1.8 k B T) 6 because of the bulkiness of the fluorine atom. Moreover, fluorinated chains are both hydrophobic and lipophobic and consequently tend to segregate when mixed with hydrocarbon chains. 7 Also, the cross sections of the two blocks are different: 0.18 nm 2 for the hydrocarbon chain and 0.28 nm 2 for the fluorocarbon chain. Because of this combination of antagonisms, semifluorinated alkanes exhibit self-assembling properties, alone 4,9 or in mixtures with other compounds, in two or three dimensions. The bulk crystalline phases of FnHm seem to depend strongly on the chain length (both n and m) leading to layered structures for which parallel, antiparallel, interdigited, and other structures have been proposed. 12 At higher temperatures, some FnHm exhibit mesophases with smectic ordering. 4,13À16 The primitive surfactant nature of FnHm diblocks is revealed by their capacity to form micelles in both fluorocarbons and hydrocarbons 17,18 and to induce gelification. 3 In aqueous solutions, they stabilize phospholipid vesicles, 19 fluorocarbon-in-water emulsions, 20 and hydrocarbon-in-fluorocarbon emulsions. 2 The synthesis and behavior of FnHm diblocks has recently been reviewed. 4 Pure FnHm can also exhibit surface freezing as observed for F12Hm (m = 8, 14, and 19). 21 FnHm monolayers have been the subject of many studies both at liquid and solid surfaces using different preparation and characterization techniques. Indeed, it is known since Gaines' pioneering work that, despite the absence of a polar headgroup, FnHm can form stable Langmuir monolayers. 22 Following these first experiments, many papers have dealt with their interfacial properties. Various authors have studied by surface pressure measurement the behavior on the surface of water of several FnHm (n = 3À12 and m = 6À20) and have shown that most of these molecules form monolayers at the airÀwater interface and have uncovered more complex behavior. 10,19,23À25 Because of their hydrophobic character, FnHm Langmuir monolayers segregate vertically, spontaneously or upon compression, when mixed with other amphiphilic compounds, such ABSTRACT: We have determined the structure formed at the airÀwater 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.
Properties of Langmuir monolayers from semifluorinated alkanes
Applied Surface Science, 2005
The aim of this study was to characterize several semifluorinated alkanes (SFA), of the general formula F(CF 2) m (CH 2) n H (in short FmHn), containing 25 carbon atoms in total (pentacosanes) differing in the m/n ratio, as Langmuir monolayers at the free water surface. The following compounds have been studied: F6H19, F8H17, F10H15 and F12H13. Surface pressure (p) and electric surface potential (DV) isotherms were recorded in addition to quantitative Brewster angle microscopy results. The negative sign of DVevidenced for the orientation of all the investigated semifluorinated pentacosanes, regardless the length of the hydrogenated segment, with their perfluorinated parts directed towards the air. As inferred from apparent dipole moment values and relative reflectivity results, the fluorinated pentacosanes with shorter perfluorinated fragment (F6H19 and F8H17) were found to be vertically oriented at the air/water interface, while those with longer perfluorinated moiety (F10H15 and F12H13) remain titled even in the vicinity of the film collapse.
Physical Review E, 2002
We show the spontaneous formation of an antiparallel monolayer of diblock semifluorinated n-alkane molecules spread at the air-water interface. We used simultaneous measurements of surface pressure and surface potential versus molecular area and performed grazing x-ray reflectivity experiments to characterize the studied monolayer, which is obtained at almost zero surface pressure and precedes the formation of a bilayer at higher surface pressure. Its thickness, equal to 2.7 nm, was found to be independent of the molecular area. This behavior may be explained by van der Waals and electrostatic interactions.
Structural Investigation of Langmuir and Langmuir−Blodgett Monolayers of Semifluorinated Alkanes
The Journal of Physical Chemistry B, 2006
The behavior of a semi-fluorinated alkane (C 10 F 21 C 19 H 39) has been studied at the air-water interface by using surface pressure and surface potential-area isotherms as well as infrared spectroscopy for the Langmuir-Blodgett films. In addition, based on the quantum chemical PM3 semiempirical approach, the dimer structure was investigated, and the double helix was found to be the most stable conformation of the dimer. The obtained results allow us to imply that the phase transition observed in the course of the surface pressure/area isotherm is due to a conformational change originating from the double helix to a vertical, single helix configuration.
Monolayers of Perfluoropolyethers with a Hydrophilic Head Group
Langmuir, 1994
Perfluoropoly(oxypropylene), F((CFz)30),((CFz)z)X, with a hydrophilic head group (X = -COOHI has been investigated as a thin film spread onto the surface ofwater and as films transferred to solid substrates via the Langmuir-Blodgett technique. Between the onset of the isotherm and collapse, the polymer forms a continuous film of uniform thickness. The density of this film is the same as in the bulk polymer, and the film thickness depends on the area per molecule. The theory of tethered polymers predicts an entropic contribution to the free energy of the system due to deformation of the polymer coils. We propose a thermodynamic analysis in which we mathematically separate the contributions to the surface pressure from the head groups and from the stretching of the polymer chains. In addition, we investigate the influence of the subphase composition (aqueous solutions of CaC12, poly(ethylenimine), HC1, FeCl3, N H 3 ) and compare the isotherm with a model system for the isolated head group. Based on the thermodynamic analysis and the comparison of isotherms, we conclude that-in the case of a chain length of approximately 100 o-bonds-there is no significant entropic contribution from the polymer chains.
Journal of Colloid and Interface Science, 2001
N-(1,1-dihydroperfluorododecyl)-N,N,N-trimethylammonium chloride (C12-TAC) and perfluorododecanoic acid (FC12) on the substrate of 0.01 M sodium chloride at pH 2.0 were investigated at the air-water interface by the Langmuir method and the ionizing electrode method. The temperature dependence of the transition pressure of each component was not larger than that of normal hydrogenated surfactant. The apparent molar entropy, enthalpy, and internal energy changes of phase transition from the disordered to the ordered state were calculated. Surface potentials ( V) were analyzed using the three-layer model proposed by R. J. Demchak and T. Fort (J. Colloid Interface Sci. 46, 191-202, 1974) for FC12 and Gouy-Chapman treatment for C12-TAC. The contributions of the ω-CF 3 group and the head group of the vertical component to the dipole moment, µ ⊥ , were estimated. The new finding was that the π-A curves are shifted to an area smaller than a molecular area of two pure components for the mole fraction (x) of perfluorododecanoic acid of x ≥ 0.3. The molecular areas negatively deviate from the additivity rule at discrete surface pressures. Assuming a regular surface mixture, the Joos equation (Joos, P., and R. A. Demel, Biochim. Biophys. Acta 183, 447, 1969) for analysis of the collapse pressure of mixed monolayer allowed estimation of the interaction parameter; ξ = −4.20 at x ≤ 0.5 and ξ = −0.24 at x > 0.5 between two fluorinated amphiphiles. C 2001 Academic Press
Physical Review E, 2004
We investigate the dynamic behavior upon lateral compression of a semifluorinated alkane F͑CF 2 ͒ 8 ͑CH 2 ͒ 18 H (denoted F 8 H 18), spread on the hydrophobic top of a suitable amphiphilic monolayer: namely, a natural ␣-helix alamethicin peptide (alam). We show, in particular, the formation of an asymmetric flat bilayer by compressing at the air-water interface a mixed Langmuir film made of F 8 H 18 and alam. The particular chemical structure of F 8 H 18 , the suitable structure of the underlying alam monolayer and its collapse properties, allow for a continuous compression of the upper F 8 H 18 monolayer while the density of the lower alam monolayer remains constant. Combining grazing incidence x-ray reflectivity, surface potential, and atomic force microscopy data allow for the determination of the orientation and dielectric constant of the upper F 8 H 18 monolayer.
The journal of physical chemistry. B, 2014
The effect of molecular orientation on the miscibility and structure of the adsorbed film of the 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC10diol)-1H,1H,2H,2H-perfluorodecanol (FC10OH) mixture at the hexane/water interface were examined by interfacial tension and X-ray reflectivity measurements. The interfacial tension and X-ray reflectivity at the hexane solution/water interface were measured as a function of total molality m and composition of FC10OH in the mixture X2 under atmospheric pressure at 298.15 K. The interfacial pressure vs mean area per molecule curves showed that two kinds of condensed monolayers (C1 and C2) and multilayer (M) states appeared depending on m and X2. In the pure component systems, it was found that FC10OH forms condensed monolayer in which the molecules orient almost normally to the interface, and FC10diol orients parallel and is densely packed in the condensed monolayer and then piles spontaneously to form multilayer. In the mixed system, the phase di...
Langmuir, 1994
A new fluorocarbon-chain-containing molecule, CF3(CF2)7C(O)N(H)CH2CH2SH, for self-assembly was synthesized using a simple and general technique. Self-assembled monolayers of this molecule were studied using ellipsometry, contact angle measurements, grazing-incidence infrared spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure measurements. The films were found to be uniform and reproducible, presenting a fluorocarbon surface (8,dV(H20) = 114") with predominantly CF3 groups exposed. The C=O and N-H dipoles are found to be oriented parallel to the gold surface, with the fluorocarbon axis oriented perpendicular to the surface.
Theory of Surface Micelles of Semifluorinated Alkanes
Langmuir, 2006
Surface structures of semifluorinated alkanes F(CF 2 ) n (CH 2 ) m H (referred to as FnHm) spread on the air/water interface are investigated theoretically. The study is focused on the disklike surface micelles that were recently identified by AFM and scattering techniques at sufficiently high surface concentrations. We show that (1) the micelles emerge as a result of liquid/liquid (rather than liquid/gas) phase separation in the Langmuir layer; (2) the micelles are islands of the higher-density phase with roughly vertical orientation of FnHm molecules (F-parts extend toward air, H-parts toward water) and the matrix is the lower density-phase where the FnHm diblocks are nearly parallel to the water surface; (3) the micelles and the hexagonal structure they form are stabilized by the electrostatic interactions which are mainly due to the vertical dipole moments of the CF 2 -CH 2 bonds in the vertical phase; and (4) the electrostatic repulsive interactions can serve to suppress the micelle size polydispersity.