Langmuir Monolayers of a Hydrogenated/Fluorinated Catanionic Surfactant: From the Macroscopic to the Nanoscopic Size Scale (original) (raw)
Related papers
Journal of Colloid and Interface Science, 2008
The monolayer formed at an air/water interface by the synthetic non-ionic surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(ω-methoxydodecakis (ethylene glycol)) (2C 18 E 12) has been characterized using Langmuir trough measurements, Brewster angle microscopy (BAM), and neutron reflectometry. The BAM and reflectometry studies were performed at four different surface pressures (π) in the range 15-40 mN/m. The BAM studies (which give information on the in-plane organisation of the surfactant layer) demonstrate that the 2C 18 E 12 molecules are arranged on the water surface to form distinct, approximately circular, 5 µm diameter domains. As the surface pressure is increased these domains retain their size and shape but are made progressively more close-packed, such that the monolayer is made more or less complete at π = 40 mN/m. The neutron reflectometry measurements were made to determine the structure of the interfacial surfactant layer at π = 15, 28, 34 and 40 mN/m, providing information on the thickness of the 2C 18 E 12 alkyl chains', head groups' and associated solvent distributions (measured along the surface normal), along with the separations between these distributions, and the effective interfacial area per molecule. Partial structure factor analyses of the reflectivity data show that the effective interfacial area occupied decreases from 217 Å 2 per 2C 18 E 12 molecule at π = 15 mN/m down to 102 Å 2 at π = 40 mN/m. There are concomitant increases in the widths of the surfactant's alkyl chains' and head groups' distributions (modelled as Gaussians), with the former rising from 12 Å (at π = 15 mN/m) up to 19 Å (at π = 40 mN/m) and the latter rising from 13 Å (at π = 15 mN/m) up to 24 Å (at π = 40 mN/m). The compression of the monolayer is also shown to give rise to an increased surface roughness, some of which is due to the thermal roughness caused by capillary waves, but with a significant contribution also coming from the intrinsic/structural disorder in the monolayer. At all surface pressures studied, the alkyl chains and head groups of the 2C 18 E 12 are found to exhibit a significant overlap, and this increases with increasing π. Given the various trends noted on how the structure of the 2C 18 E 12 monolayer changes as a function of π , we extrapolate to consider the structure of the monolayer at π > 40 mN/m (making comparison with its single chain (C n E m) counterparts) and then relate these findings to the observations recorded on the structure and solute entrapment efficiency of 2C 18 E 12 vesicles.
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
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.
Numerical Analysis of Nonionic Surfactant Monolayers at Water/Air Interfaces
The Journal of Physical Chemistry B, 2004
The time evolution of a monododecyl pentaethylene glycol monolayer at the water-air interface is investigated using velocity rescaled NVT molecular dynamics. The model we consider consists of 40 surfactant molecules and 2350 water molecules enclosed in a periodic box. The time evolution of this system is ruled by the CFF91 force field that includes intra-and intermolecular degrees of freedom for both surfactant and water molecules. The interface we consider herein is hence described in all atomic detail. We discuss the initial condition problem and study the relaxation properties to stationarity. Transient regimes of self-assembly of the surfactant chains in entangled structures in both air and water are described. From the stationary configuration, we define the interface location, determine the mass distribution across the interface, and discuss the validity of the tilt angles notion when structural roughness is considered. Using pair distribution functions, we show that, besides typical tilt angles, monododecyl pentaethylene glycol molecules might also develop domains on the interface that suggest an in-plane orientational order.
A Molecular Dynamics Study of Monolayers of Nonionic Poly(ethylene oxide) Based Surfactants
Langmuir, 2004
Molecular dynamics simulations of monolayers of nonionic, poly(ethylene oxide) based surfactants are reported. Specifically, alcohol ethoxylates and alkylphenol ethoxylates are compared in terms of the varying architecture of the molecules for the development of a structure-behavior relationship. Interfacial density profiles are used to assess the structure of the monolayers, the penetration of water and oil into the monolayers, and the solvation of the hydrophiles and hydrophobes. Chain conformational descriptors are used to examine the molecular structure of the surfactants.The simulations revealed that monolayers of alcohol ethoxylates are considerably more diffuse than their alkylphenol counterparts, with the packing being governed by the size of the hydrophile. With the exception of the branched alcohol ethoxylate, the intermixing of the bulk phases within monolayers of alcohol ethoxylates increases with increasing hydrophile length. By comparison, the packing of alkylphenol ethoxylates within the monolayer is governed by the aromatic nucleus in the molecule. No specific interaction is observed between the aromatic rings of neighboring molecules. Monolayers of alkylphenol ethoxylates are more compact than their alcohol counterparts, resulting in more effective separation of the bulk water and oil phases.
Journal of Colloid and Interface Science, 2002
Experimental data on surface tension available from the literature and generated in the present study are analyzed to estimate the applicability of adsorption models, based on the Frumkin equation, to nonionic and ionic surfactants and their mixtures. Optimization programs based on the least-squares method in media of Delphi V and Pascal VII are used. The effect of interactions between the adsorbed species on surface tension is considered in all cases. The results are compared to those obtained with the simpler Szyszkowski equation, employed in numerous studies of nonionic surfactants, when interactions are neglected. Cases where the Frumkin model can be successfully employed with ionic surfactants and mixtures are presented and the conditions of its applicability are analyzed. Related characteristic quantities (maximum adsorption, standard free energy of surfactant adsorption, energy of interaction between adsorbed species, standard free energy of counterion adsorption, degree of coverage by surfactant/counterion associates) are established as a function of: 1. The number of methylene groups in the surfactant molecule; 2. The position of the polar head on the hydrophobic tail; 3. The type of (alkali) counterion. The properties of an adsorption layer from a mixture of nonionic and ionic surface-active species are compared to those of the single surfactants. C 2002 Elsevier Science (USA)
The water soluble anionic dye, cobalt phthalocyanine disulfonate (PcCoDS), was used to prepare π-electron terminated model monolayers with a high surface free energy. We report the en face self-assembly of monomeric PcCoDS monolayers on dioctadecydiammonium bromide (DODAB). Direct surface force measurements showed that the phthalocyanine overlayers increased the adhesion between the surfactant membranes in water nearly 100-fold. This increased attraction correlated with the dye-induced aggregation of DODAB vesicles. Simultaneous force and electronic absorbance measurements indicate that the formation of strong adhesive contacts between the dye layers corresponds with phthalocyanine dimerization. Further, the adhesion increased in proportion to the dye coverage, and, at the maximum dye coverage, it is at least as strong as hydrophobic interactions that stabilize the membranes. The surface free energy of PcCoDS/ DODA membranes, determined from JKR analysis of the contact area vs applied load, is 5.2 ( 0.4 mN m -1 . Analysis of the intersurface attraction using Lifschitz theory for multilayered systems suggests that the dispersion force contributes substantially to the dye interactions. Such forces acting between assemblies of other aromatic compounds in water may similarly contribute to the stability of molecularly engineered materials.
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2011
Using computer simulations, we investigate the interfacial structure of sodium dodecyl sulphate (SDS) monolayers adsorbed at the water surface and water–oil interfaces. Using an algorithm that removes the averaging effect of the capillary waves, we obtain a detailed view of the solvation structure of water around the monolayer. We investigate surface concentrations between 45 and 33 Å 2 per surfactant, which are near experimental conditions corresponding to the critical micellar concentration and the formation of Newton black films. The surfactants induce a layering structure in water, which disappears at approximately 1 nm from the monolayer plane. The water molecules exhibit a preferred orientation with the dipoles pointing towards the monolayer. The orientational order decays slowly, but it does not influence the hydrogen bond structure of water, which is significantly disrupted in the interfacial region only. These structural changes are qualitatively the same in SDS–water and o...