Tail Ordering Due to Headgroup Hydrogen Bonding Interactions in Surfactant Monolayers at the Water−Oil Interface (original) (raw)
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We present a simple model of single chain, non-ionic surfactant adsorption at gas/liquid interface. Our model takes explicitly into account the effect of conformations the surfactant hydrocarbon chains can assume at the interface. We applied our model for the description of a dependence of surface tension on solution concentration of homologous series of n-alkanols. Aliphatic alcohols with the chain length from four to ten carbon atoms were studied. We found that our model correctly describes experimental data and predicts a distribution of chain conformation at the interface for all n-alkanols studied. We detected small but distinct even-odd effect in the distribution of conformations.
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.
The Journal of Physical Chemistry C
The anionic surfactant dodecyl sulfate (DDS) has been intercalated in an Mg−Al layered double hydroxide (LDH). Monolayer and bilayer arrangements of the alkyl chains of the intercalated surfactant can be engineered by tuning the Al/Mg ratio of the LDH. In both arrangements the anionic headgroup of the surfactant is tethered to the LDH sheets, and consequently translational mobility of the chains is absent. The degrees of freedom of the confined alkyl chains are restricted to changes in conformation. The effects of the arrangement of the intercalated surfactant chains on conformational order and dynamics have been investigated by spectroscopic measurements and molecular dynamics simulations. Infrared, Raman, and 13C NMR spectroscopies were used to investigate conformation of the alkyl chains in the monolayer and bilayer arrangements and variable contact time cross-polarization magic angle spinning (VCT CPMAS) NMR measurements to probe molecular motion. The alkyl chains in the monolayer arrangement of the intercalated DDS chains showed considerably greater conformational disorder and faster dynamics as compared to chains in the bilayer arrangement, in spite of the fact that the volume available per chain in the monolayer is smaller than that in the bilayer. Atomistic MD simulations of the two arrangements of the intercalated surfactant were carried out using an isothermal−isobaric ensemble. The simulations are able to reproduce the essential results of the experiment—greater conformational disorder and faster dynamics for the alkyl chains in the monolayer arrangement of the intercalated surfactant. The MD simulations show that these results are a consequence of the fact that the nature of conformational disorder in the two arrangements is different. In the monolayer arrangement the alkyl chains can sustain isolated gauche defects, whereas in the bilayer arrangement gauche conformers occur only as part of a kink—a gauche+−trans−gauche− sequence.
Hydrocarbon chain conformation in an intercalated surfactant monolayer and bilayer
Journal of Chemical Sciences, 2001
Cetyl trimethyl ammonium (CTA) ions have been confined within galleries of layered CdPS 3 at two different grafting densities. Low grafting densities are obtained on direct intercalation of CTA ions into CdPS 3 to give Cd 0⋅93 PS 3 (CTA) 0⋅14. Intercalation occurs with a lattice expansion of 4⋅8 Å with the interlamellar surfactant ion lying flat forming a monolayer. Intercalation at higher grafting densities was effected by a two-step ion-exchange process to give Cd 0⋅83 PS 3 (CTA) 0⋅34 , with a lattice expansion of 26⋅5 Å. At higher grafting densities the interlamellar surfactant ions adopt a tilted bilayer structure. 13 C NMR and orientation-dependent IR vibrational spectroscopy on single crystals have been used to probe the conformation and orientation of the methylene 'tail' of the intercalated surfactant in the two phases. In the monolayer phase, the confined methylene chain adopts an essentially all-trans conformation with most of the trans chain aligned parallel to the gallery walls. On lowering the temperature, molecular plane aligns parallel, so that the methylene chain lies flat, rigid and aligned to the confining surface. In the bilayer phase, most bonds in the methylene chain are in trans conformation. It is possible to identify specific conformational sequences containing a gauche bond, in the interior and termini of the intercalated methylene. These high energy conformers disappear on cooling leaving all fifteen methylene units of the intercalated cetyl trimethyl ammonium ion in trans conformational registry at 40 K.
Journal of Molecular Structure: THEOCHEM, 2010
Molecular dynamics simulations of self-assembled monolayers (SAMs) of alkanethiol derivatives interfaced with water reveal the structure of the interface and show how it influences the properties of water. Three SAMs of different character (neutral, anionic and cationic) are compared: 6-hexanethiol, 11-mercaptoundecanoic acid and 11-amino-1-undecanethiol. The simulation captures phenomena such as the hydrophobic gap, local increase of the density of water near the interface and ordering of water into layers.
The Journal of Physical Chemistry B, 1999
In this publication we used molecular dynamics computer simulations in order to get a more detailed insight into the dynamic properties of monododecyl pentaethylene glycol (C 12 E 5 ) monolayers at the water surface. We used a simulation box enclosing 36 surfactant molecules and the surfactant concentration was kept constant at an average value of 0.55 nm 2 /molecule. After reaching the thermodynamic equilibrium, it turned out that the dodecyl chains were approximately oriented diagonal to the surface with an average value of about 43°m easured in respect to the surface normal vector. This result is in fairly good agreement with a previously performed simulation containing only 25 surfactant molecules. The average orientation angle of the hydrophobic chains agrees very well with experimental data. In contrast to the unpolar part of the molecules the watersoluble glycol chains were aligned more perpendicular to the water surface with a tilt angle of only 11°. It is interesting to note that these chains are rather stiff. Within the time scale of our simulation the surfactant molecules were firmly attached at the surface and did not enter into the water phase. All alkyl and glycol chain ends, however, were characterized by a striking flexibility. The calculation of pair distribution functions and detailed pattern analysis of the oxygen atoms within the glycol chains revealed a pentagonal arrangement in the highly ordered internal segments of the E 5 chain.
Langmuir, 2009
Langmuir monolayers of the hydrogenated/fluorinated catanionic surfactant cetyltrimethylammonium perfluorooctanoate at the air/water interface are studied at room temperature. Excess Gibbs energies of mixing, ΔG E , as well as transition areas and pressures, were obtained from the surface pressure-area isotherm. The ΔG E curve indicates that tail-tail interactions are more important than head-head interactions at low pressures and vice versa. Atomic force microscopy and molecular dynamics simulations allowed a fine characterization of the monolayer structure as a function of the area per molecule at mesoscopic and nanoscopic size scales, respectively. A combined analysis of the techniques allow us to conclude that electrostatic interactions between the ionic head groups are dominant in the monolayer while hydrophobic parts are of secondary importance. Overall, results obtained from the different techniques complement to each other, giving a comprehensive characterization of the monolayer.
Solvent Isotopic Effects on a Surfactant Headgroup at the Air–Liquid Interface
The Journal of Physical Chemistry C, 2018
The geometry, arrangement, and orientation of a quaternary ammonium surfactant flanked by two methyl groups, a benzyl head, and an octyl tail were assessed at the air−water and air-deuterium oxide (D 2 O) interfaces using sum frequency generation vibrational spectroscopy. Remarkably, symmetric and asymmetric N−CH 3 stretches (at ∼2979 and ∼3045 cm −1 , respectively, in the SSP polarization combination) were visible in water but negligible in deuterium oxide. We concluded that D 2 O addition triggers the average reorientation of the dimethyl amino units parallel to the interface and possibly changes the overall conformation of the surfactant. A reduced number of gauche defects in the surfactant octyl chain is also observed in D 2 O. Tilt angles for the octyl chain (1.0− 10.8°) are consistent with an ordered monolayer at the air−liquid interface.
The Journal of Physical Chemistry C, 2021
Growth of molecular thin films with desired orders and orientations has become technologically relevant as the electronic industries seek new opportunities and applications. However, the delicate balance of interfacial and intermolecular forces and their complex influence on thin-film growths still require more understanding. Here, the effects of a hydrophobic self-assembled monolayer (SAM) surface on the crystallization of four common solvents-acetonitrile, ethanol, methanol, and water-are investigated. Despite the absence of significant substrate-molecule forces, unexpected oriented growth is observed for these molecules except water. Acetonitrile and ethanol form a sustaining vertical assembly order with long-range crystalline structures. Coincident epitaxy with small lattice mismatches is found to be essential to these orderings, which are energetically favored but without a dominant azimuthal orientation. In contrast, a preferred in-plane registry of methanol overlayers is observed for an ultrathin nominal thickness and becomes lost in slightly thicker films. Such thickness-dependent ordering of methanol assemblies can be explained with semi-commensurate epitaxy with a tensile strain of ~6.6% along hydrogen-bonded chains, whose quick relief results in the loss of order and even the phase. These rich observations suggest that SAM surfaces offer good opportunities for selective crystallization of molecular films worthy of further investigations.