Hexane-water Interface Research Articles

Tail Ordering Due to Headgroup Hydrogen Bonding Interactions in Surfactant Monolayers at the Water−Oil Interface

Interactions between surfactants, and the resultant ordering of surfactant assemblies, can be tuned by the appropriate choice of head- and tailgroups. Detailed studies of the ordering of monolayers of long-chain n-alkanoic and n-alkanol monolayers at the water-vapor interface have demonstrated that rigid-rod all-trans ordering of the tailgroups is maintained upon replacing the alcohol with a carboxylic acid headgroup. In contrast, at the water-hexane liquid-liquid interface, we demonstrate that substitution of the -CH(2)OH with the -COOH headgroup produces a major conformational change of the tailgroup from disordered to ordered. This is demonstrated by the electron density profiles of triacontanol (CH(3)(CH(2))(29)OH) and triacontanoic acid (CH(3)(CH(2))(28)COOH) monolayers at the water-hexane interface, as determined by X-ray reflectivity measurements. Molecular dynamics simulations illustrate the presence of hydrogen bonding between the triacontanoic acid headgroups that is likely responsible for the tail ordering. A simple free energy illustrates the interplay between the attractive hydrogen bonding and the ordering of the tailgroup.

Revisiting the hexane-water interface via molecular dynamics simulations using nonadditive alkane-water potentials

We present results addressing properties of a polarizable force field for hexane based on the fluctuating charge (FQ) formalism and developed in conjunction with the Chemistry at Harvard Molecular Mechanics (CHARMM) potential function. Properties of bulk neat hexane, its liquid-vapor interface, and its interface with a polarizable water model (TIP4P-FQ) are discussed. The FQ model is compared to a recently modified alkane model, C27r, also based on the CHARMM potential energy function. With respect to bulk properties, both models predict bulk density within 1%; the FQ model predicts the liquid vaporization enthalpy within 2%, while the C27r force field underestimates the property by roughly 20% (and in this sense reflects the quality of the C27r force field across the spectrum of linear and branched alkanes). The FQ hexane model realistically captures the dielectric properties of the bulk in terms of a dielectric constant of 1.94, in excellent agreement with experimental values in the range of 1.9-2.02. This behavior is also in conformity with a recent polarizable alkane model based on Drude oscillators. Furthermore, the bulk dielectric is essentially captured in the infinite frequency, or optical, dielectric contribution. The FQ model is in this respect a more realistic force field for modeling lipid bilayer interiors for which most current state-of-the-art force fields do not accurately capture the dielectric environment. The molecular polarizability of the FQ model is 11.79 A3, in good agreement with the range of experimental and ab initio values. In contrast to FQ models of polar solvents such as alcohols and water, there was no need to scale gas-phase polarizabilities in order to avoid polarization catastrophes in the pure bulk. In terms of the liquid-vapor and liquid-liquid interfaces, the FQ model displays a rich orientational structure of alkane and water in the respective interfacial systems, in general conforming with earlier simulation studies of such interfaces. The FQ force field shows a marked deviation in the interfacial dipole potentials computed from the charge densities averaged over simulation trajectories. At the liquid-vapor interface, the FQ model predicts a potential drop of -178.71 mV in contrast to the C27r estimate of -433.80 mV. For the hexane-water interface, the FQ force field predicts a dipole potential drop of -379.40 mV in contrast to the C27r value of -105.42 mV. Although the surface dipole potential predicted by the FQ model is roughly 3.5 times that predicted by the C27r potential, it is consistent with reported experimental potentials across solvated lipid bilayers in the range of 400-600 mV.

Janus Particles at Liquid−Liquid Interfaces

Following recent theoretical predictions, we report on the first experiments on the interfacial activity of so-called Janus nanoparticles (i.e., bifacial particles consisting of a gold and an iron oxide moiety). Using pendant drop tensiometry, we show that the amphiphilicity derived from the Janus character of the particles leads to a significantly higher interfacial activity compared to that of the respective homogeneous particles of the same size. The self-assembly of Janus particles at the hexane-water interface results in a significant decrease in the interfacial tension. Furthermore, we demonstrate control over the interfacial activity by tuning the particles' amphiphilicity via ligand-exchange reactions.

Fourier Transform Infrared Study of Lipoxygenase Conformation in Organic Solvent Media

The secondary structure of commercially purified soybean lipoxygenase (EC 1.13.11.12) was investigated in selected monophasic organic solvents, including chloroform, methanol, acetonitrile, hexane, and octane. The Fourier transform infrared (FT-IR) spectra of the enzyme obtained in chloroform, methanol, and acetonitrile showed an absorption band at 1617 cm(-1) indicative of significant protein aggregation, whereas spectra of lipoxygenase in hexane and octane exhibited substantially less aggregate formation. Variable-temperature infrared studies of lipoxygenase in D(2)O show that the predominately alpha-helical structure of the protein undergoes an irreversible transition to intermolecular beta-sheet at and above 65 degrees C. Chemical imaging technology employing an FT-IR spectrometer equipped with an infrared microscope and a focal-plane array detector was used to examine the changes in the secondary structure of lipoxygenase at the water-hexane interface in the presence and absence of substrate. The secondary structure of lipoxygenase at the hexane-water interface was comparable to that of the structure of lipoxygenase in D(2)O after exposure of lipoxygenase solution to hexane.

Adsorption properties of C10E8 at water/ hexane interface investigated onboard STS-107, by the FAST facility

The dynamic adsorption properties of a non-ionic surfactant C10E8 at the water-hexane interface have been investigated in microgravity using FAST (Facility for Adsorption and Surface Tension studies) onboard the Shuttle STS-107. To obtain a complete characterisation of such model system, the Capillary Pressure tensiometer implemented in the facility has been used with different experimental methodologies allowing different aspects of adsorption dynamics to be investigated. Theoretical models specifically developed have been used for the data interpretation allowing the characterisation of the adsorption mechanisms.

A dynamic liquid–liquid interfacial pressure detector for the rapid analysis of surfactants in a flowing organic liquid

Design and development of a dynamic interfacial pressure detector (DIPD) is reported. The DIPD measures the differential pressure as a function of time across the liquid–liquid interface of organic liquid drops (i.e., n-hexane) that repeatedly grow in water at the end of a capillary tip. Using a calibration technique based on the Young–Laplace equation, the differential pressure signal is converted, in real-time, to a relative interfacial pressure. This allows the DIPD to monitor the interfacial tension of surface active species at liquid–liquid interfaces in flow-based analytical techniques, such as flow injection analysis (FIA), sequential injection analysis (SIA) and high performance liquid chromatography (HPLC). The DIPD is similar in principle to the dynamic surface tension detector (DSTD), which monitors the surface tension at the air–liquid interface. In this report, the interfacial pressure at the hexane–water interface was monitored as analytes in the hexane phase diffused to and arranged at the hexane–water interface. The DIPD was combined with FIA to analytically measure the interfacial properties of cholesterol and Brij ®30 at the hexane–water interface. Results show that both cholesterol and Brij ®30 exhibit a dynamic interfacial pressure signal during hexane drop growth. A calibration curve demonstrates that the relative interfacial pressure of cholesterol in hexane increases as the cholesterol concentration increases from 100 to 10,000 μg ml −1. An example of the utility of the DIPD as a selective detector for a chromatographic separation of interface-active species is also presented in the analysis of cholesterol in egg yolk by normal-phase HPLC-DIPD.

Molecular ordering and phase transitions in alkanol monolayers at the water-hexane interface.

The interface between bulk water and bulk hexane solutions of n-alkanols (H(CH(2))(m)OH, where m=20, 22, 24, or 30) is studied with x-ray reflectivity, x-ray off-specular diffuse scattering, and interfacial tension measurements. The alkanols adsorb to the interface to form a monolayer. The highest density, lowest temperature monolayers contain alkanol molecules with progressive disordering of the chain from the -CH(2)OH to the -CH(3) group. In the terminal half of the chain that includes the -CH(3) group the chain density is similar to that observed in bulk liquid alkanes just above their freezing temperature. The density in the alkanol headgroup region is 10% greater than either bulk water or the ordered headgroup region found in alkanol monolayers at the water-vapor interface. We conjecture that this higher density is a result of water penetration into the headgroup region of the disordered monolayer. A ratio of 1:3 water to alkanol molecules is consistent with our data. We also place an upper limit of one hexane to five or six alkanol molecules mixed into the alkyl chain region of the monolayer. In contrast, H(CH(2))(30)OH at the water-vapor interface forms a close-packed, ordered phase of nearly rigid rods. Interfacial tension measurements as a function of temperature reveal a phase transition at the water-hexane interface with a significant change in interfacial excess entropy. This transition is between a low temperature interface that is nearly fully covered with alkanols to a higher temperature interface with a much lower density of alkanols. The transition for the shorter alkanols appears to be first order whereas the transition for the longer alkanols appears to be weakly first order or second order. The x-ray data are consistent with the presence of monolayer domains at the interface and determine the domain coverage (fraction of interface covered by alkanol domains) as a function of temperature. This temperature dependence is consistent with a theoretical model for a second order phase transition that accounts for the domain stabilization as a balance between line tension and long range dipole forces. Several aspects of our measurements indicate that the presence of domains represents the appearance of a spatially inhomogeneous phase rather than the coexistence of two homogeneous phases.

Assessing the efficiency of free energy calculation methods.

The efficiencies of two recently developed methods for calculating free energy changes along a generalized coordinate in a system are discussed in the context of other, related approaches. One method is based on Jarzynski's identity [Phys. Rev. Lett. 78, 2690 (1997)]. The second method relies on thermodynamic integration of the average force and is called the adaptive biasing force method [Darve and Pohorille, J. Chem. Phys. 115, 9169 (2001)]. Both methods are designed such that the system evolves along the chosen coordinate(s) without experiencing free energy barriers and they require calculating the instantaneous, unconstrained force acting on this coordinate using the formula derived by Darve and Pohorille. Efficiencies are analyzed by comparing analytical estimates of statistical errors and by considering two numerical examples-internal rotation of hydrated 1,2-dichloroethane and transfer of fluoromethane across a water-hexane interface. The efficiencies of both methods are approximately equal in the first but not in the second case. During transfer of fluoromethane the system is easily driven away from equilibrium and, therefore, the performance of the method based on Jarzynski's identity is poor.

Molecular dynamics study of the n-hexane-water interface: towards a better understanding of the liquid-liquid interfacial broadening.

By molecular dynamics simulations, we have studied the hydrophilic-hydrophobic interface between water and n-hexane liquid phases. For all temperatures studied our computed interfacial tension agrees very well with the experimental value. However, the interfacial width calculated from capillary wave theory systematically overestimates the width obtained from fitting either the total density or composition profile. We rationalize the applicability of capillary wave theory for our system by reconsidering the usual value taken for the correlation length. This is motivated by the presence of order at the interface. Possible implications for recent experimental studies on the structure of model alkane-water interfaces are discussed, including the significance of the intrinsic width parameter.

Molecular Dynamics Simulation of Surfactin Molecules at the Water-Hexane Interface

The dynamics of surfactin, a lipopeptide surfactant from Bacillus subtilis, has been studied by molecular dynamics at different interfacial concentrations in a water-hexane medium reproducing a hydrophilic/hydrophobic biphasic system. The shapes and orientations of surfactin molecules, as hydrogen bonds and Ramachandran angles, have been recorded to investigate the environment effect on the molecular structure. We demonstrate that the peptidic backbone can exhibit a large flexibility and that conformational motions and structural fluctuations depend strongly on the interfacial concentration. Moreover, we have measured the surface activity of this biosurfactant by computing the interfacial tension and lateral and rotational diffusion coefficients.

Modeling the effect of free water on enzyme activity in immobilized lipase-catalyzed reactions in organic solvents

The influence of water content on the lipase-catalyzed acidolysis of triolein (glycerol-trioleate, TO) and caprylic acid (CA) in hexane, using an immobilized enzyme was studied. An adequate water content ( R W) ranged from near zero to 0.1 g of water/g of dry enzyme. Over these values there was a decrease in the rate of incorporation of CA into triglyceride. This decrease was attributed to the progressive flooding of the carrier’s pores, in which the enzyme was immobilized. The flooding reduced the number of the enzyme molecules at the water–hexane interface and therefore, hindered the accessibility of the hydrophobic substrates (TO and CA) to the enzyme. A simple physical model based on a characterization of the immobilized enzyme particle by mercury porosimetry was developed. The model agreed well with both the experimental data and the prior published data. The model may partly explain the observed inhibition when using low molecular weight alcohols and carboxylic acids in immobilized lipase-catalyzed processes.

Surface activity of poloxamines at the interfaces between air–water and hexane–water

Abstract Poloxamines are tetra functional block copolymers, which contain four polyethylene oxide (PEO)–polypropylene oxide (PPO) chains joined to a central ethylene diamine moiety by way of the nitrogen atoms. Surface and interfacial tensions at the air–water and hexane–water interfaces have been determined over a wide concentration range for several of these block copolymeric amphiphiles. Titration data indicates that these molecules are dibasic. Two pKa values—corresponding to proton loss from the central nitrogen atoms of the molecule—have been determined by fitting titrimetric data to a thermodynamic model of the titration process. The values obtained are fairly constant for different poloxamines, which suggests that varying the block size has little impact upon these pKa values. However, the values obtained are lower than those observed for ethylene diamine and indicate that the attachment of the PEO–PPO chains to the nitrogen atoms does reduce their basicity. The Gibbs adsorption isotherm at the air–water interface is similar to the adsorption pattern reported for the poloxamers-linear PEO–PPO–PEO block copolymers [Langmuir 10 (1994) 2604] and shows that the highly branched poloxamines have similar interfacial properties as the poloxamers. Data is also reported on adsorption of poloxamines at the hexane–water interface. In the main it is concluded that those block copolymers with high PPO to PEO ratios and which are, therefore, relatively hydrophobic are more effective in reducing surface and interfacial tensions. These effects are, however, attenuated by changes in aqueous phase pH, which alters the charged nature of the poloxamine molecule.

Calculating free energies using average force

A new, general formula that connects the derivatives of the free energy along the selected, generalized coordinates of the system with the instantaneous force acting on these coordinates is derived. The instantaneous force is defined as the force acting on the coordinate of interest so that when it is subtracted from the equations of motion the acceleration along this coordinate is zero. The formula applies to simulations in which the selected coordinates are either unconstrained or constrained to fixed values. It is shown that in the latter case the formula reduces to the expression previously derived by den Otter and Briels [Mol. Phys. 98, 773 (2000)]. If simulations are carried out without constraining the coordinates of interest, the formula leads to a new method for calculating the free energy changes along these coordinates. This method is tested in two examples — rotation around the C–C bond of 1,2-dichloroethane immersed in water and transfer of fluoromethane across the water-hexane interface. The calculated free energies are compared with those obtained by two commonly used methods. One of them relies on determining the probability density function of finding the system at different values of the selected coordinate and the other requires calculating the average force at discrete locations along this coordinate in a series of constrained simulations. The free energies calculated by these three methods are in excellent agreement. The relative advantages of each method are discussed.

Noncapillary-wave structure at the water-alkane interface.

Synchrotron x-ray reflectivity is used to study the interface between bulk water and bulk n-alkanes with carbon numbers 6 through 10, 12, 16, and 22. For all interfaces, except the water-hexane interface, the interfacial width disagrees with the prediction from capillary-wave theory. The variation of interfacial width with carbon number can be described by combining the capillary-wave prediction for the width with a contribution from intrinsic structure. This intrinsic structure is determined by the gyration radius for the shorter alkanes and by the bulk correlation length for the longer alkanes.

Early events in the folding of an amphipathic peptide: A multinanosecond molecular dynamics study

Folding of the capped LQQLLQQLLQL peptide is investigated at the water–hexane interface by molecular dynamics simulations for 161.5 ns. Initially placed in the aqueous phase as a β-strand, the peptide rapidly adsorbs to the interface, where it adopts an amphipathic conformation. The marginal presence of nonamphipathic structures throughout the complete trajectory indicates that the corresponding conformations are strongly disfavored at the interface. It is further suggestive that folding in an interfacial environment proceeds through a pathway of successive amphipathic intermediates. The energetic and entropic penalties involved in the conformational changes along this pathway markedly increase the folding time scales of LQQLLQQLLQL, explaining why the α-helix, the hypothesized lowest free energy structure for a sequence with a hydrophobic periodicity of 3.6, has not been reached yet. The formation of a type I β-turn at the end of the simulation confirms the importance of such motifs as initiation sites allowing the peptide to coalesce towards a secondary structure. Proteins 1999;36:383–399. © 1999 Wiley-Liss, Inc.

Early events in the folding of an amphipathic peptide: A multinanosecond molecular dynamics study

Folding of the capped LQQLLQQLLQL peptide is investigated at the water-hexane interface by molecular dynamics simulations for 161.5 ns. Initially placed in the aqueous phase as a beta-strand, the peptide rapidly adsorbs to the interface, where it adopts an amphipathic conformation. The marginal presence of nonamphipathic structures throughout the complete trajectory indicates that the corresponding conformations are strongly disfavored at the interface. It is further suggestive that folding in an interfacial environment proceeds through a pathway of successive amphipathic intermediates. The energetic and entropic penalties involved in the conformational changes along this pathway markedly increase the folding time scales of LQQLLQQLLQL, explaining why the alpha-helix, the hypothesized lowest free energy structure for a sequence with a hydrophobic periodicity of 3.6, has not been reached yet. The formation of a type I beta-turn at the end of the simulation confirms the importance of such motifs as initiation sites allowing the peptide to coalesce towards a secondary structure. Proteins 1999;36:383-399.

X-ray scattering from monolayers of F(CF2)10(CH2)2OH at the water–(hexane solution) and water–vapor interfaces

Synchrotron x-ray reflectivity is used to study the structure of a monolayer of F(CF2)10(CH2)2OH self-assembled at the liquid–liquid interface from a solution in hexane placed in contact with water. It is demonstrated that this monolayer is in a high density (solid) phase below a transition temperature. This is in contrast to the conventional expectation that soluble surfactants form disordered monolayers at the liquid–liquid interface. Above the transition temperature the monolayer desorbs into the hexane solution, leaving behind an interface with a very low density of surfactants. Hysteresis in the formation of the monolayer occurs when the temperature is scanned through the transition temperature. The success of these measurements relied upon the development of a novel technique to flatten the liquid–liquid interface to the extent required for x-ray reflectivity. The measurements of F(CF2)10(CH2)2OH at the liquid–liquid interface are compared to x-ray surface diffraction measurements of monolayers of the same material spread at the water–vapor interface. A solid to disordered-phase phase transition also occurs in the spread monolayer though at a slightly higher temperature. This indicates that the hexane acts to disorder the solid monolayer at the water–hexane interface. A measurement of the thermal expansion coefficient of the monolayer at the water–vapor interface is consistent with literature values for bulk hydrocarbon rotator phases, in contrast with previous measurements on monolayers of perfluoro-n-eicosane supported on water.

Folding and translocation of the undecamer of poly-L-leucine across the water-hexane interface. A molecular dynamics study.

The undecamer of poly-L-leucine at the water-hexane interface is studied by molecular dynamics simulations. This represents a simple model relevant to folding and insertion of hydrophobic peptides into membranes. The peptide, initially placed in a random coil conformation on the aqueous side of the system, rapidly translocates toward the hexane phase and undergoes interfacial folding into an alpha-helix in the subsequent 36 ns. Folding is nonsequential and highly dynamic. The initially formed helical segment at the N-terminus of the undecamer becomes transiently broken and, subsequently, reforms before the remainder of the peptide folds from the C-terminus. The formation of intramolecular hydrogen bonds during the folding of the peptide is preceded by a dehydration of the participating polar groups, as they become immersed in hexane. Folding proceeds through a short-lived intermediate, a 3(10)-helix, which rapidly interconverts to an alpha-helix. Both helices contribute to the equilibrium ensemble of folded structures. The helical peptide is largely buried in hexane, yet remains adsorbed at the interface. Its preferred orientation is parallel to the interface, although the perpendicular arrangement with the N-terminus immersed in hexane is only slightly less favorable. In contrast, the reversed orientation is highly unfavorable, because it would require dehydration of C-terminus carbonyl groups that do not participate in intramolecular hydrogen bonding. For the same reason, the transfer of the undecamer from the interface to the bulk hexane is also unfavorable. The results suggest that hydrophobic peptides fold in the interfacial region and, simultaneously, translocate into the nonpolar side of the interface. It is further implied that peptide insertion into the membrane is accomplished by rotating from the parallel to the perpendicular orientation, most likely in such a way that the N-terminus penetrates the bilayer.

Vertical Standing of Amphiphilic Helical Peptides at the Hexane-Water Interface

Abstract Amphiphilic poly(L-leucine) (n=20) helixes having a trimethylammonium head group at the terminal end were stood vertical with a tilt at the hexane-water interface, although they usually laid down to the air-water interface. Structures of the transferred monolayer on a substrate were confirmed by FT-IR spectra and atomic force microscopy(AFM).

Development of an interfacial thermal lens technique: monitoring the dissolving process of amphiphilic molecules at the hexane–water interface

An interfacial thermal lens technique has been developed for monitoring amounts of molecules at a liquid–liquid interface. A thin thermal lens can be observed for amphiphilic molecules, 10-(1-pyrene)decanoic acid and 4-(1-pyrene)butanoic acid, expanded at a hexane–phosphate buffer solution interface. The thermal lens spectrophotometry offers a linear response to amounts of the expanded molecule and its minimum detectable amount is 1.0×10 −10 mol corresponding about 1/100-monolayer. The signal decay depended on the pH of aqueous phase and the faster decay at lower pH was ascribed to the dissolving process of the molecules from the interface into hexane.