R. K. Thomas

Surfactant layers at the air/water interface: structure and composition.

The use of neutron reflectometry to study the structure and composition of surfactant layers adsorbed at the air/water interface is reviewed. A critical assessment of the results from this new technique is made by comparing them with the information available from all other techniques capable of investigating this interface.

Neutron reflection study of bovine beta-casein adsorbed on OTS self-assembled monolayers

Specular neutron reflection has been used to determine the structure and composition of bovine beta-casein adsorbed on a solid surface from an aqueous phosphate-buffered solution at pH 7. The protein was adsorbed on a hydrophobic monolayer self-assembled from deuterated octadecyltrichlorosilane solution on a silicon (111) surface. A two-layer structure formed consisting of one dense layer of thickness 23 +/- 1 angstroms and a surface coverage of 1.9 milligrams per square meter adjacent to the surface and an external layer protruding into the solution of thickness 35 +/- 1 angstroms and 12 percent protein volume fraction. The structure of the (beta-casein) layer is explained in terms of the charge distribution in the protein.

Structural conformation of lysozyme layers at the air/water interface studied by neutron reflection

The adsorption of chicken egg white lysozyme at the air/water interface has been studied by specular neutron reflection. The variation of the total thickness of the lysozyme layer at the surface of water under varying solution conditions has been determined. The use of mixed H2O and D2O allowed the determination of the extent of immersion of the layer in water at all concentrations. The measured layer thickness combined with the globular dimensions of lysozyme suggests that the adsorbed lysozyme molecules retain their globular structure with no significant denaturation. Measurements were made over a lysozyme concentration range of 9×10-4 g dm-3 to 4 g dm-3 at pH 7 and at an ionic strength of 0.02 M. The thickness of the layer was determined by measuring neutron reflectivities in null reflecting water (NRW) where the signal is only from the adsorbed protein layer. Below 0.1 g dm-3 the surface coverage increases with bulk concentration but the thickness of the layer is constant at 30±3 A, suggesting that lysozyme is adsorbed sideways-on. As the bulk concentration increases, the layer thickness gradually increases to a value of 47±3 A2 at a bulk concentration of 1 g dm-3, suggesting that the molecules switch from sideways-on to longways-on orientations. The area per molecule at 1 g dm-3 was found to be 950±50 A2 which is close to the limit of 30×30 A2 for a saturated layer of longways-on molecules. The extent of mixing of the layer with water was determined directly by measuring reflectivity profiles in mixed H2O and D2O. A two layer model was found to be appropriate with an upper layer in air and a lower layer fully immersed in water. The thickness of the layer in air was found to vary from 15±5 Aat the lowest bulk concentration to 9±3 Aat the highest concentration studied. The results show that as the total layer thickness increases with bulk concentration the fraction of the layer immersed in water increases from 50 to 85%. At the highest concentration of 4 g dm-3 the adsorbed layer is better described by a two layer model consisting of a close packed top layer of thickness 47±3 Aand a loosely packed sublayer of 30±3 A.

Determination of the structure of a surfactant layer adsorbed at the silica/water interface by neutron reflection

Abstract Neutron reflection from the solid/liquid interface has been used to determine the structure of an adsorbed layer of hexaoxyethylene glycol monododecyl ether (C 12 E 6 ) on quartz. Contrast variation experiments show that C 12 E 6 adsorbs as a bilayer, the ethylene oxide chains forming the layers adjacent to the quartz and the bulk solution. The thickness of the hydrocarbon core of the layer was found to be 17±3 A and the area per molecule 23 ± 3 A 2 . The ethylene oxide chain regions were 16±3 A thick. The average hydration number of a single ethylene oxide group was found to be 2.0±0.5.

Neutron reflection from wet interfaces

Neutron reflection is one of the few newly developed techniques capable of probing structure at wet surfaces, i.e. air/liquid, solid/liquid and liquid/liquid interfaces. Although neutron scattering is not intrinsically sensitive to surfaces, the grazing incidence geometry of neutron reflection and the possibility of varying neutron refractive indices by isotopic substitution, particularly H/D substitution, make reflection extremely sensitive to selected parts of many interfacial layers. The technique is able to probe the average structure along the surface normal in a number of situations where the layer is too disordered or complex to be investigated by other methods. The application of neutron reflection to small molecules, surfactants, polymers and polymer–surfactant mixtures at the air/water interface, and examples of the behaviour of surfactants, proteins and polymers at the buried solid/liquid interface are described. Where appropriate the scope of neutron reflection relative to other new techniques is assessed.

Hydrogen bonding in the vapour phase between water and hydrogen fluoride: the infrared spectrum of the 1: 1 complex

The infrared spectrum of the 1:1 complex in the vapour phase between water and hydrogen fluoride has been observed for the first time, and measured over the range 4000-400 cm-1. Three bands of the complex have been observed, one associated with the stretching vibration of HF, one with the bending of the water molecule, and the other with two bending vibrations of the hydrogen bond itself. The first band shows that HF forms the hydrogen bond. Interpretation of its fine structure gives the frequencies of two bending vibrations at 145 and 170 cm-1. The structure of the band associated with the two bending vibrations at about 700 cm-1 has been tentatively analysed to show th at the complex is planar (C2v) and to give a value of the Coriolis constant for the interaction of the two vibrations. The value of this constant shows th at either the bending force field has an unusually large interaction force constant or one of the vibrations is anharmonic. W ith the help of an estimated value of the only remaining unknown vibration frequency of the complex, that of the intermolecular stretching vibration at 180 cm-1, the bending vibration frequencies above, and an estimate of the extinction coefficient of the band associated with the HF stretch, the enthalpy of the association of water and hydrogen fluoride is calculated to be — 26 kJ mol-1 at just above room temperature. From this a value of the potential energy well depth for the interaction has been found to be — 30 kJ mol-1.

Hydrogen bonding in the gas phase: the infrared spectra of complexes of hydrogen fluoride with hydrogen cyanide and methyl cyanide

The infrared spectra of the gas phase complexes HCN-HF, DCN-DF and four isotopic species of CH3CN-HF have been measured over the range 200 to 4000 cm-1. Two bands have been observed, one associated with the stretching vibration of HF in the complex and the other with a bending vibration of the hydrogen bond itself. At higher resolution both bands show fine structure which has been interpreted as being a series of hot bands associated with transitions from excited levels of another low-frequency bending vibration of the hydrogen bond. In the first band the peaks are P branch bandheads in the individual hot bands and in the second band they are sharp Q branches. From temperature studies of these bands and from the effects of isotopic substitution on the spacing of the fine structure the frequency of the lower bending vibration has been determined. Further structure in the first band gives the frequency of the stretching vibration of the hydrogen bond itself. A complete assignment of all the vibrations associated with the hydrogen bond has therefore been made. From the frequencies of the two bending motions (555 and 70 cm-1 for the HCN-HF complex) values of the bending force constants have been calculated. Several anharmonic constants have also been measured and the effect of anharmonicity on the breadth of bands associated with the hydrogen bond is discussed.

Solution Self-Assembly and Adsorption at the Air−Water Interface of the Monorhamnose and Dirhamnose Rhamnolipids and Their Mixtures

The self-assembly in solution and adsorption at the air-water interface, measured by small-angle neutron scattering, SANS, and neutron reflectivity, NR, of the monorhamnose and dirhamnose rhamnolipids (R1, R2) and their mixtures, are discussed. The production of the deuterium-labeled rhamnolipids (required for the NR studies) from a Pseudomonas aeruginosa culture and their separation into the pure R1 and R2 components is described. At the air-water interface, R1 and R2 exhibit Langmuir-like adsorption isotherms, with saturated area/molecule values of about 60 and 75 Å(2), respectively. In R1/R2 mixtures, there is a strong partitioning of R1 to the surface and R2 competes less favorably because of the steric or packing constraints of the larger R2 dirhamnose headgroup. In dilute solution (<20 mM), R1 and R2 form small globular micelles, L(1), with aggregation numbers of about 50 and 30, respectively. At higher solution concentrations, R1 has a predominantly planar structure, L(α) (unilamellar, ULV, or bilamellar, BLV, vesicles) whereas R2 remains globular, with an aggregation number that increases with increasing surfactant concentration. For R1/R2 mixtures, solutions rich in R2 are predominantly micellar whereas solutions rich in R1 have a more planar structure. At an intermediate composition (60 to 80 mol % R1), there are mixed L(α)/L(1) and L(1)/L(α) regions. However, the higher preferred curvature associated with R2 tends to dominate the mixed R1/R2 microstructure and its associated phase behavior.

The use of contrast variation in the specular reflection of neutrons from interfaces

Abstract Several of the useful results of the kinematic approximation are presented and applied to experimental data. From the kinematic equations it is shown that contrast variation in neutron reflection can be used to obtain a scattering length density profile across an interface unique within the limitations of resolution. It is shown to be more important for two component systems to obtain actual density profiles. Contrast variation is the only way to achieve this and an illustrative example is analysed in terms of the kinematic approximation.

Mixing Behavior of the Biosurfactant, Rhamnolipid, with a Conventional Anionic Surfactant, Sodium Dodecyl Benzene Sulfonate

The use of small angle neutron scattering, SANS, neutron reflectivity, NR, and surface tension to study the mixing properties of the biosurfactant rhamnolipid with a conventional anionic surfactant, sodium dodecyl 6-benzene sulfonate, LAS, is reported. The monorhamnose rhamnolipid, R1, mixes close to ideally with LAS at the air-water interface, whereas for mixtures of LAS with the dirhamnose rhamnolipid, R2, the LAS strongly partitions to the air-water interface relative to R2, probably because of the steric hindrance of the larger R2 headgroup. These trends in the binary mixtures are also reflected in the ternary R1/R2/LAS mixtures. However, for these ternary mixtures, there is also a pronounced synergy in the total adsorption, which reaches a maximum for a LAS/rhamnolipid mole ratio of about 0.6 and a R1/R2 mol ratio of about 0.5, an effect which is not observed in the binary mixtures. In solution, the R1/LAS mixtures form relatively small globular micelles, L(1), at low surfactant concentrations (<20 mM), more planar structures (lamellar, L(α), unilamellar/multilamellar vesicles, ulv/mlv) are formed at higher surfactant concentrations for R1 and LAS rich compositions, and a large mixed phase (L(α)/L(1) and L(1)/L(α)) region forms at intermediate surfactant compositions. In contrast, for the R2/LAS mixtures, the higher preferred curvature of R2 dominates the phase behavior. The predominant microstructure is in the form of small globular micelles, except for solution compositions rich in LAS (>80 mol % LAS) where more planar structures are formed. For the ternary mixtures, there is an evolution in the resulting phase behavior from one dominated by L(1) (R2 rich) to one dominated by planar structures, L(α), (R1, LAS rich), and which strongly depends upon the LAS/rhamnolipid and R1/R2 mole ratio.