J. Penfold

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.

The structure of the surface of pure liquids

This paper reviews the progress in the study of the structure of the surface of pure liquids made in the last 10-20 years. This area of research has benefited enormously in recent years from developments in experimental techniques (especially x-ray scattering methods based on third-generation synchrotron sources, and advanced optical techniques) and theory (particularly computer simulation and density functional theory). The review is predominantly experimentally based, and focuses very much on some recent exciting new results and developments. Specific systems or phenomena that are discussed are molecular ordering and orientation at the surface, atomic-scale layering at liquid metal surfaces, surface freezing in liquid alkanes and alcohols, surface melting and pre-melting at the water surface, thermal roughness (capillary waves) in simple liquids, and visco-elastic surface waves. Both the liquid-vapour and liquid-solid interfaces are considered. Some details of the principal experimental techniques exploited, x-ray reflectivity, diffuse x-ray scattering, grazing incidence x-ray diffraction, ellipsometry, light scattering, will be given, The experimental developments have gone very much in conjunction with substantial theoretical advances, and the principal theoretical concepts will be briefly discussed.

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.

Limitations in the Application of the Gibbs Equation to Anionic Surfactants at the Air/Water Surface: Sodium Dodecylsulfate and Sodium Dodecylmonooxyethylenesulfate Above and Below the CMC

This is a second paper responding to recent papers by Menger et al. and the ensuing discussion about the application of the Gibbs equation to surface tension (ST) data. Using new neutron reflection (NR) measurements on sodium dodecylsulfate (SDS) and sodium dodecylmonooxyethylene sulfate (SLES) above and below their CMCs and with and without added NaCl, in conjunction with the previous ST measurements on SDS by Elworthy and Mysels (EM), we conclude that (i) ST measurements are often seriously compromised by traces of divalent ions, (ii) adsorption does not generally reach saturation at the CMC, making it difficult to obtain the limiting Gibbs slope, and (iii) the significant width of micellization may make it impossible to apply the Gibbs equation in a significant range of concentration below the CMC. Menger et al. proposed ii as a reason for the difficulty of applying the Gibbs equation to ST data. Conclusions i and iii now further emphasize the failings of the ST-Gibbs analysis for determining the limiting coverage at the CMC, especially for SDS. For SDS, adsorption increases above the CMC to a value of 10 × CMC, which is about 25% greater than at the CMC and about the same as at the CMC in the presence of 0.1 M NaCl. In contrast, the adsorption of SLES reaches a limit at the CMC with no further increase up to 10 × CMC, but the addition of 0.1 M NaCl increases the surface excess by 20-25%. The results for SDS are combined with earlier NR results to generate an adsorption isotherm from 2 to 100 mM. The NR results for SDS are compared to the definitive surface tension (ST) measurements of EM, and the surface excesses agree over the range where they can safely be compared, from 2 to 6 mM. This confirms that the anomalous decrease in the slope of EMs σ - ln c curve between 6 mM and the CMC at 8.2 mM results from changes in activity associated with a significant width of micellization. This anomaly shows that it is impossible to apply the Gibbs equation usefully from 6 to 8.2 mM (i.e., the lack of knowledge of the activity in this range is the same as above the CMC (8.2 mM)). It was found that a mislabeling of the original data in EM may have prevented the use of this excellent ST data as a standard by other authors. Although NR and ST results for SDS in the absence of added electrolyte show that the discrepancies can be rationalized, ST is generally shown to be less accurate and more vulnerable to impurities, especially divalent ions, than NR. The radiotracer technique is shown to be less accurate than ST-Gibbs in that the four radiotracer measurements of the surface excess are consistent neither with each other nor with ST and NR. It is also shown that radiotracer results on aerosol-OT are likely to be incorrect. Application of the mass action (MA) model of micellization to the ST curves of SDS and SLES through and above the CMC shows that they can be explained by this model and that they depend on the degree of dissociation of the micelle, which leads to a larger change in the mean activity, and hence the adsorption, for the more highly dissociated SDS micelles than for SLES. Previous measurements of the activity of SDS above the CMC were found to be semiquantitatively consistent with the change in mean activity predicted by the MA model but inconsistent with the combined ST, NR, and Gibbs equation results.

Application of the Gibbs equation to the adsorption of nonionic surfactants and polymers at the air-water interface: Comparison with surface excesses determined directly using neutron reflectivity

Four recent papers by Menger et al. have questioned methods of analysis of surface tension (ST) data that use the Gibbs equation to obtain the surface excess (Γ) of a surfactant at the air-water interface. There have been two responses which challenge the assertions of Menger et al. and a response from Menger et al. We use directly determined values of Γ from a range of neutron reflectometry (NR) data to examine some of the issues that are relevant to these seven papers. We show that there is excellent agreement between NR measurements and careful ST analyses for a wide range of nonionic adsorbents, including surfactants and polymers. The reason it is possible to obtain good agreement near the critical micelle concentration (CMC) is that nonionic surfactants generally seem to saturate the surface before the CMC is reached and this makes it relatively easy to determine the limiting slope (and hence Γ) of the ST-log(concentration) plot at the CMC. Furthermore, there is also generally good agreement between ST and NR over the whole range of concentrations below the CMC until depletion effects become important. Depletion effects are shown to become important at higher concentrations than expected, which brings them into the range of many experiments, including techniques other than ST and NR. This is illustrated with new measurements on the biosurfactant surfactin. The agreement between ST and NR outside the depletion range can be regarded as a mutual validation of the two methods, especially as it is demonstrated independently of any model adsorption isotherms. In the normal experimental situation NR is less vulnerable to depletion than ST and we show how NR and a single ST measurement can be used to determine the hitherto undetermined CMC of the nonionic surfactant C18E12, which is found to be 1.3 × 10(-6) M.

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.

Neutron reflectivity and soft condensed matter

Abstract During the last 10–15 years neutron reflectivity has emerged as a powerful and important technique for the study of surfaces and interfaces. The selectivity and sensitivity afforded by deuterium/hydrogen exchange makes the technique particularly attractive for application to the broad field of colloid and interface science. The development of the instrumentation, specialised sample environment equipment and analysis techniques has resulted its application to complex interfaces and environments and in the study of complex multi-component systems. This review provides a summary of those developments in the last two years.

X-ray and neutron reflectivity from spread monolayers☆

X-ray and neutron reflectivity measurements on spread films of docosanoic acid on an aqueous subphase have been used to deduce structural information. At surface pressures of 30–40 mN m−1 the molecules are perpendicular to the subphase surface. If the subphase has pH greater than 5 and contains cadmium ions (about 2.5 × 10−4M) the ions bind tightly to the acid head groups and are not in a diffuse layer. At low surface pressures (film area twice the “close-packed” area) the film breaks up into islands (greater than 0.2 ωm in size) of close-packed molecules. At pH ≈ 6 the molecules in the islands are still perpendicular to the surface but at pH ≈ 3 they are tilted by about 30° to give a thinner film.