Sum frequency generation spectroscopy has been used to characterize the potential-dependent adsorption of the p-toluenesulfonate anion at the activated carbon/propylene carbonate interface of the commercial carbon YP50F. Spectra recorded from the interface between YP50F and a 1 M tetraethylammonium p-toluenesulfonate in propylene carbonate solution showed no ordered anion adsorption without an applied potential. In contrast, there is clear evidence of increasingly ordered anion adsorption with applied potential. Furthermore, there is evidence of hysteresis such that the anion remains adsorbed when the applied potential was decreased back to 0 V. Significant reversal of polarity was required before the anion signal was lost. Changes to the propylene carbonate solvent peaks during the electrochemical cycle were also observed. The data indicate that the positive electrode charges either via a counterion adsorption mechanism or via an ion-exchange mechanism.

Using specular neutron reflection, the adsorption of sodium and calcium salts of the surfactant bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT or AOT) has been studied at the mica/water interface at concentrations between 0.1 and 2 CMC. The pH dependence of the adsorption was also probed. No evidence of the adsorption of Na(AOT) was found even at the critical micelle concentration (CMC) while the calcium salt was found to adsorb significantly at concentrations of 0.5 CMC and above. This interesting and somewhat unexpected finding demonstrates that counterion identity may be used to tune the adsorption of anionic surfactants on anionic surfaces. At the CMC, three condensed bilayers of Ca(AOT)2 were adsorbed at pH 7 and 9 and four bilayers adsorbed at pH 4. Multilayering at the CMC of Ca(AOT)2 on the mica surface is an unusual feature of this surfactant/surface combination. Only single bilayer adsorption has been observed at other surfaces at the CMC. We suggest this arises from the high charge density of mica which must provide an excellent template for the surfactant.

The layering of ionic liquids close to flat, charged interfaces has been identified previously through theoretical and some experimental measurements. Here we present evidence for oscillations in ion density (‘layering’) in a long chain ionic liquid (1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) near the interface with mica using two complementary approaches. Neutron reflection at the ionic liquid–mica interface is used to detect structure at a single interface, and surface force balance (SFB) measurements carried out with the same ionic liquid reveal oscillatory density in the liquid confined between two mica sheets. Our findings imply the interfacial structure is not induced by confinement alone. Structural forces between two mica surfaces extend to approximately twice the distance of the density oscillations measured at a single interface and have similar period in both cases.

The layer structure of the dichain alkyl ammonium surfactant, didodecyldimethylammonium bromide (DDAB), adsorbed from water on to silica and mica surfaces has been determined using neutron reflection. Although sometimes considered interchangeable surfaces for study, we present evidence of significant differences in the adsorbed layer structure below the critical micelle concentration.A complete DDAB bilayer was assembled at the water/mica interface at concentrations below the critical micelle concentration (CMC). In contrast it is not until the CMC was reached that the complete bilayer structure formed on the oxidised silicon crystal. Removal of the complete bilayer on both surfaces was attempted by both washing and ion exchange yet the adsorbed structure proved tenacious.

A detailed comparison of the adsorption behavior of long straight chain saturated and unsaturated fatty acids at the iron oxide/oil interface has been considered using a combination of surface study techniques. Both depletion isotherms and polarized neutron reflectometry (PNR) show that the extent of adsorption decreases as the number of double bonds in the alkyl chains increases. Sum frequency generation spectroscopic measurements demonstrate that there is also an increase in chain disorder within the adsorbed layer as the unsaturation increases. However, for the unsaturated analogues, a decrease in peak intensity is seen for the double bond peak upon heating, which is thought to arise from isomerization in the surface-bound layer. The PNR study of oleic acid adsorption indicates chemisorbed monolayer adsorption, with a further diffuse reversible adsorbed layer formed at higher concentrations.

We present neutron reflection data from an alkylammonium surfactant (C16TAB) at the mica/water interface. The system is studied in situ in a noninvasive manner and indicates the formation of a complete adsorbed bilayer with little evidence of defects. A detailed analysis suggests that the data are not consistent with some other previously reported adsorbed structures, such as micelles or cylinders.

The adsorption of a phosphorus analogue of the surfactant AOT, sodium bis(2-ethylhexyl) phosphate (NaDEHP), at the water/alumina interface is described. The material is found to adsorb as an essentially water-free bilayer from neutron reflection measurements. This is similar to the behavior of AOT under comparable conditions, although AOT forms a thicker, more hydrated layer. The NaDEHP shows rather little variation with added salt, but a small thickening of the layer on increasing the pH, in contrast to the behavior of AOT.

Polarized neutron reflectometry has been used to investigate the detailed adsorption behavior and corrosion inhibition mechanism of two surfactants on a nickel surface under acidic conditions. Both the corrosion of the nickel surface and the structure of the adsorbed surfactant layer could be monitored in situ by the use of different solvent contrasts. Layer thicknesses and roughnesses were evaluated over a range of pH values, showing distinctly the superior corrosion inhibition of one negatively charged surfactant (sodium dodecyl sulfate) compared to a positively charged example (dodecyl trimethylammonium bromide) due to its stronger binding interaction with the surface. It was found that adequate corrosion inhibition occurs at significantly less than full surface coverage.

Neutron diffraction with isotopic substitution has been used to characterize the bulk liquid structure of the technologically relevant electrolyte solution, 1 M tetrapropylammonium bromide (TPA Br) in acetonitrile (acn), and of pure deuterated acetonitrile. Empirical potential structure refinement modeling procedures have been used to extract detailed structural information about solvent–solvent, solvent–ion, and ion–ion correlations. Analysis of the refined data shows the expected local dipolar conformation of acn in the pure solvent. This short-range dipolar ordering is also present within the solutions of TPA Br in acn, and it affects how the solvent orders itself around the ions. The solvation numbers of the TPA cations and the bromide anions are deduced, 8 and 5, respectively, as are the orientations of the solvent molecules that surround the ions. Evidence for ion association is also presented, with nearly two-thirds of the ions in the system being in associated pairs or clusters.

•We characterised the adsorption of the surfactant AOT on calcite by neutron reflection.•NaAOT and CaAOT form molecular bilayers at the calcite–water interface.•NaAOT converts to CaAOT at the slightly soluble calcite surface in water.•CaAOT adsorption could not be observed by traditional isotherm methods, but is apparent by neutron reflection.The adsorption of the surfactant Aerosol-OT (AOT) at the calcite–water interface has been investigated using batch adsorption isotherms and neutron reflection. The adsorption isotherms showed that NaAOT adsorption followed S-type adsorption behaviour with a maximum surface excess of 2.5 mg m−2 but the method could not be used for the investigation of Ca(AOT)2 adsorption owing to the changes in the bulk phase behaviour of the solution. The surface excess, determined by neutron reflection at the critical micelle concentration (CMC), was 2.5 mg m−2 for Ca(AOT)2 and 1.8 mg m−2 for NaAOT. The time dependence of the NaAOT adsorption suggests a slow conversion from the sodium to the calcium salt of AOT at the calcite–water interface by binding calcium ions released from the slightly soluble calcite. The layer thickness in both cases was 35 Å which indicates adsorption as bilayers or distorted micelles. At higher concentrations of NaAOT (∼10× CMC) adsorption of an AOT lamellar phase was evident from Bragg peaks in the specular reflection.To our knowledge, this is the first time that adsorption of a surfactant at the calcite–water interface has been investigated by neutron reflection. The technique provided significant new insight into the adsorption behaviour of AOT which would not have been accessible using traditional techniques.Figure optionsDownload full-size imageDownload high-quality image (36 K)Download as PowerPoint slide

The formation of a halogen bonded self-assembled co-crystal physisorbed monolayer containing N⋯Br interactions is reported for the first time. The co-crystal monolayer is identified experimentally by synchrotron X-ray diffraction and the structure determined. Density functional theory (DFT) calculations are also employed to assess the magnitudes of the different interactions in the layer. Significantly, compared to other halogen bonds in physisorbed monolayers we have reported recently, the N⋯Br bond here is found to be non-linear. It is proposed that the increasing importance of the lateral hydrogen bond interactions, relative to the halogen bond strength, leads to the bending of the halogen bonds.

•Adsorption isotherms of hexanoate on α-Al2O3 are determined by solution depletion.•The adsorption shows pH dependence with a maximum near the pKa of hexanoic acid.•A bilayer structure of the adsorbed layer is determined by neutron reflection.•Different binding mechanisms of the ligand on the surface have been considered.•Calculations indicate that ligand exchange may be the most significant mechanism.Neutron reflection and adsorption isotherm measurements have been used to study the adsorption behaviour of hexanoic acid onto α-alumina surfaces. Importantly, the pH dependence of the behaviour has been characterised with a pronounced maximum in adsorption identified at a pH of approximately 5, close to the pKa of the acid. The adsorbed layer is identified as a bilayer, which is reasonable given the hydrophilic nature of both side of the layer, and has a thickness of 13 Å, suggesting significant extent of interdigitation. At pH 5, the layer has much lower extent of hydration relative to the higher pH of 7, consistent with the increased total adsorption at pH 5. A number of different mechanisms for the binding of the hexanoic acid to the surface are considered. The experimental data, combined with calculations using equilibrium/binding constants of the surface and ligands, indicates that a ligand exchange reaction may be the most significant mechanism.Graphical abstract

The binding of an anionic surfactant onto an anionic surface by addition of divalent ions is reported based on experimental data from specular neutron reflection (NR) and attenuated total internal reflection IR spectroscopy (ATR-IR). Similar measurements using monovalent ions (sodium) do not show any evidence of such adsorption, even though the amount of surfactant can be much higher. This data is interpreted in terms of the so-called bridging mechanism of ion binding.

This work describes the combined use of synchrotron X-ray diffraction and density functional theory (DFT) calculations to understand the cocrystal formation or phase separation in 2D monolayers capable of halogen bonding. The solid monolayer structure of 1,4-diiodobenzene (DIB) has been determined by X-ray synchrotron diffraction. The mixing behavior of DIB with 4,4′-bipyridyl (BPY) has also been studied and interestingly is found to phase-separate rather than form a cocrystal, as observed in the bulk. DFT calculations are used to establish the underlying origin of this interesting behavior. The DFT calculations are demonstrated to agree well with the recently proposed monolayer structure for the cocrystal of BPY and 1,4-diiodotetrafluorobenzene (DITFB) (the perfluorinated analogue of DIB), where halogen bonding has also been identified by diffraction. Here we have calculated an estimate of the halogen bond strength by DFT calculations for the DITFB/BPY cocrystal monolayer, which is found to be ∼20 kJ/mol. Computationally, we find that the nonfluorinated DIB and BPY are not expected to form a halogen-bonded cocrystal in a 2D layer; for this pair of species, phase separation of the components is calculated to be lower energy, in good agreement with the diffraction results.

The adsorption behavior of a model additive, hexadecylamine, onto an iron surface from hexadecane oil has been characterized using polarized neutron reflectometry, sum-frequency generation spectroscopy, solution depletion isotherm, and X-ray photoelectron spectroscopy (XPS). The amine showed a strong affinity for the metal surface, forming a dense monolayer at relatively low concentrations; a layer thickness of 16 (±3) Å at low concentrations, increasing to 20 (±3) Å at greater amine concentrations, was determined from the neutron data. These thicknesses suggest that the molecules in the layer are tilted. Adsorption was also indicated by sum-frequency generation spectroscopy and XPS, the latter indicating that the most dominant amine–surface interaction was via electron donation from the nitrogen lone pair to the positively charged iron ions. Sum-frequency generation spectroscopy was used to determine the alkyl chain conformation order and orientation on the surface.

The formation of two-dimensional halogen-bonded layers of 4,4′-bipyridyl and 1,4-diiodotetrafluorobenzene was observed on a graphite surface at sub-monolayer coverage using synchrotron X-ray diffraction; the use of the diffraction technique enabled, for the first time, the measurement of I⋯N halogen bonding distances in a two-dimensional cocrystal and the identification of the halogen bonding interaction in the monolayer.

We report a calorimetric study of the formation of solid monolayers of fatty acid glycerides on the surface of graphite. Solid monolayer formation is observed at temperatures significantly higher than the bulk melting point for some species, while for others these melting points are rather similar. The variation of monolayer melting point as a function of alkyl chain length, number of alkyl chains and the extent of unsaturation is reported. There are a number of interesting and unexpected effects in the observed behaviour as hydroxyl groups are replaced by alkyl chains on passing from mono-, to di- and tri-glycerides.Graphical abstractHighlights► DSC identification of solid monolayer formation for glycerides on graphite surface. ► Monolayer stability was quantified in terms of the molecular architecture. ► Glycerides with longer fatty acid chain lengths form more stable solid monolayers. ► Unsaturation reduces the monolayer stability. ► Surprisingly increases as –OH groups are exchanged for alkyl chains.

Differential scanning calorimetry (DSC) and X-ray powder diffraction (PXRD) have been used to determine the phase behavior of the binary mixtures of undecanoic acid (A) and undecylamine (B) in the bulk. In addition, we report DSC data that indicates very similar behavior for the solid monolayers of these materials adsorbed on the surface of graphite. The two species are found to form a series of stoichiometric complexes of the type AB, A2B, and A3B on the acid rich side of the phase diagram. Interestingly, no similar series of complexes is evident on the amine rich side. As a result of this complexation, the solid monolayers of the binary mixtures exhibit a very pronounced enhancement in stability relative to the pure adsorbates.

In this letter, the phase behavior of a saturated alkylamide, heptanamide (C7), adsorbed on the surface of graphite using synchrotron X-ray diffraction is presented. The diffraction patterns indicate that heptanamide undergoes a solid−solid phase transition in the monolayer at 330 K from pgg symmetry at lower temperatures to p2 symmetry at high temperatures. Other alkylamides with similar carbon chain lengths do not show this phase change, making the C7 homologue unusual.

Crystalline monolayers of three aldehydes with an odd number of carbon atoms in the alkyl chain (C7, C9 and C11) at low coverages are observed by a combination of X-ray and neutron diffraction. Analysis of the diffraction data is discussed and possible monolayer crystal structures are proposed; although unique structures could not be ascertained for all molecules. We conclude that the structures are flat on the surface, with the molecules lying in the plane of the layer. The C11 homologue is determined to have a plane group of either p2, pgb or pgg, and for the C7 homologue the p2 plane group is preferred.

The adsorption of three unsaturated amides on graphite is investigated using a combination of synchrotron X-ray diffraction and differential scanning calorimetry (DSC). In this work unsaturated amides with chain lengths of six and nine carbons have been studied with one trans double bond in the alkyl chain at different positions. The structure of each unsaturated amide has been separately determined using synchrotron X-ray diffraction at submonolayer coverages. These monolayer structures are compared with the structure of the saturated amide of the same chain length. The hydrogen bond geometries in these structures are discussed, and these indicate that the position of the double bond relative to the amide headgroup has an important influence on the stability of the monolayer, a finding also supported by the DSC results. Binary mixtures of saturated and trans-unsaturated amides have also been studied using DSC and the behavior discussed using the monolayer structures of the individual species.

In this work we present a study on the adsorption of linear alkyl aldehydes physisorbed from their bulk liquid onto a graphite substrate combining calorimetry for all homologues from C6 to C13, with more detailed diffraction, incoherent neutron scattering, and scanning tunneling microscopy techniques for one (C12) representative member. We identify solid monolayer formation for some of these species for alkyl chain lengths of 6 to 13 carbons at high surface coverages. The C12 monolayer structure is determined to be most likely Pgg and this structure is discussed in terms of the importance of dipolar interactions.

Synchrotron X-ray and neutron diffraction have been used to determine the two-dimensional crystalline structures of alkylamides adsorbed on graphite at submonolayer coverage. The calculated structures show that the plane of the carbon backbone of the amide molecules is parallel to the graphite substrate. The molecules form hydrogen-bonded dimers, and adjacent dimers form additional hydrogen bonds yielding extended chains. By presenting data from a number of members of the homologous series, we have identified that these chains pack in different arrangements depending on the number of carbons in the amide molecule. The amide monolayers are found to be very stable relative to other closely related alkyl species, a feature which is attributed to the extensive hydrogen bonding present in these systems. The characteristics of the hydrogen bonds have been determined and are found to be in close agreement with those present in the bulk materials.

Combined neutron and X-ray diffraction has been used to structurally characterize the crystalline monolayer structures of fluoroalkane monolayers, C6 to C16, adsorbed on graphite at submonolayer coverages and over the temperature range from 10 K until the layers melt. In all cases the molecules are found to have their carbon backbones parallel to the graphite surface, although the experimentally determined patterns are not particularly sensitive to the orientation of the fluoroalkanes about their long axis on the surface. The layers appear to be incommensurate with the underlying substrate. A significant number of the fluoroalkanes also exhibit both a high and a low temperature phase, attributed to the onset of a rotator phase at higher temperatures. Interestingly, the temperature range of the high temperature phase is much greater than found for the alkanes. The scattering cannot distinguish the chirality of these helical molecules on the surface.

We report an investigation of the presence of thin water layers on calcium carbonate particles dispersed in cyclohexane using small-angle neutron scattering. We identify an adsorbed water layer and measure the thickness using contrast variation to optimize the sensitivity of the scattering to the water. We also report the variation in thickness of these water layers in the presence of salt solutions with the variation of salt concentration and valency. We conclude that thin water layers can be observed using SANS; however, the layers are thin and correspond to essentially the hydration of the particle surfaces.

This work reports novel Environmental Scanning Electron Microscopy (ESEM)/Energy Dispersive X-ray (EDX) measurements on the equilibrium composition and kinetics of alkyl amide adsorption at the polymer–air interface. This experimental approach provides certain advantages over other existing techniques for the study of the physisorbed molecules and offers potential for further development.

Oil-in-water (o/w) emulsions of different droplet size were filtered on membranes of various pore sizes to investigate the growth and behaviour of o/w filter cakes. The cake desorptivity S and the filter membrane resistance R were measured at various filtration pressures P. The variation of S with P shows that filter cake oil droplets of radius a   are effectively rigid for P≪γ/aP≪γ/a and fully deformable for P≫γ/aP≫γ/a, where γ is the oil–water interfacial tension. For the largest P, when S became P-independent, the filter cake remained water-permeable as expected from theory.Although relatively rigid when the driving pressures are small, emulsion droplets are found to increasingly deform under larger applied pressures. Interestingly, the gaps between the droplets never completely close even at the highest pressures.

The orientational order parameter of dense colloidal dispersions of plate-like particles as a function of volume fraction is measured using neutron diffraction. This non-invasive experimental approach directly provides the full particle orientation distribution from which the order parameters can be calculated. The orientation parameters are shown to be linked to the solids fractions of the cakes and the macroscopic permeability of the samples. However, this study suggests that, although orientation can be relevant for a given system, other factors can have a stronger influence, for example, the degree of dispersion or colloidal stability of the clay and may be the principle factor that controls permeability. In addition, we report enhanced ordering of these materials under the influence of an external cross-flow field.

Solid monolayer formation by all the linear carboxylic acids from C6 to C20 adsorbed from their liquids to a graphite surface is demonstrated. In addition, we present the two-dimensional phase behaviour of linear monocarboxylic acid mixtures adsorbed on graphite from their liquid mixtures, determined using differential scanning calorimetry. All acid mixtures with alkyl chains that differ by two or three methylene groups (Δn=2 or 3) exhibit a significant degree of phase separation in the monolayer. Generally, the mixing tendency increases with increasing alkyl chain length for a given Δn, and with more similar chain lengths (e.g. Δn=1). We report neutron diffraction data that confirms the formation of solid acid monolayers. This structural data also allows us to compare the mixing results with a recent quantitative model for 2-D mixing. The general form of the observed behaviour agrees well with the model, although the characteristic parameters that separate complete mixing, partial mixing and phase separation are different from those found with alkane and alcohol monolayer mixtures.

Differential scanning calorimetry (DSC), incoherent elastic neutron scattering, and neutron diffraction are used to demonstrate the presence of adsorbed solid multilayers of linear alcohols at the graphite–liquid alcohol interface. All alcohols studied (C5–C18) are found to form at least one monolayer. In addition all the even alcohols investigated (C6OH to C18OH) show multilayer formation. However, only the short odd alcohols (C5OH to C11OH) clearly exhibit additional features indicating multilayer formation.

The layering of ionic liquids close to flat, charged interfaces has been identified previously through theoretical and some experimental measurements. Here we present evidence for oscillations in ion density (‘layering’) in a long chain ionic liquid (1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) near the interface with mica using two complementary approaches. Neutron reflection at the ionic liquid–mica interface is used to detect structure at a single interface, and surface force balance (SFB) measurements carried out with the same ionic liquid reveal oscillatory density in the liquid confined between two mica sheets. Our findings imply the interfacial structure is not induced by confinement alone. Structural forces between two mica surfaces extend to approximately twice the distance of the density oscillations measured at a single interface and have similar period in both cases.

The formation of two-dimensional halogen-bonded layers of 4,4′-bipyridyl and 1,4-diiodotetrafluorobenzene was observed on a graphite surface at sub-monolayer coverage using synchrotron X-ray diffraction; the use of the diffraction technique enabled, for the first time, the measurement of I⋯N halogen bonding distances in a two-dimensional cocrystal and the identification of the halogen bonding interaction in the monolayer.

The formation of a halogen bonded self-assembled co-crystal physisorbed monolayer containing N⋯Br interactions is reported for the first time. The co-crystal monolayer is identified experimentally by synchrotron X-ray diffraction and the structure determined. Density functional theory (DFT) calculations are also employed to assess the magnitudes of the different interactions in the layer. Significantly, compared to other halogen bonds in physisorbed monolayers we have reported recently, the N⋯Br bond here is found to be non-linear. It is proposed that the increasing importance of the lateral hydrogen bond interactions, relative to the halogen bond strength, leads to the bending of the halogen bonds.