We have used neutron reflectivity (NR) measurements in combination with dynamic light scattering (DLS), surface tension and ellipsometry, to study the adsorption behaviour at the air/water interface of N-isopropylacrylamide-based nanogels as a function of concentration. The data provide clear evidence that the nanogels are adsorbed at the interface in a strongly deformed shape and forming a multi-layer where the thickness increases with nanogel concentration in the bulk. The combination of surface characterisation techniques and bulk studies indicate that interfacial film formation is preferred over bulk aggregation. This observation at the air/water interface supports the Derjaguin prediction, that a sphere's interaction with a plane (the thick adsorbed nanogel layer at interface) is much larger than nanogel–nanogel (sphere–sphere) association in the bulk. These findings, in particular the changes in conformations and the thick layer adsorption at the interface as a function of concentration, impact significantly on a number of applications for which nanogels are currently being investigated. These results contribute to the understanding of the behaviour of soft colloids at the interfaces.

The development of effective transdermal drug delivery systems based on nanosized polymers requires a better understanding of the behaviour of such nanomaterials at interfaces. N-Isopropylacrylamide-based nanogels synthesized with different percentages of N,N′-methylenebisacrylamide as cross-linker, ranging from 10 to 30%, were characterized at physiological temperature at the air/water interface, using neutron reflectivity (NR), with isotopic contrast variation, and surface tension measurements; this allowed us to resolve the adsorbed amount and the volume fraction of nanogels at the interface. A large conformational change for the nanogels results in strong deformations at the interface. As the percentage of cross-linker incorporated in the nanogels becomes higher, more rigid matrices are obtained, although less deformed, and the amount of adsorbed nanogels is increased. The data provide the first experimental evidence of structural changes of nanogels as a function of the degree of cross-linking at the air/water interface.

Construction of surfaces with the capability of repelling both water and oil is a challenging issue. We report the superamphiphobic properties of mineral surfaces coated with nanofluids based on synthesized Co-doped and Ce-doped Barium Strontium Titanate (CoBST and CeBST) nanoparticles and fluorochemicals of trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOS) and polytetrafluoroethylene (PTFE). Coating surfaces with these nanofluids provides both oil (with surface tensions as low as 23 mN/m) and water repellency. Liquids with high surface tension (such as water and ethylene glycol) roll off the coated surface without tilting. A water drop released from 8 mm above the coated surface undergoes first a lateral displacement from its trajectory and shape deformation, striking the surface after 23 ms, bouncing and rolling off freely. These multifunctional coating nanofluids impart properties of self-cleaning. Applications include coating surfaces where cleanliness is paramount such as in hospitals and domestic environments as well as the maintenance of building facades and protection of public monuments from weathering. These superamphiphobic-doped nanofluids have thermal stability up to 180 °C; novel industrial applications include within fracking and the elimination of condensate blockage in gas reservoirs.Keywords: barium strontium titanate (BST); carbonate rock; nanofluid; self-cleaning; stability; superamphiphobicity

A new mathematical approach has been developed for describing the interfacial behaviour of oil/water interfaces in the presence of ionic surfactants. The approach relies on the ideal behaviour of ionized surfactants at oil/water interfaces, which is previously demonstrated by Lucassen-Reynders (J. Phys. Chem., 1966, 70, 1777–1785). The new derived equation simply relates the interfacial tension to the surfactant molecular size and the cmc value of the surfactant in the aqueous phase. The predicted values are in a reasonable agreement with the measured experimental data. Formation of complex multi-layers is considered and the related development is performed. It is shown that, assuming a multi-layer interface, the proposed model gives an area per surfactant molecule similar to the values obtained by techniques such as neutron reflectivity (NR), while a monolayer assumption yields about half the value. The discussion describes the impact of dissolved oil and ionic components on the interfacial tension of the ionized surfactants at oil/water interfaces.

The conformation of charged surfactants at the oil–water interface was recently reported. With the aim to assess the role of the head group size on the conformation of the adsorbed layer, we have extended these studies to a series of nonionic dodecanol ethoxylate surfactants (C12En, ethylene oxide units n from 6 to 12). The study was performed using neutron reflectometry to enable maximum sensitivity to buried interfaces. Similarly to charged surfactants, the interface was found to be broader and rougher compared to the air–water interface. Irrespective of the head group size, the tail group region was found to assume a staggered conformation. The conformations of the head group were found to be significantly different from those of the air–water interface, moving from a globular to an almost fully extended conformation at the oil–water interface. The stretching of the head groups is attributed to the presence of some hexadecane oil molecules, which may penetrate all the way to this region. It is proposed here that the presence of the oil, which can efficiently solvate the surfactant tail groups, plays a key role in the conformation of the adsorbed layer and is responsible for the broadening of the interface.

We report the structural study of mixed monolayers of partially deuterated N,N′-di-hexadecyl-(d33)-4,13-diaza-18-crown-6 ether (d-ACE16) and palmitic acid (PA) at the oil–water interface, in order to understand the mechanism of metal ion transport through Permeation Liquid Membrane (PLM) devices. The composition of the mixed monolayers remains constant with increasing spread amount and the saturation of the interface is achieved at a relatively low spread amount. The excess PA material is accommodated in the oil phase, playing an important role in equilibrating the interfacial concentration of ACE-16. The presence of PA increases the surface concentration of ACE-16 at low spread amount and facilitates its dissolution into the oil phase at the high spread amount. The result suggests a dynamic exchange between the bulk phase and the interface ensuring a continuous turnover which reflects their relevance in PLM devices. The conclusions regarding the role of a fatty acid in regulating the surface concentration of the alkylated azacrown ether and its dominant role in the bulk transport of metal ions through the membrane are consistent with the results of macroscopic studies reported earlier.Graphical abstractHighlights► Neutron reflectivity at the oil–water interface. ► Structural studies of surfactant mixtures at the oil–water interface. ► Bulk transport of metal ions through a membrane. ► The role of fatty acid in regulating surfactant surface concentration at interfaces.

The development of novel synthetic systems with tailored catalytic activity is a high priority area with interesting applications. The use of the templating approach with nanomaterials allows the creation of three dimensional cavities in polymeric matrices with specific molecular recognition properties that can be tailored by the structure of the template used. We report an in-depth study of the effects of solvents on the catalytic activity of templated and non-templated nanogels in the Kemp elimination, a reaction for which there is no enzyme. A full morphology characterisation of the nanoparticles, using small angle neutron scattering techniques and transmission electronic microscopy, has allowed the identification of significant differences in shape, size and, more significantly, in the internal structure of templated and non-templated nanogels. Moreover the data clearly indicate a very different behaviour of the nanoparticles, as a result of variations in acetonitrile content. The data provide for the first time structural evidence of templated cavities and identify important differences in swelling behaviour, aggregation, colloidal stability between templated and non-templated nanogels, which not only explain the differences in kinetic data but also identify important characteristics required for the development of novel catalysts with improved efficiency.

A neutron reflectivity study of the phospholipid 1,2-distearoyl-sn-glycero-3-phosphocholine at the hexadecane–water interface is reported as a function of spread amount. Two isotopic contrasts have been used to determine the structure of the phospholipid molecule in the buried interfacial region. The results indicate a roughened or diffuse monolayer at low spread amounts of phospholipid at the oil–water interface. An increase in the spread amount of phospholipid results in combination of a monolayer plus micelle-like aggregates formation at the interface. There is a transition from a monolayer to a more complex monolayer and micelle-like aggregates conformation as the amount of spread phospholipid increases. The total layer thickness for these fits is about 70 Å, which is much larger than a fully extended DSPC molecule (∼30 Å). This is indicative of rough molecular packing at the oil–water interface. This roughened or diffuse interface is suggested to be because of the solvation effect of the hydrocarbon tails and the resultant hydrophobic interactions. In addition, a minor contribution may originate from the accommodation of the charges in the head groups.

The structure of the adsorbed palmitic acid at the iron oxide/oil interface has been investigated using polarized neutron reflectometry. The palmitic acid was found to be strongly adsorbed at the oxide/oil interface resulting in a monolayer of thickness 16 ± 4 Å for 150 and 500 ppm palmitic acid concentrations (16 ± 5 Å for the 1000 ppm solution). These layer thicknesses suggest tilt for the palmitic acid molecules with respect to the interface. The model also requires a second diffuse layer extending in the bulk oil. The thickness of this diffuse layer was 35 ± 17 Å for the 150 ppm solution and 45 ± 22 Å for 500 and 1000 ppm solution. The composition profiles at the interface suggest a depletion of the oil in the vicinity of the interface as the concentration of palmitic acid increases.

A Langmuir−Blodgett film of aliphatic substituted phthalocyanines on a C18 silane supporting layer coupled onto a silicon substrate has been investigated using neutron reflectometry. This multilayer structure is seen as a possible candidate for phthalocyanine−lipid biosensor devices. The results show the suitability of the C18 ligands as an anchoring layer for the phthalocyanines. The scattering length density profiles demonstrate the effectiveness of a lipid monolayer in partitioning the composition of phthalocyanine layers from that of the bulk liquid. The effectiveness of this barrier is a critical factor in the efficiency of such devices.

The adsorbed amount of partially deuterated dihexadecyl-diaza-18-crown-6 ether (d-ACE16) in the presence of different chain length fatty acids as a function of surface pressure was determined by neutron reflectometry technique. The highest adsorbed amount of the azacrown ether was observed for the mixture of ACE16 with hexadecanoic (palmitic) acid, pointing to the importance of chain length matching between the two species for optimum stabilization of the mixed monolayer. The contrast variation technique was used to estimate the contribution to the total adsorbed amount from stearic acid and ACE16. It was found that the mixed Langmuir monolayer is stable against dissolution up to a surface pressure of 20 mN m−1. Above this pressure, however, the spread and adsorbed amounts start to deviate, indicative of partial dissolution into the aqueous subphase. The consequences of this behavior for the transport of metal ions through the interfaces of permeation liquid membranes (PLMs) are discussed.

We report information regarding the structure at the interface between hexadecane and an aqueous solution of trimethyl tetradecyl ammonium bromide (C14TAB) with appropriate deuterium labeling. We also report the role of the headgroup size and the charge on structures at the oil−water interface by comparing the data for C14TAB to that for a deuterated trimethyl tetradecyl ammonium sulfate (C14TAS) solution at the oil−water interface. As the charge on the counterion increases, this results in a reduction in the limiting area per molecule and an increase in the adsorbed amount at the oil−water interface.

We report the neutron reflectometry study of partially deuterated di-hexadecyl-diaza-18-crown-6 ether (d-ACE-16) at the air−water and the oil−water interfaces. At the air−water interface, the thickness of the monolayer is smaller than that for a fully stretched d-ACE-16 molecule, suggesting a tilt of the alkyl chains with respect to the normal. At the oil−water interface, the same molecules were found to form a more diffuse layer distribution stretching across both sides of the interface. On the oil side, the molecules are densely packed within a thickness of 17 Å, the hydrophilic part of the molecule with the azacrown ether ring being immersed in the adjacent aqueous side of the interface. The latter consists of a thick 38 Å layer comprising staggered, loosely adsorbed d-ACE-16 molecules. With increasing spread amount, the adsorbed layer density increases at the oil side until saturation at ca. 2.25 × 10−6 mol m−2, above which the layer collapses.

We have used neutron reflectivity (NR) measurements in combination with dynamic light scattering (DLS), surface tension and ellipsometry, to study the adsorption behaviour at the air/water interface of N-isopropylacrylamide-based nanogels as a function of concentration. The data provide clear evidence that the nanogels are adsorbed at the interface in a strongly deformed shape and forming a multi-layer where the thickness increases with nanogel concentration in the bulk. The combination of surface characterisation techniques and bulk studies indicate that interfacial film formation is preferred over bulk aggregation. This observation at the air/water interface supports the Derjaguin prediction, that a sphere's interaction with a plane (the thick adsorbed nanogel layer at interface) is much larger than nanogel–nanogel (sphere–sphere) association in the bulk. These findings, in particular the changes in conformations and the thick layer adsorption at the interface as a function of concentration, impact significantly on a number of applications for which nanogels are currently being investigated. These results contribute to the understanding of the behaviour of soft colloids at the interfaces.