Jeff Penfold

Neutron reflection investigation of the interface between an immiscible polymer pair

Abstract Neutron critical reflection experiments are reported for the interface between polystyrene and poly(methyl methacrylate). It is shown that, contrary to predictions in the literature, the interfacial mixing is not greater than 20 ± 5 A .

Fluoro-surfactants at air/water and water/CO2 interfaces

Aqueous phase behaviour and water-in-carbon-dioxide (w/c) microemulsion formation were studied with various fluorinated sulfosuccinate surfactants. For aqueous solutions of two different compounds surface coverages n measured by neutron reflection and surface tension were consistent, giving reliable values for mean areas per molecule at the air/water (a/w) interface. At 20°C and pressures above about 250 bar, seven different surfactants were found to be effective at stabilising w/c microemulsions. With reference to recent work (J. Eastoe, n A. Downer, A. Paul, D. C. Steytler and E. Rumsey, Prog. ColloidPolym. Sci., 2000, 115, 214) it is possible n to identify a structure–performance relationship for these surfactants in water–CO2 systems. Comparison n of the phase behaviour shows that sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) forms microemulsions at the lowest pressure, e.g. for w n= 20 ([water]added/[surf]) at 30°C the w/c phase was stable n down to 120 bar. High-pressure FTIR spectroscopy indicates a fraction of the added water partitions out n of microemulsion droplets, thereby saturating the bulk CO2 n. Furthermore, high-pressure small-angle neutron n scattering (SANS) is characteristic of a simple spherical droplet structure in the microemulsions. SANS data n also indicate temperature-induced changes in radius, and this is consistent with partitioning of water to maintain saturation of the bulk CO2. For three different surfactants it has been possible to compare adsorption n at a/w and w/c surfaces, highlighting differences in packing requirements to stabilise these two different interfaces.

Lamellar structure in a thin polymer blend film

Abstract We have fully characterized the three-dimensional morphology of thin films of mixtures of polystyrene and polybutadiene cast from a toluene solution, using nuclear reaction analysis, neutron reflectometry and transmission electron microscopy. Polystyrene-rich phases wet both the air and substrate interfaces and are separated by a polybutadiene-rich phase; these layers are very well defined and the interfaces between them are sharp (down to A ). Within the polybutadiene-rich central layer, lateral phase separation is also evident, with polystyrene-rich domains of oblate spheroidal shape. Under certain circumstances thin polystyrene-rich layers exist within the polybutadiene-rich phase. We discuss possible mechanisms for this unusual morphology in terms of surface effects on the mechanism of phase separation in the ternary polymer-solvent system from which the films are cast.

Instrumentation for neutron reflectivity

Abstract The basic requirements for neutron reflectivity experiments are discussed, and some current instrumentation is described. Examples from the reflectometer CRISP are used to demonstrate the present capabilities of the technique and analysis methods in the context of surface chemistry and solid films.

The Impact of Multivalent Counterions, Al3+, on the Surface Adsorption and Self-Assembly of the Anionic Surfactant Alkyloxyethylene Sulfate and Anionic/Nonionic Surfactant Mixtures

The impact of multivalent counterions, Al(3+), on the surface adsorption and self-assembly of the anionic surfactant sodium dodecyl dioxyethylene sulfate, SLES, and the anionic/nonionic surfactant mixtures of SLES and monododecyl dodecaethylene glycol, C(12)E(12), has been investigated using neutron reflectivity, NR, and small angle neutron scattering, SANS. The addition of relatively low concentrations of Al(3+) counterions induces a transition from a monolayer to well-defined surface bilayer, trilayer, and multilayer structures in the adsorption of SLES at the air-water interface. The addition of the nonionic cosurfactant, C(12)E(12), partially inhibits the evolution in the surface structure from monolayer to multilayer interfacial structures. This surface phase behavior is strongly dependent upon the surfactant concentration, solution composition, and concentration of Al(3+) counterions. In solution, the addition of relatively low concentrations of Al(3+) ions promotes significant micellar growth in SLES and SLES/C(12)E(12) mixtures. At the higher counterion concentrations, there is a transition to lamellar structures and ultimately precipitation. The presence of the C(12)E(12) nonionic cosurfactant partially suppresses the aggregate growth. The surface and solution behaviors can be explained in terms of the strong binding of the Al(3+) ions to the SLES headgroup to form surfactant-ion complexes (trimers). These results provide direct evidence of the role of the nonionic cosurfactant in manipulating both the surface and solution behavior. The larger EO(12) headgroup of the C(12)E(12) provides a steric hindrance which disrupts and ultimately prevents the formation of the surfactant-ion complexes. The results provide an important insight into how multivalent counterions can be used to manipulate both solution self-assembly and surface properties.

Adsorption and micellisation of partially- and fully-fluorinated surfactants

Abstract Effects of chemical structure on adsorption and aggregation behaviour have been investigated for sodium perfluorononanoate (C8F17COO− Na+ or Na PFN) and 9-H perfluorononanoate (H–C8F16COO− Na+ or H Na PFN). Replacing the terminal F for H in this way gives rise to a permanent dipole moment at the hydrophobic tip of the chain. The critical micelle concentrations, determined by surface tension and electrical conductivity, were 10 and 40 mmol dm−3 for the Na PFN and H Na PFN, respectively. Tensiometric and neutron reflection (NR) methods were employed to investigate adsorption at the air-water interface. Problems associated with the experimental measurements, and data interpretation, are discussed. In particular effects on surface tensions of trace levels of Ca2+, Mg2+ and Ba2+, as well as the sequestering agent ethylenediaminetetraacetic acid (EDTA), were investigated. The background levels of a principal contaminant, Ca2+, were directly measured by atomic absorption spectrophotometry, indicating a typical ratio of Ca2+:Na+ of 1:104 in these surfactant solutions. Hence, an appropriate level of added EDTA, sufficient to chelate unwanted polyvalent metals but enough not affect the surface tension, was chosen and this was EDTA:surfactant 1:333. For both compounds the surface excess Γ obtained from NR data was consistently higher than that derived from tensiometry, using a Gibbs pre-factor of 2. However, better agreement was found if this was changed to 1.7, which would be consistent with around 30% dissociation of counterions from the film. At the critical micelle concentration (cmc) the fully fluorinated surfactant adsorbs most strongly, in that NR gave areas per molecule for the Na PFN and H Na PFN of 41 and 44 A2, respectively, whereas drop volume tensiometry gave 43 and 51 A2. Taking together all the different methods, at their cmc’s the average values were 43.1±3.4 and 48.4±5.2 A2 for the Na PFN and H Na PFN, respectively. These changes are consistent with weaker intermolecular interactions in the layer, and stronger adsorption, in the absence of the H–CF2 dipole. Aggregation in the bulk was also investigated by small-angle neutron scattering (SANS), and the data were modelled in terms of charged spherical micelles. At a volume fraction φmic∼0.05, the micellar radii and net charges were similar, being 15 and 14 A, −10 and −13, for Na PFN and H Na PFN, respectively. These values suggest that the ion dissociation in micelles is between 25 and 45%, and this is similar to the ionisation at the surface suggested by the adsorption measurements. These results will help improve understanding about properties of fully- and partially-fluorinated surfactants, which are often used in speciality applications, such as water-in-CO2 microemulsions.

Neutron and X-ray reflectometry of interfacial systems in colloid and polymer chemistry

Neutron and X-ray specular reflectometry have been applied recently to the study of the structure of a range of different interfacial systems relevant to colloid chemistry. These include surfactants and surfactant mixtures at air/liquid, liquid/liquid and solid/liquid interfaces, insoluble monolayers, polymers adsorbed from solution, and polymer thin films.

Comparison of the structural and rheological consequences of micelle formation in solutions of a model di-block copolymer

Abstract In the selective solvent, dodecane, the di-block copolymer polystyrene- b -(ethylene- co -propylene), with a narrow molecular-weight distribution, forms micelles with polystyrene cores. Small-angle neutron scattering experiments were used to investigate both core shape and dimensions and the intercore structure factors as temperature or concentration were varied. Monodisperse spherical structures with radii around 120 A were observed. Above a critical concentration these were arranged in relatively ordered structures in which preferred orientations could easily be induced. The core diameter and the intercore spacing were dependent on sample thermal history. During shear the intercore structure became less ordered. The structural results correlate well with measurements of the dynamic viscosity measured in oscillatory shear, which also show a sharp change from gel-like to liquid behaviour at this critical concentration. Data are compared to model calculations in the regions where the particle form factor or where the interparticle structure factor dominate. In the latter case a hard core potential with a soft tail is found to give reasonable agreement with the data, and to allow changes with shear rate, with concentration or with temperature to be interpreted.

Adsorption of Sophorolipid Biosurfactants on Their Own and Mixed with Sodium Dodecyl Benzene Sulfonate, at the Air/Water Interface

The adsorption of the lactonic (LS) and acidic (AS) forms of sophorolipid and their mixtures with the anionic surfactant sodium dodecyl benzene sulfonate (LAS) has been measured at the air/water interface by neutron reflectivity, NR. The AS and LS sophorolipids adsorb with Langmuir-like adsorption isotherms. The more hydrophobic LS is more surface active than the AS, with a lower critical micellar concentration, CMC, and stronger surface adsorption, with an area/molecule ∼70 Å(2) compared with 85 Å(2) for the AS. The acidic sophorolipid shows a maximum in its adsorption at the CMC which appears to be associated with a mixture of different isomeric forms. The binary LS/AS and LS/LAS mixtures show a strong surface partitioning in favor of the more surface active and hydrophobic LS component but are nevertheless consistent with ideal mixing at the interface. In contrast, the surface composition of the AS/LAS mixture is much closer to the solution composition, but the surface mixing is nonideal and can be accounted for by regular solution theory, RST. In the AS/LS/LAS ternary mixtures, the surface adsorption is dominated by the sophorolipid, and especially the LS component, in a way that is not consistent with the observations for the binary mixtures. The extreme partitioning in favor of the sophorolipid for the LAS/LS/AS (1:2) mixtures is attributed to a reduction in the packing constraints at the surface due to the AS component. Measurements of the surface structure reveal a compact monolayer for LS and a narrow solvent region for LS, LS/AS, and LS/LAS mixtures, consistent with the more hydrophobic nature of the LS component. The results highlight the importance of the relative packing constraints on the adsorption of multicomponent mixtures, and the impact of the lactonic form of the sophorolipid on the adsorption of the sophorolipid/LAS mixtures.

Solution Self-Assembly of the Sophorolipid Biosurfactant and Its Mixture with Anionic Surfactant Sodium Dodecyl Benzene Sulfonate

The self-assembly in aqueous solution of the acidic (AS) and lactonic (LS) forms of the sophorolipid biosurfactant, their mixtures, and their mixtures with anionic surfactant sodium dodecyl benzene sulfonate, LAS, has been studied using predominantly small-angle neutron scattering, SANS, at relatively low surfactant concentrations of <30 mM. The more hydrophobic lactonic sophorolipid forms small unilamellar vesicles at low surfactant concentrations, in the concentration range of 0.2 to 3 mM, and transforms via a larger unilamellar vesicle structure at 7 mM to a disordered dilute phase of tubules at higher concentrations, 10 to 30 mM. In marked contrast, the acidic sophorolipid is predominantly in the form of small globular micelles in the concentration range of 0.5 to 30 mM, with a lower concentration of larger, more planar aggregates (lamellar or vesicular) in coexistence. In mixtures of AS and LS, over the same concentration range, the micellar structure associated with the AS sophorolipid dominates the mixed-phase behavior. In mixtures of anionic surfactant LAS with the AS sophorolipid, the globular micellar structure dominates over the entire composition and concentration range studied. In contrast, mixtures of LAS with the LS sophorolipid exhibit a rich evolution in phase behavior with solution composition and concentration. At low surfactant concentrations, the small unilamellar vesicle structure present for LS-rich solution compositions evolves into a globular micelle structure as the solution becomes richer in LAS. At higher surfactant concentrations, the disordered lamellar structure present for LS-rich compositions transforms to small vesicle/lamellar coexistence, to lamellar/micellar coexistence, to micellar/lamellar coexistence, and ultimately to a pure micellar phase as the solution becomes richer in LAS. The AS sophorolipid surfactant exhibits self-assembly properties similar to those of most other weakly ionic or nonionic surfactants that have relatively large headgroups. However, the more hydrophobic nature of the lactonic sophorolipid results in a more complex and unusual evolution in phase behavior with concentration and with concentration and composition when mixed with anionic surfactant LAS.