Neutron reflection has been used to determine the surface coverage of a series of cationic gemini surfactants of the general formula [CmH2m+1N(CH3)2-(CH2)s-N(CH3)2CmH2m+1]Br2 designated Cn-Cm-Cn, where n and m denote the number of carbon atoms in the chains of the free alkyl chain and the spacer, respectively. The compounds studied were C12-C3-C12, C12-C4-C12, C12-C6-C12, C12-C12-C12 and C12-xylyl-C12, where the spacer is the xylene group, -CH2-φ-CH2, in the last of these compounds. By the use of partially deuterated forms of the surfactants in null reflecting water, neutron reflection was used to measure directly values of the area per molecule at the critical micelle concentration (cmc) of 66, 82, 95, 140, and 97 ( 3 Å 2 , respectively. Comparison with the results from surface tension measurements shows that the appropriate prefactor P in the Gibbs equation, dγ ) -PRTΓ d ln c, is approximately 2 for all except the compound with the xylyl spacer, for which P is about 3. A tentative explanation of the unexpected value of 2 is that the dicationic surfactant ions are all in the form of a 1:1 complex with the Brions in the bulk solution. Some support for this comes from the behavior of the C12-C6-C12 compound, for which P changes to 3 at concentrations much lower than the cmc, consistent with dissociation of the complex. LA981551U

The oxidation of organic films on cloud condensation nuclei has the potential to affect climate and precipitation events. In this work we present a study of the oxidation of a monolayer of deuterated oleic acid (cis-9-octadecenoic acid) at the air-water interface by ozone to determine if oxidation removes the organic film or replaces it with a product film. A range of different aqueous sub-phases were studied. The surface excess of deuterated material was followed by neutron reflection whilst the surface pressure was followed using a Wilhelmy plate. The neutron reflection data reveal that approximately half the organic material remains at the air-water interface following the oxidation of oleic acid by ozone, thus cleavage of the double bond by ozone creates one surface active species and one species that partitions to the bulk (or gas) phase. The most probable products, produced with a yield of approximately (87 +/- 14)%, are nonanoic acid, which remains at the interface, and azelaic acid (nonanedioic acid), which dissolves into the bulk solution. We also report a surface bimolecular rate constant for the reaction between ozone and oleic acid of (7.3 +/- 0.9) x 10(-11) cm2 molecule s(-1). The rate constant and product yield are not affected by the solution sub-phase. An uptake coefficient of ozone on the oleic acid monolayer of approximately 4 x 10(-6) is estimated from our results. A simple Kohler analysis demonstrates that the oxidation of oleic acid by ozone on an atmospheric aerosol will lower the critical supersaturation needed for cloud droplet formation. We calculate an atmospheric chemical lifetime of oleic acid of 1.3 hours, significantly longer than laboratory studies on pure oleic acid particles suggest, but more consistent with field studies reporting oleic acid present in aged atmospheric aerosol.

The composition of the air/solution interface of aqueous mixtures of sodiumdodecyl sulfate (NaDS) and poly(ethylene oxide) (PEO) has been studied as a function of NaDS concentration and at a fixed concentration of 0.1 wt % PEO using neutron specular reflection. With increasing surfactant concentration, the polymer is progressively displaced from the surface until, at the critical aggregation concentration (CAC), it can no longer be detected (area per segment greater than about 80 Å2. The adsorption of surfactant increases steadily with concentration and shows no sign of a discontinuity at the CAC (4.5 mM at 35 °C), which is consistent with the break in the surface tension curve being caused only by the onset of surfactant/polymer aggregation in the bulk solution. At all concentrations the adsorption of NaDS at the air/solution interface is less in the presence of polymer than that in the corresponding solutions without polymer. Nevertheless, it is shown that there is some cooperativity in the adsorption of polymer and surfactant at the interface.

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.

A combination of neutron and X-ray diffraction has been used to structurally characterise the crystalline monolayer structures of all the alkanes with odd number of carbon atoms in their alkyl chains from pentane to pentadecane adsorbed on graphite. The structures of all the molecules investigated at submonolayer coverages are isomorphous with centred rectangular unit cells containing two molecules per cell in a parallel arrangement. This is a completely different structure from the ' herringbone ' arrangement of molecules found for the shorter ' even ' alkanes, such as hexane, octane and decane. The monolayers at sub-monolayer coverages are interpreted as uniaxial commensurate with the underlying graphite while those monolayers coexisting with the liquid, while structurally similar, are fully commensurate. The difference between the two structures is a uniaxial compression in the b-direction with the monolayers coexisting with the liquids significantly more dense than at submonolayer coverages. In the low coverage structures the ' odd ' molecules have an all trans conformation with their extended axes parallel to the surface with the plane of the carbon skeleton also parallel to the graphite surface. At high coverages the carbon skeleton is no longer parallel to the graphite surface but significantly tilted. The longest alkanes, tridecane and pentadecane also show evidence of positional and/or rotational disorder at high coverages.