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  • 1. Murphy, N.
    et al.
    Niga, Petru
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. School of Physics, University College Dublin, Belfield, Dublin 4, Ireland.
    Cummings, A.
    Dunne, P.
    O'Sullivan, G.
    4d → 5p transitions in the EUV photoabsorption spectrum of Ba IV, Ba v and Ba VI2006In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 39, no 2, p. 365-373Article in journal (Refereed)
    Abstract [en]

    The photoabsorption spectra of Ba IV-Ba VI have been recorded using the dual laser plasma technique. Discrete structure due to 4d → 5p transitions has been observed in the 75-85 eV region and identified using Hartree-Fock with configuration interaction calculations. The excited states decay by autoionization involving ejection of 5s or 5p electrons and rates for the different processes and resulting linewidths were also calculated. From these calculations, synthetic spectra were produced and show excellent agreement with the experimental data.

  • 2.
    Niga, Petru
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Johnson, C. Magnus
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Frey, Jeremy G.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Crown Ethers at the Aqueous Solution-Air Interface: 2. Electrolyte Effects, Ethylene Oxide Hydration and Temperature Behaviour2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, p. 7939-7947Article in journal (Refereed)
    Abstract [en]

    Vibrational Sum Frequency Spectroscopy (VSFS) was employed to study adsorbing films of 4-Nitro Benzo-15-Crown-5 (NB15C5) and Benzo-15-Crown-5 (B15C5) at the aqueous solution–air interface. The surface of the solution is strongly influenced by the presence of crown ether species. Changes in the orientation of NB15C5 were monitored as a function of solution concentration, by targeting the ratio of peak intensities of the CN and NO2 vibrational modes. The water of hydration has also been probed as a function of crown concentration, salt concentration, and temperature. The latter study strongly suggests that the surface can be treated as a charged interface, and that the associated ordered water decreases with increasing ionic strength of the bulk solution.

  • 3.
    Niga, Petru
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    King, Wendy
    Hedberg, Jonas
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Johnson, C. Magnus
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Frey, Jeremy G.
    Rutland, Mark
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Crown Ethers at the Aqueous Solution-Air Interface: 1. Assignments and Surface Spectroscopy2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 17, p. 7930-7938Article in journal (Refereed)
    Abstract [en]

    The surface of aqueous solutions of 4-Nitro Benzo-15-Crown-5 (NB15C5) and Benzo-15-Crown-5 (B15C5) has been studied using the surface sensitive technique vibrational sum frequency spectroscopy (VSFS). The NO, CN, COC and CH vibrational modes of these compounds at the air-water interface as well as OH vibrational modes of the surface water hydrating this compound have been targeted in order to obtain molecular information about arrangement and conformation of the adsorbed crown ether molecules at the air-water interface. The CH2 vibrational modes of crown ethers have been identified and found to be split due to interaction with ether oxygen. The spectra provide evidence for the existence of a protonated crown complex moiety at the surface leading to the appearance of strongly ordered water species. The interfacial water species are influenced by the resulting charged interface and by the strong Zundel polarizability due to tunneling of the proton species between equivalent sites within the crown ring.

  • 4.
    Niga, Petru
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Tyrode, Eric
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    The Influence of Unsaturation on the Surface Structure of Fatty Acids Monolayers Studied by Vibrational Sum Frequency SpectroscopyManuscript (preprint) (Other academic)
  • 5.
    Niga, Petru
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Wakeham, Deborah
    Nelson, Andrew
    Warr, Gregory G.
    Rutland, Mark
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Atkin, Rob
    Structure of the Ethylammonium Nitrate Surface: An X-ray Reflectivity and Vibrational Sum Frequency Spectroscopy Study2010In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 26, no 11, p. 8282-8288Article in journal (Refereed)
    Abstract [en]

    X-ray reflectivity and vibrational sum frequency spectroscopy are used to probe the structure of the ethylammonium nitrate (EAN)-air interface. X-ray reflectivity reveals that the EAN-air interface is structured and consists of alternating nonpolar and charged layers that extend 31 angstrom into the bulk. Vibrational sum frequency spectroscopy reveals interfacial cations have their ethyl moieties oriented toward air, with the CH3C3 axis positioned similar to 36.5 degrees from interface normal. This structure is invariant between 15 and 51 degrees C. On account of its molecular symmetry, the orientation of the nitrate anion cannot be determined with certainty.

  • 6.
    Petru, Niga
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Self Assembly at the Liquid Air Interface2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The aim of this work is to study the interfacial properties of amphiphilic compounds at the liquid–air interface in an attempt to develop a comprehensive understanding of their orientation as well as the influence of their interaction with the solvent on the interfacial layer properties. Using Vibrational Sum Frequency Spectroscopy (VSFS) as the main tool, the molecular structure of the amphiphilic layer and the amphiphile–solvent relation can be illuminated in great detail – it is arguably the most sensitive surface spectroscopy currently available. Due to its second order nature, the VSFS technique is capable of distinguishing molecules at the interface even in the presence of a vast excess of similar molecules in the bulk.Ionic liquids (Ils) form a class of solvent which are increasingly receiving attention as ``green solvents´´. Some of these, such as ethyl ammonium nitrate (EAN), a protic IL, have the capacity to hydrogen bond extensively which is one of the important features they share with water. Since the interaction with solvent is an important consideration for self assembly and it is known that surfactant self assembly in the EAN bulk is analogous to in water, it was considered of interest to probe self assembly at EAN–air interface. To this end the interfacial structure of the pure EAN interface was probed, as was the conformation and ordering of nonionic surfactants. These studies reveal that EAN is highly ordered at the interface, exposing the ethyl moiety to the gas phase. Additionally, polarization studies have enabled the average orientation of the ethyl group to be determined. Adsorption of nonionic surfactants at the interface appears to significantly displace the EAN from the interface. The headgroup of the surfactant, a linear ethylene oxide group, appears to be highly disordered.The disorder of the linear ethylene oxide groups has led to difficulties in their surface spectroscopic fingerprinting in this and other works. In an attempt to study the interfacial behaviour of ethylene oxide and the temperature dependence of its hydration, closed loop structures of PEO attached to hydrophobic groups were also probed. This essentially locks their conformation. Such molecules are known as crown ethers and display interesting interfacial behaviour and also the ability to bind cations.

    The presence of even small amounts of adsorbed crown ethers at the water interface is shown to considerably perturb the water structure. The NO, CN, COC and CH vibrational modes of these compounds at the air-water interface as well as OH vibrational modes of the surface water hydrating this compound have been targeted in order to obtain molecular information about arrangement and conformation. The CH2 vibrational modes of crown ethers have been identified and found to be split due to their interaction with ether oxygen. The spectra provide evidence for the existence of a protonated crown complex moiety at the surface leading to the appearance of strongly ordered water species. The orientation of Nitrobenzo crown (NB15C5) was monitored as a function of solution concentration, by targeting the ratio of peak intensities of the CN and NO2 vibrational modes. The water of hydration has also been probed as a function of crown concentration, salt concentration, and temperature. The latter study strongly suggests that the surface can be treated as a charged interface, and that the associated ordered water decreases with increasing ionic strength of the bulkFinally, insoluble monolayers of fatty acids spread on a water surface have also been studied in an effort to further understand the relationship between molecular architecture and film structure. Fatty acid (Arachidic Acid – AA and Eicosenoic Acid – EA) monolayers are compared to investigate the effect on the monolayer structure of introducing unsaturation into the alkyl chain. For AA, at very large area per molecule, floating domains of crystalline nature exist rather than any classical gaseous phase. The measured conformational disorder in EA decreases continuously with monolayer compression and no crystalline domains are observed at low density. Addition of NaCl to the subphase does not affect the monolayer order for either of the compounds; instead, a dramatic increase in the signal of the water hydrating the headgroups is observed. The effect of introducing further unsaturations (up to three) was also studied in order to probe the resulting interfacial structure. Remarkably the double bonds appear to adopt the same orientation, irrespective of how many they are in the chain. By monitoring the vinyl CH stretch it was possible to study the film stability towards oxidative degradation and it was found that all three unsaturated species studied showed rapid degradation. The rate of degradation could be controlled by adjusting the film pressure. However, the monolayers could be stabilised by performing the experiments in an inert nitrogen atmosphere.

  • 7.
    Tyrode, Eric
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface Chemistry.
    Niga, Petru
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Johnson, Magnus
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Molecular Structure upon Compression and Stability toward Oxidation of Langmuir Films of Unsaturated Fatty Acids: A Vibrational Sum Frequency Spectroscopy Study2010In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 26, no 17, p. 14024-14031Article in journal (Refereed)
    Abstract [en]

    Vibrational sum frequency spectroscopy (VSFS) has been used to determine the stability toward oxidation in air of a series of unsaturated fatty acids, measuring as a function of time the changes in the chemical structure and conformational order of films spread on a Langmuir trough. The fatty acids studied consisted of a 20-carbon backbone with increasing numbers of cis double bonds in the chain: 11c-eicosenoic acid (20:1 EA, omega-9), 11c,14c-eicosadienoic acid (20:2 EA, omega-6), and 11c, 14c, 17c-eicosatrienoic acid (20:3 EA, omega-3). Measurements at constant surface pressure show that double bonds are lost from the surface region and that drops in intensity of the vinyl CH stretch are detectable within a few minutes of spreading the monolayer. The results are consistent with the fatty acid peroxidation free radical mechanism. The sum frequency spectra also reveal that what remains on the surface is conformationally more disordered with a larger number of gauche defects. The oxidation kinetics are found to be strongly dependent on the packing density of the monolayer, being more stable at higher pressures. Oxidation can be avoided by purging the system in an inert atmosphere. Finally, the molecular structure upon compression was tracked in unoxidized monolayers. The results suggest that the packing and orientation of the double bond sections of all three unsaturated fatty acids show remarkable similarities, with the direction of the double bonds approximately parallel to each other irrespective of the number of unsaturations in the chain, with the 20:3 EA probably forming "iron-angle" structures. The possibility of unsaturated chains in a "hairpin" configuration is discarded for area per molecules smaller than similar to 50 angstrom(2), which corresponds to the lowest surface pressure measured with VSFS.

  • 8.
    Tyrode, Eric
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Niga, Petru
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Johnson, Magnus
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Molecular Structure upon Compression and Stability towards Oxidation of Langmuir Films of Unsaturated Fatty Acids: a Vibrational Sum Frequency Spectroscopy StudyArticle in journal (Other academic)
  • 9. Wakeham, Deborah
    et al.
    Niga, Petru
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Ridings, Christiaan
    Andersson, Gunther
    Nelson, Andrew
    Warr, Gregory G.
    Baldelli, Steven
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Atkin, Rob
    Surface structure of a "non-amphiphilic" protic ionic liquid2012In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 15, p. 5106-5114Article in journal (Refereed)
    Abstract [en]

    The nanostructure of the ethanolammonium nitrate (EtAN)-air surface has been investigated using X-ray reflectometry (XRR), vibrational sum frequency spectroscopy (VSFS) and neutral impact collision ion scattering spectroscopy (NICISS). The XRR data decays more rapidly than expected for a perfectly sharp interface, indicating a diffuse electron (scattering length) density profile. Modelling of the XRR data using three different fitting routines produced consistent interfacial profiles that suggest the formation of interfacial EtAN clusters. Consistent with this, VSFS reveals that the EtAN surface is predominantly covered by -CH2- moieties, with the -NH3+ and -OH groups of the cation buried slightly deeper in the interface. The elemental profiles determined using NICISS also show enrichment of carbon relative to nitrogen and oxygen in the outermost surface layer, which is consistent with the surface cation orientation deduced from VSFS, and with the presence of EtAN aggregates at the liquid surface.

  • 10. Wakeham, Deborah
    et al.
    Niga, Petru
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Warr, Gregory G.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Atkin, Rob
    Nonionic Surfactant Adsorption at the Ethylammonium Nitrate Surface: A Neutron Reflectivity and Vibrational Sum Frequency Spectroscopy Study2010In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827Article in journal (Refereed)
    Abstract [en]

    The adsorbed layers of polyoxyethylene n-alkyl ether surfactants C12E4, C14E4, and C16E4 at the LEAN surface have a headgroup layer that is thin and compact (only similar to 30 vol % EAN). The headgroups do not adopt a preferred orientation and are disordered within the ethylene oxide layer. Alkyl tails contain a significant number of gauche defects indicating a high degree of conformational disorder. The thickness of the tail layer increases with increasing alkyl chain length, while the headgroup layer shows little change. Lowering the C12E4 concentration from 1 to 0.1 wt % decreases the adsorbed amount, and the headgroup layer becomes thinner and less solvated, whereas C14E4 and C16E4 adsorbed layers are unaffected by dilution over the same concentration range. The C16E4 layer thickness increases and area per molecule decreases on warming to 60 degrees C, but the adsorbed layer structures of C12E4 and C14E4 are unchanged. Both effects are attributed to surfactant solubility.

  • 11.
    Álvarez Asencio, Rubén
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Cranston, Emily
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Wakeham, Deborah
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Niga, Petru
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Werzer, O.
    Sweeney, J.
    Hausen, F.
    Hayes, R.
    Webber, G. B.
    Endres, F.
    Bennewitz, R.
    Hjalmarsson, Nicklas
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Glavatskih, Sergei
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Design (Div.).
    Atkin, R.
    Rutland, Mark
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Nanotribology: Tribotronics, ionic liquids and control of surface interactions2013In: 5th World Tribology Congress, WTC 2013, 2013, Vol. 4, p. 3106-3108Conference paper (Refereed)
    Abstract [en]

    The interfacial ordering of Ionic liquids leads to interesting nanotribological properties as revealed by colloid probe studies. The first of these is the clear correlation between the number of ion pairs trapped in the tribological contact and the friction coefficient displayed. The second is the fact that the surface electrical potential can be used to control the composition of the boundary layer and thus tune the friction. Thirdly, the interfacial ordering appears to significantly affect the fluid dynamics over large distances.

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