Browsing by Author "Kuzmenko, I"

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    Adsorption at liquid interfaces: a comparison of multiple experimental techniques
    (EDP Sciences, 2009-02) Law, BM; Brown, MD; Marchand, L; Lurio, LB; Hamilton, WA; Kuzmenko, I; Gog, T; Satija, S; Watkins, E; Majewski, J
    It has proven to be a challenging task to quantitatively resolve the interfacial pro. le at diffuse interfaces, such as, the adsorption pro. le near a bulk binary liquid mixture critical point. In this contribution we examine the advantages and disadvantages of a variety of experimental techniques for studying adsorption, including neutron reflectometry, X-ray reflectometry and ellipsometry. Short length scale interfacial features are best resolved using neutron/X-ray reflectometry, whereas, large length scale interfacial features are best resolved using ellipsometry, or in special circumstances, neutron reflectometry. The use of multiple techniques severely limits the shape of the adsorption pro. le that can describe all experimental data sets. Complex interfaces possessing surface features on many different length scales are therefore best studied using a combination of neutron/X-ray reflectometry and ellipsometry. © 2009, EDP Sciences
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    Effect of divalent cation removal on the structure of gram-negative bacterial outer membrane models
    (American Chemical Society, 2015-01-13) Clifton, LA; Skoda, MWA; Le Brun, AP; Ciesielski, F; Kuzmenko, I; Holt, SA; Lakey, JH
    The Gram-negative bacterial outer membrane (GNB-OM) is asymmetric in its lipid composition with a phospholipid-rich inner leaflet and an outer leaflet predominantly composed of lipopolysaccharides (LPS). LPS are polyanionic molecules, with numerous phosphate groups present in the lipid A and core oligosaccharide regions. The repulsive forces due to accumulation of the negative charges are screened and bridged by the divalent cations (Mg2+ and Ca2+) that are known to be crucial for the integrity of the bacterial OM. Indeed, chelation of divalent cations is a well-established method to permeabilize Gram-negative bacteria such as Escherichia coli. Here, we use X-ray and neutron reflectivity (XRR and NR, respectively) techniques to examine the role of calcium ions in the stability of a model GNB-OM. Using XRR we show that Ca2+ binds to the core region of the rough mutant LPS (RaLPS) films, producing more ordered structures in comparison to divalent cation free monolayers. Using recently developed solid-supported models of the GNB-OM, we study the effect of calcium removal on the asymmetry of DPPC:RaLPS bilayers. We show that without the charge screening effect of divalent cations, the LPS is forced to overcome the thermodynamically unfavorable energy barrier and flip across the hydrophobic bilayer to minimize the repulsive electrostatic forces, resulting in about 20% mixing of LPS and DPPC between the inner and outer bilayer leaflets. These results reveal for the first time the molecular details behind the well-known mechanism of outer membrane stabilization by divalent cations. This confirms the relevance of the asymmetric models for future studies of outer membrane stability and antibiotic penetration. © 2014 American Chemical Society. This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.
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    Hybrid microstructure of smectite clay gels revealed using neutron and synchrotron X-ray scattering
    (Springer Nature, 2023-11-07) Shoaib, M; Khan, S; Wani, OB; Mata, JP; Krzysko, AJ; Kuzmenko, I; Bluel, M; Fiddes, LF; Roth, EW; Bobicki, ER
    Aqueous suspensions of swelling clays display a nematic sol-gel transition at very low solid concentrations. The underlying microstructure of the gel has remained a point of contention since the time of Irving Langmuir and has been a major obstacle to fully realizing the potential of clays for practical applications. Here, we comprehensively probe the microstructure of a smectite clay suspension using ultra-small angle neutron/X-ray scattering and find that the nematic gel is structurally ordered and contains entities that are at least an order of magnitude larger than the individual particles. Complementary cryo-electron microscopy shows the presence of domains having particle-particle ordering responsible for nematic texture and regions of particle-particle aggregation responsible for gel-like behavior. We find that the smectic clay gels have a hybrid microstructure with co-existing repulsive nematic domains and attractive disordered domains. © 2023 The Author(s). Open Access. This article is licensed under a Creative Commons Attribution 4.0 International License.
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    X-ray and ellipsometric study of strong critical adsorption
    (American Physical Society, 2007-06) Brown, MD; Law, BM; Marchand, L; Lurio, LB; Kuzmenko, I; Gog, T; Hamilton, WA
    Carpenter [Phys. Rev. E 61, 532 (2000)] succeeded in determining a single universal model, called the P1 model, that could describe the ellipsometric critical adsorption data from the liquid-vapor interface of four different critical binary liquid mixtures near their critical demixing temperatures. The P1 model also recently has been used to describe neutron reflectometry data from a critical liquid mixture/crystalline quartz interface. However, in another recent study, the P1 model failed to simultaneously describe x-ray reflectometry and ellipsometry data from the liquid-vapor surface of the critical mixture n-dodecane + tetrabromoethane (DT). In this paper, we resolve this discrepancy between x-ray and ellipsometric data for the DT system. At large length scales (far from the interface) the local concentration is described by the P1 model in order to correctly reproduce the temperature dependence of the ellipsometric data. Close to the interface, however, the molecular structure must be correctly accounted for in order to quantitatively explain the x-ray data. An important conclusion that arises from this study is that neutron or x-ray reflectometry is most sensitive to short-range interfacial structure, but may provide misleading information about long-range interfacial structure. Ellipsometry provides a more accurate measure of this long-range interfacial structure. Complex interfacial structures, possessing both short- and long-range structure, are therefore best studied using multiple techniques. © 2007, American Physical Society