Kamil Wojciechowski

Uranyl salophenes as ionophores for phosphate-selective electrodes

Anion selectivities of poly(vinylchloride) (PVC) plasticized membranes containing uranyl salophene derivatives were presented. The influence of the membrane components (i.e. ionophore structure, dielectric constant and structure of plasticizer, the amount of incorporated ammonium salt) on its phosphate selectivity was investigated. The highest selectivity for H2PO4− over other anions tested was obtained for lipophilic uranyl salophene III (without ortho-substituents) in PVC/o-nitrophenyl octylether (o-NPOE) membrane containing 20 mol% of tetradecylammonium bromide (TDAB). Ion-selective electrodes (ISEs) based on these membranes exhibited linear response in the range 1–4 of pH2PO4− with a slope of 59 mV/decade. The introduction of ortho-methoxy substituents in ionophore structure decreased the phosphate selectivity of potentiometric sensors.

Durable phosphate-selective electrodes based on uranyl salophenes

Lipophilic uranyl salophenes derivatives were used as ionophores in durable phosphate-selective electrodes. The influence of the ionophore structure and membrane composition (polarity of plasticizer, the amount of incorporated ionic sites) on the electrode selectivity and long-term stability were studied. The highest selectivity for H2PO4− over other anions tested was obtained for lipophilic uranyl salophene III (with t-butyl substituents) in poly(vinylchloride)/o-nitrophenyl octyl ether (PVC/o-NPOE) membrane containing 20 mol% of tetradecylammonium bromide (TDAB). Moreover, phosphate-selective electrodes based on this derivative exhibited the best long-term stability (2 months). The electrode durability can be improved decreasing the amount of the ammonium salt in membrane to 5 mol%.

Biosurfactant–Protein Mixtures: Quillaja Bark Saponin at Water/Air and Water/Oil Interfaces in Presence of β-Lactoglobulin

The adsorption kinetics of mixtures of a biosurfactant Quillaja Bark Saponin (QBS) with a globular protein, β-lactoglobulin (β-LG) at the water/air and water/tetradecane interfaces was investigated by measuring dynamic interfacial tension with axisymmetric drop shape analysis (ADSA) and maximum bubble pressure (MBP) techniques. With bulk concentration of β-LG fixed at 10(-7) M, the most pronounced synergistic effects in the rate of the QBS adsorption at both interfaces were observed at low biosurfactant concentrations (5 × 10(-7)-1 × 10(-5) M). The synergistic effect due to a protein-biosurfactant complex formation is clearly noticeable, yet less pronounced than, e.g., previously studied QBS/lysozyme mixtures. The surface pressures attained at water/oil interface are higher than in the water/air system, although, at high biosurfactant/protein ratios, the presence of β-LG decelerates adsorption of the QBS/β-LG complex onto the water/tetradecane interface. In analogy to mixtures of synthetic surfactants with proteins, the adsorbed layer gets dominated by QBS at higher biosurfactant concentrations, although the presence of β-LG affects the surface pressures attained even at QBS/β-LG ratios as high as 10(4). The synergistic effects are much less noticeable in foamability and emulsion formation/stability, as probed by the modified Bikermans and dynamic light scattering (DLS) techniques, respectively.

Interaction of Quillaja bark saponins with food-relevant proteins.

The surface activity and aggregation behaviour of two Quillaja bark saponins (QBS) are compared using surface tension, conductometry and light scattering. Despite formally of the same origin (bark of the Quillaja saponaria Molina tree), the two QBS show markedly different ionic characters and critical micelle concentrations (7.7·10(-6) mol·dm(-3) and 1.2·10(-4) mol·dm(-3)). The new interpretation of the surface tension isotherms for both QBS allowed us to propose an explanation for the previous discrepancy concerning the orientation of the saponin molecules in the adsorbed layer. The effect of three food-related proteins (hen egg lysozyme, bovine β-lactoglobulin and β-casein) on surface tension of the saponins is also described. Dynamic surface tension was measured at fixed protein concentrations and QBS concentrations varying in the range 5·10(-7)-1·10(-3) mol·dm(-3). Both dynamic and extrapolated equilibrium surface tensions of the protein/QBS mixtures depend not only on the protein, but also on the QBS source. In general, the surface tension for mixtures of the QBS with lower CMC and less ionic character shows less pronounced synergistic effects. This is especially well visible for β-casein/QBS mixtures, where a characteristic maximum in the surface tension isotherm around the molar ratio of one can be noticed for one saponin product, but not for the other.

Unusual penetration of phospholipid mono- and bilayers by Quillaja bark saponin biosurfactant.

The interactions between a model phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a biosurfactant Quillaja Bark Saponin (QBS) obtained from the bark of Quillaja saponaria Molina were studied using simple models of biological membranes. QBS is known to interact strongly with the latter, exerting a number of haemolytic, cytotoxic and anti-microbial actions. The interaction of QBS dissolved in the subphase with DPPC monolayers and silicon-supported bilayers was studied above the cmc (10(-3)M). Surface pressure relaxation and surface dilatational rheology combined with quartz crystal microbalance (QCM) and neutron reflectivity (NR) were employed for this purpose. The DPPC-penetrating abilities of QBS are compared with those of typical synthetic surfactants (SDS, CTAB and Triton X-100). We show that the penetration studies using high surface activity (bio)surfactants should be performed by a subphase exchange, not by spreading onto the surfactant solution. In contrast to the synthetic surfactants of similar surface activity, QBS does not collapse DPPC mono- and bilayers, but penetrates them, improving their surface dilatational elastic properties even in the highly compressed solid state. The dilatational viscoelasticity modulus increases from 204 mN/m for pure DPPC up to 310 mN/m for the QBS-penetrated layers, while it drops to near zero values in the case of the synthetic surfactants. The estimated maximum insertion pressure of QBS into DPPC monolayers exceeds the maximum surface pressure achievable in our setup, in agreement with the surface rheological response of the penetrated layers.

Complexation of phospholipids and cholesterol by triterpenic saponins in bulk and in monolayers

The interactions between three triterpene saponins: α-hederin, hederacoside C and ammonium glycyrrhizate with model lipids: cholesterol and dipalmitoylphosphatidylcholine (DPPC) are described. The oleanolic acid-type saponins (α-hederin and hederacoside C) were shown to form 1:1 complexes with lipids in bulk, characterized by stability constants in the range (4.0±0.2)·10(3)-(5.0±0.4)·10(4) M(-1). The complexes with cholesterol are generally stronger than those with DPPC. On the contrary, ammonium glycyrrhizate does not form complexes with any of the lipids in solution. The saponin-lipid interactions were also studied in a confined environment of Langmuir monolayers of DPPC and DPPC/cholesterol with the saponins present in the subphase. A combined monolayer relaxation, surface dilational rheology, fluorescence microscopy and neutron reflectivity (NR) study showed that all three saponins are able to penetrate pure DPPC and mixed DPPC/cholesterol monolayers. Overall, the effect of the saponins on the model lipid monolayers does not fully correlate with the lipid-saponin complex formation in the homogeneous solution. The best correlation was found for α-hederin, for which even the preference for cholesterol over DPPC observed in bulk is well reflected in the monolayer studies and the literature data on its membranolytic activity. Similarly, the lack of interaction of ammonium glycyrrhizate with both lipids is evident equally in bulk and monolayer experiments, as well as in its weak membranolytic activity. The combined bulk and monolayer results are discussed in view of the role of confinement in modulating the saponin-lipid interactions and possible mechanism of membranolytic activity of saponins.

Neutron reflectivity study of alkylated azacrown ether at the air-liquid and the liquid-liquid interfaces.

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 A, 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 A 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 x 10(-6) mol m(-2), above which the layer collapses.

Interfacial tension oscillations without surfactant transfer.

The dynamic interfacial tension oscillations of different frequency and amplitude were observed by using a pendant drop method for tetraoctyl-, tetradodecyl-, tetrahexadecyl-, and tetraoctadecylammonium bromides at a nitrobenzene/water interface. Despite surfactant mass transfer from the nitrobenzene to the aqueous phase being negligible for these lipophilic derivatives, the oscillations were often accompanied by clouding of the organic phase in the vicinity of the interface. A possible mechanism of the observed phenomena is discussed in relation to spontaneous emulsification at the liquid-liquid interface.

Durability of membranes containing uranyl salophenes

Abstract Potentiometric, spectroscopic and microscopic studies concerning polymer membranes based on uranyl salophenes were performed. Uranyl salophenes—group of Schiff base complexes—revealed very good recognition abilities toward some hydrophilic anions and appeared to be very promising ionophores for phosphate-selective electrodes. The influence of the ionophore structure and polymer matrix composition on the durability and selectivity of PVC based membrane of phosphate-selective electrodes was investigated. The interaction of uranyl salophenes with water and Cl−, NO3−, H2PO4− anions was examined. When the membranes were conditioned in phosphate solution, the formation of a water–UO2–salophene complex, insoluble in both water and organic phase, and next the decomposition of the ionophore occurred. These results gave the explanation of the mechanism of deterioration of phosphate-selective membranes.

Durability of phosphate-selective CHEMFETs

Lipophilic uranyl salophenes derivatives I and II were used as ionophores in membranes of phosphate-selective CHEMFETs. High selectivity for H2PO4− over other anions was obtained for these sensors. The influence of the ionophore structure on the sensor durability was investigated. CHEMFETs based on derivative II exhibited better long-term stability due to the better solvation of this ionophore in the membrane phase. The microsensor durability can be improved decreasing the amount of the ammonium salt in the membrane to 5% mol, with only little decrease of initial selectivity.