Bond-strengthening in staphylococcal adhesion to hydrophilic and hydrophobic surfaces using atomic force microscopy

Time-dependent bacterial adhesion forces of four strains of Staphylococcus epidermidis to hydrophobic and hydrophilic surfaces were investigated. Initial adhesion forces differed significantly between the two surfaces and hovered around -0.4 nN. No unambiguous effect of substratum surface hydrophobicity on initial adhesion forces for the four different S. epidermidis strains was observed. Over time, strengthening of the adhesion forces was virtually absent on hydrophobic dimethyldichlorosilane (DDS)-coated glass, although in a few cases multiple adhesion peaks developed in the retract curves. Bond-strengthening on hydrophilic glass occurred within 5-35 s to maximum adhesion forces of -1.9 +/- 0.7 nN and was concurrent with the development of multiple adhesion peaks upon retract. Poisson analysis of the multiple adhesion peaks allowed separation of contributions of hydrogen bonding from other nonspecific interaction forces and revealed a force contribution of -0.8 nN for hydrogen bonding and +0.3 nN for other nonspecific interaction forces. Time-dependent bacterial adhesion forces were comparable for all four staphylococcal strains. It is concluded that, on DDS-coated glass, the hydrophobic effect causes instantaneous adhesion, while strengthening of the bonds on hydrophilic glass is dominated by noninstantaneous hydrogen bond formation.     

Quantitative determination of surface energy using atomic force microscopy: the case of hydrophobic/hydrophobic contact and hydrophilic/hydrophilic contact

Atomic force microscopy (AFM) has been used to determine the surface energy of chemically modified surfaces at a local scale. In order to achieve this aim, it was necessary to graft both the AFM tip and the substrate with the same chemical functional groups. Two different organothiols terminated either by hydrophilic or hydrophobic chemical functionalities were used. Grafting process classically reported shows that after UV/ozone treatment for 30 min, the tip is coated by thermal deposition with 4‐5‐nm‐thick titanium layer followed by a 30‐nm‐thick gold layer. Finally, the tip is grafted by organothiols. The thickness of the layer deposited on the tip is of the same order of magnitude as the tip radius. To avoid the use of Ti and to decrease the thickness of the gold layer, we have developed a new way of grafting by using organic molecules like (3‐mercaptopropyl)triethoxysilane (MPS) as a linkage agent. Then this way of grafting was checked. Finally, AFM force‐distance curves, between grafted tips and chemically modified surface, were carried out in contact mode. Calibration of the various parts of the apparatus and especially of the cantilever (spring constant and tip radius) is of major importance to reach quantitative data. Finally, by applying a suitable theory of contact, we were able to determine the surface energy of our system. Copyright © 2005 John Wiley & Sons, Ltd.     

Water interaction with hydrophobic and hydrophilic soot particles

The interaction of water with laboratory soots possessing a range of properties relevant for atmospheric studies is examined by two complementary methods: gravimetrical measurement of water uptake coupled with chemical composition and porosity analysis and HTDMA (humidified tandem differential mobility analyzer) inference of water uptake accompanied by separate TEM (transmission electron microscopy) analysis of single particles. The first method clarifies the mechanism of water uptake for bulk soot and allows the classification of soot with respect to its hygroscopicity. The second method highlights the dependence of the soot aerosol growth factor on relative humidity (RH) for quasi-monodisperse particles. Hydrophobic and hydrophilic soot are qualitatively defined by their water uptake and surface polarity: laboratory soot particles are thus classified from very hydrophobic to very hydrophilic. Thermal soot particles produced from natural gas combustion are classified as hydrophobic with a surface of low polarity since water is found to cover only half of the surface. Graphitized thermal soot particles are proposed for comparison as extremely hydrophobic and of very low surface polarity. Soot particles produced from laboratory flame of TC1 aviation kerosene are less hydrophobic, with their entire surface being available for statistical monolayer water coverage at RH approximately 10%. Porosity measurements suggest that, initially, much of this surface water resides within micropores. Consequently, the growth factor increase of these particles to 1.07 at RH > 80% is attributed to irreversible swelling that accompanies water uptake. Hysteresis of adsorption/desorption cycles strongly supports this conclusion. In contrast, aircraft engine soot, produced from burning TC1 kerosene in a gas turbine engine combustor, has an extremely hydrophilic surface of high polarity. Due to the presence of water soluble organic and inorganic material it can be covered by many water layers even below water saturation conditions. This soot demonstrates a gradual diameter growth factor (D(wet)/D(dry)) increase up to 1.22 at 93% relative humidity, most likely due to the presence of single particles with water soluble material heterogeneously distributed over their surface.     

Compaction of TiO2 suspension utilizing hydrophilic/hydrophobic transition of cationic thermosensitive polymers

Compaction of TiO2 suspended particles utilizing the hydrophilic/hydrophobic transition of a cationic thermosensitive copolymer of N-isopropylacrylamide (NIPAM) and N-dimethylaminopropylacrylamide (DMAPAA) was examined. The surface of suspended TiO2 particles were sufficiently covered with the adsorbed polymer molecules at room temperature, and the suspension was subsequently heated above the transition temperature of the polymer. The thrusting motion of a plunger was used in order to provide an adequate mechanical force to compact the TiO2 suspended particles. The transition temperature was dependent on the DMAPAA content in the copolymer and the pH of the suspension. The transition temperature increased with increasing DMAPAA content and decreased with increasing pH. While suspended TiO2 particles were not compacted with poly(NIPAM), hard compacted sludge was readily obtained for cationic copolymer even if the content of DMAPAA groups was as low as 0.23 mol%. Furthermore, residual polymer molecules in the supernatant were not detected even if the polymer dosage was increased. These findings were attributed to the hydrophobic interaction of thermosensitive copolymer molecules adsorbed on TiO2 particles.     

Microscopic probing of the size dependence in hydrophobic solvation

We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (C(n)H(2n+1))(4)N(+) (R(4)N(+)) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 A?, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations.     

Molecular dynamics simulations of PNIPAM‐co‐PEGMA copolymer hydrophilic to hydrophobic transition in NaCl solution

Classical molecular dynamics simulations were carried out to investigate the hydrophilic to hydrophobic transition of PNIPAM‐co‐PEGMA close to its lower critical solution temperature (LCST) in 1 M NaCl solution. PNIPAM‐co‐PEGMA is a copolymer of poly(N‐isopropylacrylamide) (PNIPAM) and poly(ethylene glycol) methacrylate (PEGMA). The copolymer consists of 38 monomer units of NIPAM with two PEGMA chains attached to the PNIAPM backbone. The PNIPAM‐co‐PEGMA was observed to go through the hydrophilic?hydrophobic conformational change for simulations at temperature slightly above its LCST. Na⁺ ions were found to bind strongly and directly with amide O, even more strongly with the O atoms on PEGAMS chains, whereas Cl^? ions only exhibit weak interaction with the polymer. Significantly a novel caged stable metal‐organic complex involving a Na⁺ ion coordinated by six O atoms from the copolymer was observed after the PNIPAM‐co‐PEGMA copolymer went through conformational transition to form a hydrophobic folded structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Prediction of molecular solvation free energy based on the optimization of atomic solvation parameters with genetic algorithm

We propose an improved solvent contact model to estimate the solvation free energy of an organic molecule from individual atomic contributions. The modification of the solvation model involves the optimization of three kinds of parameters in the solvation free energy function: atomic fragmental volume, maximum atomic occupancy, and atomic solvation parameters. All of these atomic parameters for 24 atom types are developed by the operation of a standard genetic algorithm in such a way as to minimize the difference between experimental and calculated solvation free energies. The data set for experimental solvation free energies is divided into a training set of 131 compounds and a test set of 24 compounds. Linear regressions with the optimized atomic parameters yield fits with the squared correlation coefficients (r2) of 0.89 and 0.86 for the training set and for the test set, respectively. Overall, the results indicate that the improved solvent contact model with the newly developed atomic parameters would be a useful tool for rapid calculation of molecular solvation free energies in aqueous solution.     

Probing the transition from non-localization to localization by K-shell photoemission from isotope-substituted N2

In homonuclear diatomic molecules such as N₂, the inversion symmetry of the system causes non-local, coherent behavior of the otherwise localized core holes. The non-locality of the electron emission and the remaining core hole changes in a continuous way into partially localized behaviour if a gradual breakdown of the inversion symmetry is induced by isotope substitution. This is reflected by a loss of interference and a parity mixing of the outgoing photoelectron waves. Our results represent the first experimentally observed isotope effect on the electronic structure of a diatomic molecule.     

Protein resistance of polyurethane with hydrophilic and hydrophobic soft segments

Polyurethane (PU) containing poly(propylene glycol) (PPG) or poly(tetramethylene oxide) (PTMG) soft segments have been prepared by two‐step condensation polymerization. The former (PPG‐PU) with a lower critical solution temperature (LCST) at ～21 °C can change from hydrophilic to hydrophobic, whereas the latter (PTMG‐PU) is hydrophobic at a temperature above 0 °C. The adsorption of fibrinogen, bovine serum albumin, or lysozyme on such a PU surface in aqueous solution has been investigated by use of quartz crystal microbalance with dissipation (QCM‐D) and surface plasmon resonance (SPR) in real time. PPG‐PU surface exhibits protein resistance at a temperature below the LCST of PPG, but it significantly adsorbs proteins at a temperature above the LCST. On the other hand, the hydrophobic PTMG‐PU surface adsorb the proteins at any temperatures investigated, in contrast with the hydrated poly(ethylene glycol) exhibiting excellent protein resistance. The hydration and dehydration of the polymers at different temperatures were confirmed by Raman spectroscopy. Our study demonstrates that the protein resistance of polymers is determined by their hydration. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1987–1993, 2010     

Wettability switching of SDS-doped polyaniline from hydrophobic to hydrophilic induced by alkaline/reduction reactions

In this paper, sodium dodecyl sulfate (SDS) is used as dopant to manipulate both morphology and wettability of polyaniline (PANI). When SDS amount is controlled within a certain range, micro/nano double-rough PANI with hydrophobic surface can be obtained, while insufficient or excess SDS can only lead to hydrophilic PANI. The double-rough PANI shows stimuli-responsive change in wettability to either alkaline reaction or reduction reaction from hydrophobicity to hydrophilicity. This quick response can be applied to quantitatively estimate NaOH concentrations within the range of 10(-3)-10(-1)M and to detect various reducing liquids.     

An improved coarse-grained model of solvation and the hydrophobic effect

We present a coarse-grained lattice model of solvation thermodynamics and the hydrophobic effect that implements the ideas of Lum-Chandler-Weeks theory [J. Phys. Chem. B 134, 4570 (1999)] and improves upon previous lattice models based on it. Through comparison with molecular simulation, we show that our model captures the length-scale and curvature dependence of solvation free energies with near-quantitative accuracy and 2-3 orders of magnitude less computational effort, and further, correctly describes the large but rare solvent fluctuations that are involved in dewetting, vapor tube formation, and hydrophobic assembly. Our model is intermediate in detail and complexity between implicit-solvent models and explicit-water simulations.     

Probing the interaction mode in hydrophilic interaction chromatography

This work aims at characterizing interactions between a select set of probes and 22 hydrophilic and polar commercial stationary phases, to develop an understanding of the relationship between the chemical properties of those phases and their interplay with the eluent and solutes in hydrophilic interaction chromatography. "Hydrophilic interaction" is a somewhat inexact term, and an attempt was therefore made to characterize the interactions involved in HILIC as hydrophilic, hydrophobic, electrostatic, hydrogen bonding, dipole-dipole, π-π interaction, and shape-selectivity. Each specific interaction was quantified from the separation factors of a pair of similar substances of which one had properties promoting the interaction mode being probed while the other did not. The effects of particle size and pore size of the phases on retention and selectivity were also studied. The phases investigated covered a wide range of surface functional groups including zwitterionic (sulfobetaine and phosphocholine), neutral (amide and hydroxyl), cationic (amine), and anionic (sulfonic acid and silanol). Principal component analysis of the data showed that partitioning was a dominating mechanism for uncharged solutes in HILIC. However, correlations between functional groups and interactions were also observed, which confirms that the HILIC retention mechanism is partly contributed by adsorption mechanisms involving electrostatic interaction and multipoint hydrogen bonding. Phases with smaller pore diameters yielded longer retention of solutes, but did not significantly change the column selectivities. The particle diameter had no significant effect, neither on retention, nor on the selectivities. An increased water content in the eluent reduced the multipoint hydrogen bonding interactions, while an increased electrolyte concentration lowered the selectivities of the tested columns and made their interaction patterns more similar.     

Residence time dependent desorption of Staphylococcus epidermidis from hydrophobic and hydrophilic substrata

Adhesion and desorption are simultaneous events during bacterial adhesion to surfaces, although desorption is far less studied than adhesion. Here, desorption of Staphylococcus epidermidis from substratum surfaces is demonstrated to be residence time dependent. Initial desorption rate coefficients were similar for hydrophilic and hydrophobic dimethyldichlorosilane (DDS)-coated glass, likely because initial desorption is controlled by attractive Lifshitz–Van der Waals interactions, which are comparable on both substratum surfaces. However, significantly slower decay times of the desorption rate coefficients are found for hydrophilic glass than for hydrophobic DDS-coated glass. This difference is suggested to be due to the acid–base interactions between staphylococci and these surfaces, which are repulsive on hydrophilic glass and attractive on hydrophobic DDS-coated glass. Final desorption rate coefficients are higher on hydrophilic glass than on hydrophobic DDS-coated glass, due to the so called hydrophobic effect, facilitating a closer contact on hydrophobic DDS-coated glass.     

Specific control of peptide assembly with combined hydrophilic and hydrophobic interfaces

Designed coiled-coil heterotrimers are described whose assembly is governed by both hydrophobic and hydrophilic forces. Sterically matched hydrophobic core side-chain packing of alanine and cyclohexylalanine has been shown to promote formation of a 1:1:1 heterotrimer. Manipulation of hydrophilic glutamic acid (Glu)/lysine (Lys) pairs at each of three helical contact interfaces provides a secondary recognition mechanism. Peptides with matched cores and hydrophilic contacts form stable heterotrimers (DeltaG(unf) at 25 degrees C = 17.93 kcal/mol; MW(app) = 11362 vs 11563 calcd for trimer), as do those with a single Lys/Lys (but not Glu/Glu) interface. The additional specificity engendered by simultaneous operation of two interfaces was used to design a system in which six different peptides are mixed to form three specific and independent heterotrimers in the same solution.     

Embedded hydrophilic nanogranules with radiating proton-conducting channels in a hydrophobic matrix

It is a significant challenge to develop proton exchange membranes (PEMs) possessing both desired proton conductivity and low methanol permeability for the direct methanol fuel cells (DMFC). In this work, a composite PEM was fabricated from a predominantly hydrophobic framework of three-component polymer blend (TCPB) of acrylic polymers and a dispersed proton-conducting hydrophilic copolymer network. The hydrophilic copolymer was designed to contain three co-monomer units of complementary functionalities: 2-acrylamido-2-methyl propanesulfonic acid (AMPS), 2-hydroxyethyl methacrylate (HEMA), and 2-hydroxyl-3-(diethanolamino)propylmethacrylate (DEAPMA). The resultant PEMs were macroscopically homogeneous but contained microscopic heterogeneity in the form of dispersed nanosize AMPS domains with radiating (HEMA-DEAPMA) segments in the TCPB matrix, forming an overall amphiphilic matrix. Formation of such a texture was consequential upon the association of AMPS units and hydrogen bonding between the HEMA and DEAPMA short blocks of the hydrophilic copolymer with TCPB. The polymer blend membranes therefore acquired dual functionalities, i.e., effective proton transport between AMPS granules through interconnecting (HEMA-DEAPMA) segments, resulting in proton conductivity of the order of 10(-2) S/cm, and low water uptake and inhibited methanol passage in the continuous amphiphilic matrix, resulting in methanol permeability of (1.25-8) x 10(-7) cm2/s, which is about 3-8 times smaller than that of Nafion117.     

Preparation and characterization of latices with hydrophobic core and hydrophilic shell

Anionic and non-ionic copolymer latices with a hydrophobic core and hydrophilic shell were prepared using emulsifier-free emulsion polymerization. Styrene was used as the hydrophobic monomer; acrylic acid, acrylamide, and methacrylamide were employed as the hydrophilic monomers. The amount of chemically bound hydrophilic monomers in latex and unbound homopolymers in water were determined. The salt stability and redispersability of latices in water after spray-drying were also investigated.     

DNA sequencing with hydrophilic and hydrophobic polymers at elevated column temperatures

Read length in DNA sequencing by capillary electrophoresis at elevated temperatures is shown to be greatly affected by the extent of hydrophobicity of the polymer separation matrix. At column temperatures of up to 80 degrees C, hydrophilic linear polyacrylamide (LPA) provides superior read length and separation speed compared to poly(N,N-dimethylacrylamide) (PDMA) and a 70:30 copolymer of N,N-dimethylacrylamide and N,N-diethylacrylamide (PDEA30). DNA-polymer and polymer intramolecular interactions are presumed to be a major cause of band broadening and the subsequent loss of separation efficiency with the more hydrophobic polymers at higher column temperatures. With LPA, these interactions were reduced, and a read length of 1000 bases at an optimum temperature of 70 degrees -75 degrees C was achieved in less than 59 min. By comparison, PDMA produced a read length of roughly 800 bases at 50 degrees C, which was close to the read length attained in LPA at the same temperature; however, the migration time was approximately 20% longer, mainly because of the higher polymer concentration required. At 60 degrees C, the maximum read length was 850 bases for PDMA, while at higher temperatures, read lengths for this polymer were substantially lower. With the copolymer DEA30, read length was 650 bases at the optimum temperature of 50 degrees C. Molecular masses of these polymers were determined by tandem gel permeation chromatography-multiangle laser light scattering method (GPC-MALLS). The results indicate that for long read, rapid DNA sequencing and analysis, hydrophilic polymers such as LPA provide the best overall performance.     

Entropy and enthalpy convergence of hydrophobic solvation beyond the hard-sphere limit

The experimentally well-known convergence of solvation entropies and enthalpies of different small hydrophobic solutes at universal temperatures seems to indicate that hydrophobic solvation is dominated by universal water features and not so much by solute specifics. The reported convergence of the denaturing entropy of a group of different proteins at roughly the same temperature as hydrophobic solutes was consequently argued to indicate that the denaturing entropy of proteins is dominated by the hydrophobic effect and used to estimate the hydrophobic contribution to protein stability. However, this appealing picture was subsequently questioned since the initially claimed universal convergence of denaturing entropies holds only for a small subset of proteins; for a larger data collection no convergence is seen. We report extensive simulation results for the solvation of small spherical solutes in explicit water with varying solute-water potentials. We show that convergence of solvation properties for solutes of different radii exists but that the convergence temperatures depend sensitively on solute-water potential features such as stiffness of the repulsive part and attraction strength, not so much on the attraction range. Accordingly, convergence of solvation properties is only expected for solutes of a homologous series that differ in the number of one species of subunits (which attests to the additivity of solvation properties) or solutes that are characterized by similar solute-water interaction potentials. In contrast, for peptides that arguably consist of multiple groups with widely disperse interactions with water, it means that thermodynamic convergence at a universal temperature cannot be expected, in general, in agreement with experimental results.