Attractive Hydrophobic Force Research Articles

Responsive Polymer Gels: Double-Stranded versus Single-Stranded DNA

Cross-linking of polyelectrolytes such as DNA gives gels that are osmotically highly swollen but contract upon addition of electrolytes and, in particular, upon association of oppositely charged cosolutes with the polyelectrolyte chain. The deswelling behavior of cross-linked DNA gels thus reflects the DNA-cosolute interactions and provides a basis for the development of responsive DNA formulations. Gels of both single- and double-stranded DNA have interesting applications, and a comparison between them provides the basis for understanding mechanisms. Denaturation of cross-linked ds-DNA gels was induced by heating them above the melting temperature and then cooling. This process, studied by fluorescence using ethidium bromide, appeared to be reversible when a heating/cooling cycle was performed. The swelling behavior upon addition of different cosolutes, such as metal ions, polyamines, charged proteins, and surfactants, was investigated for different DNA gel samples, including long and short ds-DNA and long and short ss-DNA. The DNA molecular weight was found to have only a slight effect on the deswelling curves, whereas conformation exhibited a pronounced effect. In general, single-stranded DNA gels exhibited a larger collapse in the presence of cations than did double-stranded DNA. This difference was more pronounced with surfactants than with the other cosolutes investigated. The difference between double- and single-stranded DNA was attributed to differences in linear charge density, chain flexibility, and hydrophobicity. For surfactants with different chain lengths, the swelling behavior displayed by ss-DNA can be interpreted in terms of an interplay between hydrophobic and electrostatic interactions, the latter being influenced by polymer flexibility. Increasing hydrophobicity of the network leads to a decreased critical aggregation concentration (cac) for the surfactant/gel complex, as a result of the strengthened hydrophobic attractive force between the surfactant and the gel chain. The swelling of DNA gels appears to be reversible and to be independent of DNA conformation. Surfactant-induced deswelling of DNA gels under some conditions appears to be quite homogeneous, whereas under other conditions, there is a separation into a collapsed region in the outer parts of the gel sample and an inside swollen part. Such "skin" formation is quite different for ss- and ds-DNA, with ss-DNA giving more pronounced skin formation over a wider range of binding ratio, beta. For example, no macroscopic separation into collapsed and swollen regions was observed at intermediate degrees of binding for ds-DNA gels, whereas a dense surfactant-rich surface phase (skin) was found to coexist with a swollen core network for ss-DNA gels with beta>0.5. One explanation for this difference is the large deformation energy required for the compression of the very stiff ds-DNA chains.

Interaction of calcium dioleate collector colloids with calcite and fluorite surfaces as revealed by AFM force measurements and molecular dynamics simulation

Spherical calcium dioleate particles (∼ 10 μm in diameter) were used as AFM (atomic force microscope) probes to measure interaction forces of the collector colloid with calcite and fluorite surfaces. The attractive AFM force between the calcium dioleate sphere and the fluorite surface is strong and has a longer range than the DLVO (Derjaguin–Landau–Verwey–Overbeek) prediction. The repulsive AFM force between the calcium dioleate sphere and the fluorite surface does not agree with the DLVO prediction. Consideration of non-DLVO forces, including the attractive hydrophobic force, was necessary to explain the experimental results. The non-DLVO interactions considered were justified by the different interfacial water structures at fluorite– and calcite–water interfaces as revealed by the numerical computation experiments using molecular dynamics simulation. The density of interfacial water at the fluorite surface is low and the fluorite surface is not strongly wetted by water molecules. In contrast to the water at the fluorite surface, water molecules at the calcite surface form tightly packed monolayer structures and the calcite surface is extensively hydrated by water molecules. The interfacial water structure agrees with the AFM force measurements and the flotation recovery data. The strong attraction between the calcium dioleate colloid and the fluorite surface, and the moderately wetted fluorite surface by water molecules explain the better flotation response of fluorite with the oleate collector colloid.

Estimate of the long-range interactions between hydrophobic surfaces as applied to flotation of sulphide minerals

Using pyrite, chalcopyrite and sphalerite for exemplification, the forces of electrostatic repulsion and hydrophobic attraction between particles are estimated. Based on the experimentally obtained values of the induction time, the limit thickness of a wetting film is calculated to demonstrate the long-range behavior of the attraction forces between sulphides and an air bubble. It is established that the surface hydrophobicity of the tested sulphides increases under the action of a hydrophobic polymer and compounded collecting agents (isopropyl ethyl thiocarbamate and butyl xanthate). This fact enables enhancing the extraction of zinc and sulphide products from tailings of copper-zinc ore concentration.

Effect of sodium dodecyl sulphate and dodecanol mixtures on foam film drainage: Examining influence of surface rheology and intermolecular forces

Intermolecular forces and surface rheology of adsorbed surfactants can jointly influence the drainage and stability of foam films significantly, but have been separately considered in the literature. Here we use mixtures of sodium dodecyl sulphate and dodecanol to produce different surface shear viscosities and to elucidate the role of intermolecular forces and surface rheology in foam film drainage. The thickness of plane parallel foam films with very small radius versus time was obtained with an improved Scheludko cell set-up used in conjunction with a high-speed video camera system operating at a frequency of 1000 frames per second. The surface tension and surface potential were measured by pendant bubble tensiometry and micro-electrophoresis, respectively. The surface shear viscosity was determined by a deep channel surface shear viscometer. The modelling considered surface tension gradient, surface shear viscosity and diffusion, and DLVO and non-DLVO intermolecular forces. Comparison between experimental data and theoretical models for foam film drainage shows that the film drainage is influenced by the Marangoni effect due to surface tension gradient, producing foam films with immobile interfaces. Intermolecular forces, in particular, non-DLVO hydrophobic attractive forces, have the most significant effect on the film drainage.

Hydrophobic attraction forces in asymmetric aqueous films between hydrophobized mica/bare mica surfaces

The water-structure-based, quasi-thermodynamic theory published several years ago of hydrophobic interaction forces in symmetric aqueous films [J.C. Eriksson, S. Ljunggren, P.M. Claesson, J. Chem. Soc., Faraday Trans. 2 (85) (1989) 163] has been generalized to encompass asymmetric films between, e.g. a hydrophobized mica surface and a bare mica surface. The interaction pressures derived on this basis are in good agreement with the experimental data recorded by Claesson et al. [P.M. Claesson, P.C. Herder, C.E. Blom, B.W. Ninham, J. Colloid Interface Sci. 118 (1987) 68]. Hence, additional support is gathered for the original claim that the hydrophobic attraction is related with hydrogen-bond-dependent cluster formation processes in water contacted with a hydrophobic solid surface.

Demonstration of a minimum in the recovery of nanoparticles by flotation: Theory and experiment

The flotation of nano- and submicron particles does not follow the conventional collection theory based on the interception and collision mechanisms, which predicts extremely low collection efficiency for particles smaller than 10 - μ m . Brownian diffusion and colloidal forces strongly influence the collection of such particles by air bubbles in flotation. In this paper, a theoretical model is presented for predicting the collection efficiency of nanoparticles. The theory incorporates mass transfer by Brownian diffusion, microhydrodynamics of particles in the vicinity of a slip surface of rising air bubbles, and colloidal interactions that come into effect at small separation distances. The governing equation was solved numerically using the Crank–Nicolson method with variable step size. A finite difference scheme with mesh refinement in the vicinity of the air bubble surface was used to discretise the stiff partial differential equation for the particle concentration. The mesh refinement produced correct numerical solutions without oscillation in the particle concentration distribution, which otherwise occurred due to the stiffness of the differential equation and coarseness of the numerical mesh. Predictions from the model were compared with experimental results obtained with a small laboratory column cell, in which colloidal silica particles with diameters in the range 40 – 160 nm were floated using fine bubbles of typical average diameter 150 μ m . The particle concentration in the pulp was about 1% by weight. Cetyltrimethyl ammonium bromide and Dowfroth 250 were used as the flotation collector and frother, respectively. Both the theory and experiment show significant effect of the electrical double-layer and non-DLVO hydrophobic attractive forces on the collection of nanoparticles by air bubbles. The theoretical and experimental results show the collection efficiency to have a minimum at a particle size in the order of 100 nm . With larger particles, the interception and collision mechanisms predominate, while the diffusion and colloidal forces control the collection of particles with a size smaller than the transition size.

Investigation on the Mechanism of Crystallization of Soluble Protein in the Presence of Nonionic Surfactant

The mechanism of crystallization of soluble, globular protein (lysozyme) in the presence of nonionic surfactant C 8E 4 (tetraoxyethylene glycol monooctyl ether) was examined using both static and dynamic light scattering. The interprotein interaction was found to be attractive in solution conditions that yielded crystals and repulsive in the noncrystallizing solution conditions. The validity of the second virial coefficient as a criterion for predicting protein crystallization could be established even in the presence of nonionic surfactants. Our experiments indicate that the origin of the change in interactions can be attributed to the adsorption of nonionic surfactant monomers on soluble proteins, which is generally assumed to be the case with only membrane proteins. This adsorption screens the hydrophobic attractive force and enhances the hydration and electrostatic repulsive forces between protein molecules. Thus at low surfactant concentration, the effective protein-protein interaction remains repulsive. Large surfactant concentrations promote protein crystallization, possibly due to the attractive depletion force caused by the intervening free surfactant micelles.

Mechanisms of Heat-induced Antigen Retrieval: Does pH or Ionic Strength of the Solution Play a Role for Refolding Antigens?

We investigated the effects of pH and ionic strength of solutions used for antigen retrieval to elucidate the mechanism of heat-induced antigen retrieval (HIAR) in immunohistochemistry. The immunostaining intensity of nuclear, cytoplasmic, cell membrane, and extracellular matrix antigens with 17 different antibodies was evaluated in formaldehyde-fixed and paraffin-embedded mouse and human tissues. Deparaffinized sections were autoclaved for 10 min in buffers with different pH values ranging from 3.0 to 10.5. To test the influence of ionic strength on immunoreactions, the sections were autoclaved for 10 min in 20 mM Tris-HCl buffers (TB) at pH 9.0 and 10.5 with or without 25, 50, and 100 mM NaCl. There were two immunostaining patterns for pH dependency of HIAR. First, the majority of antibodies recovered their antigenicity when heated in the buffers with both acidic pH (pH 3.0) and basic pH (pH 9.0 and 10.5). Second, some antibodies showed strong immunostaining only at basic pH values (pH 9.0 and 10.5). When the sections were autoclaved in TB at pH 9.0, immunostaining of all eight antibodies examined decreased as the NaCl concentration increased. On the other hand, when the sections were treated with TB at pH 10.5, all antibodies yielded stronger reactions in the buffer containing NaCl than in the buffer without NaCl; five antibodies exhibited the strongest immunoreaction at concentrations from 25 to 50 mM. These results suggest that the extended polypeptides by heating are charged negatively or positively at basic or acidic pH, and that an electrostatic repulsion force acts to prevent random entangling of polypeptides caused by hydrophobic attractive force and to expose antigenic determinants, during cooling process of HIAR solution.

Hydrophobic Attraction As Revealed by AFM Force Measurements and Molecular Dynamics Simulation

Spherical calcium dioleate particles ( approximately 10 mum in diameter) were used as AFM (atomic force microscope) probes to measure interaction forces of the collector colloid with calcite and fluorite surfaces. The attractive AFM force between the calcium dioleate sphere and the fluorite surface is strong and has a longer range than the DLVO (Derjaguin-Landau-Verwey-Overbeek) prediction. The AFM force between the calcium dioleate sphere and the mineral surfaces does not agree with the DLVO prediction. Consideration of non-DLVO forces, including the attractive hydrophobic force and the repulsive hydration force, was necessary to explain the experimental results. The non-DLVO interactions considered were justified by the different interfacial water structures at calcite- and fluorite-water interfaces as revealed by the numerical computation experiments with molecular dynamics simulation.

Colloidal forces between bitumen surfaces in aqueous solutions measured with atomic force microscope

Colloidal forces between bitumen surfaces in aqueous solutions were measured with an atomic force microscope (AFM). The results showed a significant impact of solution pH, salinity, calcium and montmorillonite clay addition on both long-range (non-contact) and adhesion (pull-off) forces. Weaker long-range repulsive forces were observed under conditions of lower solution pH, higher salinity and higher calcium concentration. Lower solution pH, salinity and calcium concentration resulted in a stronger adhesion forces. The addition of montmorillonite clays increased long-range repulsive forces and decreased adhesion forces, particularly when co-added with calcium ions. The measured force profiles were fitted with extended DLVO theory to show the repulsive electrostatic double layer and attractive hydrophobic forces being the dominant components in the long-range forces between the bitumen surfaces. At a very short separation distance (less than 4–6 nm), a strong repulsion of steric origin was observed. The findings provide a fundamental understanding of bitumen emulsion stability and a mechanism of bitumen “aeration” in bitumen recovery processes from oil sands.

Tailoring HASE rheology through polymer design: Effects of hydrophobe size, acid content, and molecular weight

Hydrophobically modified alkali-soluble emulsion (HASE) polymers are an important class of rheology modifiers for waterborne coatings applications. They are typically prepared as terpolymers by emulsion polymerization of ethyl acrylate (EA), methacrylic acid (MAA), and an associative macromonomer. The viscosity development and shear responses of HASE solutions depend on a number of factors. This article presents rheological data reflecting the impacts of three key variables: hydrophobe size, acid content, and molecular weight, on model HASE thickening and rheological performance. The relative contributions of hydrophobic association, chain expansion, and polymer chain length are discussed. In steady shear flow, all thickener solutions approached some constant low-shear viscosity at small deformation rates. At the same molar composition, larger hydrophobe size resulted in higher viscosity development and greater shear thinning behavior. The amount of acid monomer in HASE polymers can influence the balance between hydrophobic attraction and electrostatic repulsion forces. It was found that a minimum of 15 wt% MAA was required to effect dissolution and thickening of the model HASE polymers. Increasing the MAA level yielded higher zero-shear viscosity and storage modulus G’ with maximal values being obtained at 40% MAA. The molecular weight of the model thickeners was controlled by the amount of chain transfer agent (CTA) added during polymerization. When the CTA level was below 0.1 wt% based on total monomers, the polymer solutions displayed shear-thinning behavior. A small increase in CTA concentration beyond 0.1% resulted in a dramatic change to Newtonian flow, and the solution viscosity was nearly two orders of magnitude lower. The model thickeners were also tested in a vinyl acrylic architectural paint formulation. The effects of each individual factor on paint thickener efficiency, high-shear, and low-shear properties are discussed and compared with solution rheology for predictive relationships.

Transport and Retention Mechanisms of Colloids in Partially Saturated Porous Media

The transport, retention, and release of hydrophobic and hydrophilic polystyrene latex microsphere colloids were examined in 0.5-cm-thick, 26-cm-long slab chambers filled with either regular (hydrophilic) or weakly water-repellent sand. The water-repellent sand consisted of a mixture of 0.4% strongly water-repellent grains with unmodified regular sand for the remainder. The concentration of colloids in the outflow water was measured at the same time as the pore-scale distribution of colloids was recorded in still and video images. Although the type of sand affected the flow pattern in the top of the chamber, it did not affect the breakthrough for the same type of colloids. More hydrophilic colloids were eluted in the drainage water than hydrophobic colloids. Images showed that there was a greater retention of the hydrophobic colloids due to strongly attractive hydrophobic interaction forces between colloids and subsequent filtering of colloidal aggregates in the narrow passages between grains. Once filtered, these aggregates then served as preferred sites for attachment of other hydrophobic colloids. The hydrophilic colloids were retained primarily in a thin film of water at the edge of the menisci, the air–water–solid (AWS) interface. Centrifugal motion within the pendular rings observed in the videos contributed to movement of the colloids toward the AWS interface, where colloids were retained due to both low laminar flow velocities near the grain surface and straining in the thin water film at the edge of the meniscus. Except near the solid interface, sorption at the air–water (AW) interface was not observed and appeared unimportant to the retention of colloids. The findings form an essential link between colloid retention and transport processes at the interfacial, pore, and Darcy scales.

AFM Study of the Behavior of Polystyrene and Glass Particles during the Electrodeposition of Copper

In this paper, the behavior of polystyrene and glass particles on a copper electrode during the electrodeposition of copper was studied using an atomic force microscope (AFM). Polystyrene or glass particles glued to the tip of the AFM cantilever were kept in contact with the surface of the electrode. The surface forces between the polystyrene or glass particle and the copper electrode were measured before, during, and after electrodeposition. These experiments revealed that glass particles do not make contact with the electrode, probably due to the repulsive hydration force. Polystyrene particles, on the other hand, make contact with the electrode, due to the attractive hydrophobic force. The AFM experiments were correlated with sedimentation co-deposition experiments of polystyrene and glass particles with copper. It was found that 80% of the polystyrene particles added to the plating solution incorporated with copper, while only 0.25% of the glass particles co-deposited under the same conditions.

Hydrophobic Interactions in Concentrated Colloidal Suspensions: A Rheological Investigation

Rheological investigations have determined the influence of particle hydrophobicity and solution pH on particle interactions in concentrated suspensions of colloidal silica (advancing water particle contact angles in the range ∼35° to ∼90° have been explored and obtained through surface chemical modification). Suspensions of hydrophilic silica (φ = 0.55) showed negligible viscoelasticity and a pH dependent apparent yield stress. The inverse relationship between the yield stress and the particle ζ-potential squared confirmed that electrical double layer and dispersion forces control particle interactions. On increasing the particle contact angle of silica there is a substantial increase in suspension rheology, i.e., pronounced viscoelasticity and apparent yield stresses, thus suggesting the action of an attractive hydrophobic force. Attempts have been made to deconvolute the various interaction forces, quantify the hydrophobic interaction, and explain the apparent pH dependent non-DLVO behavior.

Wetting Films of the Aqueous Cationic Polyelectrolyte Solutions on Quartz

It was shown that the stability of the wetting films of aqueous cationic polyelectrolyte solutions on the flat quartz surface depends on solution concentration. At a low concentration, the films are stable owing to the electrostatic repulsive forces. At a high polyelectrolyte concentration, the films are unstable due to the hydrophobization of quartz and the appearance of the hydrophobic attractive forces in the films. In the intermediate concentration range, the films are metastable and their lifetime depends upon the competition between the electrostatic repulsive and hydrophobic attractive forces. Thus, the concentration of cationic polyelectrolyte substantially affects the wetting conditions of the quartz surface. This conclusion can also be extended to other solid surfaces negatively charged in aqueous solutions, which is inherent to the majority of natural materials.

Disruption of hydrophobic stability of biomembranes is the earliest event in several clinical disorders

Hydrophobic attractive force is the major force in maintaining the stability of biomembranes, yielding coordinated functionality to the embedded proteins that they contain. This force between the composite linear hydrocarbons of the biomembranes is a function of their length and their mutual parallel distance from each other, and is extremely sensitive to this distance. Extracellular, natural linear hydrocarbons of certain length and shape can intercalate into lipid matrix of the biomembranes, reducing their innate hydrophobic net strength in a concentration-dependent manner, making them loose, leaky, and thus gaining the credence of stimulus-generating agents. In physiological circulatory concentration, these molecules may have a role for the maintenance of cellular homeostasis. However, in stagnating physiological excess, these same agents can become acutely or chronically stimulating and, therefore, disease-precipitating. Such situations do exist in the clinical disorders of acne, atherosclerosis, acute pancreatitis, diabetes, diabetic retinopathy, homocysteinemea, and stress. A systematic approach, beginning with surface film studies with the suspect linear hydrocarbons, can be followed up with in vitro and in vivo studies. This should substantiate or negate the view presented here. Isolated information, along these lines, already exist in literature. The example of acne is a suitable starting point to elaborate this view, for sebaceous gland of the human pilosebaceous unit (PSU) contains all the exemplary, stimulus-inducing linear hydrocarbons to generate surface-reaction on the pilosebaceous ductal surface.

Miltefosine−Cholesterol Interactions: A Monolayer Study

Mixed Langmuir monolayers of miltefosine (hexadecylphosphocholine) and cholesterol have been investigated by recording surface pressure-area (pi-A) isotherms at different subphase pHs (2, 6, and 10) and temperatures (10, 20, 25, and 30 degrees C). The change of both pH and temperature within the investigated range does not modify significantly the behavior of mixed films. The most pronounced effect involves condensation of the miltefosine monolayer by cholesterol, which diminishes in the following order: pH 6 > pH 2 > pH 10. The analyses of pi-A and compressibility modulus dependencies indicate the existence of interactions in the investigated system; at pH 2 and 6, the strongest were found to occur for the mixed film of miltefosine molar fraction (XM) between 0.6 and 0.7 (mean value, 0.66). Such a composition corresponds to the stable complex formation wherein 2 miltefosine molecules and 1 molecule of cholesterol are strongly bound together, mainly by attractive hydrophobic forces between their apolar tails. However, at pH 10 the highest stability occurs for mixtures containing a smaller proportion of miltefosine (XM = 0.5), which means that on alkaline subphases the ability to condense the miltefosine monolayer by cholesterol is less efficient as it requires a higher proportion of cholesterol (1:1 as compared to 1:2 at pH 2 and 6) to attain the maximum stability of the mixed film. The attractive forces between miltefosine and cholesterol are also weaker at pH 10 due to a greater solvatation of the miltefosine polar group. A similar trend is observed on increasing subphase temperature, when monolayers are more expanded.

Effect of Degassing on the Formation and Stability of Surfactant-Free Emulsions and Fine Teflon Dispersions

The results reported here demonstrate that the formation of surfactant-free, oil-in-water emulsions is significantly enhanced by the almost complete removal of dissolved gases. However, in an apparent contradiction, the reintroduction of dissolved gases does not destabilize already formed emulsions. Removal of dissolved gases enables the formation of much more turbid and stable hydrocarbon oil in water emulsions, which can be further studied without the effects of various additives, often needed as stabilizers. The observations reported here suggest that the dispersion of oil droplets in water is opposed by an attractive, hydrophobic force, which is dependent on the presence of dissolved gas. However, once formed, the stability of the emulsions appeared to be unaffected by any attractive hydrophobic force, in either gassed or degassed conditions. Degassing was also found to enhance the dispersion of fine Teflon particles in water.

PH-Dependent Effects of Chlorpromazine on Liposomes and Erythrocyte Membranes

Chlorpromazine (CPZ) is an amphipathic antipsychotic drug that binds to erythrocytes reaching in this way the central nervous system. CPZ is a basic molecule with pK = 8.6. This paper reports on CPZ-induced lysis of red blood cells and liposomes. Haemolysis was tested under hypotonic conditions, in the pH range 5.0–10.0. Cell sensitivity towards CPZ increased with increasing pH. Increasing pH caused also a decrease in the critical micellar concentrations of CPZ. These results are interpreted in terms of a competition between repulsive electrostatic forces and attractive hydrophobic forces, that would act both in pure CPZ and in mixed CPZ-phospholipid micelles. In order to eliminate possible pH effects mediated by red blood cell proteins, experiments were carried out in which CPZ induced release of a fluorescent dye from liposomes (large unilamellar vesicles). The latter observations confirmed that membrane sensitivity towards CPZ was increased at higher pH.

Modification of the Quartz Surface by the Adsorption of a Cationic Polyelectrolyte

A capillary electrokinetics method is applied to measure the electrokinetic potential of the surface of quartz capillaries during the continuous flow of the aqueous solutions of a cationic polyelectrolyte (PE). At a low polymer concentration (10–5 g/l), the adsorption is determined mainly by the electrostatic forces and its kinetics, by the conformational rearrangement of adsorbed macromolecules. As the concentration of PE increases, the charge of quartz surface reverses; and further adsorption is due to the forces of hydrophobic and molecular attraction between macromolecules. The adsorption energy is estimated for this case. The charge reversal of the surface is associated with the presence of adsorption sites of two different types. At a low concentration of PE, the adsorption takes place on negatively charged sites of quartz surface. At higher concentrations of PE, after the neutralization of the surface, the adsorbed PE molecules become new adsorption sites, and the adsorption acquires two-layer character. After adsorption, the quartz surface is hydrophobized: the contact angle measured by the bubble method is close to 33°–34°.