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At least 793 records · Page 44Linked to original sources
PubMed2006

A Monte Carlo simulation of the aggregation, phase-separation, and gelation of model globular molecules.

Monte Carlo computer simulation on a square 3-D lattice is used to model state behavior of globular copolymers. Two types of globular molecules were defined. One consisted of a single type of subunit (a homopolymer) while the second contained a core of strongly attractive subunits and an outer layer of less strongly attractive subunits (a heteropolymer). Systems of globules were simulated at varied volume fraction (V(F)) and reduced temperature (T(R)), and state diagrams were constructed. These state diagrams contained state boundaries defined by the V(F)/T(R) combinations at which the system formed a percolating network and at which the various component subunits in the globule unfolded. Simulated systems could exist in a number of states (between 4 and 7), depending on the V(F), T(R), whether the molecule was a homo- or heteroglobule and whether the globules were allowed to interact with each other or not. All systems exhibited a gelation/crossover line that resembled a lower critical solution temperature. All systems also exhibited a critical gelation concentration, above which a continuous network was formed. The critical gelation concentration varied between about 2-4% V(F) depending on the type of system. This is comparable to experimental critical gelation concentrations of in the region of 4% (w/w) for a range of associating polymers and biopolymers such as globular proteins and polysaccharides. Other states were formed which included one where elongated, fibril-like aggregated strands were formed, and a micelle-like aggregated state. The results are discussed in terms of the known state behavior of associating polymers and biopolymers (proteins and polysaccharides).

Algorithms↗
PubMed2004

Electrohydrodynamic effect on phase separation morphology in polymer blend films.

We have investigated the effect of electrohydrodynamic (EHD) convection on the domain structure in a polystyrene (PS)/polyvinyl acetate (PVA) blend film to demonstrate the feasibility of using the EHD effect as a means of mixing and morphology control in a polymer blend film prepared by solvent evaporation. Here, polymers-toluene solutions were spread on a glass substrate with patterned electrodes to apply a dc electric field, and well-defined structures of EHD convection were formed in the polymer solutions. As a result, regular patterns were formed in the PS/PVA polymer blend film in which PVA-rich domains were confined within each unit of patterned electrodes, i.e., between positive and negative electrodes, at an appropriate electric voltage. In addition, it was demonstrated that such novel morphology is not due to the wetting/dewetting effect of polymer components to the Pt electrodes deposited on the glass substrate, by experiments with a SiO2-covered substrate.

Journal Article↗
PubMed2004

Phase separations in pH-responsive polyelectrolyte multilayers: charge extrusion versus charge expulsion.

Polyelectrolyte multilayers with continuously variable amounts of ionizable weak acid functionality were prepared by blending ionizable and nonionizable polyelectrolytes in the deposition solutions. Diluting ionizable groups in this way yielded multilayers that were more structurally stable, shown by thickness and atomic force microscopy measurements, as their internal polymer charge was varied by the pH of the external solution. Multilayers prepared with opposite surface charge to that appearing within the bulk (as a result of ionization) were more stable, as were thinner films, both results suggesting the extrusion of bulk charge to the surface. These multilayers were able to control the direction and magnitude of electroosmotic flow in microfluidics systems. Multilayers bearing only one, diluted layer ofionizable material were surprisingly effective in this respect.

Electrolytes↗
PubMed2005

Honeycomb pattern formation via polystyrene/poly(2-vinylpyridine) phase separation.

The surface morphologies and properties of polystyrene (PS)/poly(2-vinylpyridine) (PVP) blend films cast on the mica substrate from ethylbenzene solution were investigated upon controlling different weight ratios and solvent evaporation rates. A near-honeycomblike surface morphology of the PS/PVP blend film formed under controlling the solvent evaporation rate due to the effect of Marangoni-Benard convection. The results of static water contact angles, X-ray photoelectron spectroscopy, solvent selective etching, and treatment by water illustrated that the near-honeycomblike structures on the surface of PS/PVP blend films were different for different weight ratios of PS and PVP. After treatment with water for several minutes, PVP islands-like structure emerged in the holes of the film for a PS/PVP weight ratio of 4/1, and a quasihexagonal arrangement of alternate big and small PVP droplets emerged on the top layer of the film for a PS/PVP weight ratio of 7/1. The formation mechanisms of different surface structures and their response behaviors to water were discussed.

Journal Article↗
PubMed2006

Phase separation of a mixed self-assembled monolayer prepared via a stepwise method.

Self-assembled monolayers (SAMs), a molecular-level assembly that forms spontaneously, provide a vehicle for investigating specific interactions at interfaces. This is particularly true for mixed SAMs that are composed of organosilanes with different chain lengths and/or chemical functionalities because they offer an adjustable surface for constructing 3D structures containing a variety of moieties. We recently observed that coadsorbed monolayers with different organosilanes on a Si wafer were separated into several tens or hundreds of nanometer domains that were rich in individual components. Several organosilanes, such as octadecyltrichlorosilane (OTS), octadecyltrimethoxysilane (OTMS), (3-mercaptopropyl)trimethoxysilane (MPTMS), and (3-aminopropryl)trimethoxysilane (APTMS), were used for regional separation. In this study, we propose a stepwise deposition method, namely, the deposition of a second siliane on a SAM substrate that creates intentional defects in the first silane. The surface morphologies were adjusted by the deposition sequence and immersion time of the silanes. As a result, a mixed SAM prepared by the proposed method showed effectively functionalized films compared to that prepared by the one-step method.

Gold↗
PubMedDate not supplied

One-pot glyco-affinity precipitation purification for enhanced proteomics: the flexible alignment of solution-phase capture/release and solid-phase separation.

A one-pot affinity precipitation purification of carbohydrate-binding protein was demonstrated by designing thermally responsive glyco-polypeptide polymers, which were synthesized by selective coupling of pendant carbohydrate groups to a recombinant elastin-like triblock protein copolymer (ELP). The thermally driven inverse transition temperature of the ELP-based triblock polymer is maintained upon incorporation of carbohydrate ligands, which was confirmed by differential scanning calorimetry and (1)H NMR spectroscopy experiments. As a test system, lactose derivatized ELP was used to selectively purify a galactose-specific binding lectin through simple temperature-triggered precipitation in a high level of efficiency. Potential opportunities might be provided for enhanced proteomic, cell isolation as well as pathogen detection applications.

Amino Acid Sequence↗
PubMed2000

A novel mimetic enzymatic fluorescence immunoassay for hepatitis B surface antigen by using a thermal phase separating polymer.

Iron tetrasulfonatophthalocyanine (FeTSPc), a peroxidase mimic, was used as a labeling reagent and poly(N-isopropylacrylamide) (PNIP) as the separation support of the immune complex for the mimetic-enzymatic immunoassay of hepatitis B surface antigen (HBsAg). PNIP was precipitated from aqueous solution when the ambient temperature was higher than its lower critical solution temperature of 31 degrees C. In a sandwich immunoassay, the antigen (HBsAg) first reacted with mouse anti-human HBsAg antibody immobilized on PNIP (PNIP-antibody) and then further reacted with FeTSPc-labeled mouse anti-HBsAg antibody (antibody-FeTSPc) at room temperature in a homogeneous format. After changing the temperature to separate the PNIP-antibody-HBsAg-antibody-FeTSPc conjugate moiety, it was re-dissolved and determined by coupling with the fluorogenic reaction of hydrogen peroxide and p-hydroxyphenylpropionic acid. The sensitivity of this method (3 ng mL-1) was close to that of the traditional ELISA using the same reactants. However, the assay was much faster (the assay time decreased from 100-120 to 45 min). This method was applied to determine HBsAg in human serum with satisfactory results.

Hepatitis B Surface Antigens↗
PubMed2006

Stability and phase separation in mixed self-assembled monolayers.

Recent single molecule experiments rely on the self-assembly of binary mixtures of molecules with very different properties in a stable monolayer, in order to probe the characteristics of the interspersed molecule of interest in a controlled environment. However, not all efforts at coassembly have been successful. To study systematically the behavior of such systems, we derive the free energy of multicomponent systems of rods with configurational degrees of freedom, localized on a surface, starting from a generalized van der Waals description. The molecular parameters are determined by geometrical factors of the molecules and by their pairwise van der Waals interactions computed using molecular mechanics. Applying the model to two experimental situations, we are able to use the stability analysis of the respective mixtures to explain why coassembly was successful in one set of experiments (carotene and alkanethiol) and not in another (benzenethiols and alkanethiol). We outline general guidelines for suitable choices of molecules to achieve coassembly.

Journal Article↗
PubMed1973

On the Relation Between lambda Lines and Phase Separations.

It has often been assumed that the slope of the isotherm involving a pair of secondary variable vanishes along a lambda line [for example, along a lambda line in the pressure-volume plane ([unk]P/[unk]V)(T) vanishes], and therefore that the specific heat for constant extensive variable (e.g., C(V)) has the greatest possible value on the lambda line and so obeys the Buckingham-Fairbank relation. It is shown here by a heuristic theoretical argument that in (3)He-(4)He solutions ([unk]mu(4)/[unk]x(3))(T) and ([unk]mu(3)/[unk]x(3))(T) probably do not vanish and C(x3) does not have its maximum possible value, although it may become infinite when x(3) --> 0. (mu(4) and mu(3) are chemical potentials of (4)He and (3)He and x(3) is the molefraction of (3)He). Only at the tricritical point does ([unk]mu(4)/[unk]x(3))(T) finally vanish and C(x3) have a value, which cannot be exceeded without the system's becoming unstable. In the case of the transition in solid NH(4)Cl the experimental facts seem to indicate that at the higher temperatures, where the transition is of higher order, ([unk]P/[unk]V)(T) does not become zero along the transition line. A statistical thermodynamic description of tricritical points is given, and shown to accord qualitatively with the experimental results for the (3)He-(4)He solutions. There is evidence that any singular behavior at the tricritical point in (3)He-(4)He is already present along the lambda line. Finally, an analysis is made of the possible behavior of binary liquid solutions, and it is shown that a tendency of C(V) to exceed its maximum value can result in a flat top on the coexistence curve.

Journal Article↗
PubMed1982

Charcoal as a phase separating agent in ligand assays: mechanism of action and the effect of dextran and various proteins on the adsorption of of small molecules.

In ligand assays, charcoal mixed with dextran is sometimes used to separate free from bound ligand. This is done with the view that dextran "coats" the charcoal and produces a sieve effect on its surface. To further elucidate the mechanism of differential adsorption of small molecules to charcoal, studies were made on the interaction between a number of commonly measured ligands and charcoal, either unmixed or mixed with dextran, albumin, immunoglobulin (IgG) or insulin. "Coating" with dextran was shown not to have any effect on subsequent adsorption of the various ligands. However, coating charcoal with albumin, IgG or insulin did have an effect and even augmented subsequent adsorption of small molecules under certain conditions. It is concluded that the use of "dextran coating" of charcoal to give a sieve effect for the separation of small molecules from large ones is unnecessary and has no basis in fact. This is not the case for coating charcoal with proteins.

Animals↗