Biomedical subjects

Alexei R Khokhlov

Publications and source records attributed to Alexei R Khokhlov.

At least 19 recordsLinked to original sources

PubMed2007

Self-assembled networks highly responsive to hydrocarbons.

Rheological studies were performed with aqueous salt solutions of anionic surfactant potassium oleate and its mixtures with hydrophobically modified polyacrylamide. Semidilute solutions of the surfactant in the presence of salt (KCl) demonstrate viscoelastic properties due to the formation of a transient network of entangled wormlike micelles. These systems are highly responsive to hydrocarbons: the addition of n-heptane or n-dodecane reduces the viscosity of solutions by up to 4 to 5 orders of magnitude, thus inducing the transition of a gellike system to a fluid one. It is the transformation of cylindrical surfactant micelles into spherical ones upon absorption of hydrocarbon that disrupts the network. The addition of a small amount (0.5 wt %) of associating polymer leads to up to a 5000-fold increase in the zero-shear viscosity and enhances the susceptibility to hydrocarbons. SANS data show that independently of the presence of polymer the radius of wormlike micelles is roughly equal to the length of a surfactant molecule, whereas the radius of spheres formed upon the absorption of hydrocarbon is 2-2.5-fold higher. A possible structure of the spherical micelles is discussed.

Journal Article↗

PubMed2006

Why ionic liquids can possess extra solvent power.

We use the Flory-Huggins theory to demonstrate conditions of extra solvent power of ionic liquids. The short-range interactions between anions, cations, and molecules of the solute are taken into account. We find that solvent power of the ionic liquids is enhanced if non-Coulomb interactions between the anions and cations are repulsive. The mechanism responsible for the extra solvent power is related to the "shielding" of the anion-cation interactions by the molecules of the solute.

Letter↗

PubMed2006

Recognition of complex patterned substrates by heteropolymer chains consisting of multiple monomer types.

We propose a statistical mechanical model of surface pattern recognition by heteropolymers with quenched monomer sequence distribution. The chemically heterogeneous pattern consists of different adsorption sites specifically distributed on a surface. The heteropolymer sequence is complementary with respect to the pattern. The concepts of recognition probability and recognition temperature are introduced. The algorithm for calculating the recognition probability is based on efficient recurrence procedures for evaluating the single-chain partition function of a chain macromolecule consisting of multiple monomer types, which interact with multiple types of adsorption sites. The temperature dependencies of the recognition probability are discussed. We address the critical role of the commensurability between the heteropolymer sequence and the distribution of the surface adsorbing sites on the polymer adsorption. Also, we address the question of how many types of monomer units in the heteropolymer are required for unambiguous recognition of compact target patterns. It is shown that perfect pattern recognition can be achieved for the strong-adsorption regime in the case of specifically structured compact patterns with multifunctional adsorption sites and heteropolymers with multiple monomer types when the degeneracy of the ground state is suppressed. The pattern recognition ability increases with the number of different types of monomer units and complementary adsorption sites. For random heteropolymers and patterns, the free energy change associated with the recognition process decreases linearly with increasing this number. Correlated random heteropolymers are capable of recognizing related patterns on a random background.

Journal Article↗

PubMed2006

Self-assembly of (perfluoroalkyl)alkanes on a substrate surface from solutions in supercritical carbon dioxide.

Toroidal self-assembled structures of perfluorododecylnonadecane and perfluorotetradecyloctadecane have been deposited on mica and highly oriented pyrolytic graphite surfaces by exposure of the substrates to solutions of the (pefluoroalkyl)alkanes in supercritical carbon dioxide. Scanning force microscopy (SFM) images have displayed a high degree of regularity of these self-assembled nanoobjects regarding size, shape, and packing in a monolayer. Analysis of SFM images allowed us to estimate that each toroidal domain has an outer diameter of about 50 nm and consists of several thousands of molecules. We propose a simple model explaining the clustering of the molecules to objects with a finite size. The model based on the close-packing principles predicts formation of toroids, whose size is determined by the molecular geometry. Here, we consider the amphiphilic nature of the (perfluoroalkyl)alkane molecules in combination with incommensurable packing parameters of the alkyl- and the perfluoralkyl-segments to be a key factor for such a self-assembly.

Alkanes↗

PubMed2006

Semiflexible amphiphilic polymers: cylindrical-shaped, collagenlike, and toroidal structures.

A coarse-grained model is used to study the conformational properties of semiflexible polymers with amphiphilic monomer units containing both hydrophilic and hydrophobic interaction sites. The hydrophobically driven conformational transitions are studied using molecular dynamics simulations for the chains of varying stiffness, as characterized by intrinsic Kuhn segment lengths that vary over a decade. It is shown that the energy of hydrophobic attraction required for the realization of the coil-to-globule transition increases with increasing chain stiffness. For rather stiff backbone, the coil-to-globule transition corresponds to a first order phase transition. We find that depending on the chain stiffness, a variety of thermodynamically stable anisometric chain morphologies are possible in a solvent selectively poor for hydrophobic sites of amphiphilic monomer units. For flexible chains, the amphiphilic polymer forms a cylindrical globule having blob structure with nearly spherical blobs. With increasing stiffness, the number of blobs composing the globule decreases and the shape of blobs transforms into elongated cylinder. Further increase in stiffness leads to compaction of macromolecules into a collagenlike structure when the chain folds itself several times and different strands wind round each other. In this state, the collagenlike structures coexist with toroidal globules, both conformations having approximately equal energies.

Journal Article↗

PubMed2006

Morphology of hybrid polystyrene-block-poly(ethylene oxide) micelles: analytical ultracentrifugation and SANS studies.

Morphology and structure of aqueous block copolymer solutions based on polystyrene-block-poly(ethylene oxide) (PS-b-PEO) of two different compositions, a cationic surfactant, cetyl pyridinium chloride (CPC), and either platinic acid (H2PtCl6.6H2O) or Pt nanoparticles were studied using a combination of analytical ultracentrifugation (AUC), transmission electron microscopy (TEM), and small angle neutron scattering (SANS). These studies combining methods contributing supplemental and analogous structural information allowed us to comprehensively characterize the complex hybrid systems and to discover an isotope effect when H2O was replaced with D2O. In particular, TEM shows formation of both micelles and larger aggregates after incorporation of platinic acid, yet the amount of aggregates depends on the H2PtCl6.6H2O concentration. AUC reveals the presence of micelles and micellar clusters in the PS-b-PEO block copolymers solution and even larger (supermicellar) aggregates in hybrids (with CPC). Conversely, SANS applied to D2O solutions of the similar species indicates that micelles are spherical and no other micellar species are found in block copolymer solutions. To reconcile the SANS and AUC data, we carried out AUC examination of the corresponding D2O block copolymer solutions. These measurements demonstrate a pronounced isotope effect on micelle aggregation and micelle size, i.e., no micelle aggregation in D2O solutions, revealing good agreement of AUC and SANS data.

Calorimetry, Differential Scanning↗

PubMed2006

Adsorption of polyelectrolyte molecules to a nanostructured monolayer of amphiphiles.

We investigate the conformation of atactic poly(sodium 4-styrene sulfonate) (PSS) molecules both in water-ethanol solutions and adsorbed to a template provided by a nanostructured monolayer of flat-lying octadecylamines on graphite. The conformations of the PSS molecules in these solutions undergo two conformational transitions, a re-entry transition at 40 vol % and a coil-globule conformational transition at 80 vol % ethanol. On the surface the conformations are characterized by two different length scales: on a short length scale they are defined by the template, whereas the large-scale structures reflect the conformations in solution. We conclude that the conformational transitions are discontinuous on the level of single molecules and appear continuous in the ensemble.

Adsorption↗

PubMed2005

Metalated diblock and triblock poly(ethylene oxide)-block-poly(4-vinylpyridine) copolymers: understanding of micelle and bulk structure.

The paper provides new insights into the structure of Pt-containing diblock and triblock copolymers based on poly(ethylene oxide) (PEO) and poly(4-vinylpyridine) (P4VP), using a combination of atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and anomalous small-angle X-ray scattering (ASAXS). Parallel studies using methods contributing supplemental structural information allowed us to comprehensively characterize sophisticated polymer systems during metalation and to exclude possible ambiguity of the data interpretation of each of the methods. AFM and TEM make available the determination of sizes of the micelles and of the Pt-containing micelle cores, respectively, while a combination of XRD, TEM, and ASAXS reveals Pt-nanoparticle size distributions and locations along with the structural information about the polymer matrix. In addition, for the first time, ASAXS revealed the organization of Pt-nanoparticle-filled diblock and triblock copolymers in the bulk. The nanoparticle characteristics are mainly determined by the type of block copolymer system in which they are found: larger particles (2.0-3.0 nm) are formed in triblock copolymer micelles, while smaller ones (1.5-2.5 nm) are found in diblock copolymer micelles. This can be explained by facilitated intermicellar exchange in triblock copolymer systems. For both systems, Pt nanoparticles have narrow particle size distributions as a result of a strong interaction between the nanoparticle surface and the P4VP units inside the micelle cores. The pH of the medium mainly influences the particle location rather than the particle size. A structural model of Pt-nanoparticle clustering in the diblock PEO-b-P4VP and triblock P4VP-b-PEO-b-P4VP copolymers in the bulk was constructed ab initio from the ASAXS data. This model reveals that nearly spherical micellar cores of about 10 nm in diameter (filled with Pt nanoparticles) aggregate forming slightly oblate hollow bodies with an outer diameter of about 40 nm.

Micelles↗

PubMed2005

Effect of low-molecular-weight salt on the nematic ordering in solutions of rodlike polyelectrolytes.

Nematic ordering is studied in solutions of rodlike macroions in the presence of monovalent salt ions. The correlation free energy of electrostatic interactions is calculated within the framework of the Debye-Hückel approach. The phase diagram of the system is obtained. We have shown that the addition of salt destroys the weakly ordered nematic phase which is stable at small polymer concentrations. On the other hand, the salt widens the region of the phase separation between the isotropic (or weakly ordered nematic) and the highly ordered nematic phases at high polymer concentrations. An explanation of these effects is proposed.

Journal Article↗

PubMed2005

Core-shell nanostructures from single poly(N-vinylcaprolactam) macromolecules: stabilization and visualization.

Formation of core-shell poly(N-vinylcaprolactam) (PVCL) single-molecule nanostructures due to interaction of PVCL with metal ions was studied using transmission electron microscopy, 13C NMR, and light scattering. This study demonstrates that addition of CoCl2 to PVCL in its globular conformation yields unimolecular core-shell polymer particles with the core decorated with Co(II) ions. The crucial condition for formation of well-defined unimolecular nanostructures is the presence of stable globular aggregates in aqueous solution. Moreover, the metal ions should have a sufficiently high coordination number (higher than 2) to provide a cross-linking and stabilization of the core.

Caprolactam↗

PubMed2005

Adsorption of multiblock copolymers onto a chemically heterogeneous surface: a model of pattern recognition.

We present a statistical mechanical model, which is used to investigate the adsorption behavior of two-letter (AB) copolymers on chemically heterogeneous surfaces. The surfaces with regularly distributed stripes of two types (A and B) and periodic multiblock copolymers (Al)B(l))(x) are studied. It is assumed that A(B)-type segments selectively adsorb onto A(B)-type stripes. It is shown that the adsorption strongly depends on the copolymer sequence distribution and the arrangement of selectively adsorbing regions on the surface. The polymer-surface binding proceeds as a two-step process. At the first step, the copolymer having short blocks adsorbs onto the surface as an effective homopolymer, which does not feel chemical pattern. At the second step, when the polymer-surface attraction is sufficiently strong, the adsorbed chain adjusts its equilibrium conformation to reach the perfect bound state, thereby demonstrating ability for pattern recognition. The key element of this mechanism is the redistribution of strongly adsorbed copolymer diblocks A(l)B(l), which behave as surfactants, between multiple AB interfaces separating A and B stripes on the adsorbing surface. Such redistribution is accompanied by a well-pronounced decrease in the system entropy. We have found that marked pattern recognition is possible for copolymers with relatively short blocks at high polymer/surface affinities, beyond the adsorption threshold.

Journal Article↗

PubMed2005

Effect of the mobility of charged units on the microphase separation in amphiphilic polyelectrolyte hydrogels.

The effect of the migration of charged units on the structure of hydrophobically modified polyelectrolyte gels swollen by D(2)O was studied by small-angle neutron scattering on an example of gels of terpolymers of acrylic acid, n-dodecylacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid (quenched charged units) and gels of copolymers of partially neutralized acrylic acid and n-dodecylacrylate (annealed charged units). The content of charged units did not exceed 20 mol %, so that the electrostatic repulsion was too weak to disrupt the hydrophobic domains formed by self-assembled n-dodecyl chains, which was evidenced by NMR data. It was shown that upon increasing the charge content both types of gels undergo microphase separation with the formation of hydrophobic clusters consisting of several densely packed hydrophobic domains and hydrophilic regions swollen by water, where most of the charged repeat units and counterions are located. The dimensions of the nanostructure of the gels with quenched and annealed charged groups were compared. It was shown that the size of clusters in the gels with annealed charged units is much bigger than that in the gels with the same fraction of quenched charged units. This effect was attributed to a much weaker electrostatic repulsion in the corona of the hydrophobic clusters in the gels with annealed charged groups, because the charged units repelling each other are able to move farther apart.

Journal Article↗

PubMed2005

Rheology of viscoelastic solutions of cationic surfactant. Effect of added associating polymer.

Rheological studies were performed with aqueous salt solutions of viscoelastic cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) and its mixtures with hydrophobically modified polyacrylamide. The solutions of surfactant itself above the concentration of crossover of wormlike micelles exhibit two regions of rheological response. In the first region, they behave like polymer solutions in semidilute regime characterized by viscoelastic behavior with a spectrum of relaxation times. In the second region, unlike polymer solutions their relaxation after shear is dominated by a single relaxation time. Being composed of "living" micelles, the EHAC solutions easily lose their viscosity at the variation of the external conditions. For instance, heating from 20 to 60 degrees C reduces viscosity by up to 2 orders of magnitude, while added hydrocarbons induce a sudden drop of viscosity by 3-6 orders of magnitude. Polymer profoundly affects the rheological properties of EHAC solutions. The polymer/surfactant system demonstrates a 10,000-fold increase in viscosity as compared to pure-component solutions, the effect being more pronounced for polymer with less blocky distribution of hydrophobic units. A synergistic enhancement of viscosity was attributed to the formation of common network, in which some subchains are made up of elongated surfactant micelles, while others are composed of polymer. At cross-links the hydrophobic side groups of polymer anchor EHAC micelles. In contrast to surfactant itself, the polymer/surfactant system retains high viscosity at elevated temperature; at the same time it keeps a high responsiveness to hydrocarbon medium inherent to EHAC.

Cations↗

PubMed2004

Template copolymerization near a patterned surface: computer simulation.

We perform a Monte Carlo simulation of irreversible template copolymerization near a chemically heterogeneous surface with a regular distribution of discrete adsorption sites that selectively adsorb from solution one of the two polymerizing monomers and the corresponding chain segments. In the polymerization model, the chain propagation process is simulated by adding individual monomers to the end of growing macroradical. We focus in this paper on the influence of polymerization rate, adsorption energy, and the distance between adsorption sites on the chain conformation and the primary sequence of the resulting two-letter (AB) copolymers and, specifically, on the coupling between polymerization and adsorption. The conditions for the realization of conformation-dependent copolymerization are formulated. For this regime, we observe the formation of a quasiregular copolymer with two types of alternating sections. One of them contains randomly distributed A and B segments. The second one consists mainly of strongly adsorbed A segments. It is found that the average length of the random sections is proportional to the distance between the nearest neighbor adsorption sites. The average length of the A-rich sections is determined by the "adsorption capacity" of adsorption site. By varying the strength of the effective monomer-substrate interaction and the distribution of adsorption sites on the substrate, the copolymers with different surface-induced primary sequences can be designed and synthesized in a controlled fashion.

Journal Article↗

PubMed2004

Reversible collapse of brushlike macromolecules in ethanol and water vapours as revealed by real-time scanning force microscopy.

Environment-controlled scanning force microscopy allowed us to study adsorption and desorption of single poly(methacrylate)-graft-poly(n-butyl acrylate) brush molecules on mica in real time. The molecules transform reversibly from a two-dimensional, extended wormlike state to a compact globular state. The dynamics of the conformational transition was sufficiently slow in order to allow its observation by scanning force microscope in real time. The reversible transformation is effected by coadsorption of water or ethanol, the latter introduces the collapse. Adsorbing ethanol and water from the vapour atmosphere results in a change of the surface properties of mica, either favouring adsorption or desorption of the graft polymer. When the extended, tightly adsorbed poly(n-butyl acrylate) brush molecules are exposed to ethanol vapour, the macromolecules swell and contract to form compact globules. Exchanging the ethanol vapour to a humid atmosphere caused the molecules to extend again to a wormlike two-dimensional conformation. Coexistence of collapsed and extended strands within the same molecule indicates a single-molecule first-order transition in agreement with observations on Langmuir films previously reported.

Ethanol↗

PubMed2004

Nematic ordering in dilute solutions of rodlike polyelectrolytes.

Quantitative theory of orientational behavior of rodlike polyelectrolytes in dilute solution is developed. We find that in salt-free solutions many-body Coulomb interactions between macro- and counterions favor nematic ordering. It is shown that the orientationally isotropic phase of the solution becomes unstable toward nematic ordering at polymer concentration smaller than the overlap concentration. Our predictions are consistent with experimental observations for synthetic polyelectrolytes poly(p-phenylene)sulfonates in aqueous solutions.

Journal Article↗

PubMed2004

"Swiss-cheese" polyelectrolyte gels as media with extremely inhomogeneous distribution of charged species.

"Swiss-cheese" polyelectrolyte gels (i.e., gels containing a regular set of closed spherical pores) are considered as a suitable system for modeling of a medium with extremely inhomogeneous distribution of charged species. It is shown that the inhomogeneous distribution of ions in Swiss-cheese polyelectrolyte gels can be reached simply by immersion of the gels in an aqueous solution of charged species (e.g., low-molecular 1-1 salt or multivalent ions and macroions charged likely to the gel chains). If a polymer gel is kept in such a solution for a long time, the concentration of ions within relatively big voids becomes equal to that in external solution. On the other hand, due to the Donnan effect the ion's concentration in polymer matrix is always lower than that in external solution. As a result the multivalent ions distribute between water voids and polymer matrix. The extent of this distribution is characterized by partition coefficient kD (determined as ratio kD = n(s)(void)/n(s)(mat) of the concentrations n(s)(void) and n(s)(mat) of ions in water voids and in polymer matrix, correspondingly). It is shown that the partition coefficient kD can be larger than 10 for low-molecular salt, reaches 10(3) for bivalent ions, and is higher than 10(6) for tetravalent ions. In the case of polymer macroions the partition coefficient kD tends to infinity. Our calculations show that the lower limit of characteristic scales of heterogeneity (determined by water voids size starting from which the condition of total electroneutrality is fulfilled and effect of partition is the most pronounced) can be equal to tens of nanometers.

Journal Article↗

PubMed2004

Poisson-Boltzmann theory of the charge-induced adsorption of semi-flexible polyelectrolytes.

A model is suggested for the structure of an adsorbed layer of a highly charged semi-flexible polyelectrolyte on a weakly charged surface of opposite charge sign. The adsorbed phase is thin, owing to the effective reversal of the charge sign of the surface upon adsorption, and ordered, owing to the high surface density of polyelectrolyte strands caused by the generally strong binding between polyelectrolyte and surface. The Poisson-Boltzmann equation for the electrostatic interaction between the array of adsorbed polyelectrolytes and the charged surface is solved for a cylindrical geometry, both numerically, using a finite element method, and analytically within the weak curvature limit under the assumption of excess monovalent salt. For small separations, repulsive surface polarization and counterion osmotic pressure effects dominate over the electrostatic attraction and the resulting electrostatic interaction curve shows a minimum at nonzero separations on the Angstrom scale. The equilibrium density of the adsorbed phase is obtained by minimizing the total free energy under the condition of equality of chemical potential and osmotic pressure of the polyelectrolyte in solution and in the adsorbed phase. For a wide range of ionic conditions and charge densities of the charged surface, the interstrand separation as predicted by the Poisson-Boltzmann model and the analytical theory closely agree. For low to moderate charge densities of the adsorbing surface, the interstrand spacing decreases as a function of the charge density of the charged surface. Above about 0.1 M excess monovalent salt, it is only weakly dependent on the ionic strength. At high charge densities of the adsorbing surface, the interstrand spacing increases with increasing ionic strength, in line with the experiments by Fang and Yang [J. Phys. Chem. B 101, 441 (1997)].

Adsorption↗