Biomedical subjects

F A M Leermakers

Publications and source records attributed to F A M Leermakers.

At least 19 recordsLinked to original sources

PubMed2007

Negative compressibility and nonequivalence of two statistical ensembles in the escape transition of a polymer chain.

An end-tethered polymer chain compressed between two pistons undergoes an abrupt transition from a confined coil state to an inhomogeneous flowerlike conformation partially escaped from the gap. This phase transition is first order in the thermodynamic limit of infinitely long chains. A rigorous analytical theory is presented for a Gaussian chain in two ensembles: (a) the H-ensemble, in which the distance H between the pistons plays the role of the independent control parameter, and (b) the conjugate f-ensemble, in which the external compression force f is the independent parameter. Details about the metastable chain configurations are analyzed by introducing the Landau free energy as a function of the chain stretching order parameter. The binodal and spinodal lines, as well as the barrier heights between the stable and metastable states in the free energy landscape, are presented in both ensembles. In the loop region for the average force with dependence on the distance H (i.e., in the H-ensemble) a negative compressibility exists, whereas in the f-ensemble the average distance as a function of the force is strictly monotonic. The average fraction of imprisoned segments and the lateral force, taken as functions of the distance H or the average H, respectively, have different behaviors in the two ensembles. These results demonstrate a clear counterexample of a main principle of statistical mechanics, stating that all ensembles are equivalent in the thermodynamic limit. The authors show that the negative compressibility in the escape transition is a purely equilibrium result and analyze in detail the origin of the nonequivalence of the ensembles. It is argued that it should be possible to employ the escape transition and its anomalous behavior in macroscopically homogeneous, but microscopically inhomogeneous, materials.

Biopolymers↗

PubMed2006

On the binding of calcium by micelles composed of carboxy-modified pluronics measured by means of differential potentiometric titration and modeled with a self-consistent-field theory.

We perform differential potentiometric titration measurements for the binding of Ca2+ ions to micelles composed of the carboxylic acid end-standing Pluronic P85 block copolymer (i.e., CAE-85 (COOH-(EO)26-(PO)39-(EO)26-COOH)). Two different ion-selective electrodes (ISEs) are used to detect the free calcium concentration; the first ISE is an indicator electrode, and the second is a reference electrode. The titration is done by adding the block copolymers to a known solution of Ca2+ at neutral pH and high enough temperature (above the critical micellization temperature CMT) and various amount of added monovalent salt. By measuring the difference in the electromotive force between the two ISEs, the amount of Ca2+ that is bound by the micelles is calculated. This is then used to determine the binding constant of Ca2+ with the micelles, which is a missing parameter needed to perform molecular realistic self-consistent-field (SCF) calculations. It turns out that the micelles from block copolymer CAE-85 bind Ca2+ ions both electrostatically and specifically. The specific binding between Ca2+ and carboxylic groups in the corona of the micelles is modeled through the reaction equilibrium -COOCa+ <==> -COO- + Ca2+ with pKCa = 1.7 +/- 0.06.

Journal Article↗

PubMed2006

On the curvature dependence of the interfacial tension in a symmetric three-component interface.

We consider a symmetric interface between two polymers A(N) and B(N) in a common monomeric solvent S using the mean-field Scheutjens-Fleer self-consistent field theory and focus on the curvature dependence of the interfacial tension. In multi-component systems there is not one unique scenario to curve such an interface. We elaborate on this by keeping either the chemical potential of the solvent or the bulk concentration of the solvent fixed, that is we focus on the semi-grand canonical ensemble case. Following Helfrich, we expand the surface tension as a Taylor series in the curvature parameters and find that there is a non-zero linear dependence of the interfacial tension on the mean curvature in both cases. This implies a finite Tolman length. In a thermodynamic analysis we prove that the non-zero Tolman length is related to the adsorption of solvent at the interface. Similar, but not the same, correlations between the solvent adsorption and the Tolman length are found in the two scenarios. This result indicates that one should be careful with symmetry arguments in a Helfrich analysis, in particular for systems that have a finite interfacial tension: one not only should consider the structural symmetry of the interface, but also consider the constraints that are enforced upon imposing the curvature. The volume fraction of solvent, the chain length N as well as the interaction parameter chi(AB) in the system can be used to take the system in the direction of the critical point. The usual critical behavior is found. Both the width of the interface and the Tolman length diverge, whereas the density difference between the two phases, adsorbed amount of solvent at the interface, interfacial tension, spontaneous curvature, mean bending modulus as well as the Gaussian bending modulus vanish upon approach of the critical point.

Journal Article↗

PubMed2006

Can linear micelles bridge between two surfaces?

Surfactants are extensively used as stabilizers of colloidal particles, even though the use of high surfactant concentrations can induce a loss of the stability of the dispersion. The depletion mechanism is believed to be responsible for this instability. In this paper, we show that there exists an alternative interpretation, namely that wormlike micelles can bridge between two surfaces. Such a stalk-like object connecting two adsorbed bilayers is (in first order) stable when the endcap (free) energy of the wormlike micelle (in solution) is higher than the connection (free) energy of the stalk with the surface layer. As an example, we consider an aqueous solution of nonionic C(12)E(6) surfactants and use a molecularly realistic self-consistent field approach to evaluate the free-energy of bridge formation. It appears impossible to connect linear micelles to hydrophobic surfaces onto which a monolayer of surfactants exists, and stalks only occur with an exponentially low probability for very hydrophilic surfaces. However, at a wide regime of moderately hydrophilic surfaces the stalks are thermodynamically stable. In this regime, the adsorbed bilayers are typically only marginally stable. We identify a range of parameters for which such adsorbed bilayer ruptures around the stalk and then the wormlike micelle essentially connects (head-on) to the bare surface. The strength of interaction is of the order of the endcap energy which easily exceeds 10 k(B)T. The range of interactions is expected to be large as it is set by the characteristic size of the linear micelles in solution. The regime of moderately hydrophilic surfaces is relevant experimentally, and we conclude that surfactant-induced flocculation may well be the result of stalks. The depletion mechanism is only expected for systems with extremely hydrophobic and with very hydrophilic particles.

Adsorption↗

PubMed2006

Wetting phase diagrams of a polyacid brush with a triple point.

The (pre)wetting behavior of an annealed polyelectrolyte (PE) brush by an electrolyte solution that is strongly segregated from an apolar phase is analyzed. In this complex interface, there are interactions on various length scales. There are short-range interactions with the (uncharged) surface, and there are interactions on the length scale of the brush height. Using either the ionic strength or the water-surface interaction strength as the control parameters, it is possible to approach and induce a wetting transition in this system. The first-order wetting transition, promoted by favorable short-range substrate interactions with the surface, is in competition with the wetting transition controlled by the detachment of the fluid interface from the periphery of the PE brush. The electric double layer on top of the PE brush contributes with a repulsive forces to the disjoining pressure that tends to thicken the wetting film, and therefore, the transition in all cases is first order. Various phase portraits of the wetting phase diagram are envisioned. One of these features the crossing of two prewetting lines. At the crossing point three surface states coexist. This triple point is analyzed in some detail with the help of a molecular-level self-consistent field model.

Journal Article↗

PubMed2006

Symmetric liquid-liquid interface with a nonzero spontaneous curvature.

The curvature dependence of the symmetric interface between two immiscible polymer solutions in a common monomeric solvent is analyzed using a self-consistent field theory. Contrary to symmetry arguments we find that the surface tension depends in first order on a nonzero Tolman length. These interfaces further have a negative mean and a positive Gaussian bending modulus. The finite spontaneous curvature is attributed to the adsorption of the solvent at the interface.

Journal Article↗

PubMed2006

Confinement-induced symmetry breaking of interfacial surfactant layers.

Interaction forces between mesoscopic objects are fundamental to soft-condensed matter and are among the prime targets of investigation in colloidal systems. Surfactant molecules are often used to tailor these interactions. The forces are experimentally accessible and for a first theoretical analysis one can make use of a parallel-plate geometry. We present molecularly realistic self-consistent field calculations for an aqueous nonionic surfactant solution near the critical micellization concentration, in contact with two hydrophobic surfaces. The surfactants adsorb cooperatively, and form a monolayer onto each surface. At weak overlap the force increases with increasing compression of the monolayers until suddenly a symmetry braking takes place. One of the monolayers is removed jump-like and as the remaining monolayer can relax, some attraction is observed, which gives way to repulsion at further confinement. The restoring of symmetry at strong confinement occurs as a second-order transition and the force jumps once again from repulsion to attraction. It is anticipated that the metastable branch of the interaction curve will be probed in a typical force experiment. Under normal conditions pronounced hysteresis in the surface force is predicted, without the need to change the adsorbed amount jump-like.

Journal Article↗

PubMed2006

Self-consistent field modeling of linear nonionic micelles.

A self-consistent field theory is used to predict structural, mechanical, and thermodynamical properties of linear micelles of selected nonionic surfactants of the type C(n)()E(m). Upon increase in surfactant concentration the sudden micelle shape transition from spherical to cylindrical (second critical micelle concentration (cmc)) is analyzed. The cylindrical micelles consist of a body (with radius R(c) and length L) and two slightly swollen endcaps. For small L, the shape resembles a dumbbell. With increase in the length of the body, an oscillatory behavior in the grand potential of the micelle is found. The wavelength of the oscillation (lambda(d)) is proportional to the surfactant tail length n. The amplitude of these oscillations decreases exponentially with a decay length xi. In the limit of very long micelles, the grand potential converges to the endcap (free) energy E(c). This endcap energy increases approximately quadratic with the tail length and diminishes by increasing the headgroup size m. The micelle size distribution is generated showing non-monotonic features due to the presence of short dumbbells and becomes exponential when L >> 8R(c). It is also shown that the endcap energy can be estimated in first order by the grand potential of the spherical micelle that coexists with infinitely long cylindrical micelles. The persistence length l(p) of these linear micelles is evaluated to estimate the relative importance of conformational entropy for these micelles.

Journal Article↗

PubMed2006

Why surfaces modified by flexible polymers often have a finite contact angle for good solvents.

Polymers adsorbing from a dilute solution onto the solvent-vapor interface generate a nonzero surface pressure. When the same polymers are end-grafted onto a surface such that a so-called polymer brush is formed, one will find that the solvent wets this compound interface partially. The partial wetting and the finite surface pressure are intimately linked properties of the polymer-solvent-vapor combination. It is shown that the spreading parameter in the wetting problem is proportional to the surface pressure in the adsorption case. Complete wetting is only possible when this surface pressure is nonpositive. The wetting characteristics are hardly influenced by the grafting density and chain length characterizing the brush. We argue that the grafted polymer chains can bridge to the solvent-vapor interface, thereby preventing the wetting film to become macroscopically thick. We present experimental data underpinning our self-consistent field analysis. Indeed, finite contact angles should be expected in various systems in which bridging attraction contributes to the disjoining pressure in wetting films.

Journal Article↗

PubMed2006

Self-consistent-field analysis of the micellization of carboxy-modified poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers.

The micellization properties of carboxy-modified Pluronics P85 (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers) are investigated by means of a molecularly realistic self-consistent-field theory. We consider the, so-called, carboxylic acid end-standing P85 (CAE-85) case where the carboxylic group is located at the end of both PEO parts and the carboxylic acid center-standing P85 (CAC-85) case where each of the carboxylic group presents between the PEO and PPO blocks. The micellization of these copolymers depends on the pH, the added electrolyte concentration phis, and the temperature. It is shown that the aggregation number (Nagg) decreases, whereas the critical micellization concentration (CMC) increases with pH. For the case of increasing phis, the Nagg increases and the CMC decreases. The critical micellization temperature (CMT) and cloud point temperature (CPT) increase with pH at low phis and decrease at increasing phis. The changing from CAE-85 to CAC-85 leads to increasing CMC and CMT, but lower CPT.

Journal Article↗

PubMed2005

Modeling of confinement-induced phase transitions for surfactant layers on amphiphilic surfaces.

A self-consistent field model is used to consider a solution of positively charged surfactants up to its critical micellization concentration adsorbing onto two surfaces in close proximity. Each surface mimics a polystyrene sulfonate interface; that is, hydrophobic properties are combined with a (fixed) negative charge. We observe large and sudden changes in adsorption as a function of separation, which are not normally considered when interpreting surface force measurements. The parameters are chosen such that the adsorbed surfactant layer is of a monolayer type when the surfaces are far apart. A typical interaction curve is presented for a fixed surfactant chemical potential, which is extracted from the set of adsorption isotherms each with a fixed slit width. When the slit width approaches the thickness of the two surfactant layers, a first-order phase transition takes place, which is driven by the unfavorable hydrophobic-water contacts. At the transition, the average orientation of the surfactants switches from a high concentration of tails at the surface to a bilayer configuration where tail profiles from both sides merge in the center. The headgroups are pulled slightly away from the surface. The interaction force jumps from a weak electrostatic repulsion at large distances (two effectively positively charged surface layers repel each other) to a strong electrostatic attraction at short distances (the central surfactant bilayer is attracted to the oppositely charged surfaces). The amount of adsorbed surfactants tend to decrease with decreasing distance between the surfaces but suddenly increases at the transition. Because of this, we anticipate that in surface force experiments, for example, there is a hysteresis associated with this transition: the forces and also the adsorbed amounts depend not only on the distance between the surfaces but also on the history if nonsufficient equilibration times are implemented.

Journal Article↗

PubMed2005

Confinement-induced phase transition and hysteresis in colloidal forces for surfactant layers on hydrophobic surfaces.

In this paper we consider surfactant solutions near a pair of interfaces. It is well-known that strong lateral interactions between surfactant molecules give rise to a step in the adsorption isotherm. In a self-consistent field theory, such a step in the adsorbed amount shows up as a van der Waals loop. The consequence of such a loop for surface force experiments is analyzed. From adsorption isotherms at fixed confinement we extract the relevant adsorbed amounts for a fixed chemical potential as a function of the confinement. A cusped structure is found for the relation between the interaction energy and the slit width: there is a confinement-induced first-order phase transition. The corresponding interaction curve has a kink at the binodal slit distance. Metastable branches as well as an unstable branch (bracketed by the two spinodal points) are presented. The metastability is expected to give rise to force hysteresis in, e.g., atomic force microscope or surface force apparatus experiments, distinctly different from those due to mechanical instabilities of the cantilever system.

Journal Article↗

PubMed2005

Surface forces in a confined polymer melt: self-consistent field analysis of full and restricted equilibrium cases.

In full equilibrium the self-consistent field theory for a homopolymer melt confined between two surfaces predicts pronounced oscillatory interaction forces on the monomer length scale. However, when not all the polymer molecules can reversibly equilibrate with the bulk, the trapped molecules may be squeezed, adding a repulsive contribution to the interaction energy. The classical constrained or restricted equilibrium approach by Scheutjens and Fleer two decades ago to deal with this for polymers adsorbed from dilute solutions, breaks down in semidilute and concentrated polymer solutions. We present a generalized restricted equilibrium ansatz applicable also for concentrated polymer solutions. The key idea is that only the adsorbed polymer molecules, i.e., molecules that touch the surface at least once, are forced to remain inside the gap, whereas the nonadsorbing chains are free to move out of the gap when the surfaces approach each other. As in dilute solutions, the forces found in confined melt with trapped adsorbed chains become repulsive. We analyse the dependence of the interaction forces both in full as well as in restricted equilibrium cases as a function of the chain length and the interactions with the surface for a compressible polymer melt.

Journal Article↗

PubMed2005

Bending moduli and spontaneous curvature of the monolayer in a surfactant bilayer.

We developed a method to evaluate the mechanical properties of the monolayers in symmetric surfactant bilayers using self-consistent field theory. A specific boundary condition is used to impose the same curvature onto the two opposing monolayers at the surfactant chemical potential equal to that of the corresponding homogeneously curved bilayer. Typically, the spontaneous monolayer curvature not equal 0 and its value depend on the surfactant architecture. This is of importance for the thermodynamics and topology of lamellar surfactant phases. Furthermore, it may be relevant in processes involving biological membranes, for example, the fusion and budding of vesicles and the incorporation of proteins in lipid bilayers.

Letter↗

PubMed2005

Steady-state analysis of polymer adsorption at and transport across an interface between two polymer phases.

We consider a mutually incompatible polymer mixture composed of two major components AN and BN and a third minority component C(N). The interactions, parameterized by short-range Flory-Huggins interaction parameters, are chosen such that C wets the A/B interface completely at three-phase coexistence. At sub-saturated conditions the adsorption of C remains necessarily microscopic. We study such a system in a stationary off-equilibrium state: due to imposed chemical potential gradients, polymer AN diffuses from the A-rich bulk phase through the interface to the A-poor bulk phase. Polymer BN travels in the opposite direction. Symmetric conditions are selected for which the polymer CN that accumulates at the A/B-interface has no net flux in the stationary state. This system is described by the Mean Field Stationary Diffusion (MFSD) model, an approach that solves the Scheutjens Fleer self-consistent-field (SF-SCF) equations with the boundary condition that the chemical potentials in the two bulk phases are different (but constant) so that a stationary state can be described. When the chemical potentials in the two bulk phases are the same, MFSD reduces to the equilibrium SF-SCF results. From the MFSD method we obtain the stationary volume fraction profiles and segmental fluxes. By forcing the system further from three-phase coexistence, i.e. by imposing larger concentration gradients, the adsorption of C goes unexpectedly from a thin adsorption layer to a thick adsorption film. The susceptibility partial differential J(A)/partial differential Delta(phi)A of the flux of A (equal to minus the flux of B) with respect to the imposed concentration gradient changes abruptly at the transition in adsorption behaviour. Interestingly, upon variation of the concentration gradients, the fluxes of A and B are enhanced by the accumulation of C at the interface. This means that the adsorbed C-film does not behave as an inert barrier.

Journal Article↗

PubMed2004

Charged lipid vesicles: effects of salts on bending rigidity, stability, and size.

The swelling behavior of charged phospholipids in pure water is completely different from that of neutral or isoelectric phospholipids. It was therefore suggested in the past that, instead of multilamellar phases, vesicles represent the stable structures of charged lipids in excess water. In this article, we show that this might indeed be the case for dioleoylphosphatidylglycerol and even for dioleoylphosphatidylcholine in certain salts. The size of the vesicles formed by these lipids depends on the phospholipid concentration in a way that has been predicted in the literature for vesicles of which the curvature energy is compensated for by translational entropy and a renormalization of the bending moduli (entropic stabilization). Self-consistent field calculations on charged bilayers show that the mean bending modulus kc and the Gaussian bending modulus k have opposite sign and /k/>kc, especially at low ionic strength. This has the implication that the energy needed to curve the bilayer into a closed vesicle Eves=4pi(2kc+k) is much less than one would expect based on the value of kc alone. As a result, Eves can relatively easily be entropically compensated. The radii of vesicles that are stabilized by entropy are expected to depend on the membrane persistence length and thus on kc. Experiments in which the vesicle size is studied as a function of the salt and the salt concentration correlate well with self-consistent field predictions of kc as a function of ionic strength.

Computer Simulation↗

PubMed2004

Electrostatic interactions between double layers: influence of surface roughness, regulation, and chemical heterogeneities.

Electrostatic interactions between two surfaces as measured by atomic force microscopy (AFM) are usually analyzed in terms of DLVO theory. The discrepancies often observed between the experimental and theoretical behavior are usually ascribed to the occurrence of chemical regulation processes and/or to the presence of surface chemical or morphological heterogeneities (roughness). In this paper, a two-gradient mean-field lattice analysis is elaborated to quantifying double layer interactions between nonplanar surfaces. It allows for the implementation of the aforementioned sources of deviation from DLVO predictions. Two types of ion-surface interaction ensure the adjustment of charges and potentials upon double layer overlap, i.e., specific ionic adsorption at the surfaces and/or the presence of charge-determining ions for the surfaces considered. Upon double layer overlap, charges and potentials are adjusted via reequilibrium of the different ion adsorption processes. Roughness is modeled by grafting asperities on supporting planar surfaces, with their respective positions, shapes, and chemical properties being assigned at will. Local potential and charge distributions are derived by numerically solving the nonlinear Poisson-Boltzmann equation under the boundary conditions imposed by the surface profiles and regulation mechanism chosen. Finite size of the ions is taken into account. A number of characteristic situations are briefly discussed. It is shown how the surface irregularities are reflected in the Gibbs energy of interaction.

Journal Article↗

PubMed2004

Partition function, metastability, and kinetics of the escape transition for an ideal chain.

An end-tethered polymer chain squeezed between two pistons undergoes an abrupt transition from a confined coil state to an inhomogeneous flower-like conformation partially escaped from the gap. We present a rigorous analytical theory for the equilibrium and kinetic aspects of this phenomenon for a Gaussian chain. Applying the analogy with the problem of the adsorption of an ideal chain constrained by one of its ends, we obtain a closed analytical expression for the exact partition function. Various equilibrium thermodynamic characteristics (the fraction of imprisoned segments, the average compression, and lateral forces) are calculated as a function of the piston separation. The force versus separation curve is studied in two complementary statistical ensembles, the constant force and the constant confinement width ones. The differences in these force curves are significant in the transition region for large systems, but disappear for small systems. The effects of metastability are analyzed by introducing the Landau free energy as a function of the chain stretching, which serves as the order parameter. The phase diagram showing the binodal and two spinodal lines is presented. We obtain the barrier heights between the stable and metastable states in the free energy landscape. The mean first passage time, i.e., the lifetime of the metastable coil and flower states, is estimated on the basis of the Fokker-Planck formalism. Equilibrium analytical theory for a Gaussian chain is complemented by numerical calculations for a lattice freely jointed chain model.

Journal Article↗