Ion induced lamellar-lamellar phase transition in charged surfactant systems (original) (raw)
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Characterisation of phase transition in adsorbed monolayers at the air/water interface
Advances in Colloid and Interface Science, 2010
Recent work has provided experimental and theoretical evidence that a first order fluid/condensed (LE/LC) phase transition can occur in adsorbed monolayers of amphiphiles and surfactants which are dissolved in aqueous solution. Similar to Langmuir monolayers, also in the case of adsorbed monolayers, the existence of a G/LE phase transition, as assumed by several authors, is a matter of question. Representative studies, at first performed with a tailored amphiphile and later with numerous other amphiphiles, also with n-dodecanol, provide insight into the main characteristics of the adsorbed monolayer during the adsorption kinetics. The general conditions necessary for the formation of a two-phase coexistence in adsorbed monolayers can be optimally studied using dynamic surface pressure measurements, Brewster angle microscopy (BAM) and synchrotron X-ray diffraction at grazing incidence (GIXD). A characteristic break point in the time dependence of the adsorption kinetics curves indicates the phase transition which is largely affected by the concentration of the amphiphile in the aqueous solution and on the temperature. Formation and growth of condensed phase domains after the phase transition point are visualised by BAM. As demonstrated by a tailored amphiphile, various types of morphological textures of the condensed phase can occur in different temperature regions. Lattice structure and tilt angle of the alkyl chains in the condensed phase of the adsorbed monolayer are determined using GIXD. The main growth directions of the condensed phase textures are correlated with the two-dimensional lattice structure. The results, obtained for the characteristics of the condensed phase after a first order main transition, are supported by experimental bridging to the Langmuir monolayers. Phase transition of adsorbing trace impurities in model surfactants can strongly affect the characteristics of the main component. Dodecanol present as minor component in aqueous sodium dodecylsulfate solution dominate largely the fundamental features of the adsorbed monolayer of the mixed dodecanol/SDS solutions at adsorption equilibrium. A theoretical concept on the basis of the quasi-chemical model and assumption of the entropy non-ideality has been developed which can well describe the experimental results of the diffusion kinetics of surfactant adsorption from solutions. The model regards the phase behaviour of adsorbed monolayers on the basis of the experimental results explicitly supported by the first order fluid/condensed phase transition and theoretical models assuming bimodal distribution between large aggregates (domains) and monomers and/ or very small aggregates. Another simple theoretical model for the description of the coadsorption of surfactant mixtures, based on the additivity of the contributions brought by the solution components into the surface pressure is shown to be in qualitative agreement with the experimental data of mixed dodecanol/ SDS solutions. The theoretical results corroborate the fact that the formed condensed phase (large aggregates) in the mixed monolayer consists mainly of dodecanol.
Phase behavior of a charged, fluid lipid bilayer
2013
Lateral organization of cell membranes is an important problem in Biophysics. A large number of experimental studies points to the existence of lateral domains, sometimes called membrane rafts, and their relation with many biological processes, including signal transduction, membrane trafficking, cell adhesion, membrane fusion etc. Important aspects to consider are: electrostatic interaction, since charged lipids are ever present in biomembranes, the compositional lipid asymmetry and the coupling between the two faces of the membrane. In the present work, we study the lateral phase separation in an asymmetrically charged lipid bilayer consisting of neutral and negatively charged lipids that are in contact with an ionic solution. The two asymmetric monolayers are coupled only trough electrostatic interactions. We have describe the bilayer on the mean-field level by splitting the free energy into non-electrostatic and electrostatic contributions, the former using the Bragg-Williams approximation (random mixing approximation) of a binary lattice gas and the latter using the non-linear Poisson-Boltzmann theory. With the goal of modeling bilayers where only one monolayer tends to phase separate we have assigned different values of non-ideality parameter χ to each one. We calculated the entire phase diagram containing binodal and spinodal lines as a function of the of non-ideality parameter χ and monolayer-monolayer electrostatic coupling. It is shown that the domain formation may be induced or inhibited in the apposed monolayer depending on the location within the phase diagram.
Monolayer penetration by a charged amphiphile: equilibrium and dynamics
Colloids and Surfaces A-physicochemical and Engineering Aspects, 2001
The penetration of a soluble surfactant into an insoluble monolayer provides a means of understanding intermolecular interactions and their impact on equilibrium and dynamic surface pressures. In this paper, the adsorption of an ionic surfactant into an insoluble monolayer is studied theoretically and numerically. The equilibrium increase in surface pressure Dy caused by the surfactant adsorption is derived for a Davies adsorption isotherm using a Gibbs adsorption equation properly constrained for the presence of the insoluble monolayer. The dynamic surface pressure is studied using this surface equation of state for Dy assuming either diffusion controlled or mixed kinetic-diffusion controlled mass transfer. Several trends are predicted with variations of the surface coverage of the insoluble component, the concentration of soluble surfactant and the ionic strength of the surfactant subphase. Experiments in the literature have shown that, for an uncharged monolayer, Dy at equilibrium is greater the higher is the surface coverage of the insoluble monolayer into which the ionic, soluble surfactant adsorbs. Our results show that this trend can be attributed to the role of the insoluble component in presenting an entropic barrier to adsorption, thereby reducing the repulsive surface charge density at a given net surfactant coverage, allowing more surfactant to adsorb. Greater surface pressures result. Signature trends in the timescales for diffusion controlled mass transfer of an ionic surfactant as a function of initial surface coverage of the insoluble monolayer are derived. This timescale is longer than predicted by simply accounting for the area blocked by the insoluble component using a Langmuir argument, and approaches the Langmuir argument with increasing ionic strength. For mixed kinetic-diffusion controlled mass transfer, because the insoluble component blocks interface, it reduces the amount of surfactant that can adsorb. This decreases the diffusion timescale, and allows adsorption-desorption kinetics to play a controlling role. Since electrostatic repulsion reduces the adsorption of the soluble component, kinetics also play a stronger role the lower is the ionic strength or the higher the surfactant valence. Considering a surfactant with fixed physicochemistry, a shift of controlling mechanism from diffusion control to kinetic control is demonstrated with increasing bulk concentration, surface charge density or surface coverage of insoluble component.
Phase Behavior and Induced Interdigitation in Bilayers Studied with Dissipative Particle Dynamics
The Journal of Physical Chemistry B, 2003
Bilayers formed by coarse-grained models of amphiphilic surfactants are studied using dissipative particle dynamics combined with a Monte Carlo scheme to achieve the natural state of a tensionless bilayer. We address the issue of the influence of the molecular structure and the level of coarse graining on the bilayer properties by studying two models of different complexity: a single tail and a double tail surfactant. We compute the area per surfactant, the bilayer thickness, and the orientational order parameters and show how their dependence on the surfactant structure and temperature. We reproduce the gel to liquid crystalline phase transition, and we study the conditions that induce a gel interdigitated phase in the bilayer. We show how the interdigitated state in a bilayer containing double-tail surfactants can be reproduced by adding an extra bead in the surfactant headgroup, which mimics an ester-linkage to the phosphate group. In a bilayer formed by single tail surfactants, we induce interdigitation by changing the strength of the repulsive interaction between the headgroups, which corresponds with adding salt to the system. We are then able to derive a phase diagram as function of temperature and repulsion parameter for surfactants of different chain lengths.
Advances in Colloid and Interface Science, 1999
Our understanding of the Poisson-Boltzmann electrostatics of curved fluid monolayers and bilayers of ionic amphiphiles, and its status within the Helfrich bending-energy formulation, is reviewed and subsequently extended. The two limiting scenarios, namely excess salt (non-overlapping interfaces) and zero salt (counterions only) are briefly discussed to couch their generalization to intermediate screening. This general situation of arbitrary composition is treated first within the cylindrical and spherical cell models, to harmonic order in their curvatures, leading on to the geometry of undulating planar confinement, now harmonic in amplitude but covering the entire spectrum of wavelengths. The latter derivation extends previous results, and moreover, provides analytic expressions for the electrostatic free energy using theta function developments. This in turn supplies the Hamiltonian for the generalized harmonic theory of fluctuating lamellar phases with added salt. Finally, the statistical mechanical predictions for the area increase, and relative amplitudes, of bilayer thermal undulations are compared to existing light-scattering data from dilute lamellar phases doped with anionic surfactant. 0 1999 Elsevier Science B.V. All rights reserved. .se (A. Fogden) 0001-8686/99/$see front matter 0 1999 Elsevier Science B.V. All rights reserved.
Charge-induced nematic-isotropic transition in mixed surfactant solutions
Langmuir, 1993
The present paper investigates the effect of coulomb interaction on the nematic phase and the isotropic phase observed in the CTAB-SHNC mixed system. It is shown that the observed N-I transition temperatures depend quadratically on the charge Z of the rod like micelles as expected from an extension of the Maier-Saupe theory. Salt effects are shown to increase the transition temperature supporting the concept of a charge induced isotropic state introduced by the theory.
Fluctuations and defects in lamellar stacks of amphiphilic bilayers
Computer Physics Communications, 2005
We review recent molecular dynamics simulations of thermally activated undulations and defects in the lamellar Lα phase of a binary amphiphile-solvent mixture, using an idealized molecular coarse-grained model: Solvent particles are represented by beads, and amphiphiles by bead-and-spring tetramers. We find that our results are in excellent agreement with the predictions of simple mesoscopic theories: An effective interface model for the undulations, and a line tension model for the (pore) defects. We calculate the binding rigidity and the compressibility modulus of the lamellar stack as well as the line tension of the pore rim. Finally, we discuss implications for polymer-membrane systems.
Phase transitions in cationic and anionic surfactant mixtures
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1996
The phase behavior of cationic and anionic surfactant mixtures strongly depends on the molar ratio and actual concentration of surfactants. The phase diagram contains large areas of coexisting solid crystalline catanionic surfactant and liquid crystalline phases. Intermediate phases from pure solid crystalline catanionic surfactant to mixed micelles were examined by light microscopy under crossed polarizers and potentiometric and electrophoretic measurements. The mixed liquid crystalline phase is composed of both cationic and anionic surfactants. Its appearance as lamellar droplets may be explained by the increase in electrostatic repulsion at the bilayer surface.
The European Physical Journal E, 2009
We consider bilayer biomembranes or surfactants made of two chemically incompatible amphiphile molecules, which may laterally or transversely phase separate into macrodomains, upon variation of some suitable parameter (temperature, lateral pressure, etc.). The purpose is an extensive study of the dynamics of both lateral and transverse phase separations, when the bilayer is suddenly cooled down from a high initial temperature towards a final one very close to the spinodal point. The critical dynamics are investigated through the partial dynamic structure factors of different species. Using a two-order parameter field theory, where the two fields are the composition fluctuations of one component in the leaflets of the bilayer, combined with an extended van Hove approach that is based on two coupled Langevin equations (with noise), we exactly compute these dynamic structure factors. We first find that the dynamics is governed by two time scales. The longest one, τ , can be related to the thermal correlation length, ξ ∼ σ|T − T c| −1/2 , by τ ∼ ξ z , with the dynamic critical exponent z = 4, where σ is an atomic length scale, T the absolute temperature, and T c its critical value. The characteristic time τ can be interpreted as the time required for the formation of the final macrophase domains. The second time scale is rather shorter, and can be viewed as the short time during which the unlike phospholipids execute local motion. Second, we demonstrate that the dynamic structure factors obey exact scaling laws, and depend on three lengths, namely the wavelength q −1 (q is the wave vector modulus), the correlation length ξ, and a length scale R(t) ∼ t 1/z (z = 4) representing the size of macrophase domains at time t. Of course, the two lengths ξ and R(t) coincide at the final time τ at which the bilayer reaches its final equilibrium state. Finally, the present work must be considered as a natural extension of our previously published one dealing with the study of lateral and transverse phase separations from a static point of view.