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La tension interfaciale est une grandeur physico-chimique d'intérêt pour de nombreuses industries et notamment l'industrie pétrolière. Cette grandeur est l'un des paramètres qui permet d'optimiser le rendement d'un puits de pétrole. La difficulté liée à sa mesure dans les conditions réservoirs a amené à étudier les systèmes eau/tensioactif/huile par simulation moléculaire. Ce travail a permis de montrer que la Dynamique Particulaire Dissipative (DPD) était un outil adapté pour l’étude de systèmes eau/tensioactif/huile sous différents aspects, de la caractérisation de la structure des interfaces au calcul de la tension interfaciale. Cette thèse a permis la démonstration de l’influence non-négligeable de la variation des paramètres de la force harmonique, l’amplitude K et la distance d’équilibre r0 , sur le calcul de la tension interfaciale et sur la structure des interfaces à forte concentration en tensioactif. En effet, la structure des tensioactifs aux interfaces est le résultat d’une balance subtile entre les forces intra et inter moléculaire. L’étude d’une population modèle de tensioactifs non chargés a permis de montrer que la DPD reproduit bien l'évolution de la tension interfaciale en fonction de la concentration en tensioactif en solution et en fonction du coefficient de partage de tensioactifs modèles non chargés. Une méthodologie est proposée pour caractériser les systèmes contenant des interfaces et où la tension interfaciale est calculée.Des travaux prospectifs ont permis de montrer que la DPD permettait d'étudier des phénomènes liés à la tension interfaciale comme le mûrissement d'Ostwald dans les émulsions d'huile dans l'eau. Ces derniers travaux ouvrent la voie à l’étude d’autres systèmes d’intérêt pour le milieu pétrolier comme le décollement de gouttes de pétroles adsorbées sur des parois ou l’étude d’émulsions pétrolières. / The interfacial tension is a physical-chemical property that numerous industrial areas have an interest of especially the petroleum industry. This property is one of the many which helps to optimize production wells' rate of return. Measuring that property in reservoir's conditions (high pressure and temperature) is highly difficult and led to study water/surfactant/oil systems using molecular modeling. The difficulty to measure that specific physical-chemical property linked to the pressure and temperature conditions in the reservoirs led the scientists to study water/surfactant/oil systems using molecular modeling. This thesis establishes that the Dissipative Particle Dynamics (DPD) is able to study water/surfactant/oil systems. The study of the effect of the variation of the harmonic force's parameters, namely the force constant K and the equilibrium distance r0, demonstrated that their variation can heavily influence the interfacial tension computation. Actually, a subtle balance exists between the intra and inter-molecular interactions, which influences the local structure of the surfactants at the oil-water interface, modifies the interfacial tension and influences the interface stability. It was demonstrated that DPD reproduces the variation of interfacial tension with the bulk surfactant concentration and the effect of the variation of hydrophobicity of models of un-charged surfactants on interfacial tension by mean of their coefficient partition. We established a method to properly study systems containing interfaces where interfacial tension is computed. Prospective work showed that DPD was a good tool to study microscopic phenomenon which can be observed macroscopically like the Ostwald ripening in oil in water emulsions. This is a first step before studying others systems of interest for the petroleum industry such as oil/water emulsion or the adsorption of oil droplets on rock wall.

<p>The elastic properties of oil/water/surfactant interfaces play an important role in the phase behaviour of microemulsions and for the stability of macroemulsions. The aim of this thesis is to obtain an understanding of the relationship between the structure of the surfactant molecules, the structure of the interface, and macroscopic interfacial properties. To achieve this aim, we performed molecular simulations of oil/water/surfactant systems. We made a quantitative comparison of various model surfactants to determine how structural changes affect interfacial properties and film rupture. The model consists of water, oil, head, and tail beads, and surfactants are constructed by coupling head and tail beads with harmonic springs. We used a hybrid dissipative particle dynamics-Monte Carlo scheme. The former was used to simulate particle trajectories and the Monte Carlo scheme was used to mimic experimental conditions: bulk-interface phase equilibrium, tensionless interfaces in microemulsions, and the surface force apparatus.</p><p>A detailed comparison of various non-ionic model surfactants showed how structural changes affect interfacial properties:</p><p>Comparison between linear and branched surfactants showed that the efficiency of adsorption is higher for linear surfactants, although branched surfactants are more efficient at a given surface density. Linear surfactants can be more efficient also at the same surface density if the head group is sufficiently soluble in oil, because low head-oil repulsion makes the branched isomers stagger at the interface. The bending rigidity is higher for linear surfactants. Furthermore, branched surfactants make oil droplets coalesce more easily than linear surfactants do, but linear and branched surfactants have roughly the same effect on water droplet coalescence. </p><p>Comparison of linear surfactants with varying chain lengths showed that longer surfactants have a lower surface tension and higher bending rigidity. The increase in rigidity with chain length follows a power law, but the exponent is higher for surfactant monolayers at a fixed density than at a fixed tension. Longer tails and/or denser monolayers influence the stability of water droplets in a positive direction, and the stability of oil droplets in a negative direction. </p><p>Addition of cosurfactant showed that mixed monolayers have a lower bending rigidity than pure monolayers at the same average chain length and tension. Cosurfactants have a negative effect on the stability of water droplets, and a positive effect on the stability of oil droplets.</p> / Paper I reprinted with kind permission of EDP Sciences. Paper III reprinted with kind permission of the American Institute of Physics. Paper IV reprinted with kind permission of the American Physics Society.

The elastic properties of oil/water/surfactant interfaces play an important role in the phase behaviour of microemulsions and for the stability of macroemulsions. The aim of this thesis is to obtain an understanding of the relationship between the structure of the surfactant molecules, the structure of the interface, and macroscopic interfacial properties. To achieve this aim, we performed molecular simulations of oil/water/surfactant systems. We made a quantitative comparison of various model surfactants to determine how structural changes affect interfacial properties and film rupture. The model consists of water, oil, head, and tail beads, and surfactants are constructed by coupling head and tail beads with harmonic springs. We used a hybrid dissipative particle dynamics-Monte Carlo scheme. The former was used to simulate particle trajectories and the Monte Carlo scheme was used to mimic experimental conditions: bulk-interface phase equilibrium, tensionless interfaces in microemulsions, and the surface force apparatus. A detailed comparison of various non-ionic model surfactants showed how structural changes affect interfacial properties: Comparison between linear and branched surfactants showed that the efficiency of adsorption is higher for linear surfactants, although branched surfactants are more efficient at a given surface density. Linear surfactants can be more efficient also at the same surface density if the head group is sufficiently soluble in oil, because low head-oil repulsion makes the branched isomers stagger at the interface. The bending rigidity is higher for linear surfactants. Furthermore, branched surfactants make oil droplets coalesce more easily than linear surfactants do, but linear and branched surfactants have roughly the same effect on water droplet coalescence. Comparison of linear surfactants with varying chain lengths showed that longer surfactants have a lower surface tension and higher bending rigidity. The increase in rigidity with chain length follows a power law, but the exponent is higher for surfactant monolayers at a fixed density than at a fixed tension. Longer tails and/or denser monolayers influence the stability of water droplets in a positive direction, and the stability of oil droplets in a negative direction. Addition of cosurfactant showed that mixed monolayers have a lower bending rigidity than pure monolayers at the same average chain length and tension. Cosurfactants have a negative effect on the stability of water droplets, and a positive effect on the stability of oil droplets. / Paper I reprinted with kind permission of EDP Sciences. Paper III reprinted with kind permission of the American Institute of Physics. Paper IV reprinted with kind permission of the American Physics Society.

Lipid molecules are amphiphilic in nature consisting of a hydrophilic head group and hydrophobic hydrocarbon tails. The lipid bilayer consists of two layers of lipid molecules arranged with their hydrophobic tails facing each other and their hydrophilic head groups solvated by water. Lipid bilayers with hydrophilic polymer chains grafted onto the head groups have applications in various fields, such as stabilization of liposomes designed for targeted drug delivery, synthesis of supported bilayers for biomaterial applications, surface modification of implanted medical devices to prevent biological fouling and design of in vitro biosensors. The focus of this thesis lies in understanding the effects of polymer grafting on the thermodynamics and mechanical properties of lipid bilayers. Dissipative particle dynamics (DPD) has evolved as a promising method to study complex soft matter systems. The basic DPD algorithm, and its implementation are discussed in Chapter 2 of this thesis. It is important to achieve a tensionless state while studying phase transitions and deducing the mechanical properties of the bilayer. We proposed a modification of the Andersen barostat which can be incorporated in a DPD simulation to achieve the tensionless state as well as carry out simulations at a prescribed tension. In Chapter 3 of this thesis the effect of polymer grafting on single tailed lipid bilayers is studied. Simulations are carried out by varying the grafting fraction, Gf, defined as the ratio of the number of polymer molecules to the number of lipid molecules. At lowGf, the bilayer shows a sharp transition from the gel (Lβ) to the liquid crystalline (Lα) phase. This main melting transition temperature is lowered as Gf is increased. Corresponding to this, an increase in the area per head group is also observed. Above a critical value of Gf the interdigitated, LβI phase is observed prior to the main transition for the longer lipid tails. The analysis for two tailed lipids as a function of polymer chain length is extensively studied in Chapter 5. For the case of two tailed lipids, an intermediate interdigitated phase was not observed and the decrease in the melting temperature is more pronounced as the length of the polymer chain is increased. The scaling for fractional change in the area per head group, as well as the decrease in transition temperature as a function of polymer grafting are in good agreement with mean field theory predictions. The bending modulus (k) and area stretch modulus (kA) are essential for determining the shape and the mechanical stability of biological cells or lipid based vesicles. In simulations, the bending modulus k is evaluated from the Fourier transform of the out-of-plane fluctuations of the bilayer mid-plane. In Chapter 4 of this thesis, we illustrate that a surface representation based on Delanuay triangulation provides a robust parameter free representation of the bilayer surface. By evaluating the bending modulus for single tail lipids of different tail lengths, the continuum scaling relation d2 is verified. To our knowledge this is the first systematic investigation and verification of this scaling relationship using computer simulations. Using the continuum relation, =kAd2/ we find that α depends weakly on the tail lengths of the bilayer. Nevertheless we illustrate that a value of α=130 can be used to reliably estimate the bending modulus from the area stretch modulus for polymer free bilayers. Using our method, we are also able to capture the low q scalings and obtain the bending modulus of the gel (Lβ) phase. Grafted polymer was found to increase the value of the bending modulus for single tail lipids. Although the presence of polymer directly increases the area per head group, the suppressed height fluctuations dominate and the bending modulus increases for the single tail lipids. For two tail lipids a small decrease in the bending modulus was observed at low grafting fractions and short polymer chains. For large polymer lengths the bending modulus was found to increase monotonically.

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Title: The study of the association behaviour of the amphiphilic copolymers in solutions containing low molar compounds by means of computer simulations. Author: Mgr. Karel Šindelka Department: Faculty of Science, Charles University Supervisor: Doc. Ing. Zuzana Limpouchová, Csc. Abstract This doctoral thesis focuses on the study of electrostatic self- and co-assembly in complex polymer solutions containing polyelectrolyte (PE) block copolymers together with surfactants, neutral homopolymers, or oppositely charged PEs using the dissipative particle dynamics (DPD). It was shown that the electro- static self-assembly depends not only on the cooperative interactions of oppo- sitely charged PE chains, but also on the amphiphilicity of PE species or on the polymer block compatibility, among other properties. PEs with incompatible blocks create well-defined core-shell structures, while large ill-defined crew-cut aggregates form from PEs with compatible blocks In non-stoichiometric mixtures of PEs with incompatible blocks, co-assembled nanoparticles are smaller than in stoichiometric mixtures and are charged. The destabilization of larger aggregates depends on how the PE charge surplus is introduced: the effect is strongest when the density of the surplus PE charge on the PE chains is increased and weakest when the...