Hernan Ritacco

Salt-induced changes in the growth of polyelectrolyte layers of poly(diallyl-dimethylammonium chloride) and poly(4-styrene sulfonate of sodium)

Multilayers formed by the sodium salt of poly(4-styrene sulfonate), PSS, and poly(diallyldimethylammonium) chloride, PDADMAC, have been built by electrostatic self-assembly from polyelectrolyte aqueous solutions of different ionic strengths. The growth of the multilayers has been followed using dissipative quartz crystal microbalance and ellipsometry. Neutron reflectometry and XPS data indicate that the PSS and PDADMAC layers interpenetrate leading to an almost homogeneous polymer film. The results show that on increasing [NaCl] the growing process changes from a linear to a non-linear regime. The comparison of the thickness values obtained from QCM and ellipsometry has allowed us to calculate the water content of the polymer film. The results agree with those obtained by neutron reflectometry. The analysis of the QCM data has provided values of the complex shear modulus, which are typical of a rubber-like polymer system. The analysis of the mass adsorbed calculated by the ellipsometry measurements indicated that the charge compensation mechanism changes from intrinsic at low ionic strengths to mainly extrinsic at high ionic strengths. Finally, it was found that the response of a polymer film to a change in [NaCl] is rather different for films grown at low or at high ionic strengths.

Adsorption Kinetics and Mechanical Properties of Ultrathin Polyelectrolyte Multilayers: Liquid-Supported versus Solid-Supported Films

Multilayers of sodium salt of poly(4-styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium) chloride (PDADMAC) have been built layer by layer (LbL) both at the solid/aqueous interface (solid supported) and the air/aqueous interface (liquid supported). For the solid-supported multilayers, the adsorption kinetics and the complex shear modulus were measured using a dissipative quartz crystal microbalance and a null ellipsometer. A bubble tensiometer was used to measure the adsorption kinetics and the elasticity modulus of the liquid-supported multilayers. At the solid/aqueous interface, adsorption kinetics changes with the number of adsorbed layers. However, at the air/aqueous interface, PSS dynamics were the same for all adsorbed layers except the first. Conversely, the adsorption kinetics of PDADMAC at the air/water surface differed between those layers close to the interface and those far from it. Multilayers grow at the air/water interface by an intrinsic-charge-compensation process, whereas, for the same ionic strengths, solid-supported layers deposit by the extrinsic-charge-compensation process. No significant differences were found between the recoverable dilational storage modulus of the liquid-supported multilayers and the real part of the shear modulus of the solid-supported ones built at the same ionic strength. The values of the modulus are in the MPa range, which corresponds to gel-like films. This result is in agreement with the strong hydration degree of the LbL films calculated from ellipsometry measurements.

Freezing Transition and Interaction Potential in Monolayers of Microparticles at Fluid Interfaces

The structure and the interaction potential of monolayers of charged polystyrene microparticles at fluid interfaces have been studied by optical microscopy. Microparticles of different sizes have been studied over a broad range of surface particle densities. The structural characterization is based on the analysis of images obtained by digital optical microscopy. From the experimental images, radial distribution functions, hexagonal bond order correlation functions, and temporal orientational correlation functions have been calculated for different monolayer states at both the air/water and oil/water interfaces. The interaction potential has been calculated from the structure factor using integral equations within the hypernetted chain closure relationship. For particles trapped at the oil-water interface, it was found that, upon increasing the surface coverage, a freezing transition occurs, that leads to the formation of a 2D crystalline structure. We have studied the freezing densities of particle monolayers at the oil/water interface and compared them with Monte Carlo simulation results reported by H. Löwen. In contrast, at the air-water interface, freezing is inhibited due to the formation of particle aggregates.

Surface rheology: macro- and microrheology of poly(tert-butyl acrylate) monolayers

We have studied the surface shear viscoelasticity of poly(tert-butyl-acrylate) Langmuir monolayers spread at the air/water interface, by tracking the Brownian motion of tracer particles with different sizes and surface chemical nature, trapped at the same interface. Surface shear moduli have been extracted from the particles mean square displacements (MSD), using different approaches: hydrodynamic calculations of drag coefficients and direct inversion of the MSD by means of the generalized Stokes–Einstein equation. It has been found that these different theoretical approaches lead to comparable values of the shear interfacial viscosity independent of the polymer concentration and molecular weight. In addition, no effect of the size or chemical nature of the probe has been detected. The results have demonstrated the consistency of the microrheological techniques used, and confirm the existence of entanglements in PtBA monolayers, as recently deduced from dilational elasticity and viscosity measurements, [Maestro et al., Soft Matter, 2010, 6, 4407]. An unexpected result was that the interfacial viscosity values obtained from microrheology have been found to be several orders of magnitude lower than the ones obtained with macroscopic interfacial shear rheometers. At the moment there is no clear explanation for this disagreement, although it is not related to the probe size or their chemical nature. Furthermore, this discrepancy is not related to the analysis methodology used, including the calculation of the two-point correlation function used in 3D microrheology when there are heterogeneities present within the range of the probe size.

Temperature and concentration effects on the equilibrium and dynamic behavior of a Langmuir monolayer: from fluid to gel-like behavior.

The equilibrium isotherms of monolayers of poly(4-hydroxystyrene) on the air/water interface have been studied in the 5-60 degrees C range. The results indicate that the interface is a poor solvent for the monolayers over the whole temperature range. For surface pressures within the semidilute regime, the plot of the area occupied by the polymer coils versus temperature at constant surface pressure shows a sharp change of slope near 30 degrees C. Also, the surface excess entropy shows a similar change of slope at the same temperature. The surface shear viscosity can be described by a power law of the surface concentration. Within the semidilute regime, the exponent of the power law changes. The temperature dependence of the viscosity points out a change from independent particle to collective dynamics.

Dynamics in Ultrathin Films: Particle Tracking Microrheology of Langmuir Monolayers

Particle tracking has been shown to be a powerful technique for measuring bulk and interfacial rheology of flu- ids. The Brownian motion of microparticles trapped at interfaces is very sensitive to the viscosity of the subphase, and to the contact angle of the particles. The Stokes-Einstein relation is fulfilled if the friction factor is properly taken into ac- count. The diffusion coefficient of the latex microparticles spread on surfactant monolayers allows one to calculate the shear viscosity of the monolayer using Danovs theory. Good agreement was found with previous results for monolayers of pentadecanoic acid. The method has also been used to study monolayers of n-dodecanol. Moreover, the shear viscosity of a polymer monolayer has been calculated by particle tracking, and the results show good agreement with data obtained by canal viscosimetry. The temperature dependence of the shear viscosity shows the existence of a glass transition for monolayers of poly(4-hydroxystyrene).

Equilibrium and surface rheology of monolayers of insoluble polycations with side chains.

We have studied monolayers of poly(n-tetradecyl 4-vinylpyridinium-co-4-vinylpyridine) bromide with different degrees of quaternization at the air-water interface. The isotherms (surface pressure vs area) present several phase transitions: at low monolayer coverage, there is a phase transition over a characteristic area that increases on increasing the quaternization degree. This behavior can be rationalized in terms of a mean-field theory of 2D semiflexible polymeric chains and could be an indication of a disorder-order transition from a 2D isotropic liquid (IL) at low surface concentration to a 2D nematic phase (N) at higher concentrations. Low-frequency oscillatory strain experiments show that at low surface coverage the monolayers exhibit highly nonlinear behavior, even for low strain amplitude, whereas at higher surface coverage the response is linear for strains higher than 20%. In addition, stress relaxation experiments show a minimum in the characteristic times that coincide with the transition area. These unexpected results at low surface coverage might be characteristic of the system or related to the fact that the oscillatory experiments do not strictly correspond to constant surface-coverage conditions. However, they are in agreement with high-frequency viscoelasticity, obtained by surface quasielastic light scattering, that shows that the dilational viscosity is higher at low surface concentration than for concentrations beyond the surface phase transition. At higher coverage, there is a second phase transition, after which the isotherms present hysteresis, which is not observed below. Ellipsometry indicates that, after this transition, the monolayer thicken, which may be related to 3D growth into a multilayer.

The aqueous Triton X-100 – dodecyltrimethylammonium bromidemicellar mixed system. Experimental results and thermodynamic analysis

Abstract The micellization process of the aqueous mixed system triton X-100 (TX100) – dodecyltrimethylammonium bromide (DTAB) has been studied with a battery of techniques: surface tension, static and dynamic light scattering and ion-selective electrodes. Results have been also analysed with two thermodynamic procedures: the Regular Solution Theory or Rubingh’s model and the recently developed Equation Oriented Mixed Micellization Model (EOMMM). For αDTAB ≤ 0.40 (αDTAB: total molar fraction of the system without considering the water), the micelles are predominantly TX100 with scarce solubilized DTA+ ions, with TX100 acting as a nearly ideal solvent. In the range 0.50 ≤ αDTAB ≤ 0.75, it seems that none of the components acts as a solvent. Above αDTAB ≈ 0.75 there are noticeable changes in the size and electrophoretic mobility of the micelles. These phenomena have been interpreted in the light of the thermodynamic results and literature on some TX100-ionic surfactant mixtures. The case under study is an almost ideal but very asymmetric mixed surfactants system, what is very interesting in view of the very different nature and structures of the components.