G. Loglio

Dilational surface visco-elasticity of polyelectrolyte/surfactant solutions: Formation of heterogeneous adsorption layers

Recent application of the methods of surface dilational rheology to solutions of the complexes between synthetic polyelectrolytes and oppositely charged surfactants (PSC) gave a possibility to determine some steps of the adsorption layer formation and to discover an abrupt transition connected with the formation of microaggregates at the liquid surface. The kinetic dependencies of the dynamic surface elasticity are always monotonous at low surfactant concentrations but can have one or two local maxima in the range beyond the critical aggregation concentration. The first maximum is accompanied by the generation of higher harmonics of induced surface tension oscillations and caused by heterogeneities in the adsorption layer. The formation of a multilayered structure at the surface for some systems leads to the second maximum in the dynamic surface elasticity. The hydrophobicity and charge density of a polymer chain influence strongly the surface structure, resulting in a variety of dynamic surface properties of PSC solutions. Optical methods and atomic force microscopy give additional information for the systems under consideration. Experimental results and existing theoretical frameworks are reviewed with emphasis on the general features of all studied PSC systems.

Dilational viscoelasticity of fluid interfaces: The diffusion model for transient processes

Abstract The functional form of the surface dilational modulus is determined by considering the subsurface concentration changes produced by the diffusion process consequent to a non-periodic (rectangular step) perturbation of surface area (local equilibrium between surface and subsurface is assumed). The mathematical treatment is intended to present a straightforward demonstration that the same (unique) physical quantity, i.e. the dilational modulus, governs both harmonic and transient surface processes. The results definitely show that this modulus is an intrinsic physical property of a diffusive system, independent of the type of surface disturbance (within the linearity approximation).

Impact of Globule Unfolding on Dilational Viscoelasticity of β-Lactoglobulin Adsorption Layers

The dynamic surface dilational elasticity, surface pressure, and adsorbed amount of the mixed solutions of beta-lactoglobulin and guanidine hydrochloride were measured as a function of surface age and denaturant concentration. It was shown that the conformational transition from compact globules to disordered protein molecules in the surface layer leads to strong changes in the surface elasticity kinetic dependencies and thereby can be easily detected by measuring the surface dilational rheological properties. The corresponding changes of the kinetic dependencies of the surface pressure and adsorbed amount are not so pronounced but correlate with the results on surface dilational elasticity.

Non-equilibrium properties of fluid interfaces - Aperiodic diffusion-controlled regime. 1. Theory

Abstract Surface relaxation processes of liquid solutions, under not-far-from-equilibrium conditions, are interpreted from a phenomenological point of view by application of the theory of distributed systems. In the case of diffusion-controlled adsorption, exact analytical expressions describe the transient responses of bulk concentration and dynamic surface tension, consequent to trapezoidal pulses of relative surface area. The mathematical treatment shows that surface responses have a definite physical significance, as they manifest a constitutive property of the system, i.e. the surface dilational modulus.

Formation of Protein/Surfactant Adsorption Layer at the Air/Water Interface as Studied by Dilational Surface Rheology

The dynamic dilatational surface elasticity of mixed solutions of globular proteins (β-lactoglobulin (BLG) and bovine serum albumin (BSA)) with cationic (dodecyltrimethylammonium bromide (DTAB)) and anionic (sodium dodecyl sulfate (SDS)) surfactants was measured as a function of the surfactant concentration and surface age. If the cationic surfactant concentration exceeds a certain critical value, the kinetic dependencies of the dynamic surface elasticity of BLG/DTAB and BSA/DTAB solutions become nonmonotonous and resemble those of mixed solutions of proteins with guanidine hydrochloride. This result indicates not only the destruction of the protein tertiary structure in the surface layer of mixed solution but also a strong perturbation of the secondary structure. The corresponding kinetic dependencies for protein solutions with added anionic surfactants are always monotonous, thereby revealing a different mechanism of the adsorption layer formation. One can assume that the secondary structure is destroyed to a lesser extent in the latter case and hinders the formation of loops and tails at the interface. The increase of the solutions ionic strength by the addition of sodium chloride results in stronger changes of the protein conformations in the surface layer and the appearance of a local maximum in the kinetic dependencies of the dynamic surface elasticity in a relatively narrow range of SDS concentration.

Dilational surface elasticity of spread monolayers of polystyrene microparticles

The dependence of the dilational surface elasticity on the surface pressure of the spread monolayers of polystyrene microparticles is studied at the water-air interface. The surface rheological measurements together with the data from optical methods allow the division of the whole range of surface pressures into three zones characterized by different monolayer structures. The extremely high surface elasticity (∼500 mN m(-1)) at surface pressures close to 30 mN m(-1) is similar to the results for the adsorption layer of the complexes formed between silica particles and surfactant molecules and is probably caused by strong hydrophobic attraction between the particles. At the same time, some other characteristic features of the viscoelasticity of the monolayers of polysterene microparticles differ strongly from the properties of previously studied systems.

Nonequilibrium properties of fluid interfaces: aperiodic diffusion-controlled regime 2. Experiments

Abstract We have investigated the surface dilational properties of a submicellar aqueous solution of n -dodecyldimethyl-phosphine oxide. We compared (a) the steady-state responses of dynamic surface tension, induced by small-amplitude harmonic disturbances of surface equilibrium (at different frequencies), and (b) the transient response induced on the same sample by a single trapezoidal-pulse disturbance. Using the equations for a diffusive model, we determined the fit parameters from the observed response values both in the frequency and in the time domains. The results enabled us to predict satisfactorily the harmonic dynamic behaviour of the surface, from the relaxation data. The prediction-bservation agreement supports the theoretical treatment and, in particular, the assertion that the viscoelastic link, between surface excitation and response, is a constitutive property of a liquid solution. The experiments also show the existence of a linearity range and the repeatability of measurements.

Surface rheology of adsorbed surfactants and proteins

Interfacial Theological methods have been developed to a level which allow the quantification of the shear and dilational behaviour of mixed protein/surfactants systems. First attempts are published to provide an insight into the complex interfacial layer properties. The type of interaction between the components controls the changes in rheological parameters as compared to the pure protein. The understanding of the rheological behaviour, in particular the compressibility, of lung surfactant monolayers as one of the highly practical, relevant systems is in the focus of interest.

Spectrofluorimetric Evidence of the Transport of Marine Organic Matter in Antarctic Snow Via Air-Sea Interaction

Abstract This paper considers the importance of marine aerosol in the atmosphere. As a consequence of its peculiar generation mechanism, it is also considered as a possible contributor to the transport of man-made microcomponents via air-sea interaction, especially in remote sites. In view of future marine aerosol studies, the dominant presence of marine aerosol components in coastal Antarctic snow is discussed the presence of fluorescent marine organic matter in Antarctic snow is shown. Its “marum” nature is evidenced. We give a tentative interpretation of the experimental data, and make a hypothesis concerning the variations in marine aerosol composition in correspondence to various altitudes.

Exchange of matter at the interface between two liquid phases

Theoretical models of the exchange of matter at interfaces are necessary to interpret relaxation phenomena in surfactant adsorption layers. For liquid-liquid systems, the diffusion-controlled exchanges of surfactant molecules at the interfaces can be influenced by a simultaneous transfer of molecules from one bulk phase to the other. The diffusional mass exchange function is derived taking into account the transfer across the interface. The resulting mass exchange function is used to calculate the interfacial tension response of a liquid-liquid system. As an example, the interfacial response after a ramp-type area perturbation is calculated.