Libero Liggieri

Drop and bubble shape analysis as tool for dilational rheology studies of interfacial layers.

Drop and bubble shape tensiometry is a modern and very effective tool for measuring dynamic and static interfacial tensions. An automatic instrument with an accurate computer controlled dosing system is discussed in detail. Due to an active control loop experiments under various conditions can be performed: constant drop/bubble volume, surface area, or height, trapezoidal, ramp type, step type and sinusoidal area changes. The theoretical basis of the method, the fitting procedure to the Gauss-Laplace equation and the key procedures for calibration of the instrument are analysed and described.

Adsorption and partitioning of surfactants in liquid–liquid systems

The adsorption at liquid-liquid interfaces is a phenomenon with a remarkable impact on many scientific and technological fields concerning multiphasic systems. Though in principle similar to liquid-vapour, the study and the description of the dynamic aspects of the adsorption processes at liquid-liquid interfaces deserves some specific considerations. In fact, these systems are often characterised by the partitioning of the surfactant between the two liquid phases, which makes much more complex both their modelling and investigation. In some conditions, the partitioning can be the main process controlling the adsorption dynamics. This paper is aimed at reviewing the state-of-the-art of the theoretical modelling and experimental investigation of the adsorption dynamics of surfactants at liquid-liquid interfaces. After a brief introduction to the problem of adsorption dynamics, the principal models utilised to describe the process at liquid-liquid interfaces under different assumptions are critically presented, underlining the influence of the surfactant partitioning, with respect to the relative volumes of the liquid phases and of the initial partitioning conditions. The most important experimental methodologies for the measurement of the dynamic interfacial tension are also critically reviewed by pointing out the specific problems related with the investigation of the adsorption dynamics of surfactants at liquid-liquid interfaces. Moreover, the problem of the measurement of the thermodynamic quantities characterising the partitioning--mainly the partition coefficient--is also addressed, reporting some literature data. Finally, a review of the literature about the experimental work on the subject and an overview of the needs and of the open questions is given.

A diffusion-based approach to mixed adsorption kinetics

A new theoretical approach has been developed which allows mixed kinetics of non-ionic surfactants to be studied in the unitary framework of an extended diffusion-controlled problem. This treatment not only allows one to consider a large class of surface models, but also provides a better understanding of the relation existing between the presence of a potential barrier and the diffusion-controlled adsorption. In particular it is shown that the assumption of the presence of adsorption barriers does not depend on the local equilibrium hypothesis. In fact the theoretical treatment takes into account the possibility that mixed kinetics also occur in the absence of potential barriers. By this treatment a general equation is obtained which allows the adsorption as a function of time to be evaluated by numerical or analyical procedures for a given particular surface model.

Wettability of silica nanoparticle–surfactant nanocomposite interfacial layers

The hydrophobicity of a particle surface can be tuned by the addition of surfactants that change the surface free energy for their attachment to a liquid interface. In this work, we report an experimental study where the wettability properties of silica nanoparticles are modified by the adsorption of alkyltrimethylammonium surfactants (CnTAB, n = 12, 16) on the surface of the particles. We have pointed out that the wettability of the complexes is controlled by an intricate balance of electrostatic and hydrophobic interactions between the particle surface and the surfactant. These interactions play an important role in the structure of the surfactant–particle nanocomposite interfacial layer.

A new experimental method for the measurement of the interfacial tension between immiscible fluids at zero bond number

Abstract A new methodology (Pressure Derivative Method) for measuring the interfacial tension σ between immiscible fluid phases at Δρg = 0 (zero Bond number) is presented. This method, which is particularly suitable for measurements under microgravity conditions, is also applicable on earth to liquid pairs with a small density difference. The method consists, essentially, of calculating σ by a linear fitting of experimental capillary pressure and curvature data, collected during the formation of a drop of one of the two liquids inside the other one. The curvature data are derived from the injected drops volume; thus, any pair of fluids can be used, including opaque liquids, for which optical curvature measurements are not possible. By a specific mathematical treatment of the experimental data, an overall accuracy on the order of less than 1% has been obtained. During the drop growth, a maximum in the pressure is reached, which can also be used to calculate σ by the Maximum Bubble Pressure technique (MBP). This value can be utilized as an internal check on the interfacial tension data obtained by the method proposed.

Wide-frequency dilational rheology investigation of mixed silica nanoparticle–CTAB interfacial layers

The interfacial properties of aqueous dispersions are strictly connected to the transfer/accumulation of particles into the surface layer, driven by the particle hydrophobicity. The addition of surfactants adsorbing on the particles and tuning their hydrophobicity represents therefore an attractive route to control the properties of these liquid interfaces. These mixed systems present however rather complex behaviours, which are still difficult to understand, deserving both theoretical developments and experimental investigations. Here the results of a wide study on the interfacial properties of a silica nanoparticle dispersion added with CTAB are presented, which allows to elucidate some basic aspects. A set of different techniques and methodologies have been utilised to measure the dilational viscoelasticity in a wide frequency range: from 10−3 to 103 Hz. The study concerned both particle layers obtained after the spontaneous accumulation/segregation at the dispersion interface, and layers obtained after surface compression. The analysis of these data in the framework of available models provides qualitative and quantitative information about kinetic and structural features of these complex mixed layers. The results evidence different relaxation mechanisms effective on different timescales, which involves both the particles and the surfactant adsorbed on them. Besides the specific results concerned with the particular investigated system, the study confirms the surface dilational rheology investigation as a powerful method for surface science, in particular, as far different techniques are utilised to explore dilational properties in a broad frequency window.

Effect of surfactant interfacial orientation/aggregation on adsorption dynamics.

The application of new thermodynamic adsorption isotherms allow to improve the description of surfactant adsorption kinetics based on a diffusional transport. While the consideration of interfacial reorientation corrects apparently too high diffusion coefficients, interfacial aggregation avoids too small diffusion coefficients or the assumption of adsorption barriers. The adsorption kinetics of alkyl dimethyl phosphine oxides is influenced by interfacial reorientation. While the lower homologues (C8-C12) follow the classical diffusion model, the higher homologues (C13-C15) yield diffusion coefficients several times larger than the physically reasonable values. Assuming two different adsorption states, the resulting diffusion coefficients agree with those expected from the geometric size of the molecules. The model also works well for oxyethylated non-ionics, such as C10EO8. As a second example, a good theoretical description is obtained for experiments of 1-decanol solutions when a mean surface aggregation number of n = 2.5 is assumed. The same n was obtained from the description of the equilibrium adsorption isotherm of 1-decanol. Assuming that the transition from one into the other state is controlled by a rate constant (change in orientation, formation or disintegration of two-dimensional aggregates) significant changes in the kinetics curves can result. The use of additional rate constants yields an improved fitting to experimental data.

Mixed DPPC-cholesterol Langmuir monolayers in presence of hydrophilic silica nanoparticles.

Langmuir monolayers of Cholesterol (Chol) and a mixture of Chol with 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), at a ratio of 17:83 in weight, spread on pure water and on silica nanoparticle dispersions, have been investigated measuring the compression isotherms as well as the surface pressure response to harmonic area variation of the monolayer. Aim of this study was to evaluate the effects of the interaction of silica nanoparticles with Chol and the conditions for the incorporation in the monolayer. In previous works on different kind of lipid monolayers, it has been shown that hydrophilic silica nanoparticles dispersed in the sub-phase may transfer into the monolayer, driven by the interaction with the lipid molecules that make them partially hydrophobic. The results here obtained indicate that also for Chol and Chol-DPPC mixtures the presence of silica nanoparticles may have important effects on the phase behaviour and structural properties of the monolayer. As confirmed by complementary structural characterisations, BAM, AFM and ellipsometry, the principal effect of the nanoparticle incorporation is the disruption of the monolayer packing, owing to the alteration of the cohesive interactions of lipid components.

Influence of silica nanoparticles on dilational rheology of DPPC–palmitic acid Langmuir monolayers

The effect of silica nanoparticles on the dynamic behavior of monolayers composed by 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a mixture of DPPC with Palmitic acid (PA), has been investigated by comparing the dilational rheological response of these lipid layers, spread on water and on a silica nanoparticle dispersion. To this aim, the dilational viscoelasticity has been measured against the frequency of the surface area perturbation of the monolayer, according to the Oscillatory Barrier method in a Langmuir trough. These measurements were performed at different values of the surface pressure, corresponding to different degrees of compression of the monolayer. The results show that the incorporation of particles in the layer induces additional surface kinetic processes and, depending on the surface pressure, modifies both the quasi-equilibrium dilational elasticity and the high frequency limit of the viscoelastic modulus. Another important effect concerns the linearity of the dilational rheological response which is appreciably worsened by the presence of nanoparticles. With DPPC being the major component of pulmonary surfactant and PA used as a component in synthetic substitutes of it, the results here obtained are relevant in the framework of wider studies on the effect of nanoparticles on the pulmonary surfactant interfacial properties.

MOLECULAR REORIENTATION IN THE ADSORPTION OF SOME CIEJ AT THE WATER-AIR INTERFACE

Abstract As demonstrated by some recent works the adsorption properties of some n -alkyl poly-oxyethylene glycol ethers can be explained by a model assuming the co-existence of different orientation states for the adsorbed molecules. In the present paper, with the aim of giving complete evidence of the above, the adsorption isotherms and the adsorption kinetics of some n -decyl and dodecyl poly-oxytheylene glycol ethers (C 10 E 8 , C 10 E 5 , C 10 E 4 , C 12 E 5 , C 12 E 8 ) at water/air interface have been studied by using the pendant drop technique. The data have been interpreted with a model considering the possibility of two orientation states for adsorbed molecules. The results of this study show that the model accurately describes both the equilibrium and dynamic adsorption behaviour of these molecules and that adsorption at water/air is controlled by diffusion. Moreover, the analysis of the best fit isotherm parameters of homologue molecules gives interesting information about the configuration of the adsorption layer.