C. Della Volpe

Acid–base surface free energies of solids and the definition of scales in the Good–van Oss–Chaudhury theory

The overwhelming basicity of all analysed surfaces strongly dependent on the choice of liquid triplet used for contact angle measurements and the negative values sometimes obtained for the square roots of the acid-base parameters can be summarized as the main problems arising from the application of the Good-van Oss-Chaudhury (GvOC) theory to the calculation of Lewis acid-base properties of polymer surfaces from contact angle data. This paper tries to account for these problems, namely: (1) the Lewis base, or electron donor component, is much greater than the Lewis acid or electron-acceptor component because of the reference values for water chosen in the original GvOC theory. A direct comparison of the acidic component with the basic one of the same materials has no meaning. A new reference scale for water which is able to overcome this problem is suggested. For the calculation of acid-base components, a best-fit approach is proposed which does not require any starting information about the liquids or polymers and can yield estimates of the acid-base parameters for both the liquids and the polymers involved; (2) the strong dependence of the value of the acid-base components on the three liquids employed is due to ill-conditioning of the related set of equations, an intrinsic and purely mathematical feature which cannot be completely cured by any realistic improvement in experimental accuracy. To reduce or eliminate the effect, one only needs a proper set of liquids, representative of all kinds of different solvents; (3) the negative coefficients appear as a simple consequence of measurement uncertainty, combined with the possible ill-conditioning of the equation set. We cannot exclude, however, that in some cases they could have a different origin.

The determination of a ‘stable-equilibrium’ contact angle on heterogeneous and rough surfaces

Abstract The common measurement of the contact angle is performed in conditions not corresponding to true equilibrium states and gives non-equilibrium values, the advancing and receding contact angles. To solve this problem, a very simple experimental device, based on the Wilhelmy experiment, is proposed in the present paper. It is able to transfer mechanical energy to the three-phase system in a controlled way through a simple loudspeaker; the analysis of some common surfaces is made through this method showing as a new stable minimum of the surface free energy can be attained, independent on the initial conditions and corresponding to a value of the contact angle intermediate between the advancing and receding ones. A comparison is developed with literature results on heterogeneous and rough surfaces, some ‘first-order’-approximation equations proposed in the literature are examined and compared with the new results. A simple but useful theoretical treatment is also compared with the experimental results to allow a more detailed, although qualitative-level, analysis. An important consequence with respect to the calculations of solid surface free energies is indicated.

The combined effect of roughness and heterogeneity on contact angles : the case of polymer coating for stone protection

The individual effects of heterogeneity and roughness on contact angles have been repeatedly analysed in the literature, but the application of the accepted models to practical situations is often not correctly performed. In the present paper the combined effects of roughness and heterogeneity on the contact angles of water on stone surfaces protected by a hydrophobic polymer coating are considered. Two different kinds of calcareous stone with different surface roughnesses and porosities were protected against the effect of water absorption by two different polymer coatings. The contact angles of water on the protected stone surfaces were measured by the Wilhelmy and the sessile drop techniques. A comparison of the results obtained shows not only the limits of the static sessile drop technique, but also the combined effect of roughness and heterogeneity. Some considerations are developed on the application of commonly accepted models to surfaces with a combination of roughness and heterogeneity. Some other results obtained with techniques such as roughness measurements, mercury porosimetry, energy dispersive X-ray spectroscopy (EDXS), thermogravimetric analysis (TGA), water absorption by capillarity experiments (WAC), all able to show the structure and properties of the obtained films, are also compared with those obtained from contact angle measurements. It is concluded that the static contact angle is not well correlated with the degree of protection; on the contrary, the receding contact angles are well correlated with the degree of protection actually obtained. An ideal protecting agent should have a receding contact angle greater than 90°.

The effect of surface roughness of microporous membranes on the kinetics of oxygen consumption and ammonia elimination by adherent hepatocytes

In membrane hybrid liver support devices (HLSDs) using isolated hepatocytes where oxygen is transported only by diffusion to the cells, about 15-40% of the cell mass is likely to be in direct contact with the semipermeable membranes used as immunoselective barriers: quantitative effects of membrane surface properties on the kinetics of hepatocyte metabolic reactions may also affect HLSD performance. In this paper, we report our investigation of the effects of surface morphology of two microporous commercial membranes on the kinetics of oxygen consumption and ammonia elimination by primary hepatocytes in adhesion culture. Isolated rat hepatocytes were cultured on polypropylene microporous membranes with different surface roughness and pore size in a continuous-flow bioreactor whose fluid dynamics was optimized for the kinetic characterization of liver cell metabolic reactions. Collagen-coated membranes were used as the reference substratum. Hepatocyte adhesion was not significantly affected by membrane surface morphology. The rates of the investigated reactions increased with ammonia concentration according to saturation kinetics: the values of kinetic parameters Vmax and K(M) increased as cells were cultured on the membrane with the greatest membrane surface roughness and pore size. For the reaction of oxygen consumption, Vmax increased from 0.066 to 0.1 pmol h(-1) per cell as surface roughness increased from 70 to 370 nm. For the kinetics of ammonia elimination. K(M) increased from 0.23 to 0.32 mM and Vmax increased from 1.49 to 1.79 pmol h(-1) per cell with membrane surface roughness increasing from 70 to 370 nm. Cells cultured on collagen-coated membranes consistently yielded the highest reaction rates. The Vmax values of 0.18 and 2.84 pmol h(-1) per cell for oxygen consumption and ammonia elimination, respectively, suggest that cell functions are also affected by the chemical nature of the substratum.

Recent theoretical and experimental advancements in the application of van Oss-Chaudury-Good acid-base theory to the analysis of polymer surfaces. I. General aspects

The acid-base theory as developed by van Oss, Chaudury and Good is a powerful tool to analyze the surface free energy of polymeric materials; however, some problems are encountered in its application and some authors have shown that these problems can be theoretically solved considering this theory as an example of the so-called LFER theories. From this point of view, the definition of a well-defined scale of acid-base strength and the use of a wide and well-equilibrated, appropriate set of liquids is very important. In the present paper some recent results are presented which are based on the mathematical approach discussed by Della Volpe and Siboni in previous papers. The treatment is developed as a list of questions, Frequently Asked Questions (FAQs), whose theoretical implications are discussed using numerical examples chosen from the literature. Some literature data, collected by the opponents of the acid-base theory and recently published, are re-analysed using these methods, showing that they constitute a well-defined set to calculate, with a good precision, the acidbase components of the considered materials and the interfacial energies of liquids used. The present paper is the premise of a second one, in which a set of contact angles data collected by the authors and by other researchers will be analysed following the principles discussed here.

Polymeric membranes for hybrid liver support devices: the effect of membrane surface wettability on hepatocyte viability and functions.

Extracorporeal therapies based on membrane hybrid liver support devices using primary hepatocytes are an interesting approach to the treatment of acute hepatic failure. In such devices, semipermeable polymeric membranes are effectively used as immunoselective barriers between a patients blood and the xenocytes in order to prevent the immune rejection of the graft. The membranes may act also as the substratum for cell adhesion, thus favouring the viability and functions of anchorage-dependent cells such as the hepatocytes. Membrane cytocompatibility is expected to depend on the surface properties of the polymer, such as its morphology and its physico-chemical properties. In this paper, we report our investigation on the effect of the surface wettability of membranes on hepatocyte viability and functions. Polypropylene microporous membranes were modified to increase their surface wettability and were used as substrata for rat hepatocyte adhesion culture. Isolated hepatocytes were also cultured on collagen as a reference substratum. Hepatocyte viability generally improved as the cells were cultured on more wettable membranes. In agreement with the viability data, the increasing wettability of the membrane surface also improved some metabolic functions.

Air-plasma treated polyethylene fibres: effect of time and temperature ageing on fibre surface properties and on fibre-matrix adhesion

The effect of a low energy air-plasma treatment of extended chain polyethylene (ECPE) fibres (Spectra®900) on the adhesion with a matrix of epoxy resin has been studied. Surface energies of fibre and matrix were calculated by contact angles, measured with a Wilhelmy microbalance in different liquids, and the adhesion between fibres and the matrix was evaluated through a pull-out test. The results showed an increase in fibre-matrix adhesion by a factor of ca. 1.5 calculated by surface energy measurements, and by a factor of ca. 4 measured by the pull-out tests. Time (up to six months) and temperature (in the range 20–120 °C for 2 h) ageing caused some decrease in adhesion with respect to the values evaluated just after the fibre plasma treatment. The plasma treatment did not affect the fibres mechanical properties.

Thermomechanical behaviour of interfacial region in carbon fibre/epoxy composites

Abstract A study of the thermomechanical stability of the fibre-matrix interphase in carbon/epoxy composites has been carried out. The thermodynamic work of adhesion has been evaluated at room temperature by wetting measurements. The interfacial shear stress transfer level 〈τ〉 for sized and desized carbon fibre has been measured as a function of temperature by means of a single-fibre fragmentation test. As the test temperature increased 〈τ〉 values were found to decrease, with values being higher for the desized carbon fibre. The dependence of interfacial shear stress transfer on bulk matrix mechanical properties (modulus and shear strength) has also been discussed. Dynamic mechanical measurements performed on single-bundle composites confirmed the better thermomechanical stability of the desized fibre interphase.

A multiliquid approach to the surface free energy determination of flame-treated surfaces of rubber-toughened polypropylene

In the present work, rubber-toughened polypropylenes (TPOs) with different properties of the rubbery phase, consisting in different degrees of rubber dispersion and different levels of rubber crystallinity, were considered. The flame-treated surfaces of these materials and their interfaces with a commonly used primer were studied by dynamic contact angle (DCA) analysis using seven liquids and by scanning probe microscopy (SPM). The contact angle data were analysed using the concepts and the equations of the surface free energy acid-base component theory. A new approach consisting in the use of a large number of probe liquids and of a proper mathematical method is proposed; it allows higher precision in the determination of the surface energy components and the work of adhesion, the reduction of possible artefacts, and the calculation of standard deviations of obtained quantities. It was found that: (i) the characteristics of the flame-treated surfaces were largely independent of the composition and morpho...

A comparative analysis of surface structure and surface tension of hybrid silica films

Unsupported SiO2-based films have been prepared by cohydrolysis of methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) in different molar ratios. Analysis of surface structure and surface tension have been performed; FTIR-ATR (attenuated total reflectance) results were compared with results obtained from atomic force microscopy (AFM) and dynamic contact angle (DCA) techniques. The chemical nature of the exposed surface is discussed on the basis of calculations of surface tension components using different theoretical models. Due to the different sampling depth of the techniques used, a complete and original description of the surface structure of SiO2-based films is proposed. The relative ratio between the exposed groups Si–OH, Si–O− and Si–CH3 depends on the TEOS/MTES ratio. Increasing %MTES produces a decrease of Si–OH and Si–O− surface content. The surface of pure MTES derived film appears to be a flat and dense arrangement of Si–Me groups pointing outward.