S. Siboni

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°.

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.

Enzymatically-tailored pectins differentially influence the morphology, adhesion, cell cycle progression and survival of fibroblasts

Improved biocompatibility and performance of biomedical devices can be achieved through the incorporation of bioactive molecules on device surfaces. Five structurally distinct pectic polysaccharides (modified hairy regions (MHRs)) were obtained by enzymatic liquefaction of apple (MHR-B, MHR-A and MHR-alpha), carrot (MHR-C) and potato (MHR-P) cells. Polystyrene (PS) Petri dishes, aminated by a plasma deposition process, were surface modified by the covalent linking of the MHRs. Results clearly demonstrate that MHR-B induces cell adhesion, proliferation and survival, in contrast to the other MHRs. Moreover, MHR-alpha causes cells to aggregate, decrease proliferation and enter into apoptosis. Cells cultured in standard conditions with 1% soluble MHR-B or MHR-alpha show the opposite behaviour to the one observed on MHR-B and -alpha-grafted PS. Fibronectin was similarly adsorbed onto MHR-B and tissue culture polystyrene (TCPS) control, but poorly on MHR-alpha. The Fn cell binding site (RGD sequence) was more accessible on MHR-B than on TCPS control, but poorly on MHR-alpha. The disintegrin echistatin inhibited fibroblast adhesion and spreading on MHR-B-grafted PS, which suggests that MHRs control fibroblast behaviour via serum-adhesive proteins. This study provides a basis for the design of intelligently-tailored biomaterial coatings able to induce specific cell functions.

Numerical models for the evaluation of the contact angle from axisymmetric drop profiles: A statistical comparison

Axisymmetric drop shape analysis (ADSA) is a well-established methodology for estimating the contact angle value and the surface tension of liquids starting from sessile drops images. It consists of an iterative procedure in which a best fit between a theoretical axisymmetric Laplacian curve and an experimental drop profile is performed. When only an evaluation of the geometric contact angle value is needed, a similar numerical approach can be adopted by using simpler algebraic models in place of a Laplace profile, thus allowing more straightforward implementations and shorter computation times. In this work the relative merits of the different methodologies are compared. Beside the standard ADSA procedure, four different mathematical models are examined, namely the circular and elliptical models, the first-order perturbative solution of the Laplace equation, and a cubic spline model. Their relative statistical performances are tested on both calculated and experimental drop profiles. For simulated drops, the actual capability of the models to predict the correct contact angle is also investigated.

Common volume functions and diffraction line profiles of polyhedral domains

A general numerical algorithm is proposed for the fast computation of the common volume function (CVF) of any polyhedral object, from which the diffraction pattern of a corresponding powder can be obtained. The theoretical description of the algorithm is supported by examples ranging from simple equilibrium shapes in cubic materials (Wulff polyhedra) to more exotic non-convex shapes, such as tripods or hollow cubes. Excellent agreement is shown between patterns simulated using the CVF and the corresponding ones calculated from the atomic positions via the Debye scattering equation.

Hydrophobic siloxane paper coatings: the effect of increasing methyl substitution

Paper is an organic material widely used in cultural heritage and mainly composed of cellulose mixed with lignin, hemicellulose and small amounts of additives. This paper deals with siloxane coatings on pure cellulose paper, applied by sol–gel dipping in sols prepared with different siloxane precursors (tetraethoxysilane, methyl triethoxysilane, dimethyl diethoxysilane, trimethyl monoethoxysilane). The coated samples were characterized using various techniques (Fourier Transform Infrared Spectroscopy FT-IR, Nuclear Magnetic Resonance NMR and Scanning Electron Microscopy and Energy Dispersive Spectroscopy SEM–EDS), measuring their mechanical properties, flame resistance and contact angles, and a colorimetric test. The coated samples’ behavior was more hydrophobic the higher the methyl number of siloxane precursor, regardless of the coating’s thickness. Increasing the thickness improved the mechanical and thermal properties. The thickest coatings were obtained using a double coating process and a basic catalyst for the hydrolysis step, but this latter condition facilitated the formation of surface agglomerates, which make the paper too stiff and yellow.

Analysis of dynamic contact angle on discoidal samples measured by the Wilhelmy method

Contact angle measurements have been performed using the Wilhelmy microbalance on disc-shaped samples of different thicknesses and these are compared with the theoretical analysis proposed by Dryden and Andrade (D-A). In disagreement with the original D-A results, our conclusion is that not only are the force values well reproducible, but also the determination of the contact angles, by inverting the force equations, provides acceptable estimates. This conclusion is supported by the analysis of error propagation for the various physical and geometrical parameters of the model. By taking into account the thickness of the samples a correction is introduced, the significance of which is analysed with respect to some of the experimental parameters. Moreover, a second method of analysis based on an iterative procedure is proposed; it uses only the force data collected during the immersion of the diameter zone of the disc samples and can be applied to both discoidal (e.g. lenses) and plane disc-shaped samples. Finally, an accurate analysis of the meniscus shape during the immersion leads to a third method for the estimation of contact angles on discs. It requires the detection of the immersion level at which the lateral meniscus becomes perfectly planar. Its use is restricted to the case of thick discs and the precision achieved is not high.

Wettability of Porous Materials III: Is the Wilhelmy Method Useful for Fabrics Analysis?

Woven and nonwoven fabrics and the corresponding fibers have been analyzed using a Wilhelmy microbalance. A self-developed analysis software (written using Labview flow-data language) has been used to extract the advancing and receding angles and also other informations commonly neglected by the standard data analysis of Wilhelmy technique, such as the immersion cross-sectional area, the effective wet perimeter, the liquid absorption, the parallelism of the straight lines describing the immersion and the withdrawal of the sample in a typical force vs immersion graph. This approach has previously been proposed for rigid porous materials such as stones and wood with some success. In the present cases some modifications of the approach have been introduced to take into account the specificity of the materials. The results appear promising for the more rigid nonwoven fabric, but suffer from a greater uncertainty and from even unacceptable inconsistencies in the case of the softer and traditional fabric. Through the application of the proposed method and software to the Wilhelmy approach the precision of the measurements can be increased in the most common cases and may help in estimating wettability. This remains true even for soft fabrics, although in this case some very important limitations of the experiment and of the model are unable to eliminate systematic errors, with the consequence that only a qualitative estimate of contact angles can be achieved.