The Effect of Copolymer Composition on Surface Free‐Energy of Poly(2‐Hydroxyethyl Methacrylate–Crotonic Acid) Copolymers

Abstract

Poly(2‐hydroxyethyl methacrylate–crotonic acid) P(HEMA/CrA) copolymers with varying compositions were prepared from ternary mixtures of 2‐hydroxyethyl methacrylate (HEMA)/crotonic acid (CrA)/water by using ⁶⁰Co γ‐rays. The wetting forces were determined according to the Wilhelmy Plate Technique. Di‐iodomethane, ethylene glycol, and formamide were used as probe liquids. Lifshitz–van der Waals surface energy components, Lewis acid–base surface components, and total surface energies were calculated using van Oss et al. methodology. It was determined that Lifshitz–van der Waals component (γ_S ^LW) of the copolymers did not differ much from the copolymer composition. However, the electron‐donor surface free energy components (γ_S ^?) of the copolymers were decreased considerably with the increase of the CrA content of the copolymers, the surfaces of these copolymers were still found to have a basic character.     

Surface free energy of sulfur--revisited I. Yellow and orange samples solidified against glass surface

Surface free energy of two different samples of solidified sulfur (yellow and orange) was investigated, using several approaches for its determination. It was found that values determined about two decades ago for surface free energy of sulfur were overestimated. From current studies the apparent value of this energy ranges between 30 and 60 mJ/m(2), depending on the kind and age of the sulfur samples (up to 1 year old) and/or the probe liquid used for the advancing and receding contact angle measurements. The energy has been calculated from van Oss et al.'s approach (Lifshitz-van der Walls, electron-donor, and electron-acceptor components), the contact angle hysteresis approach proposed by Chibowski, the equation of Owens and Wendt (dispersion and polar components), and Neumann et al.'s equation of state, as well as from equilibrium contact angle using Tadmor's procedure. The lowest values of the energy for 3-day- and 3-month-old samples of sulfur were calculated from the equation of state; they were below the range mentioned above.     

Application of an extended DLVO theory for the calculation of the interactions between emulsified oil droplets in alcohol solutions

The stability of diluted emulsions (0.1% v/v) of n-dodecane in 1 M methanol, ethanol or propanol was studied. The effective diameter and zeta potential were determined by dynamic light scattering. The parameters were measured 5, 30, 60, 120 min and after 1 day after preparation of the emulsions by mechanical mixing at 10 000 r.p.m. for 3 min. Calculations of the free energy interactions between dodecane droplets were conducted applying van Oss et al.’s extended DLVO theory, in which acid–base interactions involving electron donor and electron acceptor parameters are also accounted for. For this purpose the interfacial tensions of oil–alcohol solutions were taken from the literature. The acid–base interactions were evaluated considering two variants. In the first we assumed a close-packed monolayer of alcohol molecules on the droplet surface, interacting by hydrogen bonds with water as well with alcohol molecules. In the second variant, it was considered that in these electrolyte-free systems (pH close to neutral) the measured zeta potentials were due to the oriented alcohol dipoles on the droplet surface. This would mean that the slipping plane is very close to the droplet surface. Both variants lead to the same conclusion that in these system the dominant role is played by attractive acid–base interactions, which is much bigger than the equally attractive apolar Lifshitz–van der Waals interaction. Repulsive electrostatic interactions play only a minor role.     

Characterization of the surface properties of xylan by FT-Raman spectroscopy and wicking technique

Using FT-Raman spectroscopy, column wicking technique and the equations of Washburn as well as van Oss et al., the surface properties of xylan, the main component in hemicelluloses, has been characterized and estimated. Raman spectrum showed that xylan has been structured by acetyl group and methyl-bonded glucurono group. Obtained results show that the surface free energy of xylan is higher in comparison with literature reported values for cellulose because the former has a larger Lifshitz–van der Waals component than the latter. However, xylan has been found to have very smaller polarity and orientation data than that of cellulose.     

Intermolecular Interactions and the Adhesion of Oleic Acid

The method presented by Good, van Oss, and Chaudhury was applied to characterize intermolecular interactions and the adhesion of oleic acid to selected model surfaces. Interfacial tensions of oleic acid were on the order 11–12 mJ/m² in aqueous solutions and 31–32 mJ/m² at air. The dispersive contribution to the surface tension of oleic acid against different neutral interfaces was determined to be 24–31 mJ/m² in air. Contact angles of oleic acid on selected hydrophilic and hydrophobic model surfaces were measured both in air and in aqueous solution. Van der Waals (dispersive) interactions determined the wetting properties of oleic acid in air both on nonpolar and basic surfaces. As expected, the adhesion of oleic acid to hydrophilic surfaces was much lower and to hydrophobic surfaces higher in aqueous environment than in air. The adhesion in aqueous environment is mainly governed by the cohesive and adhesive properties of water. It was concluded that the GvOC method in this case was only capable to give qualitative information about Lewis acid-base and van der Waals properties of surfaces and liquids, an important limiting factor being the asymmetry of oleic acid and the common probe liquids (diiodomethane and water).     

Topography and surface free energy of DPPC layers deposited on a glass, mica, or PMMA support

An investigation of energetic properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) layers deposited on glass, mica, and PMMA (poly(methyl methacrylate)) surfaces was carried out by means of contact angles measurements (advancing and receding) for three probe liquids (diiodomethane, water, and formamide). DPPC was deposited on the surfaces from water (on glass and mica) or methanol (on PMMA) solutions. The topography of the tested surfaces was determined with a help of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Using the measured contact angles, the total apparent surface free energy and its components of the studied layers were determined from van Oss et al.'s (Lifshitz-van der Waals and acid-base components, LWAB) and contact angle hysteresis (CAH) approaches. It allowed us to learn about changes in the surface free energy of the layers (hydrophobicity/hydrophilicity) depending on their number and kind of support. It was found that the changes in the energy greatly depended on the surface properties of the substrate as well as the statistical number of monolayers of DPPC. However, principal changes took place for first three monolayers.     

Zeta potential and surface free energy changes of solid-supported phospholipid (DPPC) layers caused by the enzyme phospholipase A2 (PLA2)

Stability and wetting properties changes of systems formed of phospholipid DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) layers covering silica particles or glass slides due to the phospholipase A₂ (PLA₂) action were determined by zeta potential measurements and the surface free energy evaluation, respectively. The comparison of the zeta potential and surface free energy, which was evaluated from advancing and receding contact angles via applying models of interfacial interactions, i.e. van Oss et al. (LWAB) and contact angle hysteresis (CAH), was found to be helpful for better understanding the mechanism of PLA₂ action on the lipid layers, what is discussed in the paper.     

POLAR INTERACTIONS OF HYALURONIC ACID -EXPERIMENTS AND MOLECULAR DYNAMICS SIMULATIONS-

ABSTRACT

To study the polar interactions of the cartilage component hyaluronate (HA) contact angle measurements of polymer films of sodium hyaluronate and of the free hyaluronic acid with different probe liquids and theoretical investigations with molecular dynamics simulation (MD) on polymer segments in aqueous environment were performed. For the designation of contact angles water, formamid, glycerol and α-bromnaphthalene as probe liquids were used. The surface tension components were calculated on the basis of the theory of van Oss using the Young equation. Experimental investigations were done with air dried layers of the sodium salt of HA whose surface has been formed at the interface to the air resp. glass support. Whereas the surface polymer/air is characterized by small, but non-zero values for γ^? and γ^-the surface polymer/glass tends to have γ^-monopolar properties. In opposite to the salt form of HA a strong repulsion of chains and high γ^- monopolarity was measured for the protonated form.

The molecular dynamics simulation (MD) on HA in water were carried out employing the force field CHARMM and the water model TIP3P. MD trajectories of HA tetramer subunit surrounded by approximately 950 water molecules were produced up to 3 ns. The interaction energies of HA and water, hydrogen bonding, and the orientation of water molecules at different solute atom groups were calculated. On the basis of energy and geometry criteria, the number of hydrogen bonds between the water molecules and the polymer acceptor atoms was determined to be between 10 and 15 per dimer unit.     

Wettability study of surface modified silica aerogels with different silylating agents

In wettability study, surface free energy interactions are of crucial importance for silica aerogels in which absorption of organic liquids and transportation of chemicals carried out for chemical and biotechnological applications. In present study, we have used Lifshitz–van der Waals/acid–base approach for calculation of surface free energy of aerogel sample. We have investigated that the surface free energy values of aerogels are 45.95, 51.42 and 45.69 mJ/m² by modifying their surfaces using 7 % chlorotrimethylsilane (TMCS), dimethyldichlorosilane (DMDCS) and hexamethyldisilazane (HMDZ) silylating reagents with solvent, respectively. The alcogels were prepared by two step acid–base catalyzed process where the molar ratio of precursors tetraethoxysilane:methanol:oxalic acid:NH₄OH:NH₄F was kept at optimal value of 1:16.5:0.71:0.58:0.60:0.98, respectively. To modify gel surfaces, TMCS, DMDCS and HMDZ concentration have been varied from 5 to 12 % and such alcogels were dried at ambient pressure. The aerogels have been characterized by fourier transform infrared spectroscopy, scanning electron microscopy, thermo-gravimetric and differential thermal analysis and Wetting properties of silica aerogel surfaces was studied by contact angle measurements. The surface chemical composition of DMDCS modified silica aerogels was studied by using X-ray photoelectron spectroscopy. As there is not any direct method, we have used Lifshitz–van der Waals/acid–base approach which gives, polar and non-polar components of aerogels surface free energy.     

Adhesion and components of solid surface energies

Contact angle data for sets of probe liquids allow the determination of components of solid surface energies which in turn can be used to calculate the work of adhesion of other materials to the solid surface. There is much debate currently about the correct choice of the acid–base components for the probe liquids. For many systems, the strength of adhesion measured independently correlates well with the calculated work of adhesion. Recent trends in this area include adhesion under water and the adhesion of bacterial and other cells to immersed solids.     

茶多酚的表面性能研究

通过分别测试二碘甲烷、水、甲酰胺等液体在茶多酚上的接触角,并根据van Oss-Chaudhury-Good的组合理论对茶多酚的表面性能进行研究。结果表明茶多酚的表面能随着茶多酚的含量增加而提高;且随着茶多酚的含量增加,儿茶素类物质即含酚羟基类物质的含量进一步增加,形成氢键的几率进一步增大,导致分子之间作用力和氢键力加大,因而使茶多酚的表面能提高。     

Surface free energy of ladderlike polyepoxysiloxane–diamine reaction systems

We studied three kinds of ladderlike polyepoxysiloxanes, which have different side groups grafted on the ladderlike backbones. 1,3‐Bis(aminopropyl)tetramethyl disiloxane (diamine) was used as the curing agent. The reaction between ladderlike polyepoxysiloxanes and diamine was investigated by contact angle measurements and surface free energy study. Several factors such as diamine amount, reaction time, and temperature can affect the systems' surface tension (or surface free energy), which were determined by two‐liquid geometric and three‐liquid acid‐base methods. The experimental results showed that an increase in the diamine amount in the reaction systems results in an increase in the polar part of surface free energy because of electron donate characteristics of the diamine. However, because epoxy (electron acceptor) and diamine (electron donor) react fast at elevated temperatures, increasing reaction temperature decreases the polar part of the surface free energy, while increases the nonpolar part of the surface free energy. The evolution of surface free energy with time for various epoxy–diamine reaction systems at various temperatures has also been studied. It was found that it took a relatively long time (50–60 h) to reach the equilibrium state. The experimental results can be well interpreted by the epoxy–diamine reaction mechanism and van Oss–Good's Lewis acid‐base theory. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1449–1460, 2000     

The total surface free energy and the contact angle in the case of low energetic solids

Using the literature data of the refractive index, the structural unit molar volume of polymers and their dipole moment, as well as the literature data of the polarizability, ionization potential, and dipole moment of many liquids, values of the Φ parameter for paraffin—liquid and polymer—liquid interfaces were calculated. Next, introducing these values of Φ and the earlier measured values of the contact angle for many liquids to the Young equation, values of the surface free energy (γ_S) of paraffin, polytetrafluoroethylene (PTFE), polyethylene (PE), polyethylene terephthalate (PET), and polymethacrylate (PMMA), were determined. It was found that the average values of γ_S for these solids were in agreement with those calculated on the basis of geometric, harmonic, or harmonic—geometric mean approaches. The values of the surface free energy of paraffin, PTFE, PE, PET, and PMMA were also calculated from the Young equation modified by Neumann et al. and, using the earlier measured values of the contact angle for many liquids, they were compared with the values obtained by other methods. Next, employing the mean value of the surface free energy, values of the contact angles for many liquids were calculated and compared with those measured earlier for the same liquids. It was found that for paraffin, PTFE, and PE there were big differences among the values of their surface free energies calculated from the contact angles for some liquids; however, the average values were in agreement with those obtained by other methods. The average values of the surface free energies of PET and PMMA were also in the range of the results obtained by other authors. It was also found that the average deviations of the contact angles calculated from the Young equation modified by Neumann et al. from the measured ones were slightly larger than those of the contact angles calculated from equations employing the geometric and harmonic means of the surface free energy components; the method of Neumann et al. may also be used to predict the wettability in some systems.     

Determination of free surface-energy parameters using a spatial method

A spatial method for determining acid and base parameters of the free surface energy of different solid surfaces using multidimensional approximation based on reduction of the van Oss-Chaudhury-Good equation to the equation of a plane is developed. The obtained results confirm the efficiency of the spatial method for correct determination of free surface-energy parameters for different substances.     

Ionic surfactant adsorption onto activated carbons

The adsorption of sodium dodecyl sulfate onto a set of activated carbons from aqueous solutions has been studied in the low concentration range. The adsorption isotherms are reasonably well fitted by a double Langmuir equation but the calorimetry of adsorption enthalpies shows a rather wide distribution of energies. This distribution is related to direct adsorbate-adsorbent interactions in pores of different size, without noticeable contributions from the chemical nature of the surface. The adsorbate-adsorbent interaction free energy through water is evaluated using the model proposed by van Oss and co-workers for the interfacial free energy. The obtained results indicate that the calculated free energy is in good agreement with that found from application of the double Langmuir equation to the adsorption isotherms.     

A Novel Method for Surface Free-Energy Determination of Powdered Solids

Interfacial solid/liquid interactions play a crucial role in wetting, spreading, and adhesion processes. In the case of a flat solid surface, contact angle measurements are commonly utilized for the determination of the solid surface free energy and its components. However, if such a surface cannot be obtained, then the contact angle can not be measured directly. Usually methods based on imbibition of probe liquids into a thin porous layer or column are applied. In this paper a novel method, also based on the capillary rise, is proposed for the solid surface free-energy components determination. Actually, it is a modification of the thin column wicking method; similar theoretical background can be applied together with that appropriate for the capillary rise method of liquid surface tension determination. The proposed theoretical approach and procedure are verified by using single glass capillaries, and then alumina and ground glass powders were used for the method testing. Thus obtained surface free-energy components for these solids, for both glass and alumina, agree well with the literature values.     

Changes in wetting properties of alumina surface treated with DPPC in the presence of phospholipase A2 enzyme

Wetting properties of commercial Al(2)O(3) plates contacted with dipalmitoylphosphatidylcholine (DPPC) or DPPC+enzyme (phospholipase PLA(2)) in NaCl solution were determined by thin layer wicking and with the help of Washburn equation. Van Oss et al.'s approach to interfacial free energy interactions was applied to determining the solid surface free energy components. Wicking experiments were performed both for bare and alumina plates precontacted overnight with the probe liquid saturated vapours, as well as the untreated and DPPC (or DPPC+PLA(2)) treated alumina plates. For this purpose the penetration rates of n-octane, water and formamide were measured. From these experiments it resulted that original alumina surface is strongly polar with electron-donor interactions originating from the surface hydroxyl groups. Adsorption of DPPC on Al(2)O(3) plates slightly increased the hydrophobic character of the alumina surface (considerable decrease of the electron-donor, γ(s)(-) parameter and γ(s)(AB) component was visible) in such a way that the hydrocarbon chains were directed outwards and the polar part towards the alumina surface. However, after the enzyme action the products of DPPC hydrolysis by PLA(2) (palmitic acid and lysophosphatidylcholine) increased again the hydrophilic character of Al(2)O(3) surface (a minor increase in γ(s)(AB) component and drastic increase of the electron-donor γ(s)(-) parameter was noticeable). After treatment with DPPC or DPPC+enzyme PLA(2) solution the changes of the total surface free energy of alumina and its Lifshits-van der Waals (γ(s)(LW)) component were in the range 7-10 mJ/m(2), but the most considerable and delivering more interesting information were the changes of the electron-donor (γ(s)(-)) parameter ranging from 27 to 35 mJ/m(2). Moreover, the changes of the alumina surface wettability were dependent on the time of the enzyme contacting with DPPC in NaCl solution. On the basis of the obtained results it seems that the thin layer wicking method can be an additional useful tool in investigations of the effect of phospholipid and PLA(2) action on the hydrophilic-hydrophobic character of alumina surface.     

Free energy of interaction of sodium dodecyl sulfate in aqueous solution with carbon black surfaces

The free energy of the adsorption process of an ionic surfactant from aqueous solutions onto a set of carbon blacks in the range of low concentrations was evaluated using the model proposed by van Oss and co-workers. The obtained results indicated that the free energy of interaction between adsorbent and adsorbate through water results mainly from Lifshitz-van der Waals and electrostatic interactions, and its value showed a good correspondence with that previously found from a combination of the classical measurements of adsorption isotherms and the Langmuir model.     

Wettability of plasma-polymerized vinyltriethoxysilane film

Plasma-polymerized films of vinyltriethoxysilane were surface characterized using the sessile drop technique. The surface free energy and its components were evaluated using the Owens-Wendt-Kaelble geometric mean method, Wu harmonic mean method, and van Oss, Chaudhury, and Good acid-base theory. Influence of deposition conditions on the surface free energy was demonstrated in the study. Improved wettability of the films was related to the diminished concentration of apolar methyl groups in plasma polymers. An increased concentration of carbonyl and hydroxyl groups resulted in a very small improvement of the polar component.