Lifshitz-van Der Waals Component Research Articles

Surface Heterogeneity of Passively Oxidized Silicon Carbide Particles: Hydrophobic–Hydrophilic Partition

The surface of silicon carbide (SiC) particles previously subjected to passive oxidation has been characterized using various techniques such as adsorption of positively and negatively charged surfactants in aqueous solution, immersion microcalorimetry in three probe liquids, and flow microcalorimetry in organic media. Adsorption data show that around one quarter of the surface appears negatively charged and thus hydrophilic, while three quarters appear uncharged and hydrophobic. This is attributed to dissociation of silanols groups. Immersion calorimetry in liquids having well-defined polar and nonpolar components of surface energy shows that the Lifshitz–van der Waals component of SiC is very high and that the acid and basic components are weak. The experimental results appear to be consistent with both computations of surface energy using Lifshitz theory and experimental data previously obtained with other minerals. The three indexes are discussed and it is argued that they represent different terms of the solid surface energy.

Investigation of the effect of substrata on the surface free energy components of silica gel determined by thin layer wicking method

The components of surface free energy of silica gel deposited on glass plates, aluminium or plastic sheets (commercial Merck's plates for TLC) were determined by applying two theoretical approaches: the Lifshitz-van der Waals acid-base approach (LWAB), the geometric mean of apolar and polar interactions approach (GM). The thin layer wicking method was applied. Besides, from the measured contact angles of probe liquid (diiodomethane, α-bromonaphthalene, water, formamide, glycerol and ethylene glycol) surface free energy components were calculated for the substrata on which the silica gel was deposited. In this case the equation of state (ES) was additional by applied. The study has shown that both the kind of substratum and that of the chambers used for measuring the penetration rate of the liquid do not influence the calculated values of the surface free energy components of silica gel. The components calculated from the two models of interfacial interactions: γLWs and γABs or γds and γps appeared to be practically the same if diiodomethane and α-bromonaphthalene were considered as weakly polar liquids, i.e. donor-acceptor interactions in the surface tension were taken into account. However, when γl+ and γl− of these liquids are neglected, considerable differences occur, particularly in the determined value of Lifshitz-van der Waals component. The values of the total surface free energy of substrata calculated by applying three thermodynamic approaches are very similar, but if diiodomethane and α-bromonaphthalene are considered as weakly polar liquids this similarity is more distinct.

Some Remarks on the Components of the Liquid Surface Free Energy

Measurements of the contact angle for water, glycerol, formamide, and ethylene glycol on paraffin, polytetrafluoroethylene (PTFE), polyethylene (PE), polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), and polyethylene terephthalate, for diiodomethane on PE, PVC, PMMA, and PET, and for a series ofn-alkanes fromn-heptane ton-hexadecane on PTFE were made. On the basis of the contact angles obtained on paraffin, PTFE, and PE the Lifshitz-van der Waals components of the glycerol, formamide, and ethylene glycol surface free energy were determined. Next, by using the contact angle values of water, glycerol, formamide, ethylene glycol, and diiodomethane, the values of the electron-acceptor and electron-donor parameters of the acid-base components of the surface free energy of glycerol, formamide, and ethylene glycol were calculated and compared with the values taken from the literature. It was found that the Lifshitz-van der Waals component of the formamide surface free energy and the electron-acceptor and electron-donor parameters of the glycerol, formamide, and ethylene glycol surface free energy determined in the above-mentioned way differ from those determined by van Osset al.and by Lee. They are closer to those determined on the basis of interfacial tension measurements. Using the newly determined components and parameters it was found that PVC has an electron-acceptor character in contrast to an electron-donor character in the literature.

Wettability and surface free energy of zirconia ceramics and their constituents

Measurements of contact angle for water, glycerol, formamide and diiodomethane on surfaces of zirconia ceramic stabilized with 4% magnesia, zirconia ceramic stabilized with 3% yttria, zirconia ceramic stabilized with 5% yttria, tetragonal-Y-zirconia ceramic, cubic-Y-zirconia ceramic, zirconium (IV) oxide, aluminum oxide, magnesium oxide and yttrium oxide were conducted. Using the values of contact angle, the total surface free energy of these solids, and its components resulting from different kinds of intermolecular interactions were calculated. It was found that the Lifshitz-van der Waals component of the surface free energy of zirconia ceramics only slightly depends on the amount and kind of metal oxides present in the ceramics. The acid-base component of all the studied solids is lower than 13 mJ/m2, showing a small dependence on the kind of metal oxide. For all samples studied the electron-donor parameter of the acid-base components of the surface free energy is many times higher that electron-acceptor one. It was stated that the acid-base component probably depends on the density of OH groups on the surface of the solids studied.

Surface free energy analysis of vinyl triethoxy silane-methyl methacrylate copolymers and their homopolymer blends

Methyl methacrylate (MMA) was copolymerized with vinyl triethoxy silane (VTES) using 60Co γ-radiation at varying mol fractions of VTES from 0.02 to 0.11. The homopolymers of MMA and VTES were also synthesized by the same method. Homopolymer blends having different VTES mol fractions varying from 0.03 to 0.15 were prepared. The mol fractions of VTES units in the synthesized copolymers were determined by elemental analysis of Si using inductive coupling plasma spectroscopy (ICP). The thin copolymer and homopolymer blend films were prepared by casting from tetrahydrofurane (THF) on a mercury surface. Contact angles of paraffin, water, ethylene glycol, dimethyl formamide, and glycerol drops were measured on these copolymer and homopolymer blend films. Contact angle data were evaluated for the determination of surface free energy components of the copolymer and homopolymer blend films by using Van Oss-Good methodology. It was observed that the Lifshitz-Van der Waals component and the electron donor component of the surface free energy of copolymer and homopolymer blend films were changed to a considerable extent with the change in the VTES mol fraction of both copolymers and homopolymer blends. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1551–1556, 1998

Changes in sebum levels and skin surface free energy components following skin surface washing

The forehead skin sebum casual (or usual) level, which is remarkably stable in a given individual, is responsible for the monopolar basic character of the skin surface. However, the significance of this finding, in terms of the physicochemical behaviour and wettability of the skin surface, remains unexplained. How does this vary during sebum casual level restoration following skin washing and what is its relationship with so-called “dry skin”? The present work was aimed at answering these questions. Forehead surface free energy components were assessed before and during the sebum casual level restoration following skin washing by nonionic soap and water, in 12 healthy volunteers: four oligo- (OS), four normo- (NS), and four hyper-seborrheic (HS) skins. Skin contact angles of water, formamide and diiodomethane were measured at intervals, and surface free energy components (i.e. Lifshitz–van der Waals, electron acceptor and electron donor components) were calculated using the contact angle values and acid–base theory of C.J. van Oss et al. The results showed that a low sebum-level does not deprive the skin surface of a normal Lifshitz–van der Waals component. This suggests that a low sebum-level is sufficient to render the skin normally lipophilic. The very low acidic component of the skin surface seemed to be due to the masking of acidic moieties by adsorbed water. The basic component was sebum-level dependent below a threshold value and remained to be related to the sebum free fatty acids and especially to their outward carboxylic group orientation. We conclude that the wetting properties of the skin surface could be defined by a “surface free energy component balance” or SEB. The SEB of NS and HS skins was around 0.22, and that of OS skin was 0.07. This low SEB stems from a weak hydrophilic component, allowing OS skin to be called dry skin, or better, a dry skin surface. This approach may provide a new insight into so-called “skin hydration”.

Influence of ethyl xanthate on the wettability and surface free energy of synthetic chalcocite

Contact angles of apolar and polar liquids were measured on unoxidized and oxidized surfaces of synthetic chalcocite and on chalcocite surfaces covered with potassium ethyl xanthate film formed from aqueous solutions at different concentrations. Then, using the obtained contact angle values, the Lifshitz—van der Waals component and electron—acceptor and electron—donor parameters of the Lewis acid—base component of the surface free energy of chalcocite and chalcocite covered with ethyl xanthate (EX) film were calculated. It was found that the surface free energy of chalcocite originated from apolar and polar intermolecular interaction and that covering the chalcocite surface with ethyl xanthate changed its surface free energy and its components. It was also found that the changes of the surface free energy as a function of the ethyl xanthate concentration from which the films were formed were larger for oxidized than for unoxidized chalcocite. This probably results from the polar character of oxidized chalcocite being higher than that of unoxidized chalcocite and from the difference between the amounts of sulfur present on the unoxidized chalcocite/EX film surface and on the oxidized chalcocite/EX film surface.

THE SURFACE, INTERFACIAL AND ELECTROKINETIC PROPERTIES OF BIOMINERALS

ABSTRACT The apolar (Lifshitz van der Waals) component (γ LW S) and the polar [electron-acceptor (γ + S) and electron-donor (γ − S)] parameters of the surface tension of biominerals and surfaces of dental hard tissues have been measured, as well as their ζpotentials. The interfacial tensions between their surfaces and aqueous solutions, determined using van Oss-Chaudhury-Good equations, are comparable with the interfacial free energy values obtained from the kinetics of crystallization and dissolution using a constant composition technique. The electron-donor (γ − S) parameters indicate that most of these minerals have a moderate to high hydrophobicity. The interfacial tension values are consistent with the order of the solubilities of these minerals.

Surface free energy components of silica gel determined by the thin layer wicking method for different layer thicknesses of gel

The influence of the thickness of silica gel layer on the penetration rate of selected liquids and, in consequence, on the value of surface free energy components determined by thin layer wicking method was studied. Plates of the following gel thickness were used: 0.25, 0.5, 1.0 and 2.0 mm. Measurements of the penetration rate of apolar liquids, i.e. three alkanes: octane, nonane and decane, diiodomethane, α-bromonaphthalene and two polar liquids: water and formamide were made for this purpose. From the obtained relationships x2=f(t) the suitability of Washburn’s equation in the whole penetration range (9 cm) was confirmed for all thicknesses of porous layers. However, the penetration rate of probe liquids changed with the thickness of the deposited layer, it was mainly dependent on an effective (apparent) radius of the interparticle pores. Using these results and the appropriate form of Washburn’s equation surface free energy components of silica gel 60 (for four thicknesses of layer) were calculated. It was found that values of apolar Lifshitz–van der Waals (γLWs=41.7±0.9 mJ m-2) and polar acid–base (γABs=11.5±0.5 mJ m-2): electron donor (γ-s=50.8±0.9 mJ m-2) and electron acceptor (γ+s≡0.7±0.1 mJ m-2) components of surface free energy were very similar for different layer thicknesses. Even in the case of a 2 mm thick layer reproducible values of both Lifshitz–van der Waals and acid–base component were obtained. It is important when the thin layers are prepared in laboratory conditions, i.e. from suspensions by water evaporating, and the deposited layer is of less controlled thickness. When diiodomethane and α-bromonaphthalene are considered as weakly polar liquids, the value of Lifshitz–van der Waals component of silica gel is very close to γLWs determined from n-alkanes.

Surface Thermodynamic Properties of Micas and Related Layer Silicate Minerals

ABSTRACT The surface tension values for a series of 2:1 phyllosilicate minerals with different structural characteristics and layer charges ranging from 0 to 2 per O10 group have been examined. The average Lifshitz-van der Waals component (ΓLW)value is 40.3 ± 1.9 mJ/m2, and the average Lewis acid (®) parameter is 1.2 ± 0.6 mJ/2 for those minerals with a layer charge greater than zero. In contrast, the Lewis base parameter (γe) varied greatly from a maximum of 59.7 mJ/m to a minimum of 23.7 mJ/m2. Those minerals with a layer charge of zero (three samples) had smaller γLW values of 32.0 ± 1.6 mJ/m2, slightly larger γ® values of 2.0 ± 0.4 mj/m2 and significantly smaller values of γ® of 4.5 ± 1.6 mJ/m2. The hydrophobic versus hydrophilic character of these materials is largely governed by the value of γ° which is strongly related to the value of the layer charge. Thus, the hydrophilicity of a smectite is determined by the charge unbalancing ionic substitutions which attract hydrophilic interlayer cations and render adjacent oxygen atoms of the panicle surface hydrophilic.

Influence of ethyl xanthate on the wettability and surface free energy of galena

The contact angles of water (W), glycerol (G), formamide (F), diiodomethane (D) and 1,1,2,2-tetrabromoethane (T) were measured on a non-oxidized and oxidized surface of galena previously covered with a potassium ethyl xanthate film formed from aqueous solutions at different concentrations. Then, using the obtained values of the contact angle, the Lifshitz–van der Waals component and electron-acceptor and electron-donor parameters of the Lewis acid–base component of the surface free energy of galena covered with ethyl xanthate film were calculated from the modified Young equation. It was found that the surface free energy of the galena/ethyl xanthate film depends on the way of galena surface treatment after adsorption of ethyl xanthate on its surface and results from both apolar and polar intermolecular interactions. The contribution of these interactions in the surface free energy of galena depends on the concentration of potassium ethyl xanthate solution used for film formation and the state of the galena surface (non-oxidized or oxidized). With the increase of ethyl xanthate concentration in the solution from which the film was formed, the Lifshitz–van der Waals component increases or decreases a little depending on whether or not the surface galena, after adsorption of ethyl xanthate, was washed by ethyl ether. In all cases, the electron-acceptor parameter of the acid–base component is considerably smaller than the electron-donor one and practically does not depend on the concentration of the potassium ethyl xanthate solution in contact with galena. The changes of the electron-donor parameter of the acid–base component depend on the state of the galena surface, the concentration of the potassium ethyl xanthate solutions and the way of galena surface treatment after adsorption. It is interesting that the total surface free energy, both of non-oxidized and oxidized galena, does not practically depend on the concentration of potassium ethyl xanthate in aqueous solution from which the film was formed.

The physicochemical surface characteristics of Brevibacterium linens

The physicochemical surface characteristics and interfacial behavior of two strains of Brevibacterium linens (BL-MGE and BL-9174), that may enhance cheese flavor, were assessed. Cell surface hydrophobicity was determined by measuring the contact angle of a variety of polar and non-polar wetting agents on lawns of the bacterial cells. The contact angles obtained were used to calculate the cell surface free energy components γ LW, γ AB, γ + and γ −. The Lifshitz van der Waals component and the Lewis acid-base component were approximately 35 mJ m −2 and 22 mJ m −2, respectively, for both strains. Under conditions of physiological pH and low ionic strength, neither strain exhibited affinity for an octyl ligand in hydrophobic interaction chromatography. This occurred despite a favorable free energy of interfacial interaction which was attributable almost entirely to favorable acid-base interactions between cells and octyl-sepharose. The nature of surface functional groups was evaluated using X-ray photoelectron spectroscopy. Excluding H, the mean percentage of atomic fraction for C, O, N and P for BL-MGE was 57.7, 37.8, 3.9 and 0.6%, respectively and 61.0, 31.8, 7.0 and 0.2%, respectively for BL-9174.

Wettability of psoralen powders: influence of bile salts on their contact angles and surface free energy components

The purpose of this work was to assess the surface free energy (SFE) of bare and bile-salt-precontacted pellet surfaces of psoralens (lipophilic molecules). This study was carried out according to the approach of van Oss et al. to the solid-liquid interfacial free energy using the Lifshitz-van der Waals component, electron acceptor and electron donor parameters of the SFE. With the help of a hydraulic press, powders of 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP) and 4,5',8-trimethylpsoralen (TMP) were compacted sufficiently to obtain smooth and glossy disc surfaces. One set of pressed discs was used directly after preparation and another set was treated with a bile salt (sodium cholate/sodium deoxycholate mixture) aqueous solution of 0.1 mol l −1 for 2 h at 37°C before droplet deposition. The surface free energies of bare and bile-salt-treated discs were determined by advancing contact angles using polar (water and formamide) and apolar (diiodomethane) standard liquids. The measurements were carried out at 21.5 ± 1.5°C using a surgical microscope fitted with a slanted mirror and equipped with a video camera linked to a computer. In addition, the surface and interfacial tensions of a biphasic system of octanol and water in which psoralens were solubilized were measured using the De Noüy ring method. The results were correlated with the lipophilicity and solubility of these drugs. The data of contact angle values of psoralen bare surfaces indicated the hydrophobicity order 8-MOP < 5-MOP < TMP. The percentages of Lifshitz-van der Waals ( γ s LW) components corroborated this. 8-MOP showed the same acidic and basic values ( γ s AB = 6.5 mJm −2, while γ s AB was equal to 1.8 and 0.4 mJm −2 for 5-MOP and TMP respectively. However, when the psoralens were treated with sodium cholate sodium deoxycholate ( NaC NaDC ) mixture, the presence of micelles on the surface modified the previous data by decreasing the contact angles with polar solvents and γ s LW. In contrast, the total surface free energy of all the psoralens increased, especially the electron onor component ( γ −) and γ s AB. When comparing the three psoralens, 5-MOP behaved peculiarly as it had the lowest γ s − (29.5 mJm −2) and γ s AB (21.4 mJm −2). Theoretical explanations concerning the reactivity of the psoralens vis à vis standard liquids were assumed. The results of this work enabled us to understand qualitatively and quantitatively the types of interaction between the molecules of each studied psoralen and to predict their behaviour when they permeate through a given biological membrane (e.g. human skin). From this work the adsorption of bile salts on to the hydrophobic surface was confirmed for the psoralen case. This result could constitute the beginning of a physicochemical explanation of the digestion step for orally taken psoralens.

THE SURFACE THERMODYNAMIC PROPERTIES OF SOME VOLCANIC ASH COLLOIDS

ABSTRACT The surface tension components and parameters for nine naturally occurring colloidal-sized volcanic ash samples were determined by thin-layer wicking. These materials include rhyolites, trachytes, phonolites, and an andesite, and were collected from deposits in California, Arizona, Italy and Martinique. The samples are principally volcanic glass (with minor amounts of mixed materials of various compositions) with silica contents ranging (average values)from 74%(rhyolite) to 57% (andesite). The surface tension components were determined by means of thin-layer wicking to provide contact angles which were used to solve the Young-Dupré equation. The Lifshitz-van der Waals component of the surface tension, γLWLWLWfor these ash samples varies between 26.6 mj/2and 38.7 mj/m2The Lewis acid parameter γ⌖ values are small with values ranging from 0 mJ/m2to 1.8 mJ/m2, while the Lewis base parameter, values varied between 15.6 mJ/m2and 45.6 mJ/m2Of the nine samples, five were hydrophobic and the other four were hy-drophilic. The hydrophobic ashes were produced by eruptions with a dominantly

Change in surface properties of solids caused by grinding

The surface properties (i.e. the values of the Lifshitz-van der Waals ( γ LW) components and Lewis acid ( γ +) and base ( γ −) parameters) of talc, calcite, dolomite, silica and zirconia, untreated and as ground powders, were determined by contact angle measurements (directly on flat surfaces and by thin layer wicking for powders). The values of γ LW, γ +and γ − for these solids tend to change after grinding; γ LW and γ − decreased for all these solids and γ + often increased, depending on the material.

Conditioning Film-Induced Modification of Substratum Physicochemistry—Analysis by Contact Angles

Stainless steel, germanium, polypropylene, and perspex were coated with films derived from marine and freshwater samples and from solutions of model compounds (bovine serum albumin, lactoglobulin, myoglobin, and humic acids). Contact angles of water, formamide, and diiodomethane measured on clean and conditioned interfaces were used to calculate physicochemical surface properties by the Lifshitz–van der Waals acid–base approach, including interfacial parameters for the interaction of carbon- and nitrogen-limited cells of the Gram-negative bacterium SW8 with these surfaces. The interaction parameter φ was evaluated as an indicator of the chemical constitution of conditioned interfaces. Surface free energy and its Lifshitz–van der Waals component were modified by 59 and 66% of conditioning films, respectively, but these alterations were relatively small. The acid–base components of surface free energy and the free energy of adhesion of bacteria to substrata were, however, substantially affected by the coatings. Most films did not modify the interaction parameter φ. Those which did increased the φ of polymeric surfaces and decreased it on stainless steel. The solid–water interfacial tension (γSW), which was larger than the solid–bacterium interfacial tension (γSB) on 97% of surfaces, was reduced by 61% of the conditioning films and increased by 14% of the coatings. The γSBof nitrogen-limited cells was smaller than the γSBof carbon-limited organisms on 92% of substrata. Values of γSBof carbon- and nitrogen-limited organisms were negative on 17 and 69% of the surfaces, respectively. More coatings modified the γSBof nitrogen-limited cells (97%) than the γSBof carbon-limited SW8 (72%). The free energy of adhesion was negative on 31% of interfaces but similar for both phenotypes. It was altered by 91% of conditioning films, which generally reduced its value.

96/03583 Surface free energy and floatability of low-rank coal

Measurements were made of zeta potential of low-rank coals, the surface of which was previously covered with apolar liquid to different degrees. The zeta potentials were then used for determination of the film pressure of this liquid, making it possible to calculate the apolar Lifshitz-van der Waals (γsLW) component of the coal surface free energy. Electron donor (γs−) and electron acceptor (γs+) polar components were determined by measurement of water and glycerol contact angles on coal plates. The values of γs− and γs+ were then compared with the non-dispersion component, γsn, of the surface free energy. The values of the surface free energy components confirmed that the hydrophobic properties of a coal surface increase with increasing degree of coalification. This was also confirmed by tests on the natural floatability of these coals in a Hallimond single-bubble tube.

Microbial adhesion to solvents: a novel method to determine the electron-donor/electron-acceptor or Lewis acid-base properties of microbial cells

Lewis acid-base, i.e. electron-donor/electron-acceptor, interactions are implicated in various interffacial phenomena such as phagocytosis, biofouling and microbial adhesion. Therefore, the determination of electron-donor/electron-acceptor properties of microbial cells can be of importance in many research areas. However, until now, there has only been one method to determine these properties which is based on contact angle measurements combined with the equations of van Oss. Consequently, this method requires specific and elaborate equipment. Thus to facilitate the characterization of microbial cell surfaces, we have developed a simple, rapid and quantitative technique, the MATS (microbial adhesion to solvents) method, which is based on the comparison between microbial cell affinity to a monopolar solvent and a polar solvent. The monopolar solvent can be acidic (electron acceptor) or basic (electron donor) but both solvents must have similar surface tension Lifshitz-van der Waals components. Using this new method we have shown that Streptococcus thermophilus B (STB) and Leuconostoc mesenteroides NCDO 523 (LM 523) display maximal affinity for an acidic solvent and a low affinity for basic solvents. There was not a great difference between microbial cell adherence to basic solvents and apolar solvents, except for STB suspended in a 0.1 mol l −1 potassium phosphate buffer. These results, which demonstrate that both bacteria are strong electron donors and very weak electron acceptors, are in accordance with the energetic characteristics derived from van Oss's approach.

Surface free energy analysis of polystyrene-poly(?-hydroxynonanoate) graft copolymers

Polystyrene–poly(β-hydroxynonanoate) (PS–PHN) graft copolymers having 12, 17, 27, 31, 37, 60, 89, and 94% PS contents were prepared using active PS synthesized from styrene and oligododecandioyl peroxide initiator. Then the copolymer films were cast from chlo-roform, toluene, cyclohexanone, and butyl acetate. PS and PHN homopolymer blends were also prepared as films having 8, 12, 28, 34, 44, and 64% PS in the same casting solvents. The contact angles of water, glycerol, ethylene glycol, formamide, trans-decaline, and paraffin oil drops were measured on these cast films. The contact angle results were evaluated in terms of surface free energy components using Van Oss–Good methodology. It was determined that the Lifshitz–van der Waals component (γ) of the graft copolymers and blend of homopolymers did not differ so much with the copolymer composition. However, the electron donor component of the surface free energy (γ) decreased with the decrease of PHN content to a considerable extent. It was observed that this decrease is more linear than that of homopolymer blends. The effect of the nature of the casting solvents on the surface properties of the polymers was also investigated in terms of their solubility parameter values and it was found that the more H-bonding solvents resulted in higher (γ−s) surfaces. © 1996 John Wiley & Sons, Inc.

Surface free energy and floatability of low-rank coal

Measurements were made of zeta potential of low-rank coals, the surface of which was previously covered with apolar liquid to different degrees. The zeta potentials were then used for determination of the film pressure of this liquid, making it possible to calculate the apolar Lifshitz-van der Waals ( γ s LW) component of the coal surface free energy. Electron donor ( γ s −) and electron acceptor ( γ s +) polar components were determined by measurement of water and glycerol contact angles on coal plates. The values of γ s − and γ s + were then compared with the non-dispersion component, γ s n , of the surface free energy. The values of the surface free energy components confirmed that the hydrophobic properties of a coal surface increase with increasing degree of coalification. This was also confirmed by tests on the natural floatability of these coals in a Hallimond single-bubble tube.