Thin-layer Wicking Technique Research Articles

Determination of relevance between surface free energy and adsorption capacity of cement particles

The compatibility between superplasticizer and cement was influenced by the adsorption capacity of cement particles. This study investigated the relevance between the adsorption capability and surface free energy. Adsorption capacity and surface free energy of both sulphoaluminate cement and portland cement were measured. The adsorption capacity of cement particles was measured by ultraviolet spectrophotometry. The test showed that particles of sulphoaluminate cement adsorbed more molecules of superplasticizer than portland cement particles. The weight of superplasticizer adsorbed by 2g of sulphoaluminate cement and portland cement were 0.28mg and 0.159mg respectively. Surface free energy of cement particles was calculated by contact angle and the contact angles were determined by the thin-layer wicking technique and washburn equation which is theoretical basis of thin-layer wiching technique presented by Chibowski E. The sulphoaluminate cement, portland cement's surface free energy were 51.46 mJ·m-2 and 49.36 mJ·m-2 respectively. The results showed that the higher adsorption capacity of particles was usual accompanied by higher surface free energy. The fluidity of cement paste was influenced by the adsorption capacity of cement particles because the more molecules of superplasticizer was adsorbed by cement particles there were lacking superplasticizer in the paste. The macro-behaviour of higher adsorption capacity is that the cement paste need more superplasticizer to reach the needed fluidity.

Hydrophilicity characterization of Al2O3-coated MoS2 particles by using thin layer wicking and sessile drop method

When hydrophobic particles are employed to prepare self-lubricating electroless composite coatings, they usually require surface modification to improve their wettability. The study focuses on the resultant change in wettability of the MoS2 particles with an inorganic coating. Al2O3-coated MoS2 particles were prepared by using a heterogeneous nucleation process. Then, particle surface morphology and structure were examined by using SEM, EDS and XRD. Contact angle measurements were conducted by using a thin layer wicking technique and a sessile drop method in order to determine the surface free energy components of the bare and the coated MoS2 particles. Our results showed the MoS2 particles were successfully coated with an Al2O3 layer, which is an amorphous structure. The decreased water contact angle indicates an enhanced hydrophilicity for the coated particles. This decrease depends more strongly on a significant decrease of a new parameter ratio, γ+/γ−, than on a slight increase in total surface free energy. The surface energy component of the bare MoS2 is very consistent with that of Talc, and that of the Al2O3-coated one is very similar to that of alumina, apart from the base component.

Wettability of cotton fabric by aqueous solutions of surfactants with different structures

In this paper the results of investigation into influence of surfactant structure on wettability of desized (D) and alkaline scoured (A) woven cotton fabric are presented. Two anionic surfactants sodium dodecylsulfate (SDS) and sodium dioctylsulfosuccinate (SDOSS) and a non-ionic surfactant Triton X 100 (TX100) were used. In wetting experiments the dynamic contact angle, θ a, is determined by use of thin-layer wicking technique, where the penetration rate of surfactant solutions of different concentrations into studied cotton samples, deposited on a glass plate, was measured. Surface free energy components for samples D and A were also calculated from results of thin-layer wicking. Performance of surfactants as wetting agents is discussed in terms of the spreading coefficient, S L/S, and adhesion tension. The results show that the surfactant penetration rate into the studied fabric samples depends on structure and concentration of the surfactant as well as on surface properties of the sample. In the concentration range from 1 × 10 −5 to 5 × 10 −2 mol/kg, all three surfactants spread over fabric samples forming a finite θ a, which results in negative values of S L/S. It decreases with increasing surfactant concentration causing an increase of surfactant wetting power. Irrespective to pretreatment of cotton samples, TX100 causes the highest wetting power, followed by SDOSS and SDS. It is related to their chemical structure, where the branched hydrophobic group of TX100 causes much higher increase of diffusion rate in comparison with SDOSS and SDS, which contain straight hydrophobic alkyl chains. The influence of hydrophobic group of studied surfactants on diffusion rate is stronger than of the hydrophilic one. The results also show that the mechanism of surfactant adsorption onto cotton samples from aqueous solution, which depends on attractive interactions between the surfactant and cotton surface, strongly influences the value of adhesion tension. The increase of adhesion tension with decreasing surface tension for all the studied surfactants on nonpolar sample D indicates that the mode of the surfactant adsorption increases the sample hydrophilicity resulting in the improved wettability. The opposite phenomenon was obtained in the case of polar sample A, where an increase of sample hydrophobicity, which is caused by adsorption of surfactants at concentrations lower than c.m.c., decreases adhesion tension of surfactant solutions in comparison with water and thus reduces their wetting power. The wettability of sample A is increased at concentrations above the c.m.c. where bilayer adsorption of surfactants results in increased hydrophilicity of cotton surface.

Determination of Contact Angles on Microporous Particles Using the Thin-Layer Wicking Technique

The properties of particle-stabilized emulsions, especially with regard to phase inversion, are very dependent on the contact angle that the particles experience at the oil-water interface. For the very small particles used for such emulsions (often a few tens of nm), it is impossible to measure this contact angle directly. Its value could be calculated if it were possible to determine the components of the solid surface free energy. To establish a method suitable for such particles, we have investigated the imbibition of five probe liquids into a porous bed of silica (commercial TLC plates) using the thin-layer wicking technique. For all liquids, the difference between wicking rate for bare plates and for those pre-contacted with the vapors is large but it is not due to an advancing angle effect on bare plates. Our analysis shows that it is due to the diversion of flowing liquid into blind pores which are already filled in the pre-contacted case. Thus a new model is proposed describing wicking in a porous medium with very small blind pores by introducing a parameter into the Washburn equation that corrects for this capillary condensation effect. The parameter needed is determined independently using gravimetric adsorption measurements. When this modified Washburn equation is used, the difference between advancing and receding contact angle is actually quite small. When the averages are used as the Young's contact angles, values for the surface energy components of silica are obtained that are completely consistent between the five liquids and have magnitudes expected for this type of silica surface.

Effect of electrolyte on surface free energy components of feldspar minerals using thin-layer wicking method

Application of the thin-layer wicking (TLW) technique on powdered minerals is useful for characterizing their surfaces. Albite (Na-feldspar) and orthoclase (K-feldspar) are feldspar minerals which are frequently found in the same matrix. Despite similarities in their physicochemical properties, separation of these minerals from each other by flotation is generally possible in the presence of monovalent salts such as NaCl. Both albite and orthoclase exhibit the same microflotation properties and rather close electrokinetic profiles in the absence of salt. In this study, contact angles of albite and orthoclase determined by the TLW technique yielded close values in the absence and presence of amine collector. While the calculated surface energies and their components determined using contact angle data reveal that the energy terms remain farther apart in the absence of the collector, the differences narrow down at collector concentrations where full flotation recoveries are obtained. However, the effect of addition of NaCl on contact angles and surface free energy components at constant amine concentration indicates that albite is significantly affected by salt addition, whereas orthoclase remains marginally affected. This interesting finding is explained on the basis of ion-exchange properties, the stability of the interface, flotation data, and zeta potential data in the presence of NaCl.

Effect of an external radiofrequency electric field on the surface free energy components of calcium carbonate in the presence of cationic and anionic surfactants

The surface free energy components of calcium carbonate for bare and surfactant-treated surfaces were studied by the thin-layer wicking technique. The surface precoverage from a 10-3 M aqueous solution of cetyltrimethylammonium bromide (CTMABr) and sodium dodecyl sulfate (SDS) caused a decrease in the electron donor component and an increase in the electron acceptor component. The changes were more pronounced in the case of CTMABr than for SDS. Consequently, in the presence of the cationic surfactant, the surface becomes hydrophobic while the SDS-precovered surface is still slightly hydrophilic. The presence of a radiofrequency (RF) electric field (44 MHz, 60 V peak-to-peak no-load amplitude) during wicking experiments perpendicularly to the wicking direction also decreases the electron donor component and increases the electron acceptor component. The increase is relatively higher for the surfactant-precovered surfaces. The RF treatment does not change the hydrophobicity of the bare surface but changes the hydrophobic CTMABr-precovered surface to slightly hydrophilic. On the contrary, the slightly hydrophilic SDS-preadsorbed surface becomes slightly hydrophobic in the presence of RF. It is concluded that the free energy changes appearing in the presence of RF are due to reorientation of the adsorbed surfactant molecules and/or changes in the structure of the water molecules hydrating the surface.

Effect of Tannic Acid on the ζ Potential, Sorption, and Surface Free Energy in the Process of Dyeing of Leacril with a Cationic Dye

The behavior of the surface free energy in the process of dyeing Leacril pretreated with tannic acid and subsequently dyeing with the cationic dye Rhodamine B has been studied. Also the electrokinetic behavior of these systems has been analyzed by studying the ζ potential, which has been obtained by means of the streaming potential technique. Values more significative of the ζ potential of these systems have been obtained using the three models of capillaries existing in the literature. The qualitative behavior of the ζ potential is the same for the three models of capillaries tested in this paper. These models are those of Goring and Mason, Biefer and Mason, and Chang and Robertson. The ζ potential of the systems analyzed is negative in the range of concentration of the dye in the liquid phase from 10−6to ca. 10−4M of dye. In the range of low concentrations (from 10−6to ca. 10−5M of dye) the ζ potential of the system untreated Leacril/Rhodamine B increases in absolute value due to increasing hydrophobic attractions between both the hydrophobic chains of the dye and the Leacril fibers in aqueous media. In the system Leacril treated with tannic acid/Rhodamine B, this increase is also due to the presence of hydrogen bonding between the phenolic hydroxyl groups of the tannic acid and the sulfonate and sulfate end groups of Leacril fibers. For concentrations of dye between 10−5and 10−4M of dye in solution, the ζ potential decreases in absolute value due to the electrostatic attractions between the groups negatively charged in the fiber and the cation of the dye. The ζ potential changes its sign at the highest concentrations of dye used in this work. The adsorption of Rhodamine B onto both untreated Leacril and Leacril treated with tannic acid is favored by the increasing temperature of adsorption. The behavior of the components of the surface free energy obtained by the thin-layer wicking technique led us to consider that the cationic dye Rhodamine B is adsorbed on the surface of both untreated Leacril and Leacril treated with tannic acid by Lewis acid–base interactions.

Effect of Tannic Acid on the Surface Free Energy of Cotton Fabric Dyed with a Cationic Dye

The effect of tannic acid and/or Rhodamine B on the surface free energy components of bleached cotton was studied. The zeta potential of cotton fabric was also measured in 10−6–10−2M tannic acid solutions and in the presence of 10−3M NaCl. It was found that the zeta potential of cotton is negative and that its absolute value decreases with increasing concentration of tannic acid. In the presence of NaCl the potential is more negative than in the tannic acid solutions alone. One may conclude that both H+and Cl−are potential-determining ions for the cotton surface. The surface is practically monopolar Lewis basic with an electron donor, γ−S, interaction of 50.1 mJ/m2, while its electron acceptor interaction, γ+S, equals only 0.92 mJ/m2. This polar character of the surface results from the presence of carboxyl end-groups. The apolar Lifshitz–van der Waals interaction, γLWS, amounts 45.0 mJ/m2. The cotton surface with adsorbed tannic acid from 10−3or 10−2M solutions, or Rhodamine B from 10−3M solutions, as well the surface dyed with Rhodamine after adsorption of tannic acid, shows different values of the surface free energy components. The changes are within ca. 14 mJ/m2for apolar γLWScomponent and within ca. 13 mJ/m2for polar γ−Scomponent. It is believed they result from the presence of the above mentioned polar end groups of bleached cotton, phenolic hydroxy groups in the tannic acid molecule, and two amine groups and one carboxyl group in the Rhodamine B molecule. The adsorption processes occurring in the systems are due to both electrostatic interaction and hydrogen bonding between the polar groups. It appeared that the thin-layer wicking technique adapted to the fabric materials is very useful for the surface free energy study, especially because the contact angle method cannot be applied for such materials.

Effect of a cationic dye on the surface free energy of Leacril fabric

Changes in Leacril fabric surface free energy components (Lifshitz-van der Waals γ S LW, electron donor γ s −, and electron acceptor γ s +) due to absorption of the cationic dye Crystal Violet (CV) were determined by means of the thin-layer wicking technique. It was found that the treatment of Leacril with different amounts of CV produced practically no changes in the value of the γ S LW component. However, the above treatment reduces the γ S + component practically to zero. In contrast, the γ S − component increases from 60 mJ m −2 for untreated Leacril up to 79 mJ m −2 for Leacril treated with 10 −4 M CV, with a very sharp decrease of γ s − to 67 mJ m −2 for increasing concentrations of CV (up to 10 −2 M). The initial increase in γ S − with 10 −4 M CV could be due to the presence of three disubstituted aromatic amine groups in the molecule CV. These groups are strong electron donors and their presence increases the value of the γ S − component of the Leacril. The initial value of 60 mJ m −2 of the γ S − component for the untreated Leacril could be due to the presence of a large amount of both sulphonate and sulphate end-groups on untreated Leacril. These groups are also donors of electrons. For increasing concentrations of CV in the treatment of Leacril, the electrostatic interactions between the cation of the dye and both the sulphonate and sulphate end-groups of the Leacril produce a blockade of these groups on the fabric and a decrease of the γ S − component to a value of 67 mJ m −2 for a concentration of 10 −2 M CV. The amount of CV uptake by Leacril increases with the concentration of the cationic dye in solution and with the temperature of absorption. It is concluded that the absorption of CV on Leacril takes place by means of electrostatic attractions between the disubstituted aromatic amine groups of the cationic dye used and both the sulphonate and sulphate end-groups of Leacril. These groups could impart hydrophilicity to the surface of the Leacril and hence acid-base interactions between the molecules of the cationic dye and the surface of the fabric could explain the interactions between the CV and the Leacril. The value of 67 mJ m −2 obtained for γ S − after the treatment of Leacril with 10 −2 M CV could be due to the final presence of the three (electron donor) disubstituted aromatic amine groups per molecule of the cationic dye on the Leacril surface due to the absorption process of the dye on the Leacril fabric.

Characterization of cellulose surface free energy

The thin-layer wicking technique was used to determine the surface free energy components and the surface character of three celluloses (Sigmaccll 101, Sigmacell 20, and Avicel 101), using an appropriate form of the Washburn equation. For this purpose, the penetration rates of probe liquids into thin porous layers of the celluloses deposited onto horizontal glass plates were measured. It was found that the wicking was a reproducible process and that the thin-layer wicking technique could be used for the determination of the celluloses' surface free energy components. The size of the cellulose particles was characterized with the Galai CIS-100 system and their crystallinity was measured by X-ray diffraction. The three celluloses have high apolar (yLWS = 50-56 mJ/m2) and electron donor (γs = 42-45 mJ/m2) components, while the electron acceptor component (γS+ ) is practically zero. The free energy interactions of cellulose/water/cellulose calculated from the components are positive, regardless of the cellulose crystallinity. This would mean that the cellulose surfaces have a hydrophilic character. However, the work of spreading of water has a small negative value (3-9 mJ/m2), indicating that the surfaces are slightly hydrophobic. It is believed that the work of spreading characterizes better the hydrophobicity of the surface than the free energy of particle/water/particle interaction, because in the latter case, no electrostatic repulsion is taken into account in the calculations.

Intergrowth of calcium phosphates: an interfacial energy approach

A factor which is usually ignored in discussions of the induced crystallization of one phase by the surface of another is the surface free energy of the nucleus/substratum interface. Interfacial energies of hydroxyapatite (HAP), octacalcium phosphate (OCP) and fluorapatite (FAP) microcrystals against aqueous solutions, measured using a thin-layer wicking technique, were 9.0, 4.3 and 18.5 mJ m −2, respectively. The calculated low interfacial energy, 0.93 mJ m −2, between OCP and HAP provides strong support for the suggestion that OCP is the first forming phase that induces HAP crystallization in calcium phosphate precipitation reactions. Using the constant composition kinetics method, the nucleation and growth of OCP on titanium oxide surfaces were investigated. The interfacial energy calculated from the nucleation and growth data compared favorably with that obtained by thin layer wicking. Extending the interfacial energy approach to polymeric substrata such as poly(methyl methacrylate) (PMMA), and the radiofrequency glow discharge treated PMMA demonstrates, in predicting the ability of surfaces to induce mineral nucleation, the importance of the Lewis base parameter.

The Crystallization of Fluorapatite in the Presence of Hydroxyapatite Seeds and of Hydroxyapatite in the Presence of Fluorapatite Seeds

The kinetics of growth of crystals induced by hydroxyapatite (HAP) seed crystals in supersaturated solutions of fluorapatite and of fluorapatite (FAP) seed crystals in supersaturated solutions of hydroxyapatite have been studied using the constant composition method. The reactions were investigated at relative supersaturations ranging from σFAP= 0.99 to 12.0 at pH 6.5 and for HAP, σHAP= 3.6 to 12.6 at pH 7.4. In FAP-supersaturated solutions, this phase was nucleated at the HAP surfaces and underwent growth at a rate more than three times greater than that on FAP seed crystals of equivalent surface area. Transmission electron microscopy (TEM) and electron diffraction imaging clearly demonstrated that the new needle-like HAP phase originated at the FAP surface with the 002 planes of HAP growing on FAP. In contrast, the mutual orientation of FAP crystallization on HAP seed crystals could not be established. Interfacial energies of FAP, HAP, and octacalcium phosphate (OCP) microcrystals against aqueous solutions were obtained by using a thin-layer wicking technique. The interfacial energy values measured in pure aqueous solutions were 18.5, 9.0, and 4.3 mJ m−2for FAP, HAP, and OCP, respectively. The much smaller value for OCP as compared with the other phases may explain why this phase has been so frequently implicated as a possible precursor to the formation of apatite, especially in biological mineralization reactions.

Effect ofN-Cetylpyridinium Chloride on the Surface Free Energy of Leacril Fabric

Changes in the Lifshitz–van der Waals, γLWS, electron donor, γ−S, and electron acceptor, γ+S, surface free energy components of leacril fabric due to the adsorption ofN-cetylpyridinium chloride (N-CP-Cl) were determined by the thin-layer wicking technique. It was found that the treatment of leacril with different amounts ofN-CP-Cl practically does not change the value of the γLWScomponent. The treatments reduce the γ+Scomponent practically to zero, and a very sharp decrease of γ−S(from 60.5 to 35.8 mJ/m2) was observed as the leacril was treated with increasing amounts of the above surfactant. It is concluded that the adsorption ofN-CP-Cl on leacril takes place by means of electrostatic attractions between the quaternary ammonium group ofN-CP and both the sulfonate and the sulfate groups of leacril. These groups could impart hydrophilicity to the surface of the leacril and hence acid–base interactions between the molecules of the surfactant and the surface of the fabric could explain the interactions between theN-CP-Cl and the leacril. The decrease in the γ−Sparameter in leacril fabric at concentrations ofN-CP-Cl in solution as the fabric is treated is due to the decreasing content of sulfonate and sulfate groups electron donors, in the leacril due to acid–base neutralization between the cation of the surfactant and the surface groups of the fabric. The obtained value of γ−S, 35.8 mJ/m2, after the treatment of leacril with 10−2M N-CP-Cl could be due to the presence of N+-pyridinium groups, electron donors, on the leacril surface at the highest concentrations of surfactant due to the adsorption of the surfactant onto the leacril surface.

On the use of Washburn's equation for contact angle determination

A systematic study on the possibility of Young's contact angle determination from Washburn's equation was performed using the so-called thin-layer wicking technique in which the rate of penetration of a liquid into the porous layer of a solid is measured. Commercial (Merck) SiO2 deposited on the glass plate for thin-layer chromatography was used as a model solid and n-alkanes (from pentane to hexadecane), diiodomethane, and α-bromonaphthalene were employed as the probe liquids. It was shown that the contact angle calculated from Washburn's equation was not equal to Young's contact angle of a drop of the same liquid, placed on a flat surface of the solid. Consequently, the solid surface free energy components calculated using contact angles from Washburn's equation are not the true values. However, the approach previously suggested by us has been verified again, as it gives consistent values of the surface free energy components determined from all the liquids used.

Changes in zeta potential and surface free energy of calcium carbonate due to exposure to radiofrequency electric field

A calcium carbonate (Fluka; more than 99.0% pure) suspension or a thin layer of its powder deposited on a glass slide was exposed to an r.f. electric field (44 MHz; 60 V peak-to-peak no-load amplitude) for 5–30 min. The zeta potential of the suspension in water (natural pH, 9.9 ± 0.1) or in 10 −2 M NaCl (natural pH, 9.6 ± 0.1) was then measured electrophoretically. The surface free energy components, i.e. non-polar Lifshitz—van der Waals (γ LW S) and polar (acid—base) electron donor (γ − S) and electron acceptor (γ + S) were determined by the “thin-layer wicking technique”. It was found that a negative zeta potential of CaCO 3 in water measured immediately (within 2–3 min) after the exposure to the r.f field changed in an oscillatory fashion depending on the exposure time. For the sample treated for 15 min with the r.f field the zeta potential decreased by 5–7 m V in relation to an untreated sample. Residual changes occurred also in an oscillatory fashion during the 40 min after field removal. A similar behavior was found for the suspension in 10 −2 M NaCl. The observed zeta potential changes were accompanied by changes in the electron donor component of the surface free energy and the appearance of a small electron acceptor component, which resulted from the presence of hydrogen bonding on the calcium carbonate surface. The r.f. field treatment led to a decrease in γ − S from 83 mJ m −2 (untreated sample) to 40 mJ m −2 (15-min-treated sample) and 64 mJ m −2 (25-min-treated sample) and caused the appearance of γ + S components of 1.34 and 1.77 mJ m −2. The observed changes in the zeta potential and the surface free energy components are believed to result from the changes in the surface charge of calcium carbonate and the structure of the water film at the surface.

Surface thermodynamic properties of cadmium sulfide

Measurements of the contact angle for water, formamide and diiodomethane on pellets of cadmium sulfide were made. Also, the thin-layer wicking technique was used with n-decane, water and formamide as the penetrating liquids. The measurements were used to determine apolar, Lifshitz-van der Waals (γ LW) and polar, acid-base (Lewis) (γ AB) surface free energy components of monosized, spherical CdS colloidal particles. The results were compared with those obtained on a pure, commercial sample of cadmium sulfide. They both are monopolar materials, although synthetic CdS has a slightly higher electron-donor character (Lewis base) ( γ - S =61.4 mJ/ m 2) than commercial cadmium sulfide ( γ - S =56.0 mJ/ m 2). The γ LW component has a similar value for both samples, in the range of 44–47 mJ/m 2. Δ G TOT 121, the free energy of interaction between particles in aqueous medium is for both synthetic and commercial CdS around 34.0 mJ/m 2. In spite of the (small) attractive contribution due to the Lifshitz-van der Waals interaction (Δ G LW 121), the large acid-base repulsion would ensure stability of CdS dispersions. Surface characteristics of the synthesized cadmium sulfide are similar to those of polydispersed commercial particles concerning their wetting properties. In consequence, the stability of dispersions of either kind of particles is expected to be the same. However, the use of perfectly spherical particles of this material is still valuable for testing model theories concerning interactions between particles in suspension.

Surface free energy components of monodisperse zinc sulfide

Apolar, Lifshitz-van der Waals ( γ LW ) and polar, acid-base (Lewis) ( γ AB ) surface free energy components, of monosized, spherical ZnS colloidal particles were experimentally determined using the thin-layer wicking technique. The results were compared with those obtained on a pure, commercial sample of zinc sulfide. The fact that the synthetized sample showed a very high electron-donor ( γ s −) component suggested that some species, related in principle to surface oxidation of ZnS during the synthesis reaction, could be formed on the surface of the particles. To check for this possibility, the surface free energy components of pure and oxidized ZnS, and of zinc oxide and sulfate, were determined by both the contact angle and thin-layer wicking methods. The similarity between γ s components obtained with both methods for pure ZnS lead to the conclusion that both techniques give reliable results. The high γ s − value determined for synthetic ZnS particles cannot be explained simply by surface oxidation of the colloidal particles, since partially oxidized ZnS, or even zinc oxide or sulfate, gave very similar γ s −, always lower than that found for synthetic spherical ZnS. The fact that the Lifshitz-van der Waals ( γ s LW ) component of the surface free energy is somewhat higher for the latter material is believed to be related to elemental sulfur present on the surface of the particles. The general compatibility of the results obtained by both methods suggests that the thin-layer wicking technique is useful to further characterize these or other systems, particularly if the examined material is sufficiently monodisperse.

Surface Free Energy Components of Calcium Carbonate and Their Changes Due to Radiofrequency Electric Field Treatment

Surface free energy components for calcium carbonate were determined using a thin-layer wicking technique based on Washburn's equation. It was found that the Lifshitz-van der Waals interaction for this solid, γLWS, is 48.0 mJ/m2, and the acid-base electron donor parameter, γ-S, equals 79.0 mJ/m2, while no value for the electron acceptor parameter, γ+S, was detected. These results show that calcium carbonate possesses a strong monopolar surface. After the thin layer of calcium carbonate was treated with a radiofrequency electric field (44 MHz, 60 V nonloaded peak-to-peak amplitude) for 15 or 25 rain, the electron donor parameter decreased to 40.7 and 63.9 mJ/m2, and small electron acceptor parameters appeared, 1.34 and 1.77 mJ/m2, respectively. At present, no idea can be put forward for the smaller changes in the γ-S parameter for the surface treated for the longer time with the RF field. However, it is postulated that the observed changes in acid-base interactions (hydrogen bonding) are due to some rearrangements of the surface hydroxyl groups and/or the adsorbed water molecules, leading to their partial mutual saturation, as this type of interaction is complementary.

Effect of aminoacids on the surface free energy of nitrofurantoin

Changes in nitrofurantoin surface free energy components, Lifshitz-van der Waals, γsLW, electron donor, γs−, and electron acceptor, γs+, due to adsorption of the aminoacids: lysine, alanine and glutamic acid, were determined by means of the thin-layer wicking technique. It was found that the aminoacids slightly increase the γs− component already at 10−4 M concentration. They reduce the γs+ component practically to zero, and a very sharp increase of γs− was observed when the nitrofurantoin surface was precovered from the solutions at concentrations larger tan 10−4 M. It is concluded that the adsorption of the aminoacids takes place via hydrogen bonding and electrostatic interactions between nitrofurantoin surface and aminoacid molecules. The increase in the γs− parameter is probably caused by the presence of carboxyl groups in the aminoacid molecules.

Studies on the surface free energy components of nitrofurantoin

Surface free energy components, apolar Lifshitz-van der Waals, γ S LW, and polar electron donor, γ S −, and electron acceptor, γ S −, have been determined for nitrofurantoin ( N-(5-nitro-2-furfurylidene)-1-aminohyantoin) by both contact angle measurements and the thin-layer wicking technique. The best agreement between the values obtained by these two methods was found when water, formamide, tetrabromoethane, and n-decane were used as probe liquids. The averaged values of the surface free energy components were γ S LW = 48.0, γ S − = 25.1, and γ S + = 7.0 mJ/m 2, thus giving the value 74.5 mJ/m 2 for the total surface free energy of nitrofurantoin. Based on these values, the free energy of interaction, Δ G 121, between particles of nitrofurantoin immersed in water was calculated to be −10.6 mJ/m 2, thus demonstrating the possibility of the aggregation process if the electrostatic repulsions do not prevent it. Justification for the determined free energy components is given considering the nitrofurantoin molecular structure.