Emil Chibowski

Surface free energy of a solid from contact angle hysteresis.

Nature of contact angle hysteresis is discussed basing on the literature data (Colloids Surf. A 189 (2001) 265) of dynamic advancing and receding contact angles of n-alkanes and n-alcohols on a very smooth surface of 1,1,2,-trichloro-1,2,2,-trifluoroethane (FC-732) film deposited on a silicon plate. The authors considered the liquid absorption and/or retention (swelling) processes responsible for the observed hysteresis. In this paper hysteresis is considered to be due to the liquid film left behind the drop during retreating of its contact line. Using the contact angle hysteresis an approach is suggested for evaluation of the solid surface free energy. Molecular spacing and the film structure are discussed to explain the difference in n-alkanes and n-alcohols behaviour as well as to explain the difference between dispersion free energy gamma(s)(d) and total surface free energy gamma(s)(tot) of FC-732, as determined from the advancing contact angles and the hysteresis, respectively.

Aqueous suspension of fumed oxides: particle size distribution and zeta potential

Abstract Comparative analysis of the results of the investigations, which were performed by means of photon correlation spectroscopy (PCS) and electrophoresis, of aqueous suspensions of individual fumed oxides (SiO2, TiO2, Al2O3), binary X/SiO2 (X=TiO2, Al2O3, GeO2, C) and ternary Al2O3/SiO2/TiO2, fumed oxides modified by organic and organosilicon compounds or interacting with dissolved polymers, surfactants, etc., allows us to deeper understand the dependencies of structural and other characteristics of the dispersions on features of primary particles, their morphology, the nature of the surfaces of unmodified and modified individual and mixed oxides varied due to the synthetic techniques and the concentrations of the oxide phases or modifiers, concentrations of oxides in the liquid media, adsorption of soluble polymers, influence of surfactants, polyelectrolytes, inert electrolytes, etc. Some results related to chemically similar oxides but prepared by using other synthetic methods or data obtained by means of other experimental methods, e.g. DRS, TSD, NMR, etc., were used only in minor amounts in order to elucidate some aspects of the aqueous suspensions of fumed oxides studied by using PCS and electrophoresis methods. The main reasons of non-linear dependencies of the particle size distributions (PSD) and ζ potential on the concentration of fumed oxides in the aqueous suspensions, on the amounts of the guest X phases in X/SiO2 or the modifiers, on the pH values, etc., can be (a) a large polydispersity of PSD of both unmodified and modified fumed oxides linked with a four-stepwise structural hierarchy of fumed oxide particles, which can keep (or rearrange) in the suspensions despite different pretreatments of initial silica, X/SiO2 or their dispersions; and (b) a significant non-uniformity of the surfaces of mixed oxides due to features of the grafted or dopant phase distribution on/in the matrix depending on the guest oxide concentrations and the synthetic techniques.

Superhydrophilic and superwetting surfaces: definition and mechanisms of control.

The term superhydrophobicity was introduced in 1996 to describe water-repellent fractal surfaces, made of a hydrophobic material, on which water drops remain as almost perfect spheres and roll off such surfaces leaving no residue. Today, superhydrophobic surfaces are defined as textured materials (and coatings) on (nonsmooth) surfaces on which water forms contact angles 150° and larger, with only a few degrees of contact angle hysteresis (or sliding angle). The terms superhydrophilicity and superwetting were introduced a few years after the term superhydrophobicity to describe the complete spreading of water or liquid on substrates. The definition of superhydrophilic and superwetting substrates has not been clarified yet, and unrestricted use of these terms sometimes stirs controversy. This Letter briefly reviews the superwetting phenomenon and offers a suggestion on defining superhydrophilic and superwetting substrates and surfaces.

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

A systematic study on the possibility of Youngs contact angle determination from Washburns 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 Washburns equation was not equal to Youngs 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 Washburns 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.

Characterization of cellulose surface free energy

Junta Nacional de Investigacao Cientifica e Tecnologica (JNICT) - INTERREG II/BM/03/REG II/3/97.

On the determination of the surface free energy of quartz

Abstract Studies of water vapor adsorption on quartz by the method analogous to the dynamic gas chromatography step profile method are described. The adsorption was determined by changes in the capacitance of the capacitor (detector) between the coverings of which the quartz powder was placed. Prom the adsorption isotherm the film pressure π of the water film on quartz were determined, obtaining π max = 380 erg cm 2 . An interpretation of the π changes in relation to the film thickness and the kind of wetting process has been proposed. It is concluded that the characteristic film pressure values result from the work of spreading, immersional and adhesional wetting and correspond to thicknesses of about 2, 3 and 4 statistical water monolayers, respectively. The maximum π value, however, probably corresponds to the work of quartz- water adhesion + water cohesion work. On the basis of the thus determined values of πs, π1, and πmax, the value of the polar component γqp of the quartz surface free energy was determined, using the value γ q p = 76 erg cm 2 . The calculated average of the γqp value equals 115 erg cm 2 2 .

Determination of surface free energy components of marble

From the measured values of wetting contact angles of water droplets on marble in n-alkanes the value of the dispersion component of marble surface free energy γMd was calculated. The average γMd value was 64 ergs/cm2. Measurements of water vapor adsorption on marble at 20 and 30°C were also made. From the isotherms obtained water film pressure was determined, and then, using the calculated γMd value the polar component of marble surface free energy γMp = 106.6 ergs/cm2 was determined. These values were verified by determined isoteric and “isopressure” heats.

Zeta potential, effective diameter and multimodal size distribution in oil/water emulsion

Abstract Zeta potentials, effective diameter and multimodal size distribution of model emulsions were studied as a function of time (5–120 min, 1 and 2 days), pH and temperature. The emulsion of n-tetradecane (0.1% v/v) in pure water or 10−3 M KCl were obtained by mechanical mixing in a homogenizer at 10 rpm for 3 min. The emulsions in 1 M methanol, propanol and ethanol, and also in the presence of lysozyme (4 mg 100 ml−1) or lysine hydrochloride (1 mM) were obtained by dissolving tetradecane in the alcohol, then water was added and they were ultrasonicated for 15 min. The zeta potential of the emulsion depends on pH. In the presence of lysozyme, the isoelectric point occurred in pH 9.5, while in the presence of lysine hydrochloride it was negative in the pH range 3.5–9.5. The zeta potentials of the droplets in water and in 1 mM KCl were similar on the pH scale, but in case of water iep was observed in pH 3.05. In 1 M ethanol the iep occurred in pH 4.5. The changes of the zeta potential for these three emulsions were parallel in the pH range 5–9, however the negative values in 1 M ethanol were smaller. They were practically stable during 24 h. These diluted emulsions in 1 M alcohols were stable during 2 days as determined by effective diameter. The diameter was 350–450 nm for the emulsion in ethanol and propanol and 275–290 nm in methanol during the time studied. It was found that the zeta potential of the emulsion in 1 M propanol reversed it sign in natural pH, if the emulsion temperature has increased from 20 to 30 or 40°C. Calculations of the free energy of interactions were also conducted. It was found that principal interactions responsible for stability/instability of the emulsion are Lewis acid–base, i.e. hydrogen bonding between water molecules.

The determination of the dispersion and polar free surface energy of quartz by the elution gas chromatography method

Abstract The adsorption isotherms of n-octane, n-propanol and n-butanol on brasilian quartz were determined by the elution gas chromatography method. From the isotherms, the horizontal orientation of the hydrocarbon molecules and the vertical orientation of the alcohol molecules on the quartz surface were confirmed. From the isotherms, the film pressure π max of the adsorbates was also determined. From the π max value for n-octane, the dispersion free surface energy γ s d of quartz was calculated; the result obtained agrees with the literature data. From the π max values for the alcohols, the polar component of the free surface energy γ s p of quartz was calculated. The values obtained were compared with γ s p values, calculated from the film pressure π max data for water and n-propanol, taken from the literature. Considerable differences in the calculated γ s p values were observed and possible reasons for this are discussed.

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 Tadmors 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.