Surface free energy components, apolar Lifshitz-van der Waals, γs LW , and polar electron acceptor, γs + , and electron donor, γs -, were determined for leacril (a copolymer of 90% acrylonitryle and 9% vinyl acetate) fibers with preadsorbed tannic acid solutions (10 -5 -10 -2 M). A thin layer wicking method was applied for this purpose. The components were calculated considering van Oss et al.'s (J. Chem. Rev. 1988, 88, 927) polar components of surface tension of probe liquids (formamide and water, γw -) γw + ) 25.5 mJ/m 2 for water) as well as those suggested very recently by Lee (Langmuir 1996(Langmuir , 12, 1681, who considered for water γw -) 19 mJ/m 2 and γw + ) 34.2 mJ/m 2 . The trend of changes of the calculated components is the same, except for the surface treated with 10 -2 M tannic acid. However, the electron donor components of the leacril calculated with Lee's data are somewhat lower. From the calculated work of spreading of water, results indicate that the bare leacril surface is slightly hydrophobic, the surface treated with 10 -5 M tannic acid becomes low hydrophilic, and in the range of concentrations 10 -4 -10 -3 M becomes increasingly hydrophobic. When equilibrated with 10 -2 M it is again hydrophilic. The changes can be explained by reorientation of the tannic acid molecules on the leacril surface and micelle formation above cmc, which was determined to be at ca. 3 × 10 -4 M. Adsorption measurements show that the process is very fast and physical in nature. The rate constants and empirical diffusion coefficients were evaluated at 275, 283, 293, and 303 K. Also thermodynamic functions for the adsorption process were calculated from the adsorption isotherms. It was concluded that hydrogen bonding and Lifshitz-van der Waals interactions are responsible for the adsorption, while electrostatic interactions play a rather second-order role. The leacril shows small negative zeta potentials which drop almost to zero at concentrations higher than 10 -4 M.

Data are presented on the kinetics and thermodynamics of absorption of N-cetylpyridinium chloride (N-CP-Cl) by Leacril fibers at different temperatures. The increased temperature of the system increases the surfactant content of the fibers. The experimental time-sorption isotherms are represented by the exponential kinetic equation, M, = M,,( l -e-kf), where M, and M,, refer to the amounts of surfactant taken up by Leacril at time t and equilibrium, respectively, and k refers to the rate constant. The empirical rate constant decreases with temperature. Values of half-absorption time at different temperatures are presented. Sorption equilibria of N-CP-Cl by Leacril are described by a Freundlich isotherm equation. Changes of enthalpy and entropy related to the process of absorption are calculated from the Clausius-Clapeyron equation. Isosteric heats of absorption are positive, between 20 and 120 kllmol. The global change of entropy is positive, between 126 and 500 Jim01 K. The best thermodynamic conditions for surfactant uptake by Leacril appear at the highest temperatures, when the standard free energy of absorption takes the most negative values. From the results, we find that the absorption of N-CP-Cl by Leacril in the temperature interval investigated takes place with establishment of chemical bonds, although physical contributions of an electrostatic nature between the cation of the surfactant and the sulphonate and sulfate end-groups of Leacril are the most important.

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Comparative investigations of adsorption properties of chlorhexidine (CHX) on two cellulose fibers, bleached cotton and viscose, were studied in order to obtain dry gauzes covered with known amount of this antiseptic. Adsorption isotherm results carried out at 293 and 323 K can be described by Langmuir isotherm model, nevertheless, at high concentration correlation is better to Freundlich isotherm. Electrokinetic potential evolution with CHX concentration, shows that initial negative zeta potential of cotton and viscose diminish its absolute value as the concentration of the treatment increases; both fibers present an isoelectric point at high concentration of CHX that is 0.3 mM for viscose and 0.8 mM for cotton. Electrostatic interactions between cationic groups of CHX and carboxylic acid groups of the fibers could explain adsorption at low concentration, but when it is higher than these values, possible hydrogen bonding between the amine groups of CHX and hydroxyl groups of cellulose could explain increasing adsorption when it is hindered by electrostatic repulsion as it is predicted by Freundlich model, that describes heterogeneous surface and multilayer adsorption. Adsorption kinetics isotherms reveal that the process is quick with t 1/2 values of 5.4 min for cotton and 2.8 min for viscose. Differences in adsorption behaviour between the two fibers could be attributed to structural differences as we have observed from estimation of CI index based on FTIR spectra. Values obtained 1.6 for viscose and 2.2 for cotton could explain that the amount of CHX adsorbed on viscose is higher than it is on cotton.Finally desorption experiments performed with 0.01 M of NaCl solution at room temperature and pH 6 reveals the possibility of therapeutical application of these fibers although further investigations must be done to optimize the process.

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An experimental investigation on the streaming potential and sorption of N-cetylpyridinium chloride on Leacril fibers has been carried out. The results reveal that the uptake ofN-cetylpyridinium chloride on Leacril fibers takes place by means of electrostatic attraction between the cation of the surface-active agent used and both the sulphonate and the sulfate end-groups of the Leacril. Given the hydrophobic character of Leacril and the amphiphilic nature of the N-cetylpyridinium chloride molecules, hydrophobic attractions between the fiber and the hydrophobic part of the surfactant might account for the interaction, explaining the sorption of N-cetylpyridinium chloride even when it is hindered by electrostatic repulsion. The electrical characterization of Leacril fibers has been carried out and the methods employed for the determination of zeta potentials of Leacril fibers are discussed, on the basis of streaming potential technique. In our case for this hydrophobic fiber, the linear model of Goring and Mason of a bundle of capillaries gives the most precise values of zeta potential. Also discussed is the behavior of the surface conductance of the system in the above process of sorption.

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