A. Ontiveros-Ortega

Investigation of alumina/( +)-catechin system properties. Part I: a study of the system by FTIR-UV -Vis spectroscopy

Abstract Catechin belongs to a group of flavonoids, which are present in many fruits and plants. Catechin is often employed in model investigations on antioxidant behavior of flavonoids. Recently also some anti-disease properties of flavonoids were reported. In this paper investigations were carried out on alumina/(+)-catechin solution system. FTIR spectra were taken for the catechin samples retaken from its aqueous solutions at pH 3, 5.5 (natural) and 10.5. In other series, the solutions were contacted with an alumina powder during 20 days. Also UV–Vis spectra were taken for similar solutions incubated for 1, 7, 20, 50 and 70 days with and without contact with alumina. It appeared that the alumina essentially accelerated the autooxidation processes occurring in the solutions. The results show that the processes have completed between day 20 and 50, because the UV–Vis spectra for day 50 and 70 overlapped. Moreover, it looks that this is a specific property of alumina, which behaves as a catalyst. Similar experiments with silica showed that during 50 days, this oxide affected the processes only slightly.

Interpretation of colloidal dyeing of polyester fabrics pretreated with ethyl xanthogenate in terms of zeta potential and surface free energy balance

Data are presented on the adsorption of the colloidal dye Disperse Blue 3 onto polyester fabric (Dacron 54, Stile 777), the fabric being pretreated with different amounts of the surfactant potassium ethyl xanthogenate (PEX). This study has been made by means of both the evolution of the zeta potential of the fiber/dye interface and the behaviour of the surface free energy components of the above systems. The kinetics of adsorption of the process of dyeing, using 10(-4) M of PEX in the pretreatment of the fabric, shows that increasing temperature of adsorption decrease the amount of colloidal dye adsorbed onto the fabric. This fact shows that the principal mechanism involved in this adsorption process is physical in nature. The adsorption isotherms of the colloidal dye onto polyester pretreated with different amounts of PEX, shows that the adsorption of the dye is favored with the increase in the concentration of the surfactant used in the pretreatment. This fact shows that the pretreatment with PEX is a very interesting aspect of interest in textile industry. The zeta potential of the system fabric/surfactant shows that this parameter is negative (about -25 mV) for the untreated fiber and decreases in absolute value for increasing concentration of the surfactant on the fiber, the value of the zeta potential of the system being -5 mV for 10(-2) M of PEX. This behavior can be explained for the chemical reaction nucleophilic attack between the carboxyl groups of polyester, ionized at pH 8, and the thiocarbonyl group of the xanthogenate ion. On the other hand, the zeta potential of the system polyester pretreated with PEX/Disperse Blue 3 at increasing concentrations of the surfactant and the dye shows that this parameter increases its negative value strongly with increasing concentration of the surfactant used in the treatment. This can be explained for the hydrogen bonds between the hydroxy groups of the dye and the S- ions of the thiocarbonyl group of the surfactant preadsorbed onto the fiber.

Investigation of the Polyamide 6,6 dyeing process with Acid Blue 45 dye. Part II. Surface free energy, zeta potential and dye/polyamide interactions

Synthetic fibres are now of great importance for textile production. In Part I of this study, the kinetic and thermodynamic functions of the sorption process for Acid Blue 45 on Polyamide 6,6 (Nylon 6,6) were studied; in Part II, the surface free energy components for the polymer and the dye were investigated. If van Oss et al.s approach to the surface free energy components is used, the results obtained show that the surface of Polyamide 6,6 is a monopolar electron-donor surface. Also, the dye, when compressed as pellets, shows practically only electron-donor character. But these values are debatable, as both acidic and basic sites are present on the polyamide surface. The isoelectric point for the polyamide appears at pH 3, while for the dye in its saturated aqueous solution it is at pH 2. Therefore both surfaces are negatively charged in a broad pH range. Nevertheless, the adsorption takes place, which means that the attractive forces are higher than the electrostatic repulsive forces . As the adsorption takes place in spite of the mostly electron-donor character of both surfaces and the same sign of the electrical charge, this places doubt on the validity of such determined components. But on the other hand, it may be concluded that the adsorption process on the bare polyamide surface is of a chemical nature. For example, interactions may occur between the -COOH end-groups of the polyamide and the -OH groups of the dye, as well as between the -NH2 end-groups of the polyamide and -SO3Na of the dye, which are much stronger than electrostatic repulsive forces. The two electron-donor sites do not interact repulsively. The increase in sorption with increasing temperature (Part I) and practically no desorption of the dye from the polyamide surface for lower coverages (when dyed from a solution of concentration up to 10-3 M) support the above conclusion.

Investigation of alumina/(+)-catechin system properties. Part II: ζ-potential and surface free energy changes of alumina

Abstract The effects of (+)-catechin adsorption to the alumina surface were studied by ζ-potential and surface free energy determination. The presence of catechin causes essential changes in the alumina ζ-potential, which at the concentration slightly higher than 10 −5 M reverses from the positive into negative one. At constant concentration of catechin (10 −3 M), the effect on ζ-potential of alumina as a function of pH appears in a drastic shift of the isoelectric point, from pH 8.4 to 4.6, and the equilibrium is established practically within 2 h. This is probably due to relatively low p K a =4.6 for catechin 3′-OH group deprotonation. At high alkaline environment (pH≥10), even in the presence of catechin in the solution, the hydroxyl OH − ions play principal role in the surface charge formation for the alumina. At such pH catechin molecule is double negatively charged and hence its adsorption on highly negatively charged alumina surface is rather restricted. Nevertheless, various dimeric forms of catechin, which are formed at the alkaline pH, probably adsorb on the alumina surface. This appears in small increase in apolar surface free energy component at natural and alkaline pH. On the other hand, at acidic pH 3 small increase of the electron acceptor interaction is observed. This may result from increased number of hydroxyl groups on the alumina surface originating from the adsorbed molecules of catechin, which are mostly undissociated at this pH. The interactions of catechin with alumina surface seems to be also of some specific nature, because neither changes in the ultraviolet–visible (UV–vis) absorbance (Part I) nor in the ζ-potentials had occurred in the silica suspensions in which also catechin was present.

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

Abstract 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.

Investigation of the Polyamide 6,6 dyeing process with Acid Blue 45 dye. Part I. Thermodynamics of Acid Blue 45 adsorption

Among the many synthetic fibres, polyamides are one of the most important. Improvement in the adsorption/ absorption strength of a dye to and inside the yarns is therefore very important for their practical applications. In this study, the kinetics and thermodynamics of the sorption process of Acid Blue 45 on Polyamide 6,6 (Nylon 6,6) were investigated. The sorption was conducted at 293, 303, and 313 ± 0.1 K for 48 h from 10-5 -10-4 M solutions in the presence of 10-3 M NaCl. From the kinetic experiments it was found that this time was sufficient to attain the adsorption equilibrium. The first stage of the sorption (up to 160 min) can be well described by a first-order kinetic equation by its numerical fitting to the experimental results. It is a spontaneous process in which the rate of sorption and the sorbed amount increase with increasing temperature. However, it is a relatively slow endothermic process (positive enthalpy of sorption); thus, it must be entropy-driven to produce a negative free energy of sorption. From the sorption isotherms and calculated thermodynamic functions it can be concluded that at higher temperatures (303-313 K), more polar groups of Polyamide 6,6 are accessible for the dye molecules, which may be due to a glass transition. Data on the zeta potential changes and surface free energy components of Polyamide 6,6 will be presented in Part II of this study, which may help in a better understanding of the sorption process.