Application of Methylene Blue Adsorption to Cotton Fiber Specific Surface Area Measurement: Part I. Methodology

Adsorption of methylene blue or 3,7 bis (dimethylamino) phenothiazin-5-ium ion was used to measure the specific surface areas of six cotton fibers taken from the International Calibration Cotton Standards. A kinetic study of this dye’s adsorption to the cotton fibers was first conducted to establish the C. Kaewprasit, Faculty of Liberal Arts and Science, Kasetsart Univ., Thailand; E. Hequet, Lab. of Cotton Technology, CIRAD-CA, Montpellier, France; N. Abidi; Lab. of Spectronomie-Physics, Univ. of Moulay Ismail, P.O. Box 4010, Meknes, Marocco;J.P. Gourlot, Lab. of Cotton Technology, CIRAD-CA, Montpellier, France. Received 8 May 1998. *Corresponding author (rdicrk@ku.ac.th). 165 KAEWPRASIT ET AL.: APPLICATION OF METHYLENE BLUE ADSORPTION TO COTTON adsorption isotherms. From this study, the adsorption isotherm was determined at 25°C for 24 hours with the concentration of methylene blue solution in the range 0.004 to 0.18 x 10 kg L. After 24 hours, the concentration of methylene blue at the adsorption equilibrium was analyzed by a spectrophotometer at a wavelength of 660 nm. The specific surface area was then calculated from this isotherm. The specific surface areas for these cottons, B-26, C-36, D-5, E-4, G-17, and I-26 were found to be 32.32, 32.42, 34.48, 52.72, 43.96, and 29.91 x 10 km kg, respectively. The reliability of this method seems very good. Fiber quality is a major concern for all segments of the cotton industry. Although physical characteristics serve as predictors of performance, the interpretation of measurements is not usually used to correlate fiber quality with results from processing. Specific surface area is one of the physical characteristics that is important for dyeing characteristics. The specific surface area is defined as the accessible area of solid surface per unit mass of material. Various methods are used to determine specific surface area such as: moisture adsorption (Assaf et al., 1944), N adsorption (Assaf et al., 1944; Merchant, 1957), nuclear magnetic resonance (Froix and Nelson, 1975) and liquid chromatography (Ladisch et al., 1992). Because the surrounding phase modifies the surface area, each method has inherent sources of error. The N adsorption/desorption (physisorption) isotherm is one of the most important and extensively used methods for determining specific surface area of a material. In the case of cotton fibers, N sorption measurements show that the surface area of dry cotton is in the range 0.6 to 1.0 x 10 km kg (Blair and McElroy, 1976; Rowen and Blaine, 1947). This value of specific surface area is attributed mainly to external surface. However, the internal surface of cotton fibers is also important for dyeing and it must be measured too. Thus, it is necessary to find a method that allows the measurement of the total surface of cotton fiber. Adsorption of methylene blue for specific surface area determinations has been widely adopted for solids of variable nature such as oxides, graphite, yeast, activated carbons, calcium carbonate, etc. (Pakhovchishin et al., 1991; Graham, 1955; Puri et al., 1979; De et al., 1974; Handreck and Smith, 1988; Savitsky et al., 1981; Kaoua et al., 1987; Tanada et al., 1980). This method has also been used to assess pore size and distribution in the transitional pore range for charcoals, silica, and alumina (Giles and De Silva, 1969). Davidson established the conditions for determinations made with methylene blue (1947). In the present work, the method of methylene blue adsorption in liquid phase was proposed to measure the specific surface area of cotton fibers. The objective was to estimate the specific surface area of raw cotton to compare, in the future, with the surface area determined by air flow, for example, and to correlate measurements with fiber characteristics such as fineness, maturity, tenacity, etc. To achieve this objective, wax was not removed from the fiber before the analysis because its surface would have been modified by this chemical treatment. To establish the adsorption isotherms of methylene blue on cotton fiber, a kinetic study was conducted. Cotton fibers with various mechanical properties were measured for specific surface areas. The effects of different preparations and treatment temperatures on cotton fiber before adsorption were examined. Microcrystalline cellulose was chosen in this study as a reference material to compare with cotton fibers and for demonstrating the reliability of this method.