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, θa, 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, SL/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 θa, which results in negative values of SL/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.