Effect of water hardness on surface tension and dilational visco-elasticity of sodium dodecyl sulphate solutions

The complementary drop and bubble profile analysis and maximum bubble pressure tensiometry are used to measure the dynamic surface tension of aqueous SDS solutions in the presence of hardness salts (CaCl(2) and MgCl(2) in the ratio of 2:1 at concentrations of 6 and 40FH). The presence of hardness salts results in an essential increase of the SDS adsorption activity, which indicates the formation of Ca(DS)(2) and Mg(DS)(2) in the SDS solutions. The surface tension isotherms of SDS in presence of Ca(DS)(2) and Mg(DS)(2) are described using the generalised Frumkin model. The presence of hardness salts accelerates the ageing of SDS solutions as compared with the addition of 0.01 M NaCl due to a faster hydrolysis and hence formation of dodecanol. These results are used to estimate the possible concentration of dodecanol in the studied SDS solutions. The buoyant bubble profile method with harmonic surface oscillations is used to measure the dilational rheology of SDS solutions in presence of hardness salts in the frequency range between 0.005 Hz and 0.2 Hz. The visco-elasticity modulus in the presence of hardness salts is higher as compared with its values in the presence of 0.01 M NaCl additions. The ageing of SDS solutions leads to an essential increase of the visco-elastic modulus.