الملخص الإنجليزي
Surface tension and conductivity measurements were conducted to investigate the effect of temperature and surfactant structure on the critical micelle concentration (CMC) of dodecyl ether sulfate with different degrees of ethoxylation, ranging from one to three per molecule (H25C12-(OCH2-CH2).-SO4Na where x =1 for CS-130; x =2 for CS-270; and x = 3 for CS-370) and compare their behavior with analogous simple surfactants such as sodium dodecyl sulfate (H25C12 -SO.Na; x = 0) and Brij 35 (H25C12 -(OCH2 CH2)4-OH; x = 4 and no sulfate group). At any given temperature within the range investigated (3 - 65 °C), both measurements (surface tension and conductivity) gave similar CMC values for the three ether sulfate surfactants investigated. Thus, at room temperature, the CMC was found to be 0.80 mM for the three ether sulfate surfactants, well below the CMC of sodium dodecyl sulfate (8.2 mm). This significant decrease was associated with a decrease in the hydrophilicity of EO group in the presence of the sulfate group. Ionization degrees of these ether sulfate surfactants were found to be were significantly high (B= 0.70 in the range 3-22.5 °C and B=0.89 in the range 38- 65 °C) in comparison with sodium dodecyl sulfate micelles (B = 0.18). In addition, the CMC behavior of CS series was found to be independent of the degree of ethoxylation, thereby deviating from the corresponding behavior of the analogous polyethoxylated surfactants. The variation of the CMC of the ether sulfate surfactants with temperature showed no minimum. Instead, the CMC first decreases with increasing temperature and eventually mum.
Te becomes practically temperature independant above 38 °C, reflecting a behavior similar to that observed with simple nonionic surfactants. Finally, the Gibbs free energies (46°m), enthalpies (4Hom) and entropies (45°m) of micellization as a function of temperatures were estimated and reflected an enthalpy-entropy compensation effect.
