Colloids and Surfaces A: Physicochemical and Engineering Aspects
Studies on the effects of amino acids on micellization of CTAB via surface tension measurements
Introduction
The formation of micelles by ionic and non-ionic surfactants is a well-established fact. In last few years, extensive structural, kinetic and thermodynamic studies have been performed on surfactant–water systems including the effect of additives on micellization [1], [2], [3], [4], [5].
The interaction between surfactant and proteins has been recognized for a long time [6]. Amino acids are important biological-active substances and basic structural units of proteins. The study on the effect of amino acid on the properties of surfactant will provide the important information for interaction between surfactant and protein. The study on the property of the aqueous solution of amino acids is very limited and there appears to have been only several studies in the first half of the 20th century [7], [8], [9]. The studies have intended to explore surface properties of zwitterionic solutes and showed general tendencies of the solutions. Subsequent studies on the surface properties of amino acids were aimed chiefly at biochemical subjects. Bull and Breese [10] measured the surface tension of 20 common amino acids in order to evaluate the hydrophobicity scale of the residues. Glinski et al. [11] studied the dependence of surface tension of the aqueous solutions of l-leucine on some factors, such as temperature and concentration as well as in aqueous solutions of NaOH and HCl, in order to discuss water structure around l-leucine molecules. So far the study of interactions between amino acids and surfactants have been scarcely reported via surface tension measurement.
The aim of this paper is to investigate the effects of temperatures, concentration and category of amino acids on micellization of CTAB by surface tension measurement.
Section snippets
Experimental
Analytical reagent grade tris(hydroxymethyl)aminomethane (abbreviated as Tris) and NaCl, manufactured by Shanghai Chem. Co., were used without any pretreatment. Glycine, l-alanine and l-serine were of >99% purity (Sigma) and used without further purification. Their structures are shown in Table 1. Analytical reagent grade cetyltrimethylammonium bromide (CTAB, A.R.) was purchased from Shanghai Chem. Co. There was no minimum in the γ versus c curve of CTAB. The water used to prepare solution was
Effect of the presence of glycine at different temperature
The cmcs of CTAB in the absence and presence of glycine (0.05 mol L−1) in Tris–HCl buffer solution at different temperature are presented in Table 2.
As can be seen from Table 2, the addition of glycine decreases the cmc and γcmc values of CTAB at the same temperature and the cmc values of CTAB decrease with increasing temperature regardless of in the absence or presence of glycine.
It is well known that the inorganic electrolytes can increase the surface activity of the solution of ionic
Conclusion
The cmc of CTAB in water as well as in the Tris–HCl buffer solution in the presence of glycine, l-alanine and l-serine was determined. It is observed that the cmc is dependent upon temperature and the nature of the additives as well as on the concentration of the additives. Amino acids do not correspond with the definition of the word electrolyte. However, it is appropriate to include amino acids when studying the effect of ions on the water surface, because they exist in solutions as
Acknowledgments
The authors are grateful to the financial support from China Postdoctoral Science Foundation and the Natural Science Foundation of Shandong Province of China (Q2003B01).
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