Acid-Base and Solvatochromic Indicators in Surfactant Micellar Solutions of Various Types: Is the Common Electrostatic Model Valid?
Science Direct Working Paper No S1574-0331(04)70699-2
42 Pages Posted: 6 Jun 2017 Last revised: 13 Jan 2018
Date Written: July 2003
The properties of micellar solutions of over 20 colloidal surfactants (including 9 anionic) as media for protolytic reactions were studied with the help of acid-base indicators, mainly of the two-step, or ‘bifunctional’, Decyl Fluorescein (HR ⇄ HR ⇄ R) and of the Reichardt betaine dye, 2,6-diphenyl-4-(2,4,6-triphenylpyridinio-1) phenoxide, with solvatochromic zwitterionic form R (HR ⇄ R + H), as well as of other dyes, completely bound to micelles. The validity of the traditional electrostatic model was tested on the basis of the ‘apparent’ , which describes the dissociation of the indicator, . The object of analysis were alterations in the values, caused by the transfer from water to micellar solutions , from one surfactant to another, and through the variation in the composition of salt background (inorganic and organic ions). The main assumptions of the common electrostatic model – (1) constancy of the ‘intrinsic’ contribution to the value of the given indicator in any micellar system, (2) constancy of the value of the electrical potential of the Stern layer Ψ of the given micellar surface obtained by using any indicator, and (3) the possibility of complete description of salt effects with the help of ion-exchange model, – are shown in most cases to be justified only approximately, and sometimes even insufficiently. Up-to-date attempts to modify the simple electrostatic model are shown to be rather arbitrary. The values in nonionic micelles cannot be considered to be an optimal model of in ionic micelles. That is why the possibility of using the values in micellar solutions of a zwitterionic surfactant CHN(CH)–(CH)–SO as values of the corresponding dyes in micelles of ionic surfactants is discussed. The micellar transitions caused by salts became evident in the case of hydrophobic counterions. The analysis of the values revealed the greatest hydration of the Stern region of anionic micelles as compared with other micellar surfaces. This is in agreement with their values, as well as with other solvatochromic scales. The nature of ionic pairs formed by dye ions and oppositely charged surfactant head groups is discussed. The differentiating action of micellar pseudophase upon the acid-base properties of indicator couples of various charge types and nature, i.e. the disparity in their values, manifests itself distinctly, depending on the nature of the surfactant. This effect is caused, on the one hand, by the miscellaneous character of any micellar surface, and on the other hand – by the dissimilarity among hydrophilic portions of cationic, anionic, nonionic (with oxyethylene chains), and zwitterionic surfactants, as well as of the Stern region of ionic micelles containing counter-ions of different hydrophobicity. The differentiating impact of micelles seems to be the main hindrance to exact evaluations of the interfacial electrical potential of micelles by means of acid-base indicators.
Keywords: Physical Chemistry > Colloid and Surface Chemistry, physchem/0307002
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