253 J. Indian Chem. Soc., Vol. 94, March 2017, pp. 253-259 Effect of nonionic micelles of Triton X-100 on protonation equilibria of salicylic acid derivatives M. Ramanaiah* a,c , S. Goutham Sri a , M. Balakrishna a , B. Rama Raju b and B. B. V. Sailaja c a Department of Chemistry, Aditya Institute of Technology and Management, Tekkali-532 201, Andhra Pradesh, India b School of Material Science and Engineering, Nanyang Technological University, Singapore c Department of Inorganic & Analytical Chemistry, Andhra University, Visakhapatnam-530 003, Andhra Pradesh, India E-mail : ramanaiahmalla4@gmail.com Manuscript received online 04 April 2016, accepted 13 October 2016 Abstract : To gain more information about the effect of surfactant on salicylic acid derivatives, the stoichiometric pro- tonation constants of 5-sulphosalicylic acid and 5-hydroxysalicylic acids in 0.0–2.5% (v/v) Triton X-100 (TX100)-water mixtures were determined at an ionic strength of 0.16 mol dm –3 and at 303 K. A potentiometric method was used and the calculation of constants was carried out using the computer program MINIQUAD75. These protonation constants values have been found to shift in micellar media as compared to those in pure water. The change of log values of step-wise protonation constants with mole fraction of the medium have been explained based on specific solute-solvent interactions. In this study distributions of species, percentage of species composition, protonation equilibria and effect of influential parameters on the protonation were also discussed. Keywords : Protonation equilibria, MINIQUAD75, Triton X-100, 5-sulphosalicylic acid, 5-hydroxysalicylic acid. Introduction The acidity or basicity of a compound in a given me- dium is influenced by both the electronic effects of the substituents and the solvent effects of the medium. More- over, it is sometimes extremely difficult to assess how much each effect contributes to the acidity or basicity. Small differences in acidity or basicity between similar molecules are also extremely difficult to interpret and one must be very careful in deciding which structural effect is the main influence on acidity or basicity. TX100 is a nonionic surfactant and profoundly influ- ences the bulk properties of physiological systems. They can solubilise, concentrate and compartmentalize ions and molecules 1 . Hence, the influence of anionic micellar media on the protonation equilibria of phenyl alanine and ma- leic acid are investigated in the presence of TX100. The use of aqueous micellar media is wide and varied such as in analytical chemistry, pharmaceutics, organic synthesis and several industrial applications. Amphiphilic molecules, containing both hydrophobic and hydrophilic moieties, associate in water above a certain concentration to form colloidal particles called micelles 2 . Micellar systems can shift acid-base equilibria. This shift can be explained in terms of differences between the properties of the bulk solvent and of the interfacial region and perturbation of the acid-base equilibria by the electrostatic field effect of the charged interface. The dissociation equilibria of sub- stituted benzoic acids in cationic and anionic micelles have been investigated potentiometrically 3 . It was shown that their pKa values shift to about 0.5–3.0 in anionic micelles. The acid-base equilibria of a number of phenols, amines and carboxylic acids in aqueous micellar solu- tions have been examined 4 . A number of studies has re- ported on protonation constants of -amino acids in dif- ferent media 5–8 . Acidity and basicity of a molecule is governed by its structure and solvent effects 9,10 . There- fore, with this in mind, this work investigates the effects of non-ionic micellar solution on the dissociation equili-