Journal of Colloid and Interface Science 255, 410–416 (2002) doi:10.1006/jcis.2002.8671 Thermodynamic Properties of the Aqueous Solution of Potassium Salts of Some 4-((Alkylcarbonyl)amino)-2-hydroxybenzoic Acids at 298 and 313 K Emilia Fisicaro, ∗,1 Carlotta Compari, ∗ Guido Viscardi,† and Pierluigi Quagliotto† ∗ Dipartimento di Scienze Farmacologiche, Biologiche e Chimiche Applicate, Universit` a di Parma, Parco Area delle Scienze, 27A, 43100 Parma, Italy; and †Dipartimento di Chimica Generale ed Organica Applicata, Universit` a di Torino, Corso D’Azeglio, 48, 10125 Torino, Italy Received March 5, 2002; accepted August 14, 2002 To understand the aggregation behavior of surface-active ligands with a salycilic polar head, we undertook a systematic study of some classes of anionic surfactants where the presence and the position of the –OH and the carboxylic group differ. This paper reports the di- lution heats at 298 and 313 K of aqueous solutions of potassium 4-((alkylcarbonyl)amino)-2-hydroxybenzoate (KPAS-C n where n stands for the number of carbon atoms in the chain) in KOH at 0.1 m, measured as a function of concentration. From the experi- mental data, apparent and partial molar enthalpies vs concentra- tion were obtained. By using a pseudo-phase-transition approach, the enthalpy changes upon micelle formation (H m ) and assuming that in the restricted range of temperature examined heat capacities are constant, the heat capacity changes have been obtained. Micelle formation enthalpies are seen to be additive with a group contri- bution for the methylene group of -1.5 ± 0.1 kJ mol -1 per group at 298 K and -2.3 ± 0.1 kJ mol -1 per group at 313 K, comparable with that obtained for similar anionic compounds in the same exper- imental conditions and for N-alkylnicotinamide chlorides (cationic surfactants). The –CH 2 – group contribution to the micelle forma- tion heat capacities is -53 ± 1JK -1 mol -1 . C  2002 Elsevier Science (USA) Key Words: potassium 4-((hexylcarbonyl)amino)-2-hydroxyben- zoate; 4-((heptylcarbonyl)amino)-2-hydroxybenzoate; potassium 4-((nonylcarbonyl)amino)-2-hydroxybenzoate; apparent molar en- thalpies of; partial molar enthalpies of; micelle formation enthalpy; micelle formation heat capacity; methylene group contribution. INTRODUCTION “Functional” surfactants is the name given to those com- pounds with reactive functions covalently bound to the molec- ular structure (1–4). In addition to their ability to form micelles in solution, surface-active ligands show a chelating polar head able to coordinate metal ions. For this reason, they have po- tential applications in extraction methods, ion transport across lipophilic membranes, and other analytical fields. This poten- tial has encouraged us to study the thermodynamic properties of 1 To whom correspondence should be addressed. the aqueous solutions of potassium 4-((alkylcarbonyl)amino)- 2-hydroxybenzoate (later on referred to as KPAS–C n , where n stands for the number of carbon atoms in the chain). These com- pounds are able to bind specifically iron(III) ion, due to the pres- ence of the salycilic moiety, and their lipophilicity can be modu- lated by changing the length of the alkylic chain to optimize the extraction performance as a function of the solvents used. They can be solubilized in water and, in the case of sufficiently high values of n, aggregate only in a basic environment. The system is complicated by the presence of the salicylic hydroxylic group: this group can be deprotonated, giving rise to a doubly charged polar head, either at high pH values or in the presence of a tran- sition metal. Notwithstanding their potential interest, we know of very few studies of their thermodynamics. We have already studied the enthalpic properties of the solutions of KPAS–C 7 at different pH and ionic strength (1). This paper aims to complete the picture of this class of surfactants by reporting the thermody- namic behavior of other compounds in this series at two different temperatures. EXPERIMENTAL Materials Reagents. 4-((Alkylcarbonyl)amino)-2-hydroxybenzoic acids (PAS–C n , n = 6, 7, 9) were prepared as indicated in Ref. (2) and purified by repeated crystallization from ethanol. The purity was checked by TLC: only one spot was detected for every analyzed compound. Freshly boiled doubly distilled water, stored under nitrogen, was used. Potassium hydroxide solutions (ca. 2 and 0.2 mol L -1 ) were prepared by diluting the content of Merck Titrisol ampoules with the proper quantity of water and standardized with potas- sium hydrogen phthalate (C. Erba; dried at 120 ◦ C). The surfactant solutions were prepared under nitrogen, adding to the acid the quantity of KOH needed both to neutralize the carboxylate group and to reach the required excess of hydrox- ide ion in the final solution (KOH at 0.1 m excess). Solution concentrations are expressed as molality, m (mol kg -1 ). 410 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.