Influence of aliphatic amides on the temperature of maximum density of water Andrés Felipe Torres, Carmen M. Romero ⇑ Departamento de Química, Facultad de Ciencias, Universidad Nacional de, Colombia article info Article history: Received 10 June 2016 Received in revised form 27 September 2016 Accepted 13 October 2016 Available online 14 October 2016 Keywords: Amides Temperature of maximum density Despretz constant Water structure Solute-solvent interactions abstract The influence of dissolved substances on the temperature of the maximum density of water has been studied in relation to their effect on water structure as they can change the equilibrium between struc- tured and unstructured species of water. However, most work has been performed using salts and the studies with small organic solutes such as amides are scarce. In this work, the effect of acetamide, propionamide and butyramide on the temperature of maximum density of water was determined from density measurements using a magnetic float densimeter. Densities of aqueous solutions were measured within the temperature range from T = (275.65–278.65) K at intervals of 0.50 K in the concentration range between (0.10000 and 0.80000) molÁkg À1 . The temperature of maximum density was determined from the experimental results. The effect of the three amides is to decrease the temperature of maximum density of water and the change does not fol- low the Despretz equation. The results are discussed in terms of solute-water interactions and the dis- rupting effect of amides on water structure. Ó 2016 Published by Elsevier Ltd. 1. Introduction One of the characteristic and special properties of water is the density maximum at 277.13 K [1]. The effect of dissolved sub- stances on the temperature of maximum density of water h o induces shifts in the temperature of maximum density and these changes have been related to the effect of these compounds on the hydrogen bonded structure of liquid water which changes the equilibrium between structured and unstructured species of water [2–5]. The temperature shift Dh can be represented by Eq. (1): Dh ¼ h s À h o ð1Þ where h s is the temperature of maximum density of the solution and h o is the temperature of maximum density of water. It has been observed that strong electrolytes lower the temper- ature of maximum density of water and the change is proportional to the concentration of solute. The behaviour of electrolytes for dilute and even moderate concentration is usually well repre- sented by the Despretz equation [2,6–11,5]: Dh ¼ K D m ð2Þ The concentration is usually expressed as molality m in molÁkg 1 and K D is called the Despretz constant and its magnitude depends on the nature of the solute added as well as on solute–solvent and solute–solute interactions. Negative Despretz constants corre- spond to negative shifts like those observed for strong electrolytes and are supposed to be related to disrupting effects on the solvent structure; positive changes have been attributed to the stabiliza- tion of water structure [2,12–15]. However, it has been found that this simple equation is not suf- ficient to describe the behaviour of all types of solutes in aqueous solutions, neither the behaviour observed over a wide concentra- tion range. Therefore other mathematical relationships have been proposed trying to explain the changes in the Dh as a function of molality as a result of the different interactions in solution and the parameters obtained from these equations are used as criteria for classifying solutes as forming or disrupting solutes of water structure [2–5,8–15]. Amides have been used as model compounds because these solutes are non-ionic uncharged molecules that contain amidic and apolar groups within the molecule and constitute the struc- tural units of polypeptide chains and proteins [16–21]. For this rea- son, the study of the thermodynamic properties of these solutes in aqueous solutions is important because it can contribute to under- http://dx.doi.org/10.1016/j.jct.2016.10.024 0021-9614/Ó 2016 Published by Elsevier Ltd. ⇑ Corresponding author at: Universidad Nacional de Colombia-Sede Bogotá, Facultad de Ciencias, Departamento de Química, Grupo de Termodinámica Clásica, Laboratorio de Investigaciones Básicas, Calle 44 # 45-67 Bloque B9, Bogotá Código Postal 111311, Colombia. E-mail address: cmromeroi@unal.edu.co.Tel (C.M. Romero). J. Chem. Thermodynamics 105 (2017) 173–178 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct