Fluid Phase Equilibria 300 (2011) 155–161 Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid Interactions of glycine with polyethylene glycol studied by measurements of density and ultrasound speed in aqueous solutions at various temperatures Melike Sahin, Zerin Yesil, Merve Gunel, Sadife Tahiroglu, Erol Ayranci ∗ Chemistry Department, Akdeniz University, 07058 Antalya, Turkey article info Article history: Received 22 July 2010 Received in revised form 21 October 2010 Accepted 4 November 2010 Available online 12 November 2010 Keywords: Apparent molar volume Apparent molar isentropic compression Hydrophobic hydration Glycine Polyethylene glycol abstract Density and ultrasound speed were measured accurately for binary glycine–water and ternary glycine–water–polyethylene glycol 400 (PEG400) solutions at (293.15, 298.15, 303.15 and 308.15) K. The data were used in evaluating thermodynamic properties as apparent molar volumes (V ∅ ) and appar- ent molar isentropic compressions (K S˚ ) of glycine in water and in PEG400 solutions. Infinite dilution values of these parameters, V o ∅ , and K o S˚ , were obtained from their plots as a function of molality and have been utilized in obtaining transfer volumes and transfer compressions of glycine from water to PEG400 solutions of various molalities. Apparent molar isobaric expansions were determined from the temper- ature dependence of V ∅ and V o ∅ values. All the results were interpreted in terms of glycine–water and glycine–water–PEG400 interactions. The partial hydrophobic character of PEG400 structure was found to play an important role in determining the transfer properties. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Polyethylene glycols (PEGs) of various molecular weights are being used in many areas of research and technology. In many applications they are being used in conjugation with peptides. Thus, interactions of PEGs with amino acids, building stones of peptides, are important [1,2]. A useful tool for studying such inter- actions is volumetric properties such as apparent molar volume, isentropic compression and isobaric expansion determined from density and ultrasound speed measurements at different tempera- tures as proven in our earlier works [3–5]. Although the volumetric properties of PEGs and amino acids, individually, in aqueous solu- tions have been studied extensively [6–12], there is only limited number of reports on the interactions of PEGs with amino acids studied through volumetric properties. For example Sasahara [13] reported volume changes on glycine–PEG and l-alanine–PEG in aqueous solution where PEG was with a molar mass of 4000. In another work by Sasahara and Uedaira [14], volume and compress- ibility changes on mixing aqueous solutions of amino acids and PEG, again with molar mass of 4000, were studied. In the present work, we wanted to initiate a systematic work on volumetric properties of PEG–amino acid–water systems, starting with the simplest mem- ber of -amino acids, glycine, and a PEG having a molar mass of 400, abbreviated as PEG400. We aimed at investigating the interac- tions between glycine and PEG400 through volumetric properties determined by accurate measurement of density and ultrasound ∗ Corresponding author. Tel.: +90 242 3102315; fax: +90 242 2278911. E-mail address: eayranci@akdeniz.edu.tr (E. Ayranci). speed for glycine–PEG400–water ternary systems at various tem- peratures in the interval from 293.15 K to 308.15 K. The general formula of PEG can be shown as HO–CH 2 –(CH 2 –O–CH 2 ) n –CH 2 –OH where n is the number of repeating unit which is 7.7 for PEG400 [3]. Although there are hydrophilic –OH end groups in the structure, PEG has at least partial hydrophobic character due to presence of two –CH 2 – groups against an etheric –O– group per repeating unit. Glycine is quite hydrophilic with two functional groups; an amine and a carboxylic acid group. Its isoelectric point, pI, is about 6. Thus it is found predominantly in zwitterionic form in its solutions in water. From structural information it is apparent that all kinds of interactions such as hydrogen-bonding, ion-dipole and hydropho- bic interactions are expected to involve in ternary system of glycine–PEG400–water. 2. Experimental 2.1. Materials PEG400 and glycine were obtained from Sigma. Their mass frac- tion purity was at least 0.99 pure and they were used after drying under vacuum without any further purification. Deionized water was used in all experiments. 2.2. Density and ultrasound speed measurements Densities and ultrasound speeds were measured by an Anton Paar DSA 5000 model high precision vibrating tube digital den- simeter and ultrasound speed measuring device, with automatic 0378-3812/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2010.11.005