Eur. Phys. J. E 13, 109–112 (2004) DOI: 10.1140/epje/e2004-00047-x T HE EUROPEAN P HYSICAL JOURNAL E Surface and bulk interchange energy in binary mixtures of chain molecules E. Sloutskin 1 , E.B. Sirota 2 , O. Gang 3 , X.Z. Wu 4 , B.M. Ocko 3 , and M. Deutsch 1 , a 1 Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel 2 ExxonMobil Research and Engineering Co., Route 22 E., Annandale NJ 08801, USA 3 Department of Physics, Brookhaven National Laboratory, Upton NY 11973, USA 4 San Jose Research Center, Hitachi Global Storage Technologies, 650 Harry Road, San Jose CA 95120, USA Received 14 August 2003 / Received in final form 21 December 2003 Published online 25 March 2004 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2004 Abstract. The interchange (interaction) parameter, controlling the phase behaviour of a binary mixture, is determined for the bulk and the surface of binary mixtures of different types of chain molecules, us- ing surface tensiometry and a mean-field theory. For all mixtures and concentrations studied an identical behaviour is observed at the surface, depending only on the square of the reduced chain length mis- match Δn/ n, where Δn and n are the difference in and average of the number of carbons of the two components. PACS. 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions – 68.35.Md Surface thermodynamics, surface energies – 68.03.Cd Surface tension and related phenomena 1 Introduction Binary liquid mixtures and their surfaces have been testing grounds of choice for over a century for many fundamental ideas and theories in thermodynamics and statistical mechanics [1]. A few of these are bulk [2] and surface-mediated [3] phase separation, critical adsorp- tion [4], and short- [5] and long-range [6] wetting phe- nomena [4,7]. Such mixtures are also of great practical importance for lubricants, fuels, paints etc. The phase di- agram of binary mixtures of chain molecules is dominated by a delicate balance between entropy, which drives the system towards homogeneity, and the repulsion energy be- tween the unlike constituent molecules, which drives them to segregate and phase separate. The repulsion energy is determined by the so-called interchange, or interaction, parameter ω, which is the energy change upon replac- ing one molecule of the pure phase of one species by a molecule of the other species [1]. ω is extensively used in divers areas of phase behaviour and phase stability stud- ies of mixtures, most notably in polymer blends [8], but also in gels, liquid crystals, molten metallic alloys, colloids, emulsions etc. As we show here, the surface freezing (SF) effect [9,10], provides a unique tool for studying ω at the surface and the bulk solids of the same mixture. The SF effect is the formation of a solid monolayer (for alkanes, alkenes, semi-fluorinated alkanes) or bilayer (for alcohols, alkyl-oligo-ethyleneglycols, α, β-diols) at the free surface of the molten compound at temperatures of up to a few a e-mail: deutsch@mail.biu.ac.il degrees above the bulk melting temperature. In molecules other than chain molecules, this effect is very rare [11], since almost all materials show surface melting, where the solid’s surface melts at a temperature lower than that of the bulk [12]. The molecules of the SF monolayer are the same as those of the underlying bulk melt and a free ex- change between bulk and surface is possible in all phases. By contrast, the confinement of organic molecules to the surface of water (in a Langmuir film [13]), or of a solid (in a self-assembled monolayer [14]), imposes a very high kinetic barrier on phase transitions requiring molecular rearrangement, e.g. macroscopic phase separation, and of- ten effectively prevents their observation. The SF system is, therefore, unique in allowing the study of many ther- modynamical and structural properties of a quasi-2D solid practically without kinetic barriers, and the relations be- tween bulk and surface phase behaviour. Using bulk- and surface-freezing as tools, we have stud- ied the interchange parameter ω of chain molecules in both the quasi-2D solid surface layer and the correspond- ing solid bulk rotator phases in alcohol-alcohol (dry and hydrated) and alkane-alkane (protonated-protonated and deuterated-protonated) binary mixtures, employing sur- face tensiometry [15]. ω is obtained from the measured bulk [T b (φ b l )] and surface [T s (φ b l )] freezing temperatures, where φ b l is the bulk mole fraction of the longer compo- nent in the liquid phase, using the measured (surface) and known (bulk) entropy change upon freezing, and a simple mean-field approach based on ideal solution theory and the theory of strictly-regular mixtures [1]. In spite of the Rapide Note Rapid Note