The critical crossover at the n -hexane-water interface
- PDF / 166,520 Bytes
- 3 Pages / 612 x 792 pts (letter) Page_size
- 5 Downloads / 186 Views
AL, NONLINEAR, AND SOFT MATTER PHYSICS
The Critical Crossover at the nHexane–Water Interface A. M. Tikhonov Kapitza Institute for Physical Problems, Russian Academy of Sciences, Moscow, 119334 Russia email: [email protected] Received January 21, 2010
Abstract—According to estimates of the parameters of the critical crossover in monolayers of longchain alcohol molecules adsorbed at the nhexane–water interface, all systems in which this phenomenon is observed are characterized by the same value of the critical exponent ν ≈ 1.8. DOI: 10.1134/S1063776110060142
Atoms or molecules adsorbed on the surface of a liquid or crystal frequently form a spatially inhomoge neous structure in which domains of two homoge neous phases coexist [1–4]. Both of these phases tend to intermixing, since the formation of onedimen sional interphase boundaries leads to a significant decrease in the system energy [5]. An evident conse quence of this is the impossibility of a twodimen sional (2D) firstorder phase transition in this system; instead, an infinite sequence of phase transitions (crit ical crossover) must take place [6]. This article presents the results of an analysis of experimental data obtained earlier [7, 8], which allowed a critical parameter of the crossover at the n hexane–water interface to be established. A macroscopically flat interphase boundary (inter face) between nhexane (a nonpolar organic solvent) and water (see Fig. 1) offers an example of the system, featuring the phenomenon of critical crossover. Under normal conditions, nhexane (saturated hydrocarbon with the formula C6H14, a density of ~0.65 g/cm3 at T = 298 K, and a boiling temperature of about 342 K) and water exhibit virtually no mutual solubility. It was reported earlier [7–9] that a surface electri cal double layer can form at the nhexane–water inter face owing to adsorption (from hexane solutions) of longchain molecules of fluorinated alcohols, such as 1,1,2,2tetrahydroheptadecafluorodecanol (FC10OH) and 1,1,2,2tetrahydrohenicosafluorododecanol (FC12OH), or saturated monatomic alcohols, such as ntetracosanol (C24OH) and ntriacontanol (C30OH) [7–9]. The fluorocarbon chain of FC12OH is longer by two –CF2– units than that of FC10OH, and the hydro carbon chain of C30OH is longer by six –CH2– units than that of C24OH. The main difference between the properties of fluorocarbon and hydrocarbon chain molecules is their flexibility. Indeed, the former mole cules at room temperature can be considered abso lutely rigid rods, whereas the latter are susceptible to conformation isomerization.
Synchrotron radiation reflectometry data show that molecules of the aforementioned substances at sufficiently low temperatures adsorb from a solution in liquid hydrocarbon at the nhexane–water interface in the form of a monolayer (Gibbs monolayer) with a certain set of thermodynamc parameters (p, T, c). It was found that the longchain molecules of various alcohols are ordered differently on the water surface. The density of molecules in the cond
Data Loading...
