Phase Transition Behavior of Fluorinated Monolayers at the Water−Hexane Interface

Abstract

X-ray surface scattering and interfacial tension measurements are used to study the solid-to-gas phase transition in soluble monolayers of F(CF 2 ) 8 (CH 2 ) 2 OH and F(CF 2 ) 10 (CH 2 ) 2 OH adsorbed at the water-hexane interface. X-ray reflectivity and diffuse scattering measurements determine the molecular ordering, the presence of domains, and the interfacial coverage of solid domains as a function of temperature. The temperature-dependent coverage can be analyzed by a functional form consistent with a critical transition proposed by theory.

Vaporization and layering of alkanols at the oil/water interface

“…However, for some materials, equilibrium coexistence of domains of the low and high-temperature monolayer phases was observed within some temperature range ΔT near T o , where ΔT can be as large as tens of degrees [8,9]. In these systems, it has been suggested that this phase transition can be explained as a second order transition determined by the competition of long-range and short-range interactions between the adsorbed dipolar surfactants [9][10][11][12][13].…”

X‐Ray Studies of Surfactant Ordering and Interfacial Phases at the Water‐Oil Interface

“…[5,8] When compared to the reflectivity from a pure water-hexane interface (see Figure 3), the reflection from the water-hexane (FC 10 OH) interface is greater. The increase in reflectivity over that of the pure interface is due to constructive interference of the X-rays reflected from the top and bottom of the monolayer.…”

“…A set of normalized X-ray reflectivity curves for a range of temperatures measured from the water-hexane(FC 10 OH) system is shown in Figure 5. [3,8] Analysis of data at the lowest temperature, 22.88C, has just been discussed and corresponds to a fully covered interface of FC 10 OH molecules. The peak in the reflectivity is at the same position for all temperatures.…”

“…Figure 7 illustrates, in cartoon format, the qualitative features determined from a number of studies of surfactants at the water-hexane interface. [3,4,8,11] Below the transition temperature, the interface is fully covered by a condensed phase of surfactants. From the density of the monolayer as determined by our reflectivity measurements, we know that the C 20 OH surfactants form a liquid monolayer phase while the FC 10 OH surfactants form a solid monolayer phase.…”

The critical crossover at the n-hexane-water interface

“…Both of these phases tend to intermixing, since the formation of one-dimensional interphase boundaries leads to a significant decrease in the system energy [5]. An evident consequence of this is the impossibility of a two-dimensional (2D) first-order phase transition in this system; instead, an infinite sequence of phase transitions (critical 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-hexanewater interface to be established.A macroscopically flat interphase boundary (interface) between n-hexane (a nonpolar organic solvent) and water (see Fig. 1) offers an example of the system, featuring the phenomenon of critical crossover.…”