Mixing behaviour of carboxylic acids adsorbed on graphite

Abstract

Solid monolayer formation by all the linear carboxylic acids from C 6 to C 20 adsorbed from their liquids to a graphite surface is demonstrated. In addition, we present the two-dimensional phase behaviour of linear monocarboxylic acid mixtures adsorbed on graphite from their liquid mixtures, determined using differential scanning calorimetry. All acid mixtures with alkyl chains that differ by two or three methylene groups (Dn ¼ 2 or 3) exhibit a significant degree of phase separation in the monolayer. Generally, the mixing tendency increases with increasing alkyl chain length for a given Dn, and with more similar chain lengths (e.g. Dn ¼ 1). We report neutron diffraction data that confirms the formation of solid acid monolayers. This structural data also allows us to compare the mixing results with a recent quantitative model for 2-D mixing. The general form of the observed behaviour agrees well with the model, although the characteristic parameters that separate complete mixing, partial mixing and phase separation are different from those found with alkane and alcohol monolayer mixtures.

A Tale of Tails: Alkyl Chain Directed Formation of 2D Porous Networks Reveals Odd–Even Effects and Unexpected Bicomponent Phase Behavior

“…Patterning surfaces in view of tuning its interfacial properties can be realized via the templating with organic molecules. − Bestowing self-assembled monolayers (SAMs) to the surface is an inexpensive and versatile bottom-up method. − Understanding and controlling the phase behavior on adsorption onto a solid support is important in developing robust strategies for engineering interfaces. In general, the coadsorption of the components present in the liquid mixture can lead to several possibilities. − The compounds can display eutectic behavior. , As such, the molecules are adsorbed in separate phases, each representing a structure identical to the pure system. When one of the compounds, however, gets incorporated in the lattice of the other one, mixing on the surface occurs.…”

Bulk and Adsorbed Monolayer Phase Behavior of Binary Mixtures of Undecanoic Acid and Undecylamine: Catanionic Monolayers

“…Figure presents thermograms from acid/amine mixtures in the presence of graphite where the temperature region just above the bulk melting point is highlighted. The formation of solid monolayers of adsorbed pure carboxylic acids and pure amines has been reported previsously ,− , and confirmed by diffraction studies. Monolayers formed at high coverage (well above one equivalent monolayer), approximately 40 monolayers, coexist with bulk liquid.…”

“…The bulk crystal structures have been reported previously and consist of molecular dimers. It has also been established that the dimeric form of these molecules persists into the liquid state as evident in various colligative properties. , …”

“…The formation of solid monolayers that adsorb from the liquid to graphite have been observed for a wide variety of simple molecules including linear alkanes, − alcohols, − carboxylic acids, − and amines . Carboxylic acids have also been reported to adsorb solid monolayers from solution.…”

The structures of 1-bromoheptane and 1-bromononane monolayers adsorbed on the surface of graphite

“…The apparatus and procedures for the synchrotron X-ray [24,28], Neutron [9,36] and DSC [19,37] have been described elsewhere. The instrument used for the synchrotron experiment was I11 [38] at the Diamond Light Source, Didcot, UK.…”