Hydrogen-Bonded Order in Mercury-Supported Monolayers of End-Functionalized Alkanes

Abstract

The order formed by hydrogen bonding in mercury-supported Langmuir films (LFs) of end-functionalized, surface-parallel, alkane molecules was studied by surface-specific X-ray methods and surface tensiometry. Hexadecane derivatives, substituted at one or both ends by mercapto, carboxy, or hydroxy moieties were studied. The dimensionality of the monolayer's surface-parallel order was found to correspond to the number of molecular carboxyl groups, with the zero-, one-, or two-carboxyl molecules exhibiting no, one-, or two-dimensional order, respectively. The observed structural motifs are rationalized based on the molecular conformations and bonding synthons. Similarities, or otherwise, with the corresponding structure of three-dimensional (3D) crystals and graphite-supported LFs are also discussed.

Highly anisotropic thermal expansion in molecular films of dicarboxylic fatty acids

“…Another notable difference between the 3D and our 2D crystal structure is that the b-lengths found here exceed for all n the corresponding diacid molecule's length by 18,20 ∼ 3Å. This excess was suggested 9,12,20 to arise from the incorporation of one mercury atom in each carboxyl-carboxyl bond. The elongation's coincidence with the ∼ 3.2Å diameter of the mercury atom 26 supports this suggestion, as does the LFs' ρ being commensurately higher than the pure diacid layer's, as found above in the XR measurements.…”

“…The measured R(q z ) (symbols) and ideal R F (q z ) (dashed line) are shown in Fig. 1 supported organic LFs 9,14,19,20 . It uses 6 slabs to mimic the decaying layering of the mercury near an interface 21,22 plus the minimal number of additional slabs needed to describe the LF.…”

“…A similar rotation of the backbone plane from the surface plane is highly unlikely in our case 20 , as it significantly reduces the backbone-mercury contact and thus increases the surface free energy 25 . Indeed, the XR measurements above find a layer thickness of d = 4.6 ± 0.4, corresponding to flat-lying molecules, while 35…”

“…The carboxyl endgroups hydrogen-bond the molecules to each other at both ends forming long, polymer-like chains, in which the long axis of each constituent molecule is shifted laterally from that of its neighbour due to the slight azimuthal rotation of the hydrogen bond from the backbone's axis 20,24 . Such a chain is highlighted in Fig.…”

The structure of Langmuir films of long diols on mercury

“…Mercury-supported lms are well-suited for studying bonding in end-functionalized molecules. 1,2 The high surface energy of mercury induces surface parallel orientation, and thus coplanarity, on the lms' molecules, strongly restricting the orientational degrees of freedom and promoting bonding by suitably chosen end groups. The high surface energy also provides an extremely low capillary-waves-induced surface roughness, s z 1.4 Å.…”