Two-Dimensional Order in Mercury-Supported Langmuir Films of Fatty Diacids

Abstract

The structure of mercury-supported Langmuir films of dicarboxylic acid molecules with 13 ≤ n ≤ 22 carbons is studied by X-ray methods and surface tensiometry. The molecules lie surface-parallel, forming mono-, bi-, or trilayers, depending on coverage. All films exhibit a full 2D order of the same single-molecule oblique unit cell. In particular, the distinct odd-even structure difference of 3D crystals of the same molecules is not observed. The unit cell's width and angle show a small systematic decrease with n, while the length increases commensurately with the molecular length. These results show the films to consist of closely packed, extended, polymer-like chains of diacid molecules, bound by their carboxyl end groups. Evidence is presented for the inclusion of a single mercury atom in the carboxyl-carboxyl bond. The possible conformation of this bond and implications of the parity-independent structure are discussed.

New strategy for reducing the EHL friction in steel contacts using additive-formed oleophobic boundary films

“…The slightly higher friction for molecules that have two COOH groups at 25 ℃ (Figs. 2(a) and 3) can be explained by the way that bifunctional acids can adsorb to the steel via one or both COOH polar heads [53], or react with some other molecule, and so preventing the formation of a packed and dense film with methyl tails on top [54]. In this case the free COOH groups that may be present at the surface can thus enhance the interactions with the oil in surrounding [54].…”

The structure of Langmuir films of long diols on mercury

“…The same methods employed in previous studies of LFs on mercury were used. 1,[17][18][19][20][21] The Langmuir trough allows both surface tension and X-ray measurements to be carried out. Below is a brief summary of the relevant experimental details.…”

“…A recent study 1,6 shows that mercury-supported mono-, biand tri-layers of surface-parallel a,u-alkyldiacid molecules (see Fig. 1) form a full two-dimensional surface order.…”

“…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.…”

“…Nevertheless, for the only diol molecule studied to date as a mercury-supported LF, 1,16-hexadecanediol, no order was found to emerge at any temperature Fig. 1 Two configurations of the even carbon number 1,16-hexadecandiol molecule, with both hydroxyl moieties lying in the backbone plane (dl16(k)) and with one hydroxyl group rotated from this plane (dl16(t)) (top (1) and side (2) views), and the same for the odd carbon number 1,15-pentadecandiol molecule. For the k conformation both hydroxyl groups point to the opposite side of the chain for even molecules, and to the same sides for odd molecules.…”