Decylammonium octanoate

The title compound, C10H24N+·C8H15O2 −, forms a layered structure in which intermolecular N+—H⋯O hydrogen bonds connect anions and cations, forming a two-dimensional network parallel to (010). The n-alkyl chains of the decylammonium cations pack according to an orthorhombic ‘subcell’ with approximate dimensions 5.1 × 7.3 Å, and they are significantly distorted from planarity.


Comment
The combination of alkyl carboxylic acids and primary alkyl amines is of continuing interest both in the bulk and in adsorbed monolayers. There is mainly spectroscopic evidence that a number of stoichiometric complexes can form, depending upon the molecular structure: combinations AB (1 acid: 1 amine), A 2 B and A 3 B have been reported (Backlund et al., 1994;Backlund et al., 1997;Karlsson et al., 2000;Karlsson et al., 2001;Kohler, Atrops et al., 1981;Kohler, Gopal et al., 1981;Kohler et al., 1972). Interestingly, similar complexes have not been reported on the amine-rich side of the phase diagram.
The precise nature of the complexation is still a matter of debate, but hydrogen bonding between the species is obviously strongly implicated and different structures have been proposed on this basis. However, we are not aware of any single-crystal diffraction studies for these materials.
The absence of reported single-crystal data for this class of complexes is probably attributable to difficulties in obtaining suitable crystals. Our various crystallization attempts have consistently failed, and our discovery of the crystal used for this study was serendipitous. The crystal was a thin plate that diffracted weakly, and data could be measured only to 0.95 Å resolution. Nonetheless, the data are adequate to localize the H atoms associated with the ammonium group, and these H atoms could be refined satisfactorily with restrained N-H bond lengths and individual isotropic displacement parameters.
The C-O bond lengths of 1.269 (3) and 1.253 (3) Å are also consistent with proton transfer to yield a carboxylate anion.
Both molecules adopt essentially fully extended conformations (i.e. the torsion angles along the main chain are all close to 180°), although the decylammonium chain is clearly disorted from planarity ( Fig. 1). As a measure of this distortion, we note that the terminal C atom of the chain (C10) lies 1.43 (1) Å from the mean plane defined by atoms C1, C2 and C3.
As might be expected, the crystal structure is layered, with the hydrophilic sections accommodated around the glide planes parallel to (010) at y = 1/4 and 3/4 (Fig. 2). The hydrogen bonding between the ammonium groups and carboxylate anions (Table 1) defines a 2-D network comprising 6-membered rings (Fig. 3). Projection along the n-alkyl chains of the molecules reveals an approximately orthorhombic "subcell" with approximate dimensions 5.1 × 7.3 Å (the third dimension being the translation of ca 2.54 Å along the n-alkyl chain). The plane through the C atoms of the n-alkyl chain of each octanoate anion lies almost perpendicular to the planes of the n-alkyl chains of the ammonium cations (Fig. 4). This is a common subcell arrangement for long-chain n-alkyl compounds (Dorset, 2005). The distortion from planarity of the n-alkyl chain in the decylammonium cation serves to accommodate it between two neighbouring octanoic acid molecules [symmetry codes: 1 + x,0.5 -y,-1/2 + z and 1 + x,0.5 -y,1/2 + z], optimizing dispersion interactions along the length of the n-alkyl chains within the constraints imposed by the hydrogen-bonding geometry. At the interface between layers (i.e. in the (020) planes of the structure) the methyl groups of the decylammonium cations meet the methyl groups of the octanoate anions to form C···C contacts of 3.972 (4) Å, with the H atoms approximately eclipsed.

Experimental
Octanoic acid (99%) and decylamine (99.5%) were obtained from Sigma Aldrich and used without further purification. A number of solution and melt methods were attempted to grow a single-crystal of sufficient dimensions and quality, but all supplementary materials sup-2 were unsuccessful. A crystal was finally obtained serendipidously by growth from the vapour when poorly sealed vessels containing each of the individual components were stored together inside a small container (1 litre volume) in a glove bag initially purged with N 2 and left undisturbed for a number of weeks. Crystal growth was observed on most of the plastic surfaces inside the storage container but principally on the polypropylene cap of the decylamine bottle. Elemental analysis found for the bulk sample: C 72.4, H 13.1, N, 4.8%; calculated C 71.7, H 13.0, N 4.7%.

Refinement
The crystal diffracted relatively weakly, and data were collected to a maximum θ of 22° (0.95 Å resolution). Approximately 65% of data were observed at the 2σ level to this limit. The data are adequate to support location and refinement of the H atoms associated with the ammonium group. These were refined with N-H distances restrained to 0.91 (1) Å, and with individual U iso values refined in the range 0.061 (10)-0.064 (10) Å 2 . All other H atoms were placed geometrically and refined as riding with C-H = 0.99 (CH 2 ) or 0.98 (CH 3 ) Å, and with U iso (H) = 1.2 or 1.5U eq (C). Fig. 1. Molecular structure with displacement ellipsoids drawn at 50% probability for non-H atoms.      (3) 175 (3)