Surface orientation of a pair of head-tail molecules

Dioctyl phthalate (DOP) and bis(2-ethylhexyl)chlorendate (BEHC) are two liquids with similar overall molecular architectures with two ethyl-substituted C₆ ester chains attached adjacent to a cyclic functionality. In the case of DOP this central feature is an aromatic ring,

while in BEHC the ring is a highly chlorine-substituted non-aromatic 7-member bridgehead.

A TOF spectra for DOP taken with 3 keV Ar⁺ ions at an incident angle a = 12 shows 2 major peaks which we identify as originating from recoiling surface H and C atoms.  .

The 3 keV Ar⁺ BEHC spectrum shows C(DR) and H(DR) peaks in the same positions as those from DOP.  Superimposed on the long TOF tail are other features. The peak at 12.1 µs flight time is close to the expected time of arrival of recoiling Cl atoms. The later feature at 13.6 µs flight time does not correspond to any reasonable recoiling mass. Rather we attribute this feature to a single scattering peak Cl(SS) resulting from an Ar⁺ ion being scattered from a Cl atom in the molecule. For 3 keV Ar⁺ ions single scattering into a 45^o scattering angle by collisions with H, C or O atoms is not possible. Hence the only SS peak that we can observe is due to scattering from Cl..

The overall appearance of the DOP and BEHC spectra in the C(DR) and H(DR) regions are very similar. The major difference is the appearance of Cl(DR) and Cl(SS) signals on the tail of the BEHC spectrum. A difference spectrum shows this more clearly. In the H and C regions the difference plot shows little systematic variation, but in the Cl region the appearance of the recoil and single scattering peaks are very apparent. The implication here is that both molecules are likely to be adopting similar orientations at the liquid surface. The clear appearance of Cl signals in the BEHC spectra argues that the Cl-containing ring portion of BEHC is at least partially accessible to the incident ions. On the other hand, the large H(DR) peak in the BEHC spectra means that the H-deficient Cl-substituted ring in BEHC cannot be predominantly at the surface with the tails dangling below, otherwise the H signal would be very small. If DOP were exclusively "head-up", the low H/C ratio in the head would mean that the size of the H(DR) peak relative to the C(DR) would have to be much smaller than observed. Hence, BEHC and DOP cannot have a "head-up" orientation.

A detailed analysis of atom ratios derived from the peak intensity ratios. The H/C ratio for DOP orientations with one or both tails and the ring exposed produces H/C ratios that are too low compared to the experimental value of 1.8. In order to achieve a H/C ratio that approaches the experimental value, the DOP must be "head down", although structures with one or both tails at the surface are essentially indistinguishable.

DOP - must be head down

For BEHC, orientations with most of the ring exposed have Cl/C ratios that is too high compared to experiment. On the other hand, orientations that puts most of the ring down has a Cl/C ratio that is too low. Orientations with 2-3 Cl atoms accessible to the ion beam, orientations in which the ring is rotated to expose an edge and part of the bridgehead with one or both tails in the surface, have the correct Cl/C ratio. Because the ring in BEHC contains very little H, and the C atoms are heavily shadowed/blocked by Cl atoms, the H/C ratio varies little with orientation and covers a range that includes the experimental value. Hence it is difficult to say whether one or both tails are in the surface, but it is clear that BEHC is orientated with just part of the bridgehead ring exposed.

BEHC - partly head-down but some Cl exposed

For more details see :

"Orientation of molecules at a liquid surface revealed by ion scattering and recoiling", T.J. Gannon, M. Tassotto and P. R. Watson, Chemical Physics Letters., 300, 163-168, (1999)

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Last Updated by Philip R. Watson on Monday, September 09, 2002