  Aliphatic hydrocarbons exhibit an absorption feature at 3.4 micron observed
toward sources that sample diffuse regions of the interstellar medium. The
absorbers responsible for this feature are assumed to reside in some component
of interstellar dust, but the physical nature of the particles (size, shape,
structure, etc.) is uncertain. Observations of interstellar polarization
provide discrimination. Since the grains that carry the silicate absorption
feature are known to be aligned, polarization across the 3.4 micron hydrocarbon
feature can be used to test the silicate core-organic refractory mantle grain
theory. Although the 3.4 micron feature has been observed to be devoid of
polarization for one line of sight toward the Galactic center, a corresponding
silicate polarization measurement for the same line of sight was not available.
Here, we present spectropolarimetric observations toward GCS 3-II and GCS 3-IV
toward the Galactic center, where the 9.7 micron silicate polarization has been
previously observed. We show that polarization is not detected across the 3.4
micron feature to a limit of 0.06 +/- 0.13% (GCS 3-II) and 0.15 +/- 0.31% (GCS
3-IV), well below the lowest available prediction of polarization on the basis
of the core-mantle model. We conclude that the hydrocarbons in the diffuse ISM
do not reside on the same grains as the silicates and likely form a separate
population of small grains.
