Aliphatic hydrocarbons exhibit an absorption feature at 3.4 µm 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 µm hydrocarbon feature can be used to test the silicate core-organic refractory mantle grain theory .
Although the 3.4 µm 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 µm silicate polarization has been previously observed .
We show that polarization is not detected across the 3.4 µm feature to a limit of 0.06 \pm 0.13 % ( GCS 3-II ) and 0.15 \pm 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 .