Are long wavelengths of maximum interstellar polarization due to water ice mantles on grains

The possibility that unusually long wavelengths of maximum interstellar linear polarization (lambda max) and anomalously large ratios of total to selective extinction (R) are due to mantles of water ice accreted onto interstellar grains is investigated. If the mantles were water ice, then the 3.1-micron absorption feature in the spectra of HD 147889 and NGC 2024 No. 1 would have an equivalent width greater than 0.1 micron and a central depth exceeding 2 mag. Since no absorption in excess of 0.01 mag is observed, large (lambda max) and R in front of these two stars must not be due to water-ice mantles. Ammonia ice mantles are similarly ruled out. Generalizations are also made about some other substances proposed for the composition of grain mantles causing large (lambda max) and R.