  We derive a new unidirectional evolution equation for photonic nanowires made
of silica. Contrary to previous approaches, our formulation simultaneously
takes into account both the vector nature of the electromagnetic field and the
full variations of the effective modal profiles with wavelength. This leads to
the discovery of new, previously unexplored nonlinear effects which have the
potential to affect soliton propagation considerably. We specialize our
theoretical considerations to the case of perfectly circular silica strands in
air, and we support our analysis with detailed numerical simulations.
