Lipid soluble substances can enter a cell by dissolving in the lipid portion of the membrane and diffusing through it. The greater the lipid solubility, the more readily a molecule will pass through the membrane, i.e., the greater will be the flux for a given concentration difference
Water soluble, lipid-insoluble substances by definition cannot pass through membranes by dissolving in the lipids. There must be water-filled channels or pores spanning the membrane through which these substances may diffuse.
