I have a binary mixture of two molecules able to establish hydrogen bonds and with similar volume. One of the pure has a viscosity LARGELY lower than the other, so I would expect that adding the low-viscosity (LV) one to the high-viscosity (HV) one results in a dropping in the total viscosity. This is true for addition of more than 0.02 molar fraction of LV to HV. Smaller quantities of LV results in a viscosity higher than neat HV.
How can this be interpreted?
Are these experimental data or modelling? Experimentally, adding a LV compound to a HV one should always decrease the viscosity of a mixture (especially in ILs), but then in a modelled mixture some steric hindrance could come into play, but it would be useful to get some more details on your data.
They are experimental data. This is even more peculiar because this effect appears only at temperatures lower than 25°C. Over that threshold the trend is "normal".
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You may find useful refer to: B.E. Poling , J.M. Prausnitz, J.P. O'Connell, "The Properties of Gases and Liquids", 5th ed., McGraw-Hill, 2001; sect. 9-13: "Liquid mixture viscosity".
Try to measure the viscosity of a deviating mixture at increasing shear stress. If the apparent viscosity will decrease the deviation could be explained with the formation of new structures due to hydrogen bonds which the stress destroys, i.e. the mixture will behave as a non-Newtonian fluid. You will need a suitable viscosimeter, cone and plate or coaxial cylinders for example.
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