Mobile phase viscosity

UPDATED – One of the general property of the liquids is their resistance to change a form. This resistance is called viscosity and can be expressed as a resistance to flow. In case of liquids, the viscosity can be simply expressed as “thickness”. For example, water is “thin” with low viscosity, while honey is “thick” having a higher viscosity (see example on right). More technically speaking, viscosity is classically defined as the tangential force per unit area necessary to maintain unit relative velocity between two parallel plates in a liquid unit distance apart.

Viscosity unit conversion

The unit of viscosity is called the poise (P). The SI unit for the viscosity is Pascal-second (Pa.s).

1 cP = 0.001 Pa.s

Viscosity of the mobile phase

In liquid chromatography, the viscosity of the mobile phase plays crucial role. It influences the maximum pressure used. Clearly, the mobile phase with lower viscosity shows lower instrumental pressure. Thus, higher flow rates of the mobile phase can be used and lead to the shorter analysis time.

Following tables and plots show dependence of the viscosity for a acetonitrile-water and a methanol-water mixtures at different composition of the binary solvent and temperature. Generaly, the viscosity decreases with increase in concentration of the organic modifier, acetonitrile or methanol, respectively. Further, as you can see from the following figures, the viscosity decrease with higher temperature, which forces further research in the field of high temperature liquid chromatography.

Acetonitrile-water mixture

ACN % (v/v)	0	10	20	30	40	50	60	70	80	90	100
15°C	1.10	1.18	1.23	1.30	1.09	0.98	0.89	0.81	0.70	0.54	0.40
20°C	1.00	1.14	1.10	1.13	0.99	0.90	0.81	0.69	0.56	0.50	0.37
25°C	0.89	1.01	0.98	0.98	0.89	0.82	0.72	0.59	0.52	0.46	0.35
30°C	0.79	0.90	0.87	0.86	0.80	0.74	0.65	0.52	0.45	0.43	0.32
35°C	0.70	0.73	0.78	0.76	0.72	0.68	0.59	0.47	0.43	0.39	0.30
40°C	0.64	0.72	0.70	0.68	0.65	0.62	0.54	0.44	0.41	0.36	0.27
45°C	0.58	0.61	0.64	0.61	0.59	0.58	0.50	0.43	0.38	0.33	0.25
50°C	0.54	0.60	0.60	0.57	0.55	0.53	0.46	0.41	0.36	0.31	0.24
55°C	0.51	0.53	0.56	0.53	0.51	0.49	0.43	0.38	0.34	0.29	0.23
60°C	0.47	0.52	0.53	0.50	0.49	0.46	0.41	0.35	0.37	0.27	0.22

Methanol-water mixture

MeOH % (v/v)	0	10	20	30	40	50	60	70	80	90	100
15°C	1.10	1.43	1.72	1.92	2.00	2.02	1.91	1.69	1.40	1.05	0.63
20°C	1.00	1.32	1.57	1.75	1.83	1.83	1.72	1.52	1.25	0.93	0.60
25°C	0.89	1.18	1.40	1.56	1.62	1.62	1.54	1.36	1.12	0.84	0.56
30°C	0.79	1.04	1.23	1.36	1.43	1.43	1.36	1.21	1.01	0.76	0.51
35°C	0.70	0.92	1.07	1.19	1.24	1.26	1.21	1.09	0.91	0.69	0.46
40°C	0.64	0.82	0.96	1.05	1.11	1.12	1.08	0.98	0.83	0.64	0.42
45°C	0.58	0.75	0.87	0.96	1.00	1.02	0.98	0.89	0.76	0.58	0.39
50°C	0.54	0.71	0.82	0.89	0.93	0.94	0.90	0.82	0.70	0.54	0.37
55°C	0.51	0.67	0.77	0.84	0.88	0.88	0.84	0.76	0.65	0.50	0.36
60°C	0.47	0.61	0.70	0.77	0.81	0.81	0.79	0.72	0.61	0.47	0.33

Source

These data are based on the table presented in Introduction to Modern Liquid Chromatography (L.R. Snyder, J.J. Kirkland, J.W. Dolan, Willey, 2010, 3rd ed.).

For different solvents look in the Mixtures of Water and Organic Compounds file in Landolt-Börnstein Database.