Szyszkowski equation

${\displaystyle \sigma _{m}=\sigma _{w}\left(1-0.411\log \left(1+{\frac {x}{a}}\right)\right)}$

with:

• σ_m is surface tension of the mixture
• σ_w is surface tension of pure water
• a is component specific constant (see table below)
• x is mole fraction of the solvated component

The equation can be rearranged to be explicit in a:

${\displaystyle {\begin{aligned}a&={\frac {x}{10^{\left(-1+{\frac {{\frac {\sigma _{m}}{\sigma _{w}}}-1}{-0.411}}\right)}}}\\&=x10^{\left(1+{\frac {{\frac {\sigma _{m}}{\sigma _{w}}}-1}{0.411}}\right)}\end{aligned}}}$

This allows the direct calculation of that component specific parameter a from experimental data.

The equation can also be written as:

${\displaystyle \gamma =\gamma _{0}-{\frac {RT}{\omega }}\ln(1+K_{ad}c)}$

with:

• γ is surface tension of the mixture
• γ₀ is surface tension of pure water
• R is ideal gas constant 8.31 J/(mol*K)
• T is temperature in K
• ω is cross-sectional area of the surfactant molecules at the surface

The surface tension of pure water is dependent on temperature. At room temperature (298 K), it is equal to 71.97 mN/m [4]

Meissner and Michaels published the following a constants:

The following table and diagram show experimentally determined surface tensions in the mixture of water and propionic acid.

Surface Tension of Propionic Acid solved in Water[5] xAcid [mol/mol] xWater [mol/mol] σ [mN/m] T [K] a.104 1(Pure Acid) 0 26.68 293.15 298. 0.18229 0.81771 33.08 293.15 90.7 0.17102 0.82898 33.15 293.15 85.6 0.1494 0.8506 33.54 293.15 77.2 0.12803 0.87197 34.18 293.15 69.7 0.10947 0.89053 34.56 293.15 61.4 0.0869 0.9131 35.3 293.15 51.8 0.0747 0.9253 35.77 293.15 46.3 0.06602 0.93398 36.43 293.15 43.2 0.05436 0.94564 38.51 293.15 42.2 0.0455 0.9545 40.07 293.15 40.3 0.03268 0.96732 42.34 293.15 35.1 0.02075 0.97925 46.78 293.15 32.8 0.01457 0.98543 50.17 293.15 31.4 0.00861 0.99139 53.61 293.15 25.9 0 1(Pure Water) 72.58 293.15

This example shows a good agreement between the published value a=2.6*10⁻³ and the calculated value a=2.59*10⁻³ at the smallest given mole fraction of 0.00861 but at higher concentrations of propionic acid the value of an increases considerably, showing deviations from the predicted value.

• Bohdan Szyszkowski