Variations of absorbance as functions of saturation for the modeled experiment with methane and ethane, for different values of the wetting coefficient and for the reference cases (see “REF” cases in Table 2). For each curve, the absorbance threshold 10⁻³ is reached at given saturation ratios. These modeled values are compared to the value S_crit found in the experiment by Curtis et al. (2008) and shown as red and blue boxes for methane and ethane, respectively. This allows us, for each gas, to constrain the wetting parameter m for which the value of absorbance reaches 10⁻³ when S = S_crit. The lower and upper limits of the boxes are set at half a decade below and above the reference threshold 10⁻³.