Equation of State of a Two-Dimensional Gas of Pentadecanoic Acid

We report surface pressure measurements for a two-dimensional system composed of pentadecanoic acid spread on a water substrate of $p\mathrm{H}$ 2. We observe mean-field exponents in agreement with the findings of Hawkins and Benedek obtained on pure distilled water, but we find it impossible to interpret all of our results in terms of a mean-field phase transition. The observed compressibilities are as much as a factor 20 times larger than theirs.