Casimir force in a critical film formed from an electrolytic solution

We have studied the thickness of vapor adsorbed films of the critical binary liquid mixture acetic acid plus nonane adsorbed onto a silicon wafer substrate as a function of temperature near the critical temperature. This critical film possesses opposite boundary conditions (+-) at its two surfaces and, due to the dissociation of acetic acid, both the electrostatic force and the dispersion force affect the adsorbed film thickness. On approaching the critical temperature T(c), an increase in the film thickness L is observed, implying that the sign of the universal Casimir amplitude Delta(+-) is positive, consistent with theoretical predictions. However, we find quantitative discrepancies in the value of Delta(+-) and the form of the critical Casimir pressure scaling function vartheta(+-) compared with previous experimental results. We attribute these discrepancies to the complex nature of the critical system studied in this experiment.