In this study, an environmentally friendly analysis of penicillinase-resistant antibiotics oxacillin, dicloxacillin, and cloxacillin was carried out by liquid chromatography methods using ethanol solvent that is not harmful to human health and the environment. Ionization constant (wwpKa) values of isoxazolyl penicillins in ethanol–water binary mixtures containing 20–35% (v/v) ethanol at 37 °C were obtained by green reverse phase liquid chromatography and micellar liquid chromatography methods known as green chromatography. The developed methods are based on a model that relates the chromatographic behavior of the identified analytes to pH. The linear relationship between the microscopic solvatochromic parameter ETN and the retention values of the compounds was determined depending on the change of organic solvent in the mobile phase. Using the obtained chromatographic data, conditions that ensure the simultaneous separation of the compounds for both methods were determined. The relationship between the calculated sspKa values and the macroscopic properties of ethanol was investigated. The agreement between the pKa (wwpKa) values in the water medium determined by this approach was determined by t-test at a 95% confidence level for both methods. The wwpKa values of the compounds at 25 °C were also estimated using Abraham solute parameters without any experimental work.