# Stationary Phases for Green Liquid Chromatography
## 2 Purely Aqueous Conditions: Features and Requirements  
As mentioned above, the use of pure water allows chromatographic analyses to be performed in a completely \"green\" manner. With industry and science growing faster and faster, the amount of waste generated is increasing. Therefore, we have no choice but to look for ways to ensure that modern and future-oriented analytical work does not add to environmental pollution. An extensive description of the problem and solutions was given by [PERSON] et al. [23]. The past two decades have seen a growing interest in the application of pure water as a mobile phase. One can find many experimental works and several published review papers related to this issue [6; 7; 9; 24; 25; 26; 27; 28; 29; 30; 31]. Changes to standard liquid chromatography approaches are needed in order to make the separation in pure water possible. Two paths are possible: the first includes elevated temperatures, while the second remains at ambient temperatures but focuses on selecting appropriate stationary phases. The magnitude of the temperature increase can vary between applications. However, the often-used phrase \"high-temperature\" is not exactly precise. Less precise terms such as \"higher than room temperature\" or \"higher than 100 \({}^{\circ}\)C\" and more precise terms such as \"higher than the boiling point of the mobile phase solvent\" or \"in the range between 40 \({}^{\circ}\)C and 200 \({}^{\circ}\)C\" appear in the literature [25; 26]. Changes in water temperature affect a large spectrum of physicochemical parameters, which may dishearten analysts. Usually, only the influence on the viscosity is frequently discussed in the literature. Increases in eluent strength, increases in diffusivity, and changes in the dissociation rates of ionizable compounds are equally significant, but less frequently considered. Therefore, the overall benefits and difficulties associated with high-temperature analysis are not considered exhaustively [26]. Our previous paper described the discussion about temperature-dependent changes in water properties and their effects on analyses in pure water [9]. In RP-LC, increasing the temperature produces an effect analogous to that of increasing the organic solvent concentration in the mobile phase. The studies carried out by [PERSON] and [PERSON] [32], and [PERSON] and [PERSON] [33] show that a 1% increase in ACN concentration corresponds to a 5 \({}^{\circ}\)C increase in temperature. In contrast, an equivalent increase in methanol concentration corresponds to a 3.75 \({}^{\circ}\)C increase in temperature. Of course, the precise effect depends on the stationary phases used [34; 35]. For example, in the methanol-water mobile phase, a temperature increase of 3.5 \({}^{\circ}\)C corresponds to a 1% increase in the concentration of the organic component, while a temperature increase of between 5 \({}^{\circ}\)C and 8 \({}^{\circ}\)C corresponds to a 1% increase in the concentration of acetonitrile in an acetonitrile-water mixture on polystyrene-divinylbenzene stationary phase (Hamilton PRP-1). Changing the stationary phase to Zorbax RX-C18 gives results of 1% of organic solvent concentration being equivalent to 2 \({}^{\circ}\)C for methanol-water and 3 \({}^{\circ}\)C for acetonitrile-water mobile phases [35]. The dielectric constant of water is reduced from 85 at 25 \({}^{\circ}\)C to 35 at 200 \({}^{\circ}\)C. Elevated temperatures make the water behave like an organic solvent; hence, it becomes a very effective solvent for separating weakly polar compounds [36]. Application of water at elevated temperatures allows experiments to be conducted under completely \"green\" conditions if the separated substances and the stationary phase are thermally stable [25]. If the properties of water depend on the temperature, changing the temperature makes it possible to separate substances with a full spectrum of polarity.