# X-ray Photoelectron Spectroscopy in Mineral Processing Studies
## 4 Towards in Situ XPS
### XPS Studies of Solid-Liquid Interfaces
#### 4.1.2 Cryogenic XPS  
A number of works considered above have employed low-temperature XPS for retaining surface substances unstable in vacuum at room temperature. In fact, the preparation, sometimes referred as \"freeze-drying\" [171], combined cooling and drying the sample in the entry lock chamber under controlled conditions in order to remove water but preserve volatile species of interest (sulfur, polysulfide, dixanthogens, and others). This procedure is usually preceded by washing to eliminate undesirable surface products and solution remnants since, otherwise, non-volatile solutes could uncontrollably precipitate at the interface.  
[PERSON] and co-workers [180] have proposed XPS as a tool for examination of aqueous solutions and demonstrated that their quick freezing to LN temperatures preserves the solution chemistry and prevents precipitation of solutes. More recently, [PERSON] [171, 181, 182, 183] has developed the cryogenic XPS analysis of solid/aqueous phase interfaces using fast-frozen wet particulate materials, which allows retaining the volatile species together with a thin frozen film of adjacent aqueous phase.  
In a typical \"fast-freezing\" procedure [183], an aqueous dispersion is centrifuged and the supernatant is discarded; the wet paste is placed on a sample holder cooled to a liquid nitrogen temperature, and kept in the spectrometer lock chamber under a flow of dry N\({}_{2}\) or Ar for 30-60 s prior to pumping to a vacuum of \(10^{-6}\) to \(10^{-7}\) mBar in order to freeze the paste and evaporate (sublimate) excessive water (ice). The sample is transferred to the pre-cooled manipulator in the analysis chamber; its temperature is kept below \(-150\)\({}^{\circ}\)C with the vacuum on the order of \(10^{-9}\) mbar during the experiment. After the measurement, the samples are usually warmed up in the vacuum and reexamined at room temperature. The technique allows to study mineral/solution interfaces with the water (ice) layer thickness of about 1 nm and electrolyte concentrations below 0.05-0.1 mol/L [183]. The laboratory-based cryo-XPS has been successfully employed for exploration, among many others, of ion adsorption and electrical double layer structures [182; 184], sulfur oxidation products of the reaction of goethite and lepidocrocite with aqueous sulfide [185], surfaces of chalcopyrite upon chemical and bacterial leaching [68; 186], xanthate-derived collectors adsorbed on the main sulfide minerals [102; 103], and other interfaces (see Figures 1 and 2 as examples). Interestingly, some signatures of surface nanobubbles, whose role in flotation is discussed at present [101; 186; 187; 188; 189; 190], were observed as highly inhomogeneous electrostatic charging of sulfide mineral particles reacted with butyl xanthates [102; 103] probably due to widespread cavities and rare ice islets on the hydrophobic surfaces.  
The cryo-XPS of fast-frozen pastes is a promising method for quasi in situ studies in mineral processing, especially as that can be easy implemented in commercial laboratory instruments. Some questions regarding this technique still need to be answered. Particularly, it is not quite clear yet how thick the adjacent ice films are at specific materials [183], or how very fine particles (slimes) occurring in mineral slurries [191] upon freezing and collecting the spectra will manifest themselves.