2D phase transition

After discussing specific, basic and experimental aspects of the use of thermal He as a surface probe we will review recent results obtained in the study of surface dynamics and of 2D phase transitions. 

The Broekhoff-van Dongen isotherm allows for multilayer adsorption with lateral interactions and predicts the possibility of 2D phase transitions in each layer [17]. The most spectacular evidence for 2D phase transitions concerns the adsorption of heavy noble gases on highly homogeneous non-polar surfaces of low atomic weight (typically, exfoliated graphite obtained by thermal dissociation of its intercalation compound with FeCla). This situation guarantees that the adsorbate-adsorbate interaction prevails on ad-sorbent-adsorbate interaction and makes it possible the observation of phase transitions in each layer. See Ref. [18] for a short overview of this subject. 

Two-dimensional (2D) phase transitions on surfaces or in adlayers have received increased attention in recent years [1-4] as they are related to important aspects in surface, interfadal and materials science, and nanotechnology, such as ordered adsorption, island nucleation and growth [2, 5-7], surface reconstruction [8], and molecular electronics [9], Kinetic phenomena such as catalytic activity and chirality of surfaces [10-12], selective recognition of molecular functions [13], or oscillating chemical reactions [14] are directly related to phase-formation processes at interfaces. 

Concepts of ordering and reactivity in two dimensions can be also addressed in studies with organic, ionic, or metallic (sub)-monolayers at potentiostatically controlled electrode-electrolyte interfaces. This approach offers the advantage, in comparison to a nonelectrochemical environment, that the structural and dynamic properties of the adsorbate and the substrate can be directly tuned through the applied electrode potential. The first electrochemical studies of 2D phase transitions were mostly confined to processes occurring at ideally smooth mercury electrodes. Typical examples are the formation of compact monolayers of organic molecules or salts [15, 16] and so-called... 

The present chapter starts with some general remarks on the thermodynamics and kinetics of 2D phase transitions in potentiostatically generated adlayers on well-defined metal-electrolyte interfaces. Subsequently, three main groups of systems will be considered organic and (an-)ionic... 

Adsorption phenomena including 2D phase transitions at electrochemical interfaces may be classified, according to the binding energy of the adsorbate onto the... 

Current transients and adsorption kinetics The shape of the current peaks, the hysteresis in the peak positions between the cathodic and anodic potential sweeps (particularly for B and B2) and lattice gas simulations [197, 359] suggest that monolayer formation occurs via several first-order 2D phase transitions. Single potential step experiments revealed monotonously falling transients for peak Ai (disordered Cu adlayer ( 3 x y3)R30°) and rising transients for peaks A 2 (( 3 X, /3)R30 disordered Cu... 

Historically, Frumldn-type models, which represent the Helmholtz region by a network of two or three condensers [449, 520-523] and classical thermodynamics based on a mean-field treatment [524-526], were apphed first to describe 2D phase transitions in organic adlayers at metal-electrolyte interfaces as a function of concentration, potential, and temperature (Sect. 33.2.2). In the simplest... 

The brief description of uracil adlayers on Au(hkl) also demonstrates that the understanding of 2D phase transition processes on defined solid electrodes, such as singleinterfacial processes associated with the adlayer as well as with the substrate surface. 

A subject of much attention is the nature of the 2D phase transitions. The consensus seems to be [65] that such 2D transitions as liquid-vapor and solid-vapor are discontinuous (except at the 2D critical point), whereas the melting behavior is less clear, because discontinuous changes are usually observed for the melting of patches in low-density adsorbed solids, with the possible exception of argon on graphite. 

---