Experimental evidence is brought for layering transitions at solid
(silica) surfaces in contact with liquid
mixtures, i.e., first-order transitions whereby the coverage of the
surface jumps by an amount equivalent
to one extra monolayer of 2,5-dimethylpyridine (2,5-DMP) adsorbed from
a liquid mixture with water.
These jumps are similar to surface phase changes whose aspects are
all shown on the adsorption isotherms:
coexistence of two values of adsorption for the same bulk equilibrium
composition, adsorption plateaus,
persistent metastability lines, characteristic evolution of the
plateau's length with temperature, and the
proximity of bulk demixing. Known for gas adsorption on a solid
both experimentally and theoretically,
this behavior is seen for the first time at a solid−liquid
interface.
In an adsorption study from a liquid mixture, the choice of a moderately hydrophilic silica and of a solute like 2,5-dimethylpyridine (2,5-DMP), known for its layering ability in dilute solution in water, allows the observation of several adsorption gaps on the isotherms established at different temperatures. These gaps may be attributed to surface phase transitions. Collected on a (T-Γ21) diagram, they constitute a surface phase diagram highly similar to the bulk water-2,5 DMP liquid-solid diagram at low temperature. It is the first example of a surface phase diagram at a liquid-mixture/solid interface.
The phase change possibilities in the surface phase formed in a binary liquid system (water 2,5dimethylpyridine) in contact with a solid (silica) have been examined on the basis of adsorption isotherms determined at ten different temperatures. The liquid mixture undergoes a liquid–liquid demixing but also, some 20° below the critical point, a solid phase separation. On the l.h.s. of a liquid–liquid coexistence curve 2,5dimethylpyridine, diluted in water, adsorbs step by step, and on the r.h.s. the relative adsorption is wave shaped. Both behaviors can be explained by a layering process, i.e., a solid–liquid surface demixing, which unexpectedly does not exclude a separate liquid–liquid surface demixing, which could constitute the prewetting process. This analysis is compared with known theories and its consistency is carefully checked on a thermodynamical basis.
Experimental adsorption isotherms have been drawn for the water-2,5 lutidine liquid system against silica, at several temperatures, on both sides of the liquid-liquid coexistence curve and for the coexisting phases. In the two-phase liquid system, perfect wetting, probably due to long range interactions is made by the water rich phase, whereas lutidine, probably due to chemical forces is preferentially adsorbed.In water rich monophases, at very low lutidine concentrations, lutidine forms monolayers on silica. Close to the phase separation, lutidine adsorption strongly increases and diverges for T-Tc, the critical temperature of the liquid system: a critical behaviour.In lutidine rich phases, adsorption behaviour is quite different, and T,, the wetting transition temperature ir the important parameter for the behaviour of the adsorption at coexistence.The set of experimental results is very similar to the results of the theoretical literature, and both can be used as a guide for further experimental exploration of the so-called "prewetting" transition.
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