Study of floating alcohol monolayers in contact with a reservoir drop: phase diagram and application to Langmuir-Blodgett deposition without a movable barrier
“…This result was not unexpected; the same effect was also observed when octanoic acid was not in the subphase . The slope of the solid−liquid coexistence line is equal to −0.46 (mN/m)/°C when octanoic acid is dissolved in the subphase, while it is equal to −1.24 (mN/m)/°C in the absence of octanoic acid, indicating that the dissolved acid perturbs the monolayer at low lateral pressure more than at high lateral pressure. 4 Surface tension−temperature phase diagram for dodecanol/tetradecane mixtures.…”
Section: Resultssupporting
confidence: 55%
“…The bulk phase fixes the chemical potential of the molecules in the monolayer and thus the lateral surface pressure. 3 Previous studies from our laboratory have demonstrated that surface tension and ellipsometric measurements are able to detect the first-order phase transition corresponding to the 2D melting-crystallization in the monolayer. 2,3 It is manifested as a change in the slope of the surface tension versus the temperature curve and as a sharp discontinuity in the ellipsometric angle ∆ versus the temperature curve.…”
Section: Introductionmentioning
confidence: 98%
“…For short chain fatty alcohols the monolayer crystallization temperature is about 15 °C higher than the bulk crystallization temperature. The bulk phase fixes the chemical potential of the molecules in the monolayer and thus the lateral surface pressure …”
By surface tension and ellipsometric measurements, we studied how
2D crystallization of fatty alcohol
monolayers is influenced by the presence in the subphase of a short
chain carboxylic acid, whose surface
activity is dependent upon pH. The surface crystallization of the
alcohol monolayer was appreciably
perturbed for pH values smaller than the pK
a of
the acid, that is when the protonated form of the acid
was predominant in the bulk solution. On the contrary, for pH
values above the pK
a of the acid,
the
monolayer was not influenced by the presence of the acid in the
subphase. The effect was greatly dependent
on the acid concentration in the subphase, at a fixed pH. We also
studied the effect of the acid chain length
(C6−C10) on a decanol monolayer, the effect
of the alcohol chain length with octanoic acid in the
subphase,
and the effect of the lateral surface pressure of the
monolayer.
“…This result was not unexpected; the same effect was also observed when octanoic acid was not in the subphase . The slope of the solid−liquid coexistence line is equal to −0.46 (mN/m)/°C when octanoic acid is dissolved in the subphase, while it is equal to −1.24 (mN/m)/°C in the absence of octanoic acid, indicating that the dissolved acid perturbs the monolayer at low lateral pressure more than at high lateral pressure. 4 Surface tension−temperature phase diagram for dodecanol/tetradecane mixtures.…”
Section: Resultssupporting
confidence: 55%
“…The bulk phase fixes the chemical potential of the molecules in the monolayer and thus the lateral surface pressure. 3 Previous studies from our laboratory have demonstrated that surface tension and ellipsometric measurements are able to detect the first-order phase transition corresponding to the 2D melting-crystallization in the monolayer. 2,3 It is manifested as a change in the slope of the surface tension versus the temperature curve and as a sharp discontinuity in the ellipsometric angle ∆ versus the temperature curve.…”
Section: Introductionmentioning
confidence: 98%
“…For short chain fatty alcohols the monolayer crystallization temperature is about 15 °C higher than the bulk crystallization temperature. The bulk phase fixes the chemical potential of the molecules in the monolayer and thus the lateral surface pressure …”
By surface tension and ellipsometric measurements, we studied how
2D crystallization of fatty alcohol
monolayers is influenced by the presence in the subphase of a short
chain carboxylic acid, whose surface
activity is dependent upon pH. The surface crystallization of the
alcohol monolayer was appreciably
perturbed for pH values smaller than the pK
a of
the acid, that is when the protonated form of the acid
was predominant in the bulk solution. On the contrary, for pH
values above the pK
a of the acid,
the
monolayer was not influenced by the presence of the acid in the
subphase. The effect was greatly dependent
on the acid concentration in the subphase, at a fixed pH. We also
studied the effect of the acid chain length
(C6−C10) on a decanol monolayer, the effect
of the alcohol chain length with octanoic acid in the
subphase,
and the effect of the lateral surface pressure of the
monolayer.
“…The surface tension was measured using a platinum Wilhelmy plate, and the lateral pressure was calculated at each temperature as the difference between the pure water-surface tension and the film-surface tension. As described in a previous paper, 21 the monolayers in contact with a macroscopic drop of alcohol experience a high lateral pressure ͑40-50 mN/m͒, but it is possible to reduce the lateral pressure by diluting the alcohol with an alkane. Dodecane was chosen because the molecule is long enough so that it does not spread spontaneously on the surface, but short enough to remain in a liquid phase at the temperatures of interest.…”
We show how high-resolution x-ray studies associated with surface tension measurements allow a determination of the elastic constants of crystalline 1-decanol monolayers at the water-air interface. The shape and position of the power-law diffraction peak yield, respectively, the shear and compression elastic constants. At low temperature, rotation scans of the liquid surface around a vertical axis exhibit six sharp peaks, proving the existence of the long-range order of bond orientations, as expected for a hexagonal two-dimensional ͑2D͒ crystal. We find a shear elastic constant of (210Ϯ30) mN/m and a compression modulus of (2500 Ϯ500) mN/m. These values are compatible with the existence of a 2D solid, according to the Nelson-Halperin melting theory. Between ϳ12°C and melting (T m ϭ14.3°C), the drastic broadening of the diffraction line can be interpreted either as the occurrence of a hexatic phase or as a strong decrease of the shear modulus, as expected in the framework of the weak crystallization theory.
“…The size of the molecules obliged us to develop a new method of monolayer deposition because the standard way of generating a phase transition in Langmuir films with movable barriers is not applicable for water soluble molecules [15]. A drop of pure alcohol is deposited on the water surface, a monolayer spreads spontaneously and remains in equilibrium with the reservoir drop.…”
Monolayers of short chiral alcohols at the water surface provide a simple model for chiral interactions between molecules. This paper is focused on alcohols with two particular chain lengths, 2-tridecanol (2C13) and 2-tetradecanol (2C14). Thermodynamic and structural parameters were measured, varying the ratio of left and right enantiomers within monolayers. The evolution of melting temperatures and entropies is not consistent with the formation of a racemate. Grazing X-ray diffraction, revealing the molecular stacking, shows clearly that there is no chiral separation. In these two compounds molecules spontaneously self-assemble at the water surface as a solid solution. The chiral polar head-group is partially screened by the chain thermal disorder of the rotator phase. 2-tridecanol exhibits a hexagonal rotator phase stable in time and independent of temperature. Experiments performed close to the melting point show pre-transitional effects for mixtures different from 50/50. This broadening of the peak can be related to defects in 2D crystals, phenomena already observed for the melting of some 3D systems.
PACS. 61.10.-i X-ray diffraction and scattering -64.70.Dv Solid-liquid transitions -82.65.Dp Thermodynamics of surfaces and interfaces
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