Nonequilibrium 2-Hydroxyoctadecanoic Acid Monolayers: Effect of Electrolytes

Abstract

2-Hydroxyacids display complex monolayer phase behavior due to the additional hydrogen bonding afforded by the presence of the second hydroxy group. The placement of this group at the position α to the carboxylic acid functionality also introduces the possibility of chelation, a utility important in crystallization including biomineralization. Biomineralization, like many biological processes, is inherently a nonequilibrium process. The nonequilibrium monolayer phase behavior of 2-hydroxyoctadecanoic acid was investigated on each of pure water, calcium chloride, sodium bicarbonate and calcium carbonate crystallizing subphases as a precursor study to a model calcium carbonate biomineralizing system, each at a pH of ∼6. The role of the bicarbonate co-ion in manipulating the monolayer structure was determined by comparison with monolayer phase behavior on a sodium chloride subphase. Monolayer phase behavior was probed using surface pressure/area isotherms, surface potential, Brewster angle microscopy, and synchrotron-based grazing incidence X-ray diffraction and X-ray reflectivity. Complex phase behavior was observed for all but the sodium chloride subphase with hydrogen bonding, electrostatic and steric effects defining the symmetry of the monolayer. On a pure water subphase hydrogen bonding dominates with three phases coexisting at low pressures. Introduction of calcium ions into the aqueous subphase ensures strong cation binding to the surfactant head groups through chelation. The monolayer becomes very unstable in the presence of bicarbonate ions within the subphase due to short-range hydrogen bonding interactions between the monolayer and bicarbonate ions facilitated by the sodium cation enhancing surfactant solubility. The combined effects of electrostatics and hydrogen bonding are observed on the calcium carbonate crystallizing subphase.

Impact of anions on the surface organisation of lipid monolayers at the air–water interface

“…More specifically, the role of the HCO 3 À ion may include: (1) participation in the hydrogen-bonding network, invoking enhanced condensation, (2) ion-pair formation to mitigate like-charge repulsion, and (3) Hofmeister related HCO 3 À /water interactions. [45,46] As shown in Fig. 3, the extent of the mean molecular-area shift appears to be concentration-dependent; it increases by 8-10 Å 2 /molecule for 0.1 M solution and by 18-20 Å 2 /molecule for 1 M solution.…”

Lupane-Type Pentacyclic Triterpenes in Langmuir Monolayers: A Synchrotron Radiation Scattering Study

“…The presence of two maxima in the course of a Bragg rod, obeying the relation Q z 2 = 2 Q z 1 (Figure c), is often observed for crystalline bilayers, so phase II can be identified as a bilayer in which betulin molecules are tilted from the monolayer normal. Another feature very characteristic for this phase is the presence of a Scherrer arch in the diffractogram. ,, We identified the location of the second peak of this phase as Q xy = 0.964 and Q z = 0.371 Å –1 , but it is only approximation, as this peak is smeared, interfering partially with the long signal at Q xy = 0.895 Å –1 . The presence of the Scherrer arch in the diffractogram proves that there is no identical azimuth of the tilt settled within phase II .…”

“…The intensity map registered for the betulin monolayer at 10 mN/m is presented in Figure 4. Four diffraction signals can be identified in the map: (1) a very strong signal at Q xy = 1.020 Å −1 with the intensity maximum at the horizon (Q z = 0 Å −1 ); (2) at Q xy = 0.895 Å −1 with the intensity smeared over a long-range of Q z values; (3) a higher order signal at Q xy = 1.346 Å −1 and intensity maximum at the horizon; (4) the signal smeared over the Scherrer arc, 45 the intensity maximum of which lies at Q xy = 0.964 Å −1 and Q z = 0.371 Å −1 . For a more detailed analysis of the GIXD data, the diffraction signals have been converted to Bragg peaks and Bragg rods, which are shown in Figure 5.…”

“…Another feature very characteristic for this phase is the presence of a Scherrer arch in the diffractogram. 45,47,48 We identified the location of the second peak of this phase as Q xy = 0.964 and Q z = 0.371 Å −1 , but it is only approximation, as this peak is smeared, interfering partially with the long signal at Q xy = 0.895 Å −1 . The presence of the Scherrer arch in the diffractogram proves that there is no identical azimuth of the tilt settled within phase II.…”

Quantum chemical analysis of the thermodynamics of 2D cluster formation of 2-hydroxycarboxylic acids at the air/water interface

“…Correspondingly, large differences exist also between the main monolayer characteristics of hydroxycarboxylic acids OH substituted in the midchain and C2 position in mesoscopic and microscopic scales using surface pressure-area per molecule (P-A) isotherms, Brewster angle microscopy and grazing incidence X-ray diffraction. 14,15 There is a general interest in understanding these systems because of the use of 2-hydroxycarboxylic acids in foods, 19,20 therapeutic and cosmetic products 21,22 and biomineralization 23,24 studies. It should be noted that 2-hydroxycarboxylic acids are chiral compounds.…”