Interface Langmuir monolayer

Another interesting class of phase transitions is that of internal transitions within amphiphilic monolayers or bilayers. In particular, monolayers of amphiphiles at the air/water interface (Langmuir monolayers) have been intensively studied in the past as experimentally fairly accessible model systems [16,17]. A schematic phase diagram for long chain fatty acids, alcohols, or lipids is shown in Fig. 4. On increasing the area per molecule, one observes two distinct coexistence regions between fluid phases a transition from a highly diluted, gas -like phase into a more condensed liquid expanded phase, and a second transition into an even denser... 

Keywords Interface Langmuir monolayer Adsorption process Spreading solvent Surface tension Surface isotherm Langmuir-Blodgett film Collapse pressure Amphiphilic polymer... 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

VI. PROBING MOLECULAR TRANSFER AT LIQUID-GAS INTERFACES AND LIQUID INTERFACES MODIFIED WITH LANGMUIR MONOLAYERS... 

SECM has been extended successfully to investigate chemical processes at aqueous-air interfaces and Langmuir monolayers supported on them [19,34,93]. Initial work concentrated on molecular transfer processes across the aqueous-air interface, with and without a monolayer of surfactant. A submarine UME (described in section III) was utilized which could approach the air-water interface from below. The first study employed the DPSC mode to investigate the transfer kinetics of electrogenerated Br2, from aqueous... 

Shimomura and Kunitake have reported that stable monolayers and LB films were obtained by electrostatic interaction of water soluble anionic polymers with cationic amphiphiles [58]. This polyion-complexation was also a useful method for stabilization of monolayers of unstable [59] or water soluble anionic surfactants [60]. Mixtures of water soluble cationic and anionic surfactants (1 1) also formed stable Langmuir monolayers at the air/ water interface [60]. 

Rodriguez Patino, J.M., Sanchez, C.C., Rodriguez Nino, M.R. (1999a). Is Brewster angle microscopy a useful technique to distinguish between isotropic domains in p-casein-monoolein mixed monolayers at the air-water interface Langmuir, 15, 4777 4788. 

Fig. 3.39 Surface pressure vs. area per molecule for a PS2W)-PQ4VP24o diblock adsorbed as a Langmuir monolayer at the air-water interface (Zhu et at. 1991a).

The forces acting between two surfaces in contact or near - contact determine the behavior of a wide spectrum of physical properties. These can include friction, lubrication, the flow properties of particulate dispersions, and, in particular, the adsorption and adhesion phenomena, the stability of colloidal system [1,2] and the ability to form Langmuir monolayer at the air - water interface. 

Fig. 4.11 The asymmetric rod-coil molecules form stable Langmuir monolayers at the air-water interface as shown by it-A isotherms for molecule 1 (solid line) and molecule 2 (dashed line) with deposition points indicated by arrows. (From ref. [73])...

Some the structural state of dendritic macromolecules at air - water (Langmuir monolayers) and air - solid (adsorbed monolayers, self - assembled films, and cast films) interfaces has been discussed by V. V. Tsukruk [106] and Frechet [107],... 

The first use of the SECM in a study of lateral mass transfer was reported by Unwin and Bard [79]. Several years later, the Unwin group studied the lateral mass transfer of a surfactant at the air-water (A-W) interface by analyzing transient current behavior [81]. The electroactive surfactant, A-octadecylferro-cenecarboxamide (CigFc0) was mixed with 1-octadecanol in a 1 1 ratio and spread onto water surface to form a Langmuir monolayer. A 25-pm diameter submarine tip (i.e., a Pt UME sealed in a U-shaped glass capillary with the conductive surface pointing upward) was placed 1-2 pm away from the A-W... 

Pallas. N.R. Pethica, B.A. Liquid-expanded to liquid-condensed transition in lipid monolayers at the air/ water interface. Langmuir 1985, 1, 509. 

R. Aveyard, J. H. Clint, D. Nees, and V. N. Paunov, Compression and structure of monolayers of charged latex particles at air/water and Octane/Water interfaces, Langmuir 16, 1969-1979 (2000). 

With the objective of designing new compositions of synthetic substitutes for the native lung surfactant, it is desirable to control the physical state and properties of dipalmitoylphosphatidylcholine (DPPC) spread at the air/water interface. DPPC is the main component of the native lung surfactant, which also comprises a complex mixture of lipids and specific proteins [62], Langmuir monolayers provide a model of the air/alveolar interface that is widely used for assessing the properties of lung surfactant replacement compositions. 

There has been considerable interest in the characterization of the conformation and orientation of amphiphilic molecules in Langmuir monolayers in the past few decades [1-5]. Monolayers at the air-water interface are widely employed as convenient experimental paradigms that mimic many vital biological processes. The advantages realized when using monolayers as models for biological interfaces arise primarily from the ease with which experimental variables may be manipulated. These include parameters that are not readily controlled in bulk phases or in films prepared on solid substrates, such as lateral pressure, surface area, and domain size and shape [6]. 

The applications of PM-IRRAS also include fatty acids, phospholipids, and protein conformations. Desbat and co-workers reported on the variation of the dissociation of a Langmuir monolayer of arachidic acid at the air-water interface as a function of the subphase pH and for several cations (Cd2+, Ca2 +, Mg2 +, and Na+) with the help of the PM-IRRAS method [92]. Fig. 14 shows the PM-IRRAS spectra of Langmuir monolayer of deuterated arachidic acid in the presence of CdCb as a function of the subphase pH. At low subphase pH (pH = 3.5), the spectrum only presents absorption bands related to the acid form, i.e., the C = O stretching vibration (v(C = O)) and the OH bending (<5(0-H)) located at 1720 and 1270 cm respectively. The frequency position of the v(C = O) is characteristic of a hydrogen-bonded carbonyl group. As the subphase pH is increased, the arachidic acid is progressively deprotonated to... 

---