Monolayers experiment

In whole tissue or cell monolayer experiments, transcellular membrane resistance (Rm = Pm1) lumps mucosal to serosal compartment elements in series with aqueous resistance (R = P ). The operational definition of Lm depends on the experimental procedure for solute transport measurement (see Section VII), but its magnitude can be considered relatively constant within any given experimental system. Since the Kp range dwarfs the range of Dm, solute differences in partition coefficient dominate solute differences in transcellular membrane transport. The lumped precellular resistance and lumped membrane resistance add in series to define an effective resistance to solute transport ... 

The development of amphiphilic and aggregation behaviour as a fiinction of dendrimer generation of PS-t/enstudied with several diflferent techniques the amphiphilic character at a toluene-water interface was investigated with conductivity measurements, and at a water-air interface with monolayer experiments. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to examine the aggregates formed by the different generations in aqueous solutions, while the critical association concentrations were determined with the pyrene-probe luminescence technique. 

Amphiphilic behaviour at an air-water interface was studied using monolayer experiments. Surface pressure-area isothenns were recorded for PS-rfe dr-(NH2) with = 1-16. The formed monolayers were also investigated with a Brewster angle microscope (BAM) [53], which made it possible to investigate the type of structures that are formed in the monolayer. The results are shown in Figure 15. 

Monolayer experiments were performed on the different generations of dendritic amphiphiles to estimate the molecular area. The pressure-area isotherms show a linear increase of the molecular area with the number of alkyl chains attached to the different generations dendrimer (Figure 22, Table 3). 

The result matches very well with the theoretical values based on CPK models and molecular simulation of the various dendrimers in the gas phase [33,34,72]. This similarity implieates that the hydrophilic core of the higher generation dendrimers has a more ellipsoidal conformation. The results obtained from the monolayer experiments also indicate that the higher generation dendrimer has a more flat conformation, because in that case all the attached chains can count for the observed molecular area. 

The radius of the spherical fifth generation dendrimers is about 1.25 nm [33,34]. If we assume that the molecular area of 16nm of this macromolecule (Table 3) measured by the monolayer experiments is the area of the head group [73], then a radius of 2.26 nm is calculated. This means an increase of a factor 1.8 compared with the theoretical value. It is generally believed that higher generation dendrimer are spherical. Our results show in acidic water the dendrimers have a more elliptical conformation. 

The use of this simple technique has shed light on the mechanisms by which Azone and oleic acid may exert their well-documented enhancing effects. The study of mixed monolayers of dipalmitoylphosphatidylcholine (DPPC) and Azone or oleic acid (Lewis and Hadgraft, 1990) has revealed, in some detail, possible differences in the way in which these compounds may act as enhancers. Monolayer experiments have also been used to examine the behavior of a complex mixture of lipids (representing those in the stratum comeum) and the way in which these are affected by the presence of Azone (Schuckler and Lee, 1991). Although there are many interesting results in the literature arising from such experiments, it is perhaps these that are the most relevant. Therefore, it is on these results that this section will primarily concentrate. 

Benesch, J. Svedhem, S. Svensson, S. C. T. Valiokas, R. Liedberg, B. Tengvall, P., Protein adsorption to oligo(ethylene glycol) self-assembled monolayers experiments with fibrinogen, heparinized plasma, and serum, J. Biomat. Sci. 2001, 12, 581-597... 

We assume that conditions can be controlled to minimize additional relaxation effects such as magnetic dipole-dipole interactions. As the number of relaxation mechanisms decreases, the information necessary for a line shape analysis of the spectra also decreases. Thus, a poorer signal-to-noise ratio can be tolerated, and the signal can be smoothed by curve fitting techniques. Since little is known at the molecular level about two-dimensional transport coefficients, such as the surface viscosity, large uncertainties can be tolerated. In this sense, we believe that much can be learned from monolayer experiments using spin label surfactants. 

Quantitative information on penetrated layers under dynamic and equilibrium conditions require much attention in respect to the experimental technique. There are a number of penetration experiments with different advantages and drawbacks. The classical experiment is the injection technique where a soluble component is injected into the subphase below a spread monolayer. Experiments can then be performed at constant monolayer coverage [212, 213, 214] or by compression and expansion cycles [215, 216]. Another possibility is to exchange the subphase below a spread monolayer using a laminar pumping system. Other experiments were performed by using the sweeping technique as described in [217, 218]. 

As mentioned above, lipase-catalyzed synthesis of triglycerides from glycerol and fatty acids can be performed in fair yield in monolayer experiments [93,133]. Another air/water inter facial system, foam, also gave di- and triglycerides in higher yields than were obtained in microemulsions [134]. Reactions in foams may be of more practical interest than reactions in monolayers due to the much larger interfacial area in the former type of system. 

Partly soluble triblock copolymers are also sometimes used for monolayer studies. Such investigations could provide data on desorption kinetics, and allow for comparison of the film structure, whether spread or adsorbed. However, attention should be paid to data interpretation in such cases because intricate equilibriums take place in such systems. A somewhat confusing study has been presented concerning the monolayer miscibility between PLA and PEO-PPO-PEO (also known as Pluronic) in monolayers [53]. The authors attempted to discuss interactions between the triblock copolymer and a homopolymer (PLA) on the basis of Langmuir monolayer experiments however, the results show unrealistic values for molecular areas, and therefore conclusions from those measurements cannot be quantitative. In particular, surface pressure-area isotherms for pure polymers and their mixtures reveal, in the compressed state, areas per monomer unit of the order of 3 h and below. Such low values cannot be real and most probably result either from material dissolution in the subphase or poor spreading at the air-water interface. Indeed, the isotherms do not appear smooth, which suggests low film stability and difficulties in forming a true monolayer. 

In DPPC or DPPC/PG monolayer experiments changes in surface pressure (penetration kinetics) and area/phospholipid molecule values (compression isotherms) indicated similar differences in expansion of membranes. It can be concluded that the effect of polymeric polypeptides on phospholipid monolayers depends not only on the polymer charge (positive/negative, neutral), but also on charge density. 

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