Monolayers static properties

In order to set the stage for this review of the polymer dynamics on monolayers at interfaces with emphasis on A/W, we need to lay out its static properties first. Surface tension a represents a fundamental property of a liquid surface. The change in Gibbs free energy dG for a multi-component system including the surface contribution is written as... 

As in the earlier section, we briefly present the static properties first. The results obtained with pH 2 water (with 10 mM HC1) as the subphase are displayed in Fig. 22, where TI-A and es - A isotherms for all four polymer monolayers are plotted together in each. With the exception of PHcMA, the TI-A isotherms of the polymer monolayers are nearly identical and they are consistent with the expected isotherms of condensed films. Each isotherm is shown in two regions the dilute regime where 77 < 1 mN m 1 (open sym-... 

There is a vast body of diblock copolymer studies since block choice can be such that they resemble amphiphilic surfactants. For the sake of brevity, we will skip them. Instead, we present an interesting case of triblock copolymers of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, commonly known by one of its trade names, Pluronics [117]. They have been used as non-ionic surfactants for a variety of applications such as in emulsification and dispersion stabilization. In aqueous solutions, these copolymers form micelles, and there exists a well-defined critical micelle concentration that is experimentally accessible. Several groups have investigated colloidal suspensions of these polymers [118-122], The surface properties of the adsorbed monolayers of the copolymers have been reported with respect to their structures and static properties [123-126]. 

In the recent simulation by Matties and Hentschke [36, 37], the adsorption and melting of benzene on graphite was studied via MD simulations. In addition to determining static properties such as the center of mass density distributions and tilt angles as a function of temperature by obtaining time averages, they were also able to obtain dynamic properties such as the surface diffusion constants in the monolayer and the orientational velocity autocorrelation function (OVAF). 

Comments. Dynamic properties show changes at hydration levels above 0.4 h, the point of completion of the changes in static properties. Because the later reflect a monolayer of water about the protein, the additional water seen in the dynamic measurements is "multi-layer" water. Furthermore, hydration affects the several rate properties differently. The more complex hydration dependence of dynamic compared with static properties is to be expected. Static properties, at least the thermodynamic, have a single molecular basis. In contrast, the various transition states governing the rate processes are necessarily different. 

With today s technology, the definition of the surface as it effects a material s performance in many cases means the outer one or two monolayers. It is the specific chemistry of these immediate surface molecules that determines many of the chemical and physical properties. Therefore, it is important to have available a tool that is able to characterize the chemistry of these layers. One such method that has met with considerable success is Static Secondary Ion Mass Spectrometry (SIMS). 

The difference between the static or equilibrium and dynamic surface tension is often observed in the compression/expansion hysteresis present in most monolayer Yl/A isotherms (Fig. 8). In such cases, the compression isotherm is not coincident with the expansion one. For an insoluble monolayer, hysteresis may result from very rapid compression, collapse of the film to a surfactant bulk phase during compression, or compression of the film through a first or second order monolayer phase transition. In addition, any combination of these effects may be responsible for the observed hysteresis. Perhaps understandably, there has been no firm quantitative model for time-dependent relaxation effects in monolayers. However, if the basic monolayer properties such as ESP, stability limit, and composition are known, a qualitative description of the dynamic surface tension, or hysteresis, may be obtained. 

It is very well known that the nature of the monolayer partially depends on the strength of interfacial interactions with substrate molecules and that of polymer in-tersegmental interactions. And it is normal to expect that the viscoelastic properties of polymer monolayer are also dependent on these factors. The static and dynamic properties of several different polymer monolayers at the air - water interface have been examined with the surface quasi-elastic Light Scattering technique combined with the static Wilhelmy plate method [101]. 

Other recent applications of AFM-SECM included the study of the iontophoretic transport of [Fe(CN)6]4 across a synthetic track-etched polyethylene terephthalate membrane by Gardner et al. [193]. They made the structure and flux measurements at the single pore level and found that only a fraction of candidate pore sites are active in transport. Demaille et al. used AFM-SECM technique in aqueous solutions to determine both the static and dynamical properties of nanometer-thick monolayers of poly(ethylene glycol) (PEG) chains end-grafted to a gold substrate surface [180]. 

Up to this point, the information about the monolayers has looked at the structural properties as if the lipopolymer monolayers were static and fixed above a body of water. However, a truly remarkable aspect of these monolayers is their fascinating fluidity and visoelastic properties, and the range of distinct fluid and viscoelastic behavior they exhibit under different conditions and with different lipopolymers. These properties can be studied by analyzing the viscosity and elasticity of the monolayer, as discussed in Sect. 2.2, as well as by investigating the lateral diffusion of individual lipopolymers within the monolayer, as discussed in Sect. 2.3. 

Tnteractions at surfaces have long been at the center of interest in the study of surfactant monolayers and have been thought to influence both static and dynamic surface properties considerably (1,2). Although the theoretical interpretation and even the definition of surface interactions may be controversial, the experimental method has not been in doubt. Invariably, the equilibrium surface pressure vs. molar area relationship has been used as a criterion for assessing interactions in mono-layers since interactions, no matter what their precise definition, must appear in the measurable quantity of surface tension (y) or surface pressure (7r = y° — y) at a given surface concentration (r) or molar... 

As an example of a membrane model, phospholipid monolayers with negative charge of different density were used. It had already been found ( ) and discussed O) that the physical and biological behavior of phospholipid monolayers at air-water interfaces and of suspensions of liposomes are comparable if the monolayer is in a condensed state. Two complementary methods of surface measurements (using radioactivity and electrochemical measurements), were used to investigate the adsorption and the dynamic properties of the adsorbed prothrombin on the phospholipid monolayers. Two different interfaces, air-water and mercury-water, were examined. In this review, the behavior of prothrombin at these interfaces, in the presence of phospholipid monolayers, is presented as compared with its behavior in the absence of phospholipids. An excess of lipid of different compositions of phos-phatidylserine (PS) and phosphatidylcholine (PC) was spread over an aqueous phase so as to form a condensed monolayer, then the proteins were inject underneath the monolayer in the presence or in the absence of Ca. The adsorption occurs in situ and under static conditions. The excess of lipid ensured a fully compressed monolayer in equilibrium with the collapsed excess lipid layers. The contribution of this excess of lipid to protein adsorption was negligible and there was no effect at all on the electrode measurements. 

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