Monolayers liquid droplets

As the analytical, synthetic, and physical characterization techniques of the chemical sciences have advanced, the scale of material control moves to smaller sizes. Nanoscience is the examination of objects—particles, liquid droplets, crystals, fibers—with sizes that are larger than molecules but smaller than structures commonly prepared by photolithographic microfabrication. The definition of nanomaterials is neither sharp nor easy, nor need it be. Single molecules can be considered components of nanosystems (and are considered as such in fields such as molecular electronics and molecular motors). So can objects that have dimensions of >100 nm, even though such objects can be fabricated—albeit with substantial technical difficulty—by photolithography. We will define (somewhat arbitrarily) nanoscience as the study of the preparation, characterization, and use of substances having dimensions in the range of 1 to 100 nm. Many types of chemical systems, such as self-assembled monolayers (with only one dimension small) or carbon nanotubes (buckytubes) (with two dimensions small), are considered nanosystems. 

SA monolayers were prepared by immersing the substrates in the n-alkanoic acid/HD solutions for a predetermined period of time. After removal from the solutions, any remaining liquid droplets on the surface or the edges of the substrates were blown off with a nitrogen jet. For the equilibrium monolayer characterization, relatively high solution concentration (>10 3M) were used. For time-dependent kinetic studies, stearic acid (C]g) solutions of concentrations between 10 2 and 10 6M were used. Immersion times from a few seconds up to 24 hours were used. 

The formation of self-assembled monolayers is a powerful tool for surface modification, and it is useful when we need to control surface hydrophilic-ity or prepare fimctional electrodes, for example. Surface modification with belts composed of monolayers of various hydrophihcities can yield surfaces with hydrophilicity gradients. liquid droplets can move across such surfaces against gravity due to favorable interactions with the monolayer surface (Fig. 4.38). 

Semal, S., C. Bauthier, M. Voue, J.J.V. Eynde, R. Gouttebaron, and J.D. Coninc (2000). Spontaneous spreading of liquid droplets on mixed alkanethiol monolayers Dynamics of wetting and wetting transition. J. Phys. Chem. B 104, 6225. 

Figure 9.5 Temperature dependence of the surface tension of hexacosane [32], exhibiting prefreezing of the surface monolayer at Tj. T° indicates the bulk freezing point. Temperatures in the interval between 7 and correspond to the solid monolayer on the top of bulk liquid phase. If, for example, the freezing of liquid droplet occurs at some temperature T", then the value of T" should be substituted in Equation 9.37 as ysv...

Figure 8 shows H2SO4 droplets on mica at very low humidity (<5%). The contact line aronnd the drops is smooth and circnlar, revealing that no pinning has occurred [49]. Although the area between drops is flat, it does not correspond to clean mica but to a liquid film covering it of a few monolayers thickness. This is deduced from the hysteresis in the force versns distance experiments, where the tip is brought into contact with the surface and then pulled off. 

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

When a biopolymer mixture is either close to phase separation or lies in the composition space of liquid-liquid coexistence (see Figure 7.6a), the effect of thermodynamically unfavourable interactions is to induce biopolymer multilayer formation at the oil-water interface, as observed for the case of legumin + dextran (Dickinson and Semenova, 1992 Tsapkina et al, 1992). Figure 7.6b shows that there are three concentration regions describing the protein adsorption onto the emulsion droplets. The first one (Cprotein< 0.6 wt%) corresponds to incomplete saturation of the protein adsorption layer. The second concentration region (0.6 wt% < 6 proiem < 6 wt%) represents protein monolayer adsorption (T 2 mg m 2). And the third region (Cprotein > 6 wt%) relates to formation of adsorbed protein multilayers on the emulsion droplets. 

Microemulsions are stable, clear suspensions of two immiscible liquids and a surfactant. The surfactant forms a monolayer with its hydrophilic head dissolved in the water and its hydrophobic tail in the oil. The ratio between the three and the addition of salts, other liquids, or co-surfactants allow for fine tuning of the size of the droplets, which typically range from 5 to 100 nm. 

An explanation for this gel formation is sought in the phase transition behavior of span 60. At the elevated temperature (60 °C) which exceeds the span 60 membrane phase transition temperature (50 °C) [154], it is assumed that span 60 surfactant molecules are self-assembled to form a liquid crystal phase. The liquid crystal phase stabilizes the water droplets within the oil. However, below the phase transition temperature the gel phase persists and it is likely that the monolayer stabilizing the water collapses and span 60 precipitates within the oil. The span 60 precipitate thus immobilizes the liquid oil to form a gel. Water channels are subsequently formed when the w/o droplets collapse. This explanation is plausible as the aqueous volume marker CF was identified within these elongated water channels and non-spherical aqueous droplets were formed within the gel [153]. These v/w/o systems have been further evaluated as immunological adjuvants. 

A third aspect is the close analogy with wetting phenomena. One Important issue is whether or not a thin liquid film on a surface is stable or whether it spontaneously (that is, even without a second wall nearby) disproportionates into a droplet in contact with a very thin (usually (sub)-monolayer) film. In wetting language this disproportionation (usually in the other direction, as... 

Fig. 1. General oil-droplet model of lipoproteins is presented for chylomicron, very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) structures. Apolipoproteins in the outer phospholipid membrane, designated by letters, are defined in Table II. The major differences between the lipoproteins are the size of the neutral lipid (triglyceride and esterified cholesterol) core, liquid composition in the core, and apolipoprotein composition. (E) Triglycerides, ( Q ) phospholipids, and ( -) esterified cholesterol are shown. Although not shown, unesterified cholesterol is found predominantly in the phospholipid monolayer.

The conclusions drawn on the basis of the dielectric loss analysis of liquid samples, support the interpretation that a very gradual confluence of the different types of dispersions takes place.Such an interpretation could explain the instauration of polydispersed samples in terms of the coexistence, at equilibrium, first, of micellar aggregates with w/o microemulsion droplets and, successively, of a microemulsion with l-I O-per hydrophilic group monolayer, in equilibrium with a hydrated type of microemulsion (U-water molecule per polar head of the surfactant hydrophilic groups monolayer). The latter interpretation is in accordance with Steinbach and Sucker findings that the two types of structures ( 1-HpO and U-HgO molecule), may coexist at equilibrium (23.). 

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