Thin films on surfaces of liquids

The adsorption of amphiphilic molecules at the surface of a liquid can be so strong that a compact monomolecular film, abbreviated as monolayer, is formed. There are amphiphiles which, practically, do not dissolve in the liquid. This leads to insoluble monolayers. In this case the surface excess T is equal to the added amount of material divided by the surface area. Examples of monolayer forming amphiphiles are fatty acids (CH3(CH2) c 2COOH) and long chain alcohols (CH3(CH2)nc iOH) (see section 12.1). 

Within each class the phospholipids can have alkyl chains of different length (Table 13.1) and different numbers of double bonds. Alkyl chains which only consist of single bonds are called saturated . Alkyl chains which contain at least one double bond are called unsaturated . Unsaturated alkyl chains are more flexible and have lower melting temperatures. 

14 Dimyristoyl- DMPC / DMPE amine (PE) with alkyl chains of similar 

If there is still a significant proportion of the amphiphile dissolved in the liquid we talk about Gibbs monolayers. Solubility in water is increased by using molecules with short alkyl chain or a high polarity of the headgroup. In this case T is determined from the reduction of the surface tension according to the Gibbs adsorption isotherm (Eq. 3.52). 

The most important tool for studying insoluble monolayers is a film balance [575,576], also called a Langmuir trough (Fig. 13.2). The modern version of a film balance consists of a temperature-controlled trough which contains the liquid. The liquid is called subphase . Usually water is used as the subphase. The lipids are dissolved in a solvent (often chloroform) which is volatile and not miscible with the subphase. Drops of the solution are placed on the liquid surface and after evaporation of the solvent a lipid film remains. This process is called spreading . Via movable barriers the film balance allows to manipulate the density of molecules on the liquid surface by compression or expansion of the film. When compressing the film the area per molecule decreases, when expanding the film, it increases. 

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Ellipsometry is a branch of specular reflection spectroscopy which is particularly useful for characterising thin films on surfaces in situ, e.g. in the presence of supernatant liquids. It is applicable to many problems in corrosion and passivation, lubrication, the physics and chemistry of new materials, and biological chemistry. The principles behind it have been known for a long time, but it remains a rather under-used technique. This situation is now changing because of a number of factors, such as the advent of cheap computing power, and the impetus, brought about by the great economic importance of the semiconductor industry, to produce more powerful theories to interpret the results. 

Most nonpolar substances have very small water solubilities. Petroleum, a mixture of hydrocarbons, spreads out in a thin film on the surface of a body of water rather than dissolving. The mole fraction of pentane, CsH12, in a saturated water solution is only 0.0001. These low solubilities are readily understood in terms of the structure of liquid water, which you will recall (Chapter 9) is strongly hydrogen-bonded. Dissimilar intermolecular forces between C5H12 (dispersion) and H2O (H bonds) lead to low solubility. 

A different aspect of preparation of an organized nanoparticles on a fluid is called as the rheotaxy technique. It is a well-established one to fabricate a well-crystallized film. Mobility of the atoms on the surface of the liquid substrate favors the aggregation of atoms in the growing films (41). In order to avoid a drawback of the rheotaxy, i.e., the negative effect of high surface tension of the substrate, a modification has been made by Romeo et al. (42,43), where they used substrates of elevated temperature close to but below their melting points. They prepared, e.g., ZnS Mn thin films on some low-melting metals such as Pb, Bi or Bi-Sb alloy. 

Equation (6.25) not only allows us to calculate the Hamaker constant, it also allows us to easily predict whether we can expect attraction or repulsion. An attractive van der Waals force corresponds to a positive sign of the Hamaker constant, repulsion corresponds to a negative Hamaker constant. Van der Waals forces between similar materials are always attractive. This can easily be deduced from the last equation for 1 = e2 and n = n2 the Hamaker constant is positive, which corresponds to an attractive force. If two different media interact across vacuum ( 3 = n3 = 1), or practically a gas, the van der Waals force is also attractive. Van der Waals forces between different materials across a condensed phase can be repulsive. Repulsive van der Waals forces occur, when medium 3 is more strongly attracted to medium 1 than medium 2. Repulsive forces were, for instance, measured for the interaction of silicon nitride with silicon oxide in diiodomethane [121]. Repulsive van der Waals forces can also occur across thin films on solid surfaces. In the case of thin liquid films on solid surfaces there is often a repulsive van der Waals force between the solid-liquid and the liquid-gas interface [122],... 

The mechanical characteristics of thin films on liquids are described in a similar way to the three-dimensional case. The surface elasticity [618] is defined as... 

Poly(bithiophene) films from these two ionic liquids are morphologically similar (Figure 7.14), even though the redox behavior (Figure 7.9) is markedly different, suggesting that the dominant differences in the films produced are on an atomic or sub-micron rather than macroscopic level. The morphology ofthe poly (bithiophene) films appears to be similar to that described by Roncali et al. [74] who reported a thin film on the surface of the electrode, covered by a thick brittle powdery deposit, from the galvanostatic polymerization of bithiophene in acetonitrile. The nodular structures are smaller in the poly (bithiophene) films than in the poly (thiophene), which is consistent with the formation of shorter chain polymers [73], but this does not... 

The vapor pressure of a liquid is affected by the curvature of the surface to which it adheres. Liquid on the convex surface of a particle with extremely small radius of curvature exerts a vapor pressure greater than that on a plane surface. Conversely, the vapor pressure due to a thin film on a concave surface is less than that for a film on... 

CFD [Chemical Fluid Deposition] A process for depositing thin films on solid surfaces by a chemical reaction in a liquid such as supercritical carbon dioxide. Superior to CVD in being capable of operation at almost ambient temperatures. Demonstrated for depositing platinum metal on silicon wafers, polymer substrates, and porous solids by hydrogen reduction of an organo-platinum compound at 80°C. 

In the initial state the film surface is planar until a contact with a solid particle occurs. At the moment of rupture when the menisci tips come into contact, the film thickness becomes h = 2h. The rate of mutual approach of the menisci depends on the volume rate of film thinning and on the excess liquid volume (V + Vy. 

Open tubular columns are simply capillary tubes in which the inside of the column wall is used as the support for the liquid phase. These wall-coated open tubular columns (WCOT) have the stationary phase distributed in the form of a thin film on the inside surface of the open capillary tube, the walls thus serving as the support. In order to reduce the thickness of the liquid phase film, a porous layer may be formed on the inside wall of the capillary tubing and then coated with the liquid phase to produce a support-coated open tubular column (SCOT). Porous-layer open tubular colunms (PLOT) are similar to SCOT colunms, the difference being that in the former, the stationary phase is deposited on fine crystalline particles or glass powder which is adsorbed onto the walls of the tube. In both cases, the available surface area of the wall is increased, and allows an increased amount of liquid phase to be accommodated in the same length and diameter of tubing. The whisker-walled (WW) colunm consists of whiskers chemically etched on the surface of the wall, which also result in a significant increase in the available surface area. Wall-coated, porous-layer, and support-coated capillary columns are all available as whisker-walled, i.e., WWCOT, WWPLOT, and WWSCOT, respectively. 

As revealed distinctively by experiments on the photoinduced regulation of in-plane alignment by silica plates whose surfaces were modified with azobenzenes,148-151 the azimuthal reorientation of liquid crystals was achieved by the uniaxial conformational change of command molecules localized at the topmost substrate surface. This situation was confirmed for polymer films by the results obtained with the following systems. The first consisted of the chemical modification of PVA thin films. Careful treatment of PVA films with azobenzene acid... 

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