Surface Forces in Liquids

It is important to consider the molecular interactions in liquids that are responsible for their physicochemical properties (such as boiling point, melting point, heat of vaporization, surface tension, etc.), which enables one to both describe and relate the different properties of matter in a more clear manner (both qualitatively and quantitatively). These ideas form the basis for quantitative structure activity relationship (QSAR Birdi, 2002). This approach toward analysis and application is becoming more common due to the enormous help available from computers. 

The different kinds of forces acting between any two molecules are dependent mainly on the distance between the two molecules. The difference in distance between molecules in liquid or gas can be estimated as follows. In the case of water, the following data are known  

It is the cohesive forces that maintain water, for example, in the liquid state at room temperature and pressure. This becomes obvious when one compares two different molecules, such as H20 and H2S. At room temperature and pressure, H20 is 

Change of density of a fluid (water) near the surface. 

FIGURE 2.2 Surface film of a liquid (see text for details). 

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CAPILLARITY AND SURFACE FORCES (IN LIQUIDS) (CURVED SURFACES)... 

Since the advent of sensitive techniques for the measurement of surface forces in liquids in the 1970s, it has been apparent that a liquid medium in close proximity to a solid surface is not a structureless continuum as found in bulk, but rather, the discrete molecular nature of the solid and the liquid at the interface leads... 

Surface forces become important for microchaimel flow. The nature of surface forces in liquids is different from that of gases. Therefore, the boundary conditions for liquids are different from those of gases. 

The deformation of soft surfaces can be minimized with SFM by selecting cantilevers having a low force constant or by operating in an aqueous environment. The latter eliminates the viscous force that arises from the thin film of water that coats most surfaces in ambient environments. This viscous force is a large contributor to the total force on the tip. Its elimination means that the operating force in liquid can be reduced to the order of 10 N. 

In this theory the adsorbed layers are considered to be contained in an adsorption space above the adsorbent surface. The space is composed of equipotential contours, the separation of the contours corresponding to a certain adsorbed volume, as shown in Figure 17.7. The theory was postulated in 1914 by Polanyi(18), who regarded the potential of a point in adsorption space as a measure of the work carried out by surface forces in bringing one mole of adsorbate to that point from infinity, or a point at such a distance from the surface that those forces exert no attraction. The work carried out depends on the phases involved. Polanyi considered three possibilities (a) that the temperature of the system was well below the critical temperature of the adsorbate and the adsorbed phase could be regarded as liquid, (b) that the temperature was just below the critical temperature and the adsorbed phase was a mixture of vapour and liquid, (c) that the temperature was above the critical temperature and the adsorbed phase was a gas. Only the first possibility, the simplest and most common, is considered here. 

The surface molecules are under a different force field from the molecules in the bulk phase or the gas phase. These forces are called surface forces. A liquid surface behaves like a stretched elastic membrane in that it tends to contract. This action arises from the observation that, when one empties a beaker with a liquid, the liquid breaks up into spherical drops. This phenomenon indicates that drops are being created under some forces that must be present at the surface of the newly formed interface. These surface forces become even more important when a liquid is in contact with a solid (such as ground-water oil reservoir). The flow of liquid (e.g., water or oil) through small pores underground is mainly governed by capillary forces. Capillary forces are found to play a very dominant role in many systems, which will be described later. Thus, the interaction between liquid and any solid will form curved surface that, being different from a planar fluid surface, initiates the capillary forces. 

It will be shown here that, due to the presence of surface tension in liquids, a pressure difference exists across the curved interfaces of liquids (such as drops or bubbles). This capillary force will be analyzed later. 

The available correlations in the literature are not able to represent the experimental results derived from 1500 high pressure data on the liquid hold-up. To correlate all the data, the effects of fluid inertia, surface forces, and liquid shear stress have again been accounted for, by using the corresponding dimensionless groups in the following empirical correlation [37] ... 

For systems which spread spontaneously it is well-known that a spreading drop forms a thin (< 0.1 pm) primary or precursor film [279-282], Its thickness and extension are determined by surface forces. In the precursor film, energy is dissipated by viscous friction. The liquid transport in the precursor film is driven by the disjoining pressure in the precursor film which sucks liquid from the wedge of the drop. 

Disorienting effect of the molecular motions. The molecules in liquids are in constant violent translatory and rotatory motion. It is true that there are nearly always forces at the surface, tending to orient molecules whose chemical constitution renders their fields of force un-symmetrical, in some definite position on the surface but in liquids this orientation is rarely perfect, as the molecular motions mix up the molecules at a rate comparable with their rate of orientation. In discussions of the orientation of molecules at the surfaces of liquids misunderstandings have sometimes arisen, because the problem was treated as if the molecules were stationary and the orientation fixed and definite.3... 

The surface tension forces act all around the capillary tube in the upward direction and liquid rises in the tube. This rise continues till the upward lifting force becomes equal to the weight of the liquid in the capillary tube. If the cohesive forces in liquid are greater than the solid-liquid and those of the solid surface, the liquid detaches from the surface of the solid and a fall of liquid in the tube is observed. 

In conclusion it may be noted that both experimental and theoretical results, obtained so far, point out that the linear energy of the contact line free liquid film/bulk liquid phase can be either negative or positive. The sign of the K value is determined by the interaction forces acting in the film and in the transition region film/bulk. Thus, the sign and the value of the linear energy susceptibly reflect all interactions due to surface forces in the system. 

In a liquid binary solution, this accumulation is accompanied by the corresponding displacement of another component (solvent) from the surface region into the bulk solution. At equilibrium a certain amount of the solute will be accumulated on the surface in excess of its equilibrium concentration in the bulk solution, as shown in Figure 2-6. Excess adsorption E of a component in binary mixture is defined from a comparison of two static systems with the same liquid volume Vo and adsorbent surface area S. In the first system the adsorbent surface considered to be inert (does not exert any surface forces in the solution) and the total amount of analyte (component 2) will be no = VoCo. In the second system the adsorbent surface is active and component 2 is preferentially adsorbed thus its amount in the bulk solution is decreased. The analyte equilibrium concentration Ce can only be measured in the bulk solution, so the amount VoCe is thereby smaller than the original quantity no due to its accumulation on the surface, but it also includes the portion of the analyte in the close proximity of the surface (the portion U Ce, as shown in Figure 2-6 note that we did not define V yet and we do not need to define... 

Surface tension is caused by the attractive forces in liquids. All the molecules attract each other those in the center are attracted equally, in all directions, but those at the surface are drawn toward the center because there are no liquid molecules in the other direction to pull them outward (see Fig 1.9). The effort of each molecule to get into the center causes the fluid to try to take a shape that would have the greatest number of molecules nearest the center, a sphere (Prob 1.9). Any other shape has more surface per unit volume therefore, regardless of the shape of a fluid, the attractive forces tend to pull the fluid into a sphere. The fluid thus tries to minimize its surface area. ... 

Since the first measurements of the electrostatic double-layer force with the AFM not even 10 years ago, the instrument has become a versatile tool to measure surface forces in aqueous electrolyte. Force measurements with the AFM confirmed that with continuum theory based on the Poisson-Boltzmann equation and appKed by Debye, Hiickel, Gouy, and Chapman, the electrostatic double layer can be adequately described for distances larger than 1 to 5 nm. It is valid for all materials investigated so far without exception. It also holds for deformable interfaces such as the air-water interface and the interface between two immiscible liquids. Even the behavior at high surface potentials seems to be described by continuum theory, although some questions still have to be clarified. For close distances, often the hydration force between hydrophilic surfaces influences the interaction. Between hydrophobic surfaces with contact angles above 80°, often the hydrophobic attraction dominates the total force. 

The short-range (van der Waals) forces, described in Section 8.0, exert an attraction between all molecules which are in contact with one another. The presence of these forces in liquids becomes particularly evident at surfaces. Molecules in the bulk of the liquid are subject to these forces equally and in all directions. However, the molecules located at an air-water interface experience... 

First observations of forces in liquid crystal were performed in the 80 s by Horn et al. [59]. They studied the force between two mica plates separated by a liquid crystal 5CB using the surface force apparatus. Later on, the forces due to the structure were briefly discussed by Poniewierski and Sluckin [57] and a detailed theoretical study of forces in paranematic systems was performed by Borstnik and Zumer [43]. In the meantime, much attention was paid to the fluctuation-induced forces [11,12,60,61]. Theoretical studies were followed by renewed experimental interest, studies being performed with surface forces apparatus [46] and atomic force microscopes [62]. 

Whereas the first experiments on structmral forces in liquid crystals were performed using a Surface Force Apparatus (SFA, discussed later on in part 3 of this chapter), a temperature controlled AFM was used later [4] to measure the forces between the AFM probe and the surface, mediated by a liquid crystal in between. Compared to other methods, using an AFM for measuring surface forces has some advantages and also some drawbacks. The most important advantage is, that the temperature of the sample can be controlled in a simple way to better than 0.01 K. Furthermore, the sample is not in direct contact with large mechanical parts like in the case of a SFA, and only a small amount of liquid crystal is needed for the experiment. The drawback of the AFM is, that the separation between surfaces is not measured directly... 

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