Adsorption at liquid surfaces

Adsorption at liquid surfaces can be monitored using the Gibbs adsorption isotherm since the surface energy, y, of a solution can be readily measured. However, for solid substrates, this is not the case, and the adsorption density has to be measured in some other manner. In the present case, the concentration of adsorbate in solution will be monitored. In place of the Gibbs equation, we can use a simple adsorption model based on the mass action approach. 

Adsorption experiments are conducted at constant temperature, and an empirical or theoretical representation of the amount adsorbed as a function of the equilibrium gas pressure is called an adsorption isotherm. Adsorption isotherms are studied for a variety of reasons, some of which focus on the adsorbate while others are more concerned with the solid adsorbent. In Chapter 7 we saw that adsorbed molecules can be described as existing in an assortment of two-dimensional states. Although the discussion in that chapter was concerned with adsorption at liquid surfaces, there is no reason to doubt that similar two-dimensional states describe adsorption at solid surfaces also. Adsorption also provides some information about solid surfaces. The total area accessible to adsorption for a unit mass of solid —the specific area Asp — is the most widely encountered result determined from adsorption studies. The energy of adsorbate-adsorbent interaction is also of considerable interest, as we see below. 

These insoluble monomolecular films, or monolayers, represent an extreme case in adsorption at liquid surfaces, as all the molecules in question are concentrated in one molecular layer at the interface. In this respect they lend themselves to direct study. In contrast to monolayers which are formed by adsorption from solution, the surface concentrations of insoluble films are known directly from the amount of material spread and the area of the surface, recourse to the Gibbs equation being unnecessary. 

Insoluble polar molecules (e.g., long chain fatty acids) exhibit an extreme kind of adsorption at liquid surfaces. That is, they can be made to concentrate in one molecular layer at the surface. These interfacial films often provide the stabilizing influence in emulsions since they can both lower interfacial tension and increase the interfacial viscosity. The latter provides a mechanical resistance to coalescence. Such systems also lend themselves to the study of size, shape, and orientation of molecules at an interface. Having an adsorbed layer lowers the surface tension (to Ysolution) by the surface pressure jt= ysoivent - y solution as already noted. 

Alloy films are commonly sintered during preparation by deposition on substrates heated to, say, 400°C or by subsequent annealing at such temperatures, and, consequently, rather small surface areas have to be measured, perhaps in vessels of substantial volume. Krypton adsorption at liquid nitrogen temperature was used with induction-evaporated Cu-Ni, Fe-Ni, and Pd-Ni films, and BET surface areas of 1000-2000 cm2 were recorded (48), after correction for bare glass. The total area of Cu-Ni films was measured by the physical adsorption of xenon at — 196°C (70) in addition, the chemisorption of hydrogen on the same samples enabled the quantity a to be determined where... 

In a large variety of applications, the surface of a solid plays an important role (e.g., active charcoal, talc, cement, sand, catalysis). Solids are rigid structures and resist any stress effects. Many such considerations in the case of solid surfaces will be somewhat different for liquids. The surface chemistry of solids is extensively described in the literature (Adamson and Gast, 1997 Birdi, 2002). Mirror-polished surfaces are widely applied with metals, where the adsorption at the surface is much more important. Further, the corrosion of metals initiates at the surfaces, thus requiring treatments based on surface properties. As described in the case of liquid surfaces, analogous analyses of solid surfaces can be carried out. The molecules at the solid surfaces are not under the same force field as in the bulk phase (Figure 5.1). 

Adamson, A. W., Physical Chemistry of Surfaces, 5th ed., Wiley, New York, 1990. (Graduate level. A more extended and somewhat more advanced treatment of adsorption at liquid interfaces in Chapter 4 and adsorption at solid-liquid interfaces in Chapter 11.)... 

The interfacial tension decreases with increasing amount of surface potential. The reason is the increased interfacial excess of counterions in the electric double layer. In accordance with the Gibbs adsorption isotherms, the interfacial tension must decrease with increasing interfacial excess. At charged interfaces ions have an effect similarly to surfactants at liquid surfaces. 

In the present paper, the previous analysis for moving suspensions is extended to allow reversible deposition by including the short-range Rom repulsion. The apparent surface reaction is then shown to be reversible and first order- in both directions and thus is analogous to gas adsorption at low surface coverage. Applications include solid-liquid separations, particle separation, and chromatography. 

While there have been many SHG studies at the solid electrode/liquid interface as both an in situ probe of the electrode interface and the influence of adsorption at the surface of the electrode, there have been far fewer studies of electrochenucal processes occurring at the boundary between two immiscible electrolyte solutions. 

Table 1.5. Extension of some quasi-static techniques to obtain time-dependent surface and interfacial tensions for (surfactant) solutions. ) Modified after S.S. Dukhin, G. Kretzschmar and R. Miller, Dynamics of Adsorption at Liquid Interfaces (Elsevier, 1995), 142.

Reaction at liquid surfaces covers a wide field. However, special experimental methods have been developed mainly for the study of insoluble or partly soluble monolayers at the liquid-air interface. Adsorption and reaction of monolayers at the liquid-air interface and liquid-liquid interfaces can be studied by similar techniques. It is therefore convenient to treat these together. Comprehensive information about liquid interfaces is contained in publications by Adamson, Davies , Alexander , Davies and Rideal and Gaines . ... 

Bashforth F and Adams JC (1883) An attempt to test the capillary action, Cambridge University Press and Deighton Bell Co., Cambridge Chen P, Kwork DY, Prokop RM, del-Rio 01, Susnar SS and Neumann AW (1998) Axisymmetric drop shape analysis (ADSA) and its applications , in Drops and bubbles in interfacial research, D. Moebius and R. Miller Eds., Studies in Interface Science Series, Vol. 6, Elsevier, Amsterdam Dukhin SS, Kretzschmar G and R. Miller R (1995) Dynamic of adsorption at liquid interfaces. Theory, experiments, applications, D. Moebius and R. Miller Eds., Studies in Interface Science Series, Vol. 1, Elsevier, Amsterdam Joos P (1999) Dynamic Surface Phenomena, VSP, Utrecht, 1999 Kovalchuk VI, Zholkovskij EK, Kragel J, Miller R, Fainerman VB, Wiistneck R, Loglio G and Dukhin SS (2000) Bubble Oscillations in a Closed Cell. J Colloid Interface Sci 224 245-254... 

The adsorption at liquid-vapor and liquid-liquid interfaces is generally physical in nature and the adsorbed molecules may easily be desorbed from the surface by lowering the bulk concentration of the adsorbate. For example, when we dilute the aqueous solution of ethanol with water, the adsorbed ethanol molecules will desorb, and the surface tension of the solution rises. On the other hand, chemisorbed molecules are much more difficult to desorb. 

XRD patterns were recorded on a Phillips PW 1051 diffractometer using Cu Ka radiation. SEM photographs were obtained on a Hitachi microscope. BET surface areas of the catalysts were determined using nitrogen adsorption at liquid nitrogen temperature in a... 

The BET surface area was measured by nitrogen adsorption at liquid nitrogen temperature. Prior to BET measurements, samples were outgassed at 350 C for 2 hr. XRD patterns were obtained on Rigaku diffractometer using CuKa radiation. 

This neglects the adsorption at the surface of the bubbles in the liquid pool since the holdup at the surface is generally small compared to the material in the pool. Combining equations 34-4 and 34-5 ... 

Dukhin, S.S., Kretzschmar, G., and Miller, R., Dynamics of Adsorption at Liquid Interfaces, in Studies in Surface Science , vol. 1, D. Mobius and R. Miller (Editors), Elsevier, Amsterdam, 1995... 

There are many other experimental method for studying the dynamics of adsorption at liquid interfaces. First of all, many other techniques exist to measure dynamic surface and interfacial tensions. Only a subjective selection of some more experimental developments are given in the following section. Moreover, other than surface and interfacial techniques are discussed in this chapter too, such as radiotracer, ellipsometer, electric potential, and spectroscopic methods. 

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