Adsorption at Solid Interfaces

Adsorption of surfactants on to solid surfaces is important in many of their applications, for example in detergency, when a dirt particle is surrounded by adsorbed surfactant molecules. It is also crucial to the solubilization of solid materials, for example latex and pigment particles in paints. 

If a liquid is placed on a solid, it may either spread so as to completely wet the substrate or de-wet to form droplets with a finite contact angle. The contact angle is the angle between the substrate surface and a tangent drawn to the liquid surface at the point of contact with the solid (Fig. 4.13). The equilibrium state results from a balance of three interfacial tensions. The solid/vapour surface tension ysv is balanced by the sum of the solid/liquid interfacial tension and a component of the liquid/vapour surface tension resolved parallel to the substrate, yiy cos 9  

The adsorption of amphiphiles from solution on to a solid surface is described by the Langmuir adsorption equation. It is sometimes known as the Langmuir isotherm, since it refers to adsorption as a function of concentration (or pressure, when dealing with gases) at constant temperature. The Langmuir adsorption equation assumes that the surface is homogeneous, adsorption cannot occur beyond monolayer coverage, and all adsorption sites are equivalent. In addition, the equation applies to dilute solutions where there are no snrfactant-surfactant or surfactant-solvent interactions. 

The Langmuir adsorption eqnation provides an expression for the fractional adsorbed amount (surface coverage), 0, as a function of surfactant concentration, c. It can be derived from the rates of adsorption and desorption, which are equal at equilibrium. The adsorption rate is 

a plot of 1/r versus 1/c provides Fmax and K. This is therefore the most convenient representation of the Langmuir isotherm. From the value of K obtained, it is possible to determine the Gibbs energy of 

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Another peculiarity of adsorption at solid interfaces that deserves special attention is the role of the mosaic structure (non-uniformity) of the surfaces, and primarily the role that various structural defects play in the adsorption and chemisorption phenomena. The latter is of particular importance in the chemisorption of inorganic ions at polar solid interfaces. As will be shown further, these ions are responsible for charging the surface. 

The surface density/solution concentration isotherms, not shown in this paper, reflect also the differences in the behavior of mucin and collagen upon their adsorption at solid interfaces. While the collagen isotherms on polyethylene and surface-grafted polyethylene show a plateau of adsorption at solution concentrations higher than 0.05 mg/ml, no plateau values for mucin adsorption are observed on polyethylene and surface oxidized polyethylene. 

It should be noted the result mentioned earlier holds only for the van der Waals (or Volmer) isotherm. Instead, if the Frumkin (or Langmuir) isotherm is used, the value of a obtained from the surface tension fits is about 33% greater than that obtained from molecular size [44], A possible explanation of this difference could be the fact that the Frumkin (and Langmuir) isotherm is statistically derived for localized adsorption and is more appropriate to describe adsorption at solid interfaces. In contrast, the van der Waals (and Volmer) isotherm is derived for nonlocalized adsorption, and they provide a more adequate theoretical desaiption of the surfactant adsorption at liqnid-flnid interfaces. This conclnsion refers also to the calculation of the surface (Gibbs) elasticity by means of the two types of isotherms [44]. 

J. P. Badiali, L. Blum, M. L. Rosinberg. Localized adsorption at solid-liquid interface the sticky site hard wall model. Chem Phys Lett 729 149-154, 1986. 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

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. 

Avena, M. J. and Koopal, L. K. (1999). Kinetics of humic acid adsorption at solid-water interfaces, Environ. Sci. Technol., 33, 2739-2744. 

Ruzi6, I. (1987), Time Dependence of Adsorption at Solid-Liquid Interfaces", Croat. Chem. Acta 60/3, 457-475. 

H.S. Hanna and P. Somasundaran, "Physico-Chemical Aspects of Adsorption at Solid/Liquid Interfaces, Part II. Berea Sandstond/Mahogony Sulfonate System", in Improved Oil Recovery by Surfactants and Polymer Flooding, D.O. Shah and R.S. Schecter, eds.. Academic Press, 1977, p. 253-274. 

While the bulk behavior of polyampholytes has been investigated for some time now, studies of interfacial performance of polyampholytes are still in their infancy. There are several reasons for the limited amount of experimental work the major one being the rather complex behavior of polyampholytes at interfaces. This complexity stems from a large array of system parameters governing the interaction between the polymer and the substrate. Nearly all interfacial studies on polyampholytes reported to-date involved their adsorption on solid interfaces. For example, Jerome and Stamm and coworkers studied the adsorption of poly(methacryhc acid)-block-poly(dimethyl aminoethyl methacrylate) (PMAA-fc-PDMAEMA) from aqueous solution on sihcon substrates [102,103]. The researchers found that the amount of PMAA-fo-PDMAEMA adsorbed at the solution/substrate interface depended on the solution pH. Specifically, the adsorption increased... 

Hesleitner, P. Babic, D. Kallay, N. Matijevic, E. (1987) Adsorption at solid/solution interfaces. 3. Surface charge and potential of colloidal hematite. Langmuir 3 815-820 Hesleitner, P. Kallay, N. Matijevic, E. (1991) Adsorption at solid/liquid interface. 6. The effect of methanol and ethanol on the ionic equilibrium at the hematite/water interface. Langmuir 7 178-184... 

Kallay, N. Matijevic, F. (1985) Adsorption at solid/solution interfaces. I. Interpretation of surface complexation of oxalic and citric acids with hematite. Langmuir 1 195-201... 

W. Norde, Driving forces for protein adsorption at solid surfaces, in Biopolymers at Interfaces, 2nd edn. M. Malmsten, Ed. New York CRC Press, 2003. 

Equation (46), one form of the Gibbs equation, is an important result because it supplies the connection between the surface excess of solute and the surface tension of an interface. For systems in which y can be determined, this measurement provides a method for evaluating the surface excess. It might be noted that the finite time required to establish equilibrium adsorption is why dynamic methods (e.g., drop detachment) are not favored for the determination of 7 for solutions. At solid interfaces, 7 is not directly measurable however, if the amount of adsorbed material can be determined, this may be related to the reduction of surface free energy through Equation (46). To understand and apply this equation, therefore, it is imperative that the significance of r2 be appreciated. 

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.)... 

As should be evident from the discussions in Chapters 6 and 7, adsorption phenomena play a major role in colloid and surface chemistry. We also come across other examples in Chapters 11 and 13. Adsorption, especially at solid-gas interfaces, is very important in heterogeneous catalysis, as highlighted in Vignette IX. In this chapter, the focus is the introduction of quantitative measurement and the description of adsorption at solid-gas interfaces. 

Recent studies indicate that the adsorption of metal ions is controlled only in part by the concentration of the free (aquo) metal ion of considerable importance is the ability of hydroxo and other complex ions and molecules to adsorb. There have been two apparently divergent approaches to describe the role played by hydroxo metal complexes in adsorption at solid-aqueous electrolyte interfaces. Matijevic et al. (9) have proposed that specific hydrolysis products—e.g., Al8(OH)2o4+ in the A1(III)-H20 system, are responsible for extensive coagulation and charge reversal of hydrophobic colloids. It has also been demonstrated by Matijevic that the free (aquo) species of transition and other metal ions... 

It was shown [21] that polyelectrolytes having hydrophobic functionalities show higher efficiency for such systems due to the synergetic effect of electrostatic and hydrophobic interactions promoting the polymer adsorption at solid/liquid or liquid/hquid interface. 

Some of the areas where interfacial protein layers dominate the boundary chemistry are reviewed, and we introduce some nondestructive armlytical methods which can be used simultaneously and/or sequentially to detect and characterize the microscopic amounts of matter at protein or other substrates which spontaneously acquire protein conditioning films. Examples include collagen and gelatin, synthetic polypeptides, nylons, and the biomedically important surfaces of vessel grafts, skin, tissue, and blood. The importance of prerequisite adsorbed films of proteins during thrombus formation, cell adhesion, use of intrauterine contraceptives, development of dental adhesives, and prevention of maritime fouling is discussed. Specifics of protein adsorption at solid/liquid and gas/liquid interfaces are compared. 

Interfaces can be formed between the aqueous surfactant solution and either a solid, cui oil (i.e. a liquid with which water does not mix) or a gas (vapour) phase. We shall designate them as SL, w/o or L/L and LG, respectively. Only the last two are now under consideration. We refer to sec. 11.2.7d for non-ionic surfactant adsorption at SL interfaces and to sec. II.3.12 for electrosorption of ionic surfactants. 

The use of gas flow techniques are now widely recognized as important for the study of adsorption/desorption - phenomena at solid interfaces. Such studies are particularly important for the characterization of catalytically active sites on the surfaces of solids. One particular example is a study of the adsorption of ammonia onto activated carbon. It can be shown that adsorption of ammonia consists of reversible and irreversible steps and that these steps can be attributed to physisorp-tion for the former and adsorption on particular chemical groups for the latter. The heats of adsorption for various sites can be measured and the data can be used to reveal the existence of a wide distribution of acid sites that are accessible to ammonia to varying degrees. 

The application of infrared difference spectrometry to the measurement of protein adsorption at solid/liquid interfaces is potentially... 

Apart from those mentioned, other general features of protein adsorption at solid/liquid interfaces are as follows (1) Adsorption is sensitive to pH, as it is for fluid/fluid interfaces, a maximum usually being observed near the isoelectric point of the protein (Dillman and Miller, 1973 Norde, 1976). (2) Greater adsorption occurs at hydrophobic interfaces than at hydrophilic ones (MacRitchie, 1972 Brash and... 

In studying adsorption at any interface, one is interested in determining the number of moles of the adsorbate per unit mass or unit area of the solid adsorbent since this is a measure of how much of the surface has been covered, and hence changed, by the adsorption. To find the relationship between this number of moles, n (or r ) and the... 

Another extreme is to neglect the exponential term in Eq. (5.24) at all. This leads to overestimated effect of the pH on tjo (assuming a fixed value). A model neglecting the surface potential is physically unrealistic, but non-electrostatic models of adsorption at solid-aqueous solution interface can be found even in very recent literature. According to the prevailing opinion the actual surface potential is between the above two extremes (Nemst potential and 0 = 0). The electrostatic models of oxide - inert electrolyte solution interface were discussed in detail by Westall and Hohl [25]. In this section the most common electrostatic models are combined with the 1-pK model in order to illustrate their ability to simulate the actual surface charging data. 

Equilibrium segregation and equilibrium adsorption at solid-gas interfaces have often been formally treated as identical phenomena since both obey the Gibbs adsorption theorem. However, Gibbs rigourous results are difficult to apply due to the lack of information about various parameters, especially the composition dependence of the surface tension ). Therefore, a number of alternative approachs, based on experimental results have been attempted to predict and explain surface segregation. 

Hanna, H.S., Somasundaran, P, 1977. Physico-chemical aspects of adsorption at solid/liquid interfaces, 11 Mahogany sulfonate/Berea sandstone, kaolinite. In Shah, D.O., Schechter, R.S. (Eds.), Improved Oil Recovery by Surfactant and Polymer Flooding. Academic Press, pp. 253-274. 

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