Adsorption, apparent surface tension

The first is that protein adsorption, even out of dilute solutions, changes the apparent surface tension of the imcoated polymer material very markedly. As an example, a bulk concentration of 1 mg/ml of HSA, IgG or fibrinogen gives rise to a surface tension of 53.0, 58.8 and 59.0 ergs/cm, respectively, on PTFE. These values represent an increase in surface tension of more than 30 ergs/cm over the value of uncoated PTFE particles (18). 

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. 

In the absence of specific interactions of the receptor - ligand type the change in the Helmholtz free energy (AFadj due to the process of adsorption is AFads = yps - ypi - Ysi, where Yps, YPi and ys, are the protein-solid, protein-liquid and solid-liquid interfacial tensions, respectively [5], It is apparent from this equation that the free energy of adsorption of a protein onto a surface should depend not only of the surface tension of the adhering protein molecules and the substrate material but also on the surface tension of the suspending liquid. Two different situations are possible. 

Certain negative ions such as Cl , Br, CNS , N03 and SO2 show an adsorption affinity to the mercury surface so in case (a), where the overall potential of the dme is zero, the anions transfer the electrons from the Hg surface towards the inside of the drop, so that the resulting positive charges along the surface will form an electric double layer with the anions adsorbed from the solution. Because according to Coulomb s law similar charges repel one another, a repulsive force results that counteracts the Hg surface tension, so that the apparent crHg value is lowered. 

In addition to lowering surface tension, surface-active agents contribute to emulsion stability by oriented adsorption at the interface and by formation of a protective film around the droplets. Apparently, the first molecules of a surfactant introduced into a two-phase system act to form a monolayer additional surfactant molecules tend to associate with each other, forming micelles, which stabilize the system by hydrophilic-lipophilic arrangements. This behavior has been depicted by Stutz et al. ( ) and is shown in Figures 1-5. 

Figure 2. The change in apparent critical surface tension (A) and water contact angle (B) with adsorption time for ETES films on air-dried silica (— —) and on flamed silica (—O—). (1% ETES in a-chloronaph-thalene)...

One of the approaches found most suitable to explain the sensorial properties of sweet, bitter, and sweet-bitter substances proves to be the physico-chemical approach especially as concerns hydration and surface properties (DeSimone and Fleck, 1980 Funasaki et al., 1996 Fimasaki et al., 1999 Mathlouthi and Hutteau, 1999). Thus, solution properties of sweet and bitter molecules were found informative on their type of hydration (hydrophobic or hydrophilic) and on the extent of the hydration layer (Fiutteau et al., 2003). Physico-chemical properties (intrinsic viscosity, apparent specific volume, and surface tension) and NMR relaxation rates of the aqueous solutions of sucrose, caffeine, and sucrose-caffeine mixtures were used in the interpretation of the taste modalities of these molecules and to explain the inhibition of caffeine bitterness by sucrose (Aroulmoji et al., 2001). Caffeine molecules were found to form an adsorption layer whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine gradually increases with concentration and is delayed when sucrose is added in the caffeine solution (Aroulmoji et al., 2004). 

The time, f,-, for the induction period (region I) to end is an important factor in determining the surface tension as a function of time, since only when that period ends does the surface tension start to fall rapidly. The value of f,- has been shown (Gao, 1995 Rosen, 1996) to be related to the surface coverage of the air-aqueous solution interface and to the apparent diffusion coefficient, Dap, of the surfactant, calculated by use of the short-time approximation of the Ward-Tordai equation (Ward, 1946) for diffusion-controlled adsorption (equation 5.6) ... 

The principle of the drop volume method is of dynamic character and therefore, it can be used for studies of adsorption processes in the time interval of seconds up to some minutes. At small drop times a so-called hydrodynamic effect has to be considered, as discussed in many papers (Davies Rideal 1969, Kloubek 1976, Jho Burke 1983, Van Hunsel et al. 1986, Van Hunsel 1987, Miller et al. 1994a). This hydrodynamic effect appears at small drop times under the condition of constant liquid flow into the drop and gives rise to apparently higher surface tensions. Davies Rideal (1969) discussed two factors influencing the drop formation at and its detachment from the tip of a capillary the so-called "blow up" effect and a "circular current" effect inside the drop. The first effect increases the detaching drop volume and simulates a higher surface tension while the second process leads to an earlier break-off of the drop and results in an opposite effect. A schematic of these two effects on measured drop volumes is shown in Fig. 5.10. 

The maximum bubble pressure technique is a classical method in interfacial science. Due to the fast development of new technique and the great interest in experiments at very small adsorption times in recent years, commercial set-ups were built to make the method available for a large number of researchers. Rehbinder (1924, 1927) was apparently the first who applied the maximum bubble pressure method for measurement of dynamic surface tension of surfactant solutions. Further developments of this method were described by several authors (Sugden 1924, Adam Shute 1935, 1938, Kuffiier 1961, Austin et al. 1967, Bendure 1971,... 

The surface excess obtained by the second-harmonic generation in the concentration range below the CMC, however, changes with concentration in contradiction to the usual interpretation of surface tension data. Moreover, the absolute values of the adsorption determined by two experimental methods differ by one order of magnitude. These discrepancies were explained by means of the concept of a depth-dependent distribution of surfactant molecules [66]. Different distributions can lead to identical adsorption values. The surface excess determined by the second-harmonic generation can be attributed only to the very top layer, whereas the values obtained from surface tension techniques are apparently more sensitive to the near-surface layer. 

In the case of a soluble nonionic siufactant the detected increase in a in a real process of interfacial dilatation can be a pure manifestation of siuface elasticity only if the period of dilatation,At, is much shorter than the characteristic relaxation time of surface tension x, At x (21). Otherwise, the adsorption and the surface tension would be affected by the diffusion supply of siufactant molecules from the bulk of solution toward the expanding interface. The diffusion transport tends to reduce the increase in surface tension upon dilatation, thus apparently rendering the interface less elastic and more fluid. The initial condition for the problem of adsorption kinetics involves an instantaneous (At Tjj) dilatation of the interface. This instantaneous dilatation decreases the adsorptions T and the subsiuface concentrations of the species (the subsurface... 

Interfacial tensiometry is technique which is sensitive to the adsorption of surface-active solutes. The time-dependent interfacial tension measurements served to reveal the physico-chemical changes at oil/aqueous interface during mass transfer of demulsifiers and demulsifier mixtures to the interface. The apparent spreading rate which was determined from the time-dependent tensions provided a measure for the ease of deformation of the interface and the speed of adsorption to the interface. The apparent spreading rate parameter correlated very well with the coalescence behavior and dewatering efficiency of the emulsion. 

As mentioned above, the drop volume method is of dynamic character and it can be used for adsorption processes in the time interval of seconds up to some minutes. At small drop time, the sohydrodynamic effect has to be considered [27]. This gives rise to apparently higher surface tension. Kloubek et al. [28] used an empirical equation to account for this effect. 

As with the adsorption/desorption of nitrogen at 77 K and the hysteresis effect, so with mercury porosimetiy hysteresis is observed as the applied pressure is lowered. This may be caused, not by surface tension considerations as such, but by mercury being trapped within the network of porosity. Trapping could occur if the entry into some of the porosity is via porosity of a smaller size. A comparative study of porous carbons of this trapping hysteresis may assist in an understanding of interconnectivities within the porous networks. The effect apparently has been looked upon mainly as a nuisance. 

In principle, adsorption of these PDMSs and their derivatives at air-crude oil surfaces should therefore be possible, potentially leading to close-packed adsorption layers and surface tensions reduced to about 20 mN m-. However, the solubilities of these compounds in crude oil are relatively low, which could mean that bulk phase activities are never sufficiently high to realize close-packed monolayers—reductions in air-crude oil surface tensions could therefore be correspondingly modest. Mannheimer [17] has, for example, shown that the surface tension reduction of a variety of hydrocarbon oils by a given PDMS oil is greater the greater the apparent solubility of the PDMS—in no case, however, was the surface tension reduced to the expected surface tension of a close-packed CHj-rich layer. 

The adsorbed amount and the apparent head group area obtained from the ellipsometry meaurements are shown in Table 2 together with the cross-sectional head group area as determined from the surface tension measurements for a series of fatty amide ethoxylates in comparison with fatty alcohol ethoxylates [18,19]. For the surfactants as measured by Folmer et al. [19], it can be seen that the adsorption areas as obtained from ellipsometry are up to 3.5 times larger than the areas observed from the surface tension measurements. As mentioned by the authors, there are two important differences... 

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