Desorption of surfactant

The flux of surface-active agents from the surface into the bulk of the liquid may be controlled by the slower of the following processes 1) adsorption or desorption of surfactants at the surface or 2) diffusion of surfactants from the liquid bulk to the surface. Consequently, Levich evaluated the solution for a single drop... 

Choi and Funayama [19] also measured sodium atom emission from sodium dodecylsulfate (SDS) solutions in the concentration range of 0.1-100 mM at frequencies of 108 kHz and 1.0 MHz. The sodium line intensity observed at 1 MHz was nearly constant in the concentration range from 3 to 100 mM and was considerably higher than that at 108 kHz. This frequency dependence of the intensity is opposite that for NaCl aqueous solution. The dynamical behavior of the absorption and desorption of surfactant molecules onto the bubble surface may affect the reduction and excitation processes of sodium atom emission. This point should be clarified in the future. 

Desorption kinetics can also be studied by contracting a drop from a known state to a new state. The sudden reduction in interfacial area causes desorption of surfactants, which is deduced from the IFT change over time. 

In some cases the initial contact angle of photoresist treated with surfactant solutions was significantly higher than that of the untreated reference samples but decreased fast due to the desorption of surfactant molecules. Since the drop is not mechanically stable in the first ca. 100 milliseconds, the contact angles were extrapolated to time t = 20 ms, i.e. to the first contact. The values obtained for a series of 8-10 drops were averaged. The results are shown in Fig. 11 as a function of the concentration of the surfactant solution used for pretreatment. 

Under F/T conditions, desorption of surfactant presumably occurs more readily with the poly-S emulsion as a new equilibrium is established (a slight surface-tension lowering was observed after each F/T cycle throughout the series.) This conclusion is further substantiated by the exceptional increase in minimum weight percent acid required for poly-S in SLS (black square) compared with the 80/20 MMA-EA in SLS (black dot) despite the fact that less than one-half as much SLS was used in the latter case. Thus, it appears that poly-S is a less favorable surface for surfactant adsorption and as predicted by Equation 2 more carboxylate ions are required to obtain F/T stability. 

Sodium carbonate and sodium tripolyphosphate were added to the water to obtain the desired interfacial behavior. Figure 7.48 shows that the first peak (marked as 24) represents the maximum surfactant concentration at the effluent end from slug 4 (saline water). The second peak (marked as 26) represents the maximum concentration obtained by less saline waterflooding. Note that peak 26 is higher than peak 24. The second bank of surfactant was formed from the desorption of surfactant left by the first bank of surfactant solution on the solid surfaces. 

Krishnakumar, S, and Somasundaran, P., Role of surfactant-adsorbent acidity and solvent polarity in adsorption-desorption of surfactants from nonaqueous media, Langmuir, 10, 2786, 1994. 

We now turn to a case in which the interface concentration is determined by the mass transfer process to and from the bulk fluids. We begin with the case in which the fastest of the surfactant transport processes is the adsorption desorption of surfactant between the exterior bulk fluid where it is assumed to be soluble and the interface, Bi 1. In particular, we assume that Bi (ca/V,Xjk)Pe, so that, according to (7-265),... 

Adsorption isotherms can be applied to any surface. In the following we focus our attention on surfaces covered with adsorption layers under dynamic conditions, the kinetics of adsorption and desorption of surfactants to and from soluble adsorption layers for example. Another phenomenon is the spread of surfactant molecules tangential to the surface that effect takes place if the adsorption layer is inhomogeneous (cf. Fig. 1.1). 

If a solid particle crosses the diffusion layer of a bubble or drop it also includes the long range interaction caused by a local disturbance of the adsorption layer. This leads to Marangoni effects and influences the film drainage between particle and bubble or drop. Local desorption of surfactant from one surface and its adsorption on the other also causes interaction. 

The last term is characteristic of the thermodynamics of irreversible processes. Its magnitude becomes positive if the system s processes are irreversible. Typical irreversible processes are the adsorption or desorption of surfactants at liquid interfaces. The derivative of the second term of Eq. (2C.2), as local entropy production is... 

The QCM has been used in many types of electrochemical studies involving mass changes on electrodes, including the underpotential deposition of metals, adsorption/desorption of surfactants, and changes in polymer films during redox processes. In a typical experiment A/ is monitored during a potential step or sweep. As an example of a QCM study, we con-... 

If the rate of desorption of surfactants is very much greater than the rate at which the drop traverses through a distance equivalent to its diameter (about 0.02 sec), one may expect (D6) no accumulation of surfactants at the rear end. A quantitative semiempirical expression for the degree of drop circulation as a function of the viscous forces, drop diameter, densities and the compressional modulus of the surface film (surface-tension gradient), as well as the empirical fraction of liquid circulating, has been suggested by Davies (D6). 

It has been noted above that the influence of micelles on the adsorption process is determined by their ability to be a source or sink of monomers. The capacity of this source is determined by the parameter ncm- Flowever, only a small part of these monomers can participate in the fast process. Therefore, at low micellar concentration, the influence of the slow process (the change of the total number of micelles in the system) on the kinetics of adsorption and desorption of surfactants will be more significant. 

To examine the influence of surfactants on bubble growth, consider the following problem. A spherical gas bubble is placed in a binary solution, supersaturated at the given pressure and temperature. As a result, there emerges a diffusion flux toward the bubble surface. The process is accompanied by adsorption of a surfactant, which is present in the liquid, on the bubble surface. Assume that the process is diffusion-controlled, i.e. adsorption-desorption of surfactant molecules at the interface occurs sufficiently quickly. In such a case, the surface concentration of surfactant T may be taken equal to the equilibrium value [4] ... 

Alteration of the surface tension of the DEG-l-OP-10 system with temperature is greatly influenced by the surfactant concentration (Fig. 2.2). At llkg/m of OP-10, the surface tension does not depend on temperature, and at high surfactant concentrations even increases. The OP-10 associations with DEG-1 must be of increased solubility and must easily desorb from the boundary surface in the course of raising the temperature. Thus, temperature increase may result both in decrease of the surface tension on account of increase of the thermal motion of the molecules eind in its increase due to the desorption of surfactant molecules, and this increase has a direct dependence on the sxufactant concentration. Superposition of these factors for certain surfactant concentrations can bring about independence of the system siuface tension on temperature, confirmed by experiment. In the case of siuTactant desorption, the system entropy increased due... 

When a system contains a soluble surfactant, diffusion, adsorption, and desorption of surfactant may cause interfadal tension to vary with both position and time. If, for instance, a fresh interface is formed on a stagnant pool of a surfactant solution, interfacial tension is found to decrease with time as surfactant diffuses to the interface and adsorbs. 

First, for all reduced surfactants dissolved in solution at concentrations near their critical micelle concentrations (CMC), oxidation leads to an increase in the surface tension of the solution (Fig. 1). In the case of surfactants I and n, oxidation returns the surface tension of the solution to a value that is similar to the surfactant-free solution of electrolyte (approx. 72 mN/m). The excess surface concentration of surfactant, estimated using the Gibbs adsorption equation, decreases in the case of surfactant II from 10x10 to < 0.1 X10 mol/m upon oxidation. Clearly, oxidation drives the desorption of surfactant from the surface of the solution. The increase in surface tension of... 

Inspection of Fig. 2C reveals the third mechanism by which changes in surface tension can be effected by changes in the oxidation state of ferrocenyl surfactants. At concentrations less than the CMC of IIT, oxidation of in to in " leads to a decrease in the surface tension of the solution. This behavior contrasts to that of surfactants I, n and IV, where oxidation leads to an increase in sxuface tension at low concentrations (via desorption of surfactant from the interface). 

In summary, measurements of surface tension presented in Fig. 2 demonstrate that three distinct mechanisms cause changes in surface tension upon oxidation of ferrocenyl surfactants (i) desorption of surfactant from the surface of an aqueous solution (ii) ionization of a surfactant without significant desorption from the surface of the solution and (iii) change in the phase state of a spontaneously adsorbed monolayer of surfactant, possibly driven by a change in the conformation of the surfactant within the monolayer. 

When silicon oil displaces the D-8 solution (v < 0), the dynamic advancing contact angle of oil is close to 90 °. As seen from Fig. 24, wetting tension y 12 cos 0A is very low, near zero. In this case, bulk interface tension yo = 2.5 mN/m is maintained on the meniscus due to desorption of surfactant molecules from hydrophobic walls in front of the advancing meniscus. 

The benefits of ellipsometry when compared to other techniques used for thickness determinations are its simplicity and the fact that it is very rapid and nondestructive. This makes it possible to study the kinetics of both the adsorption and desorption of surfactants. 

Mechanical instability of ITIES due to adsorption and chemical desorption of surfactants has been known for many years. For example, Aral et al. studied the instability of the water-octanol interface caused by the adsorption of sodium dodecylsulfate by potentiometry and could relate interfacial polarization and oscillations [79]. The early work has been thoroughly reviewed by Kihara and Maeda [80]. In 2001, Maeda et al. demonstrated that self-sustained oscillations could be studied for a systan comprising cetyltrimethylammonium in water and picric acid in nitrobenzene [81]. 

Desorption of surface active compounds from bulk (e.g. desorption of surfactants from bulk to surface)... 

Polystyrene colloids (PS) were synthesized by soap-less emulsion polymerization [5, 33, 34]. This prevents desorption of surfactant from the surface with time, resulting in better defined surface properties. The colloids were synthesized in presence of acrylic acid as co-monomer and the anionic radical initiator ammonium persulfate was used to start the reaction. Above a certain size the colloids are electrostatically stabilized. Since acrylic acid is a weak acid, its negative charge is pH dependent. Emulsion co-polymerization of styrene with small amounts of acrylic acid results in poly(styrene-co-acrylic acid) polymer chains provides a steric stabilization as the chains are partially situated at the water-colloid interface [35, 36]. 

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