Adsorption isotherms surfactants, measured

As there is no rigorous theory that can predict adsorption isotherms, the most suitable method for investigating the adsorption of surfactants is to determine the adsorption isotherm. The measurement of surfactant adsorption is fairly straightforward a known mass m (in grams) of the particles (substrate) with known specific surface area Ag (m g ) is equilibrated at constant temperature with surfactant solution with initial concentration Cj. The suspension is kept... 

Adsorption on Silica Gel. The adsorption isotherms of sodium dodecylbenzene sulfonate and TRS 10-410 on silica gel at 30°C and pH =5.8 are shown in Figure 2 for zero and one wt. % NaCl. Although the equivalent weights of these surfactants differ substantially (SDBS = 348 TRS-10-410=418) the isotherms are very similar in shape there is a concave toe, a shoulder, and a long flat plateau in each case. The addition of one wt.% NaCl to the solution results in a sharp reduction in the adsorption plateau (or saturation level) for SDBS (one wt.% NaCl causes salting-out of TRS-10-410, see Table I, so no adsorption isotherm was measured for TRS-10-410 and one wt % NaCl). 

Experiment C is designed to yield information on the amount of the surfactant that is actually adsorbed on the rock. This experiment measures the variation of surfactant concentration at the outlet of the core, after injection of a "slug of surfactant. The surfactant concentration in the brine depends on the position along the core and on time. The experiment is dynamic because the changing, but near equilibrium level of the adsorbed surfactant at any point along the rock sample is a function of the concentration in the solution at that point. This is described by the adsorption isotherm from a plot of M, the mass of surfactant adsorbed per gram of rock vs. Concentration. 

The mechanism of interaction of amino acids at solid/ aqueous solution interfaces has been investigated through adsorption and electrokinetic measurements. Isotherms for the adsorption of glutamic acid, proline and lysine from aqueous solutions at the surface of rutile are quite different from those on hydroxyapatite. To delineate the role of the electrical double layer in adsorption behavior, electrophoretic mobilities were measured as a function of pH and amino acid concentrations. Mechanisms for interaction of these surfactants with rutile and hydroxyapatite are proposed, taking into consideration the structure of the amino acid ions, solution chemistry and the electrical aspects of adsorption. 

The adsorption isotherms of various surfactants were measured on minerals with a different character of Ppis. The course of the isotherms on minerals, with H and OH on one hand and those with latice ions as PDIs on the other hand, is similar, with a maximum in a region close to the CMC. Some characteristic adsorp-... 

The time required to conduct an interfacial tension experiment depends largely on the properties of the surfactants and less on the chosen measurement method. A notable exception is the drop volume technique, which, due to the measurement principle, requires substantial ly more time than the drop shape analysis method. Regardless of the method used, 1 day or more may be required to accurately determine, e.g., the adsorption isotherm (unit D3.s) of a protein. This is because, at low protein concentrations, it can take several hours to reach full equilibrium between proteins in the bulk phase and those at the surface due to structural rearrangement processes. This is especially important for static interfacial tension measurements (see Basic Protocol 1 and Alternate Protocols 1 and 2). If the interfacial tension is measured before the exchange of molecules... 

The adsorption isotherm of SDS on a C18 stationary phase was also measured by determining the amount of surfactant adsorbed onto the stationary phase from frontal chromatography experiments ( 5 ). Figure 1 is a log-log plot of surface concentration vs. mobile phase concentration of SDS with a standard mobile phase of n-propanol water (3 97)(vida infra). The maximum concentration of surfactant adsorbed on the stationary phase occurs at the mobile phase concentration of ca. 10 2 M and gives a surface concentration of ca. 

The experimental results discussed pertain to foam and emulsion bilayers formed of surfactants of different kinds and provide information about quantities and effects measurable in different ways. It is worth noting that analysing the observed effect of temperature on the rupture of foam bilayers enables the adsorption isotherm of the surfactant vacancies in them to be calculated. This isotherm shows a first-order phase transition of the vacancy gas into a condensed phase of vacancies, which substantiates the basic prerequisites of the theory of bilayer rupture by hole nucleation. 

The only information needed to predict the mixture surfactant concentration to attain a specified adsorption level is the pure component adsorption isotherms measured at the same experimental conditions as the mixture isotherms. These isotherms are needed to obtain the pure component standard states. 

When semllogarithmic adsorption isotherms of surfactants are measured at different salt concentrations valuable additional information is obtained that is diagnostic for the mechanism. In some instances the l.h.s. of (3.12.3b] is negative, mainly so in the initial parts of the isotherms. This means that NaCl is expelled. The inference is that the surfactant adsorbs with its cationic charge to the negative surface, displacing Na ions. Under these conditions, the NaCl acts as an inhibitor. 

Surfactant surface activity is most completely presented in the form of the Gibbs adsorption isotherm, the plot of solution surface tension versus the logarithm of surfactant concentration. For many pure surfactants, the critical micelle concentration (CMC) defines the limit above which surface tension does not change with concentration, because at this stage, the surface is saturated with surfactant molecules. The CMC is a measure of surfactant efficiency, and the surface tension at or above the CMC (the low-surface-tension plateau) is an index of surfactant effectiveness (Table XIII). A surfactant concentration of 1% was chosen where possible from these various dissimilar studies to ensure a surface tension value above the CMC. Surfactants with hydrophobes based on methylsiloxanes can achieve a low surface tension plateau for aqueous solutions of —21-22 mN/m. There is ample confirmation of this fact in the literature (86, 87). 

Some essential discoveries concerning the organization of the adsorbed layer derive from the various spectroscopic measurements [38-46]. Here considerable experimental evidence is consistent with the postulate that ionic surfactants form localized aggregates on the solid surface. Microscopic properties like polarity and viscosity as well as aggregation number of such adsorbate microstructures for different regions in the adsorption isotherm of the sodium dedecyl sulfate/water/alumina system were determined by fluorescence decay (FDS) and electron spin resonance (ESR) spectroscopic methods. Two types of molecular probes incorporated in the solid-liquid interface under in situ equilibrium conditions... 

Adsorption isotherms are habitually obtained using the solution depletion method, which consists of comparing the solute concentrations before and after the attainment of adsorption equilibrium. Electrokinetic or zeta potentials are determined by two techniques microelectrophoresis [12,14,17] and streaming potential [13,58,59]. The former is employed to measure the mobility of small particles of chemically pure adsorbents, whereas the latter is adopted to investigate the electrophoretic behaviour of less pure coarser mineral particles. A correlation between the adsorption and electrophoretic results is usually examined with the aim of sheding light on the mechanism by means of which the surfactants are adsorbed at the solution-solid interface. This implies the necessity of maintaining the same experimental conditions in both experiments. For this purpose, the same initial operational procedure is applied. 

For colloids with a physically adsorbed surfactant or cca, the adsorption isotherm is important. The adsorbant concentration on the particle surface can be measured by infrared spectroscopy using diffuse reflectance and by ESCA. Absolute concentrations are difficult to determine with ESCA on "rough" surfaces, and a calibration point is required with other techniques. The change of the concentration of adsorbant in solution after adsorption on the colloid surfaces can be detected by elemental analysis of supernatant with plasma emission or atomic absorption if adsorbant contains specific element(s). When colloids are sterically stabilized, the effectiveness of the stabilization can be evaluated with solvent-nonsolvent techniques and with temperature studies ( 25,26). 

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