Surfactant drop method

The AOS and IOS surfactants discussed in this section are laboratory-prepared samples. The experiments were carried out by Borchardt [40] who used the spinning drop method throughout. Experimental details are given in the footnotes of the tables that follow. 

Interfacial Tension of Aqueous Surfactant Solutions by the Pendant Drop Method... 

The low interfacial tensions between two liquids have been measured for different systems by using the pendant drop method. In the case of the quaternary system Ci2ll25S 3 tNa+H20+n-Butanol+Toluene, the interfacial data as measured by pendant drop method are compared with reported literature data, using other methods (with varying NaCl concentration). In order to understand the role of co-surfactant, ternary systems were also investigated. The pendant drop method was also used for measuring the interfacial tension between surfactant-H20/n-alcohol (with number of carbon atoms in alcohol varying from 4-10). The interfacial tension variation was dependent on both the surfactant and alcohol. 

Figure 4. Variation of y of the systems. A, aqueous phase surfactant + H O oil phase toluene with pendant drop method (24 °C). Concentration of detergent = 20 g/L. NaDOC (4) NaDOC + 8% n-butanol(A) NaDDS ( ) NaDDS + 8% n-butanol (O). B, aqueous phase surfactant + H2O oil phase n-heptane by pendant drop method 25 °c). Key same as for 4A.

The interfacial tension methods are described in ISO 6889 [20], ISO 9101 [21] and ASTM D1331 -89 (2001) [ 18 ]. The method described in ISO 6889 is a simple method and applicable for the systems if the interfacial values arebetween 4 and 50 dyne cm-1, the immiscible liquids are water and organic liquids and the systems contain non-ionic or anionic surfactants but not cationic surfactants. The repeatability is within about 2 dyne cm-1. On the other hand, the drop volume method as described in ISO 9101 can be used for viscous liquids and liquids containing all types of surfactants. This method can measure the interfacial tension as low as 1 dyne cm-1 with 0.5 dyne cm-1 accuracy. If the interfacial tension is lower than 1 dyne cm-1, the spinning drop will be the suitable method. 

Surface Tension Measurement. The surface tension of the surfactant solution was determined by means of the Dynamic Contact Angle Tester FIBRO DAT 1100 (FIBRO Systems, Sweden) using the pendant drop method. It was also an output of the ADSA captive bubble contact angle measurements with surfactant solutions. 

Selective solubilization can also occur in mixtures of polar and nonpolar oils. Using their oil drop method described previously, Chen et al. measured solubilization rates of mixtures of triolein and oleic acid in solutions of pure nonionic surfactants. As Figure 9.9 shows for a drop initially containing 85/15 triolein/oleic acid by weight injected into 2 wt% Tergitol 15-S-7 at 35°C, they observed that the solubilization process consisted of two stages. In the first stage, the drop radius... 

It should be possible to describe the exchange of matter by a diffusion model for surfactant mixtures, as shown by Miller et al (1993b). They also developed another new relaxation technique, based on the pendent drop method (cf. Section 6.3.4), and studied the relaxation behaviour of SDS at the water/air interface. It could be shown that surface active impurities can alter the relaxation behaviour of the adsorption layer tremendously. The same method was also applied to detect impurities in organic solvents (cf Section 5.1.2., Fig. 5.7.). 

For DOC, it can be seen that the results of Williams (1967), for example, show an extra 2.1 g m DOC in the microlayer. If the thickness of the water film obtained with the screen device is taken to be —200 pm, the surface excess of DOC can be calculated as 2.1 X 200 X 10 = 4.2 X 10" g m . A reasonable lower limit to take for the molecular weight of this extra organic material in the surface film is that of a relatively short-chain acid or alcohol with —14 carbon atoms, equivalent to —170 g mole carbon. Using this minimum value, the area per molecule in the ambient type of films sampled by Williams (1967) can be calculated as > — 170/4.2 X lO" X 6.02 X 10 3 = > 70 A. It can be seen from Fig. 1 that for all surface film types except gaseous films, such an area per molecule has no effect on the surface tension of seawater, as measured by the spreading drop method, or on the damping of capillary waves. Moreover, only relatively water-soluble surfactants remain in the gaseous state at film pressures of —10 N m" ... 

The adsorption kinetics at liquid/liquid interfaces is a more complicated problem, as the transfer of surfactant from one phase to the other has to be taken into account. In the experiments performed by Liggieri and Ravera [197] using the expanded drop method, no preliminary saturation of the oil phase with CjoEOg was made. For this case, instead of Eq. (4.1), the expression (4.94) should be used, where K is the equilibrium distribution coefficient of surfactant between the oil and water phases, and D2 is the surfactant diffusion coefficient in the oil phase. The reduced distribution coefficient defined by = K(D2/Di) is a parameter that reflects quantitatively the adsorption dynamics at such a liquid/liquid interface. 

The interfacial tension may be determined to within about 1% accuracy with the spinning-drop method (127, 128). It is an absolute and static method that requires only small samples and, in contrast to most other methods, does not depend on the wettability of a probe, such as a ring or Wilhelmy plate. The stabilizing surfactant is commonly used at concentrations in the bulk continuous phase that are far above the critical micelle concentration (erne). This ensures that the concentration remains above the erne after adsorption on to the vastly extended interface has taken place, which is clearly needed to maintain emulsion stability. It is tempting, therefore, to assume that the interfacial tension in the finished emulsion equals that between the unemulsified bulk phases and that it remains constant when a mother emulsion is diluted with continuous phase in order to create a series of emulsions in which only O is varied (67). This may be a reasonable assumption when a pure surfactant is used, but there is evidence that this may not be so when impure commercial surfactants or surfactant mix-... 

For each of the three surfactant systems, the interfacial tension values calculated from the I.D.T. traces are higher than their equilibrium counterparts as measured by the pendant drop method. This holds true even when the interface is subject only to displacement, rather than expansion or compression this is shown in Figures 6, 8 and 10, where steady state dynamical interfacial tension values are shown. These values are obtained from the constant linear parts of the I.D.T. traces, or from the last part of the central (small capillary) section of the trace in those cases where it is not certain that the trace had yet reached its plateau (steady state) value. 

Since the shape of an oil droplet is an indication of IFT as measured by sessile drop method, the oil droplet flattening time reflects the rate of change in IFT. The results clearly show that the presence of alcohol increases the rate of achieving the final value of interfacial tension. This implies that the surfactant molecules come to the interface much faster in the presence of alcohol. Zana (20) has shown that the kinetics of micellization is more rapid in the presence of alcohol. This is presumably due to loose packing of mixed micelles containing surfactant and alcohol. Thus, it appears that the kinetics of micellization could influence the rate at which molecules saturate the surface by the breakdown of micelles to provide monomers for adsorption. 

NMR, up to 180 °C by Shinoda et al. [52]. Some work has been reported involving erne determination by calorimetry (measuring heats of dilntion or specific heats). Archer et al. [53] nsed flow calorimetry to determine the erne s of several sulfonate surfactants at up to 178 °C. NoU [5J] determined erne s for dodecyltrimethylammonium bromide and commercial surfactants in the temperature range 25—200 °C using flow calorimetry. Surface tension is the classical method for determining erne s but many surface tension methods are not suitable for use with aqueous solutions at elevated temperatures. Exceptions include the pendant, sessile, and captive drop methods which can be conducted with high-pressure cells [54, 55]. 

Abstract The interfacial properties of tracheal aspirate from infants with untreated neonatal respiratory distress syndrome (NRDS), and NRDS infants after therapy with the exogenous surfactant Curosurf were assessed. The interfacial characteristics of the aspirate (equilibrium surface tension, maximal and minimal surface tension during lateral compression-decompression cycles) were determined with the pendant drop method. Our results show that the tracheal aspirate of infants with untreated NRDS had high equilibrium, maximal and minimal surface tension values. In contrast, the samples from infants, treated with Curosurf , showed lower surface tension values, suggesting that the application of Curosurf improves the composition and the properties of the pulmonary surfactant in the infant lung. 

Keywords Alveolar surfactant Curosurf Pendant drop method Surface tension... 

In order to determine the infants lung maturity and the necessity of surfactant therapy it is of great importance to substantiate the functionality of the alveolar surfactant, derived via invasive techniques [13], Several techniques and models have been largely used to investigate inteifacial physicochemical properties in vitro and to assess clinical efficiency of ES in vivo the Langmuir monolayer technique in combination with Wilhelmy plate method for surface tension measurements and black foam film method for determination of the ability of ES for stable film formation [14]. The pendant drop method combined with the Axisym-metric Drop Shape Analysis (ADSA) has been also used for similar purposes [4,15-18]. 

The dynamic surface tension of [3-casein solutions at three concentrations 5 10, 10 and 10 mol/1 are shown in Fig. 14. As one can see the results from the two methods differ significantly. For the bubble the surface tension decrease starts much earlier. The surface tensions at long times, and hence the equilibrium surface tension from the bubble experiment are lower than those from the drop. However, the establishment of a quasi-equilibrium for the drop method is more rapid at low (3-casein concentrations while at higher P-casein concentrations this process is more rapid for the bubble method. This essential difference between solutions of proteins and surfactants was discussed in detail elsewhere [50]. In brief, it is caused by simultaneous effects of differences in the concentration loss, and the adsorption rate, which both lead to a strong difference in the conformational changes of the adsorbed protein molecules. 

Keywords Oscillating drop method, surfactant solutions, dilatioiuil elasticity, microgravity... 

The spinning drop method is commonly used in petroleum-related applications for determining the interfacial tension at the crude oil/aqueous solution interface in the presence of different surfactants. 

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