Interfacial tension values

MethylceUulose and its mixed ethers are surface-active ceUulose ethers having surface tension values as low as 44 mN/m(= dyn/cm) and interfacial tension values as low as 17 mN/m(= dyn/cm) against paraffin oU. 

Experimental correlations have been established in a given LDL formulation between foam stability and interfacial tension [33]. For example, Fig. 15 shows the effect of increasing water hardness on plate washing performance of an LAS/AES blend. A small amount of Ca2+ ion helps substantially to stabilize the foam. Under the same conditions interfacial tension is also lowered substantially. The two curves show an inverse relationship where the minimum interfacial tension value corresponds to the optimum level of foam stability as measured by plate washing [33]. 

Tests were performed at 75°C using a University of Texas Model 500 spinning drop tensiometer. Active surfactant concentration in the aqueous phase prior to addition of the oil phase was 0.5% wt. Interfacial tension values are the average of duplicate or triplicate determinations. 

Decreasing the pH of 3% NaCl (entries 2 and 3, Table 14) could decrease neutralization of crude oil organic acids. This neutralization increases both aqueous phase salinity and effective surfactant concentration. A lower effective surfactant concentration at pH 8 could account for the increased interfacial tension value. However, a similar pH change does not reduce IFT when the surfactant is AOS 1618 with a much lower di monosulfonate ratio (entries 6 and 7, Table 14). 

The Gibbs equation allows the amount of surfactant adsorbed at the interface to be calculated from the interfacial tension values measured with different concentrations of surfactant, but at constant counterion concentration. The amount adsorbed can be converted to the area of a surfactant molecule. The co-areas at the air-water interface are in the range of 4.4-5.9 nm2/molecule [56,57]. A comparison of these values with those from molecular models indicates that all four surfactants are oriented normally to the interface with the carbon chain outstretched and closely packed. The co-areas at the oil-water interface are greater (heptane-water, 4.9-6.6 nm2/molecule benzene-water, 5.9-7.5 nm2/molecule). This relatively small increase of about 10% for the heptane-water and about 30% for the benzene-water interface means that the orientation at the oil-water interface is the same as at the air-water interface, but the a-sulfo fatty acid ester films are more expanded [56]. 

Isaacs and Smolek [211 observed that low tensions obtained for an Athabasca bitumen/brine-suIfonate surfactant system were likely associated with the formation of a surfactant-rich film lying between the oil and water, which can be hindered by an increase in temperature. Babu et al. [221 obtained little effect of temperature on interfacial tensions however, values of about 0.02 mN/m were obtained for a light crude (39°API), and were about an order of magnitude lower than those observed for a heavy crude (14°API) with the same aqueous surfactant formulations. For pure hydrocarbon phases and ambient conditions, it is well established that the interfacial tension behavior is dependent on the oleic phase [15.231 In general, interfacial tension values of crude oiI-containing systems are considerably higher than the equivalent values observed with pure hydrocarbons. 

Figure 8 shows the effect of Ca 2 on the interfacial tensions of two oils (Karamay and Clearwater) in Sun Tech IV (5 g/L) and NaCI (10 g/L) solutions at 150°C. The interfacial tension values for the two oils were very similar with as much as an 8-fold reduction, depending on concentration. Interfacial tension minima in the range of 0.06 to 0.1 mN/m were observed at 0.05 and 0.5 g/L CaCI2 for both oils. 

The interfacial tension-temperature relationships at various CaCL concentrations for Karamay crude in a Sun Tech IV (5 g/L) and NaCI (10 g/L) solution are shown in Figure 9. For 0, 0.025 and 0.1 g/L Ca, an increase in interfacial tension with temperature was observed. The interfacial tension values above 150°C were about the same for these concentrations. At temperatures below 100°C, the effect of Ca was to increase interfacial tension, probably by hindering the formation of a surfactant-rich phase. This is consistent with the detrimental effect or light oil/brine interfacial tensions (increase from about 10 3 to about 10 1) reported by Kumar et al. T371. ... 

The units of interfacial tension are identical for surface tension, i.e., dyn/cm. Interfacial tension values of organic compoimds range from zero for completely miscible liquids (e.g., acetone, methanol, ethanol) up to the surface tension of water at 25 °C which is 72 dyn/cm (Lyman et al., 1982). Interfacial tension values may be affected by pH, surface-active agents, and dissolved gases (Schowalter, 1979). Most of the interfacial tension values reported in this book were obtained from Dean (1987), Demond and Lindner (1993), CHRIS (1984), and references cited therein. 

The choice between the static methods (Wilhelmy plate method and the du Noiiy ring method) should primarily be based on the properties of the system being studied, in particular, the surfactant. As mentioned in UNITD3.5, the transport of surfactant molecules from the bulk to the surface requires a finite amount of time. Since static interfacial tension measurements do not yield information about the true age of the interface, it is conceivable that the measured interfacial tension values may not correspond to equilibrium interfacial tension values (i.e., the exchange of molecules between the bulk and the interface has not yet reached full equilibrium and the interfacial tension values are therefore not static). If the surfactant used in the experiment adsorbs within a few seconds, which is the case for small-molecule surfactants, then both the Wilhelmy plate method and the du Noiiy ring method are adequate. If the adsorption of a surfactant requires more time to reach full equilibrium, then the measurement should not be conducted until the interfacial tension values have stabilized. Since interfacial tension values are continuously displayed with... 

The main advantage of the static methods is cost. The equipment needed to conduct the dynamic measurements is approximately five times as expensive as the equipment required for static measurements (- 25,000 for a drop shape and drop volume analyzer versus - 5,000 for du Noiiy and Wilhelmy instruments). This is due to the additional capability of the former instruments to determine not only interfacial tension values but also the corresponding age of the interface. For more information on equipment, costs, and suppliers, see Internet Resources. 

The basic setup to determine static interfacial tension based on either the Wilhelmy plate method or the du Noiiy ring method (see Alternate Protocol 2) is shown in Figure D3.6.1. It consists of a force (or pressure) transducer mounted in the top of the tensiometer. A small platinum (Wilhelmy) plate or (du Noiiy) ring can be hooked into the force transducer. The sample container, which in most cases is a simple glass beaker, is located on a pedestal beneath the plate/ring setup. The height of the pedestal can be manually or automatically increased or decreased so that the location of the interface of the fluid sample relative to the ring or plate can be adjusted. The tensiometer should preferably rest on vibration dampers so that external vibrations do not affect the sensitive force transducer. The force transducer and motor are connected to an input/output control box that can be used to transmit the recorded interfacial tension data to an external input device such as a monitor, printer, or computer. The steps outlined below describe measurement at a liquid/gas interface. For a liquid/liquid interface, see the modifications outlined in Alternate Protocol 1. Other variations of the standard Wilhelmy plate method exist (e.g., the inclined plate method), which can also be used to determine static interfacial tension values (see Table D3.6.1). 

Wait for the displayed interfacial tension value to stabilize and record the value. 

Wait for the displayed interfacial tension value to stabilize and record the value. Correct the interfacial tension by using the correctional term in Equation D3.6.2 (see Background Information). 

While the quasistatic method is quite accurate, it requires a long time to determine a complete adsorption kinetics curve. This is because a new drop has to be formed at the tip of the capillary to determine one single measurement point. For example, if ten dynamic interfacial tension values are to be determined over a period of 30 min, -180 min will be required to conduct the entire measurement. On the other hand, the constant drop formation method is often limited because a large number of droplets have to be formed without interruption, which may rapidly empty the syringe. Furthermore, the critical volume required to cause a detachment of droplets depends on the density difference between the phases. If the density difference decreases, the critical volume will subsequently increase, which may exacerbate the problem of not having enough sample liquid for a complete run. 

The theoretical foundation of the drop volume technique (DVT) was developed by Lohnstein (1908, 1913). Originally, this method was only intended to determine static interfacial tension values. Over the past 20 years, the technique has received increasing attention because of its extended ability to determine dynamic interfacial tension. DVT is suitable for both liquid/liquid and liquid/gas systems. Adsorption kinetics of surface-active substances at liquid/liquid or liquid/gas interfaces can be determined between 0.1 sec and several hours (see Fig. D3.6.5). 

The continuous formation of drops, however, can lead to substantial errors in obtained adsorption kinetic data. For short drop formation times, hydrodynamic effects have to be taken into account. At large flow rates, the measured drop volume at the moment of detachment must be corrected. This is because a finite time is required for the drop meniscus to be disrupted and the drop to detach. Even though the volume has already reached its critical value, fluid may still flow from the reservoir into the drop. The volume of the drop is thus larger than its measured value, which leads to larger calculated interfacial tension values. The shorter the drop formation time is, the larger the error w i 11 be. K1 oubek et al. (1976) were the first to quantify this effect by introducing a corrected critical drop volume, Vc ... 

To determine the adsorption kinetics, the effective age of the drop interface must be calculated. However, experimental data yield only interfacial tension values as a function of drop formation time. To determine the true age of the interface, both the fluid flow within the droplet and the dilation of the droplet interface must be interpreted using appropriate models. Miller et al. (1992) showed that the drop formation time, r op, can be converted into the effective age of the drop interface ... 

The drop volume and drop shape techniques provide multiple interfacial tension values as a function of the interfacial age for a given surfactant at a given concentration at a specific temperature. Examples are provided in unitd3.5 (see Fig. D3.5.6). 

Mercury Injection data revealed a porosity of 30 % and a bimodal pore size distribution with pore size maxima at 20 and 110 nm. The capillary displacement pressure (Pd) for mercury was 2.7 MPa corresponding to an equivalent value of 0.5 MPa. For the conversion from the mercury-air to the gas-water system the following parameters were used interfacial tension values of p(Hg-air) = 480 mN/m, and p(N2-water) = 70 mN/m contact angles (Hg-air) = 141°, and 6l(N2-water) = 0°. 

Particularly useful for contacting two hazardous (e.g., radioactive) liquid phases with high interfacial tension values... 

The accuracy of the calculated interfacial tension values achieved in the experiments depends on two main factors. The first is the accuracy of the densiometer, which is estimated to 3... 

Interfacial Tension Values. The results for the effect of ionic surfactant to fatty alcohol molar ratio and concentration on interfacial tensions with styrene are shown in Figure 4. Maximum interfacial... 

The interfacial tension values increase from A.l dynes/cm for SLS/ decanol to 8.3 dynes/cm for SLS/octadecanol. Conductometric titration results have indicated that all of these mixed emulsifier systems, except the one with decanol, should give a relatively stable emulsion (22,23). Interestingly, the SLS/decanol mixed emulsifier solution was the only case in which the presence of the fatty alcohol reduced the interfacial tension with styrene to below the value measured for SLS alone. Studies are in progress to investigate this phenomenon and to determine the effect of alcohol chain length on miniemulsion stability. 

The relatively large interfacial tension values given in Table I and depicted in Figure 4 may actually be an indication of the interfacial tension between the oil droplet and the mixed emulsifier interfacial layer. This hypothesis is supported by the low interfacial tensions measured for "tails" which have detached themselves from the oil drop. The interfacial tension between these detached, free-... 

Interfacial tension values between styrene and several mixed emulsifier solutions were relatively high, 5-13 dynes/cm, while the apparent interfacial tensions between the aqueous phase and the resulting interfacial layer were substantially less than 1 dyne/cm. 

The lowest interfacial tension values are produced when R 1 and the value of the numerator (or denominator) in the expression for R is greatest. This produces the largest Vw/Vs and VH fVs ratios. To reduce yow, then, R should be made to approach 1 in the case where R < 1 + by increasing the value of the numerator in the case where R > 1 increasing the denominator, rather then decreasing the numerator. 

When three different phases make contact with each other, where three surfaces intersect at a triple point, we obtain three contact angles and three interfacial tension values, as can be seen in Figure 3.7. We can obtain contact angle equilibria when we place an immiscible drop on a liquid or solid in air or a vapor phase there are many applications of contact angle measurement in industry and surface science (see Chapter 9). In these conditions, the total excess surface internal energy can be written from Equation (201) so that... 

Table 6.2 Interfacial tension values at the liquid1-liquid2 interface (Values compiled from Davies, J.T. and Rideal, E. K. (1963) Interfacial Phenomena (2nd ed.). Academic Press, New York Donahue, D. J. and Bartel I, F. E. (1952) j. Phys. Chem., 56, 480 Cirifalco, L. A. and Good, R. J. (1 957) /. Phys. Chem., 61, 904 Ivosevic, N, Zutic, V, and Tomaic, J. (1999) Langmuir 15, 7063...

The interfacial tension values reported for the caustic system in Figure 8 are comparable to the values reported recently in reference (22). Our experiments which have been conducted at a room temperature of about 25°C show that 0.1 to 0.4 weight percent concentrations of NaOH and 1.00 weight percent NaCl can lower the interfacial tension between the aqueous solution and the crude oil substantially below a value of 0.01 dynes/cm or that required for emulsification. We have previously discussed the stability of these emulsions (Fig. 5). In the experiments run on fired Berea cores, it was reported that a concentration of 0.1% NaOH and 1% NaCl in the caustic crude oil system resulted in a drastic reduction in residual oil saturation. The details of these tests are given in reference (22). 

Figure 13 exhibits both interfacial tension and electrophoretic mobility for the Huntington Beach Field crude oil against sodium orthosilicate containing no sodium chloride. The interfacial tension values are observed to be higher for the non-equilibrated sample in this case than for the caustic system reported in Figure 12. The minimum interfacial tension of 0.01 dynes/cm occurs at about 0.2% sodium silicate as opposed to a value of less than 0.002 dyne/cm at about 0.06% NaOH. It is interesting to note, however, that the maximum electrophoretic mobility is the same for the two systems. Once again, it should be noted that a maximum in electrophoretic mobility does not correspond to a minimum in interfacial tension for those samples which contained no sodium chloride.

Methods used to measure interfacial tension are reviewed by Drelich, Fang, and White [ Measurement of Interfacial Tension in Fluid-Fluid Systems, in Encyclopedia of Surface and Colloid Science (Dekker, 2003), pp. 3152-3156]. Also see Megias-Alguacil, Fischer, and Windhab, Chem. Eng. Sci., 61, pp. 1386-1394 (2006). One class of methods derives interfacial tension values from measurement of the shape, contact angle, or volume of a drop suspended in a second liquid. These methods include the pendant drop method (a drop of heavy liquid hangs from a vertically mounted capillary tube immersed... 

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