Interfacial tension surfactant concentration

As the amount of the surfactant in water is increased, the interface saturates to a point beyond which no further decrease in the interfacial tension occurs. At this concentration, the surfactant molecules start forming micelles (see Section 4.2.6). The inflection point of the interfacial tension vs. concentration curve is called the critical micelle concentration. 

CMC), the concentration at which the monomeric form, in which the surfactant exists in very dilute solution, aggregates to form a surfactant cluster known as a micelle (Chapter 3). Above this concentration the surface tension of the solution remains essentially constant since only the monomeric form contributes to the reduction of the surface or interfacial tension. For concentrations below but near the CMC the slope of the curve is essentially constant, indicating that the surface concentration has reached a constant maximum value. In this range the interface is considered to be saturated with surfactant (van Voorst Vader, 1960a) and the continued reduction in the surface tension is due mainly to the increased activity of the surfactant in the bulk phase rather than at the interface (equation 2.17). For ionic surfactants in the presence of a constant concentration of counterion, this region of saturated adsorption may extend down to one-third of the CMC. 

Figure 6 Interfacial tension vs. concentration of surfactant in the water, for the aniline/water system (Agble and Mendes-Tatsis, 2001)...

Surfactants are amphiphilic agents that show dual behavior i.e. they are both water and oil soluble. A typical structure of an anionic surfactant is shown in Figure 1 with a water-soluble head and an NAPL-soluble tail. Surfactants are also characterized by their ability to exist in the form of aggregates or micelles above a certain concentration called the critical micelle concentration. The amphiphilic nature also induces surfactants to aggregate at the water-NAPL interface, which brings about a decrease in the interfacial tension. Surfactant micelles also have the ability to solubilize significant volumes of NAPL components such as trichloroethylene (TCE), tetrachloroethylene (PCE), and 1,2-dichloroethane (DCA) among many others. 

Fig. 1. Interfacial tension/log concentration of surfactants for PNE/PFE/n-hexane systems...

Fig. III-9. Representative plots of surface tension versus composition, (a) Isooctane-n-dodecane at 30°C 1 linear, 2 ideal, with a = 48.6. Isooctane-benzene at 30°C 3 ideal, with a = 35.4, 4 ideal-like with empirical a of 112, 5 unsymmetrical, with ai = 136 and U2 = 45. Isooctane-

If an ionic surfactant is present, the potentials should vary as shown in Fig. XIV-5c, or similarly to the case with nonsurfactant electrolytes. In addition, however, surfactant adsorption decreases the interfacial tension and thus contributes to the stability of the emulsion. As discussed in connection with charged monolayers (see Section XV-6), the mutual repulsion of the charged polar groups tends to make such films expanded and hence of relatively low rr value. Added electrolyte reduces such repulsion by increasing the counterion concentration the film becomes more condensed and its film pressure increases. It thus is possible to explain qualitatively the role of added electrolyte in reducing the interfacial tension and thereby stabilizing emulsions. 

In the 1990s, the thmst of surfactant flooding work has been to develop surfactants which provide low interfacial tensions in saline media, particularly seawater require less cosurfactant are effective at low concentrations and exhibit lower adsorption on rock. Nonionic surfactants such as alcohol ethoxylates, alkylphenol ethoxylates (215) and propoxylates (216), and alcohol propoxylates (216) have been evaluated for this appHcation. More recently, anionic surfactants have been used (216—230). 

Critical Micelle Concentration. The rate at which the properties of surfactant solutions vary with concentration changes at the concentration where micelle formation starts. Surface and interfacial tension, equivalent conductance (50), dye solubilization (51), iodine solubilization (52), and refractive index (53) are properties commonly used as the basis for methods of CMC determination. 

Furthermore, in a series of polyoxyethylene nonylphenol nonionic surfactants, the value of varied linearly with the HLB number of the surfactant. The value of K2 varied linearly with the log of the interfacial tension measured at the surfactant concentration that gives 90% soil removal. Carrying the correlations still further, it was found that from the detergency equation of a single surfactant with three different polar sods, was a function of the sod s dipole moment and a function of the sod s surface tension (81). 

Finally, some general rules for the amount of surfactant appear to be vaHd (13). For anionic surfactants the average size of droplets is reduced for an increase of surfactant concentration up to the critical micellization concentration, whereas for nonionic surfactants a reduction occurs also for concentrations in excess of this value. The latter case may reflect the solubiHty of the nonionic surfactant in both phases, causing a reduction of interfacial tension at higher concentrations, or may reflect the stabilizing action of the micelles per se. 

A reduction of the o/w interfacial tension has a disadvantage because it makes the contact angle 9 more sensitive to small differences between and y. After a certain concentration of surfactant in the oil phase has brought the contact angle to 90°, the process is repeated but with the surfactant added to the oil before the phases are brought into contact. If the water droplet does not spread and its contact angle is in excess of 90°, the surfactant is added to the aqueous phase. 

The log of the reciprocal of the bulk concentration of surfactant (C in mol/ L) necessary to produce a surface or interfacial pressure of 20 raN/m, log( 1 / On= 20 i e > a 20 mN/m reduction in the surface or interfacial tension, is considered a measure of the efficiency of a surfactant. The effectiveness of surface tension reduction is the maximum effect the surfactant can produce irrespective of concentration, (rccmc = [y]0 - y), where [y]0 is the surface tension of the pure solvent and y is the surface tension of the surfactant solution at its cmc. 

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]. 

Phosphoric acid ester was used as a model for the estimation of concentration of a reagent in an adsorbed layer by optical measurements of the intensity of a beam reflecting externally from the liquid-liquid interface. The refractive index of an adsorbed layer between water and organic solution phases was measured through an external reflection method with a polarized incident laser beam to estimate the concentration of a surfactant at the interface. Variation of the interfacial concentration with the bulk concentration estimated on phosphoric acid ester in heptane and water system from the optical method agreed with the results determined from the interfacial tension measurements... 

Thus, the enhancement of heat transfer may be connected to the decrease in the surface tension value at low surfactant concentration. In such a system of coordinates, the effect of the surface tension on excess heat transfer (/z — /zw)/ (/ max — w) may be presented as the linear fit of the value C/Cq. On the other hand, the decrease in heat transfer at higher surfactant concentration may be related to the increased viscosity. Unfortunately, we did not find surfactant viscosity data in the other studies. However, we can assume that the effect of viscosity on heat transfer at surfactant boiling becomes negligible at low concentration of surfactant only. The surface tension of a rapidly extending interface in surfactant solution may be different from the static value, because the surfactant component cannot diffuse to the absorber layer promptly. This may result in an interfacial flow driven by the surface tension gradi-... 

Acid flooding can be successful in formations that are dissolvable in the particular acid mixture, thus opening the pores. Hydrochloric acid is common, in a concentration of 6% to 30%, sometimes also with hydrofluoric acid and surfactants added (e.g., isononylphenol) [130,723]. The acidic environment has still another effect on surfactants. It converts the sulfonates into sulfonic acid, which has a lower interfacial tension with oil. Therefore a higher oil forcing-out efficiency than from neutral aqueous solution of sulfonates is obtained. Cyclic injection can be applied [4,494], and sulfuric acid has been described for acid treatment [25,26,1535]. Injecting additional aqueous lignosulfonate increases the efficiency of a sulfuric acid treatment [1798]. 

In buffered surfactant-enhanced alkaline flooding, it was found that the minimum in interfacial tension and the region of spontaneous emulsification correspond to a particular pH range, so by buffering the aqueous pH against changes in alkali concentration, a low interfacial tension can be maintained when the amount of alkali decreases because of acids, rock consumption, and dispersion [1826]. 

Modem oil spill-dispersant formulations are concentrated blends of surface-active agents (surfactants) in a solvent carrier system. Surfactants are effective for lowering the interfacial tension of the oil slick and promoting and stabilizing oil-in-water dispersions. The solvent system has two key functions (1) to reduce the viscosity of the surfactant blend to allow efficient dispersant application and (2) to promote mixing and diffusion of the surfactant blend into the oil film [601]. 

The molecular collective behavior of surfactant molecules has been analyzed using the time courses of capillary wave frequency after injection of surfactant aqueous solution onto the liquid-liquid interface [5,8]. Typical power spectra for capillary waves excited at the water-nitrobenzene interface are shown in Fig. 3 (a) without CTAB (cetyltrimethy-lammonium bromide) molecules, and (b) 10 s after the injection of CTAB solution to the water phase [5]. The peak appearing around 10-13 kHz represents the beat frequency, i.e., the capillary wave frequency. The peak of the capillary wave frequency shifts from 12.5 to 10.0kHz on the injection of CTAB solution. This is due to the decrease in interfacial tension caused by the increased number density of surfactant molecules at the interface. Time courses of capillary wave frequency after the injection of different CTAB concentrations into the aqueous phase are reproduced in Fig. 4. An anomalous temporary decrease in capillary wave frequency is observed when the CTAB solution beyond the CMC (critical micelle concentration) was injected. The capillary wave frequency decreases rapidly on injection, and after attaining its minimum value, it increases... 

It follows from the above that the mechanism for electrical potential oscillation across the octanol membrane in the presence of SDS would most likely be as follows dodecyl sulfate ions diffuse into the octanol phase (State I). Ethanol in phase w2 must be available for the transfer energy of DS ions from phase w2 to phase o to decrease and thus, facilitates the transfer of DS ions across this interface. DS ions reach interface o/wl (State II) and are adsorbed on it. When surfactant concentration at the interface reaches a critical value, a surfactant layer is formed at the interface (State III), whereupon, potential at interface o/wl suddenly shifts to more negative values, corresponding to the lower potential of oscillation. With change in interfacial tension of the interface, the transfer and adsorption of surfactant ions is facilitated, with consequent fluctuation in interface o/ wl and convection of phases o and wl (State IV). Surfactant concentration at this interface consequently decreased. Potential at interface o/wl thus takes on more positive values, corresponding to the upper potential of oscillation. Potential oscillation is induced by the repetitive formation and destruction of the DS ion layer adsorbed on interface o/wl (States III and IV). This mechanism should also be applicable to oscillation with CTAB. Potential oscillation across the octanol membrane with CTAB is induced by the repetitive formation and destruction of the cetyltrimethylammonium ion layer adsorbed on interface o/wl. Potential oscillation is induced at interface o/wl and thus drugs were previously added to phase wl so as to cause changes in oscillation mode in the present study. 

Oil/water interfacial tensions were measured for a number of heavy crude oils at temperatures up to 200°C using the spinning drop technique. The influences of spinning rate, surfactant type and concentration, NaCI and CaCI2 concentrations, and temperature were studied. The heavy oil type and pH (in the presence of surfactant) had little effect on interfacial tensions. Instead, interfacial tensions depended strongly on the surfactant type, temperature, and NaCI and CaCL concentrations. Low interfacial tensions (<0.1 mN/m) were difficult to achieve at elevated temperatures. 

At a given NaCI concentration, an increase in temperature resulted in an increase in interfacial tension. In contrast, for a narrow range of CaCI concentrations, interfacial tensions decreased with increasing temperatures. Changes of the amphiphile at the oil/water interface accounted for some of the experimental observations. Since the extent of oil desaturation is dependent on interfacial tension, the tension data could be used to assess the ability of surfactants to reduce oil saturations in the reservoir for application of surfactants and foams to thermal recovery processes. 

The interfacial tension behavior between a crude oil (as opposed to pure hydrocarbon) and an aqueous surfactant phase as a function of temperature has not been extensively studied. Burkowsky and Marx T181 observed interfacial tension minima at temperatures between 50 and 80°C for crude oils with some surfactant formulations, whereas interfacial tensions for other formulations were not affected by temperature changes. Handy et al. [191 observed little or no temperature dependence (25-180°C) for interfacial tensions between California crude and aqueous petroleum sulfonate surfactants at various NaCI concentrations. In contrast, for a pure hydrocarbon or mineral oil and the same surfactant systems, an abrupt decrease in interfacial tension was observed at temperatures in excess of 120°C 1 20]. Non ionic surfactants showed sharp minima of interfacial tension for crude... 

---