Water interface, interfacial tension

Emulsifiers are a single chemical substance, or mixture of substances, that lower the tension at the oil-water interface (interfacial tension) and have the capacity for promoting emulsion formation and short-term stabilisation. 

The oil-water dynamic interfacial tensions are measured by the pulsed drop (4) technique. The experimental equipment consists of a syringe pump to pump oil, with the demulsifier dissolved in it, through a capillary tip in a thermostated glass cell containing brine or water. The interfacial tension is calculated by measuring the pressure inside a small oil drop formed at the tip of the capillary. In this technique, the syringe pump is stopped at the maximum bubble pressure and the oil-water interface is allowed to expand rapidly till the oil comes out to form a small drop at the capillary tip. Because of the sudden expansion, the interface is initially at a nonequilibrium state. As it approaches equilibrium, the pressure, AP(t), inside the drop decays. The excess pressure is continuously measured by a sensitive pressure transducer. The dynamic tension at time t, is calculated from the Young-Laplace equation... 

All molecules that, when dissolved in water, reduce surface tension are called surface-active substances (e.g., soaps, surfactants, detergents). This means that such substances adsorb at the surface and reduce surface tension. The same will happen if a surface-active substance is added to a system of oil-water. The interfacial tension of the oil-water interface will be reduced accordingly. Inorganic salts, on the other hand, increase the surface tension of water. 

As far as water/toluene interfacial tension is measured, it appears that the saturation of the interface is reached more quickly with PTBS (P0)2 star-shaped block copolymers (Table VII) this molecular architecture seems to be more efficient to fill in the interface (3 0 ... 

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

PVA can lower the surface tension of water, reduce interfacial tension at an oil/water interface and enhance tear film stability. These together with ease of sterilization, compatibility with a range of ophthalmic dmgs and an apparent lack of epithelial toxicity have led to the widespread use of PVA as a drag delivery vehicle and a component of artificial tear preparations. 

In conclusion we will note that the main difference between aqueous emulsion films and foam films involves the dependences of the various parameters of these films (potential of the diffuse double electric layer, surfactant adsorption, surface viscosity, etc.) on the polarity of the organic phase, the distribution of the emulsifier between water and organic phase and the relatively low, as compared to the water/air interface, interfacial tension. 

A number of water-soluble polymers will cause phase separation when present together at concentrations of a few percent. The most widely used polymers are polyethylene glycol (PEG) and dextran. Proteins, other macromolecules, and cell components such as mitochondria distribute in the phases or collect at the interface. Proteins are destabilized at organic solvent/water interfaces, but when each solvent is water, the interfacial tension is negligible. Some salts such as potassium phosphate will also induce phase separation when a polymer is present, but the salt concentration must be high. Two-phase aqueous systems provide a mild method for purification of proteins, and scale-up to large volumes presents no engineering problems. The polymers... 

Contrasting the behavior close to small apolar solutes, water behaves differently at an extended (planar) interface. Here, the thermodynamic features are mostly governed by water s interfacial tension, which is essentially enthalpic in nature (weakening with increasing temperature). Consequently, at some length-scale a crossover has to occur (85, 86) from an entropy to an enthalpy dominated solvation behavior. Recent studies indicate that this transition appears at a length-scale significantly below 1 nm (87, 88). 

There is little applicability of this mechanism to stabilization by small particles. For instance, using the values exemplified earlier, the energy required to remove a particle with a diameter of 200 nm (approximate actual size of the particles in the above study) and a contact angle of 150° from a water/toluene interface (interfacial tension = 0.036 N/m) is 4927 kT, while a 5 nm particle in the same system has a binding energy of 3 kT. Therefore, a 200 nm particle will be irreversibly bound to the interface, while a 5nm particle should not be held at the interface and if stabilization occurs, it must take place by a different mechanism. 

From the above, it is clear that a pre-requisite of low water/oil interfacial tensions is the complete saturation of the water-rich and oil-rich phases as well as the water/oil interface by surfactant molecules. Of course, this pre-requisite is fulfilled if one of the phases considered is a microemulsion. Furthermore, since the pioneering work of Lang and Widom [81] it is known that if a system is driven through phase inversion the interfacial tensions may become ultra-low. However, about 20 years ago, a number of experimental investigations were devoted to clarifying the origin of the ultra-low interfacial tensions [15, 17, 39, 71, 81-85]. In order to understand this correlation between phase behaviour and interfacial... 

Figure 1.15 Water/oil interfacial tension crab (plotted on log-scale) as function of the relevant tuning parameter, (a) Variation of crab with temperature T, exemplarily shown for the water-n-octane-C- oE4 system [17]. (b) Variation of crab with the composition of the amphiphilic film 8yi in the quaternary system hbO-n-octane-fS-CsG-i-CsEo at T = 25°C [90]. Both systems show that the water/oil interfacial tension runs through a distinct minimum in the middle of the three-phase region. The full line is calculated considering the bending energy difference between a curved amphiphilic film in the microemulsion and the flat film of the macroscopic interface [96].

A typical example for a stirred two-phase system with a volume fraction of 30 vol.% organic phase dispersed in water, an interfacial tension of 25 mN m-1 and a specific power input of 0.5 W L 1 shows a droplet diameter in the range of 250 pun and a specific interface of about 10 m2 L 1. These dimensions maybe estimated from simple empirical correlations between the Sauter mean diameter of the dispersed phase (zf2.3) and the characteristic Weber number (We). In case of turbulent mixing the following correlation is proposed in the literature for calculation of the mean diameter of dispersed droplets [24]... 

Water-solvent interfacial tension lowering [Figure 18.1(b)]—when the adsorption of a proper surfactant at the liquid-solvent interface results in a marked reduction of the water-displacement liquid tension. In effect, the skin on the water droplet is weakened, the droplet necks down in the presence of agitation, and a significant fraction of primary water droplets is broken away and floated to the surface of the solvent batch due to buoyant forces. 

The adsorption of surfactant molecules at an interface decreases the interfacial tension. The decrease of the water-air interfacial tension explains the foaming property. The addition of a surfactant into a biphasic liquid system renders emulsion formation possible by the decrease of the liquid-liquid interfacial tension. Wetting and detergency are two important... 

The presence of the surfactant SDS influences nanocapsule formation in two ways With increasing SDS cmicentration, the nanocapsules become smaller. At the same time, with decreasing size of the nanocapsule, the coverage of the nanoobjects (before evaporation of the solvent, the nanodroplets after the evaporatimi, the nanoparticles or nanocapsules) by SDS increases, leading to a decrease in the interfacial tension of droplel/water and copolymer/water. The interfacial tension between copolymer and water ( 0.035 N/m) without surfactant is considerably smaller than the interfacial tension between hexadecane and water ( 0.054 N/m). Thus, in the case of a low concentration of SDS and subsequent coverage of the nanoobjects by SDS, the interfacial tension of the copolymer/water interface is lower than that of the hexadecane/water interface therefore as the thermodynamically most stable structure, nanocapsules are expected to be formed (Fig. 54a). 

Wetting of soil and soiled surface The soiled surface is brought in contact with the detergent solution in water Reducing interfacial tension between soil/water and soiled surface/water interfaces Assist in adsorption and penetration of detergent into vulnerable parts... 

In particular, this parameterization approach has shortcomings in reproducing correctly the phase diagram of water, including, for example, water/vapor interfacial tension, and, as a consequence, it has flaws for what pertains the investigation of surfactant properties of lipid monolayer at the airAvater interface.Improvements of the MARTINI water model have been proposed via the use of a polarizable force field, but polarizable CG force fields remain poorly investigated so far in the field of CG simulations. 

A. P. dos Santos and Y. Levin, Langmuir, 28,1304 (2012). Ions at the Water-Oil Interface Interfacial Tension of Electrolyte Solutions. 

Assume that an aqueous solute adsorbs at the mercury-water interface according to the Langmuir equation x/xm = bc/( + be), where Xm is the maximum possible amount and x/x = 0.5 at C = 0.3Af. Neglecting activity coefficient effects, estimate the value of the mercury-solution interfacial tension when C is Q.IM. The limiting molecular area of the solute is 20 A per molecule. The temperature is 25°C. 

IHP) (the Helmholtz condenser formula is used in connection with it), located at the surface of the layer of Stem adsorbed ions, and an outer Helmholtz plane (OHP), located on the plane of centers of the next layer of ions marking the beginning of the diffuse layer. These planes, marked IHP and OHP in Fig. V-3 are merely planes of average electrical property the actual local potentials, if they could be measured, must vary wildly between locations where there is an adsorbed ion and places where only water resides on the surface. For liquid surfaces, discussed in Section V-7C, the interface will not be smooth due to thermal waves (Section IV-3). Sweeney and co-workers applied gradient theory (see Chapter III) to model the electric double layer and interfacial tension of a hydrocarbon-aqueous electrolyte interface [27]. 

Neumann and co-workers have used the term engulfrnent to describe what can happen when a foreign particle is overtaken by an advancing interface such as that between a freezing solid and its melt. This effect arises in floatation processes described in Section Xni-4A. Experiments studying engulfrnent have been useful to test semiempirical theories for interfacial tensions [25-27] and have been used to estimate the surface tension of cells [28] and the interfacial tension between ice and water [29]. 

Thus, adding surfactants to minimize the oil-water and solid-water interfacial tensions causes removal to become spontaneous. On the other hand, a mere decrease in the surface tension of the water-air interface, as evidenced, say, by foam formation, is not a direct indication that the surfactant will function well as a detergent. The decrease in yow or ysw implies, through the Gibb s equation (see Section III-5) adsorption of detergent. 

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