Particle-water interfacial tension

This paper compares the swelling of monodisperse polystyrene and polymethyl methacrylate latexes with their monomers and the estimated particle-water interfacial tensions with the theoretical curves from Morton s equation. A new model which takes into account the effect of water dissolved in the swollen particles and in the monomer phase on the swelling of relatively hydrophilic systems is presented. 

Dobler F, Pith T, Lambla M, Holl Y. Coalescene Mechanisms of polymer colloids. 1. Coalescence under the influence of particle-water interfacial tension. J CoU Interface Sci 1992 152 ... 

As discussed above, surface-active agents (or surfactants) are generally required to stabilize the colloidal dispersions. Surfactant molecules show a strong tendency to be adsorbed onto the particle surfaces, reduce the particle-water interfacial tension, and thereby help to impart adequate stabilization to the colloidal system. The Gibbs adsorption equation that is often used to describe the concentration of surfactant adsorbed at the particle surfaces is ... 

Here Vp is the volume fraction of polymer (related to the conversion), X is the number average degree of polymerisation of the polymer, x is the Flory-Huggins interaction parameter between the monomer and the polymer, R is the gas constant and T the temperature. Um is the molar volume of the monomer, y is the particle-water interfacial tension and To is the radius of the unswollen micelles, vesicles and/or latex particles. [M]a is the concentration of monomer in the aqueous phase and [M]a,sat the saturation concentration of monomer in aqueous phase. Figure 3.3 shows the contributions of the different terms of Equation 3.10 to the Vanzo equation. For a more detailed discussion see also Section 4.2 and Figure 4.5. 

Physical-chemical features of WIW emulsions compared to oil-in-water (OIW) and water-in-oil WIO) emulsions are low interfacial tension, interfacial layers of low biopolymer concentration, interfacial adsorption of lipids and high deformability of aqueous dispersed particles. Low interfacial tension in WIW emulsions reflects similar compositions of coexisting phases, where water and the biopolymers are partially cosoluble. The low-density interfacial layer is due to a trend of incompatible biopolymers to have surroundings of the same type. One more feature of WIW emulsions is a great difference in concentration between coexisting phases. This is due to the competition between the biopolymers for space in solution. The competition can be characterized (Figure 3.5) by the angle made by the tie-line with one of the concentration axes. 

Since the oil-water interfacial tension is at a minimum at the PIT, emulsions made at this temperature should have the finest particle size. The minimum work needed to create the emulsions is the product of the interfacial tension and the... 

Polymer/water interfacial tension and particle surface... 

The film formation process is extremely complex, and there are a number of theories — or more accurately, schools of theories — to describe it. A major point of difference among them is the driving force for particle deformation surface tension of the polymer particles. Van der Waals attraction, polymer-water interfacial tension, capillary pressure at the air-water interface, or combinations of the above. These models of the mechanism of latex film formation are necessary in order to improve existing waterborne paints and to design the next generation. To improve the rate of film fonnation, for example, it is important to know if the main driving force for coalescence is located at the interface between polymer and water, between water and air, or between polymer particles. This location determines which surface tension or surface energies should be optimized. 

Figure 4.3 shows several types of morphologies of the particles produced. The incompatibility of the seed polymer with the newly formed polymer results in phase separation and cluster formation. The size of the clusters increases by both polymerization and cluster aggregation. In addition, the clusters tend to reach an equilibrium morphology, which minimizes the interfacial energy of the system and depends on the polymer-polymer and polymer-water interfacial tensions [40-44]. 

It is believed that the formation of a microemulsion can enhance detergent action since the oil-water interfacial tension can be lowered considerably, which facilitates solubilization of oily dirt particles by surfactant. The microemulsion is of Winsor type III (Section 3.13.2), with small amounts of surfactant forming a middle phase microemulsion in equilibrium with excess oil and water. The oil-water interfacial tension is a minimum at the phase inversion temperature (PIT) of an oil-water-surfactant system, so it is desirable to optimize the properties of the detergent mixture so that the system is close to the PIT at the washing temperature. Microemulsions made from mixtures of nonionic surfactants are used in hard surface cleaning products. Usually they are sold in concentrated form and diluted prior to use. 

As shown in the simulation of the morphology of hybrid monomer-clay miniemulsion droplets (Figures 10.2 and 10.4), the encapsulation of clay platelets is possible provided that the clay/water interfacial tension is very high (superhydrophobic clay) and that the clay/monomer interfacial tension is low (high compatibility between clay and monomer). Another aspect that the simulations show is that the size of the clay platelets might also play a role in the encapsulation of the clay in the monomer droplets (polymer particles). 

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]. 

Emulsifiers assist the stabilizing hydrocolloids in controlling crystal structure. They accentuate the function of the homogenizer in reducing the size of the fat globules. They also reduce the interfacial tension between the fat and water phases of the mix. The result is smaller ice particles and air cells when the mix is frozen and a smoother and creamier finished product. 

Thus, in the relatively simple case of oil in water emulsions, where a surface active agent such as a soap is used as the emulsifying agent, it is known that the soap adsorbed on the surface of the oil particles decreases the interfacial tension, thus stabilizing the emulsion. The adsorbed soap ions also give a net electrostatic charge to the dispersed oil droplets, serving to repel other oil droplets, with the net effect that flocculation is hindered (and stability is increased). It is even possible to measure the amount of adsorbed soap ions and to calculate the values of the surface potential. 

---