Surface tension surfactant-polymer systems

Once a micelle is stung, polymerization proceeds very rapidly. The particle can accommodate more monomer as its polymer content increases and the water-polymer interfacial surface increases concuirently. Tlie new surface adsorbs emulsifier molecules from the aqueous phase. This disturbs the equilibrium between micellar and dissolved soap, and micelles will begin to disintegrate as the concentration of molecularly dissolved emulsifier is restored to its equilibrium value. Thus the formation of one polymer particle leads to the disappearance of many micelles. The initial latex will usually contain about 10 micelles per milliliter water, but there will be only about 10 particles of polymer in the same volume of the final emulsion. When all the micelles have disappeared, the surface tension of the system increases because there is little surfactant left in solution. Any tendency for the mixture to foam while it is being stirred decreases at this time. 

The primary peculiarity of the system under consideration is the fact that increase of the quantity of surfactant does not always result in decrease of surface tension. Thus, the surface tension of polymer containing 1% of L-19 is higher than that of a polymer containing 0.05% or 0.5% of L-19. Additionally, increase of the surfactant content of the liquid oligomer always results in the decrease of the surface tension (naturally when the surfactant concentration in the system is lower than CCMF). 

The addition of surfactants and/or polymers to a foaming system can alter any or all of the above-mentioned system characteristics and therefore enhance the stability of the foam. They may also have the effect of lowering the surface tension of the system, thereby reducing the work required for the initial formation of the foam, as well as producing smaller, more uniform bubbles. 

It must be mentioned that although the surface tension method is facile and readily applicable to the study of a mixture of surfactant/polymer, it also has its limitation. Generally, the polymer used should be less surface active than the surfactant itself. Otherwise, interpretation of the data may be difficult in terms of Tj (cac, critical aggregation concentration) and T2 (apparent cmc) in spite of a clear indication from the curve of the interaction in such a system. For example, the surface activity of polypropylene glycol makes it difficult to interpret the surface-tension results [15]. Similar to a system of mixed surfactants, the composition of polymer is also a factor in the surfactant/polymer system. The case of poly(vinyl alcohol) (PVA) is more complicated than that of other synthetic polymers, since PVA is known to be prepared by hydrolysis of the acetate group of poly(vinyl acetate). The hydrolysis is only partially done hence acetate and hydroxyl groups are present. Therefore, consideration of the compositions of polymer is important for a reliable interpretation of experimental data. 

FIGURE 13.5 Schematic plots of surface tension for (a) pure surfactant and (b) mixed surfactant-polymer system. (From Mohsenipour, A.A., turbulent drag rednction by polymers, surfactants and their mixtures in pipeline flow, PhD thesis, University of Waterloo, Waterloo, Ontario, Canada, 2011.)... 

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

Here, again, we start from compressible SCFT formalism described in Section 2.2 and consider a model system in which bulk polymer consists of "free" matrix chains (Ny= 300) and "active" one-sticker chains (Na= 100). Flory-Huggins interaction parameters between various species are summarized in Table 1. This corresponds to the scenario in which surfactants, matrix chains, and functionalized chains are all hydrocarbon molecules (e.g., surfactant is a C12 linear chain, matrix is a 100,000 Da molecular weight polyethylene, and functionalized chain is a shorter polyethylene molecule with one grafted maleic group). The nonzero interaction parameter between voids and hydrocarbon monomers reflects the nonzero surface tension of polyethylene. The interaction parameter between the clay surface and the hydrocarbon monomers, Xac= 10 (a = G, F, A), reflects a very strong incompatibility between the nonpolar polymers and... 

In this paper, the results on solution and Interfaclal properties of a cationic celluloslcs polymer with hydrophobic groups are presented. Interaction of such polymers with added surfactants can be even more complex than that of "unmodified" polymers. In the past we have reported the results of Interactions of unmodified cationic polymer with various surfactants Investigated using such techniques as surface tension, preclpltatlon-redlssolutlon, viscosity, solubilization, fluorescence, electroklnetlc measurements, SANS,etc.(15-17). Briefly, these results showed that as the concentration of the surfactant Is Increased at constant polymer level significant binding of the surfactant to the polymer occurred leading to marked Increases In the surface activity and viscosity. These systems were able to solubilize water Insoluble materials at surfactant concentrations well below the CMC of polymer-free surfactant solutions. Excess surfactant beyond that required to form stoichiometric complex was found to solubilize this Insoluble complex and Information on the structure of these solubilized systems has been presented. 

Adsorption at the aqueous-air interface from binary solutions of proteins and surfactants can be conveniently followed by surface tension measurements in which the protein concentration is kept constant and the surfactant concentration is increased to concentrations in excess of the cmc. Studies of this type were first carried out not with proteins but with polyethylene oxide in the presence of SDS [70], and it was found that plots of surface tension as a function of surfactant concentration showed a number of interesting features in comparison with the surface tension concentration plot in the absence of polymer (Fig. 4). Very similar behavior to that first observed for the polyethylene oxide-SDS system has been found for protein-surfactant systems including bovine serum albumin plus SDS [67], gelatin plus SDS [52], and reduced lysozyme plus hexa (oxyethylene) dodecyl... 

The second reason for the anomalous change of surface tension of a solid polymer containing surfactant lies in the structural and conformational conversions of the polymer itself under the influence of the surfactant. Such factors as the increase of the polymer surface tension when surfactant is added cannot be explained by the surfactant adsorption on the polymer surface only (see Fig. 2.13). Later we will consider this in detail. As was noted above, if the rate of aggregation of the surfactant molecules is higher than or equal to the rate of polymerization, the system surface tension alters during polymerization in the same way as in the coiu-se of the equilibrium process. At a high rate of polymerization, the formation of micelles of the maximum possible size can be hindered by the rapid increase of the system viscosity. In this case, when an IS substance is applied the split into two phases is not observed and the system appears to be more oversaturated by surfactant than in the first case. 

Increasing surfactant concentration does not always result in decrease of the solid polymer sm-face tension. As Fig. 2.15 shows, the surface tension of the DEG-PEPA and ED-PEPA systems passes through a maximum with surfactant growth. 

These findings are confirmed by study of the thermodynamic parameters of mixing of the cured epoxy resin with OP-20. At 6-7% content of surfactant, corresponding to the maximum surface tension of the polymer, a kink and an area of decrease of the Flory-Huggins parameter are observed in the dependence of X2,s on the surfactant concentration. This anomalous dependence can be explained in terms of the rearrangement of the intermolecular bonds in the polymer-surfactant system. With ED-20 initial resin, there are no extrema on the curve. Alteration of the macromolecular conformation affects the supermolecular structure of the polymer. Adding surfactant to ED-20 resin changes the form and causes a noticeable decrease of the size of the polymeric supermolecular formations. 

Increase of the amount of surfactant in the system does not always result in decrease of the polymer surface tension. Thus, siu-face tension decreases to the maximum extent at an L-19 content of 0.92%. Increasing the amount of surfactant to 1.9% results in a 2-fold increase of the polymer surface tension. A similar phenomenon is observed in studies of epoxy systems (see Fig. 2.15). In this case the surface tension increase also seems to be related to the surfactant structurizing effect. 

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