Effect of Polymer Polarity on the Adsorption of Sodium Lauryl Sulfate at Latex/Water Interfaces
The effect of polymer polarity on surfactant adsorption from aqueous solution is considered. The analysis assumes that the surfactant adsorption at a polymer/water interface follows a Langmuir type adsorption isotherm and the free energy of adsorption is controlled by the interfacial tension of the interface. Limiting adsorption given by the area per molecule of surfactant at the CMC of the surfactant is related to the polymer/water interfacial tension and the polarity of the polymer surface, calculated from the polar and dispersion contributions to the polymer surface energy. Available data on the area per molecule of sodium lauryl sulfate on various polymer surfaces and some preliminary data on the effect of oil phase polarity on the saturation adsorption of sodium lauryl sulfate at oil/water interfaces have been used to test the above adsorption model.
The analysis is used to interpret some of the observations relating to surfactant adsorption encountered in the emulsion polymerization of polar monomers and particle size determination of latices by the soap titration method.
The nature of the interface has been shown to exert a considerable influence on various interfacial phenomena such as interfacial tension1,2, micellization and solubilization , free energy for the transfer of a methylene group from water to organic liquids , surfactant adsorption5,6, formation and stability of polar emulsion particles7,8,9.
One area wherein polarity of the interface plays a significant role is in the emulsion polymerization of polar monomers. The effects of polarity on surfactant adsorption and the emulsion polymerization kinetics of polar monomers has received a lot of attention in recent years10 , 11 , 12 , 13 . The polarity of the monomer water and polymer-water interface are believed to govern surfactant adsorption and particle stabilization7,8,9, monomer swelling of growing particles and morphology7,14,15, and other significant features of emulsion polymerization.
Due to the central role of interfacial polarity on the emulsion polymerization of polar monomers, several workers have attempted to correlate the polarity of monomer (polymer)-water interface to monomer water solubility9,10 ,11 and monomer/water interfacial tension8 10 . Paxton6 suggested that the area per molecule of a surfactant on a polymer surface can give some useful information as to the polarity of the polymer surface. Use of monomer water solubility to estimate polarity is simple and convenient but does not provide any insight into the factors responsible for the various interfacial phenomena mentioned above. Use of monomer/water interfacial tension to estimate polarity is probably a step in the right direction. However, the polar interactions at the monomer/ water interface are probably different from those at the corresponding polymer/water interface due to the polarity associated with the unsaturation in the monomer molecule. Further, interactions such as surfactant adsorption at a fluid monomer/water interface may be more labile than at a solid polymer/water interface. Hence, we thought it would be worthwhile to expand on Paxton’s suggestion and relate the observed differences in the adsorption of a surfactant molecule on various polymer surfaces to the characteristics of the polymer surface, namely, the polar and dispersion forces acting at the polymer/water interface. In such an approach the surfactant molecule is used as a probe to investigate the nature of the polymer surface.
In this paper, we plan to discuss the energetics involved in the adsorption of a surfactant molecule from an aqueous medium at a polymer/water interface and develop an adsorption model that relates the saturation adsorption of the surfactant molecule to the polarity of the polymer surface. Polarity of various polymer surfaces and polymer/water interfacial tensions are calculated from Wu’s data16 . Theoretical predictions relating the area per molecule of a surfactant to interfacial tensions of polymer/water interfaces and polarity or polymer surfaces are tested using the available area-per-molecule data for sodium lauryl sulfate on various polymer surfaces. In limited cases, adsorption of sodium lauryl sulfate on various polymer surfaces is related to the measured interfacial tension of corresponding monomer/water interfaces. Preliminary data on the effect of oil phase polarity on the saturation adsorption of sodium lauryl sulfate at oil/water interfaces is analyzed using the above adsorption model.
KeywordsInterfacial Tension Surfactant Molecule Emulsion Polymerization Polymer Surface Sodium Lauryl Sulfate
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