This paper presents the effect of negatively charged silica nanoparticles (NPs) on the interfacial tension of the n-hexane-water system at variable concentrations of four different surfactants, viz., an anionic surfactant, sodium dodecyl sulfate (SDS), a cationic surfactant, cetyltrimethylammonium bromide (CTAB), and two nonionic surfactants, Tween 20 and Triton X-100 (TX-100). The presence of negatively charged silica nanoparticles is found to have a different effect depending on the type of surfactant. In the case of ionic surfactants, SDS and CTAB, silica NPs reduce the interfacial tension of the system. On the contrary, for nonionic surfactants, Tween 20 and TX-100, silica NPs increase the interfacial tension. The increasing/decreasing nature of the interfacial tension in the presence of NPs is well supported by the calculated surface excess concentrations. The diffusion kinetic control (DKC) and statistical rate theory (SRT) models are used to understand the behavior of dynamic interfacial tension of the surfactant-NP-oil-water system. The DKC model is found to describe the studied surfactant-NP-oil-water systems more aptly.

Nonionic surfactants are advantageous in a diversified range of applications from household cleaners, laundry detergents, and shampoo to paints, coatings, and food emulsifiers because of their low CMC and surface tension values over the ionic surfactants. Nonionic surfactants, in general, are very useful in mixed surfactant systems because of their electrical neutrality. Among the similar class of nonionic surfactants, structural difference is important in the performance. In this study, we report on the adsorption and wetting behavior of two nonionic surfactants (TX-100 and Igepal CO-630) having the same head group but structurally different tail groups. The kinetics of adsorption follows a pseudo-second-order kinetic model and a Langmuir-type isotherm for both the surfactants. The change in contact angle with the concentration of surfactant follows a trend similar to that for adsorption onto a PTFE surface. At low surfactant concentration, Igepal CO-630 shows a slightly higher adsorption density and better wetting properties than TX-100. Both surfactants show lower adsorption densities at the PTFEÀwater interface than at the airÀwater interface.

Interfacial and wetting behavior of a double chain cationic synthetic surfactant di-dodecyldimethyl ammonium bromide (DDAB), a natural surfactant (Shikakai), and their mixtures of five different ratios have been studied by surface tension and dynamic contact angle measurements. Pure Shikakai has higher surface tension and contact angle values at critical micelle concentration (CMC) than DDAB, indicating that it is inferior to DDAB and also commonly used synthetic surfactants. Addition of DDAB to Shikakai gives a gradual lowering of the CMC, surface tension and contact angle values at the CMC. When the concentration of DDAB is $50 mol% the final surface tension and contact angle values are very close to those of pure DDAB. The mixed surfactant solutions show strong non-ideal solution behaviour because of the interaction between the two surfactants. As the wetting properties of a natural surfactant are enhanced in the presence of a synthetic surfactant, the use of plant-synthetic mixed surfactant systems may be useful in wetting and in several other applications. At the same time the consumption of synthetic surfactant can also be reduced.

The wetting of hydrophobic and hydrophilic solid surfaces by surfactant solutions is an important research topic recently because of its profound practical applications. The wettability of two double-chain surfactants (cationic, didodecyldimethylammonium bromide or DDAB, and anionic, aerosol OT or AOT) solutions on poly(tetrafluoroethylene) (PTFE) and glass surfaces has been investigated here. Different physicochemical parameters such as critical micelle concentration (CMC) and surface tension, contact angle, surface excess at air−water and solid-water interfaces, work of adhesion, and free energy of wetting have been estimated for two double-chain surfactants solutions and compared with the reported results of single-chain surfactants. The double-chain surfactant solutions showed maximum lowering of surface tension values (24.36 and 26.35 mN/m for DDAB and AOT, respectively) and a change in contact angle values from pure water on PTFE (∼38°for DDAB and AOT) and glass (∼26.5 and 24°for DDAB and AOT, respectively) surfaces compared to the conventionally studied single-chain surfactants. The surfactant molecules mostly formed a monolayer adsorption on both surfaces during the wetting process. The surface excess values of both of the surfactants on PTFE−water and glass−water interfaces are 0.759 and 0.850 times lower than that of the air−water interface, respectively.

The rheological behavior of pyrophyllite−water clay slurry has been studied in the presence of three different types of surfactants: anionic, cationic, and nonionic. The slurry is non-Newtonian and thixotropic in nature, and the hysteresis area increases with increasing clay concentration. In the presence of acidic pH, surface potential is low and viscosity is high; the opposite is true for alkaline pH. The rheological behavior is different in the presence of three surfactants. The viscosity highly depends on the types of surfactant adsorbed on the clay surface and surface potential after the adsorption. The d(001) lattice spacing of the pyrophyllite clay does not change after the adsorption of surfactants, indicating that the surfactants are not adsorbing inside the clay spacing.