Surfactant adsorption contact interactions

The colloid probe technique was first applied to the investigation of surfactant adsorption by Rutland and Senden [83]. They investigated the effect of a nonionic surfactant petakis(oxyethylene) dodecyl ether at various concentrations for a silica-silica system. In the absence of surfactant they observed a repulsive interaction at small separation, which inhibited adhesive contact. For a concentration of 2 X 10 M they found a normalized adhesive force of 19 mN/m, which is small compared to similar measurements with SEA and is probably caused by sufactant adsorption s disrupting the hydration force. The adhesive force decreased with time, suggesting that the hydrophobic attraction was being screened by further surfactant adsorption. Thus the authors concluded that adsorption occurs through... 

Recently, it has been found that aqueous solutions of two different hydrocarbon chain surfactants can also show superspreading on highly hydrophobic substrates (Rosen, 2002 Zhou, 2003). In these mixtures, the two different hydrocarbon-chain surfactants also interact to produce synergistic enhancement of the total surfactant adsorption at the hydrophobic solid-aqueous solution interface relative to that at the air-aqueous solution interface, and this is accompanied by an enhanced rate of reduction of the contact angle (Zhou, 2003). SF values for these mixtures are also listed in Table 6-3. 

Surfactant adsorption on saltlike minerals, such as calcite and dolomite, is a more complex process and is less understood than adsorption on oxide surfaces. These minerals are relatively soluble and when in contact with an aqueous medium develop an interfacial region of complex composition (41—43). In addition to the two mentioned mechanisms of adsorption, covalent bonding, salt formation between surfactant and lattice ions at the solid surface, ion exchange of surfactant with lattice ions, and surface precipitation have been suggested as adsorption mechanisms (36, 43—47). The dissolution products of sparingly soluble minerals may interact with the surfactant, precipitate or adsorb at the solid surface, or lead to mineral transformations that affect surface composition and electrochemical properties (46, 48—52). All these factors can be expected to influence surfactant adsorption. 

Surfactant—solid and surfactant—surfactant hydrophobic interactions lead to minimization of solid—water and surfactant-chain—water contact and are energetically favorable. Unlike hydrophilic surfaces, hydrophobic surfaces do not lead to significant structuring of interfacial water, and the interfacial water is displaced from the surface relatively easily by the surfactant molecules. Consequently, surfactant adsorption on hydrophobic surfaces has often been found to be higher than adsorption on the corresponding hydrophilic surfaces (39, 54, 56, 57, 59—62), provided aqueous phase salinity is low. 

The Gibbs elasticity characterizes the lateral fluidity of the surfactant adsorption monolayer. For high values of the Gibbs elasticity the adsorption monolayer at a fluid interface behaves as tangentially immobile. Then, if two oil drops approach each other, the hydro-dynamic flow pattern, and the hydrodynamic interaction as well, is the same as if the drops were solid particles, with the only differenee that under some conditions they could deform in the zone of contact. For lower values of the Gibbs elastieity the... 

However, the organization of water molecules at KQ and RbQ crystal surfaces shows obvious randomness because of the relatively weak water-surface ion interactions, and these weak interactions are confirmed by the finite contact angles (Hancer et aL 2001), as well as by the large interfacial water diffusion coefficient and the short residence times from MD simulation analysis. The less-ordered interfacial water structure being an energetically less stable state suggests that surfactant adsorption would be possible with sufficient hydrophobicity created for bubble attachment and subsequent flotation. 

At the same time, the similarity between the solid and liquid phases, as evidenced by good wettability and high values of the work of wetting and the work of adhesion, suggests that the interfacial energy 012= Osl is much lower than the OsGof the same glass specimen in air. In Chapter 2, we will further address contact interactions in the case of surfactant adsorption. 

The described experimental method for measuring the contact interactions between solid particles influenced by surfactant adsorption from various electrolyte solutions allows one to observe transition from lyophilicity to lyophobicity and to study the role of the electrolyte in this transition. 

C. Contact Angie and Interaction Between Surfactant Adsorption Monolayers... 

The modification of the fluid interfaces due to surfactant adsorption strongly influences the interactions between fluid particles (droplets, bubbles) in dispersions. Frequently a thin liquid film is formed in the zone of contact of two fluid particles. The contact angle at the periphery of such a film is a measure for the interaction of the two opposite surfactant adsorption monolayers. When the latter adhere to each other, a hysteresis of the contact angle is observed, irrespective of the fact that the fluid interfaces are molecularly smooth. The properties of the thin liquid films are important for the flocculation in dispersions and the deposition (attachment-detachment) of particles at surfaces see Sec. V. 

---