Effect of Surfactant Adsorption

Surfactants lower the surface tension of water, y, and they adsorb at the solid/ liquid interface. A plot of versus log C (where C is the surfactant concentration) 

From the slope of the linear portion of the y — log C curve (just below the c.m.c.), one can obtain the surface excess (number of moles of surfactant per unit area at the L/A interface). Using the Gibbs adsorption isotherm. 

Most surfactants produce a vertically oriented monolayer just below the c.m.c. 

Smolders [14] suggested the following relationship for change of 0 with C, 

Since y v is always positive, then (d0/dhr C) wiU always have the same sign as the RHS of Equation (18.21). Three cases maybe distinguished (d /dln C) 0 Tgy TgL + Tlv cosd, when the addition of a surfactant improves wetting (d /dhi 

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Marches JR, NJ Russel, GF White, WA House (1991) Effects of surfactant adsorption and biodegradability on the distribution of bacteria between sediments and water in a freshwater microcosm. Appl Environ Microbiol 57 2507-2513. 

There are various cases of particle-interface interactions, which require separate theoretical treatment. The simpler case is the hydrodynamic interaction of a solid particle with a solid interface. Other cases are the interactions of fluid particles (of tangentially mobile or immobile interfaces) with a solid surface in these cases, the hydrodynamic interaction is accompanied by deformation of the particle. On the other hand, the colloidal particles (both solid and fluid) may hydrodynamically interact with a fluid interface, which thereby undergoes a deformation. In the case of fluid interfaces, the effects of surfactant adsorption, surface diffusivity, and viscosity affect the hydrodynamic interactions. A special class of problems concerns particles attached to an interface, which are moving throughout the interface. Another class of problems is related to the case when colloidal particles are confined in a restricted space within a narrow cylindrical channel or between two parallel interfaces (solid and/or fluid) in the latter case, the particles interact simultaneously with both film surfaces. 

These results are complicated by the fact that most experiments with drops involve the presence of surfactants which serve to stabilize droplets and aid in their formation. However, surfactant dynamics and their solubility also vary with temperature. Since the effect of surfactant adsorption can be more dramatic than that of temperature, the surfactant dynamics can greatly modify the effect of temperature gradients. Indeed anomalous thermocapillary flow was... 

If Vz,max = 1-89 X 10 cm/sec when pb = 0.01 poise, R = 52.3 x 10 cm, 0 = 00, and (5 = 0, compute the maximum velocity when J = 1 x 10 and = 2.77 poise. Also compute the velocity profile for each case. Compare these results. What is the effect of surfactant adsorption based upon this model ... 

In this section, we review the molecular theory of surface forces with special attention to the effect of surfactant adsorption and surfactant micelles on the interactions in the thin liquid films and between the particles in dispersions. 

Figure 3.10. Schematic representation of the role of the hydrophobe in determining the effectiveness of surfactant adsorption (a) n-alkyl—area determined by head group (b) branched or douhle-tailed—area determined hy bulk of tail relative to head (c) polyoxyethylene nonionic—area determined hy coiling of POE chain.

The surface forces apparatus (Section VI-3C) has revealed many features of surfactant adsorption and its effect on the forces between adsorbent surfaces [180,181]. A recent review of this work has been assembled by Parker [182]. 

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent ia which it is dissolved to a much greater extent than is expected from its concentration effects. This marked effect is the result of adsorption at the solution s iaterfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation ia the bulk of the solution, and orientation of the surfactant ions or molecules ia the micelles, which are caused by the amphipathic stmcture of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubiUty balance of the molecule. An efficient surfactant is usually relatively iasoluble as iadividual ions or molecules ia the bulk of a solution, eg, 10 to mol/L. 

In a foam where the films ate iaterconnected the related time-dependent Marangoni effect is mote relevant. A similar restoring force to expansion results because of transient decreases ia surface concentration (iacteases ia surface tension) caused by the finite rate of surfactant adsorption at the surface. 

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... 

The proposed mechanism of effect of surfactant and ultrasound is reported in Fig. 7.5. The long chain surfactant molecules attach to surface of nanoparticles due to physical adsorption. Only thin layer is adsorbed onto the CaC03 nanoparticles. Due to presence of ultrasound and use of surfactant will control the nucleation. Surfactant keeps the particles away from each other by preventing flocculation due to change in surface tension of reaction mass. The concentration of additives was changed from 0.2 to 1.0 g/L. Addition of 0.2 g/L tripolyphosphate shows the increase in the rate of precipitation which is determined from the Ca(OH)2 consumption. Polyacrylic acid shows the least rate of precipitation (0.115 mol/1), which... 

The presence of a pre-adsorbed nonionic polymer has almost negligible effects on surfactant adsorption except at high surfactant concentrations where surfactant adsorption is reduced. 

It is generally accepted that one of the attractive features of the microemulsion environment for materials synthesis is the stabilization of the produced particles by the microemulsion surfactants. However, in the specific case of alkoxide/ microemulsion systems, there have been no investigations into the manner in which this stabilization is effected. For example, when the particle size exceeds the microemulsion droplet size, are the particles expelled from the water pools, or do the particles rather induce the enlargement of the microemulsion water droplets There have been no investigations into the role of surfactant adsorption in the colloidal... 

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