Adsorption of Mixed Surfactants

Adsorption of binary surfactant mixtures at a solid-solution interface has been exploited commercially in many applications. In spite of the widespread use of hydrocarbon surfactant-fluorinated surfactant mixtures, very few articles have been published on their adsorption at a solid-solution interface. 

The interaction of hydrocarbon and fluorocarbon surfactants on the surface of dispersed particles has been studied through a flocculation and redispersion process [65-67]. Dispersions of positively charged particles can be flocculated with an anionic surfactant. An excess of the anionic surfactant forms a bilayer on the particle surface and causes redispersion of the flocculated sol. This flocculation reversal was used to study the interaction between mixed surfactants on a solid surface. A dispersion of iron(ITI) oxide hydrate particles was flocculated with an anionic hydrocarbon or fluorocarbon surfactant at pH 3.5, where the sols had a positive zeta potential. Subsequently, a second fluorocarbon or hydrocarbon surfactant was added to the flocculated sol. The extent of redispersion depended on the interaction between the two surfactants on the solid particle surface. 

Changes in zeta potential and turbidity of iron(III) oxide hydrate sols flocculated by sodium dodecyl sulfate (SDS) are shown in Fig. 5.9. When SDS was 

Changes in zeta potential and turbidity of iron(III) oxide hydrate sols flocculated with lithium perfluorooctanesulfonate (LiFOS) are shown in Fig. 5.11. The nonionic surfactants NF7 and NP7.5 redispersed the sols. However, the anionic hydrocarbon surfactant LiDS (lithium dodecyl sulfate) had no significant effect. Accordingly, sols flocculated by LiDS were redispersed by a nonionic surfactant, NF7, but not by the anionic surfactant LiFOS (Fig. 5.12). 

Esumi et al. [65-67] explained the flocculation and redispersion mechanisms by adsorption processes. Low concentrations of anionic surfactants neutralize the positive charge of iron(III) oxide hydrate sols and cause flocculation. The adsorbed anionic surfactant is oriented with its hydrophilic groups toward the particle surface and the hydrophobic groups toward water. If the second addition of a surfactant results in adsorption on the flocculated sols, the sols redisperse. The adsorption is caused by hydrophobic interactions and the surfactant is oriented with its hydrophilic groups toward water. 

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The adsorption of mixed surfactants at the air—water interface (monolayer formation) is mechanistically very similar to mixed micelle formation. The mixed monolayer is oriented so that the surfactant hydrophilic groups are adjacent to each other. The hydrophobic groups are removed from the aqueous environment and are in contact with other hydrophobic groups or air. Therefore, the forces tending to cause monolayers to form are similar to those causing micelles to form and the thermodynamics and interactions between surfactants are similar in the two aggregation processes. 

It would be of scientific interest to study the adsorption of mixed surfactant systems showing positive deviations from ideality, as has been discussed for mixed micelles and monolayers. 

Only in fundamental studies the adsorption of pure model surfactants and proteins can be investigated. Under practical conditions the adsorbing species are typically mixtures of components of different surface activity. Often mixtures are even used on purpose to reach special effects. This makes a deeper understanding of the adsorption process from solutions of surfactant mixtures important. A rather simple model for the adsorption of mixed surfactants at a liquid interface is presented here and its capacity is demonstrated on the basis of surface tension data for... 

B. Observations of hydrophobic interactions Adsorption with Attractive Electrostatic Interactions Adsorption with Repulsive Electrostatic Interactions Adsorption of Mixed Surfactants... 

Calculation examples of mixed surfactant adsorption The solid chosen as the model adsorbent was made up of a natural sand (specific area =380 cm2/g) mixed with 5% clay (Charentes kaolinite with specific area = 26.8 m2/g). This material was taken as a model of clayey sandstone reservoirs. 

Minssieux L., "Method for Adsorption Reduction of Mixed Surfactant Systems", Proc. 4th. Eur. EOR Symp., 1987, p. 293. 

When two similarly structured anionic surfactants adsorb on minerals, the mixed admicelle approximately obeys ideal solution theory (jUL - Below the CMC, the total adsorption at any total surfactant concentration is intermediate between the pure component adsorption levels. Adsorption of each surfactant component in these systems can be easily predicted from pure component adsorption isotherms by combining ideal solution theory with an empirical correspond ng states theory approach (Z3). ... 

It is obvious from the data that the total surface adsorption of 1 1 mixed solution (I ) is less than the sum of the surface adsorptions of pure surfactant and RDH solutions. Similarly, rc,F (or TcioS ) is less than rc p-(or r ps") and Troh less than Froh (e.g., for 1 1 CyFNa-... 

Adsorption of Two Surfactants. We now denote a quantity valid at the onset of micellization in the equilibrium mixed surfactant solution by the superscript c. Thus, the chemical potential of surfactant i in the mixed solution or in the mixed surface phase at the onset of micellization is given by... 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [411]. For example, in some cases the addition of a small amount of non-ionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer, which is possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [25,110], In general, some very stable foams can be formed from systems in which a liquid crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid crystal phase is present, stable foams are not produced. In this connection foam phase diagrams may be used to delineate compositions that will produce stable foams [25,110],... 

Most detergents contain electrolytes, e.g. sulphate, bicarbonate, carbonate or citrate and the presence of these electrolytes increases the adsorption of anionic surfactants at the gas/liquid interface as already mentioned. This leads to a reduction of the surface tension at an equal solution concentration [7] and to a strong decrease of the cmc. The effect can be of several orders of magnitude. Similar to this are the effects of mixtures of surfactants with the same hydrophilic group and different alkyl chain length or mixtures of anionic and non-ionic surfactants as they are mostly used in detergency [8]. Mixtures of anionic and non-ionic surfactants follow the mixing rule (eqn. 3) in the ideal case ... 

The studies of adsorption layers at the water/alkane interface give excess to the distribution coefficient of a surfactant, which is a parameter of particular relevance for many applications. Theoretical models and experimental measurements of surfactant adsorption kinetics at and transfer across the water/oil interface will be presented. The chapter will be concluded by investigations on mixed surfactant systems comprising experiments on competitive adsorption of two surfactants as well as penetration processes of a soluble surfactant into the monolayer of a second insoluble compound. In particular these penetration kinetics experiment can be used to visualise separation processes of the components in an interfacial layer. 

All hydrocarbon mixed surfactant systems with dissimilar head groups, such as ionic/nonionic, ionic/ amphoteric, and anionic/cationic, tend to have increased adsorption relative to the pure component adsorption at the same surfactant concentration. This synergisin is analogous to the effect of mixed surfactant systems in forming low CMC surfactant mixtures. It is easier to form a mixed admicelie rather than a pure component admicelie, just as it is easier to form a mixed micelle. 

Stability depends upon so many things that it is easy to alter its value. However, in most eases the general phenomenology versus formulation and eomposition is valid. The presenee of aleohol, partieularly an intermediate-solubility alcohol, such as rec-butanol or ter-pentanol, or a mixture of propanol and butanol, tends to reduce the interfacial adsorption of the surfactant, thus reducing all associated effects, in particular the repulsion that eontributes to stabilization. It is worth noting that the use of mixed surfae-tant systems, which is often advised in emulsion making manuals, can be detrimental in some eases in which a selective partitioning of surfactant species takes plaee (191, 192), and little surfactant is left at the interfaee. 

There are different models developed in the past to describe the adsorption from mixed surfactant solutions, for example recently by Siddiqui and Franses (1997), Ariel et al. (1999), Mulqueen and Blankschtein (1999, 2000), Penfold et al. (2003). The simplest model is obviously a generalised Langmuir isotherm (for ideal behaviour in the bulk and at the interface) for mixtures of two surfactants 1 and 2 with similar partial molar surface area (O can be presented in the form (Fainerman et al. 2001)... 

The ion-pair theory was invoked by Dong and coworkers to explain the behavior of adsorption of various surfactants at coassanbled Fc-teminated alkanethiol-alkylth-iolphene thiol SAMs on gold [51]. It was proposed that the adsorption of surfactants at the mixed monolayer created an energy barrier that inhibited the diffusion of counter ions present in solution from diffusing into the adsorbed layer as compensation for the oxidation of the Fc to Fc ions [51]. The creation of a barrier at an interface can be tested with electrochanical impedance spectroscopy (EIS) where an increase... 

In this paper we examine the role of mixed surfactants in the demulsification of water-in-Leduc oil emulsion by application of the spreading rate method which is then correlated with the electroacoustic results and centrifugation. Microelectrophoresis using the reverse emulsion was also used to investigate the adsorption process. The results show both a very good correspondence between the various techniques and provide insight on the synergistic adsorption behavior of the hydrophobic and hydrophilic surfactants. 

Sugihara, G. Miyazono, A. Nagadome, S. Oida, T. Hayashi, Y. Ko, J-S. Adsorption and Micelle Formation of Mixed Surfactant Systems in Water. II A Combination of Cationic Gemini-type Surfactant with MEGA-10. /, Oleo Sci. 2003,52,449-461. 

Fluorinated siufactants have been evaluated for paper uses since die early 1960s [13, 95, 96]. Perfiuorooctyl sulfonamido ethanol-based phosphates were the first substances used to provide grease repellence to food contact papers [97-99]. Fluorotelomer thiol-based phosphates and polymers followed [100-102]. Since paper fibers and phosphate-based fluorinated surfactants are both anionic, cationic bridge molecules need to be used in order to ensure the electrostatic adsorption of the surfactant onto the paper fiber. These surfactants are added to paper through the wet end press where cellulosic fibers are mixed with paper additives before entering the paper forming table of a paper machine. This treatment provides excellent... 

This chapter described the basis principles involved in stabilization of dispersions by polymeric surfactants. The first part described polymeric surfactants and their solution properties. The second part described the adsorption of polymeric surfactants and their conformation at the interface. The methods that can be applied to determine the adsorption and conformation of polymeric surfactants were briefly described. The third part dealt with the stabilization mechanism produced using polymeric surfactants. Two main repulsive forces were considered. The first arises from the unfavorable mixing of the chains on close approach of the particles or droplets, when these chains are in good solvent conditions. This is referred to as mixing or osmotic repulsion. The second force of repulsions... 

FIGURE 2.6 (a) A comparison of the surface tension isotherms of isopropyl alcohol at the aqueous solution-air interface, Olq, and liquid-liquid interface between aqueous solution and the hydrocarbon phase, cJll. (b) Isopropanol is a weakly surface-active substance soluble in both phases complete mixing is observed once a certain critical concentration corresponding to is reached, when complete miscibility of two phases is observed the asymmetric nature of the driving force for the adsorption of a surfactant at the interface from the aqueous and hydrocarbon phases. 

The word surfactant is an abbreviation of the more descriptive term suiface-active agent. A surfactant is a substance which, even at low concentrations, effectively lowers the surface tension of its medium by selective adsorption on the interface. A surfactant can be a pure chemical compound or a mixture of homologs or different chemical compounds. The characteristic feature of surfactants is their efficiency in lowering surface tension. The surface tension of a liquid can be lowered by mixing it with another liquid of lower surface tension. For example, one part of ethanol added to four parts of water decreases the surface tension of water from 73 mN/m to below 40 mN/m. However, only 0.1% of a typical surfactant is needed for the same surface tension reduction. The efficiency of surfactants in lowering surface tension is related to selective adsorption of the surfactant at the interface. The adsorption, in turn, is a result of the amphiphilic nature of the surfactant. 

It is noteworthy and perhaps puzzling that very few fundamental articles have been published on the adsorption of mixed fluorocarbon and hydrocarbon surfactants at the liquid-solid boundary, in spite of its great practical importance. One reason for this apparent dormancy may be the proprietary nature of knowledge. The results of research conducted by industrial laboratories may be found in the patent literature (Chapter 8). 

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