Adsorption of cationic surfactants

A. M. Grancaric, T. Pusic, I. Soljacic, and V. Ribitsch, Influence of Electrokinetic Potential on Adsorption of Cationic Surfactants," Textile Chemist Colorist 29(12) 33 (1997). 

Figure 3. Variation of pH of the supernatant as a function of adsorption (pH not adjusted) (a) systems presented in Fig.2 (b) adsorption of cationic surfactant, BDDAB, and its polar head, BTAB, onto precipitated silica at 298 K.

Cetyltrimethylammonium bromide CH3(CH2)i5N(CH 3)381 is a typical cationic surfactant. Dissociation of this compound results in amphiphilic cations and small hydrophilic anions. This classification of surfactants is similar to the classification of strongly interacting compounds (other than surfactants) in Sections II and III into cations (Section II A), anions (Section II B) and electroneutral and zwitterionic organic compounds (Section III). The adsorption of anionic surfactants is indeed enhanced, when the adsorbent carries high positive surface charge (at low pH for materials hsted in Tables 3.1, and 3.3-3.5), and adsorption of cationic surfactants is more pronounced at high pH, and the adsoiption of nonionic surfactants is often rather insensitive to the pH. However, the mechanisms of surfactant adsorption, and experimentally observed adsorption isotherms of surfactants are very different from the compounds discussed in Sections II and III. 

Goloub, T.P. et al.. Adsorption of cationic surfactants on silica. Surface charge effects, Langmuir, 12, 3188, 1996. 

Adsorption in this manner may also account for the increased reactivity of wool cystine disulfide bonds to attack by alkali in the presence of cationic surfactants and their decreased reactivity in the presence of anionics (Meichelbeck, 1971). The adsorption of cationic surfactants onto the wool surface, which is negatively charged in an alkaline medium, can impart a positive charge to the surface, thus increasing its attraction for hydroxide and sulfite ions, with consequent increase in its rate of reaction with these ions. In analogous fashion the acid hydrolysis of peptide bonds in the wool is increased by the presence of anionic surfactants (which... 

The theory of the transport stage of elementary microflotation at strong surface retardation is confirmed by works of Reay Ratcliff (1975), Collins Jameson (1977) and Anfruns Kitchener (1976, 1977). Numerous confirmations of the possibility of contactless and collectorless microflotation, of the importance of overcoming or removing electrostatic barrier at microflotation and of the possibility of flotation even of hydrophilic particles through adsorption of cationic surfactant on bubble surface are presented in Section 10.5. 

Thus, it cannot be excluded that OH ions can serve as ionic collectors if all conditions are satisfied (sufficiently high pH, not too high electrolyte concentration and definite electrical surface properties of the particles). A quantitative evaluation of this phenomenon appears to be impossible because of lack of a DAL theory for large Reynolds numbers. At adsorption of cationic surfactants the absolute value of the potential F, of a water-air interface can be smaller than that of the particle Pjj i - Within the r.s.c. a significant decrease in T, is... 

Fuerstenau, D. W. and Herrera-Urbina, R.. Adsorption of cationic surfactants and the flotation of minerals, in Cationic Surfactants Physical Chemistry, Rubingh, D. N. and Holland, P. M. (Eds), Surfactant Science Series, Vol. 37, Marcel Dekker, New York, 1991, pp. 407-447. 

The cationic surfactants strongly adsorb on reservoir silicates or quartz, which can be lowered by incorporating nonionic surfactants in the formulation. The adsorption of cationic surfactants on quartz is usually about 0.5 x 10 mg/cm. On the other hand, the nonionic surfactants adsorb slightly on quartz surfaces, but adsorption increases with raising temperature. The nonionic surfactants adsorb more on oil-wetted than water-wetted reservoir silicate surfaces. 

A few general features of the surfactant adsorbate structure can be extracted from the information available. At potentials negative of the point of zero charge (PZC) on hydrophilic metal and carbon electrodes, cationic surfactants adsorb head down. Positive of the PZC, anionic surfactants adsorb head down. Nonionic surfactants may adsorb head down on hydrophilic electrodes on either side of the PZC. Adsorption of cationic surfactants may be head down even at potentials positive of the PZC. This may involve adsorbed anions such as chloride on the electrode. Surface aggregate structures above the CMC may include bilayers, surface micelles, or cylinders depending on the nature of the surfactant, the electrode surface, and the applied potential. 

Adsorption of cationic surfactants (cationic soaps) on silica has been studied by Ter-Minassian-Sarage (88) and by Bijsterbosch (89), who have shown that either a monolayer or a bi-layer may be formed. Similar complexities may exist with some micelle-forming dyes, and special conditions would have to be worked out for each adsorbate to eliminate confusion. Also, the adsorption of these ionic species is affected by pH and by aluminosilicate ion impurities on the silica surface. Use of cationic dyes and surfactants is generally restricted to rapid comparison of surface areas of a series of silica powders of the same type. 

Aloulou F, Boufi S, Belgacem MN, Gandini A. Adsorption of cationic surfactants and subsequent adsolubilization of organic compounds onto cellulosic fibers. Colloid Polym Sci 2004a 283 344-350. 

Therefore, using capillary electrokinetics one can in principle investigate the adsorption of cationic surfactant on a silica surface and explain the results obtained on the basis of Langmuir-type isotherms of adsorption. 

The adsorption of DDA on Silicagel R and precipitated silica powder (Fig. 3) was investigated at concentrations lower than 1 mmol/dm, i.e., well below its CMC (14.8 mmol/dm [38]). The adsorption isotherms show constant adsorption above the equilibrium concentration of 0.6 mmol/dm. The amount of DDA adsorbed is lower than that of HDP by one order of magnitude for both substrates. DDA is dissolved at low pH, and therefore the acidity of the systems was adjusted to pH 2 with HCl solution. In such an acidic medium, the surface of the adsorbents is weakly positively charged [15]. In spite of this, the adsorption of cationic surfactants occurs to a small degree. The occurrence of the second step of the isotherm for the precipitated silica can be explained in terms of the second layer or surface hemimicelle formation. 

Fig. 6 Schematic representation for the possible mechanism of adsorption of cationic surfactant on uncharged hydrophilic oxide surface via van der Waals interactions and H-bonding ...

X-ray diffraction control of adsorption of cationic surfactants on swelling layer silicates... 

In the present work, the emphasis of the experiments will be on the adsorption of cationic surfactants on kaolinite. Apart from the arguments given in the introduction this choice was motivated by the theme of the meeting it is likely that this system functions as a model for the adsorption of pesticides in soils. Moreover, we want to learn how far thermodynamics can take us with difficult systems. Because of our intention to analyze the date thermodynamically, adsorption isotherms under a variety of conditions (temperature, salt concentration) will be supplemented by directly measured enthalpies. 

In summary, the utility of micro-SERS spectroscopy for the evaluation of potential-dependent interfacial com-petititve and displacement reactions at chargwl surface has been demonstrated. The data obtained allow the determination of the chemical identity, structure, orientation, competitive and displacement adsorption of cationic surfactants and nitrophenol in the first adsorption layer. The examples of these measurements in the field of surfactants and organic pollutants reviewed in this article were selected to illustrate the sensitivity, molecular specificity of adsorption processes, accuracy, ease of substrate preparation, and manifold applications of Raman analysis. The spatial resolution of the laser microprobe, coupled with the 10 enhancement of the Raman cross-section, means that picogram quantities of material localized to pm-sized surfaces areas can be detected and identified by SERS vibrational spectroscopy. 

The surface chemistry of the eye is probably better understood than that of the ear. The cornea is covered with a thin, fluid film the so-called tear film, which is believed to consist of an aqueous phase, approximately 10" cm thick, with an adsorbed lipid and mucin layer at the air-water interface and an adsorbed mucin layer on the corneal side. The latter renders the cornea hydrophilic and enables the tear film to spread. In dry eye syndrome local areas of dewetting occur due to increased contact angle. Dry eye is sometimes precipitated by drug therapy, and there is a search for adequate artificial tear fluids. The surface chemistry of tear film components has been discussed by Holly [331]. Adsorption of cationic surfactants present in eye drops as preservatives can lead to the production of a hydrophobic surface due to electrostatic adsorption of the cations with the hydrocarbon chains oriented towards the tear film. Such a process can itself result in dewetting and thus cationics should be excluded from artificial tear fluids. 

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