Hydrophobic surfaces, surfactant adsorption

The result of these investigations leaves the following view of adsorption of ionic surfactants onto hydrophobic surfaces. Measured adsorption isotherms (Langmuir type) and comparisons between particle sizes as measured by soap titration and... 

Alvarez-Cohen et al. [91] explicitly showed that microbial transformation rates of trichloroethylene (TCE) were proportional to the aqueous TCE concentrations and independent from zeolite-sorbed TCE concentrations. Apparently in contrast to these findings, Crocker et al. [92] reported on the direct bioavailability of naphthalene sorbed to hexadecyltrimethylammonium (HDTMA)-modified smectite clay to Pseudomonas putida 17848, but not to Alcaligenes sp. strain NP-Alk. It should be noted that sorption to the hexadecyl chains of HDTMA resembles more the solubilisation by a surfactant than adsorption to a solid surface. Possibly, hydrophobic surface structures of strain 17848 allowed the close contact with HDTMA, thereby facilitating the uptake of naphthalene by a lipophilic pathway. 

Hydrophobic polar surfaces, adsorption of ionic surfactants on, 24 140-141 Hydrophobic precipitated silica, 22 399 Hydrophobic solvents, 16 413 Hydrophobic surfaces, 1 584-585... 

On hydrophobic surfaces, similar effects probably occur and can explain the difference in adsorption of an anionic surfactant in the absence and presence of a nonionic surfactant above the CMC on carbon (78) ... 

The flotation of minerals is based on different attachment forces of hydrophobized and hydrophilic mineral particles to a gas bubble. Hydrophobized mineral particles adher to gas bubbles and are carried to the surface of the mineral dispersion where they form a froth layer. A mineral is hydrophobized by the adsorption of a suitable surfactant on the surface of the mineral component to be flotated. The hydrophobicity of a mineral particle depends on the degree of occupation of its surface by surfactant molecules and their polar-apolar orientation in the adsorption layer. In a number of papers the relationship was analyzed between the adsorption density of the surfactant at the mineral-water interface and the flotability. However, most interpretations of adsorption and flotation measurements concern surfactant concentrations under their CMC. 

Our aim, however, is to find a model that is as simple as possible and yet makes it possible to estimate adsorption from surfactant mixtures within experimental error in the type of systems investigated by us, i.e. adsorption at coverages approaching monolayers on essentially hydrophobic surfaces, the surfactants adsorbing with their... 

A w Y O represents the molar free energy of replacing direct surface/water contacts by surface/surfactant contacts. The surfactants adsorb with their hydrocarbon tail directed towards a hydrophobic surface and hence we expect this term to be independent of the surfactant type and to cause a decrease in the free energy of adsorption. 

In many cases, the adsorption of surfactants on hydrophobic surfaces may follow a Langmuir-type isotherm,... 

The increase in the hydrophilic head group size reduces the amount of adsorbed surfactant at surface saturation. On the other hand, increasing the hydrophobic tail length may increase, decrease or maintain the surfactant adsorption. If the surfactant molecules are not closely packed, the increase in the chain length of the tail increases surfactant adsorption on solid surfaces. If the adsorption of surfactant on the solid surface is due to polarisation of tc electrons, the amount of surfactant adsorbed on the surface reduces at surface saturation. If the adsorbed surfactants are closely packed on the solid surface, increasing the chain length of the surfactant tail will have no effect on the surfactant adsorption. 

The experiment described above does surely not yield the contact angles that determine the capillary forces since the surfactant layer is desorbed partially in water. It proves, however, that a noticeable hydrophobizing effect after surfactant adsorption can be found for the rather hydrophilic photoresist surfaces processed with the threshold dose. 

The scope of the chapter will include an introduction to the technique of neutron reflectometry, and how it is applied to the study of surfactant adsorption at the planar solid-solution interface, to obtain adsorbed amounts and details of the structure of the adsorbed layer. The advantages and limitations of the technique will be put in the context of other complementary surface techniques. Recent results on the adsorption of a range of anionic, cationic and nonionic surfactants, and surfactant mixtures onto hydrophilic, hydrophobic surfaces, and surfaces with specifically tailored functionality will be described. Where applicable, direct comparison with the results from complementary techniques will be made and discussed. 

The nature of the adsorption of surfactants at both the hydrophilic and hydrophobic solid surfaces has been subject to extensive studies, and a number of excellent recent reviews exist [27-29]. The structure of the adsorbed layer, at both the hydrophilic and hydrophobic surfaces, has been the subject of much conjecture. From the form of the adsorption isotherm at the hydrophilic surface, the cooperative nature of the adsorption was established, and the evolution of the structure with concentration was inferred [30] (see Fig 3). 

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