Adsorption surface hydrophobicity

Hydrophobic Interaction Chromatography. Hydrophobic interactions of solutes with a stationary phase result in thek adsorption on neutral or mildly hydrophobic stationary phases. The solutes are adsorbed at a high salt concentration, and then desorbed in order of increasing surface hydrophobicity, in a decreasing kosmotrope gradient. This characteristic follows the order of the lyotropic series for the anions ... 

Some authors have suggested the use of fluorene polymers for this kind of chromatography. Fluorinated polymers have attracted attention due to their unique adsorption properties. Polytetrafluoroethylene (PTFE) is antiadhesive, thus adsorption of hydrophobic as well as hydrophilic molecules is low. Such adsorbents possess extremely low adsorption activity and nonspecific sorption towards many compounds [109 111]. Fluorene polymers as sorbents were first suggested by Hjerten [112] in 1978 and were tested by desalting and concentration of tRN A [113]. Recently Williams et al. [114] presented a new fluorocarbon sorbent (Poly F Column, Du Pont, USA) for reversed-phase HPLC of peptides and proteins. The sorbent has 20 pm in diameter particles (pore size 30 nm, specific surface area 5 m2/g) and withstands pressure of eluent up to 135 bar. There is no limitation of pH range, however, low specific area and capacity (1.1 mg tRNA/g) and relatively low limits of working pressure do not allow the use of this sorbent for preparative chromatography. 

Xanthate compounds are widely used as collectors in flotation. Their function is to render the mineral surface hydrophobic and thus facilitate bubble attachment. The adsorption of xanthates onto sulfide minerals occurs via an electrochemical mechanism involving the reduction of oxygen and the anodic adsorption of xanthate. 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

In order to verify that the adsorbed lipid membrane indeed forms a bilayer film, another experiment is conducted with an aim to detect the formation of a monolayer lipid. It starts with a piranha-cleaned micro-tube treated with silane to render its inner surface hydrophobic. POPC liposome is then injected into the microtube. It is known that POPC lipid will form a monolayer to such a surface by orienting their hydrophobic tails toward the hydrophobic wall. The experimental results using a mode with similar sensitivity as the previous experiment are shown in Fig. 8.39. The resonance shift in this case is 22 pm, which is about half of that observed for the adsorption of a lipid bilayer. These two experiments suggest that the microtube resonator is capable of accurately determining an adsorbed biomolecular layer down to a few nm thicknesses. 

Sun et al. (1993a) reported the effects HS ion concentration on the adsorption of HS , the amount of extracted sulphur and sulphur-induced flotation of pyrite as shown in Fig. 3.11. The results show that dining sodium sulphide-induced collectorless flotation, it involves the adsorption of HS ion on the mineral and the HS" adsorbed can be oxidized into sulphur to render pyrite and arsenopyrite surface hydrophobic due to the fact that the adsorption density of HS" ion increases with the HS" ion concentration and the amount of extracted sulphur and hence the flotation rate increases with the increase of adsorption density. It suggests that the mechanism of sodium sulphide-induced collectorless flotation of pyrite takes place hy reactions ... 

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system. Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity. 

Entropy-related adsorption, denoted hydrophobic sorption (or solvophobic interaction) is the partitioning of nonpolar organics out of the polar aqueous phase onto hydrophobic surfaces. Fig. 5.6 shows a schematic model of forces that contribute to the sorption of hydrophobic organics, relevant to the subsurface environment. 

Avena and Koopal (1999) used reflectometry to study the kinetics of adsorption of Aldrich humic acid on hematite. Uptake was fast (diffusion-controlled) at low pH, but slow at pH > 5. The rate of uptake rose with ionic strength above the iep, but decreased with ionic strength below the iep. The adsorption of humic acid onto hematite rendered its surface hydrophobic and made it a suitable sorbent for hydrophobic organic compounds (Murphy et al., 1992). 

The porous structure is of an open-cell type so that water can readily penetrate the pores. During adsorption, the hydrophobic portion of the molecule is preferentially adsorbed on the hydrophobic polystyrene surface of the adsorbent through van der Waals attraction. The compounds being adsorbed do not penetrate substantially into the microsphere but remain adsorbed at the surface thus allowing the adsorbate to be rapidly eluted during the recovery step. 

We start out by considering the effect of such adsorption sites on the isotherms of apolar and weakly monopolar compounds. For these types of sorbates, hydrophobic organic surfaces and/or nanopores of carbonaceous materials are the most likely sites of adsorption. Such hydrophobic surfaces may be present due to the inclusion of particles like coal dust, soots, or highly metamorphosed organic matter (e.g., kerogen). Because of the highly planar aromatic surfaces of these particular materials, it is reasonable to assume that planar hydrophobic sorbates that can maximize the molecular contact with these surfaces should exhibit higher affinities, as compared to other nonplanar compounds of similar hydrophobicity. 

Let us evaluate some experimental data. To this end, we use a dual-mode model (Eq. 9-6). This model is a combination of a linear absorption (to represent the sorbate s mixing into natural organic matter) and a Freundlich equation (as seen for adsorption to hydrophobic surfaces or pores of solids like activated carbons) ... 

The hybrid mesoporous material has some silanol groups corresponding to the T2 type Si species. The peak intensity ratio of the T2/T3 signals of the hybrid mesoporous material is almost equivalent to the Q3/Q4 signals of siliceous MCM-41 material. This indicates that silanol density is almost same to the siliceous MCM-41 material. Adsorption isotherm of water vapor showed that the hybrid mesoporous material has more hydrophobic surface than siliceous MCM-41. This fact supports that the organic fragments are exposed on the surface and increased the surface hydrophobicity. 

Polymeric reversed phase resins are synthesized from divinylbenzene with styrene, methylstyrene or other styrenic monomers. Divinylbenzene is the major component and provides crosslinking. These resins are macroporous, and the surface area is usually in excess of 300 m2 / g. This surface area provides the adsorptive surface for retention of hydrophobic species. These resins can be used for matrix elimination of surfactants, weak carboxylic acids, fats, proteins, etc. 

Figure 4.34 In fatty acid esterification, the catalyst surface hydrophobicity is a key factor. Too few acid sites result in low activity, but too many lead to water adsorption and deactivation.

SURFACE PROPERTIES AFFECTING PROTEIN ADSORPTION HYDROPHILICITY/HYDROPHOBICITY TOPOGRAPHY ENERGY CHARGE ETC. 

In addition to the developed pore structure, small pore sizes, and large surface area, surface hydrophobicity is an extremely helpful property of active carbon [170-175], This property is particularly useful for the adsorption of organic species [176],... 

Electrostatic interactions occur between the ionic head groups of the surfactant and the oppositely charged solid surface (head down adsorption with monolayer structure) [56]. Acid-base interactions occur due to hydrogen bonding or Lewis acid-Lewis base reactions between solid surface and surfactant molecules (head down with monolayer structure) [57]. Polarisation of jt electrons occurs between the surfactant head group which has electron-rich aromatic nuclei and the positively charged solid surface (head down with monolayer structure) [58]. Dispersion forces occur due to London-van der Waals forces between the surfactant molecules and the solid surface (hydrophobic tail lies flat on the hydrophobic solid surface while hydrophilic head orients towards polar liquid) [59]. 

The results indicate that both NH4,TMA-fl and NH.,K-L are de-aluminated upon fluorination. Strong supporting evidence comes from framework I. R. data where the shifts in band position to higher wave numbers are as much as 20 cm-1. However, there is no evidence of structure stabilization. Also McBain water adsorption data give no indication of surface hydrophobicity. Therefore, it is likely that structure defects are formed in these two zeolites as a result of dealumination and cause low thermal stability. 

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