Adsorbent-adsorbate, attraction between

The second case involves non-adsorbing polymer chains in solution. It was realized by Asakura aird Oosawa (AO) [50] aird separately by Vrij [51] tlrat tlrese chains will give rise to air effective attraction between colloidal particles. This is kirowir as depletion attraction (see figure C2.6.4. We will summarize tire AO tlreory to explain tlris. 

In section C2.6.4.3 it was shown how tlie addition of non-adsorbing polymer chains induces a depletion attraction between colloidal particles. If sufficient polymer is added, tliese attractions can be strong enough to induce a phase separation of tire colloidal particles. An early application of tliis was tire creaming of mbber latex [93]. 

Precipitate particles grow in size because of the electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution. Ions common to the precipitate are chemically adsorbed, extending the crystal lattice. Other ions may be physically adsorbed and, unless displaced, are incorporated into the crystal lattice as a coprecipitated impurity. Physically adsorbed ions are less strongly attracted to the surface and can be displaced by chemically adsorbed ions. 

Short-Chain Organics. Adsorption of an organic dispersant can reduce polarizabiHty attraction between particles, ie, provide semisteric stabilization, if A < A.p < A or A < A.p < A (T = dispersant) and the adsorption layer is thick. Adsorption in aqueous systems generally does not foUow the simple Langmuir profile because the organic tails on adsorbed molecules at adjacent sites attract each other strongly. 

However, the assumption of molecule orientation normal to the surface is not convincing enough for this author, and it does not consist well with the results of the molecular d5mamics simulations for the alkane confined between solid walls. An example in Fig. 3 shows that the chain molecules near the wall are found mostly lying parallel, instead of normal, to the wall [6]. This means that the attractions between lubricant molecules and solid wall may readily exceed the inter-molecule forces that are supposed to hold the molecules in the normal direction. Results in Fig. 3 were obtained from simulations for liquid alkane with nonpolar molecules, but similar phenomenon was observed in computer simulations for the functional lubricant PFPE (per-fluoropolyether) adsorbed on a solid substrate [7], confirming that molecules near a solid wall lie parallel to the surface. 

In studies of steric stabilizers too little attention is generally paid to the dispersion force attractions between particles and the critical separation distance (H ) needed to keep particles from flocculating. Adsorbed steric stabilizers can provide a certain film thickness on each particle but if the separation distance between colliding particles is less than H the particles will flocculate. The calculation of H is not cr difficult and measurements to prove or disprove such calculations are not difficult either. For equal-sized spheres of substance 1 with radius or in medium 2 the Hamaker equation for the dispersion force attractive energy (Uj2i) at close approach is (7) ... 

The actual adsorption of vapor molecules takes place mainly on the surface of internal passages within the adsorbent particles, since that is where most of the available surface exists. The adsorption process may be either physical or chemical in nature. Physical adsorption is a readily reversible process that occurs as a result of the physical attraction between the gas molecules and the molecules of the solid surface. If the gas-solid intermolecular attraction is greater than the intermolecular attractions in the gas phase, the gas will condense on the solid surface, even though its pressure is lower than its vapor pressure at the prevailing temperature. For example, the equilibrium adsorption pressure of acetone on activated carbon may, under some conditions, be as little as 150 to 1,100 of the equilibrium vapor pressure at... 

Further inspection of Fig. 4.5 demonstrates that the O atoms are surrounded by dark zones. The STM technique probes not only the atomic topography but also the electronic structure, and the dark zones reflect the modification of the local electronic density in the vicinity of the adsorbates, this modification being responsible for the operation of indirect interactions (which may be either repulsive or attractive) between adsorbed particles mediated through the substrate. 

The selective adsorption of the hydrogen ions cannot proceed to tr ue equilibrium owing to the electrostatic attraction between the dissimilar ions, consequently chlorine ions are adsorbed in excess of their equilibrium concentration and since the hydrogen ions on adsorption have to do work in increasing the surface concentration of chlorine ions above their proper value, the true adsorption value of the hydrogen ions is not attained. 

At higher concentrations of the polymer, brushlike layers can form on the particles. These brushes can extend over sufficiently large distances to mask out the influence of van der Waals attraction between the particles, thereby imparting stability to the dispersion. This mechanism, already mentioned in Section 13.2, is known as steric stabilization. For steric stabilization, the polymer molecules must be adsorbed or anchored on the particle surfaces. 

Another source of attraction between two surfaces is possible when the surfaces are immersed in a solution of a nonadsorbing polymer (e.g., a polymer that does not adsorb on, or is repelled by, the surfaces). Although this force is generally weak, it can play a significant role in destabilization of colloidal particles under certain circumstances. 

This review indicates that good solvent conditions (in terms of either x or 0) result in a positive value for AGR. This is what would be expected from a model that assumes that the first encounter between particles with adsorbed layers is dominated by the polymers. Conversely, in a poor solvent AGR is negative and amounts to a contribution to the attraction between the core particles as far as flocculation is concerned. Under these conditions the polymer itself is at the threshold of phase separation. Van der Waals attraction between the core particles further promotes aggregation, but it is possible that coagulation could be induced in a poor solvent even if the medium decreases the effective Hamaker constant to zero. 

All adsorption processes result from the attraction between like and unlike molecules. For the ethanol-water example given above, the attraction between water molecules is greater than between molecules of water and ethanol As a consequence, there is a tendency for the ethanol molecules to be expelled from the bulk of the solution and to concentrate at die surface. This tendency increases with the hydrocarhon chain-length of the alcohol. Gas molecules adsorb on a solid surface because of die attraction between unlike molecules. The attraction between like and unlike molecules arises from a variety of intermolecular forces. London dispersion forces exist in all types of matter and always act as an attractive force between adjacent atoms and molecules, no matter how dissimilar they are. Many oilier attractive forces depend upon die specific chemical nature of the neighboring molecules. These include dipole interactions, the hydrogen bond and the metallic bond. 

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