Surface groups, ionisation

Imagine a suspension of colloidal particles in water. What causes stability, and what, imder changing solution conditions like addition of salt causes flocculation (precipitation of the suspension) Two opposing forces were considered to operate between two such particles. The one, attractive, is the quantum mechanical van der Waals force and treats an intervening liquid as if it has bulk liquid properties up to the interfaces of the particles (theme (i)). The other, repulsive, due to charges formed by dissociation of ionisable surface groups, is electrostatic in origin, and depends on salt concentration. 

The electrical charge on particles is due either to ionisation of surface groups or to adsorption of ions which confer their charge to the surface. A particle surface with a negative charge is shown in Fig. 7.3 along with the layer of positive ions that are attracted to the surface in the Stern layer, and the diffuse or... 

The separation of charges may arise in various ways. At a solid-electrolyte solution interface there is likely to be preferential adsorption of one ion or another on the solid surface, with a corresponding excess of the counterion in the adjacent solution. (Even at the air surface of an electrolyte solution there is an electrical separation, as most anions tend to approach the surface more closely than cations.) Above all, the effect will be important where a charged species is able to pass across the boundary from one phase to the other, as in the ionisation of surface groups of a colloidal particle, or at the interface salt crystal-solution, or at a metal electrode. 

Measurements of the surface tension of aqueous solutions of various sulphonated and unsulphonated phenylazonaphthol dyes showed that the degree of surface activity (that is, the lowering of surface tension) tended to increase progressively with the degree of alkyl substitution in the series of dyes [7]. The surface-active behaviour of such alkylated dye ions ensures that they become more concentrated at the interface between the dyebath and the fibre surface, just as they do at the air-water interface of the dye solution. Foaming of dyebaths can be a serious practical problem with relatively hydrophobic dye structures solubilised by means of a single ionised group. 

Cationic surfactants are surface-active agents that have one or more functional groups in their molecule that ionise in aqueous solution to produce positively charged organic ions. The most representative cationic surfactants are quaternary ammonium derivatives in which the N atom is bonded to four alkyl groups. For many years, ditallow dimethylammonium chloride (DTDMAC) has been the most widely used product of this family. Its recalcitrance to biodegradation, however, has... 

It is important to note that the ion series observed by the API-MS method may not be representative of all the products present, not the quantities thereof. Cleavage of the ethoxylate chain removes the capacity of silicone surfactants to be ionised and therefore detected by these methods. As such, for example, the cleaved silicone head group [M(D/CH2CH2CH2OH)M, 3] was never observed by API-MS. The nature of the API-MS process is such that competition between analytes for ionisation occurs, and as such compounds with higher surface activity and EO content can be expected to dominate in the resulting spectra. Suppression effects may thus preclude observation rather than confirm absence. As a consequence, the use of additional techniques such as FTICR-MS, GC-MS, HPLC and NMR to provide complementary data was also necessary. Furthermore, the high number of possible structures for each ion series observed, rendered it difficult to assign structures with confidence. Consequently simplified M2D-C3-0-(E0)n-R... 

The El mechanism has, as the rate-determining step in solution, the ionisation of the reactant forming a carbonium ion which then decomposes rapidly. For heterogeneous catalytic reactions, the important features are the occurrence of the reaction in two steps and the presence on the solid surface of carbonium ions or species resembling them closely. Again, the kinetic characterisation by way of an unimolecular process is of little value. Even the relative rates of the two steps may be reversed on solid catalysts. A cooperation of an acidic and a basic site is also assumed, the reaction being initiated by the action of the acidic site on the group X. 

The ability to obtain isotherms on individual groups also enable the calculation of the isosteric heat of adsorbtion on that site (as distinct from the heats of adsorption averaged over the whole surface which are normally obtained from adsorption isotherms). Relationships between the OH frequency shifts and heats of adsorbtion have been obtained this way (18). A relationship between the frequency shift and the ionisation potential of the adsorbing molecules has been demonstrated by several authors (20-22) and a theoretical explanation based on the Mullikan-Puranik approach to H-bonding has been given by Low and Cusamano (23). 

Two-dimensional monolayers can exist in different physical states which bear some resemblance to the solid, liquid and gaseous states in three-dimensional matter. Surface films are best classified according to the lateral adhesion between the film molecules, including end-groups. Factors such as ionisation (and, hence, the pH of the... 

Some of the commonly used techniques for measuring contact angle [215, 216, 217] are the sessile drop method, captive bubble method and Wilhelmy plate method. These techniques have been extensively used and well documented for characterisation of modified PE surfaces [218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230] for various applications. Whitesides et al. [231 ] studied the wetting of flame-treated polyethylene film having ionisable organic acids and bases at the polymer-water interface. The effect of the size of substituted alkyl groups in amide and ester moieties on the surface hydrophilicity was also studied [232]. The biocompatibility of the polyethylene film surface modified with various water-soluble polymers was evaluated using the same technique [233]. The surface properties of hy-perbranched polymers have been very recently reported [234]. 

If Si02 is used as the insulator, we then have a surface which contains numerous SiOH groups. These are affected by interactions with the H+ and OH moieties present in any aqueous solution. In this way changes of pH will change the ionisation of the SiOH groups and therefore change the Si02 surface po-... 

A reaction sequence somewhat in parallel with the Eigen-Wilkins-Werner mechanism can also be expressed for the inner-sphere surface complexation of bivalent metal cations by an ionised surface hydroxyl group (cf equation (9.9a) ... 

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