Dispersion polymer adsorption measurements

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Adsorption on Kaolinite. For kaolinite, the polymer adsorption density is strongly dependent on the solid/liquid ratio, S/L, of the clay suspension. As S/L increases, adsorption decreases. This S/L dependence cannot be due totally to autocoagulation of the clay particles since this dependence is observed even in the absence of Ca2+ at pH 7 and at low ionic strength where auto-coagulation as measured by the Bingham yield stress is relatively weak (21). Furthermore, complete dispersion of the particles in solvent by ultra-sonication before addition of... 

Flocculation rate limitation. The adsorption step was rate limiting for the overall flocculation process in this system. Polymer adsorption rate measurements for dispersed systems reported in the literature (2,26) do not lend themselves to direct comparisons with the present work due to lack of information on shear rates, flocculation rates, and particle and polymer sizes. Gregory (12) proposed that the adsorption and coagulation halftimes, tA and t, respectively, should be good indications of whether or not the adsorption step is expected to be rate limiting. The halftimes, tA and t, are defined as the times required to halve the initial concentrations of polymer and particles, respectively. Adsorption should not limit the flocculation rate if... 

The evidence of the polymer adsorption on the surface of pigment particles and the formation of protective layers has been obtained by ESA method by experiments on measuring the -potential of the TiC>2 surface in aqueous dispersions in the presence of EFIEC (Table 2). It can be seen from the table that -potential becomes less negative in the presence of EHEC and still more approaches zero as a result of the treatment of dispersions in an ultrasonic field. This indicates the activation of polymer adsorption and the formation of stabilizing layers due to mechanochemical modification of particles. From the table it is also seen that as a consequence of mechanochemical treatment, the size of pigment particles in dispersions decreases, and dispersion becomes structurally more uniform, which leads to an increase of its stability even at increased temperature (experiment 4). 

It is clear from the above theories that for full characterization of polymer adsorption and configuration at the interface, one needs to measure the following values, i.e. the amount of polymer adsorbed per unit area of the surface, r(mol m ), the fraction of segments in direct contact with the surface (in trains), p, and the segment density distribution p(z). Measurement of F and p as a function of polymer concentration is fairly straightforward. The parameter F can be directly determined by equilibrating a known amount of disperse phase (particles or droplets) of known surface area with polymer solutions with various concentrations, starting... 

In principle every method suited for measuring some property of an adsorbed polymer as a function of time can be used for studying one or more aspects of the dynamics of polymer adsorption. Some methods can only be used for polymer adsorption on flat (oxidic) siufaces, others require higher amounts of adsorbed polymers and therefore can only be applied in dispersions with sufficient surface area. Recently some reviews on experimental methods have been published [1, 35]. In this section we will discuss some frequently applied methods applicable for oxidic surfaces. 

Electron spin resonance (ESR) has been used to measure the rates of polymer adsorption and exchange [63 ]. Similarly to NMR, the method is based on a mobility criterion to measure the bound polymer fraction. However, it has the advantage that it can be readily carried out in systems rich in H such as water and that it has a much higher sensitivity. A major drawback of ESR is that the polymer must be spin labeled and again the label can influence the conformation and the adsorbed amount at the surface. For this method, only sufficiently concentrated dispersions can be used. For that reason it is only useful for relatively slow processes in which geometrical factors determining transport are of less importance. 

Techniques applied to study polymer adsorption have to be sensitive enough to detect small mass ( l-5 mg/m ) included within adsorbed polymer monolayers. One way to increase the sensitivity of the measurements is to study polymer adsorption using dispersed particles. Sensitive techniques are also now available to study thin adsorbed polymer layers at planar surfaces. 

Measurements on Flat Surfaces. Using planar substrates, polymer adsorption can be studied under well-defined conditions of surface eneigies and known surface area. These measurements provide important insists into the structure and dynamics of adsorbed pol3rmers, which are often not experimentally achievable using dispersed particles. 

The way in which these factors operate to produce Type III isotherms is best appreciated by reference to actual examples. Perhaps the most straightforward case is given by organic high polymers (e.g. polytetra-fluoroethylene, polyethylene, polymethylmethacrylate or polyacrylonitrile) which give rise to well defined Type III isotherms with water or with alkanes, in consequence of the weak dispersion interactions (Fig. S.2). In some cases the isotherms have been measured at several temperatures so that (f could be calculated in Fig. 5.2(c) the value is initially somewhat below the molar enthalpy of condensation and rises to qi as adsorption proceeds. In Fig. 5.2(d) the higher initial values of q" are ascribed to surface heterogeneity. 

As another criterion of stability, a critical flocculation temperature(OFT) was measured. The measurement of CFT was carried out as follows the bare latex suspension was mixed with the polymer solution of various concentrations at 1+8 °C by the same procedure as in the adsorption experiments. Then, the mixture in a Pyrex tube(8 ml, U.0 wt %) was warmed slowly in a water bath and the critical temperature at which the dispersion becomes suddenly cloudy was measured with the naked eye. 

A confirmation of the soundness of electronic theory was derived from a recent study, performed by Bogotaj et al. [46], They measured the zeta potential of different polymer dispersions and mucosal homogenates and found a correlation between such a parameter and the force necessary to detach a polymer dispersion from the biological substrate. The adsorption theory states that the bioadhesive bond is due to van der Waals interactions, hydrogen bonds, and other related weak interactions [44],... 

Polarity of Vinyl Acrylic Latex and Surfactant Adsorption Contact angle measurements, dispersion and polar contribution to latex film surface tension and polarity of polymer calculated according to the method of Kaelble (10) of the three latex films are whown in Table V. It is seen that the polarity of the latex film decreases with increase in butyl acrylate content of the vinyl acrylic co-polymer. The polarity of the 70/30 (VA/BA) latex is very similar to that of the polybutyl acrylate homopolymer estimated to be about 0.21 (1). ... 

The polarity and adsorption data discussed above reveal some interesting aspects of the surface chemistry of vinyl acrylic latex surfaces. It is quite likely that the polarity of the latex films, expecially of the two co-polymers, determined by contact angle measurements may not correspond exactly with their respective latex surfaces in the dispersed state due to reorientation of polymer chains during film formation. But the surfactant adsorption data shows clearly that the three latex surfaces in their dispersed state do exhibit varying polarity paralleling the trend found from contact angle measurements. The result also shows that the surface of the co-polymer latex surface is a mixture of vinyl acetate and acrylate units. This result is somewhat unexpected in a vinyl acrylic latex, prepared by a batch... 

In the latter case the total interaction, which is what can be measured, is affected by the net charge of the surface and the adsorbed layer, ion-ion correlations, bridging interactions and steric confinement of the polymer chain [116]. We note that polyelectrolytes are often present as additives in colloidal dispersions and the character of the forces generated by the polyelectrolyte adsorption layers has a paramount influence on stability of these colloidal systems. With the aim to illustrate what can be learnt about polyelectrolyte adsorption layers using the SFA, we will look at the influence of the polyelectrolyte charge density on the forces acting between surfaces coated with polyelectroytes. We will consider an example where the polyelectrolyte charge density is varied by a systematic... 

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