Latex isotherms

The first study utilizing this method was reported by Schuller in 1966 [65]. Schuller used polystyrene latex beads that were spread on a salt-containing aqueous subphase in order to keep the particles at the interface. tt-A plots of the floating particles were determined, which showed several phase regions with reproducible transition points. The author determined the particle diameters from the A-value, at which a steep rise in the isotherm occurred. Moreover, Schuller also spread millimeter-sized Styropor particles and found isotherms of similar shape [66]. By taking pictures at different surface pressure, he was able to correlate the shape with different states of order in the monolayer. Shortly after that. 

FIG. 4 tt-A isotherm of carboxylated latex particles on aqueous subphase, it measured using a WU-helmy balance. (Data taken from Ref. 155.)... 

The adsorption of fully and partially hydrolyzed (88%) polyvinyl alcohol (PVA) on 190-1lOOnm monodisperse polystyrene latex particles was investigated. The effect of molecular weight was investigated for 190 nm-size particles using the serum replacement adsorption and desorption methods. The adsorption density at the adsorption-isotherm plateau followed the relationships for the fully hydrolyzed... 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

Polymer adsorption is important in the flocculation and stabilization of colloidal sols and has been reviewed by Vincent et al. (1) and Tadros (2). Polyvinyl alcohol (PVA) has been used in these studies because of its practical application in textiles, adhesives, and coatings. The adsorption of PVA has been studied on silver iodide by Fleer (3) and Koopal (4), and on polystyrene (PS) latex particles by Garvey (5). The adsorption isotherms reported by these workers extend up to 600 ppm PVA. The adsorption at... 

The determination of adsorption isotherms at liquid-solid interfaces involves a mass balance on the amount of polymer added to the dispersion, which requires the separation of the liquid phase from the particle phase. Centrifugation is often used for this separation, under the assumption that the adsorption-desorption equilibrium does not change during this process. Serum replacement (6) allows the separation of the liquid phase without assumptions as to the configuration of the adsorbed polymer molecules. This method has been used to determine the adsorption isotherms of anionic and nonionic emulsifiers on various types of latex particles (7,8). This paper describes the adsorption of fully and partially hydrolyzed PVA on different-size PS latex particles. PS latex was chosen over polyvinyl acetate (PVAc) latex because of its well-characterized surface PVAc latexes will be studied later. 

The investigations include the effect of (i) PVA molecular weight, particularly at higher concentrations which give different adsorption isotherms (ii) latex particle size over the range 190-llOOnm using a low-molecular-weight fully-hydrolyzed PVA (iii) electrolyte on bare and PVA-covered particles of different sizes. 

Adsorption Isotherms. The adsorption isotherms were determined using the serum-replacement adsorption or desorption methods (7). For the adsorption method, the latex samples (50 or 100 cm 2% solids) containing varying amounts of PVA were equilibrated for 36 hours at 25°C, placed in the serum replacement cell equipped with a Nuclepore membrane of the appropriate pore size, and pressurized to separate a small sample of the serum from the latex. For the desorption method, the latex samples (250 cm 2.5% solids) were equilibrated for 36 hours at 25°C and subjected to serum replacement with DDI water at a constant 9-10 cm /hour. The exit stream was monitored using a differential refractometer. The mean residence time of the feed stream was ca. 25 hours. It was assumed that equilibrium between the adsorbed and solute PVA was maintained throughout the serum replacement. For both methods, the PVA concentration was determined using a An-C calibration curve. 

Figure 2. Adsorption isotherms of HPC-M, HEC-H and PVAonto polystyrene latex particles at 1 8 °C.

In this paper we present results for a series of PEO fractions physically adsorbed on per-deutero polystyrene latex (PSL) in the plateau region of the adsorption isotherm. Hydro-dynamic and adsorption measurements have also been made on this system. Using a porous layer theory developed recently by Cohen Stuart (10) we have calculated the hydrodynamic thickness of these adsorbed polymers directly from the experimental density profiles. The results are then compared with model calculations based on density profiles obtained from the Scheutjens and Fleer (SF) layer model of polymer adsorption (11). 

Adsorption Isotherm The details of the technique used have been described before (15). Basically,0.5 g of the dilute latex was added to poly(vinyl alcohol) (PVA) solutions covering the range 50-1000 ppm. These were then rotated end-over-end for 24 hours at room temperature (22+2°C) for equilibration. 

Adsorption Isotherm The adsorption Isotherm of PVA on polystyrene latex was of the high affinity type, as previously found (15). 

Adsorption on Polystyrene Latex. Figure 3 shows the adsorption isotherms of the two single surfactants, NP-EO q and SDS, on the polystyrene latex surface. Both isotherms reach a limiting value when the cmc is approached. The lines drawn in the figure are calculated from the fitting Equation 19. The adsorption free energies, as obtained from Equation 21, are shown in Table I. The table also shows the two contributions to Ap, according to Equation 21, where the first contribution is obtained from the cmc s and the second from the difference between the two terms in Equation 21. 

Figure 3. Adsorption of NP-EO q (a) and SDS (b) on polystyrene latex. The lines are isotherms calculated by fitting of Equation 19.

Styrene butadiene rubber (SBR) latex 25 000 tonnes Continuous isothermal reaction (5°C) in a series of reactors (33ni3 capacity each). 

Figure 10. Adsorption isotherms for surfactants on a PTFE latex at 25°C (-%-) ammonium perfluoro-octanoate at pH 6.0 in 10 3 mol dm"3 ammonium nitrate (-O-) sodium dodecanoate at pH 9.0 in 10 3 mol dm"3 sodium nitrate.

Recent investigations have shown that the behavior and interactions of surfactants in a polyvinyl acetate latex are quite different and complex compared to that in a polystyrene latex (1, 2). Surfactant adsorption at the fairly polar vinyl acetate latex surface is generally weak (3,4) and at times shows a complex adsorption isotherm (2). Earlier work (5,6) has also shown that anionic surfactants adsorb on polyvinyl acetate, then slowly penetrate into the particle leading to the formation of a poly-electroyte type solubilized polymer-surfactant complex. Such a solubilization process is generally accompanied by an increase in viscosity. The first objective of this work is to better under-stand the effects of type and structure of surfactants on the solubilization phenomena in vinyl acetate and vinyl acetate-butyl acrylate copolymer latexes. 

The second objective is to verify experimentally the predicted relationship between polymer polarity and surfactant adsorption by studying the adsorption of a non ionic surfactant that shows a saturation type isotherm behavior on vinyl acrylic latexes of varying polarity. 

Figure 2 shows adsorption isotherms of two sulfated ethoxy-late type anionics - Alipal EP-110 and Alipal EP-120 - on the 85/15 VA/BA latex surface. Again it is soen that the lower molecular weight EP-110 shows a C type isotherm similar to NaLS while the higher molecular weight EP-120 exhibits a normal saturation type isotherm. 

Figure 1. Adsorption isotherms of NaLS and Aerosol A-102 surfactants at 85/15 VA /BA latex /water interface (17) ((O) sodium lauryl sulfate (A) Aerosol A-102)...

Lower molecular weight (300-800) anionic surfactants such as NaLS and Alipal EP-110 adsorb at a vinyl acrylic latex in a complex C-type adsorption isotherm, leading to penetration and solubilization of latex. 

Table VI shows the adsorption data of Igepal CO-630 surfactant at the three latex/water interfaces. The isotherms were of the normal type and no thickening of the latex in the presence of surfactant was observed. A typical adsorption isotherm of Igepal CO-630 on a 85/15 VA/BA latex surface is shown in Figure 4. Area per molecule was calculated according to equation - 1 (14).

It is also shown that the adsorption of non-ionic surfactants at a vinyl acrylic latex/water interface that exhibit a saturation type isotherm can be related to the polarity of the polymer surface, in agreement with earlier sufactant adsorption studies. 

Fig. 2. Adsorption isotherms of oligonucleotide onto cationic polystyrene latex particles. Samples were mixed and incubated for 2 h at 20°C, 10 2 ionic strength at a given pH [24]...

The detectable amount of adsorbed species can be extremely low. A retention time shift of Atr=0.310 at a modest G (103 g) with w=250 pm results in only -10 17gof adsorbed mass (density 1.4 g/cm3). This mass corresponds to a very small layer, only 0.6 A thick on a 0.2 pm sphere [186]. The above approach has been used to measure protein adsorbed on latex surfaces [186-188], which is relevant to immunodiagnostic assays and biomedical implants. Complete adsorption isotherms can be measured [186] and antigen-antibody binding ratios determined [187]. 

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