Adsorption Of Nonionic Polymers Research Articles

Entropy drives the adsorption of xyloglucan to cellulose surfaces – A molecular dynamics study

The adsorption of nonionic polymers to cellulose is of large importance both in the plant cell wall during synthesis and for the development of sustainable materials from wood. Here, the thermodynamics of adsorption of the polysaccharide xyloglucan (XG) to both native and chemically modified cellulose with carboxyl groups was investigated using molecular dynamics simulations. The free energy of adsorption was calculated as the potential of mean force between an XG oligomer and model cellulose surfaces in a range of temperatures from 298 K to 360 K. It was found that the adsorption near room temperature is an endothermic process dominated by the entropy of released interfacial water molecules. This was corroborated by quantitative assessment of the absolute entropy per water molecule both at the interface and in the bulk. In the case of native cellulose, the adsorption became exothermic at higher temperatures, while the relatively strong interactions between water and the charged groups of the oxidized cellulose impede such a transition. The results also indicate that the extraction of strongly associated hemicelluloses would be facilitated by low temperature.

Aqueous Dispersions of Exfoliated Molybdenum Disulfide for Use in Visible-Light Photocatalysis

Single and few layer molybdenum disulfide (MoS2) was exfoliated from the bulk form through a liquid phase exfoliation procedure. Highly concentrated suspensions were prepared that were stabilized against reaggregation through adsorption of nonionic polymers to the sheet surface. These exfoliated particles showed strong photoluminescence at an energy of 1.97 eV which is in the visible-light region. These exfoliated MoS2 sheets were then used to catalyze the degradation of a model dye upon exposure to visible light.

Influence of polymer functional group architecture on titania pigment dispersion

The influence upon adsorption of polymers containing different functional groups on the dispersion behaviour of titania pigment particle has been investigated at pH 6.0 and 9.5. The polymers chosen are polyacrylic acid and modified polyacrylamides including homo and copolymers modified with carboxylate and/or hydroxyl groups. A range of experimental methods including polymer adsorption isotherms, zeta potential, rheology and particle size measurements were used to assess the dispersion effect of the polymers. Adsorption of polyacrylic acid and anionic polyacrylamide copolymers onto the titania pigment surface is greatest when electrostatic repulsion is absent. At low pH values, below the pigment isoelectric point (iep), the adsorption density of the anionic polymers on titania pigment was high whilst at higher pH values, above the pigment iep, the adsorption density decreased. However, the adsorption of non-ionic polymers on titania pigment was less influenced by the pH. The presence of carboxylate functional groups in both polyacrylic acid and anionic polyacrylamide copolymer altered the titania pigment dispersion behaviour, reducing the suspension yield stress and mean particle size. The polyacrylamide homopolymer had negligible effect on the pigment dispersion properties at pH 9.5, while it caused pigment particle aggregation at pH 6.0. The hydroxyl-modified polyacrylamide had also a de-stabilisation effect on the pigment dispersion at pH 6.0, but decreased the yield stress and the mean particle size at pH 9.5. The stabilisation effect of both anionic and non-ionic polymers was pH-dependent. The dispersion properties of the pigment particles are largely predicted by DLVO theory.

Effects of chemical functional groups on the polymer adsorption behavior onto titania pigment particles

The effects of functional groups on polymer adsorption onto titania pigment particles have been investigated as a function of pH and ionic strength using polyacrylic acid and modified polyacrylamides. The polyacrylamides include the homopolymer, an anionic copolymer with hydroxyl and carboxylate group substitution, and a nonionic copolymer with hydroxyl group substitution. Adsorption isotherms and infrared spectroscopy were used to examine the polymer–pigment interactions. The adsorption of the polyacrylic acid and anionic polyacrylamide on titania pigment is greatest when electrostatic repulsion is absent or reduced. At low pH values, below the pigment isoelectric point (IEP), or at high ionic strength, the adsorption density of the anionic polymers on titania pigment is high, while at higher pH values above the pigment IEP, the adsorption density decreases. But the adsorption of nonionic polymers on titania pigment is not influenced by either ionic strength or pH. Acrylamide groups were found to hydrogen bond with the titania pigment surface, independent of pH. With the inclusion of hydroxyl functional groups into the polyacrylamide chain, the polymer adsorption density increased without increased adsorption affinity. Carboxylate functional groups in the anionic polymers strongly interact with the pigment surface, producing the highest adsorption density at low pH values. All polymers exhibit Langmuir adsorption behavior with hydrogen bonding found as the dominant mechanism of adsorption in addition to electrostatic interaction occurring for the anionic polymers.

Influence of non-ionic polymers on the rheological behaviour of Na+-montmorillonite clay suspensions. Part II. Homopolymer ethyleneoxide and polypropylene oxide–polyethylene oxide ABA copolymers

The rheological behaviour of Na+-montmorillonite suspensions in the presence of non-ionic polymers has been systematically investigated. Two different series of non-ionic polymers were studied. The first consisted of ABA block copolymers of polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO or PE series), of varying ethylene oxide chain length. The second was a linear polyethylene oxide polymer of different molecular weights (PEO series). Thus the role of the polymer structure and ethylene oxide chain length on the rheological behaviour of the clay suspensions was addressed. Adsorption measurements were made with the purpose of quantifying the amount of polymer adsorbed on the Na+-montmorillonite surface. Rheological properties of concentrated clay suspensions (3–6% w/w) were determined as the polymers gradually covered the clay particles. Flow curves were obtained from shear stress–shear rate measurements, and were fitted to the Herschel–Bulkley model. The viscoelastic properties of the suspensions were determined from dynamic oscillatory measurements. These results demonstrate that adsorption of non-ionic polymers on the Na+-montmorillonite particles led to a gradual stabilisation of the suspension. The stability was enhanced for a longer ethylene oxide chain length and was found to depend on polymer structure.

Prediction of Adsorption of Nonionic Polymers from Aqueous Solutions to Solid Surfaces

In this paper, a model for predicting adsorption of nonionic polymers from aqueous solutions to solid surfaces has been presented. The model is based on continuum form of the self-consistent mean field theory. The model incorporates the effect of the hydrogen bond using Flory−Huggins model with a concentration-dependent Flory−Huggins parameter. The self-consistent field is derived using the Evans and Needham approach. The model has been validated using the reported experimental data on adsorption of poly(ethylene oxide) (PEO) from aqueous solution to silica. All the parameters of the model have been estimated from the reported data of the independent experiments specifically directed to obtain these parameters. Thus, the concentration-dependent Flory−Huggins parameter is estimated from the water activity in PEO solution, Kuhn length through the radius of gyration of PEO under ϑ conditions, and polymer−surface affinity parameter through the specific enthalpy of displacement of water by PEO in the limit of zero adsorption. The model quantitatively predicts the adsorbed amount in trains, loops, and tails and the hydrodynamic thickness of the adsorbed layer. It also correctly predicts the effect of pH and molecular weight of PEO on these quantities. The advantage of this approach is that it allows direct extension of models, describing thermodynamics of hydrogen bond in the bulk, to the interfacial region. The other important contribution of this work is that it shows that, for estimation of the adsorbed amount in the form of trains, the calorimetric technique yields results which are consistent with the NMR spin relaxation technique.

The colloidal and rheological properties of bentonite suspensions

The objective of this paper is to give an overview of the colloidal properties of clays and to describe the work performed until the present moment in the area of adsorption of non-ionic polymers onto clay and the rheology of clay suspensions. The review is particularly focused on Na +-montmorillonite clay of the smectite group (Bentonite), due to its high swelling capacity, the ability to adsorb significant amounts of polymer on its surfaces, and the formation of a gel-like structure with yield characteristics and viscoelastic properties at relatively low clay concentrations. A significant amount of research has been performed in these areas with applicability to various fields, such as soil science and drilling fluids, where interaction of organic polymers with the clay surface and the rheological response in the presence of electrolytes under different temperature and pressure conditions are of considerable relevance. In order to understand the interaction of these compounds with the clay surface and the rheological response of the resulting suspensions, it becomes essential to understand the structure of clay minerals, the interaction of clay particles in aqueous medium, the swelling behaviour and electrokinetic properties of Na +-montmorillonite clay. For this reason, the above topics are addressed, along with the general features of stabilisation of colloidal particles and the adsorption mechanism of non-ionic compounds on hydrophilic surfaces. A comprehensive review of the most relevant studies of the adsorption of non-ionic surfactants and polymers onto both silica and clay is presented. Finally, an overview of the rheological behaviour of concentrated suspensions and models used to describe the flow behaviour of clay suspensions is given, along with the effect of electrolytes, polymers, high pressure and high temperature on the flow behaviour of Na +-montmorillonite suspensions.

Surface modification of hydroxyapatite to avoid bacterial adhesion

Hydroxyapatite was surface-modified by adsorption of nonionic polymers carrying phosphate groups as anchoring groups. A combination of alcohol ethoxylate and alkyl phosphate was also used. The possibility of interfering with early microbial colonization on apatite, mimicking the tooth surface, was investigated using radiolabelledStreptococcus mutans as model bacteria. The polymers, a nonionic cellulose ether and an EO/PO block copolymer based on polyglycerol as starting alcohol, were effective in buffer but gave only limited reduction of bacterial adhesion when the apatite had been pretreated with saliva. A 1∶1 molar mixture of alcohol ethoxylate and alkyl phosphate was effective both with and without saliva, however. Studies with14C-labeled compounds, as well as microelectrophoresis experiments, indicate that an unsymmetrical double layer is formed on the apatite surface with predominantly alkyl phosphate in the inner layer and with alcohol ethoxylate pointing towards the water phase.