Oxidation of polyethylene with chromic acid/sulfuric acid generates a material (PE-C02H) having a high density of carboxylic acid and ketone functionalities in a thin surface layer on the polymer. This paper determines the extent of ionization of the surface and near-surface carboxylic acid groups of these materials in contact with water as a function of pH using three experimental techniques: measurement of attenuated total reflectance infrared (ATR-IR) spectra, measurement of contact angles, and direct potentiometric titration. On the basis of correlations between results obtained by these three techniques, we propose an equation relating the contact angle of an aqueous solution having a given value of pH and the extent of ionization (a;) of those carboxylic acid groups that are directly exposed to the solution. These carboxylic acid groups have broad titration curves and have C02H groups that are less acidic than soluble carboxylic acids. The initial ionization of these carboxylic acid groups occurs when the solution is approximately pH 6. The detailed structures of these oxidized polymer surface layers and the nature of the interactions between the carboxylic acid and carboxylate ions in them are still not completely defined. Salt effects on the extent of ionization a; at a particular value of pH are unexpectedly small and suggest that charge-charge interactions between carboxylate ions may not dominate the titration curves. This work demonstrates the usefulness of contact angle in following chemical changes occurring in organic functional groups on surfaces. Comparisons of wetting behavior of buffered and unbuffered solutions establishes the importance of using experimental protocols for measuring contact angles in which the reactive groups present in the interfacial region do not outnumber the reactive groups present in the small drop of dilute aqueous solution.

This paper examines the wetting by water of low-density polyethylene film modified at the polymer-water (air) interface by introduction of polar organic functional groups (carboxylic acids, amines, and others). Water/polymer contact angles were determined for each of these interfaces; for interfaces containing acidic or basic functional groups, the contact angle was determined as a function of the pH. The observed contact angle was related to the hydrophilicity of these functional groups: as the hydrophilicity (as measured by Hansch ir parameters) increased up to a certain point, the contact angle decreased. Beyond that point, increased hydrophilicity had little additional influence on the contact angle. This result is interpreted in terms of water adsorbed on the polar interfacial functional groups; extensive hydration of interfacial groups having large negative ir parameters moderates their effective hydrophilicity. The influence of a functional group on wettability was related to the normalized fraction of the area of the interface occupied by that type of functional group; in some instances this area fraction appears more useful in describing interfacial properties than the simpler mole fraction. Interfaces containing ionizable functional groups usually showed a contact angle that varied with pH, with the lower contact angle observed at values of pH for which the interfacial groups were in the more polar, charged form; interfaces without ionizable functional groups did not display a change in contact angle with pH. The paper rationalizes the change in contact angle with pH in terms of the relative areas of the interface occupied by the functional groups in different ionization states and in terms of the extent of ionization of acidic and basic groups. The interfacial values of pK& are compared with the values of pK& for similar functional groups in aqueous solution. In ©

799heterogeneity of a solid, which is characterized by the adsorption potential distribution function X ( A ) , on the enthalpy and entropy of adsorption. In these expressions the characteristic curve 8JA) and the adsorption potential distribution X ( A ) are expressed by the integral eq 19 and 22, respectively, which contain the micropore distribution J(x) characterizing the structural heterogeneity of a microporous solid. It is noteworthy that eq 22 describes the relationship between energetic and structural heterogeneity of microporous solids, which are defined, respectively, by the distribution functions X ( A ) and J(x).Special cases of the general expressions given by eq 12 and 13 were discussed for the discrete and y-type micropore distribution functions. It has been shown that the y-type distribution, viz., eq 30, produces a simple equation for the characteristic Curve &(A), viz., eq 31, which is useful for microporous solids. To illustrate practical utility of this equation, the benzene adsorption isotherm on activated carbon AC12 was interpreted in order to evaluate the adsorption parameters, which are necessary to calculate the thermodynamic functions. This example shows that in the case of adsorption on a nonuniform microporous solid, besides the adsorption potential distribution function X ( A ) , which characterizes its energetic heterogeneity and is used to calculate the thermodynamic functions, the micropore distribution function J(x) should be also evaluated in order to characterize the solid structural heterogeneity.Acknowledgment. I thank Drs. J. Choma and J. Piotrowska for assistance in numerical calculations and helpful discussions.Oxidation of low-density polyethylene film with aqueous chromic acid results in a material (PE-C02H) having hydrophilic carboxylic acid and ketone groups in a thin oxidatively functionalized interface. This interface is indefinitely stable at room temperature. On heating under vacuum, it rapidly becomes hydrophobic and similar in its wettability to unfunctionalized polyethylene film. The progression of the contact angle with water from the initial value (55') to the final value (103') follows kinetics that suggest that the polar functional groups disappear from the interface by diffusion. The magnitude of the apparent diffusion constant derived from these studies can be described approximately by an Arrhenius equation over a significant portion of the temperature range explored, with an Arrhenius activation energy of diffusion of -50 kcalfmol. Comparison of the properties of interfaces composed of carboxylic acid groups with those containing other species demonstrates that the structure of the interfacial groups also significantly influences the rate of reconstruction. In particular, reconstruction is slow when the interfacial functional groups are large and polar (e.g., esters of poly(ethy1ene glycol)) and when they have structures that result in low solid-air interfacial free energies (e.g., CF3 moieties). Studies of reconstruction carried out with PE-C02H in con...

The favorable bile acid binding characteristics of sevelamer provide a compelling explanation for its ability to lower LDL cholesterol in hemodialysis patients and in healthy volunteers.

No abstract

In vitro binding studies demonstrate that RenaGel has an extremely high phosphate binding capacity. At an estimated physiological concentration of 5 mM phosphate, RenaGel binds 2.6 mmole phosphate/g of phosphate binder. The in vivo binding study shows that RenaGel mixed into the diet decreased urinary phosphorus excretion in a dose dependent manner. RenaGel particles with a 23 microns mean diameter are more efficacious than the larger ones. In conclusion, the above studies indicate that RenaGel is a potent phosphate binder. RenaGel contains no calcium or aluminum and offers an alternative to existing phosphate binder treatments.

The point of zero charge (PZC), the surface acidity constants pK1 and pK2, and the concentration of the charged surface groups A10H2+ and AlO" throughout the pH range 3.0-11.0 were determined by potentiometric acid-base titrations for a series of 7-aluminas used in industry as carriers which contain various amounts of sodium and silicon. Moreover, these surface parameters were determined for a modified carrier prepared by deposition of sodium on the 7-Al203 surface. It was found that the PZC, pKu pK2, and the concentration of the positively charged groups (A10H2+) at very low pH increased with sodium content whereas the concentration of the negative groups (AlO") decreased. The sodium effect was very pronounced when it was selectively concentrated on the surface of 7-Al203 particles. The consequences of the above phenomena are discussed in relation to the use of 7-aluminas for preparation of supported catalysts by deposition of active ion via adsorption of positive or negative species.

We have differentiated the carboxylic acid groups present in the functionalized interface of "polyethylene carboxylic acid" (PE-C02H, a material prepared from low-density polyethylene film by reaction with aqueous chromic acid) into two subsets: those sufficiently close to the surface of the polymer to influence its wettability by water and those too deep to do so. This differentiation was accomplished by taking advantage of differences in rates of esterification of carboxylic acid groups in different regions of the functionalized interface and of differences in rates of hydrolysis of ester groups derived from them. The subset of functional groups influencing wettability comprises <30% of the total groups present in the functionalized interface and appears to be homogeneous in its chemical reactivity. The remaining groups (-70% of the total) do not directly influence wettability and appear to become less reactive with increasing depth in the polymer.The surface and subsurface carboxylic acid and ester moieties are both less reactive in hydrolysis and formation reactions them are these groups in organic molecules in solution. Reactivities of the interfacial functional groups depend on structure in ways having no analogy in reactions in solution. For example, the rate of base-catalyzed hydrolysis of esters present in the functionalized polymer interface is a strong function of the length of the alcohol component of the ester: n-octyl esters are more than 20000 times less reactive than methyl esters. We have also explored the acidity of the interfacial carboxylic acid groups using ATR-IR spectroscopy and contact angle measurements as probes. Both the local polarity of the environment of individual carboxylic acid groups and charge-charge interactions between carboxylate anions appear to be important in determining acidity. Comparisons of wettability of samples containing different proportions of carboxylic acid, carboxylate anion, and ester groups indicate that wettability is particularly sensitive to low concentrations of carboxylate anion. We hypothesize that this sensitivity reflects a limited ability of the functionalized interfacial region to reconstruct-perhaps only by small-amplitude rotations of its constituent carboxylate ions-in a way that minimizes its free energy by maximizing the number of these hydrophilic moieties in direct contact with the polar, aqueous liquid phase.