Biomedical subjects

Kam C Tam

Publications and source records attributed to Kam C Tam.

6 recordsLinked to original sources

PubMed2006

Insights on polymer surfactant complex structures during the binding of surfactants to polymers as measured by equilibrium and structural techniques.

This tutorial review provides new insights into the binding interactions between anionic surfactant molecules and various macromolecules in solution. The systems are of inherent scientific interest because synergistic mixing between these two components leads to complexes commonly found in applications such as detergency, cosmetic products, rheology control, paint and pharmaceutical formulations. We describe how the basic foundations, which are prerequisite to characterize a given polymer/surfactant system are evaluated together with information on the binding mechanism and structure derived from several methodologies.

Anions↗

PubMed2006

Interaction between silicates and ionic surfactants in dilute solution.

Understanding the interaction between silicate ions and surfactants is critical for the design and development of mesoporous siliceous materials. We examined the interaction between sodium silicate ions and three different cationic surfactants [namely, cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TTAB), and dodecyltrimethylammonium bromide (DTAB)] and an anionic surfactant [sodium dodecyl sulfate (SDS)] in dilute solution at room temperature. From the combination of several techniques, such as conductometric and potentiometric titrations, dynamic light scattering, and isothermal titration calorimetry, the phase behavior of the sodium silicate and CTAB system was determined. We observed that the aggregation behavior of the silicate-CTAB system is similar to that of a polymer-surfactant system. The formation of the silicate-CTAB complex is induced by the adsorption of SiOH and SiO- groups, aided by CTAB unimers. The electrostatic attraction and hydrophobic interaction are the dominant forces controlling the formation of silicate-CTAB complexes. When these complexes are saturated with CTAB unimers, free CTAB micelles are then produced. TEM micrographs revealed that a stable Si-O-Si network is absent within the silicate-CTAB complexes, and surprisingly, stable silicate-CTAB complexes with ordered structure were observed. The present finding is important for understanding the interaction between silicate and surfactant in the synthesis of mesoporous structure in the dilute solution regime.

Journal Article↗

PubMed2005

Microstructure and rheology of stimuli-responsive microgel systems--effect of cross-linked density.

The effect of cross-linked density on the rheological behavior of model pH-responsive microgel systems consisting of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with di-allyl phthalate (DAP) was examined. Neutralization of acid groups increases the osmotic pressure exerted by counter-ions trapped in the polymeric network against the ions in bulk solution, which is responsible for the swelling and increase in viscosity. The viscosity exhibits a maximum at approximately 1 wt.% DAP and it decreases to a steady value at 4 wt.% DAP, which is independent of pH and particle concentrations. Static light scattering results confirmed this optimum density as the critical point where sufficient cross-link points are present to produce permanent junctions that permit optimal swelling of the microgel particles. In addition, the variation of relative swelling with cross-linked densities of our model microgel systems agrees with the theoretical scaling law, Q alpha (yalphaN(x))(3/2) for cross-linked densities beyond this optimum point (Q is the swelling ratio, y is the acidic MAA content, N(x) is the average number of monomer units between two cross-linked points, and alpha is the degree of neutralization). By combining the results from light scattering and rheological measurements, we are able to correlate the microstructural evolution of the colloidal systems with their bulk rheological behavior.

Acrylic Resins↗

PubMed2005

Osmotic compressibility of soft colloidal systems.

A turbidimetric analysis of particle interaction of model pH-responsive microgel systems consisting of methacrylic acid-ethyl acrylate cross-linked with diallyl phthalate in colloidal suspensions is described. The structure factor at zero scattering angle, S(0), can be determined with good precision for wavelengths greater than 500 nm, and it measures the dispersion's resistance to particle compression. The structure factor of microgels at various cross-linked densities and ionic strengths falls onto a master curve when plotted against the effective volume fraction, phi(eff) = kc, which clearly suggests that particle interaction potential and osmotic compressibility is a function of effective volume fraction. In addition, the deviation of the structure factor, S(0), of our microgel systems with the structure factor of hard spheres, S(PY)(0), exhibits a maximum at phi(eff) approximately 0.2. Beyond this point the osmotic de-swelling force exceeds the osmotic pressure inside the soft particles resulting in particle shrinkage. Good agreement was obtained when the structural properties of our microgel systems obtained from turbidimetric analysis and rheology measurements were compared. Therefore, a simple turbidimetric analysis of these model pH-responsive microgel systems permits a quantitative evaluation of factors governing particle osmotic compressibility.

Journal Article↗

PubMed2004

Microstructure and rheology of stimuli-responsive nanocolloidal systems-effect of ionic strength.

The effect of ionic strength on the rheological behavior of model pH-responsive nanocolloidal systems consisting of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with diallyl phthalate (DAP) was examined. Neutralization of acid groups increases the osmotic pressure exerted by counterions trapped in the polymeric network against ions in bulk solution, which is responsible for the swelling and increase in viscosity. Swelling decreases with increasing salt concentration as a result of reduced osmotic pressure inside the microgels, which is attributed to the charge shielding effect of counterions (salt) on the negatively charged carboxylate groups. Electromotive measurements using ion-selective electrodes confirmed that not all the counterions, that is, K+, remain mobile, but a fraction of these ions can penetrate the porous microgel particles to shield the negatively charged carboxylate groups. A consequence of this is that some of the Na+ counterions inside the particles are expelled, thus regaining their translational entropy, and become mobile sodium ions in the bulk solution. We successfully developed a new scaling law that relates the swelling ratio, Q, of microgels as a function of neutralization degree, alpha, cross-linked density, Nx, molar fraction of acidic units, y, and concentration of mobile counterions, CK+ and CNa+, represented as (Nx/c0)(CK+ + CNa+Q + Q2/3 proportional, variant yNxalpha. The new scaling law no longer assumes that all the counterions are trapped inside the microgels. The proportionality reduces to the form Q proportional, variant (yalphaNx)3/2 in the absence of salt, that is, CK+ + CNa+ approximately 0. By combining the results from light scattering and rheological measurements, we are able to correlate the microstructural evolution of the colloidal systems with their bulk rheological behavior.

Journal Article↗

PubMed2004

Association behavior of poly(methyl methacrylate-block-methacrylic acid) in aqueous medium.

The atom transfer radical polymerization technique was used to synthesize the poly(methyl methacrylate-block-methacrylic acid) (P(MAA-b-MMA)) copolymer in order to study the aggregation behavior in aqueous solution over the course of neutralization. Combinations of static and dynamic light scattering (SLS, DLS) and potentiometric titration techniques were used to investigate the size and shape of the micelle at various degrees of neutralization (alpha). By comparing the effect of different polymer chain length with similar MMA/MAA ratio on the aggregation behavior during neutralization, we found relatively strong entanglement of long MMA polymer chains. The comparison between the different MMA/MAA ratios showed that longer MMA chains produced more entanglements. Conductometric titration was used to determine the counterion condensation phenomenon during the course of neutralization. At a critical micellar charge density observed at alpha approximately 0.4, Na+ ions are condensed on the polymer chains. The amount of condensed Na+ was evaluated by the conductivity change, yielding the condensation ratio when the polymer was completely neutralized.

Journal Article↗