Biomedical subjects

Wei-Ren Chen

Publications and source records attributed to Wei-Ren Chen.

7 recordsLinked to original sources

PubMed2006

Incorporating intermicellar interactions in the fitting of SANS data from cationic wormlike micelles.

Small-angle neutron scattering (SANS) from cationic wormlike micellar solutions composed of hexadecyltrimethylammonium bromide (CTABr) and hexadecylpyridinium bromide (CPyBr) in deuterated water was studied at 40 degrees C as a function of surfactant and salt concentrations. Two scattering functions of semiflexible chains incorporating excluded volume effects, with and without the intermicellar interactions, were used in SANS data model fitting. Two needed changes were made in the well-accepted models. Extensive and systematic SANS data analysis suggests the robustness of these corrected scattering functions when the intermicellar interactions are included. The influence of the headgroups and ionic strength on the contour length and micellar flexibility of these two systems was demonstrated on the basis of the quantitative structural information obtained from the model fitting. Micellar flexibility was found to depend on surfactant concentration, even when intermicellar interactions were taken into account, despite predictions to the contrary.

Cations↗

PubMed2005

Effective long-range attraction between protein molecules in solutions studied by small angle neutron scattering.

Small angle neutron scattering intensity distributions taken from cytochrome C and lysozyme protein solutions show a rising intensity at a very small wave vector Q, which can be interpreted in terms of the presence of a weak long-range attraction between protein molecules. This interaction has a range several times that of the diameter of the protein molecule, much greater than the range of the screened electrostatic repulsion. We show evidence that this long-range attraction is closely related to the type of anion present and ion concentration in the solution.

Animals↗

PubMed2005

Cluster formation in two-Yukawa fluids.

We present a different and efficient method for implementing the analytical solution of Ornstein-Zernike equation for two-Yukawa fluids in the mean spherical approximation. We investigate, in particular, the conditions for the formation of an extra low-Q peak in the structure factor, which we interpret as due to cluster formation in the two-Yukawa fluid when the interparticle potential is composed of a short-range attraction and a long-range repulsion. We then apply this model to interpret the small angle neutron scattering data for protein solutions at moderate concentrations and find out that the presence of a peak centered at Q=0 (zero-Q peak) besides the regular interaction peak due to charged proteins implies an existence of long-range attractive interactions besides the charge repulsion.

Journal Article↗

PubMed2004

Structural arrest transitions in fluids described by two Yukawa potentials.

We study a model colloidal system where particles interact via short-range attractive and long-range repulsive Yukawa potentials. Using the structure factor calculated from the mean-spherical approximation as the input, the kinetic phase diagrams as functions of the attraction depth and the volume fraction are obtained by calculating the Debye-Waller factors in the framework of the mode-coupling theory for three different heights of the repulsive barrier. The glass-glass reentrance phenomenon in the attractive colloidal case is also observed in the presence of the long-range repulsive barrier, which results in the lower and upper glass regimes. Competition between the short-range attraction and the long-range repulsion gives rise to new regimes associated with clusters such as "static cluster glass" and "dynamic cluster glass," which appear in the lower glass regime. Along the liquid-glass transition line between the liquid regime and the lower glass regime, crossover points separating different glass states are identified.

Journal Article↗

PubMed2003

Neutron- and light-scattering studies of the liquid-to-glass and glass-to-glass transitions in dense copolymer micellar solutions.

Recent mode coupling theory (MCT) calculations show that if a short-range attractive interaction is added to the pure hard sphere system, one may observe a new type of glass originating from the clustering effect (the attractive glass) as a result of the attractive interaction. This is in addition to the known glass-forming mechanism due to the cage effect in the hard sphere system (the repulsive glass). The calculations also indicate that if the range of attraction is sufficiently short compared to the diameter of the particle, within a certain interval of volume fractions where the two glass-forming mechanisms nearly balance each other, varying the external control parameter, the effective temperature, makes the glass-to-liquid-to-glass reentrance and the glass-to-glass transitions possible. Here we present experimental evidence of both transitions, obtained from small-angle neutron-scattering and photon correlation measurements taken from dense L64 copolymer micellar solutions in heavy water. Varying the temperature in certain predicted volume fraction range triggers a sharp transition between these two different types of glass. In particular, according to MCT, there is an end point (called A3 singularity) of this glass-to-glass transition line, beyond which the long-time dynamics of the two glasses become identical. Our findings confirm this theoretical prediction. Surprisingly, although the Debye-Waller factors, the long-time limit of the coherent intermediate scattering functions, of these two glasses obtained from photon correlation measurements indeed become identical at the predicted volume fraction, they exhibit distinctly different intermediate time relaxation. Furthermore, our experimental results obtained from volume fractions beyond the end point are characterized by the same features as the repulsive glass obtained before the end point. A complete phase diagram giving the boundaries of the structural arrest transitions for L64 micellar system is given.

Journal Article↗

PubMed2003

The glass-to-glass transition and its end point in a copolymer micellar system.

We present experimental evidence, obtained from small-angle neutron scattering and photon correlation measurements, indicating the existence of two types of structurally arrested (glassy) states in a copolymer micellar system with a short-range interparticle attractive interaction. Within a certain range of micellar volume fractions, a sharp transition between these two types of glass is observed by varying the temperature. Furthermore, we found an end point of this transition line beyond which the two glasses become identical in their local structure and their long-time dynamics. These findings confirm the recent mode-coupling theory predictions regarding the phase behavior of the structurally arrested states for this type of system.

Journal Article↗

PubMed2002

Small-angle neutron scattering study of the temperature-dependent attractive interaction in dense L64 copolymer micellar solutions and its relation to kinetic glass transition.

We made small-angle neutron scattering (SANS) study of a Pluronic L64 micellar system in aqueous solution at high polymer concentrations (wt %), 35%<c<53%, in a temperature range where a kinetic glass transition has been observed by photon correlation spectroscopy and zero shear viscosity measurements. We analyze SANS intensity distributions assuming that the micelles are spherical and interact among themselves by an effective pair potential, consisting of a hard core plus a narrow attractive square well, the depth of which is temperature dependent. The theory is able to account for the intensity distribution quantitatively when the micellar system is in the liquid phase (but qualitatively when the micellar system is in the glass state), giving values of four parameters: the aggregation number of the micelle N, the volume fraction occupied by the micelles phi, the fractional width of the square well epsilon, and the effective temperature T*=k(B)T/u, where -u is the depth of the square well. Thus, we are able to assign a point in the phase diagram (the T*-phi plane) for each measured micellar liquid and glass state. Comparison with a phase diagram predicted recently by mode coupling theory calculation allows us to identify the existence of the so-called liquid-to-attractive-glass transition line. We also found the evidence of glass-to-glass transition at volume fraction of 0.54 predicted by the mode coupling theory.

Journal Article↗