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J van Turnhout

Publications and source records attributed to J van Turnhout.

4 recordsLinked to original sources
PubMed2002

Analysis of the dielectric permittivity of suspensions by means of the logarithmic derivative of its real part.

Measurement of the dielectric permittivity of colloidal suspensions in the kilohertz frequency range (the so-called low-frequency dielectric dispersion) is a promising tool for the electrokinetic characterization of colloids. However, this technique is less used than would be desirable because of the difficulties associated with the measurements, the most important of which is the electrode polarization (EP). Recently (M. Wübbenhorst and J. Van Turnhout, Dielectrics Newsl. November (2000)) a method was proposed that appears capable of separating the unwanted electrode effects from the double-layer relaxation that we are interested in. The method, based on the logarithmic derivative of raw epsilon'(omega) data (epsilon'(omega) is the real part of the permittivity of the suspension for a frequency omega of the applied AC field), is first checked against the well-known theory of the AC permittivity of colloidal suspensions developed by DeLacey and White (E. H. B. DeLacey and L. R. White, J. Chem. Soc. Faraday Trans. 277, 2007 (1981)). We show that the derivative epsilon''(D)(omega)=-(pi/2)(partial differential epsilon'/partial differential ln omega) gives an excellent representation of the true imaginary part of the permittivity, epsilon''(omega). The technique is then applied to experimental data of the dielectric constant of polystyrene and ethylcellulose suspensions. We found that epsilon''(D) displays two separated behaviors when plotted against log omega in the frequency range 100 Hz-1 MHz: a monotonous decrease (associated with EP) followed by an absorption peak (associated with the double-layer relaxation, or alpha-relaxation). Interestingly, they are separated enough to make it possible to easily find the characteristic frequency of the alpha-relaxation. Fitting a relaxation function to epsilon''(D)(omega) after eliminating the part due to EP, we could calculate the real part epsilon'(omega) and compare it to the DeLacey and White (DW) theoretical predictions. A significantly better agreement between DW calculations and experimental epsilon'(omega) data is obtained when the logarithmic derivative method is used, as compared to the classical electrode-separation techniques.

Journal Article↗
PubMed1987

Oral versus im administration of high-dose medroxyprogesterone acetate in pretreated patients with advanced breast cancer.

In a multicenter trial, 123 patients with advanced breast cancer who had been treated with tamoxifen and/or chemotherapy were randomized to receive medroxyprogesterone acetate (MPA) orally 300 mg X 3 daily or im 500 mg daily for 4 weeks and 500 mg X 2 weekly thereafter. All case histories were reviewed extramurally by the criteria of the International Union Against Cancer. Five and 11 patients were not eligible and evaluable for response, respectively. Pretreatment characteristics were well balanced in both treated groups. Twenty-five of all 107 (23%) evaluable patients achieved an objective remission, whereas in a further 15% the disease became stable after previous progression. Results in both treatment arms did not differ significantly. The median duration of objective remission was 12 and 14 months for orally and im treated patients, respectively (P greater than 0.10). No statistically significant differences in the survival times of orally and im treated patients were found. Pretreatment characteristics positively correlated with an objective remission during MPA therapy in both groups were age greater than 50 years (P less than 0.02) and no previous chemotherapy (P less than 0.01). Toxicity included an increase in body weight, cushingoid effects, muscle cramps, and tremors in both groups. In four patients on im therapy, local infections developed. Mean serum MPA levels reached values above 100 ng/ml in nine orally and eight im treated patients (P greater than 0.10), and neither differed significantly in the patients responding to or failing therapy. In both MPA arms, plasma cortisol levels were suppressed. The drop in plasma cortisol levels was more pronounced in patients with objective remissions than in patients who failed (P = 0.04). In conclusion, oral and im MPA in the given doses had similar activity. Im administration of MPA should be reserved for patients not able to take oral medication.

Administration, Oral↗