Biomedical subjects

K Lunkenheimer

Publications and source records attributed to K Lunkenheimer.

7 recordsLinked to original sources

PubMed2004

Automated high-performance purification of surfactant solutions: study of convective-enhanced adsorption.

Due to the amphiphilic nature of surfactants, they generally contain traces of their more surface-active parent components irrespective of the as received surfactant's label declaration. These trace impurities are preferentially enriched in the surfactant's adsorbed layer. To attain the state of "surface chemical purity" that implies the absence of impurities in the adsorbed layer, we had developed an automatic high-performance purification apparatus years ago. Here we put forward an improved version of this successful technique that considerably reduces the time required to purify surfactant solutions by accelerating the adsorption through convection. The advantages and reliability of the new technique are demonstrated by the purification characteristics of different solutions of as-received surfactants of varying degree of purity. As the overall time gained with convective purification is of 1 order of magnitude, the improved technique is particularly favorable for surfactant solutions that are prone to hydrolysis or degradation.

Journal Article↗

PubMed2002

On the adsorption behavior of soluble, surface-chemically pure hemicyanine dyes at the air/water interface.

Equilibrium surface tension (sigma(e)) versus concentration isotherms of surface-chemically pure aqueous solutions of the homologous series of N-n-alkyl-4'-(dimethylamino)-stilbazium bromides ("hemicyanines") were measured at 295 K. The adsorption parameters of saturation adsorption and standard free energy of adsorption of the hemicyanines were determined from the evaluation of the sigma(e) vs c isotherms using a two-state approach to the surface equation of state. The adsorption parameters reveal a very pronounced phenomenon of alternation (even/odd effect), which has so far not been found to a large extent such as this. Thus, the cross-sectional areas of the odd members are almost twice those of the related even members. Surface activity of the odd members is stronger than that of the even members. UV-vis investigations showed that there is no indication of aggregate formation in the adsorption layer. The thermodynamic results give evidence for distinct differences between the surface conformations of the even- and the odd-chain hemicyanine dyes although the reasons for this are not known.

Adsorption↗

PubMed2002

Phase transition in an adsorption layer of a soluble surfactant at the air-water interface.

In this paper we provide experimental evidence for a phase transition between a liquid- and gas-like phase occurring in an adsorption layer of a soluble surfactant at the air-water interface. The equilibrium surface tension sigma(e) versus bulk concentration sigma(e) (c) isotherm of surface chemically pure sodium 2-[4-(4-trifluoromethyl-phenylazo) phenoxy]-ethane sulfonate was measured at a temperature of 295 K up to the solubility limit of the amphiphile. The sigma(e) (c) isotherm could be fitted by Frumkin's equation of state. The lateral interaction energy is just above the limit for which Frumkin's model predicts a phase transition. The corresponding surface pressure pi versus surface area A isotherm possesses striking similarities to first-order phase transitions in the Langmuir monolayer. The fact that the difference in the two-dimensional density is only a factor of 2 indicates that the system is very close to the critical point. The surface phases were further characterized by surface second harmonic generation. The major structural difference between the two surface phases is the amphiphile's molecular orientation. A mean orientation of the amphiphile of about 80 degrees was found in the gas analogous phase, whereas a molecular tilt of 38 degrees has been identified in the liquid-like phase.

Adsorption↗

PubMed2000

Effect of the Supporting Electrolyte on the Adsorption of Octanoic Acid at the Mercury/Electrolyte Interface.

The adsorption and the changes in the interfacial composition of octanoic acid at the mercury/electrolyte interface was studied by measuring the differential capacitance at different concentrations of the supporting electrolyte, at various supporting electrolyte systems and at various temperatures. The adsorption was followed by means of capacity-potential curves in the short-term region and capacity-time transients in the long-term region at selected potentials, in all the potential ranges. A decrease of the capacitance with time was observed in most cases, followed either by a constant capacitance value or by its increase. In the short-term region, anion-surfactant complexes are formed, where the anions act as bridges between the perpendicularly oriented surfactant molecules. The larger is the negative charge of the anion, the more negative will be the charge of the anion-surfactant complex leading to a shift of the potential of maximal adsorption to more positive values. The formation of metastable condensed films is best when the hydration of the anion and its size are not too large. In the long-term region the observed increase of the capacity with time can be explained as an exchange of the metastable condensed film by a hemimicellar surface state. Here, the anions act as cores of the hemimicelles, and the hydrophilic acid groups of the amphiphiles contact the solution. Two contrary effects determine the formation of the hemimicelles. The greater is the specific adsorption of the anions, the larger is the formation of hemimicelles and the increase of the capacity. With an increase in the ability of the anions to break the water structure (lyotropic or Hofmeister series), the formation of hemimicelles will be decreased. Copyright 2000 Academic Press.

Journal Article↗

PubMed1998

Influence of Surface-Active Trace Impurities on the Surface Properties of Aqueous Solutions of Oligoethylene Glycol Monooctyl Ethers.

In this paper the influence of surface-active trace impurities in aqueous oligoethylene glycol monooctyl ether solutions on their adsorption behavior at the air/water interface is considered. The equilibrium surface tension (final sigmae) vs logarithm of concentration (ln c) isotherms of surfactants when used "as received" usually reveal a broad concentration range characterized by a constant slope, dfinal sigmae/d ln c. This linear dependence of the equilibrium surface tension on the logarithm of the surfactant concentration is observed for various surfactants at various fluid interfaces, which contradicts the fundamental Gibbs equation given in Eq. [1]. The same behavior was also observed for aqueous solutions of chemically well-defined oligoethylene glycol monooctyl ethers. In this case the paradoxical behavior did vanish after purification of aqueous solutions until the state of "surface-chemical purity" was reached using an apparatus for programmed high-performance purification of surfactant solutions. The gas chromatographic analyses of the original samples used showed that they contained impurities of different chemical structure and surface activity. Analysis of material aspirated from the surface of these alcohol ethoxylate solutions indicates that these trace impurities are enriched or impoverished at the solution's surface. A clear correlation between the surface activity parameter and the enrichment/impoverishment was found. Thus, these findings prove that the paradoxical adsorption behavior (dfinal sigmae/d ln c = constant) of the oligoethylene glycol monooctyl ethers is caused by surface-active trace impurities. These results support the hypotheses suggested by K. Lunkenheimer and Ch. Wedler [Tenside, Surfactants, Detergents 30, 342 (1993)] that generally it is the effect of surface-active trace impurities that gives rise to paradoxical adsorption behavior with surfactant systems at fluid interfaces. Copyright 1998 Academic Press.

Journal Article↗

PubMed1998

Measurements of the Surface Elasticity in Medium Frequency Range Using the Oscillating Bubble Method.

Various experimental techniques are available for the investigation of dynamic surface tension, and a generally accepted theoretical model of these dynamics has been established. However, reliable rheological parameters of a fluid surface are very scarce. Therefore, comparisons of rheological parameters resulting from slow and faster processes or from theoretical calculations are required. In particular, a comprehensive experimental verification of the complex surface elasticity modulus which characterizes the dynamic behavior of a fluid surface in an appropriate manner is desirable. For this reason a new version of oscillating bubble method was developed which allows exact measurements of the complex elasticity modulus in the frequency range 3-500 Hz. With this method the assumptions of the theory of dynamic surface tension can be verified for medium frequencies. The new experimental results, in particular the experimental determination of the Gibbs elasticity, reveal that these assumptions are only approximately valid for faster processes. However, with a slight modification of the established model the experimental results can be explained. These experiments were carried out with solutions of tridecyl dimethyl phosphine oxide, fatty acids, n-alkanols, and triton X-100 at different surfactant concentrations. Copyright 1998 Academic Press.

Journal Article↗

PubMed1990

Detection of Lassa virus RNA in specimens from patients with Lassa fever by using the polymerase chain reaction.

Suitable oligonucleotide primers and probes were synthesized to amplify Lassa virus (Josiah strain)-specific nucleoprotein and glycoprotein gene fragments by using reverse transcription combined with the polymerase chain reaction (PCR). Our primers did not amplify the related lymphocytic choriomeningitis virus. By using PCR, about 50 50% tissue culture infective doses could be detected in the supernatant of infected cells. Furthermore, in all five serum specimens and four of five urine specimens of patients with acute Lassa fever, viral RNA could be demonstrated. Negative results were obtained with all serum and urine specimens of healthy subjects. Our data suggest that PCR may be applied as an alternative to virus isolation in the rapid diagnosis of Lassa fever.

Base Sequence↗