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High-Pressure Properties and Structure of Liquids.

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Correlation functions are very difficult to determine. The
amount N M E of frozen CH3 groups in relation to the total
number N M was determined as a function of the temperature
by separation of the nuclear magnetic absorption curve.
Hence it is possible to calculate the number of methyl groups
that pass from the frozen state into rotational motion
as a result of a temperature increase 6T for PC and PMSt.
[l] H. L. Hodgkin and A . F. Huxley, I. Physiology 117, 500
(1952).
[2] G. Adam, Z. Naturforsch. 236, 181 (1968).
[ 3 ] K. S . Cole and J . W. Moore, J. gen. Physlol. 4 4 , 123 (1960).
[41 I . Tasaki, I. Singer, and K. Takenaka, J. gen. Physiol. 48,
1095 (1965).
It is thus possible, from the nuclear magnetic resonance experiment, to find the number of CH3 groups that are forced
by a temperature change 6 T from a rotational movement
below vc to a rotational movement above vC. This number is
related to the number of CH3 groups found in a frequency
range 6, around a frequency vC that is fixed by the temperature.
High-pressure Properties and
Structure of Liquids
[VB 166 1El
Lecture at Hamburg (Germany) on June 14, 1968
German version: Angew. Chem. 80, 851 (1968)
~
.-
~
[ * ] Doz. Dr. R. Kosfeld
Institut fur Physikalische Chemie der
Technischen Hochschule
51 Aachen, Templergraben 59 (Germany)
Electrical Excitation of the Axon Membrane as
Co-operative Ion Exchange
By G. Adcim [*I
The widely accepted theory proposed by Hodgkin and
Huxleyril for nerve stimulation fails to explain the mechanism of regulation of the state of the axon membrane by the
membrane potential or the cation activities.
If, on the other hand, the axon membrane is regarded as a
two-dimensional cation exchanger interacting with the internal and external electrolyte reservoirs, a physico-chemical
mechanism can be advanced for the control of the electrical
state of the membranef21. In this mechanism, calcium ions
are bound in the lattice sites of the cation exchanger in the
resting state. On depolarization or on lowering of the calcium
activity in the outside medium, the resting state becomes
thermodynamically unstable, and calcium is co-operatively
displaced from the lattice sites by monovalent cations. The
movements of the ions during this cation exchange give rise
to an ion current flowing inward, as observed e.g. in the
voltage-clamp experiment.
This co-operative cation exchange is described as a twodimensional phase transition. Its kinetics can be theoretically
described for small depolarizations o n the basis of the concept of nucleation and nuclear growth.
The resulting kinetic theory has been applied to the. measurements of Cole and Moore[3Jof the ion current in the voltage
clamp experiment. Within the range of validity of the theory,
i.e. for small depolarizations, the experimentally determined
dependence of the ion current on time and on the membrane
potential, particularly the threshold behavior that characterizes axon stimulation, is quantitatively described by the
equations derived.
Three parameters must be matched. Two of these have simple
molecular meanings, and ha;e the following numerical
values: a, = 21 x 21 .&2 = area per binding site for the cooperatively exchangeable cations; w = 5.1 kcal/mole = interaction energy between two binding sites in the two different
bonding states.
It is probable, from the order of magnitude of these parameters, that the structural units responsible for the axon stimulation are membrane-bound proteins, as was suggested by
Tasaki e f al. [ 4 J .
Lecture a t Konstanz (Germany) on June 27, 1968
[ V B 167 IE]
German version: Angew. Chem. 80, 806 (1968)
[*] Dr. G. Adam
Institut fur Physiologische Chemie der Universitdt
8 Miinchen 15, Goethestrasse 33 (Germany)
Angew. Chem. internat. Edit.
Yo!. 7 (1968) No. I0
By E. Kuss[*I
In contrast to solids, compressed gases and liquids can be
studied at pressures up to 10 kbar over a wide range of densities and hence valuable information about the intermolecular
forces and the structure of liquids, which is still largely unknown,can be obtained from the physical properties. Properties
of interest in this connection are the pVT data, the equations of
state, the thermal conductivity, the viscosity, the spin-lattice
interaction of the nuclear magnetic resonance, and the electric birefringence.
The theoretical calculation of the second, third fourth, and
higher virial coefficients from fundamental potential functions leads to very complicated mathematical expressions and
in some cases to very different constants for the potential
functions. Owing to the mathematical difficulties, the potential U(r) is always taken to be spherically symmetrical, i.e.
the effect of the shape of the molecule is not yet theoretically
expressible.
The effect of the molecular structure on the pVT data of
liquids has been studied experimentally up to 2000 atm, and
in some cases even up to 5000 atm. A piezometric method
and a buoyancy method have been developed for this purpose. Since no suitable material was available for use as a
float, the test substance was introduced into a hollow body
and the buoyancy was measured in mercury. Measurements
that have not yet been completed show that some substances
whose viscosities are extremely pressure-dependent have a
very low compressibility (e.g. 2,4-bis-(l-phenylethyl)methoxybenzene).
The viscosity measurements were carried out in a falling
sphere viscometer (accuracy & 2 yo)and a capillary viscometer
(accuracy f 1 yo),both of which are suitable for fully automatic operation. The effects of chain length, “degree of
branching”, and other parameters of the molecular structure
on the viscosity-pressure behavior were determined for
numerous substances. On the basis of the relationships found,
it was possible to synthesize substances whose viscosities at
2000 atm are up to 8 x 106 times as great as a t atmospheric
pressure [I]. According to Benedek and PurceN[21, a comparison of measurements of the pressure-dependence of the
viscosity and the nuclear magnetic relaxation leads to the
conclusion that the translational degrees of freedom are more
strongly reduced than the rotational degrees of freedom by
rising pressure.
The very small measured effect in the electric birefringence
requires the complete elimination of parasite birefringence
of the high pressure windows in high-pressure measurements.
Benzene showed a marked anomaly in the pressure dependence of the Kerr constant [31, which suggests a modification
of the r-electron cloud or an unusual change in the liquid
structure under pressure.
[VB 170 IE]
Lecture a t Berlin (Germany) on July 5, 1968
German version: Angew. Chem. 80, 806 (1968)
[“I Prof. Dr. E. Kuss
Institut fur Erdolforschung der Technischen Hochschule
3 Hannover, Am kleinen Felde 30 (Germany)
111 E. Kuss, Chemie-1ng.-Techn. 37, 465 (1965); Angew. Chem.
internat. Edit. 4 , 944 (1965).
[21 G. B. Benedek and E. M . Purcell, J. chern. Physics 22, 2003
(1954).
[3] E. Kuss and H. H. Heydemann, Z. physik. Chem. N.F. 43,
91 (1964).
829
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