Hantz reactions

from Wikipedia, the free encyclopedia
QS-Chemie-Logo.svg
 This item has been on the quality assurance side of the editorial chemistry entered. This is done in order to bring the quality of the articles on the subject of chemistry in terms of form and content to the level desired in Wikipedia. We are grateful for your help , please take part in the discussion  ( new entry ) or revise the article accordingly.

As Hantz reactions is a class of reaction-diffusion systems referred to, in which reactions between two electrolytes in a gel , a spatial precipitation patterns result with a particular surface. The precipitation surface forms a barrier for the further diffusion of the electrolytes, which results in typical patterns of colloidal precipitation zones.

In some gels (e.g. in a polyvinyl alcohol gel), periodically organized microscopic structures arise even within the precipitate.

The experimental design is similar to one of the Liesegang rings : The precipitate is created by the reaction between electrolytes in a hydrogel , whereby one "inner" electrolyte is dissolved in the gel itself and the other, "outer" electrolyte diffuses into the gel from a highly concentrated solution. The precipitation zones form behind the diffusion front of the "outer" electrolyte and the precipitates are held by the gel at the point of origin.

They are named after the Hungarian biophysicist Peter Hantz .

Variants of reaction and types of patterns formed

The first representative of this reaction class was the reaction NaOH (external electrolyte) + CuCl 2 (internal electrolyte). Later it turned out that NaOH + AgNO 3 , CuCl 2 + K 3 [Fe (CN) 6 ], NaOH + AlCl 3 and NH 3 + AgNO 3 also showed a similar behavior in several hydrogels. The patterns that appear also depend on the type of hydrogel that contains the internal electrolyte.

Precipitation patterns that form in these reactions are extraordinarily rich. In addition to shapes such as triangular patterns, helices and cardioids (primary patterns), regular colloidal layers of precipitation with a periodicity of less than 20 micrometers (microscopic, secondary patterns) can also occur.

Primary pattern

Sequence of the formation of primary patterns. Two consecutive snapshots are displayed. Dark gray represents the precipitation, light gray the hydrogel with the inner electrolyte, white the semi-permeable “passive” edges and black the advancing, “active” precipitation front segments. Single arrows indicate the direction of growth of the active precipitation segments, while double arrows indicate their direction of regression.
3D primary, macroscopic patterns in the NaOH + CuCl 2 reaction in PVA gel

The arrangement that best shows the sequence of events that lead to the formation of primary patterns is that in which the external electrolyte penetrates a thin layer of gel between two sheets of glass. In this case, both the diffusion front and the "active" (advancing) precipitation front have a one-dimensional shape. If there are impurities or obstacles in the gel, the precipitation can stop at these points and the advancing precipitation front following the diffusion front becomes split up.

As the segments of the fractured precipitation front (the active segments) advance, they get shorter, resulting in triangular regions that are filled with precipitation. At the same time, complementary, triangular areas are created that are free from precipitation (see illustration). The precipitation stops temporarily or permanently in these empty areas, because the sloping, passive edges of the previously formed precipitation surface act as a membrane and block the diffusion of the external electrolyte (one of the reagents).

The mechanism behind the regression of the active precipitation front segments is not fully understood. It is believed that an intermediate product forms on the active segments. The concentration of the intermediate is reduced at the edges of the active segments (where the active segments and passive edges meet) while a critical concentration is required for precipitation to take place. This critical concentration is not reached at the edges of the active segments; therefore the next precipitation levels will be shorter.

When the external electrolyte is poured into a glass tube on top of a gel column, the diffusion front is roughly the shape of a (two-dimensional) disk. In this case, the active precipitation fronts involved in pattern formation can perform more complicated movements, leading to more complex precipitation patterns that depend on the external and internal electrolyte concentration. This includes the formation of multi-armed helices, cardioids embedded in one another, Voronoi tessellations ( e.g. for reaction-diffusion computers), so-called target patterns and other, even more complex forms.

Secondary, microscopic patterns

Secondary, microscopic patterns in the NaOH + AgNO 3 reaction in PVA gel, scale bar = 25 μm

Under certain conditions, for example when the cation of the internal electrolyte is Cu 2+ or Ag 2+ , regular microscopic layers of colloidal grains are formed. This phenomenon is particularly noticeable when the reactions take place in poly (vinyl) alcohol gel (PVA gel) and the speed of the precipitation front falls below about 0.3 μm / s. The finest secondary microscopic patterns were observed in the NaOH + AgNO 3 reactions, in which the periodicity dropped below 10 μm. The chemical mechanism of this pattern formation is not fully understood; but computer simulations based on phase separation, described by the Cahn-Hilliard equation with a moving source front, show the most important properties of the structure of the microscopic patterns. In this model, the moving source front leaves behind a dissolved substance (an unstable phase), which then separates into empty or precipitated layers (stable phases).

There may also be defects in the regular microlayers that may interact even during anterior spread. These microscopic patterns have also aroused interest in various areas of micro- and nanotechnology.

See also

Web links

Individual evidence

  1. ↑ Laura M. Barge, Noreen L. Thomas et al .: From Chemical Gardens to Chemobrionics . In: Chemical Reviews . tape 115 , no. 16 , 2015, p. 8652-8703 , doi : 10.1021 / acs.chemrev.5b00014 .
  2. ^ Péter Hantz: Pattern Formation in the NaOH + CuCl2 Reaction . In: Journal of Physical Chemistry B . tape 104 , no. 17 , 2000, ISSN  1520-6106 , p. 4266-4272 , doi : 10.1021 / jp992456c .
  3. ↑ a b P. Hantz: Pattern Formation in a New Class of Precipitation reactions (PDF), Thesis. Université de Genève, 2006.
  4. ^ Benjamin PJ de Lacy Costello, Péter Hantz, Norman M. Ratcliffe: Voronoi diagrams generated by regressing edges of precipitation fronts . In: The Journal of Chemical Physics . tape 120 , no. 5 , 2004, ISSN  0021-9606 , p. 2413-2416 , doi : 10.1063 / 1.1635358 .
  5. ↑ András Volford, Ferenc Izsák, Mátyás Ripszám, István Lagzi: Pattern Formation and Self-Organization in a Simple Precipitation systems . In: Langmuir . tape 23 , no. 3 , 2007, ISSN  0743-7463 , p. 961-964 , doi : 10.1021 / la0623432 .
  6. ^ A b Edwin C. Constable, Catherine E. Housecroft, Artur Braun, Laurent Marot, Daniel Mathys, István Lagzi, Rita Tóth, Roché M. Walliser: Understanding the formation of aligned, linear arrays of Ag nanoparticles . In: RSC Advances . tape 6 , no. 34 , 2016, ISSN  2046-2069 , p. 28388-28392 , doi : 10.1039 / C6RA04194A .
  7. ^ Stefan C. Müller, John Ross: Spatial Structure Formation in Precipitation Reactions . In: Journal of Physical Chemistry A . tape 107 , no. 39 , 2003, ISSN  1089-5639 , p. 7997-8008 , doi : 10.1021 / jp030364o .
  8. ↑ Péter Hantz, Julian Partridge, Győző Láng, Szabolcs Horvát, Mária Ujvári: Ion-Selective Membranes Involved in Pattern-Forming Processes . In: Journal of Physical Chemistry B . tape 108 , no. 47 , 2004, ISSN  1520-6106 , p. 18135-18139 , doi : 10.1021 / jp047081w .
  9. ↑ A. Adamatzky: Advances in Unconventional Computing 2016th
  10. ↑ Andy Adamatzki, Ben De Lacy Costello, Tetsuya Asai, reaction-diffusion computer, Elsevier 2005, p 42
  11. ^ Péter Hantz: Regular microscopic patterns produced by simple reaction diffusion systems . In: Physical Chemistry Chemical Physics . tape 4 , no. 8 , 2002, ISSN  1463-9084 , p. 1262-1267 , doi : 10.1039 / B107742B .
  12. ^ Péter Hantz, István Biró: Phase Separation in the Wake of Moving Fronts . In: Physical Review Letters . tape 96 , no. 8 , 2006, p. 088305 , doi : 10.1103 / PhysRevLett.96.088305 .
  13. ↑ BA Grzybowski: Chemistry in Motion: Reaction-Diffusion Systems for Micro- and Nanotechnology 2009.