“Maybe I should have thrown the data away”
Jacques Benveniste, 1935-2004
But being a scientist and believing in his data he could not.
Does water have memory?
The ‘memory of water’ is a popular phrase that is mostly associated with homeopathy and Jacques Benveniste  following his and others’ allergy research work . These research teams reported that solutes subjected to sequential physical processing and dilution show biological effects different from those apparent using just the water employed for the dilutions. The subject has drawn a lot of controversy with many scientists simply rejecting it outright without studying the evidence. The topic has recently been the subject of a number of papers in the journal Homeopathy (July, 2007)c  and has been reviewed . Although there is much support for water showing properties that depend on its prior processing (that is, water having a memory effect), the experimental evidence indicates that such changes are due primarily to solute and surface changes occurring during this processing. The experimentally corroborated memory phenomena cannot be taken as supporting the basic tenets of homeopathy , although they can explain some effects3.
Is water special?
The main evidence against water having a memory is that of the very short (~ps) lifetime of hydrogen bonds  between the water molecules . Clearly in the absence of other materials or surfaces (see later), the specific hydrogen bonding pattern surrounding a solute does not persist when the solute is removed any more than would a cluster around any specified water molecule, or else water would not know which of its myriad past solutes took preference. Indeed the atoms that make up the water molecule only remain together for about a millisecond in liquid water due to proton exchange (see water ionization ). A recent NMR study shows no stable (>1 ms, >5 ?M) water clusters are found in homeopathic preparations . It should, however, be noted that the lifetime of hydrogen bonds does not control the lifetime of clusters in the same way that a sea wave may cross an ocean, remaining as a wave and with dependence on its history, but with its molecular content continuously changing. Also, the equilibrium concentration of any clusters are governed by thermodynamics not kinetics.
An extraordinary paper authored by Nobel prize-winning Luc Montagnier has shown memory effects in aqueous DNA solutions that depend on interactions with the background electromagnetic field. These effects require the prior processing and dilution of the solutions and are explained as resonance phenomena with nanostructures derived from the DNA and water .
As applied to homeopathy, the ‘memory of water’ concept should also be extended to the memory of aqueous ethanol preparations, which are also used. Addition of ethanol to water adds an important further area of complexity. Ethanol forms solutions in water that are far from ideal and very slow to equilibrate . Although usually considered a single phase, such solutions may contain several distinct phases  and more generally consist of a complex mixture dominated by water-water and ethanol-ethanol clusters, where hydrogen bonding is longer-lived than in water alone . They also favor nanobubble (that is, nanocavity) formation . Thus, the peculiar behavior of aqueous solutions (as mostly discussed on this page) is accentuated by the presence of ethanol.
Does the glassware matter?
The process of silica dissolution has been much studied ,  ever since it was proven by Lavoisier over 200 years ago and fits with this argument. This may explain why glass is preferred over polypropylene tubes. It should be noted that dissolved silica is capable of forming solid particles with complementary structures (that is, imprints) to dissolved solutes and macromolecules and such particles will ‘remember’ these complementary structures essentially forever.
Is gas important?
Water does store and transmit information, concerning solutes, by means of its hydrogen-bonded network. Changes to this clustering network brought about by solutes may take some time to re-equilibrate. Agitation (succussion) may also have an effect on the hydrogen bonded network (shear encouraging destructuring) and the gaseous solutes (with critical effect on structuring ,  and possible important production of structuring nanobubbles  (nanocavities) , , and such effects may well contribute to the altered heats of dilution with such materials . Such mechanically induced hydrogen bond breakage may also give rise to increased (but low) hydrogen peroxide formation  [see equations] and such effects have been reported to last for weeks . It may be relevant to note that the presence of hydrogen peroxide can take part in and catalyze further reactions with other reactive species such as molecular oxygen and dissolved ozone18,  (not often recognized but present in nanomolar amounts) which may well vary with the number of succussion steps and their sequence, which may offer an explanation for the changes in efficacy of homeopathic preparations with the number of dilutions . Also of note are the known effects of low concentrations of reactive oxygen species on physiological processes such as the immune response; with the recent discovery of the importance of low levels of hydrogen peroxide being particularly relevant .
Does dilution happen as predicted?
Dilution is never perfect, particularly at low concentrations where surface absorption may well be a major factor, so that dilution beyond the levels that can be analytically determined remains unproven. Remaining material may be responsible for perceived differences between preparations and activity. Of course the water used for dilution is not pure relative to the putative concentration of the ‘active’ ingredient; even the purest water should be considered grossly contaminated  compared with the theoretical homeopathic dilution levels. This contamination may well have a major influence, and itself be influenced by the structuring in the water it encounters. Although it does, at first sight, seem unlikely that solutes in diluted ‘homeopathic’ water should be significantly different from a proper aqueous control, it has recently been cogently argued that the concentrations of impurities can change during the dilution process by reactions initiated by the original ‘active’ material , and this process has been mathematically modeled21.
A further consideration about ‘the memory of water’ is that the popular understanding concerning how homeopathic preparations may work not only requires this memory but also requires that this memory be amplified during the dilution; this amplification, necessitated by the increase in efficacy with extensive dilution, being even harder to explain. Samal and Geckeler have published an interesting, if controversial, paper  concerning the effect of dilution on some molecules. They found that some molecules form larger clusters on dilution rather than the smaller clusters thermodynamically expected. Just the presence of one such large ?m-sized particle in the ‘diluted’ solution could give rise to the noticed biological action (of course, some such preparations may be totally without action, being without such clustered particles).a 
However, it remains to explain this particular phenomenon, which appears to disobey the second law of thermodynamics. A possible explanation is that such biologically-active molecules can cooperatively form icosahedral expanded water networks (ES) to surround and screen them by the formation of face-linked icosahedra, similar to as expected in the minimal energy related poly-tetrahedral Dzugutov clusters . So long as such an icosahedral network structure requires the help of more than one neighboring such cluster to stabilize its formation then, in more concentrated solution, the molecules dissolve normally. However, as they are diluted (typically beyond about one clathrate-forming group per twelve icosahedral water clusters; 3,360 water molecules) no neighboring such clusters are available and the clusters coalesce to form larger clusters of biologically-active molecules within their own ES-related water network (so releasing some of the water). This tendency for particle formation is ultimately due to the hydrophobic effect and the tendency to form a small surface with the water. Overall the balance is expected to be rather fine between water cluster stabilization and particle cluster stabilization.
Solutions are more complex than expected
Water is not just H2O molecules. It contains a number of molecular species including ortho  and para water molecules , water molecules with different isotopic compositions such as HDO and H218O, such water molecules as part of weakly-bound but partially-covalently linked molecular clusters containing one, two, three or four hydrogen bonds, and hydrogen ion and hydroxide ion  species. Apart from such molecules there are always adventitious and self-created solutes in liquid water. Distilled and deionized water contain significant and varying quantities of contaminating ions. Often the criteria for ‘purity’ is the conductivity, but this will not show ionic contaminants at nanomolar, or even somewhat higher, concentrations due to the relatively high conductivity of the H+ and OH– ions naturally present. Other materials present will include previously dissolved solutes, dissolved gasses dependent on the laboratory atmosphere, gaseous nanobubbles  , material dissolved or detached from the containing vessels12, solid particles and aerosols (also dependent on the laboratory history) entering from the gas phase, and redox materials produced from water molecules18 and other solutes produced on standing  d  and homeopathic processing 21. Liquid water is clearly a very complex system even before the further complexity of molecular clusters, gas-liquid and solid-liquid surfaces, reactions between these materials, the consequences of physical and electromagnetic processing and the addition of ethanol are considered. Any or a combination of these factors may cause ‘memory’ of past solutes and processing in water. Some of these solutions are capable of causing non-specific clinical effects whereas others may cause effects specifically linked to the solution’s (and laboratory) history, as outlined below3.
There are numerous examples of the slow equilibration in aqueous solution. Thus, it can take several days for the effects of the addition of salts to water to finally stop oscillating  and such solutions are still changing after several months showing a large-scale (~100 nm) domain structure . Also, water restructuring after infrared radiation persists for more than a day , and water photoluminescence changes over a period of days . Changes to the structure of water are reported to last for weeks following exposure to resonant RLC (resistance inductance capacitance) circuits . Conductivity oscillations (~ 0.5 Hz) at low concentrations of salts also show the poor tendency to equilibrium in such solutions . Succussion, by itself, has been shown to be ‘remembered’ for at least 10 minutes as solitons (that is, standing waves) .
It has been found that clathrate hydrate nucleation is faster in solutions that once formed the clathrates but where it had been subsequently dissociated for periods up to several hours . Thus the solution shows a ‘memory effect’ of its previous history, although it is likely that this is due to retained super-saturated gas concentrations . Other interesting examples of the memory of water are the Mpemba effect  and the observation that hot water pipes are more likely to burst than adjacent cold water pipes . In both effects, water seems to remember whether it has been recently hot or cold even when subsequently cooled. The Mpemba effect  is a well proven phenomenon that also seems to be caused by unexpected solute and time effects and is described and explained elsewhere .
Explanation of homeopathy on the basis of water crystals (IE,  , ) is unconvincing as such crystals appear to be artifacts and, even as proposed, the effect of body fluid ions would be to immediately ‘dissolve’ them.
There is a strange occurrence, similar to the ‘memory of water’ but unconnected to it, in enzyme chemistry, where an effectively non-existent material still has a major effect; enzymes prepared in buffers of known pH retain (remember) those specific pH-dependent kinetic properties even when the water is removed such that no hydrogen ions are present ; these ions seemingly having an effect in their absence somewhat against common sense at the simplistic level.b 
Possible scenarios for the memory effect in homeopathic solutions
Various possible scenarios for the retained efficacy of homeopathic solutions are presented In the Table below3.
|Mechanisms for ‘the memory of water’ as applied to homeopathy|
|Specific clinical effects||Non-specific clinical effects|
|Remaining material on surfaces|
Aerosol material reintroduced
Bacterial material introduced
Remaining particle clusters
|Silicates, dissolved and particular|
nanobubbles  and their material surfaces
Redox molecules produced from water
Natural water clustering
Stabilized water clustering
Ions, including from glassware
Ethanol solution complexity
a A related phenomenon may be the occurrence of conductivity oscillations (~ 0.5 Hz) at similar concentrations of salts at the low concentration limit of obedience to Kohlrauch’s law (Onsager’s formula) ?m = ?mo – ?c½, where ?mo is the limiting molar conductivity, ? is a constant and c is the molar concentration33.
b This example of pH memory was later explained briefly as the enzymes’ acidic and basic groups retaining their charge when in an anhydrous environment44. This explanation is accepted but remains unproven independently, is derived from a circular argument and does not inform on how the charge is retained. There remains some puzzle to the extent that a single group in a molecule can either be charged or not charged; it cannot be fractionally charged. Thus the enzyme might be expected to behave as containing a mixture of charged and uncharged groups rather than, as found, fractionally charged groups as in the hydrated enzyme. Perhaps there is sufficient hydration water retained to ensure this, but I do not believe that this has been shown. Whatever, the ‘puzzle’ of the enzyme’s memory disappears with the appearance of an acceptable explanation.
c These papers are freely available on-line  where they are followed by a mixed bag of comments.
d This paper on autothixotropy has been criticized on two grounds . (1) The structures arising in the water can be destroyed by shaking whilst the solution preparation involved much shaking. However, the described destruction is at the macroscopic level (» µm) whereas the structuring could still arise on the microscopic level; (2) The autothixotropic effect requires the presence of some ions in the water. However, the distilled water used (in contrast to deionized water that does not show autothixotropy) contains ions.
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