The adsorption of uncharged polymers by clays is largely "entropy-driven." Polymer confor- mation changes from a random coil in solution to an extended form at the surface in which adsorbed polymer segments or trains alternate with loops and tails extending away from the surface. Although the net inter- action energy, e, per segment-surface contact is small (--1 kT unit), the total energy of adsorption is large because the fraction of train segments, p, is commonly between 0.3 and 0.5. The adsorption isotherms are typically of the high-affinity type, and there is an apparent lack of desorption on dilution. Positively charged polymers (polycations) are adsorbed largely through electrostatic interactions between the cationic groups of the polymer and the negatively charged sites at the clay surface. Here e >> 1 kT unit and p > 0.7, leading to an almost complete collapse of the polymer chain onto the surface. Indeed, beyond a given level of adsorption charge reversal can occur in that the clay-polycation system effectively behaves as an anion exchanger. Little adsorption occurs with negatively charged polymers (polyanions) due to initial charge repulsion between the polymer and the clay surface. Acid pH, a high ionic strength, and the presence of polyvalent cations in the system enhance and promote polyanion adsorption. Uncharged polymers and polycations can enter the interlayer space of expanding 2:1 type layer silicates but polyanions generally fail to intercalate. The interactions of clays with biopolymers, such as proteins, nucleic acids, and polysaccharides, can be rationalized in similar terms. When intercalation occurs, the interlayer biopolymer is further stabilized against microbial (enzymatic) degradation giving rise to practical applications of clay-polymer complexes as flocculants and soil conditioners. Polyanions are effective as flocculants because of their large "grappling distance," whereas uncharged polymers are better suited as soil conditioners because they can spread over adjacent clay/soil particle surfaces like a coat of paint.

Clay-Polymer Interactions: Summary and Perspectives

The aqueous alteration of ultramafic rocks through serpentinization liberates mantle carbon and reducing power. Serpentinization occurs in numerous settings on present day Earth, including subduction zones, mid-ocean ridges, and ophiolites and has extended far into Earth’s history, potentially contributing to the origins and early evolution of life. Serpentinization can provide the energy and raw materials to support chemosynthetic microbial communities that may penetrate deep into Earth’s subsurface. Microorganisms may also influence the composition and quantity of carbon-bearing compounds in the deep subsurface. However, conditions created by serpentinization challenge the known limits of microbial physiology in terms of extreme pH, access to electron acceptors, and availability of nutrients. Furthermore, the downward transport of surface carbon and subsequent mixing with calcium-rich fluids at high pH contributes to the precipitation and immobilization of carbonate minerals. The following chapter will explore the physiological challenges presented by the serpentinite environment, data from studies of serpentinite-hosted microbial ecosystems, and areas in need of further investigation.

### Physical and chemical consequences of serpentinization

Serpentinization is an alteration process of low-silica ultramafic rocks, characteristic of the lower oceanic crust and upper mantle. These rocks are rich in the minerals olivine and pyroxene. Water-rock reactions result in the oxidation of ferrous iron from olivine and pyroxene, resulting in the precipitation of ferric iron in magnetite (Fe3O4) and other minerals, and in the release of diatomic hydrogen (H2). At low temperatures (< ~150 °C) the reaction results in extremely high pH, commonly above 10. The combination of H2 and CO2 or CO under highly reducing conditions leads to formation of methane and other hydrocarbons through Fischer-Tropsch Type (FTT) synthesis (McCollom and Seewald 2001; Charlou et al. 2002; Proskurowski et al. 2008; McCollom 2013). Serpentinization also results in volume changes in the altered materials, making serpentinites less dense than …

/pdf/serpentinization-carbon-and-deep-life-4umyd2vfyy.pdf

Serpentinization, Carbon, and Deep Life

It is proposed that abiotic synthesis of organic compounds occurs in metastable states. These states are permitted by kinetic barriers which inhibit the approach to stable equilibrium in the C-H-O-N system. Evidence for metastable equilibrium among organic compounds in sedimentary basins is reviewed, and further evidence is elucidated from hydrous pyrolysis experiments reported in the literature. This analysis shows that at hydrothermal conditions, organic compounds are formed or destroyed primarily through oxidation/reduction reactions, and that the role of temperature is to lower the kinetic barriers to these reactions. These lines of evidence allow the development of a scenario in which abiotic synthesis can occur at hydrothermal conditions through the reduction of CO2 and N2. This scenario can be tested quantitatively with distribution of species calculations as functions of temperature, pressure, hydrogen fugacity (fH2) and initial composition. One example of such a test is given for an early, sudden outgassing of the Earth, in which CO2, H2O, and N2 are transported from the mantle to the atmosphere by hydrothermal solutions. Activities of metastable aqueous organic species which form as a consequence of this process are evaluated at conditions appropriate for seafloor hydrothermal systems, and are found to maximize at about 200 °C and between the oxidation states set by two mineral assemblages common in the oceanic crust.

Geochemical constraints on the origin of organic compounds in hydrothermal systems

How and where did life on Earth originate? To date, various environments have been proposed as plausible sites for the origin of life. However, discussions have focused on a limited stage of chemical evolution, or emergence of a specific chemical function of proto-biological systems. It remains unclear what geochemical situations could drive all the stages of chemical evolution, ranging from condensation of simple inorganic compounds to the emergence of self-sustaining systems that were evolvable into modern biological ones. In this review, we summarize reported experimental and theoretical findings for prebiotic chemistry relevant to this topic, including availability of biologically essential elements (N and P) on the Hadean Earth, abiotic synthesis of life's building blocks (amino acids, peptides, ribose, nucleobases, fatty acids, nucleotides, and oligonucleotides), their polymerizations to bio-macromolecules (peptides and oligonucleotides), and emergence of biological functions of replication and compartmentalization. It is indicated from the overviews that completion of the chemical evolution requires at least eight reaction conditions of (1) reductive gas phase, (2) alkaline pH, (3) freezing temperature, (4) fresh water, (5) dry/dry-wet cycle, (6) coupling with high energy reactions, (7) heating-cooling cycle in water, and (8) extraterrestrial input of life's building blocks and reactive nutrients. The necessity of these mutually exclusive conditions clearly indicates that life's origin did not occur at a single setting; rather, it required highly diverse and dynamic environments that were connected with each other to allow intra-transportation of reaction products and reactants through fluid circulation. Future experimental research that mimics the conditions of the proposed model are expected to provide further constraints on the processes and mechanisms for the origin of life.

Origins of building blocks of life: A review

http://www.casinapioiv.va/content/dam/accademia/pdf/acta20/acta20-eschenmoser.pdf

The search for the chemistry of life's origin

The condensation of mononucleotides has been carried out in aqueous solution at neutral pH in the presence of cyanamide. Oligodeoxyribonucleotides up to five units have been formed when montmorillonite was present.

https://science.sciencemag.org/content/sci/173/3995/444.full.pdf

Possible Prebiotic Condensation of Mononucleotides by Cyanamide

https://science.sciencemag.org/content/sci/174/4013/1038.full.pdf

Formaldehyde and Ammonia as Precursors to Prebiotic Amino Acids

Organogenic elements, mainly carbon, nitrogen, phosphorus, and sulfur are present in the particulate material and in a breccia rock from Tranquillity Base in amounts ranging from 5 to 4200 parts per million. The major compounds of carbon released by heating are carbon monoxide and carbon dioxide; the former predominates. Small amounts of other compounds of carbon have also been observed. Sulfur can be released as hydrogen sulfide by treatment with acid. The carbon isotopic δ 13L C values are definitely nonterrestrial (+ 13 to + 18.5 per mil).

http://science.sciencemag.org/content/sci/167/3918/765.full.pdf

J. Ibanez

Papers

Possible Prebiotic Condensation of Mononucleotides by Cyanamide

Formaldehyde and Ammonia as Precursors to Prebiotic Amino Acids

Organogenic elements and compounds in surface samples from the sea of tranquillity.