Help guest species

The concentration of SDA in the synthesis solution has not been discussed so far. In most porosil syntheses, the synthesis solution is saturated with Si02 at the temperature of synthesis. The SDA concentration is important first for the nucleation process and, secondly, it is needed to maintain crystal growth. Whereas the nucleation of the porosil depends on the presence of an SDA, crystal growth of a stable crystallite might continue with only a fraction of cages occupied by SDA or help guest species. In this way, DOH membranes have been synthesized for the separation of hydrogen gas in gas mixtures [59]. 

A help gas stabilizes double and mixed hydrates. There is some uncertainty whether these inclusion hydrates are in fact stable without a guest species in the small 512 cages. All the clathrate hydrate structures are known to be stabilized and their melting points raised by including H2S with the guest compounds, which for this reason is called the helpgas [782-786]. Since we know of no experiments where the gas hydrates were prepared in the absence of air or some other gas, this help-gas phenomenon may occur to a more or less degree in all laboratory... 

The influence of a host on the reactivity of guests is often, bnt by no means exclusively, illustrated using cycloaddition reactions, such as the Diels-Alder reaction. The rationale for this preference is simple these reactions occur without the help of harsh reagents or reaction conditions and without the formation of unwanted by-products, thus eliminating the risk of undesired reactivity of the host. For the same reason, photochemical reactions are often favored, as the reagent light can both be tuned in such a way as to avoid absorption by the host structure and because no additional chemical species need be introduced into the system. 

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