neophyte wrote:
It's a complex interplay of lipid bilayer intercalating/disrupting ability versus persistence. Alcohol sanitizers (above 70% alcohol concentrations) will immediately disrupt the integrity of the viral (or bacterial) lipid bilayers and thus disrupt the presentation of lipid bilayer-imbedded proteins responsible for attachment to cells and subsequent infection (or in the case of bacteria - disrupt cellular integrity and internal structures and allow leakage of vital internal fluids and ions). Then the alcohol evaporates, and carries away the water in the bacteria or virus, thus dessicating them.

Quaternary ammonium compounds, like myristalkonium chloride in Barbacide, act the same way, but are at much lower concentration in the solvent (water), and thus take longer to build up to an effective level in the lipid bilayer. Hence the instruction to allow adequate time, without rinsing, to insure the full germicidal effect. As the solvent evaporates, the effective concentration rises, as does the germicidal activity.

In general, quaternary ammonium compounds are quite stable in water. For example, Lysol has a shelf life of about 2 years, although it has other ingredients that may protect it from contamination with microbes after it is opened. I think it is that contamination that reduces germicidal effectiveness over time, rather than chemical instability of the active ingredient.