  Ice desorption affects the evolution of the gas-phase chemistry during the
protostellar stage, and also determines the chemical composition of comets
forming in circumstellar disks. From observations, most volatile species are
found in H2O-dominated ices. The aim of this study is first to experimentally
determine how entrapment of volatiles in H2O ice depends on ice thickness,
mixture ratio and heating rate, and second, to introduce an extended
three-phase model (gas, ice surface and ice mantle) to describe ice mixture
desorption with a minimum number of free parameters. Thermal H2O:CO2 ice
desorption is investigated in temperature programmed desorption experiments of
thin (10 - 40 ML) ice mixtures under ultra-high vacuum conditions. Desorption
is simultaneously monitored by mass spectrometry and reflection-absorption
infrared spectroscopy. The H2O:CO2 experiments are complemented with selected
H2O:CO, and H2O:CO2:CO experiments. The results are modeled with rate equations
that connect the gas, ice surface and ice mantle phases through surface
desorption and mantle-surface diffusion. The fraction of trapped CO2 increases
with ice thickness (10 - 32 ML) and H2O:CO2 mixing ratio (5:1 - 10:1), but not
with one order of magnitude different heating rates. The fraction of trapped
CO2 is 44 - 84 % with respect to the initial CO2 content for the investigated
experimental conditions. This is reproduced quantitatively by the extended
three-phase model that is introduced here. The H2O:CO and H2O:CO2:CO
experiments are consistent with the H2O:CO2 desorption trends, suggesting that
the model can be used for other ice species found in the interstellar medium to
significantly improve the parameterization of ice desorption.
