  Variations in the observed dust emission and extinction indicate a systematic
evolution of grain properties in the transition from the diffuse interstellar
medium (ISM) to denser molecular clouds. The differences in the dust spectral
energy distribution (SED) observed from the diffuse ISM to denser regions,
namely an increase in the spectral index at long wavelengths, an increase in
the FIR opacity, and a decrease in temperature, are usually assumed to be the
result of changes in dust properties. We investigate if evolutionary processes,
such as coagulation and accretion, are able to change the dust properties of
grains in a way that is consistent with observations. We use a core-mantle
grain model to describe diffuse ISM-type grains, and using DDA we calculate how
the accretion of mantles and coagulation into aggregates vary the grain optical
properties. We calculate the dust SED and extinction using DustEM and the
radiative transfer code CRT. We show that the accretion of an aliphatic carbon
mantle on diffuse ISM-type dust leads to an increase in the FIR opacity by a
factor of about 2 and in the FIR/submm spectral index from 1.5 to 1.8, and to a
decrease in the temperature by about 2 K. We also show that the coagulation of
these grains into aggregates further decreases the temperature by 3 K and
increases the spectral index up to a value of $\sim$2. The FIR opacity is
increased by a factor of 3 (7) for these aggregates (with an additional
ice-mantle) compared to the diffuse ISM-dust. Dust evolution in the ISM
resulting from coagulation and accretion, leads to significant changes in the
optical properties of the grains that can explain the observed variations in
the dust SED in the transition from the diffuse ISM to denser regions.
