Update documentation

doc/literature.bib

@@ -507,3 +507,15 @@ @article{zhang2024solving
   publisher = {Elsevier},
   doi = {https://doi.org/10.1016/j.compchemeng.2024.108612}
 }
+@article{sips1948,
+	title = {On the {Structure} of a {Catalyst} {Surface}},
+	author = {Sips, Robert},
+	year = {1948},
+	journal = {The Journal of Chemical Physics},
+	pages = {490--495},
+	volume = {16},
+	issn = {0021-9606},
+	doi = {https://doi.org/10.1063/1.1746922},
+	number = {5},
+	month = may
+}

doc/modelling/binding/multi_component_sips.rst

@@ -12,4 +12,6 @@ The Sips binding model is a combination of the :ref:`Freundlich<freundlich_ldf_m
     \end{aligned}
 
 
+Here, :math:`c_{\text{ref}}` is a :ref:`reference concentration <reference_concentrations>`, :math:`n_i` is the Freundlich exponent, :math:`k_{a,i}, k_{d,i}` are the adsorption and desorption rates, and :math:`q_{\text{max},j}` is the adsorption capacity.
 For more information on model parameters required to define in CADET file format, see :ref:`multi_component_sips_config`.
+For more information on the model and its origin, please refer to :cite:`sips1948`.