Retention phenomena of Reversed Phase Chromatography (RPC) are described in many ways by different authors. One of the groups, trying to understand the fundamentals of RPC, developing the software DryLab, was LCResources Inc. in USA under the leadership of Lloyd Snyder. Other members of the group were John Dolan and Tom Jupille in the USA and Imre Molnár in Berlin, Germany.

DryLab also uses a great deal of the philosophy described in the "Solvophobic Theory" of Csaba Horváth, which was developed in the time of 1975-1977 at Yale University (see Literature, Publications Dr. I. Molnar).

The fundamental point in this theory is that retention in RPC is enforced by the water, as part of the eluent. The dissolution of nonpolar molecules in water needs large amounts of energy. For example is the retention factor k (called also capacity factor), which is proportional to the energy needed in this dissolution process,  is in case of dibenzanthracene on a C8-phase
in water             k = ca. 4000 and
in acetonitrile  k = ca.        1  
(see example files, PNAH.dlb).

Horváth and his team found, that the only possible explanation for this extremely wide scale of retention is the change in the surface tension from water to acetonitrile (AN) or methanol (MeOH). 

Water is strongly lipophobic. However the lipophobicity of water can be easily and continuously reduced by admixing methanol (MeOH) or acetonitrile (AN) to it.

That is what we do in gradient elution.

Gradient elution starts most frequently with water or with water rich eluents. Upon injecting the sample into such a mobile phase (eluent), the water enforces a reduction of cavities around the more or less hydrophobic sample components and brings them onto the surface of the C8 or C18 column packings. By increasing the amount of the organic component (MeOH or ACN) the retention force from water will become weaker, the surface tension of the eluent is reduced from ca. 72 dyn/cm in water to ca. 20 dyn/cm in MeOH or AN  and the retention time is reduced at the same time. The process is opening fascinating separation capabilities of complex mixtures in a highly reproducible way.

DryLab calculates the retention in gradient elution precisely for every component. Based on only two gradient runs, DryLab can show by switching to the isocratic mode, how the k-values are reduced with increasing %organic in the mobile phase.

The amazingly easy nature of RP gradient elution is exhibiting itself in the continuous reduction of the retention force of the eluent by increasing the amount of the organic  component (MeOH or ACN) and reducing the amount of water. Finest differences in accessible solvophobic molecular surface areas, combined with steps in the gradient are sufficient to perform reasonably good separations with almost any mixture in Life Science.

Modeling RPC-separations in DryLab is based on the measurement of both the retention time and the capacity factor. The consequent calculation of sample positions in the corresponding chromatograms enables the chromatographer to look at experiments in a virtual mode and generate an overview of separation choices in minutes. If measurements are precise and reproducible, the retention behavior in complex mixtures can be modeled with high precision in seconds, saving valuable time in the expensive laboratory environment.