In chromatography process including HPLC, the components of a mixture are separated by differences in their distribution between two phases, i.e. the stationary phase and mobile phase. The substances must interact with the stationary phase to be retained and separated by it. Retention of mixture in the column results from a combination of reversible physical interactions like adsorption at a surface, absorption in an immobilized solvent layer, and electrostatic interactions between ions. When the stationary phase is a porous medium, accessibility to its regions may be restricted and a separation can result from size differences between the sample components. More than one interaction may contribute simultaneously to a separation mechanism. The general requirements are that all interactions must be reversible, and that the two phases can be separated in such a way that a distribution of sample components between phases and mass transport by one phase can be established. Reversibility of the interactions can be achieved by purely physical means, such as by a change in temperature or by competition; the latter condition is achieved by introducing substances into the mobile phase that have suitable properties to ensure reversibility for the interactions responsible for retention of the sample components. It is an absolute requirement that a difference in the distribution constants for the sample components in the chromatographic system exist for a separation to be possible.

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what about chemical interactions?

can you explain to me what does this mean? "When the retention is discussed in NP-HPLC, a few important factors have to be considered, namely (i) the polar substituents in the solute molecular and its molecular area, (ii) so-called secondary solvent effects referring to the solute-solvent interactions in both the mobile and adsorbed phases which could cause useful changes in retention and even new chromatographic selectivity, (iii) the potential localization of polar molecular with respect to both solutes and solvents and the preferential adsorption of solutes and solvents on polar sites of the ligands"

Polar compounds often have an ionizable group in a molecule whose bearing charge makes the molecule even more polar and difficult to retain and separate. HPLC columns consisting mixed-mode stationary phases that provide multiple types of interactions with analytes suitable for separations of polar and non-polar compounds at both analytical and preparative scales in isocratic and gradient modes. Ionizable compounds interact with the stationary phase by reverse-phase, ion-exchange or ion-exclusion mechanisms. In addition to hydrophobic interactions, neutral compounds participate in different polar interactions with highly polar column functional groups. The behaviour of polar groups can be modified by varying the mobile phase. Basic functional groups on such column form salts with different acid residues (sulfate, perchlorate, trifluoroacetate, etc.), and each salt participates differently in polar interaction with neutral analytes. Analytes themselves can be ionized in many ways depending on the pH of the mobile phase, and retention time of your compounds can also be substantially altered by changing the pH of the mobile phase. All these properties offer multiple ways to adjust selectivity of the column. For further details you may go through one of my book chapters (attached) published in the Book entitled "Cellular and Biochemical Sciences

Please find further part (pages) of the book chapter as attachment

what does you mean by Reversibility of the interactions?

Liquid chromatography including HPLC is a physical separation method in which the components of a mixture are separated by differences in their distribution between two phases i.e. stationary and mobile phases. The substances must interact with the stationary phase to be retained and separated by it. Retention results from a combination of reversible physical interactions that can be characterized as adsorption at a surface, absorption in an immobilized solvent layer, and electrostatic interactions between ions. When the stationary phase is a porous medium, accessibility to its regions may be restricted and a separation can result from size differences between the molecules of the sample components. More than one interaction may contribute simultaneously to a separation mechanism. The general requirements are that all interactions must be reversible, and that the two phases can be separated in such a way that a distribution of sample components between phases and mass transport by one phase can be established. Reversibility of the interactions can be achieved by purely physical means, such as by a change in temperature or by competition; the latter condition is achieved by introducing substances into the mobile phase that have suitable properties to ensure reversibility for the interactions responsible for retention of the sample components. Since this competition with the sample components is itself selective, it provides a general approach to adjusting the outcome of a chromatographic procedure to obtain a desired separation.

Normal-phase HPLC (NP-HPLC) separates analytes based on their affinity for a polar stationary surface such as silica, hence it is based on analyte ability to engage in polar interactions (such as hydrogen-bonding or dipole-dipole type of interactions) with the sorbent surface. NP-HPLC uses a non-polar, non-aqueous mobile phase (e.g. Chloroform), and works effectively for separating analytes readily soluble in non-polar solvents. The analyte associates with and is retained by the polar stationary phase. Adsorption strengths increase with increased analyte polarity. The interaction strength depends not only on the functional groups present in the structure of the analyte molecule, but also on steric factors. The effect of steric hindrance on interaction strength allows this method to resolve (separate) structural isomers. The use of more polar solvents in the mobile phase will decrease the retention time of analytes, whereas more hydrophobic solvents tend to induce slower elution (increased retention times). Very polar solvents such as traces of water in the mobile phase tend to adsorb to the solid surface of the stationary phase forming a stationary bound (water) layer which is considered to play an active role in retention. This behavior is somewhat peculiar to normal phase chromatograhy because it is governed almost exclusively by an adsorptive mechanism (i.e. analytes interact with a solid surface rather than with the solvated layer of a ligand attached to the sorbent surface.

For further academic details, you may go the the related text books on HPLC