Adsorption critical packing parameter

The properties (e.g. cleaning and stabilizing capabilities) of surfactants depend on both solution properties (temperature, time, presence of salts and cosurfactants) and their own characteristics, especially CMC, the Krafft point and their chemistry. The surfactant chemistry and especially the balance between hydrophobic and hydrophilic parts is quantified using tools like the CPP or HLB (critical packing parameter, hydrophilic-lipophilic balance, respectively). For example, it is often observed that detergency increases with concentration especially up to CMC and is often best at CPP values around 1. We will meet the important concept of CPP again in Chapter 7 where we will see that surfactant adsorption on solid surfaces is connected to CPP. 

The isotherm for bromide adsorption (Fig. 14c) shows that increasing the potential in the double-layer region causes a continuous increase in the bromide coverage. The data shown in Fig. 15 and in other experiments, however, are not consistent with the notion that the additionally adsorbed bromide leads to a continuous compression of the incommensurate bromide adlayer, as this would cause the sharp peak at/ = 0.67 to shift gradually to higher wave vector. An alternative explanation of our results is that bromide forms a series of high-order commensurate (HOC) structures on the Pt(lll) surface, that is, at all potentials the structure corresponds to a close-packed monolayer but that the periodicity depends critically on the ratio of the bromide and Pt lattice parameters. At 0.2 V, the unit cell corresponds to a (3 x 3) structure with a basis of four Br atoms. At 0.6 V, the unit cell is a (7 x 7) structure containing 25 Br atoms, that is, with a 7 5 ratio of the Br and Pt lattice parameters. 

We may conclude that the molar masses obtained under critical conditions reflect, correctly, homo- and co- polymer characteristics, however deviations may be observed. These deviations maybe caused by the method itself, due to parameters, who influence is not sufficiently well know, such as preferential solvation of polymer chains, unexpected differences in the structure or composition of the analyzed samples, or differences in adsorptive properties of column packings. Nevertheless, the data obtained are valuable due to very limited accessibility to such data by other analytical methods. 

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