Soap-acid association structure

When the water molecules are attached to the soap/acid association structure, structural changes may be expected. In the present investigation, three of these are examined. 

Figure 3. The position of 14 water molecules added to the expanded soap/acid association structures. Water molecule 2 is the symmetric identical to 1, 4 the corresponding pair to 3 and so forth. Key o, water e, acid , soap.

In addition, water molecules marked 3 on Fig. 3 and its symmetric location 4 (not included) gave binding energies of 27.6 Kcal/mole water. These molecules may be bound to the unexpanded soap/acid association structure with similar energies involved. The bonding of these two water molecules obviously would provide sufficient initial energy for the expansion of the soap/acid association complex to accommodate water molecules 1 and 2. 

In the presence of excess fatty acid, different soap crystalline phase compounds can form, commonly referred to as acid-soaps. Acid-soap crystals are composed of stoichiometric amounts of soap and fatty acid and associate in similar bilayer structures as pure soap crystals. There are a number of different documented acid-soap crystals. The existence of crystals of the composition 2 acid-1 soap, 1 acid-1 soap, and 1 acid-2 soap has been reported (12-14). The presence of the acid-soaps can also have a dramatic impact on the physical and performance properties of the finished soap. The presence of acid-soaps increases the plasticity of the soap during processing and decreases product firmness, potentially to the point of stickiness during processing. Furthermore, the presence of the acid-soap changes the character of the lather, decreasing the bubble size and subsequently increasing lather stability and creaminess. 

Phospholipids, which are common in mammalian membranes, contain a phosphoric acid ester. A simple example is phosphatidic acid which replaces the fatty acids associated with soaps. The molecules of these acids contain two hydrocarbon hydrophobic chains as indicated in the lipid-water systems of Fig. 1.17. The similarity between the structure and behaviour of lipids and soaps has led to a resurgence of interest in the properties of soap molecules. 

The structure in the micellar region of the phase diagrams of potassium soaps has been analysed by Reiss-Husson and Luzzati (1969) using X-ray methods. A common feature in soaps of saturated fatty acids is that spherical micelles exist at low concentrations, and at increased concentrations a transition into rod micelles occurs. Sodium oleate, however, was found to give rod-shaped micelles at all concentrations. The micellar association and phase behaviour have been reviewed by Wenner-strom and Lindman (1979) and Lindman and Wennerstrom (1980). 

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