Table A-1. Surfactants used in this thesis, names, structures, HLB and est. molecular weight.

^*POE {polyoxyethylene} (#) is the average number of ethylene oxide repeats.
⁺Castor oil is triricinoleoyl glycerol. Ricinoleic acid is a C₁₈ fatty acid, C₁₈H₃₄O₃, whose structure is CH₃(CH₂)₅CH(OH)CH₂CH=CH(CH₂)₇COOH.

Nonionic Surfactants Used in This Thesis

Table A-1 summarizes the surfactants used in this dissertation. For the microemulsion studies, nonionic surfactant structures covered include sorbitan esters, POE sorbitan esters, nonylphenyl POE ethers, other alkyl POE ethers, and POE triglycerides. Surfactants were chosen for the range of structures available for analogous compounds, allowing excursions on small variations of molecular structure to be performed in order to elucidate the effect of surfactant structure on solubilization.

Sorbitan esters and alkyl ethers are provided by ICI Americas, Inc., Wilmington, Delaware, and alkylphenyl ethers are provided by Rhone-Poulenc, Inc., Atlanta, Georgia. We are indebted to these companies for their generous support in providing samples.

Structural Variation and Polydispersity of the Nonionic Surfactants

It should be noted that all commercial surfactants used are not isomerically pure, but rather a diverse collection homologous surfactant molecules. This results from the synthesis reactions used in creating the surfactants, where the extent of reaction cannot be controlled completely.

For studies of the influence of surfactant structure on properties, one must consider structural variability in both the hydrophilic and the hydrophobic domains of the surfactant molecule. For the hydrophobic domain, variation occurs in alkyl chain length, saturation, and ring structures. Many surfactants are created by reacting a hydrophilic structure with a fatty acid, to create an ester. The purity of the fatty acid used will clearly effect the homogeneity of the resulting surfactant. Some surfactants use relatively pure fatty acids and will have only a few percent of impurities. Commonly, an oleate (C_18:1, one double bond) will have a small amount of stearate (C₁₈) and linoleate (C_18:2) as impurities. A linear saturated fatty acid will usually be contaminated with the saturated fatty acids with two more and two less carbons.

In the hydrophobic group, variation is seen in different lengths of polyethylene oxide (PEO) polymer, and in the presence of other hydrophilic structures such as sorbitan. Variation in PEO chain length results when there are multiple reaction sites (hydroxy groups) possible for the esterification (sorbitan esters have multiple sites). Sorbitan ester surfactants are available as monoesters, triesters, and sesquiesters. The monoesters will have some di- and triesters present, and the triesters will have some mono- and diesters. The sesquiesters, such as Arlacel 83, sorbitan sesquioleate, is really a mixture of sorbitan mono- and dioleates, with an average of 1.5 oleic acid residues per sorbitan.

The most common variability is found in the hydrophilic component of the surfactants. The hydrophilic domain in most nonionic surfactants is a polymer created by reacting ethylene oxide. The polymerization reaction cannot be controlled precisely, so the final product is not a homogeneous, monomolecular product. These polyoxyethylene compounds are assigned a number for the number of repeats of ethylene oxide residues. This number can be considered either an average, or the most probable (peak) number of ethylene oxide residues per molecule. The distribution of POE lengths for a given average may be quite wide, and some investigators [Shinoda et al., 1971] have studied the effect of the width of the distribution on surfactant properties by distilling the surfactants and narrowing the distribution.

Another variability present in the hydrophilic component of the sorbitan esters is also due to a reaction whose extent cannot be controlled precisely. Sorbitan (a ring) is produced by dehydration of sorbitol, where the two end hydroxyl groups of the six carbon molecules react. A fraction of the sorbitan rings will react further, with an additional dehydration leading to a isosorbide, which is a multiple ring type structure. This sorbide should be less hydrophilic, as it has fewer oxygens available to hydrogen bond.