Aggregation humic materials

It has also been shown [254] that a commercial petroleum sulfonate surfactant which consists of a diverse admixture of monomers does not exhibit behavior typically associated with micelle formation (i.e., a sharp inflection of solvent properties as the concentration of surfactant reaches CMC). These surfactants exhibit gradual change in solvent behavior with added surfactant. This gradual solubility enhancement indicates that micelle formation is a gradual process instead of a single event (i. e., CMC does not exist as a unique point, rather it is a continuous function of molecular properties). This type of surfactant can represent humic material in water, and may indicate that DHS form molecular aggregates in solution, which comprise an important third phase in the aqueous environment. This phase can affect an increase in the apparent solubility of very hydrophobic chemicals. 

Humic and fulvic adds are ill-defined and heterogeneous mixtures of naturally-occurring organic molecules that possess surface active properties The molecules that comprise this mixture are also known to form aggregates of colloidal dimensions Humic and fulvic adds are shown to be able to solubilize hydrophobic organic compounds (HOC) in a manner that is consistent with known, micelle-forming surfactants, but not at organic carbon concentrations that are environmentally relevant In addition, it is found that some HOCs are not solubilized to the same extent as other HOCs Some implications of the micellar nature of humic materials are briefly discussed ... 

There have been approximately two dozen papers published in the peer-reviewed literature since 1992 that have used the concept of fractals to characterize humic materials [16-38, 42], Most of the studies were done in a burst of interest during a 7-year period in the 1990s there have been five papers published in the peer-reviewed literature on the application of fractals to the study of humic materials other than reviews [15, 32, 43, 44] since 1999. Two common applications of fractal geometry have been to study the mass distribution of colloidal humic materials and the aggregation processes that produced them. 

The early fractal studies of humic materials alluded to the application of sample fractal dimensions for the description of aggregation mechanisms operant in these systems. In the first paper reporting a fractal nature for humic materials in solution, Osterberg noted that the observed fractal dimensions were indicative of an RLA process [26]. 

In 1999, Homer [32] noted that sparse data existed to correlate the chemical characteristics of a humic material with its fractal dimension or aggregation mechanism. For example, a preliminary study is described that may be the only attempt to correlate the carbon types present in a humic sample to its fractal properties. Humic acids with a greater proportion of aromatic carbon than carbohydrate carbon (determined from a C NMR spectrum) had larger fractal dimensions than those with smaller... 

Humic materials have been shown to be mass fractal aggregates in solution or in suspension, and surface fractals in the solid state. The fractal dimensions of both fractal types are generally between 2 and 3. But there has been no systematic study of how the fractal dimension varies with environmental variables (pH, ionic strength, humic concentration, sample source, identity and concentration of multivalent cations, aggregate size, etc.). 

A basic recognition has emerged that the aggregation of humic material can be described using a fractal approach. DLA is generally operant under acidic conditions and RLA becomes more important as the pH increases. 

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