Abstracts for the Publications of Paul Huibers

    Pressure-Jump Studies on Micellar Relaxation Time and its Effect on Various Technological Processes

    in Surfactants in Solution, (A.K. Chattopadhyay, K.L. Mittal, Editors), Marcel Dekker; New York, 1996.

    P.D.T. Huibers, S.G. Oh, D.O. Shah

    The stability of micelles can be characterized by several methods. Jump techniques have proven most useful, where the relaxation of a physical parameter is observed as the system goes from one thermodynamic state to another. Fast (t1 < 100 microseconds) and slow (t2 > 1 millisecond) relaxation processes have been observed in surfactant solutions. The pressure-jump technique has been used to characterize slow relaxation for many ionic surfactant solutions and mixtures.

    Micellar stability influences many processes of interest (eg. foaming, wetting, solubilization, etc.). It can be shown that a maximum in micellar relaxation time (which is related to the micellar stability) correlates to reduced foamability, a minimum in fabric wetting rate, a maximum in gas bubble size and emulsion droplet size, and a maximum in oil and hydrophobic solids (dye) solubilization rates in micellar solutions. (C) 1996 Marcel Dekker.

    Enhanced Solubilization in Water-in-Oil Microemulsions Using Mixtures of Nonionic Surfactants

    in Dynamic Properties of Interfaces and Association Structures, (V. Pillai, D.O. Shah, Editors), AOCS Press, 1996.

    P.D.T. Huibers, D.O. Shah

    An important aspect of microemulsion design is the ability to solubilize the maximum amount of one liquid (dispersed phase) into another (continuous phase), with a minimum amount of surfactant. Many studies have been done to characterize microemulsions made with single surfactants or surfactant/cosurfactant combinations. Work presented here will describe the synergistic effects of mixtures of various nonionic surfactants in forming cosurfactant free water-in-oil microemulsions. The surfactants used are polyoxyethylenated sorbitan esters (Span, Tween) and nonylphenyl ethoxylates. Structures of the surfactants are related to the ability to solubilize a maximum amount of water in w/o microemulsion systems. (C) 1996 AOCS Press.

    The Influence of Micellar Stability on Technological Processes

    in Dynamic Properties of Interfaces and Association Structures, (V. Pillai, D.O. Shah, Editors), AOCS Press, 1996.

    S.G. Oh, P.D.T. Huibers, D.O. Shah

    A systematic research program has been carried out to determine the role of micellar stability on processes involving micellar solutions. Micellar stability or lifetime was determined using the pressure-jump technique, which directly measures the slow relaxation time (t2) of SDS micelles as a function of surfactant concentration. The slow relaxation time of SDS micelles was maximum at 200 mM concentration at 25 C, indicating that the most stable micelles were formed at this concentration. As concentration of SDS was changed, t2 varied from 10-4 to 10 seconds. The effect of this five orders of magnitude change in slow micellar relaxation time on various technological processes such as foaming, bubble dynamics, emulsification, wetting, detergency and the rate of oil solubilization in micellar solutions has been studied. The most stable micelles resulted in the least foaming, the largest bubble size, the longest wetting time of textiles, the largest oil-in-water emulsion droplet size, the most rapid solubilization of oil and the greatest detergency. All these results can be explained by the dynamic surface tension or interfacial tension of the solution, which is influenced by the flux of surfactant monomer from the bulk phase to the interface, which in turn is influenced by the micellar stability. (C) 1996 AOCS Press.

    Prediction of Critical Micelle Concentration Using a Quantitative Structure-Property Relationship Approach. 1. Nonionic Surfactants.

    Langmuir 12, 1462-1470 (1996)

    P.D.T. Huibers, V.S. Lobanov, A.R. Katritzky, D.O. Shah, M. Karelson

    A quantitative structure-property relationship study was performed on the critical micelle concentration (CMC) of nonionic surfactants using the CODESSA program. A known correlation between the logarithm of the CMC and counts of linear alkane carbon atoms and ethoxy groups in linear alkyl ethoxylates was improved (R2=0.997) by adding square terms and cross-terms of these molecular descriptors. A general three-parameter structure-property relationship was developed for a diverse set of 77 nonionic surfactants (R2=0.984) employing topological descriptors calculated for the hydrophobic fragment of the surfactant molecule. The three descriptors represent contributions from the size of the hydrophobic group, the size of the hydrophilic group, and the structural complexity of the hydrophobic group. (C) 1996 ACS Press.

    Key Words: critical micelle concentration; cmc; nonionic surfactants; quantitative structure-property relationships; QSPR; molecular connectivity; topology; property prediction; CODESSA; MOPAC.

    Prediction of Critical Micelle Concentration Using a Quantitative Structure-Property Relationship Approach. 2. Anionic Surfactants.

    Journal of Colloid and Interface Science 187, 113-120 (1997)

    P.D.T. Huibers, V.S. Lobanov, A.R. Katritzky, D.O. Shah, M. Karelson

    Relationships between the molecular structure and the critical micelle concentration (cmc) of anionic surfactants were investigated using a quantitative structure- property relationship (QSPR) approach. Measured cmc values for 119 anionic structures were considered, representing sodium alkyl sulfates and sodium sulfonates with a wide variety of hydrophobic and hydrophilic structures. The best multiple linear regression model involved three terms (descriptors) and had a correlation coefficient of R2 = 0.940. Very good correlations (R2 = 0.988) were obtained using three descriptors for a subset of 68 structures, with structural variation only in the hydrophobic domain. From the descriptors used in these regressions, one can conclude that the cmc is primarily dependent on the size (volume or surface area) of the hydrophobic domain, and to a lesser extent on the structural complexity of the surfactant molecule. (C) 1997 Academic Press.

    Key Words: critical micelle concentration; cmc; anionic surfactants; quantitative structure-property relationships; QSPR; molecular connectivity; dipole moment; topology; property prediction; CODESSA; MOPAC.

    Predicting Surfactant Cloud Point from Molecular Structure

    Journal of Colloid and Interface Science 193, 132-136 (1997)

    P.D.T. Huibers, D.O. Shah, A.R. Katritzky

    A general empirical relationship has been developed for estimating the cloud point of pure nonionic surfactants of the alkyl ethoxylate class. For a set of 62 structures, composed of linear alkyl, branched alkyl, cyclic alkyl, and alkylphenyl ethoxylates, cloud points can be estimated to an accuracy of ±6.5oC using the logarithm of the number of ethylene oxide residues, and three topological descriptors that account for hydrophobic domain variation. (C) 1997 Academic Press.

    Key Words: cloud point; nonionic surfactant; topological indices; information-theoretical indices; QSPR; quantitative structure-property relationships; CODESSA.

    Models for the Wavelength Dependence of the Index of Refraction of Water

    Applied Optics 36, 3785-3787 (1997)

    Paul D.T. Huibers

    Models for the index of refraction of water are reviewed and reduced to wavelength-dependent formulas. A simple three-term nonlinear model derived from Quan and Fry [Appl. Opt. 34, 3477-3480 (1995)], originally developed for the visible region, fits the available data well over an extended range covering the UV to the near-IR (200-1100 nm). (C) 1997 Optical Society of America.

    Key Words: water, index of refraction, refractive index, wavelength dependence.

    The Importance of Sub-Angstrom Distances in Mixed Surfactant Systems for Technological Processes

    Colloids and Surfaces A: Physicochemical and Engineering Aspects 128, 197-208 (1997)

    Shiao, S.Y., Patist, A., Free, M.L., Chhabra, V., Huibers, P.D.T., Gregory, A., Patel, S., Shah, D.O.

    The spacing between atoms and molecules at interfaces and within materials is extremely important in determining the properties of such interfaces and materials. In surfactant monolayers at the air-water interface, it has been shown that small changes in molecular packing lead to large changes in the interfacial properties. Changes in intermolecular distance as small as 0.04 Angstroms have been attributed to the effect of mixing surfactants of different chain lengths. This paper discusses the effect of these sub-angstrom distance changes on foaming, micellar stability, melting points, bubble size, surface viscosity, lubrication, environmental remediation, enhanced oil recovery, and microemulsion stability that result from chain length compatibility. (C) 1997 Elsevier Science B.V.

    Keywords: Chain length compatibility; Mixed surfactant systems; Surfactants in technological processes

    Evidence for Synergism in Nonionic Surfactant Mixtures: Enhancement of Solubilization in Water-in-Oil Microemulsions

    Langmuir 13, 5762-5765 (1997)

    Paul D.T. Huibers and Dinesh O. Shah

    It is well known that certain mixtures of surfactants can provide better performance than pure surfactants for a wide variety of applications, and thus it is expected that enhanced solubilization of water in water-in-oil (w/o) microemulsions will also be achieved with certain surfactant mixtures. The formation of w/o microemulsions involves dissolving an aqueous phase into an oil phase, creating a transparent and thermodynamically stable suspension of droplets with diameters in the range of 10-100 nm. It is desirable to accomplish this with a minimum amount of surfactant, and in order to achieve this goal, mixtures of surfactants can be used. We intend to address different surfactant mixtures that exhibit synergism in the solubilization of water in w/o microemulsions, showing the existence of at least two different mechanisms of synergism. The model system is composed of nonylphenyl ethoxylate as the surfactant and cyclohexane as the oil phase. (C) 1997 ACS Press.

    Multispectral Determination of Soap Film Thickness

    Langmuir 13, 5995-5998 (1997)

    Paul D.T. Huibers and Dinesh O. Shah

    An analysis of light absorbance measurements of soap films is presented, allowing the determination of film thickness, by taking advantage of constructive and destructive interference caused by the internal reflection of light within a transparent film. Using a standard diode array UV/visible absorbance spectrophotometer, the thinning of soap films over time can be precisely measured. Film thickness as large as 50 microns to below 5 nm can be determined by this technique. (C) 1997 ACS Press.

    Correlation of Aqueous Solubility of Hydrocarbons and Halogenated Hydrocarbons with Molecular Structure

    Journal of Chemical Information and Computer Sciences 38, 283 (1998).

    Paul D.T. Huibers and Alan R. Katritzky

    The aqueous solubilities of a set of 109 hydrocarbons and 132 halogenated hydrocarbons (total 241) are correlated by a three term equation using descriptors calculated solely from molecular structure, with a correlation coefficient (R) of 0.979 and a standard error (s) of 0.386 log units. This equation allows the estimation of aqueous solubilities of hydrocarbons and halogenated hydrocarbons (including PCBs). The key descriptor is the molecular volume, modified by topological and electrostatic terms. The use of descriptors calculated only from molecular structure eliminates the need for experimental determination of properties for use in the correlation, and allows for the estimation of aqueous solubility for molecules not yet synthesized or isolated. (C) 1998 ACS Press.

    Key Words: aqueous solubility, halogenated hydrocarbons, quantitative structure-property relationships

    Solvent Replacement for Green Processing

    Environmental Health Perspectives 106(Suppl 1), 253-271 (1998).

    Sherman, J., Chin, B., Garcia-Valls, R., Huibers, P.D.T., Hatton, T.A.

    The implementation of the Montreal Protocol, the Clean Air Act and the Pollution Prevention Act of 1990 have resulted in increased awareness of organic solvent use in chemical processing. The advances made in the search to find "green" replacements for traditional solvents are reviewed, with reference to solvent alternatives for cleaning, coatings, and chemical reaction and separation processes, and to the development of solvent databases and computational methods that aid in the selection and/or design of feasible or optimal environmentally benign solvent alternatives for specific applications. (C) 1998 NIH/NIEHS.

    Solvent Replacement for Green Processing. Part II: Computer Simulation

    As computers become more pervasive and increasingly powerful, one would expect that specialized programs and databases would be developed to assist in a wide variety of research efforts. This is true in the search for solvent alternatives, and in this section we review the application of computers in solvent substitution studies, covering computer-aided molecular design of new solvents, methods developed for the prediction of physical properties, methods for predicting less precise chemical characteristics such as toxicity and carcinogenicity, and computer-aided design of alternative synthetic pathways. These tools may assist the scientist in two ways; 1) the optimization of the design process can be performed with more complex constraints than could otherwise be handled, and 2) the elimination in some cases of the need for time-consuming and costly physical or chemical property measurements by the use of estimation techniques for properties of interest.

    For relevant links see http://web.mit.edu/huibers/www/greenchem.html

The Effect of Polar Head Charge Delocalization on Micellar Aggregation Numbers of Decylpyridinium Salts, Revisited

Journal of Colloid and Interface Science 206, 342-345 (1998).

Paul D.T. Huibers and Paul T. Jacobs

The distribution of charge in an isomeric series of decylpyridinium bromide surfactants is calculated using the AM1 semiempirical quantum mechanical molecular model. The aggregation numbers of the surfactants are shown to increase with a decrease in the residual partial charge in the alkyl tails, suggesting a change in the packing of the surfactants. The critical micelle concentration increases with a decrease in the partial charge of the head groups, indicating increased solubility of the surfactant molecule as charge is more widely distributed throughout the molecule.

Key Words: head group charge distribution; alkyl pyridinium bromides; semiempirical quantum chemical methods; MOPAC; AM1; aggregation number, critical micelle concentration.

Reprints can be requested from Dr. Paul Huibers, Massachusetts Institute of Technology, Department of Chemical Engineering #66-317, Cambridge, MA 02139-4307.

Last modified October 20, 1998 by Paul Huibers.
See my home page or email me at( huibers@mit.edu)