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Brian Pethica

Senior Scientist, Chemical Engineering

B.Sc., Ph.D., D.Sc. London University/Imperial College
Ph.D., Sc.D. Cambridge University
Diploma, Advanced Management Program, Harvard School of Business Administration

Room: H112 Engineering Quad
Phone: 609-258-1670
Email: bpethica@princeton.edu

Honors and Awards

  • Lecturer in Pharmacology, Birmingham University Medical School
  • Senior Assistant in Research, Dept. of Colloid Science, Cambridge University
  • Lecturer/Senior Lecturer in Physical Chemistry, University of Manchester Institute of Science and Technology
  • Dean of Arts and Sciences, Clarkson University, NY
  • Professor of Chemistry and Professor of Biophysics, Clarkson University NY
  • Royal Society Visiting Lecturer, Institute of Advanced Studies, Mexico
  • Head of Laboratory, Unilever Research, Port Sunlight, UK
  • President and CEO, EBI Medical Systems, NJ

Research Interests

My diverse interests over the years, from chemisorption on metals in ultrahigh vacuum to the biophysics of cell membranes, have been connected by underlying themes in thermodynamics, surface chemistry and colloid science – much of which is now successfully rebranded as nanoscience.

Current Activities

1. In collaboration with Professor Debenedetti

The conformational behavior of biopolymers in solution and the glassy state as functions of temperature, water content and the presence of additives, studied by calorimetric and spectroscopic methods. Preservation of the integrity of biopharmaceutical proteins for long periods at the higher ranges of storage temperatures, or for short periods at more elevated temperatures chosen for the elimination of viral contaminants.

2. In collaboration with Professors Debenedetti and Prud’homme

Thermodynamic properties of the clathrate hydrates of small molecular weight paraffins.
Precipitation of high molecular weight paraffins and asphaltenes from model and heavy oils.

These two topics relate to the prevention of blockages in oil and gas pipelines

3. Intermolecular forces in surfaces:

The properties of molecules at interfaces are of very broad practical importance, but our understanding of the lateral forces underlying these properties is not well advanced. In particular, there is little critical experimental thermodynamic data available on the two-dimensional forces between adsorbed molecules at liquid interfaces, from simple gases to polymers, nor on the modification of these forces by the adsorbing phases. These data should be available to challenge the computational models on surface properties now increasingly available.

The studies generate two groups of related data. In the first, the adsorption of surface-active solutes or vapors at vapor/water or oil/water interfaces is studied by measurements of the interfacial tensions as a function of solute or vapor activity/fugacity over a range of temperatures. In the second, experiments on molecules spread as insoluble monolayers at the air/water and oil/water interfaces lead directly to pressure/area isotherms, the classic source of thermodynamic parameters.

Results to date have shown that for the adsorption of alkane gases larger than propane at the air/water interface, the two-dimensional virial coefficients at the surface are not explained by the standard intermolecular potentials derived for these substances in the gas phase. Polarization of the adsorbed species by the electric fields of the surface dipoles is a likely explanation for the changes in the potentials of mean force induced by adsorption.

Isotherms for spread monolayers of n-pentadecanoic acid at the air/water (dilute HCl) interface have been obtained by micromanometry out to the high-area gaseous region over a range of temperatures, leading to estimates of the two-dimensional virial coefficients and the contributions of hydrocarbon chains and head groups to the intermolecular forces. The results do not agree with calculations from an earlier model based on less precise published data for several n-alkyl surfactant monolayers. The model is being improved by inclusion of chain conformational terms and possible field polarization effects. Parallel experiments on ionized monolayers are directed to the properties of ionic double layers

The spread monolayer studies at the oil/water interface focus primarily on phospholipids as relevant to the forces involved in biomembrane stuctures, and are supported by related experiments on an adsorbed diglyceride to provide evidence on the contribution of the ester dipoles and phospholipid zwitterions to the pair potentials. The phospholipid results have been used by Dill and colleagues to develop a successful statistical mechanical model for interactions in the dilute gaseous region of the phase diagram over a range of temperatures. The extensive thermodynamic data for the degenerate phase transitions seen with the phospholipids at higher monolayer densities are yet to be interpreted.

4. The measurability of electric potentials. Non-additivity of electric and dispersion forces.

As indicated by Gibbs and further shown by Guggenheim, electric potentials between regions of different chemical composition cannot be measured. Correspondingly, the application of classical electrostatics to the physical chemistry of electrolyte systems – for example in the Poisson-Boltzman equation – is correspondingly non-physical except for very dilute solutions well away from a phase boundary. Recent work has rejected an earlier proposed method to circumvent the Gibbs-Guggenheim prohibition based on experiments suggested by analysis of the thermodynamics of electrical condensers. The proposal made use of experimental quantities not previously considered in analysis of the prohibition, notably adsorption at the condenser plate interfaces and the forces between the plates. It is now shown that the proposed experimental method does not avoid the prohibition. From the thermodynamic analysis of electrical condensers an explicit experimental procedure has been identified for testing whether or not electrical and dispersion forces are simply additive, as is assumed almost universally in electrochemical theory. Partly-related data from Kelvin contact potential studies in the literature suggest that non-additivity is the likely outcome.

5. Adsorption and micellar properties of surfactants in aqueous solutions.

This work is presently focused on the effects of high charge densities and hydrogen bonding on micelles and adsorption at air/ water and oil/water interfaces. The complex micellar and solubility properties of monoalky phosphate ester salts give unexpected properties with interesting applications of the Phase Rule. These surfactants can nominally provide very high charge densities due to two-stage ionization. It transpires that the high charge densities of adsorbed or spread phosphate ions are nearly matched by counterion adsorption, giving properties similar in some respects to the zwitterionic phospholipids. The micellar and precipitation patterns shown by the soluble monoesters salts are explained by the formation and precipitation of quarter salts, accompanied with an associated pH shift.

Studies with alkyl guanidine solutions show unusual behavior, apparently related to the efficient hydrogen bonding available to the quaternary cationic guanidine head group. Adsorption of the dodecyl derivative at the air/water interfaces gives very high surface pressures. Micelle formation has not yet been demonstrated.

Selected Publications

J. Phys.Chem C 2011. 115, 8056, with S.R. Middleton, N.R. Pallas and J. Mingins.
Thermodynamics of ionized monolayers: Surface manometry on very low density spread monolayers of sodium octadecyl sulphate at the air/water interface and analysis of ionic double layer contributions to the isotherms.

Phys.Chem.Chem.Phys. 2010, 12, 7445.
The thermodynamics of protein folding: a critique of widely used quasi-thermodynamic interpretations and a restatement based on the Gibbs-Duhem relation and consistent with the Phase Rule.

Phys.Chem,Chem.Phys. 2009, 11, 5028, with N.R.Pallas.
Intermolecular forces in lipid monolayers. Two-dimensional virial coefficients for pentadecanoic acid from micromanometry on spread monolayers at the air/water interface.

Chem.Commun. 2009, 4441, with T.Y.Cho, N.Byrne, D.J.Moore, C.A.Austen and P.Debenedetti
Structure-energy relations in hen egg white lysozyme observed during refolding from a quenched unfolded state.

Phys.Chem.Chem.Phys. 2007, 9, 6253.
Are electrostatic potentials between regions of different chemical composition measurable? The Gibbs-Gugenheim pprinciple reconsidered, extended and its consequences revisited.

J. Phys.Chem. B 2007, 111, 74, with N.R. Pallas.
Lateral intermolecular forces between biomembrane lipids in two dimensions: 1,2 dipalmitin at the heptane/water interface compared to phospholipids.

Langmuir 2005, 21, 944, with M.L.Glasser .
Lateral intermolecular forces in the physisorbed state. Surface field polarization of benzene and n-hexane at the water and mercury/ vapor interfaces.

Langmuir, 2004, 20, 7943, with J. Mingins.
Intermolecular forces in spread phospholipid monolayers at oil/water interfaces

J. Phys.Chem. B 2006, 110, 24936, with J. Carnali.
Quartersalt formation defining the anomalous temperature dependence of the aqueous solubility of sodium monodecyl phosphate,

Energy and Fuels 2006, 20, 250, with X.Guo, J.S. Huang, D.H. Adamson and R.K. Prud’homme.
Effect of cooling rate on crystallization of model waxy oils with microcrystalline poly(ethylene/butane)

J.Phys.Chem. B 2004, 108, 16717, with Y.Zhang, P.B.Debenedetti, and R.K.Prud’homme.
Differential scanning calorimetry studies of clathrate hydrate formation.

Ind.Eng, Chem.Research 2006, 45, 5134, with S.L.Lee, A.F. Hafeman, P.G.Debenedetti and D.J.Moore.
Solid state stabilization of α-chymotrypsin and catalase with carbohydrates.